bluuebunny/crossref_metadata_2025_split huggingface dataset:
Cloud processing of the cloud condensation nucleus spectrum and its climatological consequences
Authors: Bower K. N. , Choularton T. W. 
Published: February 2007

AbstractA modeling study of the effects of aqueous phase cloud chemistry in changing the activated cloud condensation nucleus (CCN) spectrum is presented. The chemistry model is contained within a model of the development of a hill cap cloud. The aqueous phase oxidation of sulphur (IV) to sulphur (VI) by the oxidants ozone and hydrogen peroxide is considered for each of the explicit cloud‐droplet categories. All gases enter cloud droplets at a finite rate which is calculated in the model.Significant modification of the CCN spectrum emerging downwind of the processing cloud is observed. This leads to the formation of strongly bimodal aerosol size distributions. The degree of modification is strongly correlated to the concentration of sulphur dioxide, and in oxidant limited situations tot eh concentration of hydrogen peroxide.The cloud‐droplet chemistry is seen to have the largest effect on the smallest activated CCNs. This enables such particles to activate much more readily after cloud processing at critical supersaturations up to 20 times lower than originally required. A simple cloud model is used to investigate the sensitivity of this secondary activation.With updraughts typical of a stratocumulus‐type cloud, droplet effective radii are seen to be lowered by up to 3 μ, 500 m above cloud base. This result is insensitive to the concentration of sulphur dioxide present in the processing cloud down to very low concentrations. If reproduced on a global scale this result cloud have important climatological consequences, as recently indicated by Slingo (1990). However, the reduction in effective radius is expected to be important only on local scales close to the sources of new aerosol, where the processing of the CCN spectrum by clouds may be occurring for the first time. Additional cloud processing will lead to changes in the direct radiative properties of the aerosols but will have little effect upon the radiative properties of clouds subsequently forming on them.
Nonlinear evolution of ordinary frontal waves induced by low‐level potential vorticity anomalies
Authors: Malardel S. , Joly A. , Courbet F. , Courtier P. H. 
Published: February 2007

AbstractLinear, semi‐geostrophic (SG) theory reveals the instability of steady fronts with low‐level potential vorticity anomalies. Joy and Thorpe (1990) have shown in this context the most unstable normal modes to have sub‐synoptic wavelenghts. The present study uses a primitive equation (PE) model to construct, at these wavelenghts and along the same fronts, the PE normal modes and extends the evolution tot he nonliner regime. IT is shown that PE normal modes have a structure similar to the original SG modes at the same given wavenumber.In the nonlinear experiments, two different kinds of behaviour are found, depending on the initial wavelenght of the perturbation, the frontal baroclinicity and the width of the potential vorticity anomaly. The first kind, and main finding of this study, is characterized by the inability of a barotropically unstable mode (in the energy sense) to lead to large pressure falls in the vortex. Such a mode, with its wavelenght smaller than the Rossby radius, is successful in breaking the frontal flow but saturates within two days. The other occurs when the wavelengh is larger than the Rossby radius. Then, it is shown that the initially significant barotropic contribution to the growth vanishes and the wave enters a phase of classical baroclinic growth. It is only when this second phase occurs that the frontal change in structure is accompanied by significant deepening of the surface low. IT saturates in a way similar to larger‐scale baroclinic waves, by increasing the upper‐level jet and shear.
An observational study on the energetics of the onsent monsoon vortex, 1979
Authors: George L. , Mishra S. K. 
Published: February 2007

AbstractTime variations, of the vertically integrated, and vertical distribution of time‐mean zonal and eddy components of the kinetic and available potential energy, and the associated energy conversions, are examined in order to understand the formation and growth mechanisms associated with the monsoon onset vortex during 14‐17 June 1979. For the computations, the area 0‐22.5°N and 48.75°E‐78.75°E and the vertical layer 1000‐300 hPa are considered. The dynamical characteristics of the large‐scale monsoon current surrounding the onset vortex are also investigated. Strengthening of the meridional and vertical shear of the mean zonal wind is noticed during the period. The vortex is formed to the north of the westerly jet, in the zone of maximum cyclonic shear. The northward shift of the large‐scale monsoon current is noticed in association with northward movement of the onset vortex.It is identified from the mean energetics that both barotropic and baroclinic energy conversions, and also the boundary terms, maintain the vortex. Baroclinic energy transfer to eddy kinetic energy (KE) takes place not only in the upper levels but also in the lower levels. Further, the energy conversion from zonal kinetic energy (KZ) to KE dominates over the energy conversion from eddy available potential energy (AE) to KE. The barotropic energy conversion due to the down‐gradient eddy transport of meridional momentum is more than that due to zonal momentum transport. The transfer of the vertical wave‐energy flux, due to the eddy pressure interaction, away from the middle level also contributes significantly to the maintenance of KE in the upper and lower levels. The generation rate of the zonal and eddy available potential energy and the frictional dissipation of KE are computed b the residue method; the values are 0.54 W m‐2, 0.25 W m−2 and 0.65 W m−2, respectively.The meridional and vertical transports of momentum and heat are examined in relation to the structure of the vortex. The requirement of minimum vertical resolution for the vortex modelling studies is indicated.
Multi‐year statistics of selected variable from the ISCCP C2 data set
Authors: Waare Bryan C. 
Published: February 2007

