bluuebunny/crossref_metadata_2025_split huggingface dataset:
Large‐eddy simulation: A critical review of the technique
Authors: Mason P. J. 
Published: February 2007

AbstractLarge‐eddy simulation is being increasingly used as a means of both predicting the properties of specific turbulent flows and providing flow details which can be used like data to test and refine other turbulence‐closure models. Although such simulations derive their credibility from the explicit resolution of large‐scale turbulent eddies, they depend upon a small‐scale turbulence closure and must to some degree inherit the many uncertainties associated with turbulence closure. Most results obtained to date have been very encouraging, with examples of good agreement with observations and insensitivity to the small‐scale turbulence closure. However, there are some counter examples of incorrect prediction in circumstances where the small‐scale closure is critical. The evidence from the simulations suggests that this critical behaviour may only occur in the regions of large‐eddy simulation where matches to boundaries, or regions of statically stable fluid, are encountered. In such regions an empirical approach dominates, and there is evidence that the small‐scale turbulence closure must represent stochastic fluctuations of the unresolved stresses. Although there is a need to refine the small‐scale turbulence models, it is clear from the promise of current applications that, with the expected growth in computer power, this technique will have increasing breadth of application. Here the underlying assumptions and current practical implementations of the technique are critically reviewed.
Dispersion and removal of pollutants during the passage of an atmospheric frontal system
Authors: Gimson Neil R. 
Published: February 2007

AbstractResults are presented from a numerical mesoscale modelling study of the effects of meteorological phenomena on the distribution of contaminants, both airborne and deposited onto the surface, arising from hypothetical idealized release episodes. An Eulerian transport/diffusion/removal model is coupled to the Meteorological Office non‐hydrostatic mesoscale model, with removal tuned to represent the wet deposition of pollutants likely to be emitted during a nuclear power station accident.
Diagnostic study of a narrow cold‐frontal rainband and severe winds associated with a stratospheric intrusion
Authors: Browning K. A. , Reynolds R. 
Published: February 2007

AbstractA diagnostic study of a severe wind event associated with a cold front is presented based on output from an operational numerical weather prediction model together with radar and satellite imagery and detailed surface observations. An intrusion of stratospheric air, initially with a potential vorticity of 2 PV units, is shown, as part of a backward‐tilted filament, to have plunged down to the lower troposphere where it was associated with a narrow cold‐frontal rainband at its leading edge and produced severe wind gusts (up to 70 knots). A close correspondence in time and space is revealed between these surface events and the appearance in the model of a pocket of diluted stratospheric air (with PV = 1) at the 2 km level. The stratospheric intrusion produced two distinguishable wind surges: one behind the narrow cold‐frontal rainband and another, mainly dry, about 200 km farther behind. The strength of the winds was attributable to the downward transport of momentum as part of the large‐scale dynamics; local moist processes played only a minor role. At coarse resolution the radar echo from the narrow cold‐frontal rainband resembled line convection but it is shown that there were significant differences between this event and the archetypal form of line convection often seen in the British Isles.
Thermal compression waves. I: Total‐energy transfer
Authors: Nicholls Melville E. , Pielke Roger A. 
Published: February 2007

AbstractTotal energy is considered a fundamental quantity that is transferred poleward by the general circulation of the atmosphere. In this study we investigate the mechanism for the upscale transfer of total energy from a localized heat source, such as a thunderstorm. Using a fully compressible numerical model it is shown that thermal compression waves can effectively transfer total energy at the speed of sound. It is suggested that physical interpretations of the energy cycle should take into consideration this mechanism for transferring total energy.
Predictability experiments with persistence forecasts in a red‐noise atmosphere
Authors: Fraedrich Klaus , Ziehmann‐Schlumbohm Christine 
Published: February 2007

AbstractIndividual and lagged ensemble forecasts of persistence in a red‐noise atmosphere are analysed to obtain information on predictability experiments performed in an imperfect model/ensemble environment. By examining the lead‐time‐dependent error budgets of individual and ensemble forecasts, various measures of predictability are analytically determined: the initial and saturation error, the error growth rates, the limit of predictability, error and squared error distributions depending on initial conditions, and the systematic and non‐systematic error, etc. Furthermore, weather‐regime dependent predictability can be studied directly by using different autocorrelation time‐scales of the red‐noise atmosphere. Finally, ensembles of lagged forecasts are constructed to analyse the relation between the forecast errors of the ensemble mean and the dispersion within the ensemble. Despite the simplicity of this external predictability experiment, the error budgets show features that may be qualitatively compared with those of numerical weather‐prediction and climate‐model systems.
Precipitation recycling in the Amazon basin
Authors: Eltahir E. A. B. , Bras R. L. 
Published: February 2007

