No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
The Role of Radiation in Influencing Tropical Cloud Distributions in a Radiative-Convective Equilibrium Cloud-Resolving Model
Journal of the Atmospheric Sciences
Abstract
Observations by Johnson et al. depict regions of active tropical convection as possessing increased relative humidity through a deep layer and reduced low-level static stability when compared to nonconvecting regions. Shallow cumulus clouds, congestus clouds, and deep convection all coexist within these convecting regions. This investigation explores the effect that radiation might have on the tropical cloud distributions by using large-domain (20 000 km) radiative–convective equilibrium cloud-resolving model (RCE-CRM) experiments that reproduce similar moisture, stability, and cloud structures to those observed.
Radiation is found to significantly increase the amount of shallow and intermediate-level clouds (tops between 1.5 and 5 km) by increasing low-level stability and thus promoting additional low-level cloud detrainment. The mechanism by which radiation stabilizes the low levels is found to differ between convectively suppressed and active regions. In convectively suppressed regions, strong relative humidity gradients within the trade inversion layer produce a low-level cooling maximum that further stabilizes the stable layer, much as proposed by Mapes and Zuidema. In convectively active regions, sufficiently moist columns with no relative humidity gradients are also found to produce a low-level cooling maximum that stabilizes the lower levels. This cooling maximum is due to the complicated effects of the water vapor continuum and is sensitive to the absolute moisture path. Because of the dependence on absolute moisture, the radiative enhancement of shallow and intermediate-level clouds in convectively active regions is potentially sensitive to SSTs.
... This paper therefore describes the results of simple RCE experiments in which equilibrium is achieved over a lower boundary of fixed SST for a range of tropical cloud regimes in which there is no large-scale dynamical forcing. As such, this means that convection is driven solely by surface heat and moisture fluxes and radiational cooling, and that no largescale convergence, mean wind or vertical wind shear were imposed, being similar in this respect to numerous previous RCE studies (e.g., Tompkins and Craig 1998a;Pakula and Stephens 2009). ...
... The model PW values are thus 4-5 mm (;10%) drier than observations over 300-K waters. This can in part be attributed to the two-dimensionality of the grid that artificially enlarges the areas of subsidence (Bretherton and Smolarkiewicz 1989;Tompkins 2000;Pakula and Stephens 2009). If the PW is averaged over what may be referred to as the convectively disturbed regions, defined after Stephens et al. (2008) to be those regions of the grid where the vertically integrated condensate is greater than 0.01 kg m 22 and the outgoing longwave radiation (OLR) is less than 260 W m 22 , the values range from 44.1 to 45.7 mm (experiment DIST-PW in Table 3), which is more in keeping with observations of deep convective regions in the tropics. ...
The impacts of enhanced aerosol concentrations such as those associated with dust intrusions on the trimodal distribution of tropical convection have been investigated through the use of large-domain (10 000 grid points), fine-resolution (1 km), long-duration (100 days), two-dimensional idealized cloud-resolving model simulations conducted under conditions of radiative-convective equilibrium (RCE). The focus of this research is on those aerosols that serve primarily as cloud condensation nuclei (CCN). The results demonstrate that the large-scale organization of convection, the domain-averaged precipitation, and the total cloud fraction show only show a weak response to enhanced aerosol concentrations. However, while the domainwide responses to enhanced aerosol concentrations are weak, aerosol indirect effects on the three tropical cloud modes are found to be quite significant and often opposite in sign, a fact that appears to contribute to the weaker domain response. The results suggest that aerosol indirect effects associated with shallow clouds may offset or compensate for the aerosol indirect effects associated with congestus and deep convection systems and vice versa, thus producing a more moderate domainwide response to aerosol indirect forcing. Finally, when assessing the impacts of aerosol indirect forcing associated with CCN on the characteristics of tropical convection, several aspects need to be considered, including which cloud mode or type is being investigated, the field of interest, and whether localized or systemwide responses are being examined.
... Insights from these simulations can thus inform and improve parameterization of the same phenomena in atmospheric general circulation models (AGCMs; . While RCE simulations are idealized, they provide a good analog for the tropical region as a whole, where mean large-scale ascent is diminishingly small and horizontal advection plays only a minor role in driving cloud formation (Ghan et al., 2000;Pakula & Stephens, 2009;Wing et al., 2018). Global RCE simulations have been found to be representative of the tropical atmosphere and can be successful for examining climate change responses (Popke et al., 2013). ...
... Insights from these simulations can thus inform and improve parameterization of the same phenomena in atmospheric general circulation models (AGCMs; . While RCE simulations are idealized, they provide a good analog for the tropical region as a whole, where mean large-scale ascent is diminishingly small and horizontal advection plays only a minor role in driving cloud formation (Ghan et al., 2000;Pakula & Stephens, 2009;Wing et al., 2018). Global RCE simulations have been found to be representative of the tropical atmosphere and can be successful for examining climate change responses (Popke et al., 2013). ...
Single‐column models (SCMs) simulations are sometimes used to evaluate model physics and aid parameterization development. However, few studies have systematically compared SCM behavior—where column boundary conditions are specified—with that of corresponding 3D models, where columns interact dynamically. Here we address this by comparing forced responses of an SCM in radiative‐convective equilibrium (RCE) with those of a multi‐column model (MCM) where the model domain is in RCE but individual columns are not, examining what factors affect the models' comparability. We find that convective organization in the MCM depends at least as much on the convection scheme as on other mechanisms known to organize convection (e.g., radiative feedback). Moreover, convective organization emerges as a robust factor affecting SCM–MCM comparability, with more aggregated states in 3D associated with larger behavior deviations from the 1D counterpart. This is found across five convection schemes and applies to simulated mean states, linear responses to small tendency perturbations, and adjustments to doubled‐CO2 forcing. Nevertheless, we find that even when convection is organized, behavior differences between pairs of schemes in the SCM are largely preserved in the MCM. This indicates that when model physics produces accurate behavior in a 1D setup, it will be more likely to do so in a 3D setup. However, our idealized RCE framework implies that these conclusions may not apply to situations with strong large‐scale forcing or encountered over land. Lastly, we demonstrate the practical value of linear responses by showing that they can accurately predict an SCM's tropospheric adjustment to doubled‐CO2 forcing.
... Over the tropical oceans, the congestus mode is associated with a mid-level stable layer near the melting (0 • C) level (e.g., Johnson et al., 1999;Jensen and Del Genio, 2006). This is thought to arise from radiative interactions accompanying intrusions of dry air from poleward latitudes (e.g., Mapes and Zuidema, 1996;Redelsperger et al., 2002;Pakula and Stephens, 2009) or melting processes in organized stratiform precipitation (Mapes and Houze, 1995), although recent findings argue that the melting mechanism is not essential for creating the stable layer (Nuijens and Emanuel, 2018). How these two possible mechanisms explain the presence of the congestus mode across the different Amazon regimes is not obvious. ...
