bluuebunny/crossref_metadata_2025 huggingface dataset:
Developments of a targeted ensemble prediction system
Authors: Hersbach H. , Mureau R. , Opsteegh J. D. , Barkmeijer J.
Published: June 2003

Based on results presented in a previous paper, an Ensemble Prediction System, especially designed for the short‐range to early‐medium range for the European domain (TEPS), was further developed. Between 1 January 1999 and 27 February 2000 this system was run for two cases per week. Results were compared to the ensemble prediction system (EPS) that was operational at the European Centre for Medium‐Range Weather Forecasts at the time. The focus was on weather parameters. Several configurations were tested. Besides perturbations that optimize total error growth for the European domain at 48 h, perturbations relevant for shorter forecast times were included as well. Firstly, in accordance with EPS, evolved northern hemispheric singular vectors (ETEPS) and secondly, targeted singular vectors with an optimization time of 12 h (STEPS) were added. Compared to EPS, ensemble spread was increased in the short range, having a beneficial effect on statistical properties such as reliability and the frequency of outliers. Spread–skill correlations improved as well. Best performance was found for the STEPS configuration. The impact on Brier scores, relative operating characteristic curves and cost‐loss analyses was less clear. From the singular vector point of view, the positive impact of using targeted perturbations is evident. However, after integration (STEPS or ETEPS), it was difficult to find synoptic cases in which this clear impact is still present. Copyright © 2003 Royal Meteorological Society
Sensitivity of extratropical cyclone mesoscale structure to the parametrization of ice microphysical processes
Authors: Forbes Richard M. , Clark Peter A.
Published: July 2003

The ability of numerical weather prediction models to represent and forecast mesoscale structure accurately in extratropical cyclones depends on the resolution and dynamical formulation of the model and the way in which the subgrid‐scale processes are parametrized. This study investigates the sensitivity of extratropical cyclone forecasts to aspects of the microphysical parametrization in a mesoscale version of the Met Office Unified Model. Case‐studies of rapidly developing cyclones over the North Atlantic Ocean from the Fronts and Atlantic Storm‐Track Experiment period are used to study the forecast sensitivity to the ice sublimation and deposition rate, and the ice terminal fall speed. Modifying these parameters not only changes the balance between ice content, liquid‐water content and water vapour in the model but also affects the mesoscale dynamics of the frontal rain bands associated with the cyclone. In particular, the cooling due to sublimating ice beneath slantwise ascending frontal updraughts has a significant effect on the dynamics of the front and on the development of further post‐frontal rain bands. The study highlights the need for a microphysical parametrization that produces an appropriate cooling profile from sublimating ice particles for the forecasting of mesoscale structure in extratropical cyclones. © Crown copyright, 2003. Royal Meteorological Society
The influence of PV inversion on polar‐vortex dynamics and passive‐tracer simulations in atmosphere‐like regimes
Authors: Methven John
Published: July 2003

Both the stratospheric polar‐vortex edge and the midlatitude tropopause are characterized by strong potential vorticity (PV) gradients on isentropic surfaces, a jet and persistent wave activity. Passive tracers in these regions also develop strong gradients, where contours of mixing ratio lie almost parallel to PV contours. Both sets of contours stretch and fold producing narrow filaments. This behaviour is associated with time‐dependent large‐scale straining flow. The dominance of the large spatial scales in velocity on the tracer evolution depends upon the smoothing effects of PV inversion.

A quasigeostrophic shallow‐water model is used to investigate regimes under which a polar vortex with a sharp PV gradient and persistent wave activity can be maintained. In all experiments waves grow on a zonal jet in a Rossby wave critical layer forced by orography. A minimal set of parameters is varied: the Rossby radius of deformation L, orographic amplitude Zt, radiative relaxation time‐scale τr and diffusion time‐scale τd. Although the quasi‐geostrophic approximation is not strictly valid globally, it is used here because the PV inversion operator depends on a single parameter, L. Finite amplitude wave‐mean flow theory is used to diagnose a zonally symmetric background state and wave activity that can evolve only through non‐conservative processes (which act on slower time‐scales than advection). For given τr and τd, a curved band in (L, Zt) parameter space can be identified with an ‘atmosphere‐like regime’ where both the background jet speed and wave activity are greater in the turbulent late‐time near‐equilibrium state than during the initial turnover of the critical layer. The ‘tracer scale factor’ (the ratio of the smallest resolved velocity‐scale to the narrowest trustworthy tracer scales from contour advection) is shown to fall as L is decreased across the atmosphere‐like regime, demonstrating that passive tracers become more difficult to simulate accurately as the smoothing effect of PV inversion lessens. Copyright © 2003 Royal Meteorological Society
A 4D‐Var re‐analysis of FASTEX
Authors: Desroziers G'erald , Hello Gwena'elle , Thépaut Jean‐Noël
Published: July 2003