AbstractPrecipitation 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, ρ, is the basic variable in describing the recycling process, ρ 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 studying 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. The same analysis is repeated using a different data set from the Geophysical Fluid Dynamics Laboratory. 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 influence of Antarctica on the momentum budget of the southern extratropics
Authors: Juckes M. N. , James I. N. , Blackburn M. 
Published: February 2007

AbstractThe antarctic plateau acts as a strong heat sink for the global climate, cooling the atmosphere and radiating energy to space. A cold dense atmospheric boundary layer is formed. Strong surface winds are formed as the boundary layer drains off the plateau. These drainage winds and the eddy fluxes necessary to maintain them are analysed in a general circulation model (GCM). The drainage flow is well represented in the GCM. The associated mean meridional circulation is analysed in isentropic coordinates. The momentum budget over Antarctica reveals a balance between the Eliassen‐Palm flux convergence and the Coriolis torque exerted by the mean meridional mass flux. Both vertical and horizontal components of the Eliassen‐Palm flux contribute, the vertical component being the greater.
On large‐scale circulations in convecting atmospheres
Authors: Emanuel Kerry A. , David Neelin J. , Bretherton Christopher S. 
Published: February 2007

AbstractThe dominant thinking about the interaction between large‐scale atmospheric circulations and moist convection holds that convection acts as a heat source for the large‐scale circulations, while the latter supply water vapour to the convection. We show that this idea has led to fundamental misconceptions about this interaction, and offer an alternative paradigm, based on the idea that convection is nearly in statistical equilibrium with its environment. According to the alternative paradigm, the vertical temperature profile itself, rather than the heating, is controlled by the convection, which ties the temperature directly to the subcloud‐layer entropy. The understanding of large‐scale circulations in convecting atmospheres can, therefore, be regarded as a problem of understanding the distribution in space and time of the subcloud‐layer entropy. We show that the subcloud‐layer entropy is controlled by the sea surface temperature, the surface wind speed, and the large‐scale vertical velocity in the convecting layer, and demonstrate how the recognition of this control leads to a simple, physically consistent view of large‐scale flows, ranging from the Hadley and Walker circulations to the 30–50‐day oscillation. In particular, we argue that the direct effect of convection on large‐scale circulations is to reduce by roughly an order of magnitude the effective static stability felt by such circulations, and to damp all of them.
Structure of a frontal cyclone
Authors: Browning K. A. , Roberts N. M. 
Published: February 2007

AbstractA diagnostic study of a mid‐latitude cyclone has been carried out using routinely available numerical weather‐prediction model products and imagery. The cyclone was intense but not exceptionally so, and the results are believed to have some generality. The detailed description of the cyclone structure, just before it began to deepen rapidly, integrates concepts from a number of researchers into a common framework. A dominant feature of the cyclone, close to its centre, was the ‘cloud head’: a region of cloud with a sharp convex outer edge, which formed poleward of the main polar‐front cloud band. The cloud head was caused by two flows that entered it from the east, ascending and fanning out within it. One flow (the ‘cold conveyor belt’) brought low wet‐bulb potential temperature (θw) air back into the cloud head from low levels ahead of the warm front. The other flow was due to high‐θw air that peeled off from the base of the main warm‐sector airflow (i.e. part of the ‘warm conveyor belt’) and travelled in the boundary layer back towards the cyclone centre, first undercutting dry air that had earlier descended from the upper troposphere (called a ‘dry intrusion’), and then ascending at the upper boundary of the cloud head, above the cold conveyor belt. The transverse circulation that gave the ascent within the cloud head also led to the cold front fracturing along its length into two separate sharp surface cold fronts, with a more diffuse frontal region in between (‘frontal fracture’). The two sharp surface cold fronts were associated with narrow cold‐frontal rainbands (‘line convection’), one of the line‐convection segments forming the southern edge of the cloud head. The overrunning of the dry intrusion in the region of the frontal fracture led to a structure (known as a ‘split front’) in which an upper‐level humidity front began to run ahead of the position of the surface cold front. A feature of the cyclone at this intermediate stage in its evolution was that a large proportion of the precipitation was being generated by ascent of relatively cold air (the cold conveyor belt) in the cloud head as part of the thermally indirect circulation at the left exit of a developing secondary upper‐level jet.
The influence of surface texture on the effective roughness length
Authors: Schmid H. P. , Bünzli B. 
Published: February 2007

