Quarterly Journal of the Royal Meteorological Society

Published by Wiley
Online ISSN: 1477-870X
Print ISSN: 0035-9009
An analysis is presented of the life cycles of three different South Pacific cyclones. Emphasis is given to two of the cyclones which propagate south-eastward along the western edge of the South Pacific convergence zone (SPCZ) and reach middle latitudes before decaying. The analysis is based on imagery from the GOES-West Satellite and on data from the European Center for Medium Range Weather Forecasts (EMWF). A comparison between estimates of the mean sea level pressure in the vicinity of the cyclones taken at observation stations and from ECMWF predictions shows good agreement when the cyclone disturbances are above grid-scale. The propagation characteristics of the cyclones are discussed in detail.
During Intensive Observing Period (IOP) 2a of the Mesoscale Alpine Programme (MAP) a significant convective system developed in the Lago Maggiore area. Its evolution is simulated using the Meso-NH model run over three nested domains with a 2 km horizontal resolution for the finest grid. Model results are assessed by comparing computed precipitation and model radar reflectivities with observed parameters. A control simulation initialized with European Centre for Medium-Range Weather Forecasts analyses and run with the standard microphysical scheme of the model yields fairly realistic results. Neglecting the ice processes in the microphysical scheme degrades the numerical results, whereas, significant improvement is obtained when heavy rimed particles are considered to be hail instead of graupel.
Heat and moisture budgets are computed during part of the FGGE Special Observing Period-1 for an area containing the South Pacific convergence zone, deriving 12-hour precipitation rate estimates as residuals and comparing them with those derived from a GOES IR technique. Heat budget estimates are generally in good agreement with the IR estimates; the axis of maximum precipitation derived from tne heat budget is well aligned with the lowest values of outgoing long wave excitation, which represent high, cold cloud tops resulting from deep cumulus convection. The vertical advection term is found to be the dominant term in the heat budget; this, together with radiative cooling, yields the approximate balance between adiabatic cooling and diabatic heating.
Calculations of the transfer of kinetic energy from the troposphere to the stratosphere by means of the pressure-interaction term are carried out for the mid-winter stratospheric warming of 1967-1968. This term is computed in the wave number domain for the 100 mb level by means of the Eliassen and Palm (1960) approximation. There appears to be a distinct correlation between the stratospheric kinetic energy per wave number at the 10 mb level and the energy transfer by that wave number at the 100-mb level. A comparison of the magnitude of the energy flux for 1963 versus that for this later warming suggests that this term is of the same order as the remaining terms in the kinetic energy equation calculated by Perry (1967). Results obtained using this approximation appear to give good agreement with those obtained by direct calculation of the vertical velocity field.
During the winter of 1969-1970 when a rather intense stratospheric warming of the type restricted to the upper stratosphere occurred, we were fortunate in having a considerable number of rocketsonde observations throughout the Northern Hemisphere. In addition, the Satellite Infrared Spectrometer (SIRS) on board Nimbus 3 yielded large amounts of radiation temperature data which, when combined with the above, enabled us to construct hemispheric synoptic analyses at the 10-, 5-, and 2-mb levels, for seven days during the warming event. Employing these analyses along with the daily objective analyses from 1,000-30 mb, we computed certain of the major energy transformations for the troposphere and lower and middle stratosphere. It was found that the middle stratospheric warming event was of a direct baroclinic type, forced from below. Also, we noted that if the warming is to propagate into the lower stratosphere, it is necessary to reverse the normal eddy kinetic to eddy potential energy transfer in that region so that it operates in a direct baroclinic sense.
The paper describes the results of a series of 30 observations of stratospheric aerosol made with a ground-based lidar on the North Pacific Coast during a period relatively uninfluenced by major volcanic penetrations and displaying a relative temporal minimum in particulate content. The objectives were to provide a record of aerosol behavior during this intervolcanic period, to compare this behavior with that revealed by previous studies using a variety of techniques, and to provide comparative data on the stratospheric aerosol by conducting joint lidar and aircraft observations. Determination of scattering profile ratios from lidar signal profiles and analysis of experimental errors are described. Analysis of the data shows that significant temporal variability of the aerosol was observed, probably of nonvolcanic origin. Much of the variability was confined to the 23-30 km height region, above the major peak in scattering ratio. The evidence is that this is not due to influxes of extraterrestrial material. Vertical motions of the centroid of the scattering ratio peak were recorded during the 1973 stratospheric warming, and illustrate the value of lidar's ability to monitor temporal variations of vertical structure.
A potential vorticity (PV) index is defined as a measure of the zonally averaged, raid-latitude PV gradient on the 300 K isentropic surface in the northern hemisphere, and use is made of the 1978/79 wińter ECMWF FGGE Hl-b data set to study the evolution of that index and its relation to teleconnection patterns of 500 mb geopotential-height anomaly. This index vacillates with a dominant period of about two weeks. For some time lags, the isoline patterns of the cross-correlation function between the PV index and 500mb geopotential-height anomalies are similar to some teleconnection patterns studied by several investigators using large climatological data sets. Scandinavia, Greenland and the Pacific are important centres of ‘action’ in the northern hemisphere in the 1978/79 winter. the formation of blocking highs at these locations occurs at rather specific phases during the vacillatory evolution of the PV index, especially as the flow transits from a low to a high PV index. When the above study is duplicated using a zonal mean wind gradient at 500mb as a zonal index, the results do not reflect the teleconnection patterns as crisply as the PV index does for the FGGE year.
Data obtained from stratospheric sounding units on board the NOAA-6 satellite were used to investigate the three-dimensional evolution of the final warming that takes place in the stratosphere of the Southern Hemisphere during spring, with particular attention given to the events of spring 1982. Evidence is presented for a strong influence of the topography of the Southern Hemisphere on the evolution of the final warming. An association was found between the location of anticylones in the upper stratosphere, warm pools of air in the lower stratosphere, and a climatological split of the westerly jet stream in the upper troposphere.