A complete re‐analysis of the Fronts and Atlantic Storm‐Track EXperiment (5 January–27 February 1997) is performed. It provides a reference description of the life cycle of an ensemble of cyclogeneses over the north Atlantic. Many aspects make this re‐analysis noteworthy. In particular, the FASTEX period is characterized by an exceptional increase in the number of dropsoundings or radiosoundings concentrated on the storm track. Another striking point of the re‐analysis is the use of a four‐dimensional variational (4D‐Var) assimilation scheme on such a long period associated with the French Arpège (Action des Recherche Petite Echelle et Grand Echelle) global stretched model, that allows a locally high resolution in the area where dense networks of dropsondes were deployed. A twin re‐analysis with the same characteristics but without any FASTEX data is also performed. It shows that the impact of the special FASTEX data is quite noticeable in the resulting analyses and is lower, but still perceptible, in the associated short‐range forecasts (6 h–36 h). The impact of FASTEX observations on a particular FASTEX case is also shown. Copyright © 2003 Royal Meteorological Society
Large‐amplitude nonlinear stability results for atmospheric circulations
Authors: Cullen M. J. P. , Douglas R. J.
Published: June 2003

It is shown how large‐amplitude stability results for flows governed by potential‐vorticity conservation can be obtained by geometric arguments using rearrangements of functions. The method allows for non‐smooth solutions, and also gives a framework for the rigorous treatment of the effects of mixing by increasingly fine‐scale filamentation. It is, thus, different from the energy‐Casimir method. It is applied to the semi‐geostrophic shear‐flow problem in a channel, where the results can be compared with other methods. It is shown that, under the definitions used, there are no non‐zonal nonlinearly‐stable states in this problem. In the baroclinic case, it is shown how potential‐vorticity conservation reduces the ‘available energy’ for the transient flow. Copyright © 2003 Royal Meteorological Society
A simple model of the hurricane boundary layer
Authors: Smith Roger K.
Published: July 2003

A simple, steady, moist, axisymmetric, constant‐depth, slab model for the hurricane boundary layer is investigated. High‐resolution solutions of the boundary‐layer equations are obtained by integrating inwards from some large radius, at which it is assumed that geostrophic balance and convective–radiative balance exist. In all the solutions obtained, the tangential wind speed in the boundary layer approaches that above the boundary layer in the inner‐core region and the maximum wind speed in the boundary layer is comparable with, or even marginally higher than that above. A new feature of one of the solutions described is the existence of spatial oscillations in vertical velocity at the top of the boundary layer, inside the radius of maximum tangential wind speed. These oscillations may be interpreted as frictionally damped inertial waves. They are accompanied by annular regions in which the tangential flow alternates between supergradient and subgradient. The existence of boundary‐layer‐induced oscillations in vertical velocity in reality would have implications for the organization of convection in the core region of a hurricane. It is shown that an approximation to determine the radial flow in the boundary layer suggested by Willoughby overestimates the vertical motion at the top of the boundary layer by a factor of about two, but the analysis leads us to question the utility of the approximation.

We investigate also the thermodynamic structure of the boundary layer and the radial distribution of surface fluxes for vortices with the same maximum tangential wind speed above the boundary layer and the same radius of maximum wind (RMW), but having different widths. It is found that the equivalent potential temperature (θe) in the boundary layer continues to increase with decreasing radius inside the RMW. Moreover, the negative radial gradient of θe in the inner‐core region, which is related to that of virtual temperature above the boundary layer in the eyewall region, is relatively insensitive to the vortex width, but the maximum values of θe increase with the width. The strength and radial distribution of the latent‐heat flux is insensitive to the vortex width in the inner‐core region, but varies markedly with width in the outer part of the vortex. Realistic radial distribution of relative humidity are obtained only when shallow convection is represented in the model. The inclusion of dissipative heating in the thermodynamic equation leads to an increase in θe of the order of 1.5 K in the inner‐core region of the vortex and to a reduction in the boundary‐layer relative humidity of 5%. Copyright © 2003 Royal Meteorological Society
The vorticity method: Extension to mesoscale vertical velocity and validation for tropical storms
Authors: Lee J. L. , Kuo Y.‐H. , Macdonald A. E.
Published: July 2003

