bluuebunny/crossref_metadata_2025 huggingface dataset:
The Pressure force induced by neutral, turbulent flow over hills
Authors: Wood Nigelo , Mason Pual
Published: February 2007

The pressure force induced by neutral, turbulent flow over small‐scale hills is considered. The influence that the level of turbulence closure has on liner, analytical expressions for the force is evaluated. The two‐dimensional linear expressions are then extended to three dimensions and a heuristic extension to moderate and steep slopes, for which nonlinear effects are important, is proposed. The predictions of these various expressions are compared with results from a three‐dimensional numerical model employing a 1 1/2‐order turbulence‐closure model, and very good agreement is found.

The numerical‐model results supports the idea that, sufficiently far above the hills, the areally averaged velocity profile varies approximately logarithmically with height but with an enhanced, or effective, roughness length and a friction velocity equal to the square root of the total surface force per unit area and unit density. Using the theoretical results for the pressure force, discussed above, and approximate expression for the effective roughness length is obtained that agrees well with values from the model, and also matches with existing expressions that are valid for two‐dimensional hills with either very low or very large slopes. Qualitative agreement is also found with the limited number of observed values of roughness length in hilly terrain. These results suggest that the effective roughness length is an accurate and convenient method of parametrizing the drag due to subgrid‐scale orography in large‐scale numerical‐weather‐prediction and climate models.
Dynamical structure of a wide cold‐frontal cloudband observed during FRONTS 87
Authors: Lagouvardos K. , Lemaitre Y. , Scialom G.
Published: February 2007

Conditional symmetric instability (CSI) is examined as a possible mechanism for generating cloudbands within a cold‐frontal system. The data used in this study were collected during the Franco‐British field experiment FRONTS 87. A dense network of rawinsounding measurements was used to test the various hypotheses underlying CSI, such as the sign of the moist potential vorticity, the relative slope of absolute momentum and moist isentropic surfaces or the alignment of bands with the vertical shear of the wind inside the unstable domain. Radar data provided from the French dual‐Doppler radar system RONSARD was used to retrieve the three‐dimensional airflow within a cloudband observed during the last frontal event of FRONTS 87. A new retrieval method, called MANDOP, was used to estimate the circulations within this band and to test whether the retrieved circulations were consistent with the CSI theoretical expectation. A roll‐like circulation present in the vicinity of the observed cloudband seems to support the hypothesis that CSI was the formative mechanism.
Large‐eddy simulation: A critical review of the technique
Authors: Mason P. J.
Published: February 2007

Large‐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

Results 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

A 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

Total 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

Individual 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

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, 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

The 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

The 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

A 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

It 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

Microphysical 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

An 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

The 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

We 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

Recent 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.
Zonal asymmetries in SAMS stratospheric methane and nitrous oxide
Authors: Ziemke J. R. , Stanford J. L.
Published: February 2007

Stratospheric and Mesospheric Sounder (SAMS) methane (CH4) and nitrous oxide (N2O) constituent measurements were taken a decade before the Upper Atmosphere Research Satellite (UARS) observations and are generally of lesser quality. However, SAMS data are important because of the limited lifetime of the UARS and because they provide a unique, historical data series for these gases involved in greenhouse and ozone‐depletion effects. While most prior SAMS analyses focused on zonal means, this paper assesses the strengths and weaknesses of analysing zonally asymmetric perturbations in SAMS data. It is shown that wave‐1 perturbations can be successfully investigated, provided sufficient care is taken and aliasing considered.

At the lowest SAMS level, 20 hPa (∼ 28 km altitude) zonal‐wave‐1 CH4 and N2O data reveal similar features for latitudes 45°N–65°N during stratospheric warming events and break‐up of the polar vortex. Large wave‐1 anomalies in the upper stratosphere (2 and 0.6 hPa) were found to be out of phase with the corresponding anomalies at 20 hPa. In one episode in early 1981 (during stratospheric sudden warming) southward winds over North America transported air with low mixing ratios from polar latitudes, while northward winds over Siberia transported air with high mixing ratios from subtropical latitudes. the effect produced strong wave‐1 amplitudes in both CH4 and N2O mixing ratios.

