Article

Observations of Diurnal Variation in a Cloud-capped Marine Boundary Layer

June 1991
Journal of the Atmospheric Sciences 48(12):1474-1482

June 1991
48(12):1474-1482

DOI:10.1175/1520-0469(1991)048<1474:OODVIA>2.0.CO;2

Authors:

Observations are presented of the turbulent structure of a cloud-capped boundary layer during the 1987 FIRE marine stratocumulus project. The measurements were made from San Nicolas Island, off the coast of California, using instruments attached to the cable of a tethered balloon. A marked diurnal variation is demonstrated through a direct comparison of observations made around local noon and approximately 12 hours later. Emphasis is given to the decoupling of the boundary layer during the morning into a separate cloudy layer and surface mixed layer.

Turbulent and boundary layer characteristics during VOCALS-REx

Article

Full-text available

Feb 2021
ATMOS CHEM PHYS

Boundary layer and turbulent characteristics (surface fluxes, turbulent kinetic energy – TKE, turbulent kinetic energy dissipation rate – ϵ), along with synoptic-scale changes in these properties over time, are examined using data collected from 18 research flights made with the CIRPAS Twin Otter Aircraft. Data were collected during the Variability of the American Monsoon Systems (VAMOS) Ocean–Cloud–Atmosphere–Land Study Regional Experiment (VOCALS-REx) at Point Alpha (20∘ S, 72∘ W) in October and November 2008 off the coast of South America. The average boundary layer depth is found to be 1148 m, with 28 % of the boundary layer profiles analyzed displaying decoupling. Analysis of correlation coefficients indicates that as atmospheric pressure decreases, the boundary layer height (zi) increases. As has been shown previously, the increase in zi is accompanied by a decrease in turbulence within the boundary layer. As zi increases, cooling near cloud top cannot sustain mixing over the entire depth of the boundary layer, resulting in less turbulence and boundary layer decoupling. As the latent heat flux (LHF) and sensible heat flux (SHF) increase, zi increases, along with the cloud thickness decreasing with increasing LHF. This suggests that an enhanced LHF results in enhanced entrainment, which acts to thin the cloud layer while deepening the boundary layer. A maximum in TKE on 1 November (both overall average and largest single value measured) is due to sub-cloud precipitation acting to destabilize the sub-cloud layer while acting to stabilize the cloud layer (through evaporation occurring away from the surface, primarily confined between a normalized boundary layer height, z/zi, of 0.40 to 0.60). Enhanced moisture above cloud top from a passing synoptic system also acts to reduce cloud-top cooling, reducing the potential for mixing of the cloud layer. This is observed in both the vertical profiles of the TKE and ϵ, in which it is found that the distributions of turbulence for the sub-cloud and in-cloud layer are completely offset from one another (i.e., the range of turbulent values measured have slight or no overlap for the in-cloud and sub-cloud regions), with the TKE in the sub-cloud layer maximizing for the analysis period, while the TKE in the in-cloud layer is below the average in-cloud value for the analysis period. Measures of vertical velocity variance, TKE, and the buoyancy flux averaged over all 18 flights display a maximum near cloud middle (between normalized in-cloud height, Z*, values of 0.25 and 0.75). A total of 10 of the 18 flights display two peaks in TKE within the cloud layer, one near cloud base and another near cloud top, signifying evaporative and radiational cooling near cloud top and latent heating near cloud base. Decoupled boundary layers tend to have a maximum in turbulence in the sub-cloud layer, with only a single peak in turbulence within the cloud layer.

Aerosol impacts on the life cycle of boundary layer clouds

Article

Full-text available

Nov 2008

Irina Sandu

Anthropogenic aerosols may have a noticeable impact on the life cycle of boundary layer clouds, via their effects on radiation and precipitation efficiency. It is however difficult to document such impacts from observations. The interactions between aerosol particles and the dynamics of boundary layer cloud systems (typically marine stratocumulus) have therefore been explored with high resolution numerical models (LES), that now include detailed parameterizations of turbulence, radiative transfer and microphysics. In this study, the focus is on the coupling between aerosol impacts on cloud microphysics and the diurnal cycle of stratocumulus clouds. LES simulations of a 36 hours cycle are performed with aerosol concentrations typical of pristine and polluted air masses, successively. Although the simulations start from the same initial state, they rapidly diverge. The increased concentration of cloud condensation nuclei yields to an increased droplet concentration, a reduction of the droplet sizes and the inhibition of the droplet sedimentation and precipitation formation. The liquid water content at cloud top hence increases and the cloud top entrainment is strengthened. Moreover, the absorption of solar radiation at cloud base is no longer balanced by the droplet and drizzle evaporation, and the decoupling of the cloud layer is reinforced. Overall, the polluted cloud layer is better coupled during the night and more decoupled during the day than its pristine counterpart. Measurable signatures of these impacts are identified to help at designing observational studies of aerosol impacts on the dynamics on boundary layer clouds.

The Semi-Direct Aerosol Effect

Article

Jan 2003

Benjamin Thomas Johnson

A Moist PBL Parameterization for Large-Scale Models and Its Application to Subtropical Cloud-Topped Marine Boundary Layers

Article

Mar 2001

A new general purpose boundary layer parameterization that permits realistic treatment of stratocumulus-capped boundary layers (SCBLs) with coarse vertical resolution is described. It combines a 1.5-order turbulent closure model with an entrainment closure at the boundary layer top. Three different implementations of the entrainment closure, in which the boundary layer height is respectively prognosed, reconstructed from thermodynamic values at the grid points, or restricted to lie on a flux level of the host model grid, are tested in a single-column modeling framework at both fine and coarse vertical resolution. The first two approaches permit a stratocumulus top and base to lie between grid levels and evolve continuously with time, but are more complicated to implement in a three-dimensional model. The model performs very well in cases of dry convection, whatever the inversion implementation and the vertical resolution. With 15-mb or better vertical resolution, all approaches properly simulate mixing in SCBLs. including daytime cloud thinning and a transition to decoupling and conditional instability as SST increases. With coarser resolution, details of the implementation influence the simulated cloud thickness, which is systematically underestimated with the restricted inversion approach. A method for computing vertical advective fluxes at the boundary layer top that explicitly accounts for the inversion is presented; an essential component of the reconstructed inversion implementation, this vertical advection scheme also improves SCBL simulation at low resolution with a restricted inversion. For comprehensive simulation of boundary layer convection, this scheme should be coupled with a parameterization of shallow cumulus convection; this will be described in a forthcoming paper.

The Stability of Cloud Top with Regard to Entrainment: Amendment of the Theory of Cloud-Top Entrainment Instability

Article

Feb 1993

Peter G. Duynkerke

The stability of a uniformly saturated cloud layer separated from an overlying nonturbulent unsaturated layer by a thin inversion is considered. The stability of the interface can be described by entraining a parcel of air from above the inversion into the cloud layer below and by subsequently studying the effect of the mixing on the buoyancy of the parcel. From the relevant momentum equation for the parcel it is shown that the important quantity to consider is the total buoyancy (total mass of the parcel times the virtual potential temperature difference between parcel and environment) of the parcel per unit mass of entrained air. The total buoyancy is a more general and useful concept than all other parameters discussed in the literature. -Author

Improving Stratocumulus Cloud Amounts in a 200‐m Resolution Multi‐Scale Modeling Framework Through Tuning of Its Interior Physics

Article

Full-text available

Mar 2024

High‐Resolution Multi‐scale Modeling Frameworks (HR)—global climate models that embed separate, convection‐resolving models with high enough resolution to resolve boundary layer eddies—have exciting potential for investigating low cloud feedback dynamics due to reduced parameterization and ability for multidecadal throughput on modern computing hardware. However low clouds in past HR have suffered a stubborn problem of over‐entrainment due to an uncontrolled source of mixing across the marine subtropical inversion manifesting as stratocumulus dim biases in present‐day climate, limiting their scientific utility. We report new results showing that this over‐entrainment can be partly offset by using hyperviscosity and cloud droplet sedimentation. Hyperviscosity damps small‐scale momentum fluctuations associated with the formulation of the momentum solver of the embedded large eddy simulation. By considering the sedimentation process adjacent to default one‐moment microphysics in HR, condensed phase particles can be removed from the entrainment zone, which further reduces entrainment efficiency. The result is an HR that can produce more low clouds with a higher liquid water path and a reduced stratocumulus dim bias. Associated improvements in the explicitly simulated sub‐cloud eddy spectrum are observed. We report these sensitivities in multi‐week tests and then explore their operational potential alongside microphysical retuning in decadal simulations at operational 1.5° exterior resolution. The result is a new HR having desired improvements in the baseline present‐day low cloud climatology, and a reduced global mean bias and root mean squared error of absorbed shortwave radiation. We suggest it should be promising for examining low cloud feedbacks with minimal approximation.

The Turbulence Structure in a Continental Stratocumulus Cloud from Millimeter-Wavelength Radar Observations

Article

Full-text available

Aug 2000

The turbulent-scale vertical velocity structure in a continental stratocumulus cloud is studied using a 3-mm wavelength Doppler radar operating in a vertically pointing mode. The radar observations provided 30-m sampling in the vertical with 2-s averages of 10000 samples. Vertical velocity measurements were made continuously for an 8-h period and were further supported by measurements of cloud-base height from a laser ceilometer and liquid water path from a microwave radiometer. During the beginning of the observational period, the cloud layer extended between 200 and 800 m. The vertical velocity variance profiles evolved systematically over the period from a well-defined peak in the upper part of the cloud layer of 0.7 m2 s2 to a peak in the lower part of the cloud layer of 0.2 m2 s2 as the layer became decoupled later in the observing period. The vertical velocity skewness during the well-coupled conditions was negative through most of the cloud, consistent with the presence of relatively narrow downdrafts. A positive skewness in the top 100 m of the cloud is consistent with relatively narrow penetrating updrafts at this level.The radar vertical velocities are used to compare the directly observed updraft fractional coverage and mass flux with those obtained from the bulk statistics. These comparisons are consistent with similar comparisons made using a large eddy simulation model. The fractional coverage and the mass flux associated with coherent updraft structures are obtained for a range of criteria used to define the updrafts. A more detailed analysis of the vertical velocities in the cloud confirms the existence of well-defined downdrafts extending through the entire cloud depth. These downdrafts are estimated to have horizontal dimensions of about 200 m and appear to originate on the downshear side of updrafts. The reduction of radar reflectivity at cloud top in the downdrafts is consistent with the entrainment of drier air. This study further illustrates the utility of millimeter-wavelength radars for studying turbulence in boundary layer clouds and particularly in defining the vertical structure of coherent eddies.

A robust method to determine global distribution of atmospheric boundary layer top from COSMIC GPS RO measurements

Article

May 2010

In this study, we introduce a robust method for precise determination of atmospheric boundary layer (ABL) top from COSMIC global positioning system radio occultation measurements. We apply a wavelet covariance transform to compute the convolution of COSMIC-observed bending angle/refractivity profile with a Haar function and use the maximum covariance to identify the ABL top, making detection of even small transitions possible. Results obtained were compared with radiosonde N profiles for verification of the ABL top. This procedure developed was used to study the global distribution of ABL top with special reference to the inter-tropical convergence zone.

Evidence of a Diurnal Cycle in Precipitation over the Southern Ocean as Observed at Macquarie Island

Article

Full-text available

Feb 2020

Due to a lack of observations, relatively large discrepancies exist between precipitation products over the Southern Ocean. In this manuscript, surface hourly precipitation observations from Macquarie Island (54.62oS, 158.85oE) are analysed (1998-2016) to reveal a diurnal cycle. The precipitation rate is at a maximum during night/early morning and a minimum in the afternoon at Macquarie Island station. Seasonally, the diurnal cycle is strongest in summer and negligible over winter. Such a cycle is consistent with precipitation arising from marine boundary layer clouds, suggesting that such clouds are making a substantial contribution to total precipitation over Macquarie Island and the Southern Ocean. Using twice daily upper air soundings (1995-2011), lower troposphere stability parameters show a stronger inversion at night, again consistent with precipitation arising from marine boundary layer clouds. The ERA-Interim precipitation is dominated by a 12 hourly cycle, year around, which is likely to be a consequence of the twice-daily initialisation. The implication of a diurnal cycle in boundary layer clouds over the Southern Ocean to derived A-Train satellite precipitation products is also discussed.