The vorticity method infers vertical velocity from vorticity variations in space and in time based on the vorticity equation. The viability of the large‐scale vorticity method has been demonstrated in previous studies based on the quasi‐geostrophic vorticity equation. However, the extension of the vorticity method to the mesoscale vorticity equation, including the vertical stretching and tilting terms, is hindered by inseparable variables when formulated in Cartesian coordinates. In this study, the separation of variables for the mesoscale vorticity method is achieved by formulating the mesoscale vorticity equation in Lagrangian coordinates in terms of the characteristic line composed of three nonlinear ordinary differential equations. A numerical method based on backward integration is also presented to solve the vertical velocity along the characteristic of the mesoscale vorticity equation. Observing‐system simulation experiments were undertaken to quantify both the numerical accuracy of the vorticity method, and the sensitivities of the kinematic and vorticity methods to errors present in horizontal wind fields. With perfect horizontal winds, the vertical velocities derived from the kinematic and vorticity methods are identical to the degree of accuracy governed by the truncational errors. When error perturbations are included, the vertical motions derived from the vorticity method are less susceptible to errors in horizontal winds than the kinematic method. This conclusion is consistent with the analytical analysis and numerical results with the large‐scale vorticity method shown in previous studies. The mesoscale vorticity method with the tilting term is shown to be more accurate than the large‐scale vorticity method in deriving vertical velocities in tropical storms. Numerical experiments demonstrate that the mesoscale vorticity method can benefit from high‐temporal‐resolution data observed by Doppler radars to infer the vertical velocity in tropical storms, where the inner circulation is dominated by the rotational winds. Copyright © 2003 Royal Meteorological Society
Considerations on minimum friction velocity
Authors: Akylas E. , Tsakos Y. , Tombrou M. , Lalas D. P.
Published: June 2003

The concept of the minimum friction velocity is studied using three different methods of scalar averaging for the calculation of the stresses. Particular emphasis is given to the extraction of the influence of a non‐zero ambient wind shear observed in field measurements. Data from three different experimental sites in Athens with high roughness values are analysed in order to provide information concerning the dependence of the dimensionless minimum friction velocity on the dimensionless roughness length. Data from the BOREX‐95 experiment have also been re‐analysed according to the methodologies presented in this study. The results are compared to the large‐eddy simulations that are considered to be a reference study on shear‐free convection. Copyright © 2003 Royal Meteorological Society
Forecast 4D‐Var: Exploiting Model Output Statistics
Authors: Hansen James A. , Emanuel Kerry A.
Published: July 2003

Forecasts of certain weather elements are improved by linearly relating observed elements to past observations, climatological information, and numerical weather‐prediction‐model output. Model Output Statistics (MOS) is a statistical post‐processing of model output that is capable of forecasting some subgrid scale, synoptically‐forced events and of correcting some systematic, but state dependent, model bias. This tendency of accounting for model error is exploited by feeding MOS forecasts back into the state estimation problem. MOS forecasts and their associated uncertainty are treated as ‘observations’ of the future system state, and a four‐dimensional variational assimilation procedure is employed to improve the original analysis and resulting model forecast. In a simple‐model scenario, it is found that this approach has a small negative impact on the magnitude of forecast errors relative to MOS with a decrease in the number of very good forecasts, but a large positive impact on the variance about the forecast errors: forecast busts are reduced. As a further step, a second round of MOS is performed on the new model forecasts in a manner identical to the original MOS approach. This second application of MOS results in a significant reduction in both the forecast errors and the variance about those errors relative to the first application of MOS, and includes an increase in very good forecasts. Copyright © 2003 Royal Meteorological Society
Initial stages in the morphological evolution of vapour‐grown ice crystals: A laboratory investigation
Authors: Bacon Neil J. , Baker Marcia B. , Swanson Brian D.
Published: June 2003

We describe experiments to investigate the first stages in the evolution of small (100–200 µm) ice crystals levitated in air at temperatures and humidities characteristic of fully glaciated stratiform clouds. We find that in these conditions particle morphology is not uniquely determined by environmental temperature and humidity as has commonly been assumed; other parameters, such as the mode of ice initiation, appear to have important effects on particle shape. Crystals grown from frozen droplets at vapour excesses of less than about 0.05 g m−3 adopt isometric compact habits at temperatures above −22°C, while polycrystals dominate at lower temperatures. In contrast, crystals grown under similar conditions from frost ‘seeds’ usually develop into pristine hexagonal prisms with varying aspect ratios. An increasing fraction of frozen droplets develop into ‘florid’ crystals, containing thin side‐planes, at higher supersaturations. This mix of particle types is similar to that found in field studies of cold clouds. The change in morphology from compact hexagon to ‘florid’ with increasing growth rate defines a facet instability transition, from which we infer a lower limit for the critical supersaturation for layer nucleation on the prism facet to be 2.4% at −15°C and 3.1% at −25°C. We develop a simple analytic model for the size dependence of facet stability and show its predictions compare well with the data. Copyright © 2003 Royal Meteorological Society
Modelling the atmospheric response to doubled CO2 and depleted stratospheric ozone using a stratosphere‐resolving coupled GCM
Authors: Gillett N. P. , Allen M. R. , Williams K. D.
Published: July 2003