Cross‐correlations between wave‐1 CH4 and N2O are large and positive in middle and high latitudes (consistent with ideal tracer behaviour for both constituent gases) but weak over the tropics. the cause of the latter remains an open issue.
On the Hamiltonian formulation of the quasi‐hydrostatic equations
Authors: Roulstone I. , Brice S. J.
Published: February 2007

A variational approach to the Euler‐Lagrange equations for the quasi‐hydrostatic model is presented together with an explicit Hamiltonian (Poisson bracket) formulation. the bracket acts on functionals of the zonal and meridional angular momentum. Features of the relationships between the Hamiltonian formulations of the equations for an ideal fluid, the hydrostatic primitive equations and the quasi‐hydrostatic equations are discussed.
Observations of the mesoscale sub‐structure in the cold air of a developing frontal cyclone
Authors: Browning K. A. , Clough S. A. , Davitt C. S. A. , Roberts N. M. , Hewson T. D. , Healey P. G. W.
Published: February 2007

Observations from 58 dropwindsondes released in a mesoscale array during the FRONTS 92 experiment are interpreted in the context of satellite imagery to derive the mesoscale structure and evolution of parts of a frontal cyclone developing over the eastern North Atlantic. A conceptual model involving the intertwining of ‘dry intrusion’ and ‘cloud head’ flows is corroborated and is used to provide the framework for interpreting the detailed mesoscale behaviour. In the cold air, two distinct dry‐intrusions were responsible for two cold fronts, trailing south‐westwards from the tip of the cloud head. Both were surface features at the beginning of our study but the leading one evolved into an upper front with mid‐level convection as the dry intrusion responsible for it overran the warm sector. Areas of both high and low potential vorticity were indicated within the dry intrusions. Upon encountering a critical level of zero system‐relative velocity at the top of the moist boundary layer, the dry‐intrusions' arrival was associated with the development of multiple dry and moist laminae near the top of the boundary layer. the vertical wavelength of the laminae was about 1 km and they extended over 200 km in the front‐normal direction, with a slope of typically 1 in 60. Although most parts of the laminae were subsaturated, their circulations combined with the double structure of the dry intrusions to produce multiple shallow cloud‐lines within the boundary layer. These formed as an extension of the south‐western tip of the cloud head associated with the developing cyclone. Possible mechanisms for generating the observed structures are discussed.
Simulation of daily variability of surface temperature and precipitation over europe in the current and 2 × Co2 climates using the UKMO climate model
Authors: Gregory J. M. , Mitchell J. F. B.
Published: February 2007

As the spatial scales simulated by general circulation models are refined and the parametrizations they employ are improved, greater accuracy can be achieved in their simulations of regional climate and events on short time‐scales, and greater confidence attached to their predictions of these for perturbed climates. In the present paper, we examine the daily (interdiurnal) variability of surface temperature and precipitation in Europe simulated by a version of the UK Meteorological Office (Hadley Centre) general circulation model, having 11 levels in the vertical and a horizontal resolution of 2.5°latitude × 3.75° longitude, coupled to a slab ocean, in the current climate and the equilibrium climate for a doubled atmospheric concentration of carbon dioxide. We evaluate the simulation of current conditions by comparison with climatologies and time‐series of observed data. In the modified climate, we find reductions in temperature variability in winter, related to a lessened land‐sea temperature contrast, and increases in summer, caused by changes in the surface heat balance. Precipitation in all seasons shows significant tendencies both to more dry days and to heavier events, with less frequent moderate daily amounts; this is connected with an increase in convective activity. We conclude that simulation of daily variability is strongly dependent on particular local physical processes and hence on the parametrizations used to represent them. While acknowledging deficiencies in the model's performance, we consider that our results are indicative of possible changes for a future climate.
Patterns of interannual variability defined by a cluster analysis and their relation with ENSO
Authors: Efimov V. V. , Prusov A. V. , Shokurov M. V.
Published: February 2007