An averaged time series for the surface data for the 15 × 15 km FIFE site was prepared for the summer of 1987. Comparisons with 48-Hour forecasts from the ECMWF model for extended periods in July, August and October 1987 identified model errors in the incoming short-wave radiation in clear skies, the ground heat flux, the formulation of surface evaporation, the soil-moisture model, and entrainment at boundary-layer top. The model clear-sky short-wave flux is too high at the surface by 5–10%. The ground heat flux is too large by a factor of 2 to 3 because of the large thermal capacity of the first soil layer (which is 7 cm thick), and a time truncation error. The surface evaporation was near zero in October 1987, rather than of order 70 W m−2 at noon. The surface evaporation falls too rapidly after rainfall, with a time-scale of a few days rather than the 7-10 days (or more) of the observation. On time-scales of more than a few days the specified ‘climate layer’ soil moisture, rather than the storage of precipitation, has a large control on the evapotranspiration. The order 2g Kg-1 in forecasts from an experimental analysis with nearly realistic surface fluxes; this because there is insufficient downward mixing of dry air.
The vertical structure of monsoon thermal forcing by precipitating convection is diagnosed in terms of horizontal divergence. Airborne Doppler-radar divergence profiles from nine diverse mesoscale convective systems (MCSs) are presented. The MCSs consisted of multicellular convective elements which in time gave rise to areas of stratiform precipitation. Each of the three basic building blocks of the MCSs - convective, intermediary, and stratiform precipitation areas - has a consistent, characteristic divergence profile. Convective areas have low-level convergence, with its peak at 2-4 km altitude, and divergence above 6 km. Intermediary areas have convergence aloft, peaked near 10 km, feeding into mean ascent high in the upper troposphere. Stratiform areas have mid-level convergence, indicating a mesoscale downdraught below the melting level, and a mesoscale updraught aloft. Rawinsonde composite divergence profiles agree with the Doppler data in at least one important respect: the lower-tropospheric convergence into the MCSs peaks 2-4-km above the surface. Rawinsonde vorticity profiles show that monsoonal tropical cyclones spin-up at these elevated levels first, then later descend to the surface. Rawinsonde observations on a larger, continental scale demonstrate that at large horizontal scales only the 'gravest vertical mode' of MCS heating is felt, while the effects of shallower components of the heating (or divergence) profiles are trapped near the heating, as predicted by geostrophic adjustment theory.
All available, routinely collected surface data are used to document the development of the ‘Papal Front’, which crossed southern Germany on 3 May 1987 causing casualities and damage. Objective analyses on isentropic surfaces, initialized with the European aerological data, indicate that the development of the violent mesoscale front, which exhibited squall-line characteristics, began when a distinct mid-troposopheric anomaly of potential vorticity reached the north-western rim of the Alpine arc. The forced uplift ahead of the associated shortwave trough combined with ageostrophic motions due to a jet streak at higher levels apparently provided a trigger for deep convection as was observed over the Alpine foreland. Numerical simulations with a dry hydrostatic model indicate the relative importance of the synoptic-scale forcing and the Alpine orography for the generation of the ‘Papal Front’. A westerly ‘orographic jet’ over the foreland below crest height only develops in the simulation with full Alps and is corroborated by the few mountain wind observations that are available. A comparison with studies of frontal propagation along other mountain ranges reveals that the routine network in the Alpine region and its surroundings is at least of similar density, but so far hardly used for frontal case-studies. It is concluded that a series of ‘routine-data case-studies’ could help to determine the variability of frontal progression along the Alps and, thus, provide guidance when assessing the significance of single events, particularly those sampled during special observation programmes.
In this study a potential-vorticity (PV) streamer which crossed the Alps during the Special Observing Period of the Mesoscale Alpine Programme (MAP) on 6 November 1999 is analysed. In situ aircraft data, wind-profiler data, model output data from the French non-hydrostatic mesoscale model (Meso-NH), and European Centre for Medium-Range Weather Forecasts (ECMWF) analyses are used to describe the structure and evolution of the streamer. In addition lower-stratospheric humidity which was remotely sensed by a Differential Absorption Lidar (DIAL) installed on board the aircraft is analysed. The comparison of measured and simulated lower-stratospheric humidity data show good agreement in structure and magnitude. The data analysis also reveals that waves are generated by the orography and transmitted into the lower stratosphere where they modify the streamer's PV structure. The impact of orography and diabatic processes on the streamer's evolution is simulated with Meso-NH in a series of simulations. These simulations demonstrate that the separate effects of orography and diabatic processes are significant and comparable in magnitude, and that their combined influence leads to the formation of a PV cut-off. As a synergetic effect it is found that PV filaments are generated north of the Alpine barrier. The influence of orography and diabatic processes reduces the lower-stratospheric and upper-tropospheric PV by about 25% above the Alps, compared with the situation where neither effect is present.
Using three cloud generators, three-dimensional (3D) cloud fields are reproduced from microphysical cloud data measured in situ by aircraft. The generated cloud fields are used as input to a 3D radiative transfer model to calculate the corresponding fields of downward and upward irradiance, which are then compared with airborne and ground-based radiation measurements. One overcast stratocumulus scene and one broken cumulus scene were selected from the European INSPECTRO field experiment, which was held in Norwich, UK, in September 2002. With these data, the characteristics of the three different cloud reproduction techniques are assessed. Besides vertical profiles and histograms of measured and modelled liquid water content and irradiance, the horizontal structure of these quantities is examined in terms of power spectra and autocorrelation lengths. 3D radiative transfer calculations are compared with the independent pixel approximation, and their differences with respect to domain-averaged quantities and 3D fields are interpreted.
