bluuebunny/crossref_metadata_2025_split huggingface dataset:
Variational inversion of simulated TOVS radiances using the adjoint technique
Authors: Thepaut Jean‐Noël , Moll Patrick 
Published: February 2007

AbstractThe adjoint technique allows the variational approach for assimilating various types of observations in meteorology, including raw radiances measured by satellite, to be treated exactly and at a ‘reasonable’ cost.We have developed the tangent linear and adjoint operators of the International TOVS Processing Package radiative transfer equation. These two tools are used for treating two aspects of variational inversion of satellite sounding radiances.First of all, we apply this technique to the computation of the error covariance matrix of the retrieval/analysis of simulated TOVS sounding radiances. Using this matrix, an original method to interpret satellite sounding radiances in terms of temperature and humidity information is then presented. We can thus evaluate the number of independent parameters which are significantly estimated from radiances, given a direct radiative transfer model, observation and background error statistics. It is found that 19 HIRS channels provide six independent pieces of information while four MSU channels provide three independent pieces of information. When grouping HIRS and MSU channels, only seven independent pieces of information are available. It is shown that in cloudy conditions MSU channels bring additional significant information to HIRS channels especially at the surface. We also study the impact of cloudiness on the quality of the retrieved profiles.Numerical feasibility of 1‐D variational inversion using the adjoint of the International TOVS Processing Package radiative transfer model is demonstrated. Impact of the choice of the inner product on the efficiency of the method is studied. We then discuss the quality of the inversion, which turns out to be strongly related to the quality of the atmospheric information present in the sounding radiances and to the background and observation error statistics. the gain brought by MSU channels at the surface in cloudy conditions is discussed.
The temporal distribution of amazonian rainfall and its implications for forest interception
Authors: Lloyd C. R. 
Published: February 2007

AbstractA model for the temporal distribution of rainfall is derived from measurements made in central Amazonian rainforest. A contrast is made with a rainfall distribution model which has been used in a stand‐alone comparison of land‐surface sub‐models of two General Circulation Models (GCMs). In terms of the rainfall interception loss, it is shown that the central Amazonian model produces an overestimate of 35%, but is within the error bars of the measured interception loss, whereas the latter model overestimates the interception loss by more than 100%. Two other time distributions of rainfall from contrasting regions of the Amazon are derived from published data in order to investigate the possible interception variation due to rainfall pattern variability. the results illustrate the need for GCM modellers to monitor their synthesized rainfall regime against real data and to be aware of the sensitivity of the forest interception loss to rainfall distribution.
Application of the semi‐Lagrangian method to a multilevel spectral primitive‐equations model
Authors: Ritchie Harold 
Published: February 2007

AbstractIt has previously been shown that semi‐Lagrangian schemes can be applied to spectral models of the shallow‐water equations using large time‐steps (e.g., see Ritchie 1988). The present study considers the extension of this work to a multilevel spectral primitive‐equations model.As a first step, an Eulerian vorticity‐divergence spectral model is converted to an Eulerian model based on a vector momentum form of the equation of motion. From the latter, several semi‐Lagrangian models are prepared: one using an interpolating semi‐Lagrangian treatment of advection in the horizontal (referred to as 2DISL) while retaining Eulerian advection in the vertical, another using a 3‐dimensional interpolating semi‐Lagrangian formulation (referred to as 3DISL), and another which combines the 2DISL scheme in the horizontal and a non‐interpolating semi‐Lagrangian treatment in the vertical (referred to as NISLV). Medium‐range intercomparison experiments are performed using models that include simple physical parametrizations. It is shown that the semi‐Lagrangian semi‐implicit approach can be applied accurately and stably to produce medium‐range (5‐day) forecasts using time‐steps that are far larger than those permitted by the Courant‐Friedrichs‐Lewy (CFL) stability criterion for the corresponding Eulerian model.The NISLV version is found to be more accurate than the 3DISL one which apparently has excessive damping in the vicinity of the tropopause, where all the model fields change abruptly in the vertical.
A three‐dimensional generalization of Eliassen's balanced vortex equations derived from Hamilton's principle
Authors: Craig George C. 
Published: February 2007

AbstractA new set of equations is derived for nearly circular flow in gradient balance. For precisely axisymmetric motion, the system reduces to the well‐known balanced vortex equations of Eliassen. The derivation is based on the assumption that the radial component of velocity is small in comparison to the azimuthal component. By applying this approximation to Hamilton's principle for a continuum of fluid parcels, while preserving the time and particle‐labelling symmetries of the primitive equations, it is ensured that the resulting system possesses conservation laws for energy and potential vorticity. In potential radius coordinates, the Lagrangian equations of motion take the form of the geostrophic and hydrostatic balance conditions. The system is also presented in Eulerian form, and a practical integration scheme, based on a linear elliptic equation for geopotential tendency, is described. Finally, the set is rederived by a conventional scale analysis in order to determine constraints on the diabatic forcings which are required for consistency with the approximation.
Pressure drag and momentum fluxes due to the Alps. II: Representation in large‐scale atmospheric models
Authors: Clark Terry L. , Miller Martin J. 
Published: February 2007