Diurnal cycle of the semi-direct effect from a persistent absorbing aerosol layer over marine stratocumulus in large-eddy simulations

Article

Full-text available

Feb 2020
ATMOS CHEM PHYS

The rapid adjustment, or semi-direct effect, of marine stratocumulus clouds to elevated layers of absorbing aerosols may enhance or dampen the radiative effect of aerosol–radiation interactions. Here we use large-eddy simulations to investigate the sensitivity of stratocumulus clouds to the properties of an absorbing aerosol layer located above the inversion layer, with a focus on the location, timing, and strength of the radiative heat perturbation. The sign of the daily mean semi-direct effect depends on the properties and duration of the aerosol layer, the properties of the boundary layer, and the model setup. Our results suggest that the daily mean semi-direct effect is more elusive than previously assessed. We find that the daily mean semi-direct effect is dominated by the distance between the cloud and absorbing aerosol layer. Within the first 24 h the semi-direct effect is positive but remains under 2 W m−2 unless the aerosol layer is directly above the cloud. For longer durations, the daily mean semi-direct effect is consistently negative but weakens by 30 %, 60 %, and 95 % when the distance between the cloud and aerosol layer is 100, 250, and 500 m, respectively. Both the cloud response and semi-direct effect increase for thinner and denser layers of absorbing aerosol. Considerable diurnal variations in the cloud response mean that an instantaneous semi-direct effect is unrepresentative of the daily mean and that observational studies may underestimate or overestimate semi-direct effects depending on the observed time of day. The cloud response is particularly sensitive to the mixing state of the boundary layer: well-mixed boundary layers generally result in a negative daily mean semi-direct effect, and poorly mixed boundary layers result in a positive daily mean semi-direct effect. The properties of the boundary layer and model setup, particularly the sea surface temperature, precipitation, and properties of the air entrained from the free troposphere, also impact the magnitude of the semi-direct effect and the timescale of adjustment. These results suggest that the semi-direct effect simulated by coarse-resolution models may be erroneous because the cloud response is sensitive to small-scale processes, especially the sources and sinks of buoyancy.

A Study of Near-Surface Boundary Layer Characteristics During the 2015 Chennai Flood in the Context of Urban-Induced Land Use Changes

Article

Full-text available

Jun 2019
PURE APPL GEOPHYS

Chennai metropolitan region along with the northern part of Tamil Nadu and southern part of Andhra Pradesh witnessed extreme rainfall events leading to urban flooding during November–December 2015. In order to understand the near-surface and boundary layer (BL) characteristics during the event in the context of land use and land cover (LULC) change, three decades of satellite images are analysed. In addition, the Weather Research Forecasting (WRF) model is used to perform finer-scale simulations, considering different land use (LU) data sets as input. For this purpose, LU datasets from the United Sates Geological Survey (USGS), Moderate Resolution Imaging Spectro-radiometer (MODIS), and Indian Space Research Organization (ISRO) are considered along with the Noah and Noah multi-physics (NMP) land surface model (LSM). Impact of Noah-based LSMs on the near-surface and BL characteristics over Chennai are examined besides the change in LULC during the flood event. Significant improvement of 1–2 °C is obtained in case of near-surface temperature in the simulation considering recent LU and the NMP LSM. Some WRF-simulated variables like near-surface temperature, relative humidity (RH) and convective available potential energy (CAPE) are compared with available observations for qualitative and quantitative analysis. The distorted variations of the near-surface and boundary layer parameters including temperature, BL height, sensible heat flux and CAPE, are mostly observed during phases with prevalent low-pressure systems due to the presence of large-scale forcing. In other phases, (where low-pressure systems are absent), with dominance of localised effects, noticeably higher values of the variables viz. near-surface air temperature, wind speed, RH and moisture flux, CAPE and BL height are attributed to the increased impervious layers inside the city boundary due to urbanization and its growth.

Exploring the differences in cloud properties observed by the Terra and Aqua MODIS sensors

Article

Full-text available

Jan 2009
ACP

The aerosol-cloud interaction in different parts of the globe is examined here using multi-year statistics of remotely sensed data from two MODIS sensors aboard NASA's Terra (morning) and Aqua (afternoon) satellites. Simultaneous retrievals of aerosol loadings and cloud properties by the MODIS sensor allowed us to explore intra-diurnal variation of liquid cloud fraction (CF) and optical thickness (COT) for clean, moderately polluted and heavily polluted clouds in different seasons. Data analysis for six-years of MODIS retrievals revealed strong temporal and spatial patterns in intra-diurnal variation of cloud fraction and optical thickness over different parts of the global oceans and the land. For the vast areas of stratocumulus cloud regions, the data shows that the presence of aerosols can more than double afternoon reduction of CF and COT pointing to the possible predominance of semi-direct over the indirect effects of aerosols in stratocumulus clouds. A positive relationship between AOD and morning-to-afternoon variation of trade wind cumulus cloud cover was also found over the northern Indian Ocean, though no clear correlation between the concentration of Indo-Asian haze and intra-diurnal variation of COT was established. Over the Amazon region during wet conditions, aerosols are associated with an enhanced convective process in which morning shallow warm clouds are organized into afternoon deep convection with greater ice cloud coverage. Analysis presented here demonstrates that the new technique for exploring intra-diurnal variability in cloud properties by using the differences in data products from the two daily MODIS overpasses is capable of capturing some of the major features of morning-to-afternoon variations in cloud properties and can be used for improved understanding of aerosol radiative effects.

METEOROLOGICAL PROCESSES IN THE BOUNDARY LAYER AND THEIR IMPLICATIONS ON THE SURFACE TEMPERATURE – THE BASICS

Research

Full-text available

Aug 2015

Prasanjit Dash

'Meteorological processes in the planetary boundary layer - the basics' was documented as a term paper while pursuing Doctorate at Inst. of Meteorology, Forschungszentrum Karlsruhe, Germany, 2002. In the meanwhile, Forschungszentrum Karlsruhe is now called as Karlsruhe Institute of Technology, KIT, in partnership with University of Karlsruhe.] The views and opinions contained in this term-paper are mine, based on my then understanding, and should not be construed as either an official NOAA or KIT or CSU statement.

Study of the Cloud Condensation Nuclei activation: measurements, analysis and instrument development.

Article

Full-text available

Oct 2003

Regis Dupuy

The goal of this thesis is to improve the understanding of the interactions between aerosols and clouds. With this intention, we have to cover as broad and various subjects as instrumental development, statistical analysis and modeling. Thus, we were brought to install and develop a sampler of cloudy air at the puy of Dome observatory. It now allows a continuous sampling even in cloudy conditions. It was then followed with two campaign of measurements during the winters 2000 and 2001 wich end in a collection of more than 600000 samples. Thanks to this broad panel of weather conditions and sources of air mass, we could quantify one of the key points of relationship between aerosol and cloud studies by obtaining a relationship between the particle total concentration and, on the one hand the cloud droplet concentration, and on the other hand the cloud effective radius for constant liquid water contents. Moreover, the use of a cloud droplet activation model called ExMix enabled us to have a better comprehension of particles incorporation into droplets trends. In this manner, we compared modeled and measured trends with increasing water contents and with increasing cloud effective radius for constant liquid water contents. However, that raised the important lack of aerosol physicochemical measurement with a higher frequency we can get until now. It shows, for example, a need to better take into account the aerosol organic phase. That's why an analyzer of volatility and granulometry of aerosols has been developed, built and tested during the last part of this thesis.

EVALUATING THE SEMI-DIRECT AEROSOL EFFECT: THINNING OF MARINE STRATOCUMULUS DUE TO ABSORPTION OF SOLAR RADIATION BY AEROSOL

Article

Full-text available

Countergradient Fluxes of Conserved Variables in the Clear convective and Stratocumulus-topped Boundary Layer: The role of the Entrainment Flux

Article

Jul 2004

Large-eddy simulations of a clear convective boundary layer (CBL) and a stratocumulus-topped boundary layer are studied. Bottom-up and a top-down scalars were included in the simulations, and the principle of linear superposition of variables was applied to reconstruct the fields of any arbitrary conserved variable. This approach allows a systematic analysis of countergradient fluxes as a function of the flux ratio, which is defined as the ratio between the entrainment flux and the surface flux of the conserved quantity. In general, the turbulent flux of an arbitrary conserved quantity is counter to the mean vertical gradient if the heights where the vertical flux and the mean vertical gradient change sign do not coincide. The regime where the flux is countergradient is therefore bounded by the so-called zero-flux and zero-gradient heights. Because the vertical flux changes sign only if the entrainment flux has an opposite sign to the surface flux, countergradient fluxes are predominantly found for negative flux ratios. In the CBL the flux ratio for the virtual potential temperature is, to a good approximation, constant, and equal to -0.2. Only if the moisture contribution to the virtual potential temperature is negligibly small will the flux ratio for the potential temperature be equal to this value. Otherwise, the flux ratio for the potential temperature can have any arbitrary (negative) value, and, as a consequence, the fluxes for the potential temperature and the virtual potential temperature will be countergradient at different heights. As a practical application of the results, vertical profiles of the countergradient correction term for different entrainment-to-surface-flux ratios are discussed.

Moisture Transport, Lower-Tropospheric Stability, and Decoupling of Cloud-Topped Boundary Layers

Article

Jan 1997

Decoupling during the `Lagrangian' evolution of a cloud-topped boundary layer advected equatorward by the trade winds in an idealized eastern subtropical ocean is studied using a mixed-layer model (MLM). The sea surface temperature is gradually warmed while the free tropospheric sounding remains unchanged, causing the boundary layer to deepen, the surface relative humidity to decrease, and surface latent heat fluxes to increase. Diurnally averaged insolation is used.For entrainment closures in which entrainment rate is related to a large-eddy convective velocity scale w(, the MLM predicts an increasingly prominent layer of negative buoyancy fluxes below cloud base as the sea surface temperature warms. Decoupling of the mixed layer can be inferred when the MLM-predicted negative buoyancy fluxes become too large for the internal circulations to sustain. The authors show that decoupling is mainly driven by an increasing ratio of the surface latent heat flux to the net radiative cooling in the cloud, and derive a decoupling criterion based on this ratio. Other effects such as drizzle, the vertical distribution of radiative cooling in the cloud, and sensible heat fluxes, also affect decoupling but are shown to be less important in typical subtropical boundary layers. A comparison of MLM results with a companion numerical study with a cloud-resolving model shows that the decoupling process is similar and the same decoupling criterion applies. A regional analysis of decoupling using Lagrangian trajectories based on summertime northeast Pacific climatology predicts decoupling throughout the subtropical stratocumulus region except in coastal zones where the boundary layer is under 750 m deep.A `flux-partitioning' entrainment closure, in which the entrainment rate is chosen to maintain a specified ratio of some measure of negative subcloud buoyancy fluxes to positive buoyancy fluxes within the cloud and near the surface, was also considered. By construction, such an MLM never predicts its own breakdown by decoupling. The changed entrainment closure had only a minor influence on the boundary layer evolution and entrainment rate. Instead, the crucial impact of the entrainment closure is on predicting when and where the mixed-layer assumption will break down due to decoupling.

A robust method to determine global distribution of atmospheric boundary layer top from COSMIC GPS RO measurements

Data

May 2010
Atmos Sci Lett

Anthropogenic radiative forcing of marine stratocumulus clouds under different thermodynamic conditions—An LES model study

Article

Nov 2012
ATMOS RES

A LES model with bin microphysics was used to investigate the aerosol indirect effects of marine stratocumulus clouds that develop under different thermodynamic conditions. The diurnal contrasts of cloud development were also examined in detail. Three observed CCN spectra that represent maritime, continental and polluted air masses were used as input CCN spectra. Two observed thermodynamic soundings and two derived ones from the observed soundings to vary the inversion altitude were used as initial thermodynamic conditions. With these initial conditions the model was run for the daytime and nocturnal conditions to make the total number of model runs to be 24. For both daytime and nocturnal conditions, the cloud depth and liquid water path (LWP) varied with the thermodynamic soundings. For a given thermodynamic sounding, LWP tended to be similar or slightly smaller for polluted. However, cloud top radiative cooling is stronger for polluted due to smaller sizes of cloud droplets and therefore turbulent mixing is stronger for polluted. However, there were significant differences in LWP between the daytime and nocturnal clouds. For the daytime condition, the cloud became decoupled from the surface layer and moisture supply was limited for all soundings. In contrast, with the absence of solar radiation, cloud top radiative cooling was much stronger, turbulent mixing was also much stronger and therefore no decoupling occurred and clouds were thicker with greater LWP in the nocturnal runs. To note is the derived thermodynamic sounding (named MH) that produced the thickest clouds: the clouds were too thick to be maintained as a single layer cloud and became multi-layered for the daytime condition while the maritime stratocumulus cloud broke up into a cumuliform cloud due to heavy drizzle for the nocturnal condition. From the daytime run results, the anthropogenic cloud radiative forcing was calculated by subtracting the net cloud radiative forcing for maritime from that for polluted. It amounted to be − 21.6 W m− 2 for MH but was more than a factor of two larger for all three other soundings, − 56.4, − 55.4 and − 55.7 W m− 2. For the MH sounding, the LWP was noticeably smaller for polluted than for maritime, the relative difference of the effective radii between maritime and polluted was small and therefore albedo difference between the two was also small, compared to those for the other three soundings. Notable is the similar magnitude of the anthropogenic cloud radiative forcing for these three soundings despite the significant differences in cloud depths among the clouds produced by these soundings. This may imply that there may be an optimal range of cloud depth that can produce a strong anthropogenic cloud radiative forcing. The cloud depths were smaller than 150 m for the thermodynamic sounding that produced the shallowest clouds but it seemed to be within the optimal range.