A key a priori information used in 4DVar is the knowledge of the system's evolution equations. In this paper we propose a method for taking full advantage of the knowledge of the system's dynamical instabilities in order to improve the quality of the analysis. We present an algorithm, four-dimensional variational assimilation in the unstable subspace (4DVar-AUS), that consists in confining in this subspace the increment of the control variable. The existence of an optimal subspace dimension for this confinement is hypothesized. Theoretical arguments in favor of the present approach are supported by numerical experiments in a simple perfect non-linear model scenario. It is found that the RMS analysis error is a function of the dimension N of the subspace where the analysis is confined and is minimum for N approximately equal to the dimension of the unstable and neutral manifold. For all assimilation windows, from 1 to 5 days, 4DVar-AUS performs better than standard 4DVar. In the presence of observational noise, the 4DVar solution, while being closer to the observations, if farther away from the truth. The implementation of 4DVar-AUS does not require the adjoint integration. Comment: 16 pages, 4 figures
In the afternoon of15 July 2007, a thunderstormwas initiatedwithin a line of cumulus clouds which formed parallel to the crest of the Black Forest mountains during the Intensive Observation Period (IOP) 8b of the Convective and Orographically induced Precipitation Study (COPS). This paper extends the analysis of processes that led to convection initiation (CI), i.e. the transition from shallow to deep convection, on this day with the data from several COPS instruments that have not been considered in previous studies. In particular, the boundary-layer structure, lids and the water-vapour field in the pre-convective environment of the event are discussed. For this purpose, we investigated measurements of water-vapour lidars, temperature lidars and wind lidars, profiles from radiosondes, in situ aircraft data and gridded data of weather stations as well as GPS integrated-water-vapour data and satellite imagery. Thermally driven circulation systems formed over both the Black Forest and the Vosges mountain ranges which resulted in local convergence zones. These superimposed with the large-scale convergence in the Black Forest area. In the presence of sufficient moisture and updraught, clouds formed close to the mountain crests. The related latent-heat release allowed larger thermals to be produced, which may have had a positive feedback on stabilizing these convergence zones as a whole. We believe that differences in the moisture field explain why convection remained shallow and sparse over the Vosges mountains because these differences were responsible for differences in convective inhibition (CIN). The stationary location of the convergence zone over the southern Black Forest was probably decisive for CI because it constantly transported sensible and latent heat into the area inwhich CI took place.
Time series of RMS analysis errors in potential temperature at the bottom level of the original LETKF (black line), the spin-up LETKF with Gaussian noises and epsilon=0.01 (thin red line with ), with Gaussian noises and epsilon=0.05 (red line), with Gaussian noises and 10 iterations (black dashed line), with 3D-Var noises and epsilon=0.05 (green line) and 4D-Var (blue line).
The number of iteration required by the spin-up LETKF with Gaussian noises and epsilon=0.01 (thin red line with ), with Gaussian noises and epsilon=0.05 (red line) and with 3D-Var noises and epsilon=0.05 (green line)
A scheme is proposed to improve the performance of the ensemble-based Kalman Filters during the initial spin-up period. By applying the no-cost ensemble Kalman Smoother, this scheme allows the model solutions for the ensemble to be "running in place" with the true dynamics, provided by a few observations. Results of this scheme are investigated with the Local Ensemble Transform Kalman Filter (LETKF) implemented in a Quasi-geostrophic model, whose original framework requires a very long spin-up time when initialized from a cold start. Results show that it is possible to spin up the LETKF and have a fast convergence to the optimal level of error. The extra computation is only required during the initial spin-up since this scheme resumes to the original LETKF after the "running in place" is achieved.
Lyapunov spectra for a range of values of h. The inset shows the maximal Lyapunov exponent γmax, as a function of h.
We propose a method to account for model error due to unresolved scales in the context of the ensemble transform Kalman filter (ETKF). The approach extends to this class of algorithms the deterministic model error formulation recently explored for variational schemes and extended Kalman filter. The model error statistic required in the analysis update is estimated using historical reanalysis increments and a suitable model error evolution law. Two different versions of the method are described; a time-constant model error treatment where the same model error statistical description is time-invariant, and a time-varying treatment where the assumed model error statistics is randomly sampled at each analysis step. We compare both methods with the standard method of dealing with model error through inflation and localization, and illustrate our results with numerical simulations on a low order nonlinear system exhibiting chaotic dynamics. The results show that the filter skill is significantly improved through the proposed model error treatments, and that both methods require far less parameter tuning than the standard approach. Furthermore, the proposed approach is simple to implement within a pre-existing ensemble based scheme. The general implications for the use of the proposed approach in the framework of square-root filters such as the ETKF are also discussed.
TheWind Infrared Doppler lidar (WIND) instrument was flown on board the aircraft Falcon of the Deutsches Zentrum für Luft- und Raumfahrt on two missions during the Special Observing Period (SOP) of the Mesoscale Alpine Programme (MAP). During the the first flight two complete sections of horizontal wind speed and direction were sampled up to a height of 7 km from Innsbruck to the Po basin and back. From the second mission 11 WIND soundings from 11 km downwards are presented along a route from a jet stream of up to 45 m s<sup>-1</sup> above Berlin towards the Alps. A routine radiosounding from Milano and episode-type simulations with the Meso-NH modelling system are used for detailed comparisons and to obtain comparative statistics.
We introduce an innovative wavelet-based approach to dynamically adjust the local grid resolution to maintain a uniform specified error tolerance. Extending the work of Dubos and Kevlahan (2013), a wavelet multi-scale approximation is used to make dynamically adaptive the TRiSK model (Ringler et al. 2010) for the rotating shallow water equations on the sphere. This paper focuses on the challenges encountered when extending the adaptive wavelet method to the sphere and ensuring an efficient parallel implementation using MPI. The wavelet method is implemented in Fortran95 with an emphasis on computational efficiency and scales well up to O(10^2) processors for load-unbalanced scenarios and up to at least O(10^3) processors for load-balanced scenarios. The method is verified using standard smooth test cases (Williamson et al. 1992) and a nonlinear test case proposed by (Galewsky te al. 2004). The dynamical grid adaption provides compression ratios of up to 50 times in a challenging homogenous turbulence test case. The adaptive code is about three times slower per active grid point than the equivalent non-adaptive TRiSK code and about four times slower per active grid point than an equivalent spectral code. This computationally efficient adaptive dynamical core could serve as the foundation on which to build a complete climate or weather model.