AbstractA three‐dimensional numerical model with interactive grid nesting is used to study the pressure drag and associated momentum fluxes due to the Alpine complex (for a specified upstream wind and static stability profile). Four basic questions are addressed which are relevant to the problems of flow description in the vicinity of mountains and the impact of orography in the context of mesoscale numerical weather prediction and general circulation models:

What is the dependence of basic quantities such as the pressure drag on the model's horizontal resolution?
Is there a need for non‐hydrostatic dynamics to account properly for the flow characteristics near steep mountains?
Is there an effective role for enhanced orography such as an envelope formulation?
Might a ‘gravity‐wave‐drag’ parametrization scheme be able to represent adequately the highest resolution fluxes and surface drag in a much coarser resolution model?
A series of experiments with both hydrostatic and non‐hydrostatic models at resolutions of 80, 40, 20, 10 and 5 km is analysed, and two of these experiments are repeated with an envelope orography.It is concluded that hydrostatic simulations are sufficient to describe the basic dynamics even at 5 km resolution; however, there is a strong dependence of the drag and momentum fluxes on resolution, with the drag at 80 km resolution being almost half that at the highest resolution. It is also shown that the use of an envelope orography at coarser resolutions can compensate significantly for the drag under‐estimation, but is less effective in representing the vertical stress profiles or the direction of the stress vectors, which are systematically in error at the coarsest resolution. The potential role of gravity wave drag is confirmed and possible improvements considered. It is suggested that both envelope orography and gravity wave drag may make a significant contribution until resolutions are as high as 10 to 20 km.
A model of occult deposition applicable to complex terrain
Authors: Coe H. , Choularton T. W. , Carruthers D. J. , Gallagher M. W. , Bower K. N. 
Published: February 2007

AbstractA model is presented of the turbulent deposition of cloud water and dissolved species to regions of complex topography where cloud forms in contact with the ground. A three‐dimensional airflow model (FLOWSTAR) which includes a treatment of atmospheric stability and boundary‐layer turbulence is used. Versions of the model are described which include either simple calculations of liquid‐water content or detailed calculations of the cloud microphysics. Both a simple equilibrium model of cloud‐water chemistry and a detailed model including the explicit calculation of the chemical processes in individual droplet size categories are used.The deposition velocities of the cloud droplets to the terrain are calculated from recent measurements of cloud‐water deposition to forest and moorland.It is found that good estimates of deposition may be obtained using the simplest cloud‐water and chemistry models with deposition velocities parametrized in terms of a mean droplet radius. The results indicate that the deposition is a very strong function of position within the terrain. The implications of this for the deposition of phytotoxic species to the flora and the deposition of radio‐nuclides following a nuclear accident are considered.
Mesoscale dynamics of cold fronts: Structures described by dropsoundings in FRONTS 87
Authors: Thorpe Alan J. , Clough Sid A. 
Published: February 2007

AbstractThe FRonts 87 project was a European experiment to make mesoscale observations to deduce the dynamics of active cold fronts approaching north‐west Europe, with a minimum of orographic influences. In this paper data from dropsoundings made by the Meteorological Office's C‐130 research aircraft are analysed. The aircraft flew a pattern with 4 or 5 runs oriented approximately at right angles to the front, but with each run displaced in the along‐front direction by about 100 km. The dropsondes gave soundings to a height of approximately 7 km with a cross‐frontal spacing of 20 km at best and 60 km on average. Between 30 and 50 soundings were made in each event, over an area of 500 × 500 km2. These data are unique in their mesoscale resolution over a synoptic‐scale region; this was made possible by the ability to track up to 5 sondes in the air at any time.Several new aspects of cold‐front dynamics have been identified from these data; the approach here is to treat each run as an instantaneous cross‐frontal section. The discussion is centred on the hypotheses postulated before the experiment; these were intended to allow verification or refutation of aspects of models of frontal dynamics such as those embodied in semi‐geostrophic theory. A minimal and purely objective analysis has been performed on the data presented here, but the high resolution of the dropsonde observations permits evaluation of differentiated quantities, such as potential vorticity (PV), with some confidence. Issues raised here include the degree of thermal‐wind balance, the structure of conserved variables such as equivalent potential temperature and absolute momentum, the cross‐frontal circulation and the role of diabatic processes such as the evaporation, of snow, and the potential vorticity structure on the mesoscale and its interpretation. The intention is to present an integrated view of the dynamical structure of fronts in the light of theoretical rather than conceptual or airflow models.
Localized forcing of slantwise motion at fronts
Authors: Holt M. W. , Thorpe A. J. 
Published: February 2007