Explicit cloud-top entrainment parameterization in the global climate model ECHAM5HAM

Article

Full-text available

Jan 2011
ACP

New developments in the turbulence parameterization in the general circulation model ECHAM5-HAM are presented. They consist mainly of an explicit entrainment closure at the top of stratocumulus-capped boundary layers and the addition of an explicit contribution of the radiative divergence in the buoyancy production term. The impact of the new implementations on a single column model study and on the global scale is presented here. The parameterization has a "smoothing" effect: the abnormally high values of turbulence kinetic energy are reduced, both in the single column and in the Californian stratocumulus region. A sensitivity study with prescribed droplet concentration shows a reduction in the sensitivity of liquid water path to increasing cloud aerosol optical depth. We also study the effect of the new implementation on a Pacific cross-section. The entrainment parameterization leads to an enhanced triggering of the convective activity.

Diurnal Variability of the Marine Boundary Layer during ASTEX

Article

Aug 2001

The diurnal cycle of the marine boundary layer over the Atlantic Stratocumulus Transition Experiment (ASTEX) domain is examined during a two-week period of June 1992 in several fields, including: fractional low cloudiness, various parameters that enter into the heat and moisture budgets (e.g., temperature, water vapor mixing ratio, vertical motion), the budgets themselves, radiative heating, and the vertical eddy flux of moist static energy. Results show fractional low cloudiness varies over this region from a maximum of 54% in the predawn hours to a minimum of 39% in the midafternoon. These changes in low cloudiness are accompanied by an opposite trend in the boundary layer moisture, which shows a predawn drying and an afternoon moistening. Large-scale vertical motion also varies diurnally with a peak amplitude of 0.12 cm s1 at 1.8 km, which is a 20% variation from the mean subsidence rate. Cloud-radiative processes play an important role in determining the diurnal characteristics of the vertical eddy flux of moist static energy, with a peak amplitude in the cloud layer at 1.2 km and a variation at this level from 24 W m2 at 1300 LST to 100 W m2 at 0400 LST. These diurnal changes suggest that the early morning peak in eddy flux results in a drying of the subcloud layer as moisture transport into the cloud layer is enhanced, leading to the observed low-cloud maximum. In contrast, during the daylight hours, solar heating within the cloud leads to a well-mixed cloud layer, a stabilizing of the subcloud layer with respect to the cloud layer, a decoupling of the cloud and subcloud layers, and ultimately, a thinning or reduction of the low clouds.Comparison of the diurnal cycle of summertime low-level divergence and vertical motion from this study with earlier published results from other regions in the North Atlantic shows a complex diurnal pattern with alternating phase in adjacent latitude belts across this region. The sense of these phase changes suggests that the northern branch of the Hadley circulation from roughly 5°N to 25°N over the eastern Atlantic is pulsing diurnally with enhanced mass flow during the early morning hours.

Relationships between Marine Stratus Cloud Optical Depth and Temperature: Inferences from AVHRR Observations

Article

May 2007

Studies using International Satellite Cloud Climatology Project (ISCCP) data have reported decreases in cloud optical depth with increasing temperature, thereby suggesting a positive feedback in cloud optical depth as climate warms. The negative cloud optical depth and temperature relationships are questioned because ISCCP employs threshold assumptions to identify cloudy pixels that have included partly cloudy pixels. This study applies the spatial coherence technique to one month of Advanced Very High Resolution Radiometer (AVHRR) data over the Pacific Ocean to differentiate overcast pixels from the partly cloudy pixels and to reexamine the cloud optical depth-temperature relationships. For low-level marine stratus clouds studied here, retrievals from partly cloudy pixels showed 30%-50% smaller optical depths, 1°-4°C higher cloud temperatures, and slightly larger droplet effective radii, when they were compared to retrievals from the overcast pixels. Despite these biases, retrievals for the overcast and partly cloudy pixels show similar negative cloud optical depth-temperature relationships and their magnitudes agree with the ISCCP results for the midlatitude and subtropical regions. There were slightly negative droplet effective radius-temperature relationships, and considerable positive cloud liquid water content-temperature relationships indicated by aircraft measurements. However, cloud thickness decreases appear to be the main reason why cloud optical depth decreases with increasing temperature. Overall, cloud thickness thinning may explain why similar negative cloud optical depth-temperature relationships are found in both overcast and partly cloudy pixels. In addition, comparing the cloud-top temperature to the air temperature at 740 hPa indicates that cloud-top height generally rises with warming. This suggests that the cloud thinning is mainly due to the ascending of cloud base. The results presented in this study are confined to the midlatitude and subtropical Pacific and may not be applicable to the Tropics or other regions.

The Temperature Dependence of the Liquid Water Path of Low Clouds in the Southern Great Plains

Article

Oct 2000

Satellite observations of low-level clouds have challenged the idea that increasing liquid water content with temperature combined with constant physical thickness will lead to a negative cloud optics feedback in a decadal climate change. The reasons for the satellite results are explored using 4 yr of surface remote sensing data from the Atmospheric Radiation Measurement Program Cloud and Radiation Testbed site in the southern Great Plains of the United States. It is found that low-cloud liquid water path is approximately invariant with temperature in winter but decreases strongly with temperature in summer, consistent with satellite inferences at this latitude. This behavior occurs because liquid water content shows no detectable temperature dependence while cloud physical thickness decreases with warming. Thinning of clouds with warming is observed on seasonal, synoptic, and diurnal timescales; it is most obvious in the warm sectors of baroclinic waves. Although cloud top is observed to slightly descend with warming, the primary cause of thinning is the ascent of cloud base due to the reduction in surface relative humidity and the concomitant increase in the lifting condensation level of surface air. Low-cloud liquid water path is not observed to be a continuous function of temperature. Rather, the behavior observed is best explained as a transition in the frequency of occurrence of different boundary layer types. At cold temperatures, a mixture of stratified and convective boundary layers is, observed, leading to a broad distribution of liquid water path values, while at warm temperatures, only convective boundary layers with small liquid water paths, some of them decoupled, are observed. Our results, combined with the earlier satellite inferences, suggest a reexamination of the commonly quoted 1.5°C lower limit for the equilibrium global climate sensitivity to a doubling of CO2, which is based on models in which liquid water increases with temperature and cloud physical thickness is constant.

A New Boundary Layer Mixing Scheme. Part I: Scheme Description and Single-Column Model Tests

Article

Sep 2000

A new boundary layer turbulent mixing scheme has been developed for use in the UKMO weather forecasting and climate prediction models. This includes a representation of nonlocal mixing (driven by both surface fluxes and cloud-top processes) in unstable layers, either coupled to or decoupled from the surface, and an explicit entrainment parameterization. The scheme is formulated in moist conserved variables so that it can treat both dry and cloudy layers. Details of the scheme and examples of its performance in single-column model tests are presented.

Test of the Volume-of-Fluid Method on Simulations of Marine Boundary Layer Clouds

Article

Jun 2000

The impact of using grid-averaged thermodynamic properties (i.e., neglecting their subgrid variability due to partial cloudiness) to represent forcings for condensation or evaporation has long been recognized. In particular, numerical difficulties in terms of spurious oscillations and/or diffusion in vicinity of a cloud environment interface have been encountered in most of the conventional finite-difference Eulerian advection schemes. This problem is equivalent to the inability of models to accurately track the cloud boundary within a grid cell, which eventually leads to spurious production or destruction of cloud water at leading or trailing edges of clouds. This paper employs a specialized technique called the "volume of- fluid" (VOF) method to better parameterize the subgrid-scalc advection process that accounts for the transport of material interfaces. VOF also determines the actual location of the partial cloudiness within a grid box. Consequently, relevant microphysical parameterizations in mixed cells can be consistently applied in "cloudy" and "clear" regions. The VOF technique is incorporated in a two-dimensional hydrodynamic model to simulate the diurnal cycle of the marine stratocumulus-capped boundary layer. The fidelity of VOF to advection-condensation processes under a diurnal radiative forcing is assessed by comparing the model simulation with data taken during the ISCCP FIRE observational period as well as with results from simulations without VOF. This study shows that the VOF method indeed suppresses the spurious cloud boundary instability and supports a multiday cloud evolution as observed. For the case without VOF, the spurious instability near the cloud top causes the dissipation of the entire cloud layer within a half of a diurnal cycle.

Balance of Forces in the Atmospheric Boundary Layer

Chapter

Feb 2012

Xuhui Lee

In this chapter, we divide the lower atmosphere into several sublayers, each with different degrees of turbulent mixing and different mechanisms of turbulence production.

Improving stratocumulus cloud amounts in a 200-m resolution multi-scale modeling framework through tuning of its interior physics

Preprint

Full-text available

Jan 2023

High-Resolution Multi-scale Modeling Frameworks (HR) -- global climate models that embed separate, convection-resolving models with high enough resolution to resolve boundary layer eddies -- have exciting potential for investigating low cloud feedback dynamics due to reduced parameterization and ability for multidecadal throughput on modern computing hardware. However low clouds in past HR have suffered a stubborn problem of over-entrainment due to an uncontrolled source of mixing across the marine subtropical inversion manifesting as stratocumulus dim biases in present-day climate, limiting their scientific utility. We report new results showing that this over-entrainment can be partly offset by using hyperviscosity and cloud droplet sedimentation. Hyperviscosity damps small-scale momentum fluctuations associated with the formulation of the momentum solver of the embedded LES. By considering the sedimentation process adjacent to default one-moment microphysics in HR, condensed phase particles can be removed from the entrainment zone, which further reduces entrainment efficiency. The result is an HR that is able to produce more low clouds with a higher liquid water path and a reduced stratocumulus dim bias. Associated improvements in the explicitly simulated sub-cloud eddy spectrum are observed. We report these sensitivities in multi-week tests and then explore their operational potential alongside microphysical retuning in decadal simulations at operational 1.5 degree exterior resolution. The result is a new HR having desired improvements in the baseline present-day low cloud climatology, and a reduced global mean bias and root mean squared error of absorbed shortwave radiation. We suggest it should be promising for examining low cloud feedbacks with minimal approximation.

The Diel Profile of Hydroperoxymethyl Thioformate: Evidence for Surface Deposition and Multiphase Chemistry

Article

Aug 2020

Dimethyl sulfide (DMS; CH3SCH3), a biogenically produced trace gas emitted from the ocean, accounts for a large fraction of natural sulfur released to the marine atmosphere. The oxidation of DMS in the marine boundary layer (MBL), via the hydrogen abstraction pathway, yields the short-lived methylthiomethylperoxy radical (MSP; CH3SCH2OO). In the remote MBL, unimolecular isomerization of MSP outpaces bimolecular chemistry leading to the efficient formation of hydroperoxymethyl thioformate (HPMTF; HOOCH2SCHO). Here, we report the first ground observations and diurnal profiles of HPMTF mixing ratios, vertical fluxes and deposition velocities to the ocean surface. Average daytime HPMTF mixing ratios, fluxes, and deposition velocities were recorded at 12.1 pptv, -0.11 pptv m s⁻¹, and 0.75 cm s⁻¹, respectively. The deposition velocity of HPMTF is comparable to other soluble gas phase compounds (e.g., HCOOH and HNO3), resulting in a deposition lifetime of 30 hours under typical windspeeds (3 m s⁻¹). A box model analysis incorporating the current mechanistic understanding of DMS oxidation chemistry, and geostationary satellite cloud imagery data suggests that the lifetime of HPMTF in the MBL at this sampling location is likely controlled by heterogenous loss to aerosol and uptake to clouds in the morning and evening.

