bluuebunny/crossref_metadata_2025_split huggingface dataset:
The role of vertically varying cloud fraction in the parametrization of microphysical processes in the ECMWF model
Authors: Jakob Christian , Klein Stephen A. 
Published: February 2007

AbstractGeneral‐circulation models (GCMs) have generally treated solely the radiative impacts of vertically varying cloud fraction by using a cloud‐overlap assumption. In this study, the microphysical impacts of vertically varying cloud fraction are addressed by developing a subgrid‐scale precipitation model which resolves the vertical variation of cloud fraction. This subgrid model subdivides the grid boxes into homogeneous columns which are either clear or completely cloudy. By comparing the column‐averaged microphysical quantities from the subgrid‐scale precipitation model with the parametrization in the European Centre for Medium‐Range Forecasts (ECMWF) model, the ability of the ECMWF model to account for the subgrid nature of cloud and precipitation microphysics is assessed. It is found that the ECMWF model overestimates the evaporation of precipitation in the tropical mid‐troposphere. This results from (a) an incorrect parametrization of the area of the grid box covered by precipitation, and (b) the inadequacy of assuming a single value for the precipitation rate in the grid box.In addition to assessing the ability of the ECMWF model to parametrize the subgrid nature of cloud microphysics, the subgrid precipitation model is used to show that the cloud‐overlap assumption has a large impact on the evaporation of precipitation. In light of the current trend towards more sophisticated cloud and precipitation parametrizations in GCMs, more attention should be paid to the impact of vertical cloud‐fraction variations on the parametrized microphysics.
The generation of turbulence and entrainment by buoyancy reversal
Authors: Lock A. P. , Macvean M. K. 
Published: February 2007

AbstractIn the stratocumulus‐capped atmospheric boundary layer, the evaporation of cloud droplets in mixtures of cloudy and entrained air can, under certain circumstances, reduce their buoyancy to less than that of the surrounding unmixed cloudy air. With a negative buoyancy excess, these parcels will sink away from cloud‐top and therefore generate turbulence within the boundary layer. Dimensional arguments are used to formulate a scaling for the generation of turbulence by this mechanism, referred to as buoyancy reversal, which is found to agree remarkably well with results from idealized high‐resolution three‐dimensional large‐eddy simulations (LES). Furthermore, simple arguments relating the turbulence generation by buoyancy reversal to entrainment at cloud‐top give a simple parametrization for the entrainment rate which is also found to agree very well with the LES.
Fast and slow Kelvin waves in the Madden‐Julian oscillation of a GCM
Authors: Matthews Adrian J. , Slingo Julia M. , Hoskins Brian J. , Inness Peter M. 
Published: February 2007

AbstractThe structure of the Madden‐Julian Oscillation (MJO) in an 1800‐day integration of the Hadley Centre Unified Model was analysed, and interpreted within a Kelvin‐wave framework. the model was forced with constant equinoctial (March) boundary conditions so that a ‘clean’ MJO signal could be separated from the effects of the seasonal cycle and forced interannual variability. the simulated MJO was fairly realistic in terms of its large‐scale spatial structure and propagation characteristics, although its period of 30 days (corresponding to an average phase speed of 15 m s−1) was shorter than that observed. the signal in deep convection was less coherent than in observations, and appeared to move eastwards as a sequence of discrete convective anomalies, rather than by a smooth eastward propagation. Both ‘fast’ and ‘slow’ equatorial Kelvin waves appeared to play an important role in the eastward propagation of the simulated MJO. Enhanced convection over the Indian Ocean was associated with a ‘fast’ equatorial Kelvin wave that propagated eastwards at 55 m s−1 over the Pacific. On reaching the west coast of South America, a component of this Kelvin wave propagated northwards and southwards as a trapped wave along the mountain ranges of Central America and the Andes, in agreement with observations. the anomalous surface easterlies over the tropical eastern Pacific associated with this fast Kelvin wave enhanced the climatological mean easterlies and led to positive convective anomalies over the eastern Pacific consistent with the wind‐induced surface heat exchange (WISHE) mechanism. However, WISHE was not able to account for the eastward development of the convective anomalies over the Indian Ocean/western Pacific region. By splitting the equatorial divergence anomalies of the simulated MJO into their ϱu/ϱx and ϱv/ϱy components, the role of Kelvin‐wave dynamics in the ‘slow’ (15 m s−1) average eastward propagation of the simulated MJO was examined. Although the two components were of comparable magnitude, the ϱu/ϱx component exhibited a pronounced eastward propagation which tended to be disrupted by the ϱv/ϱy component, thus supporting the paradigm of an underlying, but strongly modified, Kelvin‐wave mechanism.
A microphysically based precipitation scheme for the UK meteorological office unified model
Authors: Wilson Damian R. , Ballard Susan P. 
Published: February 2007