AbstractA cold front is here modelled as a baroclinic zone in which there is a local low‐level forcing of ageostrophic motion. Such forcing may result from the outflow from line‐convection at the surface front, or from synoptic‐scale frontogenesis, and is represented by a localized fixed region in which anomalies of the inertia and/or buoyancy of the air are imposed, either continuously or impulsively.The response to this forcing is described when the baroclinic flow is either unstable or weakly stable to symmetric instability. Such conditions are thought to be typical of cold fronts. The flow is examined using classical parcel theory, a (new) more complete parcel theory, a primitive‐equation numerical model, and the so‐called Geometric (or balanced) Model. The predicted motion involves a lens‐shaped area oriented along the sloping isentropes, with geometry dictated by the background potential vorticity and the imposed anomalies in inertia or buoyancy. The lens has zero or weakly negative potential vorticity, in the case of a background flow which is symmetrically stable.This lens, grown in a symmetrically stable atmosphere, is offered as an alternative explanation for the observed state of (moist) slantwise neutrality at a cold front, in contrast to that explanation which assumes an irreversible adjustment to symmetric instability.
Sensitivity of the southern hemisphere circulation to leads in the Antarctic pack ice
Authors: Simmondsxs Ian , Budd W. F. 
Published: February 2007

AbstractTo assess the sensitivity of the southern hemisphere circulation to changes in the fraction of open water in the sea ice we have conducted four experiments with a July 21‐wave General Circulation Model (GCM) with this fraction set to 5, 50, 80 and 100%. The mean surface temperatures and the surface atmospheric temperatures over the sea ice increased as the water fraction increased and the largest changes were simulated adjacent to the coast.Significant anomalies in the surface heat fluxes, particularly those of sensible heat, accompanied the decrease in the sea ice concentration. Substantial atmospheric warming was simulated over and in the vicinity of areas in which leads were considered. In all but one experiment there were anomalous easterlies between about 40 and 60°S with westerly anomalies further to the south. The surface pressure at high latitudes appears to change in a consistent fashion with the fraction of open water, with the largest changes occurring in the Weddell and near the Ross Seas.Some of the feedbacks which may enhance the responses here, but which are not included in our model, are discussed.
The influence of static stability on the effective roughness lengths for momentum and heat transfer
Authors: Wood Nigel , Mason Paul 
Published: February 2007

AbstractThe area‐averaged properties of the planetary boundary layer over heterogeneous terrain are considered. Previous studies which dealt with the average momentum transfer properties in neutral static stability conditions are extended to include the influence of stratification and also area‐averaged properties for heat transfer. Results from numerical simulations demonstrate the utility of values of effective roughness length for both momentum and heat transfer. A heuristic model is found to show good agreement with the numerical simulations and to provide a simple method for estimating these roughness lengths for use in boundary‐layer parametrization.Significant anomalies in the surface heat fluxes, particularly those of sensible heat, accompanied the decrease in the sea ice concentration. Substantial atmospheric warming was simulated over and in the vicinity of areas in which leads were considered. In all but one experiment there were anomalous easterlies between about 40 and 60°S with westerly anomalies further to the south. The surface pressure at high latitudes appears to change in a consistent fashion with the fraction of open water, with the largest changes occurring in the Weddell and near the Ross Seas.Some of the feedbacks which may enhance the responses here, but which are not included in our model, are discussed.
The evaporation of frontal and other stratiform precipitation
Authors: Clough S. A. , Franks R. A. A. 
Published: February 2007

AbstractThe evaporation of precipitation of differing types is considered by use of one‐dimensional models. It is shown that the evaporation of snow and crystalline ice precipitation takes place in shorter times and much shallower depths than for rain, while for graupel it falls between the two. The main factors responsible for the differences in behaviour are bulk density and terminal velocity of particles, which combine to produce evaporation depths for snow less than one tenth those for rain. Further, the evaporation depth predicted for snow is shown to be rather insensitive to detailed characteristics of the snow particles within the observed range.Some of the implications of this difference are explored by means of a model describing the interaction of evaporating precipitation with its environment when subjected to atmospheric descent. The model, originally due to Kamburova and Ludlam (1966), has been extended to study the evolution of a variable particle spectrum. It is shown that for rainfall rates of the order of 1–10 mm h−1 the evaporation of snow is sufficient to maintain the atmosphere in a near‐saturated condition despite descent rates of the order of 10–30 cms−1, typical of those observed on the mesoscale in stratiform environments like rainbands; rain exhibits no comparable behaviour.These results suggest that the evaporation of snow plays a major role in the dynamics of fronts and other mesoscale systems with stratiform cloud. This view is supported by comparison with dropsonde observations from the FRonts 87 experiment, which show a clear indication of enhanced mesoscale descent in the region of maximum precipitation.
Cloud parametrization at different horizontal resolutions
Authors: Kristjánsson Jón Egill 
Published: February 2007