We investigate the atmospheric response to doubled CO2 and stratospheric ozone depletion in three versions of a general‐circulation model with differing vertical resolution and upper‐boundary heights. We find that an approximate doubling of the vertical resolution below 10 hPa reduces the temperature response to a doubling of CO2 from 3.4 K to 2.5 K. Much of this difference is associated with changes in the cloud response. All model versions show an increase in the Arctic Oscillation index in response to a doubling of CO2, but the increase is no larger in the model with an upper boundary at 0.01 hPa than in the standard model with a top level at 5 hPa. All models also show general stratospheric cooling in response to doubling CO2. However, unlike some other authors, we find no cooling in the Arctic winter vortex below around 10 hPa in the stratosphere‐resolving model, and a weakening of the zonal winds throughout this region. This effect is due to enhanced upward propagation of planetary waves from the troposphere, and is an effect found only in the northern hemisphere, probably because of its larger zonal asymmetries. All models show a small but significant surface cooling in response to a reconstruction of 1998 stratospheric ozone depletion, and an increase in the Antarctic Oscillation index in the southern summer. The cooling extends through most of the atmosphere, and reaches a maximum in the region of the Antarctic ozone hole in November and December. © Royal Meteorological Society, 2003. K. D. Williams's contribution is Crown copyright.
Analysis of semi‐Lagrangian trajectory computations
Authors: Staniforth Andrew , White Andy , Wood Nigel
Published: June 2003

An important aspect of semi‐Lagrangian models is the determination of departure points. This involves the calculation of particle displacements and is done by integrating a discrete form of the trajectory, or kinematic, equation. The way this equation is discretized affects the stability and accuracy of the equation set. Additionally, recent work in the context of a time‐centred, semi‐implicit, semi‐Lagrangian scheme has shown that the form of the discrete kinematic equation reflects the preservation or violation of a dynamical equivalence between momentum and angular momentum formulations. Here that work is extended to the case of off‐centred semi‐implicit time weighting as used in most semi‐implicit, semi‐Lagrangian models. Two types of discretization of the trajectory equation are identified which preserve dynamical equivalence: some existing discretizations are found to be approximations to them. It is also shown how to incorporate physical forcings and/or predictor–corrector dynamics into the formulation. Two simple model problems (for solid‐body rotation and wave motion) are used to provide some further insight into the accuracy and stability properties of various trajectory schemes, including: dynamically‐equivalent schemes, approximations to these, and several existing schemes. Copyright © Crown copyright, 2003. Royal Meteorological Society
On the use of a predictor–corrector scheme to couple the dynamics with the physical parametrizations in the ECMWF model
Authors: Cullen M. J. P. , Salmond D. J.
Published: July 2003

Methods of coupling the time integration of the resolved dynamics with the parametrized processes in atmospheric models are an active development area. Many centres have demonstrated strong sensitivity to variations in the methods of coupling in their models. The European Centre for Medium‐Range Weather Forecasts has recently introduced a revised method of coupling which gives significant forecast benefits. Theoretically, optimal methods are difficult to establish because of the mixture of timescales represented within the parametrizations. Implicit methods are well‐suited to achieving coupling between different processes, but are not practical because of the nonlinear switching present in most parametrization schemes. In this paper we show that a predictor–corrector scheme can give some of the advantages of a fully‐implicit scheme. We show that the use of more than one physics evaluation per time step significantly improves the accuracy in a model problem. We also demonstrate the effect of further iterations which, in principle, would converge towards a fully‐implicit scheme. A second iteration has only a small effect on overall performance, but gives a large reduction in the amount of convection. This indicates that the current formulation of convection is not compatible with this type of integration scheme. Copyright © 2003 Royal Meteorological Society
The influence of spatial variability of boundary‐layer moisture on tropical continental squall lines
Authors: Clark D. B. , Taylor C. M. , Thorpe A. J. , Harding R. J. , Nicholls M. E.
Published: July 2003

Previous work has described how rainfall from several storms showed persistent patterns with a length‐scale of the order of 10–20 km, and suggested that a land–atmosphere feedback was responsible. As a first step in examining this mechanism, a two‐dimensional model is used to simulate the response of a squall line to idealized variability of boundary‐layer moisture. Ensembles of simulations are used so as to better account for the stochastic element of convection. Extra boundary‐layer humidity results in increased rainfall slightly downshear of the location of the anomalous humidity. The rainfall increase tended to be largest at the upshear edge of the rainfall anomaly. This was explained in terms of the horizontal gradient of humidity resulting in the growth of an especially intense cell just within the patch, at the expense of a weaker cell immediately before the patch. The largest rainfall increases occur when the humidity anomaly is some 10 km or so long, which is consistent with this length‐scale matching the scale of the convective cells, so that a single cell prospered at the expense of two adjacent cells, one each on the upshear and downshear sides. A humidity anomaly resulted in greater spatial variability of rainfall, which was consistent with the feedback mechanism previously proposed to explain rainfall persistence at convective length‐scales. Copyright © 2003 Royal Meteorological Society
Improvements to the representation of orography in the Met Office Unified Model
Authors: Webster S. , Brown A. R. , Cameron D. R. , Jones C. P.
Published: June 2003

Three improvements to the representation of orography for use in numerical weather‐ and climate‐prediction models are presented. The first improvement is to replace the US Navy dataset with a new digitally generated dataset as the definition of the true earth topography. There are large differences on all scales between the two datasets and these lead to large differences in the mean and subgrid‐scale fields that are derived from them. The second improvement is to filter the mean and subgrid‐scale orography (SSO) fields to remove grid‐scale and near‐grid‐scale features and thus suppress forcing on scales that the model cannot treat well. The third improvement is to implement a new, simple parametrization of the effects of SSO in which the total surface pressure drag is calculated using the analytical expression for linear hydrostatic flow over a two‐dimensional ridge in the absence of friction and rotation. The surface pressure drag is partitioned into gravity‐wave and blocked‐flow components that depend on the Froude number of the flow impinging on the SSO. The new scheme attributes about 70% of the total drag to flow blocking.