AbstractCoarse spatial resolution monthly statistics of selected variables of the International Satellite Cloud Climatology Project (ISCCP) C2 data set are analysed. The total cloud amount statistics are compared with those from conventional surface observations available from the Consolidated Ocean‐Atmosphere Data Set (COADS). These statistics include annual means and intra‐annual variances and correlations with the COADS sea surface temperature and surface relative humidity. In general the ISCCP C2 total cloud amount statistics are found to be in very good agreement with those of the COADS. In particular a number of correlation patterns are evident which are explainable in terms of known climatology, and which might be utilized as verification diagnostics for forecast and climate models.Preliminary analyses of the ISCCP C2 cloud‐top pressure and cloud water content have also been carried out. The annual means and intra‐annual standard deviations of these parameters appears to be consistent with surface cloud‐type observations. Furthermore, the intra‐annual variations of cloud‐top pressure and cloud water all have broad regions with significant correlations with sea surface temperature.
Marine aerosol concentrations and estimated fluxes over the sea
Authors: Smith M. H. , Park P. M. , Consterdine I. E. 
Published: February 2007

AbstractAerosol particulate fluxes, generated at the sea surface by the action of the wind, have been derived from a comprehensive series of measurements taken in the Outer Hebrides, off the west coast of Scotland. These observations cover wind speeds from essentially zero to values in excess of 30 m s−1, and show that the particle production, over the radius range from 1 to 25 μm, is very strongly dependent upon the prevailing wind speed, especially for the larger particles. These fluxes, estimated for a height of about 10 m above the ocean surface, may be approximated by an expression comprising the sum of two log‐normal distributions (in particle radius R) of the form:

where the values of fI and R0i remain essentially unchanged with wind speed, but with the values of Ai being strongly dependent upon this parameter.Alternative formulations of the aerosol flux, based upon combining field observations of the dependence of whitecap coverage upon prevailing wind speed with estimates of aerosol production from laboratory‐simulated whitecaps, provide poorer approximations to the aerosol observations, especially at the higher wind speeds when a substantial spume‐drop contribution may be expected.
Evolution of a mesoscale upper tropospheric vorticity maximum and comma cloud from a cloud‐free two‐dimensional potential vorticity anomaly
Authors: Browning K. A. 
Published: February 2007

AbstractA diagnostic case study is presented in which output from the limited‐area forecast version of the Meteorological Office operational Unified Model was used together with water vapour and cloud imagery from the Meteosat satellite. The case chosen for study was an archetypally simple example in which a long, narrow strip of air with high potential vorticity (PV) in the upper troposphere—strictly a tropopause depression—became unstable and rolled up into fairly ling‐lived mesoscale vortices. For much of the time the vortices were cloud‐free and they were dynamically pure examples of very small upper‐air PV anomalies moving relative to the strata beneath. As such they provided a good test of theoretical deductions from the PV invertability principle and of the performance of the dynamical components of the forecast model.The evolution of the mesoscale vortices as revealed in the satellite imagery was well handled by the Unified Model, indicating that the small scales were reproduced well by the model dynamics even though they were not fully, defined by the routine observations fed into the model. As the vortices began to form, the stretching deformation along the axis of the initial PV strip was very small, and the 900 km spacing of the vortices was broadly consistent with theoretical expectations for the wavelength of instabilities growing on such a PV strip. A detailed analysis of one of the mesoscale vortices throughout its 3‐day lifetime showed that its maximum potential vorticity and absolute vorticity decreased with time as it travelled within the circulation of large‐scale anticyclonic gyre. By the end of the second day, when cloud first formed, the vortex had become a weak upper tropospheric shear line, 500 km long, sandwiched between a pair of anticyclonic mesoscale vortices 900 km apart.The Meteosat imagery revealed a characteristic two‐component structure both in the water‐vapour channel and, when the cloud eventually formed, in the infrared channel too. This structure, which resembled that seen on other occasions, is explained in terms of a particular combination of vertical motion and advection in the region of sharp humidity gradients alongside dry stratospheric air that had intruded into the upper troposphere.
Cumulus entrainment, fine‐scale mixing, and buoyancy reversal
Authors: Grabowski Wojciech W. 
Published: February 2007