The diurnal cycle of the smoky marine boundary layer observed during August in the remote southeast Atlantic

Article

Full-text available

Nov 2019
ATMOS CHEM PHYS

Ascension Island (8∘ S, 14.5∘ W) is located at the northwestern edge of the south Atlantic stratocumulus deck, with most clouds in August characterized by surface observers as “stratocumulus and cumulus with bases at different levels”, and secondarily as “cumulus of limited vertical extent” and occurring within a typically decoupled boundary layer. Field measurements have previously shown that the highest amounts of sunlight-absorbing smoke occur annually within the marine boundary layer during August. On more smoke-free days, the diurnal cycle in cloudiness includes a nighttime maximum in cloud liquid water path and rain, an afternoon cloud minimum, and a secondary late-afternoon increase in cumulus and rain. The afternoon low-cloud minimum is more pronounced on days with a smokier boundary layer. The cloud liquid water paths are also reduced throughout most of the diurnal cycle when more smoke is present, with the difference from cleaner conditions most pronounced at night. Precipitation is infrequent. An exception is the mid-morning, when the boundary layer deepens and liquid water paths increase. The data support a view that a radiatively enhanced decoupling persisting throughout the night is key to understanding the changes in the cloud diurnal cycle when the boundary layer is smokier. Under these conditions, the nighttime stratiform cloud layer does not always recouple to the sub-cloud layer, and the decoupling maintains more moisture within the sub-cloud layer. After the sun rises, enhanced shortwave absorption in a smokier boundary layer can drive a vertical ascent that momentarily couples the sub-cloud layer to the cloud layer, deepening the boundary layer and ventilating moisture throughout, a process that may also be aided by a shift to smaller droplets. After noon, shortwave absorption within smokier boundary layers again reduces the upper-level stratiform cloud and the sub-cloud relative humidity, discouraging further cumulus development and again strengthening a decoupling that lasts longer into the night. The novel diurnal mechanism provides a new challenge for cloud models to emulate. The lower free troposphere above cloud is more likely to be cooler, when boundary layer smoke is present, and lower free-tropospheric winds are stronger and more northeasterly, with both (meteorological) influences supporting further smoke entrainment into the boundary layer from above.

A regional real-time forecast of marine boundary layers during VOCALS-Rex

Article

Full-text available

Aug 2010
ACP

This paper presents an evaluation and validation of the Naval Research Laboratory's COAMPS real-time forecasts during the VOCALS-Rex over the area off the west coast of Chile/Peru in the Southeast Pacific during October and November 2008. The analyses focus on the marine boundary layer (MBL) structure. These forecasts are compared with lower troposphere soundings, surface measurements, and satellite observations. The predicted mean MBL cloud and surface wind spatial distributions are in good agreement with the satellite observations. The large-scale longitudinal variation of the MBL structure along 20° S is captured by the forecasts. That is, the MBL heights increase toward the open ocean, the moisture just above the inversion decreases, and the MBL structure becomes more decoupled offshore. The observed strong wind shear across the cloud-top inversion in coastal area at 20° S was correctly predicted by the model. Our results show that the sporadic cloud spatial and temporal distribution in the 15 km grid mesh is caused by grid-scale convection likely due to a lack of a shallow cumulus convection parameterization in the model. Both observations and model forecasts show wind speed maxima near the top of MBL along 20° S, which is consistent with the west-ward upslope of the MBL heights based on the thermal wind relationship. The forecasts produced well-defined diurnal variations in the spatially averaged MBL structure, although the overall signal is weaker than those derived from the in situ measurements and satellite data. The MBL heights are generally underpredicted in the nearshore area. The analysis of the sensitivity simulations with regard to grid resolution suggests that the under-prediction is likely associated with over-prediction of the mesoscale downward motion and cold advection along the coast.

Parameterization of Convective Boundary Layer Turbulence and Clouds in Atmospheric Models

Chapter

Nov 2009

Earth’s climate system is largely controlled by the transfer of properties across different interfaces. For this reason, boundary layers, which occupy a very small fraction of the atmosphere and ocean, are disproportionably important. Atmosphere-ocean and atmosphere-land fl uxes of water, momentum, and energy are mediated by these boundary layers.

Observations of stratocumulus cloud break-up over land

Article

Jan 1999

JD Price

Detailed measurements were made during three episodes of boundary-layer stratocumulus clouds occurring at Cardington, Bedfordshire, UK, using the UK Meteorological Office tethered-balloon system. Two cases were of cloud which dispersed or partly dispersed during the day, and the other was of persistent cloud. Analysis of these data reveals that one cloud deck dispersed owing to solar heating of the boundary layer warming the cloud above its dew-point, and the other dispersed because of a combination of the above and entrainment of drier and warmer air into the top of the cloud caused by strong wind shear. This was the only incidence where entrainment was seen to affect significantly the morphology of a cloud deck. Data for the persistent case suggest that insufficient solar radiation penetrated into the boundary layer to cause significant warming and that the dynamics of the cloud were governed by radiative cooling at cloud top. The differences in the initial morphology of the cloud decks that caused them to evolve differently are discussed.

Diurnal Cycles of Cumulus, Cumulonimbus, Stratus, Stratocumulus, and Fog from Surface Observations over Land and Ocean

Article

Mar 2014

A worldwide climatology of the diurnal cycles of low clouds is obtained from surface observations made eight or four times daily at 3- or 6-h intervals from weather stations and ships. Harmonic fits to the daily cycle are made for 5388 weather stations with long periods of record, and for gridded data on a 5° × 5° or 10° × 10° latitude–longitude grid over land and ocean areas separately. For all cloud types, the diurnal cycle has larger amplitude over land than over ocean, on average by a factor of 2. Diurnal cycles of cloud amount appear to be proprietary to each low cloud type, showing the same cycle regardless of whether that type dominates the diurnal cycle of cloud cover. Stratiform cloud amounts tend to peak near sunrise, while cumuliform amounts peak in the afternoon; however, cumulonimbus amounts peak in the early morning over the ocean. Small latitudinal and seasonal variation is apparent in the phase and amplitude of the diurnal cycles of each type. Land areas show more seasonality compared to ocean areas with respect to which type dominates the diurnal cycle. Multidecadal trends in low cloud cover are small and agree between day and night regardless of the local climate. Diurnal cycles of base height are much larger over land than over the ocean. For most cloud types, the bases are highest in the midafternoon or early evening.

On fire at ten

Article

Dec 1996
ADV GEOPHYS

This chapter presents an overview of the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment (FIRE) science, covering all the 10 years of the program's existence. Following a workshop held at the National Center for Atmospheric Research in May 1983, a Research Plan was published. According to this plan, the central objectives of FIRE are first “to quantify the capabilities of the current models for large-scale cloud systems and for their effects on radiation, and to obtain the data and understanding necessary to improve these models,” and, second, “to check and improve the interpretation of global statistics on cloud parameters which will be collected by ISCCP. From the beginning, FIRE has been aimed at improving both the large-scale models and satellite cloud retrieval techniques. The five major elements that made up the 1983 FIRE Research Plan are provided. These elements identify a need to focus on two particular cloud types namely, stratus and cirrus, which are singled out because of their strong effects on the Earth's radiation budget.

Radiative Transfer and Turbulence in the Cloud-topped Marine Atmospheric Boundary Layer

Article

Full-text available

Aug 1992

The effects of longwave and shortwave radiative heating on the coupling between stratocumulus clouds and the boundary layer is investigated using a one-dimensional second-moment turbulence-closure model. The decoupling of a stratiform cloud from the subcloud layer is often a precursor to cloud break up and the transition to scattered cumulus clouds or clear sky. Coupling between cloud and subcloud layers is found to be very sensitive to cloud depth and subcloud layer sensible and latent heat fluxes. In particular, a strong moisture flux can maintain weak coupling between the cloud and subcloud layers so that the lower part of the cloud layer may continue to develop despite the formation of a stable temperature gradient between the top of the subcloud layer and cloud base. These results highlight the difficulty of predicting the formation, evolution, and dissipation of marine stratocumulus clouds. -from Authors

Doppler Radar Measurements of Turbulence in Marine Stratiform Cloud during ASTEX

Article

Full-text available

Aug 1995

A cloud-sensing Doppler radar is used with a vertically pointing antenna to measure the vertical air motion in clouds during the Atlantic Startocumulus Transition Experiment. The droplet fall velocity contamination was made negligible by using only measurements during the time the reflectivity was below -17 dBZ. During one day of measurements, the daytime character of the vertical velocity variance is different that that of the nighttime case. In the upper part of the cloud, the variance had a distinct maximum for both day and night; however, the nighttime maximum was about twice as large as the daytime case. lower down in the cloud, there was a second maximum, with the daytime variance larger than the nighttime case. The skewness of the vertical velocity was negative near cloud top in both the day and night cases, changing to positive skewness in the lower part of the cloud. This behavior near cloud top indicates that the upper part of the cloud is behaving like an upside-down convective boundary layer, with the downdrafts smaller in area and more intense than the updrafts. In the lower part of the cloud, the behavior of the motion is more like a conventional convective boundary layer, with the updrafts smaller and more intense than the downdrafts. The upside-down convective forcing in the upper part of the cloud is due to radiative cooling, with the daytime forcing less because of shortwave warming. 18 refs., 10 figs.

Mesoscale shallow convection in the atmosphere

Article

Nov 1996

This paper is a review of the observational, experimental, theoretical, and numerical studies of mesoscale shallow convection (MSC) in the atmosphere. Typically, MSC is 1 to 2 km deep, has a horizontal length scale of a few to a few tens of kilometers, and takes distinctive planforms: linear and hexagonal. The former is called a cloud street, roll, or band, while the latter is called mesoscale cellular convection (MCC), comprising three-dimensional cells. MSC is characterized by its shape, horizontal extent, convective depth, and aspect ratio. The latter is the ratio of the horizontal extent to that in the vertical. For cells the horizontal extent is their diameter, whereas for rolls it is their spacing. Rolls usually align along or at angles of up to 10° from the mean horizontal wind of the convective layer, with lengths from 20 to 200 km, widths from 2 to 10 km, and convective depths from 2 to 3 km. The typical value of aspect ratio ranges from 2 to 20. Rolls may occur over both water surface and land surfaces. Mesoscale convective cells may be divided into two types: open and closed. Open-cell circulation has downward motion and clear sky in the cell center, surrounded by cloud associated with upward motion. Closed cells have the opposite circulation. Both types of cell have diameters ranging from 10 to 40 km and aspect ratios of 5 to 50, and both occur in a convective layer with a depth of about 1 to 3 km. Both the magnitude and direction of horizontal wind in the convective layer change little with height. MSC results from a complex and incompletely understood mix of processes. These processes are outlined, and their interplay is examined through a review of theoretical and laboratory analyses and numerical modeling of MSC.

What Controls Stratocumulus Radiative Properties? Lagrangian Observations of Cloud Evolution

Article

Sep 1997

Marine stratocumulus clouds have a large impact on the earth's radiation budget. Their optical properties vary on two distinct timescales, one associated with the diurnal cycle of solar insolation and another with the downstream transition to trade cumulus. Hypotheses regarding the control of cloud radiative properties fall broadly into two groups: those focused on the effects of precipitation, and those concerned with the environment in which the clouds evolve. Reconciling model results and observations in an effort to develop parameterizations of cloud optical properties is difficult because marine boundary layer clouds are not in equilibrium with their local environment.The authors describe a new technique for the observation of boundary layer cloud evolution in a moving or Lagrangian frame of reference. Blending satellite imagery and gridded environmental information, the method provides a time series of the environmental conditions to which the boundary layer is subject and the properties of clouds as they respond to external forcings. The technique is combined with in situ observations of precipitation off the coast of California and compared with the downstream evolution of cloud fraction in five cases that were observed to be precipitating with three cases that were not. In this small dataset cloud fraction remains almost uniformly high, and there is no relationship between the presence of precipitation and the evolution of cloud fraction on 1- and 2-day timescales.Analysis of a large number of examples shows that clouds in this region have a typical pattern of diurnal evolution such that clouds that are optically thicker than about 10 during the morning are unlikely to break up over the course of the day but will instead show a large diurnal cycle in optical depth. Morning cloud optical thickness and the resultant susceptibility to breakup have a much larger impact on diurnally averaged cloud radiative forcing than do diurnal variations in cloud properties. Cloud response is significantly correlated with lower tropospheric temperature stratification at all times, though the best correlation exists when cloud response lags stability by at least 16 h. Sea surface temperature is also correlated with cloud properties during the period in which cloud response is measured and the 12 h prior. The authors suggest that sea surface temperature plays two competing roles in determining boundary layer cloudiness, with rapid changes in SST promoting cloudiness on short timescales but tending to lead to a more rapid transition to the trade cumulus regime.

