bluuebunny/crossref_metadata_2025_split huggingface dataset:
Maintenance of the African easterly jet
Authors: Thorncroft C. D. , Blackburn M. 
Published: February 2007

AbstractThe maintenance of the African easterly jet (AEJ) has been examined using a zonally symmetric general circulation model with simple parametrizations. It is shown that the AEJ is maintained in association with two diabatically forced meridional circulations: one associated with surface fluxes and dry convection in the Saharan heat‐low region and one associated with deep moist convection in the intertropical convergence zone equatorward of this. the heat‐low heating, which reaches the height of the AEJ around 700 mb, is particularly important in maintaining the AEJ and its associated meridional gradients in potential vorticity. It is concluded that the mean observed AEJ results from a combination of the diabatically forced meridional circulations which maintain it and easterly waves which weaken it.
On the numerical modelling of middle atmosphere tides
Authors: Zhu Xun , Yee Jeng‐Hwa , Strobel Darrell F. , Wang Xueliang , Greenwald Raymond A. 
Published: February 2007

AbstractA linear, spectral, tidal model for middle atmosphere thermal tides has been developed. the spectral model is based on horizontal vorticity and divergence equations, and the vertical structure equation for the geopotential is solved with appropriate boundary conditions. Such an approach yields a stable temperature field and a consistent velocity field for the diurnal tide around the apparent singularity at latitudes of ±30°. Thermal forcing consists of solar near‐infrared heating by water vapour in the troposphere based on an accurate and efficient algorithm and the solar radiation heating by ozone and molecular oxygen in the stratosphere, mesosphere and lower thermosphere. the calculated diurnal and semidiurnal temperatures are in good agreement with the recent temperature measurements derived from the National Aeronautics and Space Administration Upper Atmosphere Research Satellite (UARS) and Light Detection and Ranging around the midlatitude and high‐latitude stratopause where ozone heating is a major source of tidal variability. Some discrepancies between the current model output and the wind measurements derived from the High‐Resolution Doppler Imager and Wind Imaging Interferometer on board UARS still exist. A sensitivity study shows the need for improved modelling of tropospheric heating rates and parametrization of eddy diffusion coefficients in the upper mesosphere in order to reduce these discrepancies.
Release of potential shearing instability in warm frontal zones
Authors: Chapman D. , Browning K. A. 
Published: February 2007

AbstractPotential shearing instability (PSHI) exists in the atmosphere when the Richardson number in a stably stratified shear layer would be less than 1/4 if the layer were lifted to saturation. Sometimes the air entering a warm frontal zone near the position of the surface warm front is initially unsaturated, its Richardson number is greater than 1/4 but PSHI exists over a deep layer. This paper presents a study of a warm frontal zone using observations from the high‐resolution Doppler radar at Chilbolton, supported by radiosondes and output from the mesoscale version of the operational United Kingdom Meteorological Office Unified Model. Very large‐amplitude Kelvin‐Helmholtz billows were observed to form where the PSHI was realised following the onset of saturation. the billows were probably intense enough to be a hazard for air traffic and an important source of momentum transfer. Rough estimates from the radar of vertical momentum fluxes due to the billows are shown to be broadly consistent with measurements by the same radar of the change in velocity profile across the frontal zone from start to end of the billow train. the present version of the Unified Model does not parametrize vertical mixing due to shearing instability in the free atmosphere; this may be an important omission.
The representation of soil moisture freezing and its impact on the stable boundary layer
Authors: Viterbo Pedro , Beljaars Anton , Mahfouf Jean‐François , Teixeira João 
Published: February 2007