Results of the COADS data processing aimed at a classification of the structure of the large‐scale lowfrequency variability of the sea surface temperature, the sea‐level atmospheric pressure and the surface wind‐speed components are presented. Using cluster analysis, patterns of interannual variability in the temporal and spatial domains are obtained. Clusters consisting of points possessing similar temporal evolution are compared with the known dipole‐type patterns of interannual variability, and special attention is given to selection of the global ENSO‐related patterns. of particular interest are the teleconnection links between the equatorial and the extratropical regions in the Pacific Ocean. Clusters consisting of times with similar spatial patterns, as well as extended canonical correction analysis patterns, show the typical sequence of the spatial patterns during the ENSO cycle, especially for the ‘mature’ warm and cold phases. For these cases, typical features of the field anomalies, including the extratropical peculiarities in the Pacific Ocean, are considered and summarized.
Dependence of surface exchange coefficients on averaging scale and grid size
Authors: Mahrt L. , Sun Jielun
Published: February 2007

The value of the effective exchange coefficient for area‐averaged fluxes can depend significantly on the averaging scale. This dependence implies that the exchange coefficient in numerical models should depend on grid size. the main goal of this study is the assessment of the importance of such scale‐dependence. When the large‐scale flow is weak, the mesoscale motions on scales smaller than the averaging width (grid size) may generate significant turbulence which is not represented by the area‐averaged wind vector (resolved flow). In this case, the effective exchange coefficient must be larger than that predicted by similarity theory in order to predict the grid‐averaged turbulent flux.

This study reviews different approaches for representation of spatially‐averaged surface fluxes over heterogeneous surfaces. the scale‐dependence of the effective exchange coefficient is posed in terms of spatially averaging the bulk aerodynamic relationship. This scale‐dependence is evaluated in terms of observed fluxes from three different field programs. the effective drag coefficient is found to be more scale‐dependent than the effective exchange coefficients for heat and momentum. the effective conductance is found to be less scale‐dependent than the effective exchange coefficient and effective resistance. Except for the case of weak flow over strong surface heterogeneity, the exchange coefficient is nearly independent of grid size.
Linear theory of momentum fluxes in 3‐D flows with turning of the mean wind with height
Authors: Broad Adrian S.
Published: February 2007

Current orographically forced gravity‐wave‐drag parametrization schemes in numerical weather‐prediction models are based on two‐dimensional linear momentum‐flux theory. the vertical flux of horizontal‐momentum vector is considered parallel to the surface‐stress vector and independent of height unless the Richardson number falls below a quarter, or a critical level exists where the mean wind becomes zero in the direction of the flux vector. Non‐zero gravity‐wave drag is then taken to be parallel and opposite to the flux vector. Consideration of the full three‐dimensional linear problem reveals a number of fundamental differences: (i) the drag force is perpendicular to the mean wind where there is no wavebreaking, and (ii) the variation of the stress vector with height is dependent on the wave‐number decomposition of the underlying orography. For a general orographic profile having a continuous Fourier spectrum spanning an azimuthal angle of 180°: (a) critical levels exist at every height in the atmosphere for any wind profile, and (b) the momentum‐flux vector decreases in magnitude and turns with height.
An improved linear model of tropical surface wind variability
Authors: Saji N. H. , Goswami B. N.
Published: February 2007

For coupled modelling studies, the atmospheric component is required to simulate the large‐scale part of the observed surface winds accurately, as only this part is responsible for driving low‐frequency interannual variability in the oceans. A simple linear model of the tropical atmosphere is developed in which surface winds are viewed as the response of the atmospheric boundary layer to pressure perturbations produced by deep convection and sea surface temperature (SST) gradients. An empirical parametrization of total convection as a nonlinear function of total SST is adopted. The large‐scale variability of the simulated surface winds is examined for the period 1974–1991. The model not only captures the large‐scale low‐frequency part of the surface winds given by the first two empirical orthogonal functions (EOFs), it also successfully simulates the equatorial surface wind anomalies and their evolution during the entire period 1974–1991.