Radiosonde observations collected during the GoAmazon2014/5 campaign are analyzed to identify the primary thermodynamic regimes accompanying different modes of convection over the Amazon. This analysis identifies five thermodynamic regimes that are consistent with traditional Amazon calendar definitions of seasonal shifts, which include one wet, one transitional, and three dry season regimes based on a k-means cluster analysis. A multisensor ground-based approach is used to project associated bulk cloud and precipitation properties onto these regimes. This is done to assess the propensity for each regime to be associated with different characteristic cloud frequency, cloud types, and precipitation properties. Additional emphasis is given to those regimes that promote deep convective precipitation and organized convective systems. Overall, we find reduced cloud cover and precipitation rates to be associated with the three dry regimes and those with the highest convective inhibition. While approximately 15 % of the dataset is designated as organized convection, these events are predominantly contained within the transitional regime.
... (ii) Evaporation from the ocean surface, driven by the larger scale convectively forced circulation and the localscale winds associated with the convection in the lower troposphere (e.g. Pakula and Stephens, 2009), is the principal source of moistening over most of the domain. ...
... Though studies involving satellite and in situ observations (e.g., Zhang 1993;Sherwood and Wahrlich 1999;Masunaga et al. 2005;Rapp et al. 2005;Su et al. 2006;Rondanelli and Lindzen 2008) have shed light on how deep convection might change in a warming environment, examination of the response of convection to a perturbation in the climate system requires use of a model. Several studies have employed cloudsystem-resolving models (CSRMs) to this end, using both horizontally uniform sea surface temperature (SST; e.g., Tompkins and Craig 1998a,b;Bretherton et al. 2005;Kuang and Hartmann 2007;Stephens et al. 2008;Pakula and Stephens 2009) and imposed horizontal SST gradients (Grabowski et al. 2000;Tompkins 2001a,b). Many modeling studies have employed radiative convective equilibrium (RCE) as a useful framework, exploiting the fact that deep convection produces an adjustment to an equilibrium state that is rapid compared to the time scale of forcing mechanisms. ...
This paper explores the response of the tropical hydrologic cycle to surface warming through the lens of large-domain cloud-system-resolving model experiments run in a radiative-convective equilibrium framework. Simulations are run for 55 days and are driven with fixed insolation and constant sea surface temparatures (SSTs) of 298 K. 300 K, and 302 K. In each experiment, convection organizes into coherent regions of large-scale ascent separated by areas with relatively clear air and troposphere-deep descent. Aspects of the simulations correspond to observed features of the tropical climate system, including the transition to large precipitation rates above a critical value of total column water vapor, and an increase in convective intensity with SST amidst weakening of the large-scale overturning circulation. However, the authors also find notable changes to the interaction between convection and the environment as the surface warms. In particular, organized convection in simulations with SSTs of 298 and 300 K is inhibited by the presence of a strong midtropospheric stable layer and dry upper troposphere. As a result, there is a decrease in the vigor of deep convection and an increase in stratiform precipitation fraction with an increase in SST from 298 to 300 K. With an increase in SST to 302 K, moistening of the middletroposphere and increase in lower-tropospheric buoyancy serve to overcome these limitations, leading to an overall increase in convective intensity and larger increase in upper-tropospheric relative humidity. The authors conclude that, while convective intensity increases with SST, the aggregate nature of deep convection is strongly affected by the details of the thermodynamic environment in which it develops. In particular, the positive feedback between increasing SST and a moistening upper troposphere found in the simulations, operates as a nonmonotonic function of SST and is modulated by a complex interaction between deep convection and the environmental relative humidity and static stability profile. The results suggest that projected changes in convection that assume a monotonic dependence on SST may constitute an oversimplification.
Abstract. Radiosonde launches collected during the GoAmazon2014/5 campaign are analyzed to identify the primary thermodynamic regimes accompanying different modes of convection over the Amazon. This analysis identifies five thermodynamic regimes that are consistent with traditional Amazon calendar definitions for seasonal shifts, which include a wet, transitional, and three dry-season clusters. A multisensor ground-based approach is used to project associated bulk cloud and precipitation properties onto these regimes to assess the propensity for each regime for characteristic cloud frequency, cloud types, and precipitation properties. Additional emphasis is given to those regimes that promote deep convective precipitation and organized convective systems. Overall, we find reduced cloud cover and precipitation rates to be associated with the driest regimes and those with the highest convective inhibition CIN. While approximately 15 % of the dataset is designated as organized convection, these events are predominantly contained within transitional regime days.
A two‐column radiative–convective equilibrium (RCE) model is used to study the depth of convection that develops in the subsiding branch of a Walker‐like overturning circulation. The model numerically solves for two‐dimensional non‐rotating hydrostatic flow, which is damped by momentum diffusion in the boundary layer and model interior, and by convective momentum transport. Convection, clouds and radiative transfer are parametrized, and the convection scheme does not include explicit freezing or melting.
While integrating the model towards local RCE, the level of neutral buoyancy (LNB) fluctuates between mid‐ and high levels. Evaporation of detrained moisture at the LNB locally cools the environment, so that the final RCE state has a stable layer at mid‐levels (550 hPa ≈ 50–100 hPa below 0 °C), which is unrelated to melting of ice. Preferred detrainment at mid‐ and high levels leaves the middle‐to‐upper troposphere relatively dry.
A circulation is introduced by incrementally lowering the sea‐surface temperature in one column, which collapses convection: first to a congestus mode with tops near 550 hPa, below the dry layer created in RCE; then to congestus with tops near 650 hPa; and finally to shallow cumulus with tops near 850 hPa. Critical to stabilizing congestus near 650 hPa is large radiative cooling near moist cumulus tops under a dry upper atmosphere. This congestus mode is very sensitive, and only develops when horizontal temperature gradients created by evaporative and radiative cooling can persist against the work of gravity waves. This only happens in runs with ample momentum diffusion, which are those with convective momentum transport or large domains.
Compared to the shallow mode, the congestus mode produces a deep moist layer and more precipitation. This reduces radiative cooling in the cloud layer and enhances stability near the cloud base, which weakens the circulation, and leads to less precipitation over the warm ocean.
Through convection-permitting ensemble and sensitivity experiments, this study examines the impact of the diurnal radiation cycle on the pregenesis environment of Hurricane Karl (2010). It is found that the pregenesis environmental stability and the intensity of deep moist convection can be considerably modulated by the diurnal extremes in radiation. Nighttime destabilization of the local and large-scale environment through radiative cooling may promote deep moist convection and increase the genesis potential, likely enhancing the intensity of the resultant tropical cyclones. Modified longwave and shortwave radiation experiments found tropical cyclone development to be highly sensitive to the periodic cycle of heating and cooling, with suppressed formation in the daytime-only and no-radiation experiments and quicker intensification compared with the control for nighttime-only experiments.
Motivated by the importance of the effective radius (re) of
the droplets to radiative transfer, this paper presents parameterization
schemes, which provide a measure of re in stratiform liquid
water clouds (in the 13° to +13°C temperature range), for use in
general circulation models (GCMs) or mesoscale models. The first scheme
developed here is based on theory and numerical calculations of droplet
condensational growth, while the second is based on Twomey's analytical
approach. Both methods are evaluated against detailed model
calculations, and a method for implementing either scheme in general
circulation models and remote sensing applications is described. The new
parameterization produces accurate (within a few percent) estimates of
the effective droplet radius as a function of height, while the cloud
optical thickness compares favorably (often to within <10%) with the
model calculations. Twomey's scheme gives reasonable estimates of
optical thickness, but tends to underestimate the droplet concentration
and overestimate the effective radius for typical maritime and
continental CCN spectra.