The Diurnal Cycle in the Tropics

Article

Apr 2001

A global archive of high-resolution (3-hourly, 0.5° latitude-longitude grid) window (11-12 μm) brightness temperature (T b) data from multiple satellites is being developed by the European Union Cloud Archive User Service (CLAUS) project. It has been used to construct a climatology of the diurnal cycle in convection, cloudiness, and surface temperature for all regions of the Tropics. An example of the application of the climatology to the evaluation of the climate version of the U.K. Met. Office Unified Model (UM), version HadAM3, is presented. The characteristics of the diurnal cycle described by the CLAUS data agree with previous observational studies, demonstrating the universality of the characteristics of the diurnal cycle for land versus ocean, clear sky versus convective regimes. It is shown that oceanic deep convection tends to reach its maximum in the early morning. Continental convection generally peaks in the evening, although there are interesting regional variations, indicative of the effects of complex land-sea and mountain-valley breezes, as well as the life cycle of mesoscale convective systems. A striking result from the analysis of the CLAUS data has been the extent to which the strong diurnal signal over land is spread out over the adjacent oceans, probably through gravity waves of varying depths. These coherent signals can be seen for several hundred kilometers and in some instances, such as over the Bay of Bengal, can lead to substantial diurnal variations in convection and precipitation. The example of the use of the CLAUS data in the evaluation of the Met. Office UM has demonstrated that the model has considerable difficulty in capturing the observed phase of the diurnal cycle in convection, which suggests some fundamental difficulties in the model's physical parameterizations. Analysis of the diurnal cycle represents a powerful tool for identifying and correcting model deficiencies.

Aerosol Impacts on the Diurnal Cycle of Marine Stratocumulus

Article

Full-text available

Aug 2008

Recent large-eddy simulation (LES) studies of the impact of aerosol on the dynamics of nocturnal marine stratocumulus revealed that, depending on the large-scale forcings, an aerosol-induced increase of the droplet concentration can lead to either an increase or a decrease of the liquid water path, hence contrasting with the cloud thickening that is expected from a reduction of the precipitation efficiency. In this study, the aerosol impacts on cloud microphysics are examined in the context of the boundary-layer diurnal cycle using 36-h LES simulations of pristine and polluted clouds. These simulations corroborate previous findings that during nighttime aerosol-induced liquid water path changes are sensitive to the large-scale forcings via enhancement of cloud-top entrainment such that, ultimately, the liquid water path may be reduced when the free-tropospheric-entrained air is drier. During the day, however, enhanced entrainment, inhibition of drizzle evaporation below cloud base, and reduced sensible heat flux from the surface lead to a more pronounced decoupling of the boundary layer, which significantly amplifies the liquid water path reduction of the polluted clouds. At night the sign of the liquid water path difference between pristine and polluted clouds depends upon large-scale forcings, while during the day the liquid water path of polluted clouds is always smaller than the one of the pristine clouds. Suggestions are made on how observational studies could be designed for validation of these simulations.

Modeling the Impact of Marine Stratocumulus on Boundary Layer Structure

Article

Full-text available

Apr 1995

An ensemble-average closure model intended for mesoscale studies is applied to a marine stratocumulus-capped PBL. The intention is to test this model, in particular, for cases where cloud and subcloud layers are decoupled. The test is based on one case from the First ISCCP Regional Experiment, where solid cloud-capped and clear sky areas were found in close proximity.The model results compare favorably both with the measurements and with results from more complex model formulations. They show the response of the entire boundary layer dynamic structure to stratocumulus formation as well as longwave and shortwave radiative heat transfers. The net result is that the entire turbulent layer in the cloud-capped case is more vigorously mixed, more neutrally stratified, and deeper compared to a cloud-free PBL developing under similar conditions. Surface fluxes of sensible and latent heat, from the measurements as well as simulations thus vary relatively little between the areas in spite of the observed substantial sea surface temperature difference.All simulations presented here reveal cloud decoupling during daytime. The multilayer structure is, however, seen almost only in profiles of second-order moments. The mean profiles indicate one single, deep well-mixed layer, while the turbulence profiles clearly show two separate well-mixed layers. The turbulent flux of water vapor from the surface thus generally never penetrates to the cloud layer during daytime but may eventually cause formation of a shallow layer of cumuli below the main cloud layer.

An Observational Study of the Role of Penetrating Cumulus in a Marine Stratocumulus-Topped Boundary Layer

Article

Full-text available

Aug 1995

Isolated cumuli penetrating through marine stratocumulus clouds were documented during the Atlantic Stratocumulus Transition Experiment. This paper aims at understanding the role of the penetrating cumulus in regulating stratocumulus and boundary-layer structure through analysis of data from the NCAR Electra aircraft. When penetrating cumulus clouds are present, the boundary layer is generally decoupled from the near-surface air except in the cumulus region. Therefore, air in the cumulus region includes air entrained at the cloud top, as well as air modified by surface processes. In the stratocumulus region, however, entrained inversion air and moist surface air are confined to separate layers. As a result, large horizontal variations are found in scalars, such as ozone and water vapor. Turbulence statistics and conditional sampling of entrainment events in the cumulus and stratocumulus regions indicate that stronger entrainment may occur at the cumulus top compared to the surrounding stratocumulus. This analysis is, however, complicated by insufficient sampling of cloud-top jump conditions in both regions.Convergent flow in the lower boundary layer and compensating diverging flow in the upper boundary layer were identified along the flight trark. This flow field, together with the vertical coupling of surface air with the cloud layer in the cumulus region, helps to transport moisture upwards from the sea surface and disperse it to the surrounding stratocumulus sheet, thus helping to maintain the stratocumulus cover.

The Diurnal Cycle of the Marine Stratocumulus Layer: A Higher-Order Model Study

Article

Full-text available

Dec 1985

Philippe Bougeault

A third-order turbulence model of the marine stratocumulus layer, including cloud ensemble relations and detailed radiative computation, is used to study a case observed over the North Sea during the JASIN experiment. It is shown that the model is able to reconstruct the observed structure of the boundary layer, starting from only the large-scale information. A strong diurnal cycle, induced by the absorption of solar radiation of the cloud layer, is studied in some detail. The role of intermittent cumuli observed below the stratocumulus, and the significance of the cloud-top instability criterion are also investigated.

Satellite-based assessment of marine low cloud variability associated with aerosol, atmospheric stability, and the diurnal cycle

Article

Sep 2006

This study examines variability in marine low cloud properties derived from semiglobal observations by the Tropical Rainfall Measuring Mission (TRMM) satellite, as linked to the aerosol index (AI) and lower-tropospheric stability (LTS). AI is derived from the Moderate Resolution Imaging Spectroradiometer (Terra MODIS) sensor and the Goddard Chemistry Aerosol Radiation and Transportation (GOCART) model and is used to represent column-integrated aerosol concentrations. LTS is derived from the NCEP/NCAR reanalysis and represents the background thermodynamic environment in which the clouds form. Global statistics reveal that cloud droplet size tends to be smallest in polluted (high-AI) and strong inversion (high-LTS) environments. Statistical quantification shows that cloud droplet size is better correlated with AI than it is with LTS. Simultaneously, the cloud liquid water path (CLWP) tends to decrease as AI increases. This correlation does not support the hypothesis or assumption that constant or increased CLWP is associated with high aerosol concentrations. Global variability in corrected cloud albedo (CCA), the product of cloud optical depth and cloud fraction, is very well explained by LTS, while both AI and LTS are needed to explain local variability in CCA. Most of the local correlations between AI and cloud properties are similar to the results from the global statistics, while weak anomalous aerosol-cloud correlations appear locally in the regions where simultaneous high (low) AI and low (high) LTS compensate each other. Daytime diurnal cycles explain additional variability in cloud properties. CCA has the largest diurnal cycle in high-LTS regions. Cloud droplet size and CLWP have weak diurnal cycles that differ between clean and polluted environments. The combined results suggest that investigations of marine low cloud radiative forcing and its relationship to hypothesized aerosol indirect effects must consider the combined effects of aerosols, thermodynamics, and the diurnal cycle.

Growth behavior of the marine submicron boundary layer aerosol

Article

Aug 1997

A box model for investigating the chemistry and growth of submicron particles in the marine boundary layer was developed. Processes simulated by the model were gas phase chemistry, in-cloud sulfate production, gas-to-particle transfer of condensable vapors, coagulation, dry deposition of particles and gases, and entrainment between the boundary layer and the free troposphere. According to model simulations, the most influential factor for the growth of nuclei and Aitken mode particles is the production rate of methane sulfonic acid (MSA) and other low-volatility compounds in the gas phase. Processes controlling SO2 concentrations dictate the amount of non-sea-salt sulfate produced in the boundary layer but are less important for particle growth. The ratio of MSA to non-sea-salt sulfate in the particulate phase may vary largely, even when a constant MSA yield from dimethylsulfide (DMS) oxidation is assumed. Clouds decrease nuclei lifetime but do not affect their growth significantly, unless the time between two cloud passages is very short. Sources other than DMS may produce condensable vapors that assist particle growth to some extent. With our current knowledge of the concentrations of condensible matter in the marine boundary layer, however, it seems unlikely that small nuclei are able to grow into cloud condensation nuclei size over their lifetime. More information is needed on heterogeneous surface reactions that may occur between submicron particles and vapors such as SO2, as well as on potential transport limitations between condensable vapors and particles caused by thermodynamics or organic surfactants.

Enhanced formation and development of sulfate particles due to marine boundary layer circulation

Article

Nov 1995

Production and the subsequent development of sulfur-derived particles in the marine boundary layer have been of great interest due to their potential role in modifying the radiative properties of marine stratiform clouds. In this work these phenomena were studied using a dynamic air parcel model, with the emphasis placed on examining how the boundary layer dynamics affects the system. Our simulations suggest that a homogeneous or steady boundary layer assumption may lead to a considerable underestimation of the number of nuclei formed. Further, we showed that if the observed particle size distributions and nonsea-salt fraction of cloud condensation nuclei (CCN) production are to be explained by in situ particle production and growth, the sulfuric acid accommodation coefficient onto nuclei must be greater than 0.1 under conditions typical for clean marine areas. These high values require either a sulfuric acid vapor source in addition to SO2(g) oxidation or a H2SO4(g) accommodation coefficient onto more aged particles which is less than that onto nuclei. Both these options were tested and can be considered viable based on our present understanding of the marine environment. Finally, we showed that a nighttime interaction of the boundary layer air with clouds does not significantly affect nuclei formation or growth, but may lead to significant scavenging of fresh nuclei if their sulfuric acid accommodation coefficient is small. Cloud interactions during photochemically active periods may completely inhibit new particle production, and the growth of preexisting nuclei is likely to be significantly decelerated.

A numerical model of the cloud-topped planetary boundary-layer: radiative forcing of aerosols in stratiform clouds

Article

Nov 1998
ECOL MODEL

Andreas Bott

In a numerical sensitivity study with the microphysical stratus model MISTRA the impact of aerosol particles on the time evolution of stratiform clouds is investigated. Four model runs with different aerosol size distributions are presented. Two size distributions are typical for maritime and continental air masses. The third model run consists of a mixture of maritime and rural aerosol particles, while in the fourth case study rural aerosol particles with a reduced water solubility are utilized. The numerical results show that the microphysical structure of the clouds is strongly affected by the physico-chemical properties of the aerosol particles. In the maritime case, with a relatively low aerosol concentration, the clouds consist of few but large droplets. In contrast to this, the rural and the mixed case yield many, but relatively small, cloud droplets. Due to the low water solubility of the aerosol particles, in the fourth model run only a small number of aerosol particles becomes activated so that the microstructure of the cloud is again very similar to the maritime case. The different droplet spectra have a direct influence on the radiative forcing of the clouds. In situations with many small droplets the reflectivity of the clouds is distinctly higher than in cases with few but large cloud droplets. The effective cloud droplet radii are larger in the model runs with few large cloud droplets than in the other cases. In all simulations the radii show a strong diurnal variation.

Atmospheric planetary boundary-layer research in the US: 1991-1994 (95RG00185)

Article

Jul 1995

With the increasing emphasis on coupled climate systems, planetary-boundary-layer (PBL) research has also gained increasing attention. Atmospheric and oceanic PBLs serve as the interface between all of the system components: atmosphere, ocean, land, and biosphere. Boundary-layer clouds also play important roles in climate, e.g., trade-wind cumulus in the hydrological cycle and subtropical marine stratus in the Earth's radiation budget. Therefore, proper PBL parameterization schemes are needed to accurately link all of the climate system components and to represent cloud formation and dissipation.