AbstractMany numerical weather prediction models are moving towards more prognostic schemes for the prediction of ice and liquid water contents within clouds. This paper describes a large‐scale cloud and precipitation scheme developed for the UK Meteorological Office's Unified Model. It uses physically based transfer equations to predict ice as a prognostic variable. We review similar schemes and then describe our new scheme, giving examples of its performance in mesoscale forecasts compared with the current operational scheme and with observations. the microphysical processes occurring in a frontal cloud are well modelled. the prediction of supercooled stratocumulus cloud is much improved.
The role of sound waves in sea‐breeze initiation
Authors: Tum A. B. C. , van Delden A. J. 
Published: February 2007

AbstractThe dynamics of the pressure distribution associated with the sea‐breeze circulation is investigated. Analysis of hourly observations of surface pressure reveals that, on a typical sea‐breeze day, large surface‐pressure falls are recorded very far inland. Using a linear numerical model we show that this can only be understood when sound waves are taken into account. Sound waves are excited when air in the boundary layer over land expands due to diabatic heating. the numerical study reveals that vertically travelling sound waves induce a pressure increase through the entire atmosphere above the heated layer within a few minutes, while horizontally travelling sound waves induce a surface‐pressure decrease over land and a surface‐pressure increase over sea, with the resulting horizontal surface‐pressure gradient initiating the sea breeze. Because the signal travels inland at the speed of sound (about 300 m s‐1), points at a distance of more than 1000 km from the coast experience a surface‐pressure decrease within an hour after the initiation of diabatic heating over land. the pressure decrease near the earth's surface over land, as well as the pressure increase aloft, is in accord with the observations at mountain stations and the analyses of the observed daytime surface‐pressure changes in summer over the continent and adjacent seas. the implications of these findings for hydrostatic modelling of sea breezes are investigated. It appears that after one hour the hydrostatic sea breeze is 10% stronger than the non‐hydrostatic sea breeze, while the hydrostatic return current is about 20% weaker.
Feedbacks between sea ice and baroclinic waves using a linear quasi‐geostrophic model
Authors: Tansley C. E. , James I. N. 
Published: February 2007

AbstractBoth the high‐latitude atmospheric circulation and the oceanic sea‐ice cover exhibit significant synopticscale variability. Areas of open water within the ice cover, caused by divergence of the ice, may be generated by convergent, divergent or offshore wind fields. There is a large anomalous upward heat flux over such open water areas, leading to the possibility of complex feedbacks between the ice cover and the baroclinic weather systems that pass over it. the effect of this feedback on the development of baroclinic systems has been investigated in an idealized manner through normal‐mode analysis of a system of quasi‐geostrophic equations. Results indicate that the growth rate, scale, structure and propagation speed of high‐latitude systems may be strongly modified.
Near‐surface fluxes of cloud water evolve vertically
Authors: Kowalski A. S. , Vong R. J. 
Published: February 2007