Certain interesting differences between precipitation forcing and SST induced forcing of surface winds is brought out in this study. The precipitation forcing was found to be dominant over the central and western Pacific, while the SST forcing was found to be dominant over the eastern Pacific. The resultant wind response was also found to reflect this behaviour. The most interesting result is the change in balance of the precipitation‐related and SST‐gradient‐related terms in the forcing of anomalous and climatological winds. Our results indicate that convective heating predominates over SST‐gradient‐induced effects in forcing anomalous winds, but the balance reverses in the case of the climatological winds. We also find that SST‐gradient effects are critical in the simulation of the climatological wind field.
Sensitivity of a multi‐layer quasi‐geostrophic β‐channel to the vertical structure of the equilibrium meridional temperature gradient
Authors: Pavan Valentina
Published: February 2007

A multi‐layer, quasi‐geostrophic, Boussinesq model on a β‐channel is used to study the sensitivity of the mid‐latitude storm‐track to changes in the vertical structure of the zonal mean equilibrium meridional temperature gradient. Motivation for this study is taken from the observation that the doubled CO2 climatology of general‐circulation models presents a weaker gradient in the lower troposphere and a stronger gradient in the upper troposphere than in a ‘control’ climatology.

It is observed that the dynamics of the quasi‐geostrophic model are generally more sensitive to the lower than to the upper tropospheric temperature gradient as a consequence of the importance of shallow eddies. In order to give a good representation of the eddy dynamics it is necessary to use a sufficiently high resolution, which, in the case chosen, proved to be between 10 and 20 tropospheric layers. If only three layers are used the general features of the response are still reproduced, but the model does not represent the sensitivities of the eddy meridional momentum‐flux convergence adequately. It is argued that this is connected with the particular choice taken when parametrizing the surface Ekman friction. When a higher‐resolution model is used, the eddy momentum‐flux convergence proves to be more sensitive to changes in the upper than in the lower meridional equilibrium temperature gradient but is generally much less responsive to these changes than other quantities like the surface eddy potential‐vorticity flux or eddy kinetic energy.

For each variable the two sensitivities become close to each other as the baroclinicity of the equilibrium flow is increased because of the prevalence of deep eddies in the dynamics, sensitive only to the total radiative equilibrium wind vertical gradient. For very high values of the baroclinicity of the equilibrium flow the sensitivities approach a constant value in accordance with a simple scaling argument.
On the relationship between the quasi‐biennial oscillation, total chlorine and the severity of the antarctic ozone hole
Authors: Butchart Neal , Austin John
Published: February 2007

A 3‐dimensional model of the dynamics and radiation of the stratosphere and mesosphere is used to simulate austral spring conditions. An idealized representation of the southern hemisphere planetary waves is prescribed at the 316 mb model lower boundary, and the tropical zonal winds are relaxed towards an idealized time‐dependent representation of the quasi‐biennial oscillation (QBO). As the initial phase of the QBO is changed, the October mean lower‐stratospheric temperatures polewards of 75°S respond systematically, with the two extreme responses found for phases only 1/4 cycle apart rather than for opposite phases. The integrations are repeated for these two phases but with fully interactive photochemistry and different levels of chlorine, corresponding to conditions in about 1980, the mid to late 1980s, and those anticipated in about the year 2010. Increasing the chlorine amount produced systematically deeper ozone holes with areas significantly larger for conditions in the year 2010 than for those in the late 1980s, broadly in agreement with the secular trend observed by the Total Ozone Mapping Spectrometer. For the QBO phase giving lowest temperatures the simulated ozone holes are consistently deeper but with reduced sensitivity for high chlorine amounts. Differences in polar ozone amounts between the two phases are comparable with the observed interannual variability. In the model the results show that the QBO modulates the severity of the antarctic ozone hole directly through the wave‐induced transporting circulation rather than indirectly through the temperature‐dependent heterogeneous chemistry. Further, it is argued that the phases of the QBO producing the two extreme responses in the springtime ozone amounts over Antarctica are likely to depend, in reality, on details of the tropospheric planetary‐wave flux. The model results are also shown to be consistent with the observed downward trend in October antarctic temperatures since 1979.
The climatology of the stratospheric ‘THIN AIR’ model
Authors: Kinnersley Jonathan S.
Published: February 2007

