bluuebunny/crossref_metadata_2025_split huggingface dataset:
The influence of PV inversion on polar‐vortex dynamics and passive‐tracer simulations in atmosphere‐like regimes
Authors: Methven John 
Published: July 2003

AbstractBoth 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

AbstractA 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

AbstractIt 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

AbstractA 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

AbstractThe 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

AbstractThe 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

AbstractForecasts 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

AbstractWe 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

AbstractWe 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

AbstractAn 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

AbstractMethods 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

AbstractPrevious 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

AbstractThree 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

AbstractNew 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