We describe the remapped particle-mesh method, a new mass-conserving method for solving the density equation which is suitable for combining with semi-Lagrangian methods for compressible flow applied to numerical weather prediction. In addition to the conservation property, the remapped particle-mesh method is computationally efficient and at least as accurate as current semi-Lagrangian methods based on cubic interpolation. We provide results of tests of the method in the plane, results from incorporating the advection method into a semi-Lagrangian method for the rotating shallow-water equations in planar geometry, and results from extending the method to the surface of a sphere.
A multifactor parameterization is described to permit the efficient calculation of collision efficiency (E) between electrically charged aerosol particles and neutral cloud droplets in numerical cloud and climate models. The four parameter representation summarizes the results obtained from a detailed microphysical model of collision efficiency, which accounts for the different forces acting on the aerosol in the path of falling cloud droplets. The parameterizations range of validity is for aerosol particle radius 0.4 to 10 micron, aerosol particle density 1 to 2.0 g.cm-3, aerosol particle charge from neutral to 100 elementary charges and drop radii 18.55-142 micron. It yields collision efficiencies well within an order of magnitude of the detailed models values, from a data set of of 3978 E values. 95 percent of these values have modeled to parameterized ratios between 0.5 and 1.5 for aerosol particle size range 0.4 to 2 micron and about 96 percent in the second size range. This parameterization speeds up the collision efficiency calculation by a factor of about 103, as compared with the original microphysical model, permitting the inclusion of electric charge effects in numerical cloud and climate models. In the following pages parameterization code in C language is provided for readymade use.
A formula is derived for the evaluation of the total volume of aerosol in a column, and hence for the aerosol columnar mass loading, from multispectral extinction data. This formula is exact in the 'anomalous diffraction' approximation, and reasonably accurate for Mie scattering, over a fairly wide range of refractive indices typical of real aerosols.
We introduce a novel large-eddy model for cirrus clouds with explicit aerosol and ice microphysics, and validate its central components. A combined Eulerian/Lagrangian approach is used to simulate the formation and evolution of cirrus. While gas and size-resolved aerosol phases are treated over a fixed Eulerian grid similar to the dynamical and thermodynamical variables, the ice phase is treated by tracking a large number of simulation ice particles. The macroscopic properties of the ice phase are deduced from statistically analysing large samples of simulation ice particle properties. The new model system covers non-equilibrium growth of liquid supercooled aerosol particles, their homogeneous freezing, heterogeneous ice nucleation in the deposition or immersion mode, growth of ice crystals by deposition of water vapour, sublimation of ice crystals and their gravitational sedimentation, aggregation between ice crystals due to differential sedimentation, the effect of turbulent dispersion on ice particle trajectories, diabatic latent and radiative heating or cooling, and radiative heating or cooling of ice crystals. This suite of explicitly resolved physical processes enables the detailed simulation and analysis of the dynamical–microphysical–radiative feedbacks characteristic of cirrus.
A coupled model, which combines the Biosphere-Atmosphere Transfer Scheme (BATS) with an advanced atmospheric boundary-layer model, was used to validate hypothetical aggregation rules for BATS-specific surface cover parameters. The model was initialized and tested with observations from the Anglo-Brazilian Amazonian Climate Observational Study and used to simulate surface fluxes for rain forest and pasture mixes at a site near Manaus in Brazil. The aggregation rules are shown to estimate parameters which give area-average surface fluxes similar to those calculated with explicit representation of forest and pasture patches for a range of meteorological and surface conditions relevant to this site, but the agreement deteriorates somewhat when there are large patch-to-patch differences in soil moisture. The aggregation rules, validated as above, were then applied to remotely sensed 1 km land cover data set to obtain grid-average values of BATS vegetation parameters for 2.8 deg x 2.8 deg and 1 deg x 1 deg grids within the conterminous United States. There are significant differences in key vegetation parameters (aerodynamic roughness length, albedo, leaf area index, and stomatal resistance) when aggregate parameters are compared to parameters for the single, dominant cover within the grid. However, the surface energy fluxes calculated by stand-alone BATS with the 2-year forcing, data from the International Satellite Land Surface Climatology Project (ISLSCP) CDROM were reasonably similar using aggregate-vegetation parameters and dominant-cover parameters, but there were some significant differences, particularly in the western USA.
The specific thermal enthalpy of a moist-air parcel is defined analytically following a method in which specific moist entropy is derived from the Third Law of thermodynamics. Specific thermal enthalpy is computed by integrating specific heat content with respect to absolute temperature and including the impacts of various latent heats (i.e., solid condensation, sublimation, melting, and evaporation). It is assumed that thermal enthalpies can be set to zero at $0$ K for the solid form of the main chemically inactive components of the atmosphere (solid-$\alpha$ oxygen and nitrogen, hexagonal ice). The moist thermal enthalpy is compared to already existing formulations of moist static energy (MSE). It is shown that the differences between thermal enthalpy and the thermal part of MSE may be quite large. This prevents the use of MSE to evaluate the enthalpy budget of a moist atmosphere accurately, a situation that is particularly true when dry-air and cloud parcels mix because of entrainment/detrainment processes along the edges of cloud. Other differences are observed when MSE or moist-air thermal enthalpy is plotted on a psychrometric diagram or when vertical profiles of surface deficit are plotted.