AbstractIt has recently become common practice to parametrize vertical momentum transfer, due to subgridscale motions over inhomogeneous areas, by a grid‐cell averaged effective roughness length, z, based on the concept of a blending height. Here, it is argued that the requirement that z be determined above a blending height is not a sufficient condition for its parametrization. Results from a numerical experiment are presented which suggest that z is strongly modulated by the texture of surface variability. These results demonstrate that the neglect of subgridscale surface texture may lead to an underestimation of z by up to an order of magnitude. Surface texture, or the spatial relationship between different roughness patches, is interpreted as being of a ‘second‐order roughness’, by which the regular ‘first‐order roughness’ is enhanced owing to the asymmetry of local advection across surface‐type transitions. It is submitted for further discussion that texture analysis provides a framework for the objective quantification of this net effect of subgridscale processes or ‘second‐order roughness’. In the appendix, the flow models used in this study are verified by comparing them with the micrometeorological observations of Bradley.
Melting and freezing in a mesoscale convective system
Authors: Braun Scott A. , Houze Robert A. 
Published: February 2007

AbstractMicrophysical and thermodynamic fields retrieved from dual‐Doppler synthesized winds are used to examine the distribution of melting and freezing rates within a midlatitude squall line with trailing stratiform precipitation. Previously, the effect of the heating/cooling processes associated with the release/consumption of the latent heat of fusion have emphasized the melting layer in the stratiform precipitation region, where the layer is typically marked by a radar bright band. The retrieved fields reveal the melting and freezing in the convective as well as stratiform region. Two‐dimensional retrieved fields indicate that the strong melting effect is not confined to the stratiform region, but extends across the entire breadth of the storm. The peak rates of cooling in the stratiform precipitation region are −2 to −3 K h−1, in general agreement with previous studies. In contrast, cooling rates in the convective precipitation region reach −14 K h−1. The heating by freezing is concentrated in a vertical column associated with the convective updraughts, with peak heating rates of 8 K h−1 just above the 0 °C level. For the mature stage of the storm, the convective region is found to account for 56% of the total cooling by melting, and 87% of the total heating by freezing within the squall line.A one‐dimensional retrieval model is applied to mean vertical‐velocity profiles to retrieve area‐mean profiles of the melting and freezing rates. In the convective region, average cooling rates of −10 K h−1 are found. The strong cooling by melting is expected to exert a strong influence on the development of convective downdraughts, the cold pool, and the gust front. The heating by freezing occurs through a deep layer, with a peak average heating rate of about 4 K h−1 just above the 0 °C level. In the stratiform precipitation region, a melting rate of about −2 K h−1 produced a near 0 °C isothermal layer approximately 500 m deep, similar to isothermal layers observed in stratiform precipitation areas associated with fronts.
A model of ammonia/ammonium conversion and deposition in a hill cap cloud
Authors: Bower K. N. , Wells M. , Choularton T. W. , Sutton M. A. 
Published: February 2007

AbstractAn aqueous‐phase cloud‐chemistry model is used to investigate the conversion of ammonia gas to ammonium aerosol in an air stream as it passes through a hill cap cloud. The sensitivity of the conversion to the scavenging of soluble gases such as SO2 and HNO3, and to the sulphur chemistry occurring in the cloud, is investigated. The subsequent effect of conversion on the pattern of deposition of reduced nitrogen (NHx) to the surface before, during and after processing by the cap cloud is also considered.The fraction of ammonia present in the inflowing air stream converted to ammonium depends critically on the ratio of ammonia to the concentration of all other species present in solution with which it is capable of forming stable ammonium salts. If sulphate, generated by the oxidation of S(IV) in solution by ozone (and hydrogen peroxide when available), is the only such species, ammonium production is directly linked to sulphate production. When aqueous ammonia is in deficit to acidic species (e.g. HSO), at least 85% of the input ammonia is converted to ammonium. When ammonia is in excess, the fraction of ammonia converted to ammonium depends upon the degree of this excess.It is calculated that conversion of ammonia to ammonium changes patterns of deposition of reduced nitrogen to the surface significantly. Within the cap cloud the efficient deposition of ammonium in cloud droplets (compared with the less efficient deposition of submicron aerosol) dominates over the dry deposition of ammonia gas, most of which is absorbed into the aqueous phase of the cloud. This generates a total reduced‐nitrogen flux within the cloud that is typically several times larger than the pre‐cloud flux. To the lee of the cap cloud, deposition of reduced nitrogen is also predicted to be much lower than pre‐cloud values. This lowered deposition results firstly from the depletion of gas‐phase ammonia, and secondly from the inefficient deposition of aerosol particles which individually carry an increased loading of ammonium after passage through the cap cloud (and so collectively constitute a larger proportion of the total loading of reduced nitrogen). This conversion enables the NHx loading of processed air streams to be transported over much longer distances before being deposited. The depleted concentrations of ammonia may enable further emissions of ammonia into the air stream as it passes over recently fertilized terrain, thus replenishing some of the NH, lost over cloud‐covered hills.
Variability of summer rainfall over tropical north Africa (1906–92): Observations and modelling
Authors: Rowell David P. , Folland Chris K. , Maskell Kathy , Ward M. Neil 
Published: February 2007