Recent progress is reviewed in the understanding of convective interaction with water vapor and changes associated with water vapor in warmer climates. Progress includes new observing techniques (including isotopic methods) that are helping to illuminate moisture-convection interaction, better observed humidity trends, new modeling approaches, and clearer expectations as to the hydrological consequences of increased specific humidity in a warmer climate. A theory appears to be in place to predict humidity in the free troposphere if winds are known at large scales, providing a crucial link between small-scale behavior and large-scale mass and energy constraints. This, along with observations, supports the anticipated water vapor feedback on climate, though key uncertainties remain connected to atmospheric dynamics and the hydrological consequences of a moister atmosphere. More work is called for to understand how circulations on all scales are governed and what role water vapor plays. Suggestions are given for future research.
Radiative-convective statistical equilibria are obtained using a
two-dimensional model in which radiative transfer is interactive with
the predicted moisture and cloud fields. The domain is periodic in x
(the horizontal direction), with a width of 640 km, and extends from the
ground to 26 km. The lower boundary is a fixed-temperature
water-saturated surface. The model produces a temperature profile
resembling the mean profile observed in the tropics. A number of
integrations of several months' duration are described in this
preliminary examination of the model's qualitative behavior. The model
generates a Quasi-Biennial Oscillation (QBO)-like oscillation in the
x-averaged winds with an apparent period of approximately 60 days. This
oscillation extends into the troposphere and influences the convective
organization. In order to avoid the associated large vertical wind
shears, calculations are also performed in which the x-averaged winds
are constrained to vanish. The convection then evolves into a pattern in
which rain falls only within a small part of the domain. The moisture
field appears to provide the memory that localizes the convection. If
the vertical shears are fixed at a modest nonzero value, this
localization is avoided. Comparing calculations with surface
temperatures of 25 and 30 C, the planetary albedo is found to decrease
with increasing temperature, primarily due to a reduction in low-level
cloudiness.
This paper examines feedbacks between the radiative heating of clouds and convection in the context of numerical radiative-convective equilibrium experiments conducted using both 2D and 3D versions of a cloud-resolving model. The experiments are conducted on a large domain, and equilibria develop as jux- taposed regions of dry and moist air that are connected and sustained by circulations between them. The scales of such variability are large and differ significantly between the 2D and 3D versions of the experi- ments. Three sensitivity experiments were conducted which, when compared to the control experiment, provide insight into the relative influences of cloud-radiation feedback mechanisms on the equilibrium state achieved. It emerges from the experiments conducted that radiation feedbacks operate via two main pathways, with the radiative heating by high clouds being the governing process of both. The predominant bimodal nature of the moist equilibrium is established by gradients in radiative heating that, in turn, are determined by high cloud differences between wet and dry regions that, in turn, are controlled by convec- tion. Convection, on the other hand, is also influenced by the localized effects of cloud radiative heating by these extended layers of high clouds. The results of the experiments demonstrate how high cloud radiative heating, which is a by-product of the convection itself, provides a feedback that acts to regulate the high clouds produced in the wet convective areas of the equilibrium.
From the analysis of surface, upper-air, and satellite observations it is suggested that the hydrological cycle associated with the Madden Julian oscillation acts in the mode of a self-regulating oscillator. The regulation occurs as a feedback between hydrological processes in the atmosphere; radiation processes; and the dynamical movement of air over the tropical oceans controling variations of rainfall, cloudiness, and sea surface temperature (SST) on time scales varying between 30 and 60 days. The conjectured feedback occurs in three main phases: (i) the destablization phase: the atmosphere becomes increasingly unstable by the combination of radiative cooling of the upper troposphere, the gradual build up of shallow convection, and the warming of the SSTs under near-clear-sky and calm conditions; (ii) the convective stage: large-scale convection develops over the region resulting in widespread heavy precipitation, deepening of the oceanic mixed layer, cooling of the SST, and moistening of the upper troposphere; and (iii) the restoring phase: the combination of continued cooling of the SSTs maintained by the strong low-level winds and reduced solar heating, with the radiative heating of the upper atmosphere by high clouds sustained by high humidity, are major factors in stabilizing the atmosphere, suppressing convection, bringing an end to the cooling of the SSTs, and eventually leading to a calming of the winds, dissipation of the thick upper-level clouds, and a restoration of the cycle to its warming phase.
A detailed multiple-scattering model has been employed to investigate the sensitivity of radiation profiles and flux divergences to changes in macrostructure and microstructure of basic water cloud types. The study has been performed on a range of cloud types including variable distributions of liquid water content (LWC) and drop-size distributions. Total shortwave heating rates vary from 1 to 5°C h1 and are larger in higher clouds. IR cooling rates in the upper regions of cloud also increase with increasing elevation and are dominated by the atmospheric window contribution. Thus the typical instrument discriminating the IR radiation between 7 and 14,m will measure almost the entire IR radiative cooling or heating of low-level water clouds. Both shortwave heating and IR cooling within cloud layers are primarily dependent on LWC and its vertical distribution and are more or less independent of drop-size distribution. Cloud albedo does vary with drop-size distribution but is virtually independent of LWC distribution for fixed total water.
This study investigates the radiative, cloud, and thermodynamic characteristics of the atmosphere separated into objectively defined cloud regimes in the tropical western Pacific (TWP). A cluster analysis is applied to 2 yr of daytime-only data from the International Satellite Cloud Climatology Project (ISCCP) to identify four major cloud regimes in the TWP region. A variety of data collected at the Department of Energy’s Atmospheric Radiation Measurement Program (ARM) site on Manus Island is then used to identify the main characteristics of the regimes. Those include surface and top-of-the-atmosphere radiative fluxes and cloud properties derived from a suite of ground-based active remote sensors, as well as the temperature and water vapor distribution measured from radiosondes.
The major cloud regimes identified in the TWP area are two suppressed regimes—one dominated by the occurrence of mostly shallow clouds, the other by thin cirrus—as well as two convectively active regimes—one exhibiting a large coverage of optically thin cirrus clouds, the other characterized by a large coverage with optically thick clouds. All four of these TWP cloud regimes are shown to exist with varying frequency of occurrence at the ARM site at Manus. It is further shown that the detailed data available at that site can be used to characterize the radiative, cloud, and thermodynamic properties of each of the regimes, demonstrating the potential of the regime separation to facilitate the extrapolation of observations at one location to larger scales. A variety of other potential applications of the regime separation are discussed.