Statistics of the saturation mixing ratio of H2 O at the temperature minimum and at 100 mb are extracted from about 16 000 radiosonde ascents between latitudes 20°N and 20°N on 140 days, sampling the year December 1978 through November 1979 (FGGE). ‘Window’ radiance temperatures measured by satellite (TIROS and Japanese GMS) between 15°N and 15°N latitude 85° to 185° longitude, at the highest available resolution (about 5 km × 5 km) on one day (Julian day 28 1979), are analysed to extract frequencies of occurrence of temperatures in ranges below −70°C. (Radiosondes and cloud imagery on this day are representative of conditions in these latitudes in the southern summer.) Low radiance temperatures are rare, the frequency of occurrence in the range −86°C to −90°C is 2.5×10−4, whereas about 1 in 4 of the radiosonde ascents in the area at this time have temperatures below −86°C. It is estimated that the cloud turrets with these low radiance temperatures could provide a sufficient source of the driest stratospheric air, but the temperature structure required, by Danielsen's hypothesis, for subsequent dehydration is not observed by radiosonde. All published in situ observations of H2 O mixing ratio in the tropical and northern temperate stratosphere are examined in conjunction with the zonal means observed by the LIMS satellite in December 1978 and May 1979. Constraints imposed by the observations on descriptions of air motion within the stratosphere are discussed.
Area as , but showing the 250 hPa wind speed (dotted contours every 2.5 m s−1 above 37.5 m s−1), and the height of the 1.5 PVU level, computed with the usual Ertel's version PV (θ) (shaded areas from 5000 to 8000 every 1000 m, then every 500 m up to 9500 m).
Cross-section as in Figures 5(a), comparing Ertel's formulation PV (θ) (plotted every 0.25 PVU), PV θ and PV v (plotted every 0.25 PVUS).
Cross-sections as in Figure 5(a), comparing Ertel's formulation P V (θ) (plotted every 0.25 PVU), P V θ and P V v (plotted every 0.25 PVUS). 
A new potential vorticity is derived by using a specific entropy formulation expressed in terms of a moist-air entropy potential temperature. The new formulation is compared with Ertel's version and with others based on virtual and equivalent potential temperatures. The new potential vorticity is subject to conservative properties ensured by the Second Law applied to the moist-air material derivatives. It is shown that the upper tropospheric and stratospheric (dry) structures are nearly the same as those obtained with Ertel's component. Moreover, new structures are observed in the low troposphere, with negative values associated with moist frontal regions. The negative values are observed in the frontal regions where slantwise convection instabilities may take place, but they are smaller than those observed with the equivalent potential vorticity. The main purpose of the article is to diagnose the behaviour of the new potential vorticity from numerical output generated by the ARPEGE NWP model, with the help of isobaric charts and vertical cross-sections. Two inversion methods are suggested. The first method could be based on the invertibility principle verified by the virtual potential vorticity, with a possibility to control and modify separately potential vorticity components in the (dry) upper and (moist) lower atmospheric levels. The other method may consist of an inversion process directly applied to the new moist-air entropy potential vorticity, because the negative values and the solenoidal term are smaller than those observed with equivalent potential vorticity, as shown by numerical evaluations.
Warm conveyor belts (WCBs) are key flow structures associated with extratropical cyclones. They transport moist air from the cyclone’s warm sector poleward and upward close to the tropopause level, leading to the formation of elongated cloud bands, intense latentheating and surface precipitation. In this study a comprehensive dataset of airborne lidar observations of moisture andwind fromdifferent campaigns has been investigated with a trajectory-based approach to identify ‘lucky encounters’ with WCBs. On 19 July 2007, an upstream flight over the Iberian Peninsula during the European THORPEX Regional Campaign (ETReC 2007) in Central Europe intersected two WCBs: one in the upper tropospheric outflow region about 3 days after starting the ascent, and the other one in the boundary layer inflow region over Spain justprior to the strong ascent.Comparisonof the lidarhumiditymeasurements with analysis fields from the European Centre forMedium-RangeWeather Forecasts (ECMWF) reveals significant positive deviations, equivalent to an overestimation of the modelled humidity, in this low-tropospheric WCB inflow region (of about 1 g kg−1 (14%) on average and with peak deviations up to 7 g kg−1). It is noteworthy that this substantial bias occurs in a potentially dynamically highly relevant airmass that will be subsequently lifted within aWCBto the upper troposphere.ALagrangian moisture source diagnostic reveals that these largemoisture deviations occur within airmasses that, according to the ECMWF analyses, are coherently transported from the western Mediterranean towards Spain and experience intense moisture uptake over the Ebro valley. It is suggested that inaccuracies in surface evapotranspiration, horizontal moisture advection, and turbulent vertical transport of moisture in the atmospheric boundary layer potentially contribute to the erroneous lowtropospheric humidity in the inflow region of this particular summertime WCB over Spain in theECMWFanalyses.
A set of about 2500 Doppler wind lidar (DWL) profiles was measured by the DLR Falcon aircraft during the life cycle of Typhoon Sinlaku in the western North Pacific as part of the THORPEX Pacific Asian Regional Campaign (T-PARC) 2008. These DWL profiles were assimilated in the global models of the European Centre for Medium-Range Weather Forecasts (ECMWF) and the Naval Research Laboratory (NRL). The beneficial impact of DWL observations is demonstrated with data denial experiments of both models and differences of the observation impact on analyses and forecasts are analysed. Additionally, the impact is quantified using the adjoint observation impact calculation. These calculations confirm the beneficial impact of DWL observations. The total relative contribution of DWL observations is about twice as high in the NRL system compared to the ECMWF system, which may be due to the lower number of satellite observations assimilated in the NRL system. In the NRL system, the DWL impact per observation is higher than that of other wind observations, whereas in the ECMWF system the DWL impact per observations is similar to other aircraft observations and lower than that of radiosondes. The results confirm preceding studies assimilating airborne DWL observations in numerical weather prediction models and underline the high expectations for future space-borne DWL instruments.