AbstractThe 1993 to 1996 version of the European Centre for Medium‐Range Weather Forecasts model had a pronounced near‐surface cold bias in winter over continental areas. the problem is illustrated in detail with help of tower observations. It is shown that a positive feedback exists in the land surface boundary‐layer coupling that has the potential to amplify model biases. If the surface is cooled too much the boundary layer becomes too stable, reducing the downward heat flux and making the surface even colder. This positive feedback is believed to be stronger in the model than in the real atmosphere, resulting in diurnal temperature cycles that are too large and in excessive soil cooling on a seasonal time‐scale in winter.An important contributor to the excessive winter cooling turns out to be the lack of soil moisture freezing in the model. the importance of this process is obvious from soil temperature observations. the seasonal soil temperature evolution shows a clear ‘barrier’ at 0°C due to the thermal inertia of freezing and thawing. A more quantitative illustration is the result of a simple calculation. This shows that the amount of energy necessary to freeze/thaw 1 m3 of wet soil, would cool/warm this soil by about 50 K if the phase transition was not taken into account.To reduce the winter cold bias in the model, three model changes have been tested and are described: (i) the introduction of the process of soil moisture freezing; (ii) revised stability functions to increase the turbulent diffusion of heat in stable situations; and (iii) an increase of the skin‐layer conductivity. the effect of these changes on the seasonal evolution of soil and 2 m temperatures is investigated with long runs that have terms that relax towards the operational analysis above the boundary layer. In this way the impact can be studied on the temperature forecasts for the winter of 1995/1996, during which the operational model showed considerable soil temperature drift over Europe. Also, short periods of data assimilation (including 10‐day forecasts) have been carried out to study the diurnal time‐scales and the impact on model performance.The model changes eliminate to a large extent the systematic 2 m temperature biases for the winter of 1995/1996 over Europe and make the soil temperature evolution much more realistic. the soil moisture freezing, in particular, plays a crucial role by introducing thermal inertia near the freezing point, thereby reducing the annual temperature cycle in the soil. the process of soil moisture freezing leads to a considerable warming of the model's near‐surface winter climate over continental areas.
Skill and relative economic value of the ECMWF ensemble prediction system
Authors: Richardson D. S. 
Published: February 2007

AbstractThe economic value of the European Centre for Medium‐Range Weather Forecasts (ECMWF) operational ensemble prediction system (EPS) is assessed relative to the value of a perfect deterministic forecast. The EPS has substantial relative value throughout the medium range. Probability forecasts derived from the EPS are of greater benefit than a deterministic forecast produced by the same model. Indeed, for many users, the probability forecasts have more value than a shorter‐range deterministic forecast. Based on the measures used here, the additional information in the EPS (reflecting the uncertainty in the initial conditions) provides a benefit to users equivalent to many years' development of the forecast model and assimilation system.The impact of ensemble size on forecast value is considered. The difference in performance between ensembles with 10 and with 50 members may appear relatively small, based on standard skill measures, yet the larger ensembles have substantial benefit to a range of users. Further increases in ensemble size may be expected to provide additional value.
On the representation of gravity waves in numerical models of the shallow‐water equations
Authors: Mohebalhojeh A. R. , Dritschel D. G. 
Published: February 2007

AbstractGravity waves, or imbalanced motions, that develop during the evolution of vortical flows in numerical models of the shallow water (SW) equations are examined in detail. The focus here is on nearly‐balanced flows, with small but non‐zero gravity‐wave activity. For properly initialized flows, it is reasonable to expect small GW activity when Froude numbers Fr < 1 and Rossby numbers Ro ≲ 1.The guiding principle in the present study is that an accurate representation of potential vorticity (PV) is the pre‐requisite to a fair assessment of the generation of gravity waves. The contour‐advective semi‐Lagrangian (CASL) algorithm for the SW equations is applied, as it shows a remarkable improvement in the simulation of PV. However, it is shown that the standard CASL algorithm for SW leads to a noticeable numerical generation of gravity waves. The false generation of gravity waves can equivalently be thought of as the false, or numerical, breakdown of balance.In order to understand the maintenance of balance in the SW equations, a hierarchy of CASL algorithms is introduced. The main idea behind the new hierarchy is to implement PV inversion partially, balancing algorithms directly within the SW algorithm, while still permitting imbalanced motions. The results of the first three members of the hierarchy, CA0 (standard CASL), CA1, and CA2, are described and are compared with the results of two other SW algorithms, a pseudo‐spectral and a semi‐Lagrangian one. The main body of results is obtained for a highly ageostrophic regime of flow, with|Ro|max ∼ 1 and Frmax ∼ 0.5, where sub‐index 'max' denotes maximum over the domain. Other flow regimes in the relevant parts of the Ro‐Fr parameter space are also explored. It is found that, for a given resolution and Froude number, there is an optimal CASL algorithm, i.e. one which gives rise to the least numerical generation of gravity waves.
The parametrization of drag induced by stratified flow over anisotropic orography
Authors: Scinocca J. F. , McFarlane N. A. 
Published: February 2007