These improvements have been incorporated into a new version of the Met Office Unified Model. A series of numerical weather‐prediction experiments demonstrates that the introduction of the new SSO scheme is the most significant change. In particular, significant improvements to forecast skill, attributable to the SSO scheme's flow‐blocking drag component, are found at low levels in the northern hemisphere and the Tropics for an extended northern hemisphere wintertime forecast trial. Furthermore, there are no significant degradations in skill at upper levels, in the southern hemisphere or for summertime trials. Copyright © Crown copyright, 2003. Royal Meteorological Society
An experiment to determine atmospheric CO concentrations of tropical South Atlantic air samples
Authors: Kirchhoff V. W. J. H. , Aires C. B. , Alvala P. C.
Published: June 2003

New observations of atmospheric carbon monoxide, CO, are described, from tropical South Atlantic air samples. A new observational site, Maxaranguape, was set up in a clean remote environment right next to the ocean on the north‐east coast of Brazil, to obtain CO mixing ratios and auxiliary data (meteorological parameters, ozone (O3), carbon dioxide (CO2) and methane (CH4)) during three sequential seasonal cycles. The seasonal variations of temperature, humidity and precipitation are shown for the new site. Chromatographic separation followed by mercury oxide detection is used to measure CO. The seasonality of the CO data was clearly established. Minima are seen during April, May and June showing wet‐period averages of 56.1 parts per billion by volume (ppbv), with standard deviation 8.7 ppbv; during dry‐period months, August to November, the average was 77.7±16.5 ppbv. For comparison, CO concentrations were also measured over continental areas in Brazil. Much larger values have been found in moderate ‘burning’ regions, such as the south of the state of Mato Grosso and the north‐western part of the state of Parana, where 200 ppbv in the dry season has been observed. Since normally the air masses have travelled for several days over the ocean, the air masses over the site present low chemical activity. Daily variations of CO2 are very small, of the order of a few percent relative to the diurnal mean. Only on rare occasions, when the wind direction changes, is the sampled air contaminated from flowing over the inhabited shoreline to the south, and then CO2 varies inversely with O3. The monthly mean CH4 data does not show a clear seasonal variation, possibly because the amplitude of the CH4 variation is only of the order of 1%, which is close to the precision of the measuring instrument. Copyright © 2003 Royal Meteorological Society
The deep‐atmosphere Euler equations with a mass‐based vertical coordinate
Authors: Wood Nigel , Staniforth Andrew
Published: July 2003

The terrain‐following hydrostatic pressure‐coordinate system for the shallow‐atmosphere Euler equations is generalized for deep atmospheres using a mass‐based vertical coordinate. A consequent benefit is that an existing (shallow atmosphere) hydrostatic primitive‐equations model, which uses a pressure‐based terrain‐following vertical coordinate, could be modified for non‐hydrostatic deep‐atmosphere applications, without the need to change the scientific and computing infrastructure substantially in which it is embedded. © Crown copyright, 2003. Royal Meteorological Society
Stratospheric temperatures in Antarctic winter: does the 40‐year record confirm midlatitude trends in stratospheric water vapour?
Authors: Roscoe H. K. , Colwell S. R. , Shanklin J. D.
Published: June 2003

Water vapour is a potent greenhouse gas, and the observed increases in water vapour in the stratosphere act to cool it. Possible changes in stratospheric temperatures are important for future ozone loss because colder temperatures in the edge region of the Antarctic ozone hole act to increase polar stratospheric clouds there, and so delay recovery of the ozone hole. Trends in lower‐stratospheric temperature within the core of the Antarctic vortex in winter should be a unique indicator of trends in stratospheric water vapour, because neither changes in CO2 nor in ozone have a large effect on temperature in the lower stratosphere in the dark. Here, measured stratospheric temperatures southward of 70°S in winter are presented and their quality and corrections discussed. The character and magnitude of the long‐term changes at Halley (76°S) are similar from 100 to 70 hPa and at 50 hPa, whether corrected for sonde changes or not, and are also similar to those at other Antarctic sites. We found no significant trend in temperatures at Halley between 1960 and 2000, which is inconsistent with the change calculated from the trend in lower‐stratospheric water vapour in northern hemisphere midlatitudes between 1960 and 2000. Over the shorter interval between 1980 and 2000 at Halley, the change in temperature was −1.8±0.6 K, in agreement with the change calculated from the trend in stratospheric water vapour in northern hemisphere midlatitudes between 1980 and 2000. The differences between these periods are discussed in terms of: possible fortuitous agreement between 1980 and 2000; the poorer representation and quality of the measurements of stratospheric water vapour between 1960 and 1980; and a possible large variation in the rate of oxidation of CH4 to H2O in the upper stratosphere between 1960 and 1980. Such a variation in oxidation rate was observed by satellite between 1992 and 1999. Copyright © 2003 Royal Meteorological Society
Simulation of satellite channel radiances in the Met Office Unified Model
Authors: Ringer M. A. , Edwards J. M. , Slingo A.
Published: July 2003