The mesoscale structure of a mature polar low was studied on the basis of high-resolution airborne measurements and numerical modelling. A polar low was measured by light detection and ranging (lidar) and dropsonde observations over the Norwegian Sea on 3 and 4 March 2008. Lidar observations provided cross-sections of water-vapour mixing ratio, backscatter ratio and horizontal wind speed around the polar low and through its centre. Mesoscale structures, such as shallow convection in a cold-air outbreak, a dry intrusion in the eye-like centre of the cyclone and deep convection surrounding it could be identified. Numerical simulations were performed with the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecast System (IFS) and a high-resolution, polar version of the Weather Research and Forecasting (WRF) model. WRF simulations reproduced these structures and showed that the polar low had a warm, upper-level core with descending motions. The eye-like centre had a diameter of about 100–150 km and was characterized by rather stable stratification, horizontally constant potential temperatures and calm winds. Beyond the centre, wind speeds increased rapidly. The observed radial wind and temperature profiles support previous idealized simulations. Several WRF sensitivity tests showed the influence of the initialization time and sensible and latent heat fluxes from the surface on the simulated polar-low development. The polar-low simulations were more accurate in runs starting at the mature stage. Heat fluxes from the surface were important for the polar-low energetics, especially at the final stages.
The Convective and Orographically-driven Precipitation Study (COPS) carried out in summer 2007 over northeastern France and southwestern Germany provided a fairly comprehensive description of the low-troposphere water-vapour field, thanks in particular to the deployment of two airborne differential absorption lidar systems. These lidar observations were assimilated using the 3D-Var assimilation system of the Application of Research to Operations at MEsoscale (AROME) numerical weather prediction mesoscalemodel. The assimilation was carried out for the period 4 July�3 August by running a three-hour forward intermittent assimilation cycle. First, the impact of the lidar observations was assessed by comparing the analyses with a set of more than 200 independent soundings. The lidar observations were found to have a positive impact on the analyses by reducing the dry bias in the first 500 m above ground level and by diminishing the root-mean-square error by roughly 15% in the first km. Then the impact of the lidar observations was assessed by comparing the precipitation forecasts (obtained with and without the lidar observations for the period 15 July�2 August) with the gridded precipitation observations provided by the Vienna Enhanced Resolution Analysis. In general, the impact was found to be positive but not significant for the 24 h precipitation and positive and significant for the 6 h precipitation, with an improvement lasting up to 24 h. Some selected case studies show that the improvement was obtained through a better depiction of convection initiation or through a more accurate positioning of the precipitation systems.
This study is based on the NCAR King Air aircraft and radiosonde observations on 31 October 1986 during the FIRE in Wisconsin over Oshkosh. Aircraft step-up and spiral descent flights are used to obtain kinematic and thermodynamic data. In the step-up maneuver, six different penetrations were made between 1528 and 1616 UTC. Each penetration was about 30 km long separated in the vertical by about 300 m. The time difference between the two spiral soundings was about 43 min. The aircraft descended at a rate of 1.5 m/s during these spiral soundings. Kinematic, cloud physical, and radiometric observations from various instruments are used to estimate the different terms in the moisture- and heat-budget equations. The results show that the advection terms, estimated using the mean longitudinal wind and vertical velocities, and radiative fluxes are important in forming budgets for the cirrus layers. Ice-crystal growth is significant in the upper layers. The maintenance of cirrus can be attributed to relatively warm and moist air advection, radiative cooling at upper levels, and moisture advection in the vertical. Turbulent heat and moisture fluxes are found to be significant in the low levels of cirrus.
Measurements of the thermal radiation emitted by natural water surfaces at a wavelength of 1·55 cm have been measured from aircraft at up to 12 km altitude. The dependence of the radiation on surface roughness is studied in view of its possible importance to satellite observation of sea state.
Autumnal streamer-type elongated troughs are related to heavy precipitation observed at the southside of the European Alps. In order to study the development of this configuration, the typical structural evolution of the ambient flow in the event of heavy rain is investigated by lagged analyses. The precipitation amounts, both observations at the Alpine southside and from ERA40 forecasts by the ECMWF, are chosen as parameters to be correlated with atmospheric fields provided by the ERA40 data. Indeed, the resulting statistics indicate that the preferable position of elongated streamers is quasi-north–south aligned and extending southward over the Mediterranean Sea with its southern end located between the Pyrenees and northern Africa. A significant orographic impact on the streamer is revealed in the regression fields of the upper-level potential vorticity showing an indentation above the western Alps. The lower-tropospheric regression fields show a zonally elongated pressure signal due to the orographic influence. Up to two days in advance of heavy rain events a regressed meridional humidity flux occurs towards the south-western Alps, which at the event’s peak time is found impinging upon the entire Alpine ridge. The regressed vertical velocity fields exhibit two maxima south of the Alps, a strong one at the western and a weaker one over the eastern Alps.
Cooling by melting precipitation in Alpine valleys is investigated. In this case, the latent heat required for melting the falling snow is continuously removed from the valley air until the snowline reaches the valley bottom. Numerical simulations with a highly idealized two-dimensional model indicate an even smaller volume effect, which might be partly due to numerical inaccuracies. We also examined the impact of an air-mass exchange between the upper part of the valley atmosphere and the environment. Our results also suggest that resolving the turbulent eddies that form below the melting layer is important for realizing the volume effect, which implies that mesoscale numerical models with parametrized turbulence will systematically underestimate the cooling effect in Alpine valleys.
The development and conduct of the Mesoscale Alpine Programme (MAP) is summarized – a 10-year research initiative shared by meteorological services, research laboratories and university institutes from 14 countries. The overall investment is estimated to have exceeded 35 million Euros. Inventories are given of scientific results as published in some 220 peer-reviewed publications and over 40 completed PhD theses spanning a full decade. The close linkage between field measurements and various types of simulation experiments is emphasized by considering the role of natural laboratories (observing the atmosphere) and numerical laboratories (undertaking simulation experiments). Various forms of cooperation between laboratories and different domains such as nations, institutions and generations are used as the structuring elements of the survey. A comparison with surveys from other large field programmes helps to put the overall achievements of MAP into perspective.