AbstractA new parametrization of drag arising from the flow over unresolved topography (UT) in a general‐circulation model (GCM) is presented. It is comprised of three principle components: a parametrization of the source spectrum and drag associated with freely propagating hydrostatic gravity waves in the absence of rotation, a parametrization of the drag associated with low‐level wave breaking, and a parametrization of low‐level drag associated with upstream blocking and lee‐vortex dynamics. Novel features of the scheme include: a new procedure for defining the UT in each GCM grid cell which takes account of the GCM resolution and includes only the scales represented by the parametrization scheme, a new method of representing the azimuthal distribution of vertical momentum flux by two waves whose direction and magnitude systematically vary with the flow direction and with the anisotropy of the UT in each GCM grid cell, and a new application of form drag in the lowest levels which can change the direction of the low‐level flow so that it is more parallel to unresolved two‐dimensional topographic ridges.The new scheme is tested in the Canadian Centre for Climate Modelling and Analysis third generation atmospheric GCM at horizontal resolutions of T47 and T63. Five‐year seasonal means of present‐day climate show that the new scheme improves mean sea level pressures (or mass distribution) and improves the tropospheric circulation when compared with the gravity‐wave drag scheme used currently in the GCM. The benefits are most pronounced during northern hemisphere winter. It is also found that representing the azimuthal distribution of the momentum flux of the freely propagating gravity‐wave field with two waves rather than just one allows 30‐50% more gravity‐wave momentum flux up into the middle atmosphere, depending on the season. The additional momentum flux into the middle atmosphere is expected to have a beneficial impact on GCMs that employ a more realistic representation of the stratosphere.
On the dynamics of hurricane‐like vortices in vertical‐shear flows
Authors: Smith Roger K. , Ulrich Wolfgang , Sneddon Graeme 
Published: February 2007

AbstractA simple two‐layer analogue model is used to elucidate aspects of vortex motion in a vertically sheared zonal flow. The model is based on the idea that a vortex can be considered as the sum of a pair of barotropic vortices, one whose vorticity, or potential vorticity, resides in the upper layer and the other whose vorticity resides in the lower layer. Each vortex has an associated tangential velocity distribution in the other layer, which advects the vortex in that layer. The strength of this velocity distribution is characterized by a coupling parameter, which, in the case of quasi‐geostrophic vortices, is related to the Rossby depth scale. Besides their mutual advection, the component vortices are differentially advected by vertical shear. The model leads to a set of coupled ordinary differential equations for the motion of each component vortex, which may be solved analytically in certain circumstances. The calculations indicate two types of flow behaviour according to the strength of the component vortices, the degree of vertical coupling and the strength of the shear. For weak shear and/or strong vortices and strong coupling, the vortices rotate around each other as their mean centre translates with a fraction of the mean zonal flow. For Strong shear and/or weak vortices and weak coupling, the vortices undergo a partial rotation while they are in proximity, but become progressively separated by the shear. The calculations are an aid to understanding the range of behaviour of vortices in shear in numerical calculations by other authors, and it is reasonable to presume that the processes represented by the model are fundamental processes in tropical cyclones also.The analogue model is evaluated in the context of quasi‐geostrophic theory, where the breakdown into the component vortices can be accomplished and where the complete problem can be solved numerically without approximation. The results of the quasi‐geostrophic model are contrasted with those of other recent studies of baroclinic vortices in the absence of vertical shear.
A general, linearized vertical structure equation for the vertical velocity: Properties, scalings and special cases
Authors: Inverarity G. W. , Shutts G. J. 
Published: February 2007

AbstractA general, linear vertical structure equation for the vertical velocity component, including explicit forcing terms in the momentum, thermodynamic and continuity equations, is derived for horizontally‐homogeneous flows. The basic flow is assumed to depend on height alone and is in geostrophic and hydrostatic balance. Scale analysis is used to show that this equation incorporates a variety of familiar special cases including the lee‐wave equation, Eady's equation and the quasi‐geostrophic omega equation, the different flow regimes being identified in terms of the Rossby, Froude and Richardson numbers. Using the vertical structure equation, a wave‐stress conservation principle is derived that is valid for basic flows whose magnitude and direction vary with height. In addition to providing some unification to the many flavours of vertical velocity equation in the literature, this derivation was motivated by the need to provide a starting point for a wide class of analytical problems in the study of baroclinic instability and inertia‐gravity wave dynamics.
The dynamical consequences for tropopause folding of PV anomalies induced by surface frontal collapse
Authors: Ziemiański M. Z. , Thorpe A. J. 
Published: February 2007

AbstractIt has been noted previously that during frontal collapse dynamical processes lead to the formation of potential vorticity (PV) anomalies in the vicinity of the surface front. These processes can either be associated with diffusion in the presence of the tight temperature gradients or with intrusion into the atmosphere of the vanishingly thin layer adjacent to the surface. This paper explores the dynamical consequences of these PV anomalies on the parent baroclinic wave cyclone. The method used is to find the vertical motion attributable to these anomalies. The location and magnitude of this vertical motion is clearly key to the dynamical influence of the anomalies. As we are dealing with a three‐dimensional evolving cyclone with an emergent tropopause fold, the calculation of vertical motion needs to be capable of accounting for the role of the highly deformed tropopause. Hence we develop and use a nonlinear balance model approach. The result shows that the PV anomalies near the surface front induce downward vertical motion at the tropopause fold, and thus they act to amplify this feature. This role of diffusion associated with surface frontal collapse is the key finding of this paper. An analysis is made of the role of upper‐ and lower‐level PV anomalies on the overall role of the wave development.
The formation of vertical Vortices in the convective boundary layer
Authors: Kanak Katharine M. , Lilly Douglas K. , Snow John T. 
Published: February 2007