Soundings taken from the tropical western Pacific warm pool region during TOGA COARE reveal the common occurrence of temperature and moisture perturbations near the 0°C level. The perturbations frequently are characterized by shallow layers of increased stability (or occasionally temperature inversions) and reversals or inflections in the vertical profile of specific humidity. Similar temperature and moisture inversions have been observed elsewhere in the Tropics and midlatitudes but have not received much attention. Isothermal layers are known to exist just below the melting level in stratiform rain regions; however, not all stable layers observed over the warm pool are confined to precipitation systems. The perturbation in the specific humidity profile accounts for the often-observed double-peak structure in the apparent moisture sink Q2 in the Tropics. Stratification of the data based on relative humidity criteria indicates that the stable layers near the 0°C level generally fall into two main classifications: anomalously cool-moist conditions at and slightly below the 0°C level and anomalously warm-dry conditions at and just above. The former occur primarily within or in close proximity to precipitating systems, suggesting they are a result of the direct effects of melting. Soundings in the latter group typically occur outside convective areas. Mechanisms for formation of the warm-dry stable layers are unclear at this time, but advective, radiative, gravity wave, and melting effects may all play some role. In some cases they may simply be remnant melting layers from past convection. There is evidence to indicate that the stable layers near the 0°C level affect tropical cloud populations. Convection impinging upon or penetrating the stable layers may detrain significantly near the 0°C level, thereby contributing to perturbations in the moisture profile there. Midlevel cloud layers that are commonly observed in the Tropics may be evidence of this detrainment. Another primary finding is the frequent occurrence of a trade wind stable layer over the warm pool. Though not widely recognized, this finding is consistent with the prevalence of trade cumulus clouds in the western Pacific region. The trade inversions often coexist with, but are distinct from, the inversions near the 0°C level.
The authors hypothesize that radiation is responsible for the thermal structure of dry tongues. The radiative effects of humidity structures in the troposphere are reviewed and illustrated. A composite-derived radiative heating perturbation, acting for 3.5 days in an idealized model of a dry tongue 300 km in width (values consistent with case studies), reproduces fairly well the high vertical wavenumber components of the composite thermal structure. Dynamics acts to spread the effect of the radiative heating perturbation over a wider area and to concentrate the temperature perturbations near the dry tongue base, as observed. The deep layer-mean warmth of the composite dry tongue arises from a slight correlation between dry tongue occurrence in this dataset and a 1°C global-scale intraseasonal variation of tropical tropospheric temperature.A dry tongue affects convective clouds both directly, through its thermal structure, and indirectly, through dry air entrainment. Low-level dry tongues can prevent deep convection outright while the stable layers associated with dry tongues at higher altitudes may cause convection to detrain mass. Humidity drops, stable layers, and a proxy for layer clouds all have similar altitude distributions.
It has long been known that trade wind cumulus and deep cumulonimbus represent primary components of the broad spectrum of cumulus clouds in the Tropics, which has led to the concept of a bimodal distribution of tropical clouds. However, recent analyses of shipboard radar data from Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (COARE) provide evidence of abundant populations of a third cloud type, cumulus congestus. Congestus clouds constitute over half the precipitating convective clouds in COARE and contribute over one-quarter of the total convective rainfall. Global Atmospheric Research Program Atlantic Tropical Experiment studies reveal a similar midlevel peak in the distribution of radar-echo tops. These findings lead to the conclusion that shallow cumulus, congestus,and cumulonimbus are all prominent tropical cumulus cloud types. They are associated with trimodal distributions of divergence, cloud detrainment, and fractional cloudiness in the Tropics. The peaks in the distributions of radar-echo tops for these three cloud types are in close proximity to prominent stable layers that exist over the Pacific warm pool and the tropical eastern Atlantic: near 2 km (the trade stable layer), ;5 km (near 08C), and ;15-16 km (the tropopause). These stable layers are inferred to inhibit cloud growth and promote cloud detrainment. The 08C stable layer can produce detrainment from cumulonimbi (at- tendant shelf clouds) and help retard the growth of precipitation-laden and strongly entraining congestus clouds. Moreover, restriction of growth of congestus clouds to just above the 08C level limits further enhancement of cloud buoyancy through glaciation. The three cloud types are found to vary significantly during COARE on the timescale of the 30-60-day intraseasonal oscillation. The specific roles of clouds of the congestus variety in the general circulation are not yet clear, but some (the shallower ones) contribute to moistening and preconditioning the atmosphere for deep convection; others (the deeper ones) contribute an important fraction of the total tropical rainfall, and both likely produce many midlevel clouds, thereby modulating the radiative heating of the tropical atmosphere.
Gravity waves play an important role in the redistribution of heat and
moisture in a deep convecting cloud field. We explore this role in a
two-dimensional numerical experiment on a simple moist convecting system
consisting of an isolated long-lasting nonprecipitating cloud in a calm
atmosphere with no surface forcing. The cloud develops a horizontally
averaged density variation with height which is neutrally buoyant with
respect to a moist adiabatic. The buoyancy difference between the cloud
and the undisturbed sounding produces circulations that can be
understood as spreading gravity waves which adjust the environmental
buoyancy to be equal to the cloud buoyancy by compensating subsidence.
Unlike the circulations inside clouds, this adjustment takes place
without turbulent mixing. Hence, the `buoyancy adjustment time'
T1 during which the environment comes into rough buoyant
equilibrium with the clouds is much shorter than the `mixing time'
T2 which it takes a tracer, initially concentrated at some
level, to become homogenized through the layer in a field of clouds.
Associated with this gravity wave response are circulations which cause
the preferential detrainment of `cloud-processed' air at heights at
which the buoyancy of clouds relative to their far environment is
decreasing with height. We confirmed these ideas in less idealized
numerical experiments in which different soundings, precipitation, and
surface heat fluxes were included.
1] Calculations of radiative flux profiles require measurements of thermodynamic and cloud properties (temperature, humidity, liquid and ice water content). Instruments capable of making these measurements have only recently become available. The U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Program operates a comprehensive set of atmospheric remote sensing instruments at sites around the world, including three in the tropical western Pacific region. We have processed several months of ARM observations from two of these sites, Manus and Nauru, to calculate time series of vertical cloud property profiles and associated radiative fluxes and heating rates. Maxima in cloud occurrence are found in the boundary layer and the upper troposphere at both sites. Manus, which was much more convectively active than Nauru during the study period, also exhibits a midlevel cloud feature near the melting level. The two sites exhibit very different diurnal cycles. Manus experiences an afternoon maximum in high clouds while Nauru experiences a weak afternoon minimum. Nauru experiences a strong afternoon maximum in boundary layer clouds. Calculated fluxes at the surface and the top of the atmosphere are found to be in reasonable agreement with measurements. Below 15 km, radiative processes lead to cooling in the average profile, with local maxima near the surface and approximately 8 km. On average, high and midlevel clouds have a net warming effect though not enough to offset the clear-sky cooling. The prevalent boundary layer clouds at Nauru have a net cooling effect in and above the cloud layer and a net warming below. This data set will be an important tool for describing radiative processes in the tropics and assessing the simulation of these processes in dynamical models.
A radiative-convective boundary layer model was developed by coupling a thermodynamic model of a partially mixed convective boundary layer (CBL) with a radiation model, and energy balance constraints were used to study coupled boundary layer (CBL) equilibrium over three timescales (about 1 day, about 10 days, and more than 100 days). It is shown that the variation in cloud top decreases with greater coupling to the atmosphere and the ocean. The slope of the latent heat flux with increasing SST decreases with more tropospheric coupling, and reverses sign with a coupled ocean.