One focus of the Mesoscale Alpine Programme (MAP) was the study of upper-tropospheric potential vorticity (PV) anomalies that take the form of narrow meridionally elongated troughs termed ‘PV streamers’. A systematic effort was undertaken within the MAP framework to: establish a streamer climatology, develop appropriate instrumentation and undertake a measurement programme to better ascertain their structure, study both their intrinsic dynamics and their modification by the Alps, and perform model experiments to examine their significance for numerical weather prediction. Here an overview is given of the progress made toward achieving these objectives. In particular it is shown that PV streamers translating toward the Alpine region are dynamically distinctive and constitute an identifiable precursor of and contribute seminally to heavy precipitation events on the south side of the Alps and possess rich mesoscale sub-structures that can be examined with novel water vapour absorption lidar instrumentation. It is also shown that the accurate representation of a streamer might well be a critical prerequisite for accurate quantitative prediction of mesoscale precipitation. Copyright  2007 Royal Meteorological Society
Precipitation recycling is the contribution of evaporation within a region to precipitation in that same region. The recycling rate is a diagnostic measure of the potential for interactions between land surface hydrology and regional climate. In this paper we present a model for describing the seasonal and spatial variability of the recycling process. The precipitation recycling ratio, rho, is the basic variable in describing the recycling process. Rho is the fraction of precipitation at a certain location and time which is contributed by evaporation within the region under study. The recycling model is applied in studyiing the hydrologic cycle in the Amazon basin. It is estimated that about 25% of all the rain that falls in the Amazon basin is contributed by evaporation within the basin. This estimate is based on analysis of a data set supplied by the European Centre for Medium-range Weather Forecasts (ECMWF). The same analysis is repeated using a different data set from the Geophysical Fluid Dynamics Laboratory (GFDL). Based on this data set, the recycling ratio is estimated to be 35%. The seasonal variability of the recycling ratio is small compared with the yearly average. The new estimates of the recycling ratio are compared with results of previous studies, and the differences are explained.
The observed kinetic energy balance is calculated over North America and compared with that computed from forecast fields for the 13-15 January 1979 cyclone. the FGGE upper-air rawinsonde network serves as the observational database while the forecast energetics are derived from a numerical integration with the GLAS fourth-order general circulation model initialized at 00 GMT 13 January. Maps of the observed and predicted kinetic energy and eddy conversion are in good qualitative agreement, although the model eddy conversion tends to be 2 to 3 times stronger than the observed values. Both the forecast and observations exhibit the lower and upper tropospheric maxima in vertical profiles of kinetic energy generation and dissipation typically found in cyclonic disturbances. an interesting time lag is noted in the observational analysis with the maximum observed kinetic energy occurring 12 h later than the maximum eddy conversion over the same region.
An analytic formula is derived which describes approximately the scattering of electromagnetic waves in a haze. This formula is compared, and found to agree well, with the exact results from Mie theory that are available in the literature. The formula is used to arrive at a simple, nonredundant, but complete parametrization of the aerosol model.
Contributions to a dynamic climatology of the equatorial components Mx and My of global angular momentum are presented on the basis of ECMWF re-analysis data. The covariance functions of Mx, My and the related torques are evaluated using the observations. It is shown that the torques due to the equatorial bulge, and the torques due to the gradient of the geopotential, balance. Prognostic equations for Mx and My are derived on the basis of this result as are the equations for the covariance functions. All terms in these equations are discussed on the basis of the data. The autocovariance functions of both equatorial components decay rapidly for lags less than 5 days, and rather slowly for larger lags. The cross-covariance function of Mx and My implies westward motion of the angular-momentum vector. Atmospheric tides leave a clear imprint on the wind terms.
For the purpose of total ozone measurement, the atmosphere's particulate scattering spectrum in the 305 to 340 nm interval is approximated by a parabolic function. The inclusion of this function in the usual total ozone equations gives rise to expressions which allow the experimental determination of the “true” total ozone as well as the otherwise unknown expansion coefficients of the function.The theory can be both illustrated and tested by the graphical methods which are described. The parabolic model is found to be very well supported by an experimental data sample. Of note is the finding that this sample's double wavelength-pair total ozone values differ from the true total ozone value by as much as 0.024 atm-cm. Some implications of the theory are discussed.
A partial survey of recent theoretical work on the flow over obstacles is presented. A model for linear, stationary mountain waves with arbitrary basic flow and two-dimensional topography is described in detail. It contains the option of a nonlinear lower boundary condition. The wave drag is computed by three methods and the complete flow field is obtained. The sensitivity of the solutions to small changes of the upstream flow is demonstrated as a critical factor limiting the applicability of the model. The strong downslope wind storms of the Boulder area are briefly discussed in the general context of foehn phenomena. Computations indicate that resonance lee waves do not seem to be part of the mechanism of these winds.
The vertical structure equation is a singular Sturm-Liouville problem whose eigenfunctions describe the vertical dependence of the normal modes of the primitive equations linearized about a given thermal profile. The eigenvalues give the equivalent depths of the modes. The spectrum of the vertical structure equation and the appropriateness of various upper boundary conditions, both for arbitrary thermal profiles were studied. The results depend critically upon whether or not the thermal profile is such that the basic state atmosphere is bounded. In the case of a bounded atmosphere it is shown that the spectrum is always totally discrete, regardless of details of the thermal profile. For the barotropic equivalent depth, which corresponds to the lowest eigen value, upper and lower bounds which depend only on the surface temperature and the atmosphere height were obtained. All eigenfunctions are bounded, but always have unbounded first derivatives. It was proved that the commonly invoked upper boundary condition that vertical velocity must vanish as pressure tends to zero, as well as a number of alternative conditions, is well posed. It was concluded that the vertical structure equation always has a totally discrete spectrum under the assumptions implicit in the primitive equations.