A system is described which allows satellite channel radiances to be simulated within both the numerical weather prediction and climate versions of the Met Office Unified Model. It is based on a spherical harmonics version of the radiation code that calculates radiative fluxes and heating rates in the climate model and it uses the same pre‐processing software and molecular absorption database. In the results shown here, fractional cloud cover is represented by decomposition into separate columns. The methodology is described and results of off‐line tests of the code against high spectral resolution calculations are presented. Examples of infrared window and water vapour channel brightness temperatures simulated using the global forecast model are then compared with Meteosat 7 imagery. The utility of radiance simulations for model validation studies is illustrated by these comparisons, and other applications are also discussed. © Crown copyright, 2003. Royal Meteorological Society
Dependence of aqua‐planet simulations on time step
Authors: Williamson David L. , Olson Jerry G.
Published: June 2003

Aqua‐planet simulations with Eulerian and semi‐Lagrangian dynamical cores coupled to the NCAR CCM3 parametrization suite produce very different zonal average precipitation patterns. The model with the Eulerian core forms a narrow single precipitation peak centred on the sea surface temperature (SST) maximum. The one with the semi‐Lagrangian core forms a broad structure often with a double peak straddling the SST maximum with a precipitation minimum centred on the SST maximum. The different structure is shown to be caused primarily by the different time step adopted by each core and its effect on the parametrizations rather than by different truncation errors introduced by the dynamical cores themselves. With a longer discrete time step, the surface exchange parametrization deposits more moisture in the atmosphere in a single time step, resulting in convection being initiated farther from the equator, closer to the maximum source. Different diffusive smoothing associated with different spectral resolutions is a secondary effect influencing the strength of the double structure. When the semi‐Lagrangian core is configured to match the Eulerian with the same time step, a three‐time‐level formulation and same spectral truncation it produces precipitation fields similar to those from the Eulerian.

It is argued that the broad and double structure forms in this model with the longer time step because more water is put into the atmosphere over a longer discrete time step, the evaporation rate being the same. The additional water vapour in the region of equatorial moisture convergence results in more convective available potential energy farther from the equator which allows convection to initiate farther from the equator. The resulting heating drives upward vertical motion and low‐level convergence away from the equator, resulting in much weaker upward motion at the equator. The feedback between the convective heating and dynamics reduces the instability at the equator and decreases the convection there. The behaviour of the parametrizations depends on the amount of water inserted into the troposphere during a discrete time step. This is as important as the rate of insertion. Experiments are described that support this explanation. Copyright © 2003 Royal Meteorological Society
The behaviour of large‐scale model cloud schemes under idealized forcing scenarios
Authors: Wilson Damian , Gregory David
Published: July 2003

The behaviour of widely used large‐scale cloud schemes is assessed in the idealized framework of a single grid box forced by uniform warming or cooling. The general advantages and disadvantages of using diagnostic and prognostic schemes are compared, as are characteristics of observational relationships and specific parametrization schemes used in general‐circulation models. It is noted that all parametrization schemes fail to produce, in some way, the correct behaviour when the grid box is forced and initial conditions altered. The reasons for this fundamental result are discussed. Closures for prognostic cloud schemes under uniform forcing are investigated, and a closure developed which closely meets the theoretically ideal behaviour. © Crown copyright, 2003. Royal Meteorological Society
A comparison of two methods for developing the linearization of a shallow‐water model
Authors: Lawless A. S. , Nichols N. K. , Ballard S. P.
Published: July 2003

We develop the linearization of a semi‐implicit semi‐Lagrangian model of the one‐dimensional shallow‐water equations using two different methods. The usual tangent linear model, formed by linearizing the discrete nonlinear model, is compared with a model formed by first linearizing the continuous nonlinear equations and then discretizing. Both models are shown to perform equally well for finite perturbations. However, the asymptotic behaviour of the two models differs as the perturbation size is reduced. This leads to difficulties in showing that the models are correctly coded using the standard tests. To overcome this difficulty we propose a new method for testing linear models, which we demonstrate both theoretically and numerically. © Crown copyright, 2003. Royal Meteorological Society
The dynamics of NAO teleconnection pattern growth and decay
Authors: Feldstein Steven B.
Published: July 2003

This investigation performs both diagnostic analyses with NCEP/NCAR re‐analysis data and forced, barotropic model calculations to examine the dynamical mechanisms associated with the growth and decay of the North Atlantic Oscillation (NAO) teleconnection pattern. The diagnostic calculations include projection and composite analyses of each term in the stream‐function‐tendency equation.