AbstractOccult deposition to vegetation, via mechanical interception of wind‐blown cloud water, can be a significant fraction of total ionic chemical deposition for some forests. Applying micrometeorological methods to the estimation of cloud water deposition requires particular consideration since cloud droplets are not conservative but are subject to material change (phase change); sedimentation also affects fluxes of droplets. the budget equation for liquid water (LW) in orographic cloud predicts that LW fluxes will diverge due to condensation during mean ascent. For hilltop measurements such as these, other factors can contribute to changes in the vertical flux with height above the surface.Fluxes measured concurrently at two heights exhibit a persistent and significant divergence, while laterally separated measurements are found to agree. A LW budget equation is presented and simplified by scale analysis. Surface uptake is estimated by extrapolation of the measured fluxes. Estimated surface deposition is found to be substantially different from the flux measured at a reference height (10 m), often by a factor of two and occasionally with a different sign. This difference in estimated surface uptake extends to estimates of chemical as well as water deposition. the turbulent flux of LW is shown to be dependent on two criteria for describing ‘steadycloud’ conditions, thus presumably minimizing the effects of entrainment. an often used model relating droplet deposition to the ‘deposition velocity’ for momentum is found to be inappropriate for application in complex terrain. Deposition of LW is estimated to range from 5 to 50 mg m−2 s−1 during the third field campaign of the Cloud and Aerosol CHemistry Experiment (CACHE‐3). with an average of 19 mg m−2 s−1 (nearly 2 mm per in‐cloud day) in late summer at this Pacific coastal site in North America.
Targeted observations in FASTEX: Adjoint‐based targeting procedures and data impact experiments in IOP17 and IOP18
Authors: Langland R. H. , Gelaro R. , Rohaly G. D. , Shapiro M. A. 
Published: February 2007

AbstractThe Fronts and Atlantic Storm‐Track EXperiment (FASTEX) provided an opportunity for testing targeted‐observing procedures in a real‐time framework during January and February 1997. This study describes the use of singular vectors (SVs) for objective targeting during FASTEX, and the evaluation of the impact obtained from targeted dropsonde data, satellite wind data, and other observations on 1‐2 day forecast skill in intensive observation periods (IOPs) 17 and 18.In IOP17, targeted dropsondes improve a 42 h forecast of L41 (Low 41; cyclones were numbered in sequence throughout FASTEX) in terms of sea‐level pressure, but the forecast skill is degraded in the upper troposphere. It is suggested that the degraded forecast may be caused by an incomplete survey of the SV target area, that improved the analysis in one region, but made the analysis less accurate in an adjacent part of the target area where no dropsonde data were provided. In a series of experiments, the best 42 h forecast of L41 is obtained by the addition of a few radiosonde profiles provided specially for FASTEX at off‐times, that provide observational data in the most sensitive part of the SV target area. the analysis differences introduced by the radiosonde profiles are much smaller in magnitude than those from the dropsonde data, but have a larger forecast impact, because they occur in an area that has larger error growth rates in this forecast.In a series of experiments for IOP18, the best 24 h forecast of L44 is obtained using a combination of targeted‐dropsonde data and satellite wind data. Both data types can also be used separately to improve this forecast. the assimilation of satellite wind data and ship‐based soundings in areas of weak initial‐condition sensitivity (‘null’ areas) is shown to have minimal impact on the forecast error. the target areas identified by SVs in these two IOPs occur in strongly baroclinic regions, tending to favour the right‐entrance and left‐exit regions of the upper‐level jet, but with greatest sensitivity near 600 hPa.
Dynamical effects of ice sublimation in a frontal wave
Authors: Clough Sid A. , Lean Humphrey W. , Roberts Nigel M. , Forbes Richard M. 
Published: February 2007