Infrared radiative properties of Arctic stratus clouds in the 10.1-12.7 micron wavelength region are determined from a series of aircraft measurements. The average emissivity of the clouds when scattering is neglected is unity for cloud depths greater than 350 m. Under the assumption that the cloud layers are homogeneous the cloud droplet volume absorption coefficient is estimated as 17 + or - 5 per km. Several mass absorption coefficients are estimated for several assumed liquid water distributions. It is also concluded that the cloud reflectance is not larger than 2 (+ or - 5%).
This study presents a comprehensive analysis of the variability of water vapour in a growing convective boundary-layer (CBL) over land, highlighting the complex links between advection, convective activity and moisture heterogeneity in the boundary layer. A Large-eddy Simulation (LES) is designed, based on observations, and validated, using an independent data-set collected during the International H2O Project (IHOP_2002) field-experiment. Ample information about the moisture distribution in space and time, as well as other important CBL parameters are acquired by mesonet stations, balloon soundings, instruments on-board two aircraft and the DLR airborne water-vapour differential-absorption lidar. Because it can deliver two-dimensional cross-sections at high spatial resolution (140 m horizontal, 200 m vertical), the airborne lidar offers valuable insights of small-scale moisture-variability throughout the CBL. The LES is able to reproduce the development of the CBL in the morning and early afternoon, as assessed by comparisons of simulated mean profiles of key meteorological variables with sounding data. Simulated profiles of the variance of water-vapour mixing-ratio were found to be in good agreement with the lidar-derived counterparts. Finally, probability-density functions of potential temperature, vertical velocity and water-vapour mixing-ratio calculated from the LES show great consistency with those derived from aircraft in situ measurements in the middle of the CBL. Downdraughts entrained from above the CBL are governing the scale of moisture variability. Characteristic length-scales are found to be larger for water-vapour mixing-ratio than for temperature
Data assimilation refers to the problem of finding trajectories of a prescribed dynamical model in such a way that the output of the model (usually some function of the model states) follows a given time series of observations. Typically though, these two requirements cannot both be met at the same time--tracking the observations is not possible without the trajectory deviating from the proposed model equations, while adherence to the model requires deviations from the observations. Thus, data assimilation faces a trade-off. In this contribution, the sensitivity of the data assimilation with respect to perturbations in the observations is identified as the parameter which controls the trade-off. A relation between the sensitivity and the out-of-sample error is established which allows to calculate the latter under operational conditions. A minimum out-of-sample error is proposed as a criterion to set an appropriate sensitivity and to settle the discussed trade-off. Two approaches to data assimilation are considered, namely variational data assimilation and Newtonian nudging, aka synchronisation. Numerical examples demonstrate the feasibility of the approach.
A unique airborne differential absorption lidar (DIAL) for water vapour observations was developed at the Deutsches Zentrum für Luft- und Raumfahrt (DLR). Installed on board the DLR Falcon 20 aircraft, the system measured a dataset of about 3900 water vapour profiles during the T-PARC field campaign. These highresolution humidity observations were assimilated into the European Centre for Medium-Range Weather Forecasts (ECMWF) global model using a version of the operational four-dimensional variational data assimilation system. The assimilation system is able to extract the information for DIAL observations, and verification with independent dropsonde observations shows a reduction in the analysis error when DIAL water vapour observations are assimilated. The forecast influence of the humidity observations is found to be small in most cases, but the observations are able to affect the forecast considerably under certain conditions. Systematic errors are investigated by comparison between humidity model fields, DIAL and dropsonde observations. Overall, DIAL observations are roughly 7–10%drier thanmodel fields throughout the troposphere. Comparison with dropsonde observations suggests that the DIAL observations are too dry in the lower troposphere but not above it.
Variational data assimilation in continuous time is revisited. The central techniques applied in this paper are in part adopted from the theory of optimal nonlinear control. Alternatively, the investigated approach can be considered as a continuous time generalisation of what is known as weakly constrained four dimensional variational assimilation (WC--4DVAR) in the geosciences. The technique allows to assimilate trajectories in the case of partial observations and in the presence of model error. Several mathematical aspects of the approach are studied. Computationally, it amounts to solving a two point boundary value problem. For imperfect models, the trade off between small dynamical error (i.e. the trajectory obeys the model dynamics) and small observational error (i.e. the trajectory closely follows the observations) is investigated. For (nearly) perfect models, this trade off turns out to be (nearly) trivial in some sense, yet allowing for some dynamical error is shown to have positive effects even in this situation. The presented formalism is dynamical in character; no assumptions need to be made about the presence (or absence) of dynamical or observational noise, let alone about their statistics. Comment: 28 Pages, 12 Figures
Data assimilation schemes are confronted with the presence of model errors arising from the imperfect description of atmospheric dynamics. These errors are usually modeled on the basis of simple assumptions such as bias, white noise, first order Markov process. In the present work, a formulation of the sequential extended Kalman filter is proposed, based on recent findings on the universal deterministic behavior of model errors in deep contrast with previous approaches (Nicolis, 2004). This new scheme is applied in the context of a spatially distributed system proposed by Lorenz (1996). It is found that (i) for short times, the estimation error is accurately approximated by an evolution law in which the variance of the model error (assumed to be a deterministic process) evolves according to a quadratic law, in agreement with the theory. Moreover, the correlation with the initial condition error appears to play a secondary role in the short time dynamics of the estimation error covariance. (ii) The deterministic description of the model error evolution, incorporated into the classical extended Kalman filter equations, reveals that substantial improvements of the filter accuracy can be gained as compared with the classical white noise assumption. The universal, short time, quadratic law for the evolution of the model error covariance matrix seems very promising for modeling estimation error dynamics in sequential data assimilation.
Top-cited authors
Anton Beljaars
  • European Center For Medium Range Weather Forecasts
Erik Andersson
  • European Center For Medium Range Weather Forecasts
Jean-François Mahfouf
  • Centre National de Recherches Météorologiques
Magdalena Alonso Balmaseda
  • European Center For Medium Range Weather Forecasts
Sai Sanmith Uppala
  • Swinburne University of Technology