This book presents descriptions of numerical models for testing cumulus in cloud fields. It is divided into six parts. Part I provides an overview of the problem, including descriptions of cumulus clouds and the effects of ensembles of cumulus clouds on mass, momentum, and vorticity distributions. A review of closure assumptions is also provided. A review of "classical" convection schemes in widespread use is provided in Part II. The special problems associated with the representation of convection in mesoscale models are discussed in Part III, along with descriptions of some of the commonly used mesoscale schemes. Part IV covers some of the problems associated with the representation of convection in climate models, while the parameterization of slantwise convection is the subject of Part V.
This paper describes an analysis of large-scale [O(1000 km)] convectively coupled gravity waves simulated using a two-dimensional cloud-resolving model. The waves develop spontaneously under uniform radiative cooling and approximately zero-mean-flow conditions, with wavenumber 2 of the domain appearing most prominently and right-moving components dominating over left-moving components for random reasons. The analysis discretizes the model output in two ways. First, a vertical-mode transform projects profiles of winds, temperature, and heating onto the vertical modes of the model’s base-state atmosphere. Second, a cloud-partitioning algorithm sorts sufficiently cloudy grid columns into three categories: shallow convective, deep convective, and stratiform anvil.
Results show that much of the tilted structures of the waves can be captured by just two main vertical spectral “bands,” each consisting of a pair of vertical modes. The “slow” modes have propagation speeds of 16 and 18 m s−1 (and roughly a full-wavelength vertical structure through the troposphere), while the “fast” modes have speeds of 35 and 45 m s−1 (and roughly a half-wavelength structure). Deep convection anomalies in the waves are more or less in phase with the low-level cold temperature anomalies of the slow modes and in quadrature with those of the fast modes. Owing to the characteristic life cycle of deep convective cloud systems, shallow convective heating peaks ∼2 h prior to maximum deep convective heating, while stratiform heating peaks ∼3 h after. The onset of deep convection in the waves is preceded by a gradual deepening of shallow convection lasting a period of many hours.
Results of this study are in broad agreement with simple two-mode models of unstable large-scale wave growth, under the name “stratiform instability.” Differences here are that 1) the key dynamical modes have speeds in the range 16–18 m s−1, rather than 23–25 m s−1 (owing to a shallower depth of imposed radiative cooling), and 2) deep convective heating, as well as stratiform heating, is essential for the generation and maintenance of the slow modes.
This paper outlines a radiation parameterization method for deriving broadband fluxes that is currently being implemented in a number of global and regional atmospheric models. The rationale for the use of the 2-stream method as a way of solving the radiative transfer problem for broadband solar and longwave fluxes is presented. This rationale is based on assessment of these models in the context of a novel method of classifying radiative transfer problems that more clearly identifies the types of problems encountered in calculating globally distributed broadband fluxes. The delta-Eddington model (DEM) and the constant-hemispheric 2-stream models (CHMs) are shown to be superior to other 2-stream methods of solution under this classification and also superior to 4-stream solutions for the many classes of problems relevant to modeling the global atmosphere. These two methods are used to construct a radiation model of broadband solar and IR fluxes based on the k-distribution data of Fu and Liou. When tested against available line-by-line (LBL) and other reference model calculations of broadband fluxes, it is shown that (i) comparisons of CHM top-of-atmosphere (TOA) clear-sky longwave fluxes with fluxes obtained from LBL models agree within approximately 1 2 W m-2. The agreement with LBL clear-sky fluxes at the surface, typically within 5 W m-2, is compromised by the specific form of continuum absorption parameterization adopted. (ii) The clear- and cloudy-sky solar fluxes and heating rates agree remarkably with a reference doubling adding multiple scattering model. The rms TOA flux difference under all-sky conditions is approximately 6 W m-2; the layer-mean heating rate difference is 0.1 K day-1. (iii) The effect of IR scattering by clouds is shown to produce a bias when neglected that generally exceeds the model-to-model differences presented. Neglect of IR scattering produces a global bias in the calculated outgoing longwave radiation (OLR) of approximately -8 W m-2 (i.e., the nonscattering models calculated an OLR that is larger than what the scattering models calculated by this amount). Locally, the TOA bias may approach 20 W m-2. The associated bias in surface longwave fluxes varies in magnitude between 2 and 5 W m-2. It was also shown how the computational effort required to produce broadband fluxes varies linearly with the number of model layers. This is an important characteristic given the increasing tendency for increasing the vertical resolution of atmospheric models.
This paper presents observational evidence in support of the existence of a large diurnal cycle (one daily maximum and one daily minimum) of oceanic, tropical, deep cumulus convection. The more intense the deep convection and the more associated it is with organized weather systems, the more evident is a diurnal cycle with a maximum in the morning. At many places heavy rainfall is 2–3 times greater in the morning than in the late afternoon-evening. Many land stations also show morning maxima of heavy rainfall. The GATE observations show a similar diurnal range in heavy rainfall, but the time of maximum occurrence is in the afternoon. This occurrence is 6–7 h later than in most other oceanic regions and is probably a result of downwind influences from Africa and the fact that the GATE heavy rainfall was often associated with squall lines.
Diurnal variations in low-level, layered and total cloudiness show a much smaller range. The variability of deep convection and heavy rainfall is not readily observable from those satellite pictures which cannot well resolve individual convective cells nor is it easily obtained from surface observations of the percent of sky coverage which are heavily weighted to the presence of low-level and layered clouds. A comparison of previous observational studies is made. It is hypothesized that the diurnal cycle in deep convection with a morning maximum is associated with organized weather disturbances. This diurnal cycle likely results from day versus night variations in tropospheric radiational cooling between the weather system and its surrounding cloud-free region.
A new three-dimensional cloud resolving model (CRM) has been developed to study the statistical properties of cumulus convection. The model was applied to simulate a 28-day evolution of clouds over the Atmospheric Radiation Measurement Program (ARM) Southern Great Plains site during the summer 1997 Intensive Obser- vation Period. The model was forced by the large-scale advective tendencies and surface fluxes derived from the observations. The sensitivity of the results to the domain dimensionality and size, horizontal grid resolution, and parameterization of microphysics has been tested. In addition, the sensitivity to perturbed initial conditions has also been tested using a 20-member ensemble of runs. The model captures rather well the observed temporal evolution of the precipitable water and precipitation rate, although it severely underestimates the shaded cloud fraction possibly because of an inability to account for the lateral advection of clouds over the ARM site. The ensemble runs reveal that the uncertainty of the simulated precipitable water due to the fundamental uncertainty of the initial conditions can be as large as 25% of the mean values. Even though the precipitation rates averaged over the whole simulation period were virtually identical among the ensemble members, the timing uncertainty of the onset and reaching the precipitation maximum can be as long as one full day. Despite the predictability limitations, the mean simulation statistics are found to be almost insensitive to the uncertainty of the initial conditions. The overall effects of the third spatial dimension are found to be minor for simulated mean fields and scalar fluxes but are quite considerable for velocity and scalar variances. Neither changes in a rather wide range of the domain size nor the horizontal grid resolution have any significant impact on the simulations. Although a rather strong sensitivity of the mean hydrometeor profiles and, consequently, cloud fraction to the microphysics parameters is found, the effects on the predicted mean temperature and humidity profiles are shown to be modest. It is found that the spread among the time series of the simulated cloud fraction, precipitable water, and surface precipitation rate due to changes in the microphysics parameters is within the uncertainty of the ensemble runs. This suggests that correlation of the CRM simulations to the observed long time series of the aforementioned parameters cannot be generally used to validate the microphysics scheme.
