bluuebunny/crossref_metadata_2025_split huggingface dataset:
Correction for attenuation of radar reflectivity using polarization data
Authors: Smyth T. J. , Illingworth A. J. 
Published: February 2007

AbstractAttenuation by heavy rain is a major operational problem in radar estimation of rainfall rates, and one which is increasingly severe at wavelengths shorter than 10 cm. Gate‐by‐gate correction algorithms, including those using both reflectivity (ZH) and differential reflectivity (ZDR), are inherently unstable. In addition values of ZH and ZDR are affected by hail, which causes little attenuation. Methods dependent upon differential phase shift (KDP) do not give unique solutions but depend upon raindrop size distributions. A new technique which uses both KDP and differential attenuation between the horizontally and vertically polarized radiation (AH‐v) is proposed. The total AH‐v is measured from the negative ZDR in light precipitation behind the attenuating region. Theory and observation show that corrections based on the KDP ‐ AH‐v scheme should be unaffected by hail, stable and accurate. A simpler algorithm may be feasible at C‐band in which the total attenuation is derived from the observed total differential attenuation. It appears that the most severe cases of attenuation result from heavy rain consisting of large oblate raindrops. Accordingly, the attenuation would be appreciably reduced if operational radars used vertical polarization rather than the commonly employed horizontal polarization.
Retrieval of ageostrophic wind from a radiosounding network and a single ST radar
Authors: Le Bouar E. , Petitdidier M. , Lemaitre Y. 
Published: February 2007

AbstractPrevious studies have shown that ageostrophic circulations play an important role in frontogenesis phenomena. They result from the maintenance of the balanced state of the atmosphere which tends to be broken down by the large‐scale forcing of the atmosphere. Their existence explains how a frontal discontinuity can be produced in a few days. From an experimental and forecasting point of view it would be interesting to obtain these circulations in real time. Unfortunately, the ageostrophic wind is not a measurable quantity and its retrieval from experimental data is difficult, particularly at the mesoscale, since instrumental noise leads to a need for significant data correction.In this context, this paper provides answers to some of the main problems related to the retrieval of ageostrophic winds in frontal systems. It focuses on the feasibility of retrieval while retaining, at small enough scale, details of physical interest. The framework of this study is the future implementation of regular meteorological wind‐profiler networks with a sampling time of one hour.This paper proposes to overcome numerous practical problems using an approach based on the variational Analyse du Vent AGéostrophique (AVAG) analysis. Tests of such an approach are done on real data extracted from the Mesoscale Frontal Dynamic Project/FRONTS 87 data base for a typical two‐dimensional frontal case (the IOP 7 case). They confirm that the AVAG analysis is able to provide a non‐noisy and consistent ageostrophic circulation. Comparisons with classical methods demonstrate the robustness of AVAG and its capability to obtain physically meaningful ageostrophic wind fields. This study also estimates the best and most suitable experimental sampling for practical applications. Sensitivity tests on the quality of the retrieved wind field with respect to (i) the sampling time used, (ii) the availability of additional thermodynamic data and (iii) the geometrical pattern of the network are presented. These tests provide evidence of the importance of accounting for thermodynamical‐ and dynamical‐field along‐front variations.
Intercomparison and evaluation of atmospheric transport in a Lagrangian model (STOCHEM), and an Eulerian model (UM), using 222Rn as a short‐lived tracer
Authors: Stevenson D. S. , Collins W. J. , Johnson C. E. , Derwent R. G. 
Published: February 2007

AbstractTransport of the short‐lived (half‐life 3.83 days) isotope 222Rn, which is emitted from unfrozen soils, is used to compare transport in several versions of the UK Meteorological Office Lagrangian chemistry‐transport model and in the Unified Model, the UK Meteorological Office general circulation model. The same 222Rn experiment is repeated for all the model versions, illustrating the impact on global transport of various model improvements: adding boundary‐layer schemes, including sub‐grid scale convection, increasing model spatial resolution, and increasing the temporal resolution of the meteorological fields used for driving the off‐line model. Results from all model versions are compared with a limited observational data set, and also with results from the same 222Rn simulations carried out with several global atmospheric transport models as part of the World Climate Research Program in December 1993 (Jacob et al. 1997). Versions of the Lagrangian chemistry‐transport model that include sub‐grid scale convection, transport 222Rn in a manner that is similar to the Unified Model and most general circulation models, supporting the simple and computationally inexpensive Lagrangian approach taken.
Conventional and Bayesian approach to climate‐change detection and attribution
Authors: Hasselmann K. 
Published: February 2007

AbstractThe conventional multi‐variate, multi‐fingerprint theory of climate‐change detection and attribution, expressed in terms of existing frequency distributions, is reviewed and generalized to a Bayesian approach based on subjective probabilities. Bayesian statistics enable a quantitative determination of the impact of climate‐change detection tests on prior subjective assessments of the probability of an externally forced climate change. The Bayesian method also provides a potentially powerful tool for enhancing statistical detection and attribution tests by combining a number of different climate‐change indicators that are not amenable to standard signal‐to‐noise analyses because of inadequate information on the associated natural‐variability statistics. The relation between the conventional and Bayesian approach is illustrated by examples taken from recent conventional analyses of climate‐change detection and attribution for three cases of climate‐change forcing by increasing greenhouse‐gas concentrations, increasing greenhouse‐gas and aerosol concentrations, and variations in solar insolation. The enhancement of detection and attribution levels through a joint Bayesian analysis of a number of different climate‐change indices is demonstrated in a further example. However, this advantage of the Bayesian approach can be achieved only within the framework of a subjective rather than objective analysis. The conventional and Bayesian approach both exhibit specific advantages and shortcomings, so that a parallel application of both methods is probably the most promising detection and attribution strategy.
Comparison of several numerical schemes applied to advection equations
Authors: Sokol Zbyněk 
Published: February 2007