AbstractThe dynamical role of ice sublimation in weather systems is briefly reviewed. Observations are presented from the Fronts and Atlantic Storm‐Track Experiment (FASTEX) Intensive Observation Period 16 that show variations of static stability, humidity and mesoscale circulation corresponding to those associated theoretically with sublimation of ice precipitation. It is thus suggested that the observations display the mechanism proposed by Clough and Franks in which forward mesoscale flows are associated with moist adiabatic descent supported by the sublimation cooling. This mechanism was suggested as an important stage in the evolution of many mesoscale rain bands.A set of three model simulations of the event has been made with versions of The Mel. Office's Unified Model. Of these a mesoscale model integration with 11 km resolution and 45 levels clearly displays the symptoms, and is diagnosed to demonstrate its consistency with the Clough‐Franks mechanism. An integration omitting the cooling due to sublimation differs significantly from the full model experiment in the structure of low‐level wind fields, frontal troughs and mesoscale precipitation distribution. Il is also demonstrated that the static‐stability transition, mesoscale circulation and mid‐tropospheric potential‐vorticity perturbations are substantially weakened in this integration, thus confirming that the Clough‐Franks mechanism is also operating in the numerical weather prediction (NWP) model.We deduce from these studies that ice precipitation and its sublimation has a major role in determining mesoscale circulation and structure in mid‐latitude weather systems, affecting stratification and the formation of features such as fronts and rain bands. These are substantially affected by the fall and evaporation of ice crystals, which are both important to temperature and moisture transports and the behaviour of NWP models on time‐scales of hours to days. In our integrations dynamical feedback due to sublimation cooling coincided with extreme negative potential‐vorticity values and potentially conditional symmetric instability, hence the anticyclonic motion occurring in the cloud head or deep cloud of the moist warm sector as in this case. In moist warm sectors a substantial role for sublimation may be anticipated more generally, particularly for air trajectories receiving most ice precipitation.We suggest that the described phenomenon be referred to as sublimation enhanced descent or SED.It is concluded that in view of tins demonstrated sensitivity substantial attention should be given to refining microphysical parametrizations in NWP models, and that radar and sounding observations from the FASTEX experiment provide a suitable basis for validating these schemes.
On the impact of Doppler radar derived wind fields in a mesoscale non‐hydrostatic model
Authors: Nuret Mathieu , Chong Michel , Lafore Jean‐Philippe , Bousquet Olivier , Gouget Viviane 
Published: February 2007

AbstractFine‐scale three‐dimensional wind fields retrieved from airborne Doppler radar observations within an oceanic tropical mesoscale convective system (MCS), during the Tropical Ocean/Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment, are used to enhance the initial conditions of a n on‐hydrostatic mesoscale model. They are incorporated into the French CANARI‐ALADIN analysis system, based on the optimal interpolation technique. The fine‐mesh (4 km) horizontal wind components are first averaged over subdomains of 20 km × 20 km in order to provide a series soscale wind profiles. Because of the absence of dense sounding data, the synthesized vertical‐velocity profiles are transformed into humidity profiles by considering that updraughts contain nearly saturated or saturated air, while downdraughts are associated with unsaturated air.Comparison of the initial state using conventional data with the Doppler‐enhanced initial state clearly identifies the benefits of the addition of the observed mesoscale circulation features. In terms of the precipitation forecast, the control run that uses the conventional initial State dramatically fails to predict the existence of the MCS precipitation core. On the other hand, the reference run with the data‐enhanced initial state succeeds in forecasting it up to 12 hours in a way that is well consistent with the satellite imagery. In terms of the wind forecast, the mesovortex signature that could be identified from the radar observations becomes a persistent feature with the radar‐enhanced initial conditions. Sensitivity tests reveal the specific roles of wind and humidity data. As in previous studies, the initial state of humidity is fundamental for the forecast of the MCS precipitation by sustaining the convective activity, and also by forming and maintaining a vortex‐like circulation. These results suggest that moist convective processes play a major role. The inclusion of wind data is necessary for improving the system propagation by advective processes.
Use of GPS/MET refraction angles in three‐dimensional variational analysis
Authors: Zou X. , Wamg B. , Liu H. , Aathes R. A. , Matsumura T. , Zhu Y.‐J. 
Published: February 2007