Evolution of the Subtropical Marine Boundary Layer: Comparison of Soundings over the Eastern Pacific from FIRE and HaRP

Article

Full-text available

Jun 1994

The mean time rates of change of temperature, total water mixing ratio and ozone along airflow trajectories in the lower troposphere over the eastern Pacific are inferred by comparing aircraft soundings from the First ISCCP Regional Experiment (FIRE) and the Hawaiian Rainband Project (HaRP). Through the use of the estimated mean fluxes of temperature and total water mixing ratio, it is found that the tendency for stratus layers to grow or dissipate is very sensitive to the assumed turbulence structure below the capping inversion. A mixed-layer model that assumes a well-mixed boundary layer up to the capping inversion predicts a solid cloud layer extending all the way to Hawaii, whereas a model that allows decoupling predicts rapid dissipation of the stratus layer. It is concluded that stratus dissipation here is due to the slowdown of turbulent mixing throughout the layer below the capping inversion, not the drying out of a well-mixed layer; hence, the mixed-layer model cannot be expected to predict realistic cloud dissipation. The differences in ozone concentration observed in the boundary layer during HaRP and FIRE suggest a chemical loss of ozone of 3–8ppb day⁻¹, corresponding to a lifetime of 3–9 days. This implies that ozone cannot be treated as a conserved tracer when dealing with ozone budgets over periods of days. The ozone sink is probably of photochemical origin, and it requires further investigation.

Interactions among longwave radiation of clouds, turbulence, and snow surface temperature in the Arctic: A model sensitivity study

Article

Jul 2001

Recent observations have shown that low-level clouds have significant impact on snow surface temperature in the Arctic by enhancing downwelling longwave radiative flux at the surface. This study focuses on the detailed interactions among the longwave radiation, clouds, turbulent structure, and snow surface temperature. The approach is to perform sensitivity simulations with a coupled one-dimensional planetary boundary layer (PBL) turbulence closure-snow model. The numerical experiments show that the responses of the snow surface temperature to the cloud longwave radiation may depend on how the clouds are related to the boundary layer turbulent structure. For a cloud-topped boundary layer without upper level clouds, longwave radiative cooling at the cloud top is a main source for the turbulence. In this situation the energetic turbulent eddies effectively transport the radiatively cooled air near the cloud top down to the ground, resulting in a strong sensible heat flux at the surface. When multilayered clouds exist, the radiative cooling at the lower-level cloud top is significantly reduced due to the enhanced downward longwave radiative flux above the cloud, leading to a higher temperature of surface air and weaker positive sensible heat flux at the surface. Consequently, the snow surface temperature is higher in the presence of the multilayered clouds than in cases of only boundary layer clouds. Therefore the boundary layer clouds, dependent on their vertical distribution, may not only increase the snow surface temperature by increasing the downwelling longwave radiative flux but also create negative feedback mechanisms to reduce this temperature increase. A key process in this feedback mechanism is turbulent transport as it directly links the longwave radiative cooling at the cloud top to the surface air temperature.

Observations of turbulence-induced new particle formation in the residual layer

Article

Full-text available

May 2010

Aerosol particle measurements in the atmospheric boundary layer performed by a helicopter-borne measurement payload and by a lidar system from a case study during the IMPACT field campaign in Cabauw (NL) are presented. Layers of increased number concentrations of ultrafine particles were observed in the residual layer, indicating relatively recent new-particle formation. These layers were characterized by a sub-critical Richardson number and concomitant increased turbulence. Turbulent mixing is likely to lead to local supersaturation of possible precursor gases which are essential for new particle formation. Observed peaks in the number concentrations of ultrafine particles at ground level are connected to the new particle formation in the residual layer by boundary layer development and vertical mixing.

Measuring Second- through Fourth-Order Moments in Noisy Data

Article

Full-text available

Oct 2000

The authors derive expressions for correcting second- through fourth-order moments of measured variables that are contaminated by random uncorrelated noise. These expressions are then tested by applying them to an artificially produced time series as well as measurements from two upward-pointing ground-based lidar systems: a differential absorption lidar that measures water vapor density and a high-resolution Doppler lidar that measures vertical wind velocity. Both sets of measurements were obtained in a convective boundary layer, and contain sufficient noise to significantly affect measurements of second- and fourth-order moments (as well as integral scale and skewness) throughout the boundary layer. It is shown that the corrections derived here can be used to obtain useful measurements of these moments from instruments such as lidars, which are inherently noisy. The authors also obtain information on higher-order moments of the noise as well as the correlation between noise and atmospheric measurements.

Estimating Spatial Velocity Statistics with Coherent Doppler Lidar

Article

Full-text available

Mar 2002

The spatial statistics of a simulated turbulent velocity field are estimated using radial velocity estimates from simulated coherent Doppler lidar data. The structure functions from the radial velocity estimates are processed to estimate the energy dissipation rate epsilon and the integral length scale L-i, assuming a theoretical model for isotropic wind fields. The performance of the estimates are described by their bias, standard deviation, and percentiles. The estimates of epsilon(2/3) are generally unbiased and robust. The distribution of the estimates of L-i are highly skewed; however, the median of the distribution is generally unbiased. The effects of the spatial averaging by the atmospheric movement transverse to the lidar beam during the dwell time of each radial velocity estimate are determined, as well as the error scaling as a function of the dimensions of the total measurement region. Accurate estimates of L-i require very large measurement domains in order to observe a large number of independent samples of the spatial scales that define L-i.

Comparison of Eddy Dissipation Rates from Spatial Spectra of Doppler Velocities and Doppler Spectrum Widths

Article

Full-text available

Sep 1986

Doppler radars offer unique data from which it is possible to estimate the turbulent eddy dissipation rates, epsilon. If the inertial subrange extends to lengths longer than the radar resolution volume size, epsilon can be obtained from the Doppler spectrum width. Spatial spectra of mean Doppler velocities can also yield epsilon estimates but only if a significant portion of the analysis length is contained within the inertial subrange. Dissipation rate estimates obtained with the two independent measurement techniques are compared. At close range and vertical incidence, agreement between the two independent estimates of epsilon is within 10 percent. Furthermore, the slope of the spatial energy densities is very close to -5/3 predicted by Kolmogorov (1941).

Collisions of Cloud Droplets in a Turbulent Flow. Part V: Application of Detailed Tables of Turbulent Collision Rate Enhancement to Simulation of Droplet Spectra Evolution

Article

Full-text available

Feb 2008

The present study is a continuation of the series of studies dedicated to the investigation of cloud droplet collisions in turbulent flow with characteristics that are typical of real clouds. Detailed tables of collision kernels and collision efficiencies calculated in the presence of hydrodynamic interaction of droplets are presented. These tables were calculated for a wide range of turbulent parameters. To illustrate the sensi- tivity of droplet size distribution (DSD) evolution to the turbulence-induced increase in the collision rate, simulations of DSD evolution are preformed by solving the stochastic kinetic equation for collisions. The results can be applied to cloud modeling. The tables of collision efficiencies and collision kernels are available upon request. Some unsolved problems related to collisions of droplets and ice hydrometeors in turbulent clouds are discussed in the conclusion.

Length Scales in the Convective Boundary Layer

Article

Full-text available

Jun 1986

We calculated integral scales for horizontal and vertical velocity components, temperature, humidity and ozone concentration, as well as for their variances and covariances from aircraft measurements in the convective atmospheric boundary layer over both ocean and land surfaces. We found that the integral scales of the second-order moment quantities are 0.67± 0.09 that of the variables themselves. Consequently, only the second-order moment integral scales are presented here. These results are used to calculate the averaging lengths necessary to measure second-order moment quantities to a given accuracy. We found that a measurement length of 10 to 100 times the boundary-layer height is required to measure variances to 10% accuracy, while scalar fluxes require a measurement length of 102 to 104 and stress a measurement length of 103 to 105 times the boundary layer height. We also show that the ratio of the wavelength of the spectral peak to the integral scale can be used to estimate the sharpness of the spectral peak.

Aircraft observation of marine stratocumulus during JASIN

Article

Full-text available

Oct 1982
Q J ROY METEOR SOC

Observations of marine stratocumulus obtained by three research aircraft during the JASIN experiment are presented. Detailed measurements were made of the thermodynamic, cloud physics and radiation fields. These showed an essentially well-mixed boundary layer with cloud liquid water contents close to their adiabatic values. The mean drop radius increased steadily towards cloud top, where evidence of inhomogeneous mixing was found. Both the longwave and shortwave radiative components of the cloud layer energy budget were measured. Good agreement was obtained between the observations and several radiation schemes and in particular the measured cloud shortwave absorption was close to the theoretical values. At midday, the net longwave cooling of the cloud layer was found to be approximately compensated by the shortwave warming, although the regions of net warming and cooling were separated vertically, thereby promoting convection within the cloud. The wider implications of these results to studies of stratiform cloud are discussed.

Doppler Lidar Measurements of Vertical Velocity Spectra in the Convective Planetary Boundary Layer

Article

Full-text available

Aug 2009

We utilized a Doppler lidar to measure spectra of vertical velocity w from 390m above the surface to the top of the daytime convective boundary layer (CBL). The high resolution 2μm wavelength Doppler lidar developed by the NOAA Environmental Technology Laboratory was used to detect the mean radial velocity of aerosol particles. It operated continuously during the daytime in the zenith-pointing mode for several days in summer 1996 during the Lidars-in-Flat-Terrain experiment over level farmland in central Illinois, U.S.A. The temporal resolution of the lidar was about 1s, and the range-gate resolution was about 30m. The vertical cross-sections were used to calculate spectra as a function of height with unprecedented vertical resolution throughout much of the CBL, and, in general, we find continuity of the spectral peaks throughout the depth of the CBL. We compare the observed spectra with previous formulations based on both measurements and numerical simulations, and discuss the considerable differences, both on an averaged and a case-by-case basis. We fit the observed spectra to a model that takes into account the wavelength of the spectral peak and the curvature of the spectra across the transition from low wavenumbers to the inertial subrange. The curvature generally is as large or larger than the von Kármán spectra. There is large case-to-case variability, some of which can be linked to the mean structure of the CBL, especially the mean wind and the convective instability. We also find a large case-to-case variability in our estimates of normalized turbulent kinetic energy dissipation deduced from the spectra, likely due for the most part to a varying ratio of entrainment flux to surface flux. Finally, we find a relatively larger contribution to the low wavenumber region of the spectra in cases with smaller shear across the capping inversion, and suggest that this may be due partly to gravity waves in the inversion and overlying free atmosphere.

Measurements of Turbulent Energy Dissipation Rate with a CW Doppler Lidar in the Atmospheric Boundary Layer

Article

Full-text available

Aug 1999

The results of a theoretical and experimental study of the feasibility of the turbulent energy dissipation rate ε(T) measurements with a continuous wave (CW) CO2 Doppler lidar in the atmospheric boundary layer are presented. Three methods of probing ε(T) are considered: 1) Doppler spectrum width, 2) the temporal spectrum (temporal structure function) of wind velocity measured by the Doppler lidar, and 3) spatial structure function. In these methods, information on the dissipation rate is extracted by means of analysis of the corresponding statistical characteristics of wind velocity in the inertial subrange of the turbulence, taking into account the spatial averaging of the measured wind velocity fluctuations over sounded volume. In the first and third methods, the spatial structure of the turbulence is analyzed directly. In the second method, to determine ε(T) from the measured temporal characteristics, it is necessary to use a model for the spatiotemporal correlation function of wind velocity. As a result of the study, it has been shown that in the case of large longitudinal size of sounded volume and weak side wind. Taylor's hypothesis of 'frozen' turbulence cannot be accepted for the correlation function. The strict limitation on the longitudinal size of the sounded volume and therefore sounding height is the main restriction of the first method. The third method is free of such limitations. It allows one to obtain the information on the dissipation rate profile throughout the entire boundary layer. Comparison of the developed theory for statistical characteristics of wind velocity measured by the Doppler lidar with the obtained experimental data has demonstrated their good agreement. The vertical profiles of the turbulent energy dissipation rate retrieved from Doppler lidar data with the use of the methods described above do not contradict the known experimental results. This fact confirms the feasibility of application of lidar remote sensing methods to the study of the small-scale turbulence in the atmospheric boundary layer.

Tracer concentration profiles measured in Central London as part of the REPARTEE campaign

Article

Full-text available

Jan 2011
ACP

There have been relatively few tracer experiments carried out that have looked at vertical plume spread in urban areas. In this paper we present results from two tracer (cyclic perfluorocarbon) experiments carried out in 2006 and 2007 in central London centred on the BT Tower as part of the REPARTEE (Regent's Park and Tower Environmental Experiment) campaign. The height of the tower gives a unique opportunity to study vertical dispersion profiles and transport times in central London. Vertical gradients are contrasted with the relevant Pasquill stability classes. Estimation of lateral advection and vertical mixing times are made and compared with previous measurements. Data are then compared with a simple operational dispersion model and contrasted with data taken in central London as part of the DAPPLE campaign. This correlates dosage with non-dimensionalised distance from source. Such analyses illustrate the feasibility of the use of these empirical correlations over these prescribed distances in central London.