AbstractThe nature and causes of seasonal (July to September (JAS)) rainfall variability over tropical north Africa are investigated using a combination of empirical and modelling approaches. Concentration is focused on three regions: the Sahel, Soudan and Guinea Coast.The variability of seasonal rainfall through the twentieth century is analysed for each region. The well known dipole of anomalies between the Sahel and Guinea Coast is evident, and new analysis reveals that this fluctuates primarily on time‐scales of five years or less. Attention is then focussed on the causes of rainfall variability in each region, by examining the relationships with global sea‐surface‐temperature (SST) patterns; a partitioning of the data into low‐ and high‐frequency components is found to be particularly useful.So as to attribute more convincingly the primary cause of seasonal rainfall variability to global SST forcing, a suite of general circulation model experiments are performed, aimed at simulating JAS rainfall anomalies for ten past years between 1949 and 1990. Each is forced by the observed SST patterns for the appropriate year. In almost every case, the model quite skilfully simulates the magnitude and pattern of JAS rainfall anomalies across tropical north Africa, thus strengthening the idea that global SST variations are indeed responsible for most of the variability of seasonal rainfall. The relative impact of two additional sources of variability is also investigated. First the role of internal atmospheric variability is quantified (using an ‘analysis of variance’ technique), and found to be small in all three regions. Second, and perhaps more controversially, the possible effects of land‐surface‐moisture feedback are explored. This is done by replacing the normal interactive soil‐moisture scheme with a model‐derived climatology; results suggest that in some years moisture evaporated from the land surface may play a key role, but that in general SST forcing still dominates. Finally, an assessment of the model's skill at sub‐seasonal time‐scales reveals that fluctuations of monthly rainfall about each year's seasonal mean (intraseasonal variations) are insensitive to SST forcing, in part due to a larger influence of internal atmospheric variations.
Rossby number expansions, slaving principles, and balance dynamics
Authors: Warn T. , Bokhove O. , Shepherd T. G. , Vallis G. K. 
Published: February 2007

AbstractWe consider the problem of constructing balance dynamics for rapidly rotating fluid systems. It is argued that the conventional Rossby number expansion—namely expanding all variables in a series in Rossby number—is secular for all but the simplest flows. In particular, the higher‐order terms in the expansion grow exponentially on average, and for moderate values of the Rossby number the expansion is, at best, useful only for times of the order of the doubling times of the instabilities of the underlying quasi‐geostrophic dynamics. Similar arguments apply in a wide class of problems involving a small parameter and sufficiently complex zeroth‐order dynamics.A modified procedure is proposed which involves expanding only the fast modes of the system; this is equivalent to an asymptotic approximation of the slaving relation that relates the fast modes to the slow modes. The procedure is systematic and thus capable, at least in principle, of being carried to any order—unlike procedures based on truncations.We apply the procedure to construct higher‐order balance approximations of the shallow‐water equations. At the lowest order quasi‐geostrophy emerges. At the next order the system incorporates gradient‐wind balance, although the balance relations themselves involve only linear inversions and hence are easily applied. There is a large class of reduced systems associated with various choices for the slow variables, but the simplest ones appear to be those based on potential vorticity.
The role of snow sublimation in frontogenesis
Authors: Parker Douglas J. , Thorpe Alan J. 
Published: February 2007

AbstractRecent work on the influence of moist processes at synoptic fronts has shown how condensation heating can be a strong and even a dominant factor in their development. At mesoscale lines of convection the heating is again dominant, but it is thought that cooling by the evaporation of precipitation is also a crucial factor in their development and propagation. Although most simple models of synoptic fronts do not attempt to simulate diabatic cooling accurately, data from the FRONTS 87 experiment have identified evidence of a strong mesoscale downdraught below a synoptic frontal surface. There has been speculation that such a downdraught is forced by the sublimation of falling snow.Since snow is more efficient at cooling the atmosphere, for a given precipitation rate, than evaporation of rain, rainfall parametrizations are likely to underestimate the influence of such cooling. In this paper, a suitable parametrization of diabatic cooling, based on the assumption of small stability to slantwise convection, is used to simulate a band of snow sublimation in developing fronts. the semi‐geostrophic equations are employed in the study of fronts which are formed by deformation or horizontal shear of the geostrophic winds.It is found that the cooling has little influence on the large scales of motion, and does not greatly accelerate the formation of frontal singularity. However, the cross‐frontal flows in the vicinity of the cooling are modified strongly, with the formation of a narrow downdraught of comparable dimensions and intensity to that suggested by the experimental data. the cooling has a weak influence on all the fields at a larger distance from the front; this is discussed in the context of the large‐scale, or geostrophic, response to lines of convection.