AbstractThe Spectral Statistical Interpolation (SSI) analysis system of the National Centers for Environmental Prediction (NCEP) is modified to include GPS/MET data (meteorological data from the Global Positioning Satellite system) using a GPS ray‐tracing operator. The new system is tested by incorporating 30 actual GPS/MET observations of refraction angles obtained during the GPS/MET experiment. This is the first time that real radio occupation refraction angles and refractivities have been incorporated into a three‐dimensional variational analysis system. We examine the magnitude and the vertical distribution of the analysis adjustments that result from using refraction‐angle observations in the NCEP SSI analysis system. The average magnitudes of the adjustments in the temperature and specific‐humidity fields are approximately 0.4 degC and 0.6 g kg−1, respectively. Individual changes can be as large as 4 degC and 4g kg−1, respectively. The greatest adjustments to the temperature occur in the middle and upper troposphere and stratosphere, while the major changes in specific humidity occur in the lower troposphere.An assessment of the impact of the GPS/MET observations on the analysis, verified by conventional (mostly radiosonde) data, is difficult because of the small number of GPS/MET data used. Nevertheless, it is found that, even over data‐rich regions (regions containing many radiosonde observations), and even when the verification data were the radiosonde data themselves, the use of GPS/MET refraction angles makes a slight improvement, overall, to the analysed temperatures and winds. The impact on the water‐vapour analyses, again as measured against radiosonde data, is mixed, with improvements shown in some layers and degradation in others. Compared with the background field, the use of refraction angles from one occultation results in an analysis whose simulated refraction angles are much closer to the withheld GPS/MET refraction angles at the two nearby occultation locations, and whose temperature and moisture profiles are also closer to those resulting from the direct assimilation of the two withheld occultations.Although the forward model used in this study, with the ray tracing being carried out in a two‐dimensional plane, is much cheaper than a more accurate three‐dimensional forward model, it is still quite expensive. In order to further reduce the computational requirement for the assimilation of GPS/MET data, we test a scheme in which the GPS/MET‐retrieved refractivities (instead of refraction angles) are used above a selected height for each occupation. These heights are determined objectively based on the departures from spherical symmetry of the model field. It is shown that the mixed use of GPS/MET refraction angles and refractivities produces an analysis result similar to the one using refraction angles alone, while the computational cost is reduced by more than 30%.
Cloud‐resolving simulation of convective activity during TOGA‐COARE: Sensitivity to external sources of uncertainties
Authors: Guichard F. , Redelsperger J.‐L. , Lafore J.‐R 
Published: February 2007

AbstractA one‐week convective period of the Coupled Ocean‐Atmosphere Response Experiment (10‐17 December 1992), prior to a westerly wind burst, has been simulated with a cloud‐resolving model. Large‐scale advection derived from observations is used to force the model, in the same way as usually done in single‐column models. Our aim is to evaluate this explicit simulation against observed large‐scale thermodynamic and radiative fields, and to investigate the sensitivity of model results to observational uncertainties. Precipitation, apparent heat source and moisture sink are fairly well reproduced by the model as compared to those diagnosed from observations. Temperature (T) and moisture (qv) fields are also reasonably well captured except for a moderate cold and moist bias. Simulated moist static energy is too high below 6 km and too low above, possibly because convection is slightly less active in the model than observed.In order to investigate the sensitivity of mode results to observational uncertainties, results are analysed with the moist static energy budget together with independent observational radiative datasets. This analysis suggests that the atmospheric radiative rate that is in equilibrium with the applied large‐scale advection and observed surface fluxes is too weak and that its diurnal cycle is not realistic. The most likely reason for this problem is found to be related to uncertainties in the large‐scale advection diagnosed from observations. This analysis also indicates that the simulated high‐cloud cover is too large in the model. It is greatly improved by increasing the ice‐crystal fall speed. Additional tests show a large sensitivity of the simulated moist static energy, and thus T and qv, to the range of uncertainties previously found for large‐scale advection. The vertical structure of the model bias is not significantly modified by changing the intensity of these forcings, but it is most sensitive to their vertical structures.It is argued that it is crucial to get some insights into the range of uncertainties of external forcings (large‐scale advection, surface fluxes and atmospheric radiative‐heating rate) so as to assess the relevance of any evaluation of simulated temperature and moisture when a model, either resolving clouds or parametrizing them, is forced with large‐scale advection deduced from observations.