AbstractThe formation of buoyancy reversal due to entrainment of dry environmental air, and its implication for cumulus dynamics, are discusses. Concepts originating from laboratory experiments with reacting turbulent flows, and from numerical simulations of homogeneous and isotropic turbulence, are applied to distinguish between large‐scale entraining eddies developing at the interface, subsequent development of smaller‐scale motions, and final homogenization by microscale processes. The physics of the microscale homogenization by molecular diffusion and sedimentation of cloud droplets is discussed. It is shown that, as a result of droplet sedimentation, much smaller negative buoyancy (buoyancy undershoots) may be generated on cloud microscale as compared with the value predicted by a classical nonlinear mixing diagram of cloudy and cloud‐free air.Highly idealized numerical experiments aimed at simulating the temporal evolution of a small convective cloud were performed with and without the effects of molecular mixing that results in buoyancy reversal. It is argued that these experiments provide two limiting cases, and that the dynamics of a real cumulus cloud is located somewhere between them. The major effect of buoyancy reversal as suggested by these experiments is to increase the intermittency associated with cloud evolution. The temporal variation of cloud‐top height, maximum undraughts, and minimum downdraughts increases significantly when buoyancy reversal is allowed.It is argued that results of numerical experiments, together with other laboratory and theoretical studies, cast serious doubts on the concept of cumulus entrainment being driven by the cloud‐top entrainment instability.
Comparison between the land surface response of the ECMWF model and the FIFE‐1987 data
Authors: Betts Alan K. , Ball John H. , Beljaars Anton C. M. 
Published: February 2007

AbstractAn 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.
Anomaly in the Alpine diurnal pressure signal: Observations and theory
Authors: Frei Ch. , Davies H. C. 
Published: February 2007

AbstractAn examination is undertaken of the diurnal surface‐pressure pattern across the main east‐west ridge of the European Alps. A case study for clear‐sky, quiescent synoptic conditions reveals a pronounced across‐ridge asymmetry—to the north the primary feature is an approximate 0.5 hPa semi‐diurnal oscillation whilst on the southern side there is an approximate 2.5 hPa diurnal oscillation. A one‐year statistical climatology and a two‐month synoptic climatology confirm the existence, and indicate the prevalence of this asymmetry. The across‐ridge variation in the amplitude of the solar‐diurnal (i.e S1) component is about 1 hPa and it has a lateral scale of around 200 km.From a theoretical standpoint physical arguments and dynamical considerations are advanced to suggest that the S1 anomaly is not readily attributable to purely in situ effects. In addition a heuristic model is deployed to examine the possible nature of the Alpine modification of the planetary diurnall tide. The model, comprising a cylindrical island embedded in ‘reduced‐gravity’ shallow water, reveals a terrain‐induced modification in the form of a mesoscale, terrain‐trapped Kelvin wave that interferes destructively (constructively) with the global S1 signal to the north (south) of the terrain. The pattern's amplitude, phase and spatial structure are consistent with the main features of the observed signal.
Importance of diabatic processes on ageostrophic circulations observed during the FRONTS 87 experiment
Authors: Lagouvardos K. , Lemaitre Y. , Scialom G. 
Published: February 2007

AbstractThe impact of condensation and evaporation on real cross‐frontal circulations is studied, using data from the FRONTS 87 field experiment. The Sawyer‐Eliassen equation is solved numerically in its primitive equation form, for one intensive observing period of FRONTS 87. Three different schemes of parametrization of condensation heating are used. Comparison between diagnosed and observed ageostrophic circulations shows that latent heat release plays an important role in enhancing the ascending branch of the ageostrophic circulation ahead of the frontal zone. In particular, when a parametrization scheme for the low‐level moisture convergence is applied, a cell of strong ascending motion is formed ahead of the frontal zone, in good agreement with the observations. Further, a simple parametrization of evaporative cooling below the frontal discontinuity leads to a reinforcement of downdraughts in the cold sector of the front, as observed in regions where precipitation originating from the strong line convection evaporates. In comparison with their adiabatic counterparts, the current solutions show strong similarities with the observed circulation and indicate the predominance of diabatic effects in the frontal zone.
Nonlinear balance on an equatorial beta plane
Authors: Raymond David J. 
Published: February 2007