In Part I of this paper, the primary focus is on the water and heat budgets of the control experiment, which is designed to simulate the convective-radiative equilibrium response of the model to an imposed vertical velocity and a fixed sea surface temperature at 28°C. The simulated atmosphere is conditionally unstable below the freezing level and close to neutral above the freezing level. The equilibrium water budget shows that the total moisture source, Ms, which is contributed by surface evaporation (0.24 Ms) and the large-scale advection (0.76 Ms), all converts to mean surface precipitation P̄s. Most of Ms is transported vertically in convective regions where much of the condensate is generated and falls to surface (0.68 P̄s). -from Authors
Using a compositing technique, the temporal progression of tropical convective systems and the mean atmospheric state in their vicinity is constructed from a time series of geostationary satellite and operational rawinsonde data. The technique establishes the stage in the life cycle of convection prevailing at a given place and time, by a simple objective method using time series of satellite brightness temperature (T(b)) histograms collected from a region surrounding the site. Soundings are classified according to their placement in the convective life cycle, and composites formed that represent the areal-mean state of the convecting atmosphere at each stage, for several scales of horizontal averaging. The temporal structure found here for the mesoscale-mean atmosphere closely resembles existing observations of the horizontal structure of a tropical squall line, albeit with certain reductions in amplitude and stabilization rate. This supports the generality of previous findings that the physical mechanisms documented for squall line systems are characteristic of other forms of tropical convection, and quantifies their imprint on thermodynamic mean fields at large spatial scales. The results show an instability decay time during convection of about 3 h at the 120-km horizontal scale. This time grows with scale, as does the duration of the mature stage of convection, which is of similar magnitude. The results also show a column-integrated loss of water vapor and moist static energy following convection. These results may be of use in model validation and theoretical treatments of convective interaction with dynamics.
The author's studies of distributions of atmospheric water in relation to air circulations are reviewed and summarized here. These studies began in the 1950's during student days at M.I.T. and at the Weather Radar Branch, Air Force Cambridge Research Laboratories. They were extended during 1961–1963 at the Travelers Research Center, Hartford, Connecticut, with encouragement and support from Helmut Weickmann, then a principal scientist at the U.S. Army Signal Research and Development Laboratory, Fort Monmouth, New Jersey. The author's work on this subject was substantially completed at the National Severe Storms Laboratory, Norman, Oklahoma.The studies show the nature of probable connections among distributions of water vapor, cloud, rain, and snow with vertical and horizontal winds, divergence of the wind, compressibility of the atmosphere, and the strength and distribution of various microphysical processes. The findings also aid interpretation of observations and they offer lessons for efforts toward artificial augmentation of precipitation.
The conservation and distribution of water substance in atmospheric circulations is considered within a frame of continuity principles, model air flows, and models of microphysical processes. The simplest considerations of precipitation involve its vertical distribution in an updraft column, where condensate appears immediately as precipitation with uniform terminal fallspeed. The study also treats steady two-dimensional air circulations in which time-dependent distributions of water vapor, cloud and precipitation respond to model microphysical processes.
The approach throughout is essentially kinematic, although results provide numerous insights into the dynamical properties of a cloudy or stormy atmosphere. Water distributions are explained in relation to the air’s horizontal divergence, vertical velocity and compressibility, and physical pictures are presented frequently. The findings are compared with various observations on precipitating weather systems.
Detailed summaries of this paper by Sections are presented in Sections 1 and 15.
The response of convection to changing sea surface temperature (SST) in the absence of large-scale flow is examined, using a three-dimensional cloud resolving model. The model includes a five-category bulk microphysical scheme representing snow, ice, graupel, rain, and cloud amounts in addition to an interactive radiation scheme for the shortwave and infrared. Long integrations are made to achieve a radiative-convective equilibrium state for SSTs of 298, 300, and 302 K, for which cloud and convection statistics are analyzed.The main conclusion of the paper is that, despite significant temperature sensitivities in many of the conversion terms between bulk water categories, convection is very insensitive to changing SST in the absence of large-scale flow. This is a result of the moist adiabatic temperature profile that the tropical atmosphere is constrained to take. A parcel of air rising through a deep convective cloud experiences approximately the same range of temperatures but at higher altitudes as SST increases. Thus the vertical profiles of cloud fraction and other cloud-related statistics are simply shifted in height, but not changed in overall magnitude.The small changes in cloud properties that do occur lead to a small reduction in cloud fraction as SST increases. This appears to be due to an increase in graupel amounts with respect to snow, giving smaller cloud fractions since graupel has a higher fall velocity. The radiative effects of the changes in atmospheric properties are examined and it is found that the model atmosphere exhibits no supergreenhouse effect since atmospheric relative humidity is not altered significantly by the SST changes. The water vapor feedback effect is largely canceled by the change in temperature. Clouds have a negligibly small, but highly nonlinear, feedback in the model climate, in the absence of large-scale flow.
The spatial organization of deep moist convection in radiative–convective equilibrium over a constant sea surface temperature is studied. A 100-day simulation is performed with a three-dimensional cloud-resolving model over a (576 km)2 domain with no ambient rotation and no mean wind. The convection self-aggregates within 10 days into quasi-stationary mesoscale patches of dry, subsiding and moist, rainy air columns. The patches ultimately merge into a single intensely convecting moist patch surrounded by a broad region of very dry subsiding air.
The self-aggregation is analyzed as an instability of a horizontally homogeneous convecting atmosphere driven by convection–water vapor–radiation feedbacks that systematically dry the drier air columns and moisten the moister air columns. Column-integrated heat, water, and moist static energy budgets over (72 km)2 horizontal blocks show that this instability is primarily initiated by the reduced radiative cooling of air columns in which there is extensive anvil cirrus, augmented by enhanced surface latent and sensible heat fluxes under convectively active regions due to storm-induced gustiness. Mesoscale circulations intensify the later stages of self-aggregation by fluxing moist static energy from the dry to the moist regions. A simple mathematical model of the initial phase of self-aggregation is proposed based on the simulations.
In accordance with this model, the self-aggregation can be suppressed by horizontally homogenizing the radiative cooling or surface fluxes. Lower-tropospheric wind shear leads to slightly slower and less pronounced self-aggregation into bands aligned along the shear vector. Self-aggregation is sensitive to the ice microphysical parameterization, which affects the location and extent of cirrus clouds and their radiative forcing. Self-aggregation is also sensitive to ambient Coriolis parameter f, and can induce spontaneous tropical cyclogenesis for large f. Inclusion of an interactive mixed-layer ocean slows but does not prevent self-aggregation.