The results of the analyses reveal a complete life cycle of growth and decay within approximately two weeks. The positive NAO phase is found to develop after anomalous wavetrain propagation across the North Pacific to the east coast of North America. This contrasts with the negative NAO phase which appeared to develop in situ.

Both high‐frequency (period <10 days) and low‐frequency (period >10 days) transient eddy fluxes drive the NAO growth. After the NAO anomaly attains its maximum amplitude, the high‐frequency transient eddy fluxes continue to drive the NAO anomaly in a manner that is consistent with a positive feedback process. The decay of the NAO occurs through both the divergence term and the low‐frequency transient eddy fluxes. The temporal and spatial properties of the divergence term are found to be consistent with Ekman pumping. These results illustrate many important differences between the NAO and Pacific/North American (PNA) teleconnection patterns, perhaps most striking being that the NAO life cycle is dominated by nonlinear processes, whereas the PNA evolution is primarily linear, In addition, the relation between the NAO and the zonal index is discussed. Copyright © 2003 Royal Meteorological Society
Effects of vertical differencing in a minimal hurricane model
Authors: Zhu Hongyan , Smith Roger K.
Published: July 2003

The minimal three‐layer hurricane model developed by the authors is reformulated with a Charney–Phillips grid (CP‐grid) in the vertical instead of the Lorenz grid (L‐grid) that was used in the original model. It has been shown by others that the popular L‐grid supports a computational mode in the temperature field that can lead to inaccuracies, especially when moist processes are involved. Here we compare calculations of the hurricane model using the L‐grid and CP‐grid. We present evidence that the computational mode is excited in the L‐grid model during the period of rapid vortex intensification, which begins when grid‐scale latent‐heat release occurs in the core region. Thus it would appear that the solution in the mature stage of evolution is contaminated by this mode. We show also that the vortex asymmetries that develop during the mature stage are sensitive to the choice of vertical grid and argue that those that occur in the CP‐grid formulation are more realistic. It is possible that the computational mode is a spurious feature of many earlier studies of asymmetries in hurricane models in which moist processes are represented. Copyright © 2003 Royal Meteorological Society
Flow regimes in the winter stratosphere of the northern hemisphere
Authors: Gray L. J. , Sparrow S. , Juckes M. , O'neill A. , Andrews D. G.
Published: July 2003

A stratosphere–mesosphere model is used to investigate the response of the northern‐hemisphere stratospheric flow regime to changes in tropospheric wave forcing and equatorial wind direction. An ensemble approach is employed to assess changes in variability. The flow regime of the modelled atmosphere is broadly determined by the strength of the imposed tropospheric forcing. However, there exists a region of parameter space, where the tropospheric forcing is intermediate, in which the tropospheric control of the stratospheric response is substantially weakened. Under these circumstances the ambient background flow of the stratosphere into which the tropospheric planetary waves propagate is important. We define three flow regimes based upon the likelihood of stratospheric warming events, which may have a correspondence in the real atmosphere. With low tropospheric forcing major warming events never occur. With strong tropospheric forcing major warmings always occur. With intermediate forcing, which we suggest may be the case for most of the time in the northern hemisphere, there is reduced tropospheric control, greater uncertainty in the stratospheric response and an increased window of opportunity for influence from other factors such as early‐winter initial conditions, the Quasi‐Biennial Oscillation (QBO) and possibly the solar cycle. We emphasize the importance of using Eliassen–Palm (EP) flux diagnostics not only to examine the extent of ‘wave activity’ but also to examine where the interaction of the waves with the mean flow is strongest. The latter suggests that flow modification in the upper stratosphere as the Aleutian high is developing is a key aspect of the early stages of the stratospheric warming. The model shows strong evidence that the flow in the mid and upper stratosphere modifies the EP fluxes at the lower boundary and that the wave fluxes through the lower boundary should be considered as part of the response to forcing rather than as the forcing per se. We also highlight the role of travelling anticyclones in the build up of the Aleutian high, the subsequent sudden stratospheric warming and, hence, the modelled variability. These anticyclones are features of the subtropical upper stratosphere with maximum amplitude at the stratopause. Warm disturbed winters are associated with travelling anticyclones that have larger amplitude than average and are more effective in penetrating polewards. Copyright © 2003 Royal Meteorological Society
Layer‐wise attribution of vertical motion and the influence of potential‐vorticity anomalies on synoptic development
Authors: Dixon M. A. G. , Thorpe A. J. , Browning K. A.
Published: June 2003