AbstractExtension of the nonlinear balance model to include the equator is examined here from a theoretical perspective. It is found that this model is ill‐posed, in that zonally symmetric equatorial Kelvin waves of arbitrary amplitude can occur. Two possible solutions are outlined. The zonally‐averaged zonal momentum equation provides auxiliary information that can be imposed on the flow. This equation can be solved simultaneously with the other parts of the problem. Alternatively, when small, isolated disturbances can be considered to have a negligible effect on the mean flow; one can simply assume that zonal means remain unchanged.
Thermal compression waves. II: Mass adjustment and vertical transfer of total energy
Authors: Nicholls Mel Ville E. , Pielke Roger A. 
Published: February 2007

AbstractA fully compressible model is used to simulate the mass adjustment that occurs in response to a prescribed heat source. Results illustrate the role that thermal compression waves have in this process. The vertical mass transport associated with compression waves decreases rapidly with height. Most of the mass transport occurs in the horizontal, with the vertical structure of the disturbance similar to that of a Lamb wave. The vertical transfer of total energy in a thermally driven mixed layer is also examined. It is shown that the upward transport of total energy is accomplished by a compression effect rather than by the exchange of warm and cold air by buoyant thermals. Model results are analysed to determine budgets of total energy, mass and entropy. It is demonstrated that buoyant thermals are predominantly responsible for a transfer of entropy, rather than total energy. In the light of these results the notion of ‘heat transport’ in a fluid is discussed.
Passage of a frontal zone over a two‐dimensional ridge
Authors: Egger Joseph , Hatt Hans 
Published: February 2007

AbstractThe passage of a frontal zone over a two‐dimensional triangular ridge is investigated using a semigeostrophic model. The model atmosphere consists of three layers of uniform density and potential vorticity. The densest (coldest) layer represents the airmass behind the cold front, the next layer the frontal zone itself, and the least dense (warmest) layer represents the flow ahead of and above the frontal zone. At ground level, the frontal zone is contained between two surface fronts. A prescribed pressure field imposes translation at a speed U, and confluence with a deformation rate α. A one‐layer version of the model has also been considered. The main conclusions are that (a) the mountain‐induced shift of the position of a front does not depend on U or α; it is mainly negative on the windward slope and the upper part of the leeward slope, but is positive in the downstream plain and on the lower part of the leeward slope (there are no effects far‐afield), and (b) frontal collapse must occur for α > 0. The collapse is delayed over most of the mountain, compared to the no‐mountain case, but occurs earlier downstream from the mountain. Prior to the collapse, there is up‐slope frontogenesis when part of the frontal zone is in the plain. Frontolysis prevails if all of the frontal zone is on the up‐slope and if α is not too large. Frontogenesis occurs on the down‐slope and in the plain.
The shapes of snow crystals—Fitness for purpose?
Authors: Mason B. J. 
Published: February 2007

AbstractSnow crystals exhibit six quite sharp changes of habit between 0 and −25°C as between hexagonal plates, columns/needles and stellar dendrites. The questions arise as to why, and do these changes, unique in crystal physics, serve a useful purpose?.Calculations of the growth rates of the various forms, and also of ice spheres, as they fall through atmospheres with specified profiles of temperature and supersaturation, reveal that the transitions from plates to stellar dendrites that grow only between −12 and −16°C, and from plates to columns below −25°C, permit the more effective release of precipitation from layer clouds. The interruption of the plate regime by the growth of columns and needles between −8 and −3°C leads to the formation of larger precipitation elements that better survive evaporation below cloud base than would plate crystals, by virtue of having much higher collection efficiencies for supercooled cloud droplets.
Physical retrieval of liquid water contents in a North Atlantic cyclone using SSM/I data
Authors: Prigent C. , Sand A. , Klapisz C. , Lemaitre Y. 
Published: February 2007

AbstractIn the framework of the Mesoscale Frontal Dynamics Project/FRONTS 87, a cyclone over the North Atlantic Ocean is studied with the help of the Special Sensor Microwave/Imager (SSM/I) on board the satellite of the Defense Meteorological Satellite Program. After a brief recall of the relevant studies on the frontal system of 11 November 1987 in the context of the second intensive observing period of Fronts 87, the SSM/I images are analysed to show the potential of microwave observations for diagnosing the mesoscale structure of the front. Then, considering the poor accuracy of the SSM/I regression‐type algorithms in estimating the integrated cloud‐liquid‐water contents, we propose to retrieve this quantity with the help of a radiative‐transfer model, coupled with a realistic first guess of the atmospheric characteristics in the meteorological area of concern. A physically consistent estimation is deduced from a nonlinear iterative inversion procedure, which aims at reducing the difference between simulated and measured brightness temperatures. The mesoscale structures of the retrieved fields of water vapour, cloud liquid water and rain rate appear consistent with the Fronts 87 meteorological analysis of this situation, and they complement the understanding of the processes involved in the dynamics of the front.