Doppler Lidar Measurements of Turbulent Structure Function over an Urban Area

Article

Full-text available

May 2004

Analysis of radial wind velocity data from the Salford pulsed Doppler infrared lidar is used to calculate turbulent spectral statistics over the city of Salford in the United Kingdom. The results presented here, first, outline the error estimation procedure used to correct the radial wind velocity measurements from the Salford lidar system; second, they correct the data for the spatial averaging effects of the Salford lidar pulse; and finally, they use the corrected data to calculate turbulent spectral statistics. Using lidar data collected from the Salford Urban Meteorological Experiment (SALFEX), carried out in May 2002, kinetic energy dissipation rates, radial velocity variance, and integral length scales are calculated for the boundary layer above an urban canopy. The estimates of the kinetic energy dissipation rate from this method are compared to calculations using more traditional spectral methods. The estimates of the kinetic energy dissipation rate for the two methods are correlated and both show an increase in dissipation rate through the day. The procedure followed for the correction of the spatial averaging effects of the lidar pulse shape actually uses the Salford lidar pulse shape profile.

Statistical Theory of Turbulence. II

Article

Jan 1935

G.I. Taylor

Dissipation of Energy in Locally Isotropic Turbulence

Article

Jul 1991

A. N. Kolmogorov

In my note (Kolmogorov 1941 a ) I defined the notion of local isotropy and introduced the quantities B d d ( r ) = [ u d ( M ′ ) − u d ( M ) ] 2 , ¯ [ u n ( M ′ ) − u n ( M ) ¯ ] 2 , where r denotes the distance between the points M and M' , u d (M) and u d (M') are the velocity components in the direction MM' ¯¯ at the points M and M' , and u n (M) and u n (M') are the velocity components at the points M and M' in some direction, perpendicular to MM' .

An Analysis of the Performance of the UFAM Pulsed Doppler Lidar for Observing the Boundary Layer

Article

Feb 2009

The performance of the 1.5-mu m pulsed Doppler lidar, operated by the U. K. Universities Facility for Atmospheric Measurement (UFAM) over a 51-day continuous and unattended field deployment in southern England, is described and analyzed with a view to demonstrating the capabilities of the system for remote measurements of aerosols and velocities in the boundary layer. A statistical assessment of the vertical pointing mode in terms of the availability and errors in the data versus range is presented. Examples of lidar data are compared to theoretical predictions, radiosondes, the UFAM radar wind profiler, and an ultrasonic anemometer.

Energy Dissipation Rates of Free Atmospheric Turbulence

Article

Nov 1974

W. Y. Chen

Energy spectra of high-altitude atmospheric turbulence data were analyzed. Kolmogorov's similarity theory for the inertial subrange was applied to infer energy dissipation rates from the spectra. The dissipation rates increase as the third power of the truncated root-mean-square values of turbulence: thus, the former can be inferred directly from the later with relative case.In the stratosphere the dissipation rates can occasionally he of the same order as those in the surface layer; a single curve can hardly represent the dissipation state in the atmosphere.

Preliminary results of the routine comparison of wind profiler data with the Meteorological Office Unified Model vertical wind profiles

Article

Dec 2001

An aim of the Met Offices wind profiler project is to provide routine comparisons between numerical weather prediction models and European wind profiler data. This paper describes early results of routine comparisons between wind profiles from a selection of European wind profilers with similar data from the Met Office's Unified Model (UM) at mesoscale resolution.It was found that data from Aberystwyth and Cabauw compared very well with model analyses. The profiler at La Ferté Vidame had a stable relationship with these model data but showed signs of a small bias. The profiler at Dunkeswell compared well with model analyses below 2.5km but required further assessment to determine its performance at greater altitudes. The wind profiler at Camborne had the poorest relationship with model analyses. Investigations have revealed a problem with this systems configuration that has now been addressed.

Observations of Small-Scale Turbulence and Energy Dissipation Rates in the Cloudy Boundary Layer

Article

May 2006

Tethered balloon borne measurements with a resolution in the order of 10 cm in a cloudy boundary layer are presented. Two examples sampled under different conditions concerning the clouds' stage of life are discussed. The hypothesis tested here is that basic ideas of classical turbulence theory in boundary layer clouds are valid even to the decimeter scale. Power spectral densities S(f) of air temperature, liquid water content, and wind velocity components show an inertial subrange behavior down to 20 cm. The mean energy dissipation rates are 10-3 m2 s-3 for both datasets. Estimated Taylor Reynolds numbers (Relambda) are 104, which indicates the turbulence is fully developed. The ratios between longitudinal and transversal S(f) converge to a value close to 4/3, which is predicted by classical turbulence theory for local isotropic conditions. Probability density functions (PDFs) of wind velocity increments Deltau are derived. The PDFs show significant deviations from a Gaussian distribution with longer tails typical for an intermittent flow. Local energy dissipation rates tau are derived from subsequences with a duration of tau = 1 s. With a mean horizontal wind velocity of 8 m s-1, tau corresponds to a spatial scale of 8 m. The PDFs of tau can be well approximated with a lognormal distribution that agrees with classical theory. Maximum values of tau 10-1 m2 s-3 are found in the analyzed clouds. The consequences of this wide range of tau values for particle turbulence interaction are discussed.

Estimation of Velocity Error for Doppler Lidar Measurements

Article

Oct 2001

Rod Frehlich

Various methods of estimating the magnitude of the random error of Doppler lidar velocity measurements are compared for typical operating conditions using computer simulations of lidar data. Under certain conditions, the magnitude of the random estimation error can be determined from data without the need for in situ measurements for both ground-based and space-based wind measurement.

Radar Measurements of Turbulent Eddy Dissipation Rate in the Troposphere: A Comparison of Techniques

Article

Feb 1995

Stephen A Cohn

Two independent radar methods for estimating the turbulent eddy dissipation rate ϵ are applied to a common dataset, and the results are compared. The first method estimates ϵ from backscattered power and relies on the effects of turbulent mixing of atmospheric refractive index gradients. It requires additional measurements of temperature and humidity from a balloon sounding. The second makes use of broadening of the backscattered Doppler spectrum by turbulent motions. The turbulent eddy dissipation rate ϵ is a measure of the energy cascade through scales of inertial subrange turbulence. Data were collected with the Millstone Hill UHF radar in Westford, Massachusetts, and with Cross-chain Loran Atmospheric Sounding System thermodynamic soundings launched from Hanscom Field about 25 km away. Encouraging similarities are found both in the magnitude and shape of the measured profiles, though differences are also found. Some differences may be explained by characteristics of the measurement techniques...

Turbulence in the Evolving Stable Boundary Layer

Article

May 1979

The turbulence structure observed in seven early evening runs of the 1973 Minnesota experiments is presented and discussed. Wind and temperature sensors mounted on a 32 m tower and on the tethering cable of a large balloon spanned the entire depth of the rapidly evolving nocturnal boundary layer. Spectral shapes and the vertical profiles of turbulence variances and covariances, dissipation rates for turbulent kinetic energy and temperature variance, and energy-containing range length scales show remarkable order when plotted in dimensionless coordinates, even though properties varied widely among the runs. Observed dissipation rates and boundary layer depth agree well with predictions of the Brost-Wyngaard (1978) model. It is shown that the slight (0.0014) terrain slope and possibly baroclinity affected the boundary-layer evolution, and that although the turbulence structure was probably in equilibrium with the wind and temperature fields, these were strongly evolving during the runs.

Marine Stratocumulus Layers. Part 1: Mean Conditions

Article

Mar 1982

The mean radiational, dynamical and thermodynamical structure of the marine stratocumulus-topped mixed layers of the California coast is described for two days in June 1976 using data from the NCAR Electra aircraft. We suggest that the synoptic conditions found may be typical of about half of the shallow stratocumulus-topped boundary layers that occur in this region during summer. The inversion was low near the coast and increased in height to the west, consistent with the average westward increase in sea-surface temperature. North-south inversion height change was largely due to entrainment and mean mesoscale vertical motions. Below the inversion, strong winds (12-20 m s1 from the north) and horizontal inhomogeneities resulted in large advection terms in mean field equations. The sloping inversion often produced large vertical shears of the actual and geostrophic wind velocities across the inversion. Because of low liquid-water contents (0.1 g kg1), temperature and water vapor could be measured in cloud with in situ instrumentation without significant errors due to wetting.The longwave radiative extinction length was found to be relatively short; 63% of the cloud-top jump in radiation flux occurred within 40 m. Radiative heat loss was largely balanced by shear-driven entrainment. Compositing vertical gradients provided by individual aircraft ascents and descents is shown to overestimate vertical gradients at the inversion.

The Determination of the Kolmogoroff Constants for Velocity, Temperature, and Humidity Fluctuations from Second- and Third-Order Structure Functions

Article

Nov 1971
J FLUID MECH

Second- and third-order structure functions were computed from velocity, temperature and humidity fluctuations in the wind over the ocean. Universal inertial-convective subrange constants (Kolmogoroff constants) were computed from these structure functions. The constant for velocity is consistent with other recent observations. The temperature and humidity constants are found to be equal within the measurement error and have values of about 0·8.

Some aspects of turbulence structure through the depth of the convective boundary layer

Article

Oct 1979

Marine Stratocumulus Layers. Part II: Turbulence Budgets

Article

Apr 1982

This paper discusses the turbulence profiles and budgets for two days of radiation, dynamical and thermodynamical observations by the NCAR Electra in shallow marine stratocumulus off the California coast in June 1976.The boundary layer is characterized by relatively high wind speeds (12-20 m s1) and low liquid water contents (0.1 g kg1); the clouds are not very convective and seem to have little influence on the turbulence budgets. In cloud, drizzle has a significant impact on the liquid water budget and occasionally even on the total water budget even though no drizzle is observed at the surface. The stresses, velocity variances, and their budgets behave as in a neutral boundary layer, sometimes with an additional peak in the cross-wind variance at the inversion due to shear production.There is scant evidence of direct production of vertical velocity variance at cloud top due to radiative cooling or latent heat release; it is maintained principally by the pressure-scrambling terms through redistribution of the shear-produced energy. We find, however, that while the Rotta parameterization for pressure scrambling in the stress budgets works well near the surface and sometimes throughout the layer, it is unsatisfactory in the variance budgets.Fluctuations of temperature and moisture on a scale of several hundred meters in cloud satisfy the Clausius-Clapeyron equation. When the boundary layer is well mixed in equivalent potential temperature and total water substance, the vertical turbulent fluxes of these quantities are usually almost linear. The efficiency of cloud-top radiative cooling in producing mixed-layer convection is also discussed.

Aircraft observations of the Ekman layer during the Joint Air-Sea Interaction Experiment

Article

Apr 2007
Q J ROY METEOR SOC

S. Nicholls

Results are presented from an observational study of the mid-latitude atmospheric boundary layer over the sea. the data were obtained mainly by instrumented aircraft as part of the Joint Air-Sea Interaction Experiment (JASIN). The eight occasions chosen for study are characterized by small surface buoyancy fluxes and the conditions are well described as near neutral and barotropic. In the absence of low-level inversions, a well-mixed Ekman layer is observed on each occasion but is limited to a depth of approximately 0.2u */f. Accurate measurements of horizontal gradients (including pressure) together with improved wind observations and turbulent flux measurements enabled each of the terms of the momentum balance to be evaluated throughout the depth of this layer. These terms were found to balance quite closely, were well described by Ekman scaling and were consistent with the requirements of the measured turbulent kinetic energy balance. the latter suggests that the main shear production terms are dissipated locally and that little is exported to upper levels in the Ekman layer to enable significant deepening by entrainment although only a slight increase in instability appears to be needed to alter this balance. Conditions in which such boundary layer structure might be observed are suggested. Values of various coefficients used in schemes for relating surface fluxes to mean quantities (CD, CE, Cg, A, B) are derived and compared with previous measurements. Spectral analysis reveals that most of the turbulent transport is confined to a distinct high wavenumber region whose characteristics vary as a function of Ekman layer depth and stability parameters. This is superimposed on larger-scale fluctuations which do not vary appreciably within the Ekman layer and which therefore dominate the variances in the upper regions as the intensity of the smaller-scale turbulence decreases strongly with height. Finally, further implications of this interpretation are discussed with particular reference to the heat and water vapour balance. These imply that removal of water vapour from the Ekman layer is accomplished by transfer related to cloud activity, marking a significant change in the mechanism of turbulent transport at the top of this layer. the relationship between these and similar results obtained concurrently by different methods is also discussed.