A methodology is presented for attributing vertical motion to potential vorticity (PV) in a explicit manner using the quasi‐geostrophic omega equation. The methodology is then used to determine the contribution individual PV anomalies make to the synoptic development. It is shown how the total vertical motion may be decomposed into self‐generating and interaction terms. The former correspond to vertical motion that arises from one PV region alone, whilst each of the interaction terms represent vertical motion due to the interaction of two different regions of PV. The method is applied to a number of cyclones studied in the Fronts and Atlantic Storm‐Tracks Experiment in order to determine the dynamical role of individual PV regions in cyclogenesis. For purposes of comparison, cyclones were chosen to be representative of the types (A, B and C) of an extended Petterssen–Smebye scheme devised by Deveson, Browning and Hewson. In terms of our new PV method, the Type A and B cyclones exhibit only subtle differences, with the upper‐level layer of PV playing a larger role in the case of the Type B cyclone. A much larger difference is apparent between the Type A and B cyclones on the one hand, and the Type C cyclone on the other: in the Type C cyclone, the upper‐level PV self‐generating term plays an important role, whereas in the Type A and B cases the influence of the upper‐level PV is largely restricted to the interaction terms. In all of the cases, the vertical motion attributable to the interaction between the low‐level PV and boundary thermal gradient was found to be large. Hence, from the viewpoint of instantaneous attribution, we conclude that the low‐level PV is always important. Copyright © 2003 Royal Meteorological Society
Small‐scale rainfall mechanisms for an idealized convective southerly flow over the Alps
Authors: Gheusi François , Stein Joel
Published: June 2003

Rain‐producing flows over the south side of the Alps are investigated with an idealized high‐resolution (2.5 km) numerical configuration where only three basic ingredients of typical heavy‐precipitation situations have been retained: (1) a uniform and stationary southerly flow far upstream from the Alps, which drives (2) a conditionally unstable atmosphere towards (3) the real topography of the Alps and the surrounding secondary mountains. Attention is given to small‐scale mechanisms of rainfall over the Po Plain and the Alpine slopes. These are linked with two orographically induced low‐level wind jets: the easterly barrier‐wind jet along the southern flank of the Alps and a south‐easterly jet formed over the sea along the south‐western flank of the Apennines and continuing over land up to the Alps. The specific role of the secondary relief upstream from the Alps in inducing or influencing these low‐level flow features, and also in triggering organized convection, is studied by means of experiments on the sensitivity to topography. The idealized configuration also allows the advantages of a convection‐resolving model for modelling precipitation distributions at the Alpine valley scale to be considered. Copyright © 2007 Royal Meteorological Society
Dynamical equivalence and the departure‐point equation in semi‐Lagrangian numerical models
Authors: White A. A.
Published: July 2003

The kinematics and Newtonian dynamics of a continuous fluid may be presented in a number of ways. Given the kinematic relation between velocity and position, the dynamics may be formulated in terms of momentum or in terms of angular momentum and the scalar product of velocity and position. It is also possible to carry out the formulation in terms of momentum, angular momentum and the scalar product of velocity and position, in which case the kinematic relation becomes a deducible property. The three formulations are dynamically equivalent in the sense that the flow evolution is demonstrably the same in each. It seems desirable to build this elementary symmetry into discrete models. Two‐time‐level, semi‐Lagrangian models are considered here, with emphasis on the departure‐point equation (from which trajectory origins at the earlier time‐level are determined). Once a rule has been chosen for the approximation of trajectory time‐averages of vector and scalar products, dynamical equivalence implies a particular form of the departure‐point equation. One such rule delivers a doubly‐implicit equation that is a simple time‐centred discretization of the kinematic relation. A second rule leads to a form that differs only slightly from the singly‐implicit equation of Gospodinov and co‐workers. It is noted that dynamical equivalence relates to the consistency and non‐ambiguity of results rather than to their accuracy. © Crown copyright, 2003. Royal Meteorological Society
Benefits of increased resolution in the ECMWF ensemble system and comparison with poor‐man's ensembles
Authors: Buizza R. , Richardson D. S. , Palmer T. N.
Published: July 2003

In November 2000 the resolution of the forecast model in the operational European Centre for Medium‐Range Weather Forecasts Ensemble Prediction System was increased from a 120 km truncation scale (EPS) to an 80 km truncation scale (High‐resolution EPS or HEPS). The HEPS performance is compared with that of EPS and with different flavours of poor‐man's ensembles. Average results based on Brier skill scores and the potential economic value of probabilistic predictions for 57 winter and 30 summer cases indicate that the new HEPS system is about 12 hours more skilful than the old EPS. Averages over 39 winter cases indicate that HEPS forecasts perform better than five‐centre ensemble forecasts. Results also show that if forecasts are transformed into parametrized Gaussian distribution functions centred on the bias‐corrected ensemble mean and with re‐scaled standard deviation, HEPS‐based parametrized forecasts outperform all other configurations. Diagnostics based on parametrized forecast probabilities indicate that the different impact on the probabilistic or deterministic forecast skill is related to the fact that HEPS better represents the daily variation in the uncertainty of the atmosphere, and is not simply a reflection of improved mean bias or of a better level of spread. Copyright © 2003 Royal Meteorological Society