An investigation is conducted to document the role convectively generated cold pools play in determining the spatial organization of tropical deep convection. Using a high-resolution cloud-resolving model, the evolution of cold pools produced by deep convection is examined, in the situation of limited large-scale wind shear, and a homogeneous underlying sea surface temperature. Ignoring the cold pools resulting from multiple deep convective events, the mean model cold pool attained a minimum temperature and water vapor mixing ratio depression of 1 K and 1.5 g kg -1, respectively; a horizontal velocity increase of 4.8 m s -1; and the latent and sensible heat fluxes are increased by a factor of 1.9 and 2.6, respectively. The cold pools had a mean lifetime of approximately 2.5 h and attained maximum radii ranging from 3 to 18 km, with a mean of 8.6 km. Taking the organization of convection into account, these figures are consistent with observational studies of convective wakes. The composite cold pool showed that development occurred in three distinct stages. As seen in observations, the air in the vicinity of deep convection has a higher equivalent potential energy than average. In the first stage, before the downdraft develops and reaches the subcloud layer, the area below the convection is cooled and moistened by the evaporation of rainfall. The downdraft then injects cold and dry air into the boundary layer, and the spreading cold pool is consequentially moister than average just inside the gust front but drier in the central regions. Finally, mass conservation requires that air from above the boundary layer be entrained into the wake of the expiring downdraft-thus causing the central regions of the cold pool to recover very quickly in temperature-but increases further the moisture perturbation. These features are confirmed by a number of observational studies. The key to the triggering of new deep convective cells lies with the band of high equivalent potential temperature, but negatively buoyant air, situated inside the boundary of the spreading cold pools. It is this air that forms the new convective cells. The radius a: which this occurs is determined by the time taken for surface fluxes to remove the negative temperature perturbation, thereby reducing convective inhibition energy. In summary, the primary mechanism by which cold pools organize tropical deep convection in low wind shear conditions is principally thermodynamical, and not dynamical as previously assumed.
Separate diagnostic models of shallow and inversion-penetrating, trade
wind cumulus clouds are combined with large-scale BOMEX heat and
moisture budgets to obtain the thermodynamic effects and properties of
the shallow clouds. The deeper trade wind cumulus effects are obtained
using a modified version of Nitta's (1975) spectral diagnostic method,
while a bulk model based on the bubble theory of convection is used for
the shallow clouds.Results show that the buoyancy of the small cumuli is
an order of magnitude less than the buoyancy of the deeper clouds, and
suggest that their effects on the temperature stratification are small
compared with radiative cooling and subsidence warning due to deep
clouds. Also, the results show that the primary role of the small cumuli
is to support the growth of deeper clouds through the transport of water
vapour from the subcloud layer into the lower cloud layer. The deduced
shallow cloud properties are found to be consistent with observations
and Sommeria's (1976) numerical simulation of populations of small
tropical cumuli.The implications of these results for cumulus
parameterization schemes am discussed.
The large-scale organization of tropical deep convection is investigated in idealized two-dimensional cloud-resolving simulations. A 20 000 km periodic computational domain allows interactions among moist convection, mesoscale organization, and surface exchange on a wide range of scales. A uniform 10 m s−1 easterly wind and a uniform sea surface temperature of 30°C are assumed. A prescribed temperature tendency mimicking the mean radiative cooling of the tropical troposphere is used in this pilot study. The large-scale organization of convection resembles the observed tropical atmosphere as well as results from some previous numerical studies using parametrized convection. The simulation highlights mesoscale convective systems with horizontal scales of several hundred kilometres moving east to west with speeds similar to the mean flow, and an envelope of convection with a horizontal extent of a few thousand kilometres and propagating west to east. The propagation speed of the large-scale envelope compares well with the phase velocitites of convectively coupled Kelvin waves observed in the equatorial waveguide. Convective momentum transport and the impact of convective systems on temperature and moisture near the surface are key processes responsible for the large-scale organization of convection. These results are discussed in the context of recent observations and numerical studies of convection organization in the tropics.
Variability of atmospheric water vapour is the most important climate feedback in present climate models. Thus, it is of crucial importance to understand the sensitivity of water vapour to model attributes, such as physical parametrizations and resolution. Here we attempt to determine the minimum vertical resolution necessary for accurate prediction of water vapour.
To address this issue, we have run two single‐column models to tropical radiative—convective equilibrium states and have examined the sensitivity of the equilibrium profiles to vertical resolution. Both column models produce reasonable equilibrium states of temperature and moisture.
Convergence of the profiles was achieved in both models using a uniform vertical resolution of around 25 hPa. Coarser resolution leads to significant errors in both the water vapour and temperature profiles, with a resolution of 100 hPa proving completely inadequate. However, fixing the boundary‐layer resolution and altering only the free‐tropospheric resolution significantly reduces sensitivity to vertical resolution in one of the column models, in both water and temperature, highlighting the importance of resolving boundary‐layer processes. Additional experiments show that the height of the simulated tropopause is sensitive to upper‐tropospheric vertical resolution.
At resolutions higher than 33 hPa, one of the models developed a high degree of vertical structure in the vapour profile, resulting directly from the complex array of microphysical processes included in the stratiform cloud parametrization, some of which were only resolved at high resolutions. This structure was completely absent at lower resolutions, casting some doubt on the approach of using relatively complicated cloud schemes at low vertical resolutions.
Previous heat budget studies relying on infra-red radiation calculations have not used a water vapour pressure dependent continuum absorption in the 830-1,250 cm−1 region of the infra-red spectrum. The purpose of this study was to investigate the differences in the infra-red divergence profiles caused by inclusion of the vapour pressure dependent continuum absorption.
The results indicate only a very small change in infra-red divergence from previous work for a typical midlatitude temperature, water vapour profile. However, for tropical temperature and moisture profiles, the new continuum data result in approximately 30 per cent more total tropospheric infra-red cooling than obtained using earlier continuum absorption data. In addition, it is shown that within a realistic range of tropical atmosphere moisture content, the continuum infra-red cooling may act to either stabilize or destabilize the lower layers of the troposphere.
1] Radiative-convective equilibrium experiments with a two-dimensional cloud resolving model illustrate the influence of a lofted absorbing dust layer on the organization of tropical convection. At quasi-equilibrium, the dust-covered region of the model exhibits increased occurrence of deep convection compared to the dust-free region but with reduced convection in the dust-free region controlled in part by a large-scale monsoon-like circulation forced by the aerosol radiative heating. The dry air associated with the dust layer inhibits convection initially over most of the dust-covered region with convection occurring predominantly at the lateral boundaries of the layer. This behavior reproduces features which have been observed in cases of Saharan dust transport over the tropical Atlantic.