An improved two‐and‐a‐half dimensional chemical–dynamical–radiative model shows remarkably realistic behaviour in a number of respects. The stratospheric/mesospheric westerly jet slopes equatorwards with height and is split at mid‐latitudes in the southern hemisphere. The parametrized orographic gravity waves exert a mid‐latitude drag and are important in splitting the jet, and are possibly important in pre‐conditioning the stratosphere for a sudden warming. A large easterly drag is required in the winter high‐latitude mesosphere in order to obtain a tropical mesospheric westerly jet. The summer easterly jet maximum lies above the tropospheric jet, due to gravity‐wave absorption in the lower stratosphere. Very realistic annual and semi‐annual cycles are produced in the model, with a mesospheric semi‐annual oscillation forced by tidal drag and gravity waves. Planetary‐wave temperatures are also modelled well, and wave amplitudes are correlated with the westerly jets. In the lower stratosphere, zonalmean Ertel's potential vorticity gradients strongly influence the wave activity, horizontal convergence and heating rates, with descent being strongest equatorwards of the vortex edge. This forms the basis for an explanation of the inter‐hemispheric asymmetry in the ozone column. Nitrous oxide distributions indicate too much equatorial ascent but show a ‘double peak’ connected with the tropical circulation, and steep horizontal gradients near the winter vortex edge.
Vortex dynamics and the evolution of water vapour in the stratosphere of the southern hemisphere
Authors: Lahoz W. A. , O'Neill A. , Heaps A. , Pope V. D. , Swinbank R. , Harwood R. S. , Froidevaux L. , Read W. G. , Waters J. W. , Peckham G. E.
Published: February 2007

The seasonal evolution of water vapour in the stratosphere of the southern hemisphere is studied by using water vapour measurements made by the Microwave Limb Sounder on the Upper Atmosphere Research Satellite. This evolution is interpreted with the aid of meteorological fields produced at the UK Meteorological Office by data assimilation. The processes governing the distribution of water vapour are clarified by focusing on the physical and dynamical conditions in and around the stratospheric polar vortex. Sustained diabatic descent in the vortex causes isopleths of water vapour mixing ratio to dip down markedly in the polar vortex, strengthening radial gradients of water vapour in the westerly jet, while stretching and folding of material lines in anticyclones adjacent to the polar vortex leads to awidening zone of weak horizontal gradients of water vapour. The circulation is discussed in terms of the dynamics of interacting vortices. Two different flow regimes are identified: (a) mid and late southern winter, with a strong polar vortex and one or more eastward‐travelling anticyclone; and (b) spring, with a relatively weak polar vortex and a quasi‐stationary anticyclone. The phenomenon of merger of anticyclones, already observed in the stratosphere of the northern hemisphere, is documented for the southern hemisphere. A detailed study of the tracer transport during merger is made by computing the isentropic advection of many thousands of particles. A comparison is made of the seasonal evolution of water vapour in the stratosphere of the southern and northern hemispheres.
A non‐hydrostatic version of the NMC's regional Eta model
Authors: Gallus William A. , Rančić Miodrag
Published: February 2007

A non‐hydrostatic version of the regional Eta model used operationally at the National Meteorological Center (NMC) has been developed by implementing the ideas of Juang (1992) and Laprise (1992), who independently recommended a hydrostatically based, vertical coordinate for a fully compressible set of equations. The grid‐point model dynamics is based on perturbation equations in the η‐vertical coordinate; the base state may be taken from the operational NMC model, and thus updated with time.

The non‐hydrostatic model uses a stepwise treatment of terrain present in the operational version, eliminating the pressure‐gradient‐term error associated with sigma‐coordinate models over steep topography. The compressible equations are written in a form that allows conservation of energy in a horizontally closed domain with appropriate advective schemes.

A two‐dimensional version of the model, without parametrizations of physical processes, has been used successfully to simulate ascending warm bubbles and collapsing cold bubbles at high resolutions.