AbstractAs faster computers emerge, resolution continues to improve in weather‐prediction and climate models. How does this affect the problems that we are dealing with in the models? In this study we have concentrated on one of these problems, namely the treatment of clouds. Among the questions addressed are: How do cloud‐related processes (condensation, precipitation) depend on grid resolution? How should we take this into account? Can we simulate orographic clouds when we have a grid resolution of hundreds of kilometres (climate models)? To tackle these questions simulations are performed with a limited‐area model that is run with horizontal grid resolutions of 150,50 and 25 km. The model has a sophisticated treatment of condensation and clouds. For verification, use is made of satellite‐derived cloud water content and cloud cover, and surface observations of precipitation.It is found that when resolution is enhanced, precipitation systematically increases while cloud cover decreases. The main reason is that much stronger vertical motions develop at high resolution, yielding dense, narrow clouds, with clear air between. The consequences of these results for cloud parametrization are discussed. It is further shown that cloud‐cover simulations over mountainous terrain can be improved by paying regard to the sub‐grid‐scale orography.
On the relation between total ozone and meteorology
Authors: Vaughan G. , Price J. D. 
Published: February 2007

AbstractA relationship is derived between absolute vorticity in the lower stratosphere and the total‐ozone column. This relationship shows that reversible deformations to the flow in the lower stratosphere on synoptic scales or smaller (such as trough/ridge patterns) cause perturbations in total ozone which depend on the amplitude and depth of the vorticity anomaly. Examples are presented showing excellent correspondence between total ozone and vorticity charts, even on mesoscales. Grid correlations show that at least half the variance in total ozone is related to variations in the vorticity just above the tropopause. Some applications of the ozone‐vorticity relationship are discussed.
A GCM Case Study On the Maintenance of Short‐Term Subtropical Wind Maxima In the Summer Hemisphere During SOP‐1, FGGE
Authors: Hurrell James W. , Vincent Dayton G. 
Published: February 2007

AbstractA recent version of the GLA fourth‐order GCM is used to investigate the short‐term relationship between tropical heating and upper tropospheric subtropical westerly maxima in the summer hemisphere. Emphasis is focused on a 15‐day forecast of the circulation corresponding to 6‐20 January 1979. During this time, observational data from the FGGE suggested that strong convection over the South Pacific was helping to maintain a nearby subtropical jet. the dominant term in the zonal momentum budget was the Coriolis force applied to the diabatically driven meridional circulation, although the mean advection of zonal momentum and transient eddy momentum and sensible‐heat transports made important contributions. A ‘full‐physics’ forecast made with the GCM is able to simulate these processes to a reasonable degree of accuracy.To examine the impact of direct meridional overturning on the rotational flow in the South Pacific, the latent energy supplied to the model atmosphere is reduced in two ways. First, negative sea‐surface‐temperature anomalies are imposed in the area of maximum convection. Second, temperature changes resulting from precipitation processes are ignored in the model thermodynamic equation. Results from both types of experiments are consistent, and they substantiate our earlier observational findings that, at least for this case study, summertime subtropical westerly maxima are largely forced by divergent outflow from the tropics.
An Analysis of Simulated Summer Blocking Episodes
Authors: Kung Ernest C. , Min Wei , Susskind Joel , Park Chung‐Kyu 
Published: February 2007

AbstractUsing the high‐resolution 2 × 2.5 degree version of the Goddard Laboratory for Atmospheres general circulation model, experiments were performed for a 46‐day summer period from 1 July to 15 August 1979. During the simulation, sea‐surface‐temperature (s.s.t.) anomalies were updated daily with observed s.s.t.s to provide a realistic surface heating field. A parallel control run used climatology s.s.t.s. in lieu of observed s.s.t.s.Both the s.s.t. updating experiment and control run were able to simulate the persistent blocking patterns in Eurasia and the eastern Pacific during July. However, only the s.s.t. updating experiment was able to generate the realistic blocking episodes in the last 10‐day period during the 46‐day simulation. Forecast skill was examined in terms of 500 mb anomaly correlation and error kinetic energy. When the blocking patterns develop or become reinforced, a high forecast skill is apparent in nearly half of the northern hemisphere including the area of blocking and its downstream region. Unlike the major winter blockings, which may be treated as the manifestation of wave number one and two, the summer blockings are localized, and have a smaller scale than that of winter blockings. However, summer blockings treated in this study are persistent, and possess a considerable predictability.