AbstractA study of the dynamical formation mechanisms of atmospheric boundary‐layer vortices is being conducted. These vortices are typically manifest as dust devils although there is evidence that they exist with some frequency in the absence of visible flow tracers. For example, in 1997 MacPherson and Belts pointed out instrument observations of invisible boundary‐layer vertical vortices over the boreal forest. Several possible mechanisms for vortex formation under a variety of different dynamical regimes are described. Most observational investigators have reported that dust devils form in environments characterized by low wind speeds. The most intriguing unknown is the source of vorticity for the formation of vortices for convection in the absence (or near absence) of mean winds. Vertical vortex formation in convection without a mean shear has not often been documented in laboratory or numerical simulations.A Large‐Eddy Simulation of the convective boundary layer is performed, without mean winds, with the purpose of examining vertical vortex‐format ion mechanisms. The current work emphasizes the conjectured larger (convective) scale vorticity‐generating mechanisms. The results indicate that vertical vortices form at some, but not all of the vertices of the simulated pattern of open convective cells. The columnar vortices may be essentially vertical or tilt with height, and may or may not extend to the surface. It is suggested that the vertical vorticity initially available to the vortices comes from the tilting of horizontal vorticity associated with local unidirectional shears due to the convective‐cell circulations, with convective‐cell asymmetries being important to the process. Subsequent evolution apparently results in the co‐location of vertical vorticity centres with local updraught maxima.
Some impact of pollutants on the development and optical properties of stratocumulus clouds
Authors: Ghosh S. , Jonas P. R. , Wood R. 
Published: February 2007

AbstractTwo major recent field programmes—the European Cloud Radiation Experiment (EUCREX) and the Aerosol Characterization Experiment II (ACE‐2)‐have extensively analysed the dynamical, microphysical and radiative attributes of stratocumulus clouds contaminated by continental air. Although an extensive set of dynamical and microphysical data are now available, there are no accounts of any matching theoretical modelling studies. To fully understand and numerically model the interplay between the dynamics, microphysics, radiative and chemical properties of the two clouds chosen for our case‐studies would require a full three‐dimensional large‐eddy simulation (LES) model coupled to a full‐size resolving microphysical model where the computational costs would be prohibitive. In this study we have ‘optimized’ the classic Kessler parametrization scheme so that it is effectively able to distinguish between clean and contaminated clouds. We perform LES runs with the optimized scheme to study the morphology and the dynamics of the clouds and then use a second one‐dimensional microphysical parcel model run with identical environmental conditions to study the effects of pollution on the clouds as well as the droplet spectral evolution. This procedure yields extremely good agreement with observations at modest computational expense. It is shown that nitric acid (HNO3) vapour in the parts per billion by volume (p.p.b.v.) range affects cloud formation by increasing the number of cloud droplets and decreasing the mean size compared to an acid‐free simulation. The effects of HNO3 contamination on the EUCREX case‐study is evident owing to the proximity of this cloud to sources of air pollutants. With 10 p.p.b.v. of HNO3, we are able to achieve good agreement with the observations of the droplet effective radii as well as with observations of the optical‐depth variation. For the ACE‐2 cloud which formed further away from sources of pollutants, even on a typical ‘polluted’ day when the ambient HNO3 was ∼5 parts per trillion by volume, the drop concentration was found to be insensitive to changes in the HNO3.
Deriving cloud overlap statistics from radar
Authors: Hogan Robin J. , Illingworth Anthony J. 
Published: February 2007

AbstractThe predictions of general‐circulation models (GCMs) are sensitive to the assumed cloud overlap within a vertical column of model grid boxes, but until now no reliable observations of the degree of cloud overlap have been available. In this note we derive the overlap characteristics of clouds from 71 days of high vertical resolution 94 GHz cloud radar data in the UK. It is found that, contrary to the assumption made in most models, vertically continuous clouds tend not to be maximally overlapped. Rather, the overlap of clouds at two levels tends to fall rapidly as their vertical separation is increased, and for levels more than 4 km apart, overlap is essentially random. A simple inverse‐exponential expression for the degree of overlap as a function of level separation is proposed that could, once results become available from a variety of other locations and seasons, be implemented in current GCMs with relatively little difficulty.