The dynamics of stratocumulus: Aircraft observations and comparisons with a mixed layer model

Article

Oct 1984
Q J ROY METEOR SOC

S. Nicholls

Results are presented from a detailed case study of daytime stratocumulus over the North Sea using an instrumented aircraft. The measurements include turbulence fluctuation data, radiation fluxes and droplet spectra and were made both in and out of cloud. The mean structure is discussed and a diagnostic, one-dimensional, mixed layer model is formulated to predict the variation of turbulent fluxes with height and to assess the sensitivity of these solutions to various physical processes. These predictions are compared with observations enabling the effectiveness of some widely used model assumptions to be tested. Water transport by gravitational settling is found to be important throughout the depth of the cloud. Mixing within the cloud is driven by convection generated primarily by radiative effects: strong cooling from cloud top with warming beneath although the net heating of the cloud layer is close to zero. Turbulent kinetic energy is exported downwards via negatively buoyant elements from the zone of intense cooling near cloud top into the body of the cloud. This appears to be achieved by the combined action of the turbulent transport and velocity-pressure correlation terms in the turbulent kinetic energy balance equation which are therefore important in maintaining mixing in the lower part of the cloud. The cloud and sub-cloud layers are found to be decoupled, i.e. they appear as two separated mixed layers. Reasons for this are examined and some further consequences for cloud evolution are investigated. In the particular case studied, potential instability could be generated in the lower layer leading to low-level cumulus formation. These rise into the stratocumulus layer thereby reconnecting the two previously separated regions. The implications for stratiform cloud modelling are discussed and some recommendations for future work made.

Measurements of dissipation rate in frontal zones

Article

Jul 2001
Q J ROY METEOR SOC

This paper demonstrates that it is possible to use measurements of the Doppler spectrum width from a high-resolution radar to obtain quantitative estimates of dissipation rate during the passage of precipitating frontal zones. Using the high-resolution Chilbolton radar, this method has been applied, for the first time, to infer the rate at which kinetic energy is dissipated by turbulence within mid- and lower-tropospheric frontal zones. Analysis of detailed measurements for one cold-frontal region showed multiple shallow sheets of high dissipation rate, with spatially averaged values around 1 x 10−3 m2s−3. Comparable values were also observed in a warm-frontal zone. The magnitude of the observed dissipation rate supports the hypothesis that mixing played a major role in determining the mean properties of the fronts, in that the turbulence acted to destroy the frontal shear layer on a time-scale of several hours. These issues have particular relevance to the representation of fronts within future very high-resolution numerical weather-prediction models.

An aircraft study of turbulence dissipation rate and temperature structure function in the unstable marine atmospheric boundary layer

Article

Dec 1980

The dissipation rate of turbulent kinetic energy, ε, and the temperature structure function parameter, C T2, have been measured over water from the near surface (Z = 3 m) to the top of the boundary layer. The near surface values of ε and C T2 were used to calculate the velocity and temperature Monin-Obukhov scaling parameters u * and T *. The data collected during unstable lapse rates were used to evaluate the feasibility of extrapolating the values of ε and C T2 as a function of height with empirical scaling formulae. The dissipation rate scaling formula of Wyngaard et al. (l971 a) gave a good fit to an average of the ε data for Z < 0.8 Z i. In the surface layer the scaling formula of Wyngaard et al. (1971b) disagreed with the C T2 values by as much as 50%. This disagreement is due to an unexpected reduction in the measured values of C T2 forZ < 30 m. At this point it is not clear if the discrepancy is a unique property of the marine boundary layer or if it is simply some unknown instrumental or analytical problem. The mixed layer scaling results were similar to the overland results of Kaimal et al. (1976).

Evidence of Organized Large Eddies by Ground-Based Doppler Lidar, Sonic Anemometer and Sodar

Article

Sep 1998

In an experiment investigating the planetary boundary layer (PBL) wind and temperature fields, and PBL inversion height recorded by various instruments, the results reveal the presence of organized large eddies (OLE) or rolls. The measurements by lidars, anemometers, soundings and sodar gave an overview of the characteristics of the rolls and sources of energy production that maintain them. The experimental results obtained on two consecutive days are compared to model outputs. The agreement is excellent, showing that thermal stratification and wind shear are important factors in the structure and dynamics of OLE. A heterodyne Doppler lidar (HDL) is shown to be a useful tool in the study of OLE.

Coherent Doppler Lidar Measurements of Wind Field Statistics

Article

Feb 1998

Coherent Doppler lidar measurements of wind statistics in the boundary layer are presented. The effects of the spatial averaging by the lidar pulse are removed using theoretical corrections and computer simulations. This permits unbiased estimates of velocity variance, spatial velocity structure functions, energy dissipation rate, and other point statistics of the velocity field.

A New Tethered Balloon-Borne Payload for Fine-Scale Observations in the Cloudy Boundary Layer

Article

Mar 2003

A new scientific payload is introduced for fine-scale measurements of meteorological (wind vector, static air temperature, humidity, and air pressure) and microphysical (aerosol particles and cloud droplets) properties, suspended below a tethered balloon. The high resolution sensors and the tethered balloon are described. Measurements in a lifted fog layer from a first field campaign are presented.The detailed investigation of the fog/haze and the temperature inversion layer demonstrates the damping influence of the fog on temperature fluctuations, while thewind fluctuations are significantly decreased by theevolving temperature inversion, whichwas about 30 m above the fog layer.From spectral analysis the noise floors of the high-resolution sensors are determined to10-6 kg m-3 for the LWC (liquid water content) and 4 mK for the fast temperature sensor (UFT-B). The correlation betweentemperature and LWC structures in shallow haze layers is investigated. The release of latent heat and the corresponding warming in the haze of about 0.1 K could be quantified.

Estimation of turbulent energy dissipation rate from Doppler lidar data

Conference Paper

Jan 1995

Along with measurements of mean wind fields, Doppler lidars are used for estimation of the turbulence parameters. ¹⁻⁴ In particularly, the attempts to use the Doppler lidars for measurement of the dissipation rate of the turbulent kinetic energy ε T and the wind field structure constant from estimations of mean spectrum width of Doppler signal are discussed in Refs.2-4. This approach is true for small sizes of lidar sensing volume. If longitudinal size of sensing volume Δ z is comparable or exceeds the outer scale of turbulence L V this method can not be used. We discuss the feasibility of estimations of dissipation rate from Doppler lidar data at arbitrary size of lidar probe volume.

Retrieving Stratocumulus Drizzle Parameters Using Doppler Radar and Lidar

Article

Jan 2005

Stratocumulus is one of the most common cloud types globally, with a profound effect on the earth's radiation budget, and the drizzle process is fundamental in understanding the evolution of these boundary layer clouds. In this paper a combination of 94-GHz Doppler radar and backscatter lidar is used to investigate the microphysical properties of drizzle falling below the base of stratocumulus clouds. The ratio of the radar to lidar backscatter power is proportional to the fourth power of mean size, and so potentially it can provide an accurate size estimate. Information about the shape of the drop size distribution is then inferred from the Doppler spectral width. The algorithm estimates vertical profiles of drizzle parameters such as liquid water content, liquid water flux, and vertical air velocity, assuming that the drizzle size spectrum may be represented by a gamma distribution. The depletion time scale of cloud liquid water through the drizzle process can be estimated when the liquid water path of the cloud is available from microwave radiometers, and observations suggest that this time scale varies from a few days in light drizzle to a few hours in strong drizzle events. Radar and lidar observations from Chilbolton (in southern England) and aircraft size spectra taken during the Atlantic Stratocumulus Transition Experiment have both been used to derive the following power-law relationship between liquid water flux (LWF) (g m-2 s-1) and radar reflectivity (Z) (mm6 m-3): LWF = 0.0093Z0.69. This relation is valid for frequencies up to 94 GHz and therefore would allow a forthcoming spaceborne radar to measure liquid water flux around the globe to within a factor of 2 for values of Z above -20 dBZ.

Deriving turbulent kinetic energy dissipation rate within clouds using ground based radar

Article

Jan 2004

Antenna parameters for incoherent backscatter heterodyne lidar

Article

May 1979

B. J. Rye

The antenna and beam geometry of lidar systems employing heterodyne reception of incoherent backscatter signals are discussed. Particular emphasis is placed on systems where the target extends uniformly across the transmitted beam using topographic targets or atmospheric backscatter. The geometry is assumed to be circularly symmetrical, but otherwise arbitrary obscurations are permitted. The effects of atmospheric scintillation are neglected. Parameters are defined which characterize the system efficiency, and the conditions under which these parameters may be maximized are considered.

Comparison of balloon-carried atmospheric motion sensors with Doppler lidar turbulence measurements

Article

Mar 2009
REV SCI INSTRUM

Magnetic sensors have been added to a standard weather balloon radiosonde package to detect motion in turbulent air. These measure the terrestrial magnetic field and return data over the standard uhf radio telemetry. Variability in the magnetic sensor data is caused by motion of the instrument package. A series of radiosonde ascents carrying these sensors has been made near a Doppler lidar measuring atmospheric properties. Lidar-retrieved quantities include vertical velocity (w) profile and its standard deviation (w). w determined over 1 h is compared with the radiosonde motion variability at the same heights. Vertical motion in the radiosonde is found to be robustly increased when w>0.75 m s−1 and is linearly proportional to w. ©2009 American Institute of Physics

Errors associated with dual Doppler turbulence measurements

Article

Jun 2005

In July 2004 a field trial was undertaken to simultaneously deploy two similar Doppler lidar systems with the aim of measuring the turbulence characteristics of the atmospheric boundary layer over an urban area. This paper outlines the characteristics of the lidar systems, details the deployment configuration of the lidars, and discusses the dispersion model parameters that can be obtained using this procedure. An error analysis is undertaken to highlight the possible problems associated with the derived data. A case study from the trial is shown to compare the dual-lidar derived data to dispersion model results from the NAME dispersion model.

Estimations of atmospheric boundary layer fluxes and other turbulence parameters from Doppler lidar data

Article

Dec 1992

Techniques for extraction of boundary layer parameters from measurements of a short pulse (~0.4 μs) CO2 Doppler lidar (λ=10.6 μm) are described. The lidar is operated by the National Oceanic and Atmospheric Administration (NOAA) Wave Propagation Laboratory (WPL). The measurements are those collected during the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE). The recorded radial velocity measurements have a range resolution of 150m. With a pulse repetition rate of 20 Hz it is possible to perform scannings in two perpendicular vertical planes (x-z and y-z) in approximately 72 s. By continuously operating the lidar for about an hour, one can extract stable statistics of the radial velocities. -from Authors

Modeling of Direct Detection Doppler Wind Lidar. I. The Edge Technique

Article

Oct 1998

Jack A. McKay

Analytic models, based on a convolution of a Fabry-Perot etalon transfer function with a Gaussian spectral source, are developed for the shot-noise-limited measurement precision of Doppler wind lidars based on the edge filter technique by use of either molecular or aerosol atmospheric backscatter. The Rayleigh backscatter formulation yields a map of theoretical sensitivity versus etalon parameters, permitting design optimization and showing that the optimal system will have a Doppler measurement uncertainty no better than approximately 2.4 times that of a perfect, lossless receiver. An extension of the models to include the effect of limited etalon aperture leads to a condition for the minimum aperture required to match light collection optics. It is shown that, depending on the choice of operating point, the etalon aperture finesse must be 4-15 to avoid degradation of measurement precision. A convenient, closed-form expression for the measurement precision is obtained for spectrally narrow backscatter and is shown to be useful for backscatter that is spectrally broad as well. The models are extended to include extrinsic noise, such as solar background or the Rayleigh background on an aerosol Doppler lidar. A comparison of the model predictions with experiment has not yet been possible, but a comparison with detailed instrument modeling by McGill and Spinhirne shows satisfactory agreement. The models derived here will be more conveniently implemented than McGill and Spinhirne's and more readily permit physical insights to the optimization and limitations of the double-edge technique.

Discrete Spectral Peak Estimation in Incoherent Backscatter Heterodyne Lidar. II: Correlogram Accumulation

Article

Feb 1993

For pt.I see ibid., vol.31, no.1, p.16-27 (1993). The correlogram, an alternative to formation of the periodogram from the time series data sample. is described. The correlogram consists of a set of autocorrelation estimates for some number of lags obtained directly from the data sample. Correlogram accumulation entails adding or averaging the sets of autocorrelation estimates prior to the spectral estimation stage. The correlogram is formed in the time domain directly from the data, using a minimal number of multiplications and additions so that it may be formed and accumulated at the speed that the data are acquired

Jan 1996

M K Hill
B J Brooks
S J Norris
M H Smith
I M Brooks
G De Leeuw

Hill, M. K., B. J. Brooks, S. J. Norris, M. H. Smith, I. M. Brooks, and G. de Leeuw, 2008: A Compact Lightweight Aerosol Spectrometer Probe (CLASP). J. Atmos. Oceanic Technol., 25, 1996–2006.

Observations of Diurnal Variation in a Cloud-capped Marine Boundary Layer

Abstract

No full-text available

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