This paper offers a critical review of the topic of cloud–climate feedbacks and exposes some of the underlying reasons for the inherent lack of understanding of these feedbacks and why progress might be expected on this important climate problem in the coming decade. Although many processes and related parameters come under the influence of clouds, it is argued that atmospheric processes fundamentally govern the cloud feedbacks via the relationship between the atmospheric circulations, cloudiness, and the radiative and latent heating of the atmosphere. It is also shown how perturbations to the atmospheric radiation budget that are induced by cloud changes in response to climate forcing dictate the eventual response of the global-mean hydrological cycle of the climate model to climate forcing. This suggests that cloud feedbacks are likely to control the bulk precipitation efficiency and associated responses of the planet’s hydrological cycle to climate radiative forcings.
The paper provides a brief overview of the effects of clouds on the radiation budget of the earth–atmosphere system and a review of cloud feedbacks as they have been defined in simple systems, one being a system in radiative–convective equilibrium (RCE) and others relating to simple feedback ideas that regulate tropical SSTs. The systems perspective is reviewed as it has served as the basis for most feedback analyses. What emerges is the importance of being clear about the definition of the system. It is shown how different assumptions about the system produce very different conclusions about the magnitude and sign of feedbacks. Much more diligence is called for in terms of defining the system and justifying assumptions. In principle, there is also neither any theoretical basis to justify the system that defines feedbacks in terms of global–time-mean changes in surface temperature nor is there any compelling empirical evidence to do so. The lack of maturity of feedback analysis methods also suggests that progress in understanding climate feedback will require development of alternative methods of analysis.
It has been argued that, in view of the complex nature of the climate system, and the cumbersome problems encountered in diagnosing feedbacks, understanding cloud feedback will be gleaned neither from observations nor proved from simple theoretical argument alone. The blueprint for progress must follow a more arduous path that requires a carefully orchestrated and systematic combination of model and observations. Models provide the tool for diagnosing processes and quantifying feedbacks while observations provide the essential test of the model’s credibility in representing these processes. While GCM climate and NWP models represent the most complete description of all the interactions between the processes that presumably establish the main cloud feedbacks, the weak link in the use of these models lies in the cloud parameterization imbedded in them. Aspects of these parameterizations remain worrisome, containing levels of empiricism and assumptions that are hard to evaluate with current global observations. Clearly observationally based methods for evaluating cloud parameterizations are an important element in the road map to progress.
Although progress in understanding the cloud feedback problem has been slow and confused by past analysis, there are legitimate reasons outlined in the paper that give hope for real progress in the future.
As part of the EUROCS (EUROpean Cloud Systems study) project, cloud‐resolving model (CRM) simulations and parallel single‐column model (SCM) tests of the sensitivity of moist atmospheric convection to midtropospheric humidity are presented. This sensitivity is broadly supported by observations and some previous model studies, but is still poorly quantified. Mixing between clouds and environment is a key mechanism, central to many of the fundamental differences between convection schemes.
Here, we define an idealized quasi‐steady ‘testbed’, in which the large‐scale environment is assumed to adjust the local mean profiles on a timescale of one hour. We then test sensitivity to the target profiles at heights above 2 km.
Two independent CRMs agree reasonably well in their response to the different background profiles and both show strong deep precipitating convection in the more moist cases, but only shallow convection in the driest case. The CRM results also appear to be numerically robust. All the SCMs, most of which are one‐dimensional versions of global climate models (GCMs), show sensitivity to humidity but differ in various ways from the CRMs. Some of the SCMs are improved in the light of these comparisons, with GCM improvements documented elsewhere. © Crown copyright, 2004.
In the tropics the assumed existence of a balanced atmospheric state of radiative-convective equilibrium is a useful and widely utilized concept. Given an atmospheric state of radiative-convective equilibrium which is perturbed, this paper attempts to identify the mechanisms that determine the time-scale for the restoration of the balanced state. The perturbation could arise from large-scale atmospheric wave motions, local-scale convective downdraughts or sea surface temperature perturbations for example. The resulting state immediately after the perturbation is applied can be one of suppressed or convective conditions, and it is the complex response under convective conditions that is explored in this paper.
A three-dimensional cloud resolving model is operated to a radiative-convective equilibrium state to which sea surface temperature perturbations are then applied. It is found that the variability of the model state variables, such as temperature and total water vapour amount, can be divided by processes into an exponential adjustment to the new balanced state on a long time-scale (15 days), superimposed by short time-scale variability (<4 days) that is governed almost solely by the convective mass-flux. The determination of the long time-scale trend to equilibrium is then investigated with further numerical experiments, which demonstrate that radiation determines the adjustment time-scale via its control of the subsidence velocities in the clear-sky regions surrounding convection. Some implications of the results for cumulus parametrization are discussed.
Results of a three-dimensional numerical model are analysed in a study of turbulence and entrainment within mixed layers containing stratocumulus with or without parameterized cloud-top radiative cooling. The model eliminates most of the assumptions invoked in theories of cloud-capped mixed layers, but suffers disadvantages which include poor resolution and large truncation errors in and above the capping inversion. For relatively thick mixed layers with relatively thick capping inversions, the cloud-top radiative cooling is found to be lodged mostly within the capping inversion when the cooling is confined locally to the upper 50 m or less of the cloud. It does not then contribute substantially towards increased buoyancy flux and turbulence within the well mixed layer just below. The optimal means of correlating the entrainment rate, or mixed-layer growth rate, for mixed layers of variable amounts of stratocumulus is found to be through functional dependence upon an overall jump Richardson number, utilizing as scaling velocity the standard deviation of vertical velocity existing at the top of the mixed layer (near the center of the capping inversion). This velocity is found to be a fraction of the generalized convective velocity for the mixed layer as a whole which is greater for cloud-capped mixed layers than for clear mixed layers.
Monthly mean precipitable water data obtained from passive microwave radiometry were correlated with the National Meteorological Center (NMC) blended sea surface temperature data. It is shown that the monthly mean water vapor content of the atmosphere above the oceans can generally be prescribed from the sea surface temperature with a standard deviation of 0.36 g/sq cm. The form of the relationship between precipitable water and sea surface temperature in the range T (sub s) greater than 18 C also resembles that predicted from simple arguments based on the Clausius-Clapeyron relationship. The annual cycle of the globally integrated mass of Scanning Multichannel Microwave Radiometer (SMMR) water vapor is shown to differ from analyses of other water vapor data in both phase and amplitude and these differences point to a significant influence of the continents on water vapor. Regional scale analyses of water vapor demonstrate that monthly averaged water vapor data, when contrasted with the bulk sea surface temperature relationship developed in this study, reflect various known characteristics of the time mean large-scale circulation over the oceans. A water vapor parameter is introduced to highlight the effects of large-scale motion on atmospheric water vapor. Based on the magnitude of this parameter, it is shown that the effects of large-scale flow on precipitable water vapor are regionally dependent, but for the most part, the influence of circulation is generally less than about + or - 20 percent of the seasonal mean.
Radiativeconvective feedbacks in idealized states of radiativeconvective equilibrium
- N Wood
- L Pakula
--, N. Wood, and L. Pakula, 2004b: On the radiative effects of
dust on tropical convection. Geophys. Res. Lett., 31, L23112,
doi:10.1029/2004GL021342.
--, S. van den Heever, and L. Pakula, 2008: Radiativeconvective feedbacks in idealized states of radiativeconvective equilibrium. J. Atmos. Sci., 65, 3899-3916.