AbstractEight numerical‐solution methods of advection equations are compared using two tests: the rotational flowfield test and the deformational flow‐field test. Beside the leap‐frog difference scheme used as a reference method, all the other methods are positive definite or monotonicity preserving. the positive definiteness is either the property of the advection scheme or it is obtained by applying the flux transport limiter. the following schemes are studied: the leap‐frog scheme completed by a flux transport limiter, Bott's scheme, the piecewise parabolic method, one version of the total variation diminishing (TVD) scheme and three modifications of essentially non‐oscillatory (ENO) schemes. the aim is to compare the schemes with respect to the application of TVD and ENO schemes in atmospheric modelling with high resolution. the test results confirm that at least one of the ENO schemes tested may be applied, but apparently the best results are obtained by Bott's scheme.
Effects of resolution on the simulation of stratocumulus entrainment
Authors: Stevens D. E. , Bretherton C. S. 
Published: February 2007

AbstractThree‐dimensional large‐eddy simulations (LESS) of a smoke cloud beneath a strong inversion are used to investigate how grid resolution affects simulated entrainment‐rate. These simulations are generalizations of a 1995 GEWEX Cloud System Studies intercomparison of models of smoke clouds. Radiative cooling at the top of the smoke cloud induces convection in the smoke cloud in a manner closely analogous to that in marine stratocumulus‐capped boundary‐layers.The simulations are compared to a laboratory analogue for a range of inversion strengths and radiative forcings. the inverse Richardson‐number scaling of non‐dimensional entrainment‐rate found in the laboratory experiments is largely captured by the numerical model, but the modelled entrainment‐rates for strong inversions are two large by a factor of two. Better agreement is obtained when the numerical model resolves the undulations of the inversion driven by the underlying convection. By this criterion, most LES studies of subtropical maritime stratocumulus made so far are under‐resolved.For determining the bulk entrainment‐rate, a nested‐grid model which has fine resolution only in the entrainment zone is as effective as a model with uniformly fine resolution. If horizontal resolution is much coarser than the vertical resolution, modelled entrainment‐rates are less than those obtained with simulations which have comparable vertical and horizontal resolutions.
The sensitivity of large‐eddy simulations of shallow cumulus convection to resolution and subgrid model
Authors: Brown A. R. 
Published: February 2007

AbstractResults are presented from a series of large‐eddy simulations of shallow cumulus convection, including some at much higher resolution than previously performed. Many of the ensemble‐averaged turbulence statistics which are relevant for mass‐flux parametrization schemes are shown to be reasonably insensitive to increases in resolution, but the cloud fields change markedly. the implications for the best interpretation of large‐eddy simulation results are discussed.
Linear evolution of baroclinic waves in saturated air
Authors: Fantini Maurizio 
Published: February 2007

AbstractA primitive‐equation model which includes a moisture parametrization that assumes that ascending air is always saturated, has been used to study the characteristics of moist baroclinic waves, exploring their dependence on the Richardson number, besides zonal and meridional wavelength. Growth rates are shown to increase, in consequence of latent‐heat release, in a larger measure for the meridionally structured waves than for the two‐dimensional ones. Approximate analytic relationships for the normal‐modes growth rates, based on the concept of a weighted average of dry and saturated parameters, compare favourably with the numerical results. Asymmetric horizontal structures, previously obtained in quasi‐geostrophic approximation, are shown to be consistent with potential‐vorticity generation by diabatic heating in the updraught.
Frontogenesis and the development of secondary wave cyclones in FASTEX
Authors: Chaboureau Jean‐Pierre , Thorpe Alan J. 
Published: February 2007

AbstractTwo major mechanisms of frontogenesis‐deformation and shear‐are important in frontal wave cyclone development. Horizontal deformation can suppress the nonlinear wave development. Using an analytic model, Bishop and Thorpe have shown that large strain rates inhibit any wave‐slope amplification. For real cases, this ambient strain can be measured using the vorticity‐divergence attribution method developed by Bishop. This technique permits us to confirm the crucial role of such strain on the evolution of cases of wave development during the Fronts and Atlantic Storm Track Experiment (FASTEX).Horizontal shear in the presence of an along‐front thermal gradient is also an important mechanism of frontogenesis. Using an Eady model, Joly and Thorpe have shown that, in cases of large along‐front thermal gradient, frontal waves have growth rates smaller than the front itself, and thus would not develop. the domain‐independent attribution method developed by Bishop is here extended to a geopotential‐field partition. This leads, via a nonlinear balance condition, to the estimation of the ambient along‐front potential‐temperature gradient. the role of such an along‐front potential‐temperature gradient is discussed. as well as the relative contributions of the two frontogenesis mechanisms for the FASTEX cases.
A model study of corona emission from hydrometeors
Authors: Schroeder Vicki , Baker M. B. , Latham John 
Published: February 2007

AbstractThe maximum measured electric fields in thunderclouds are an order of magnitude less than the fields required for electric breakdown of the air. Several explanations for lightning initiation in these low‐intensity fields exist. One explanation is that electric breakdown first occurs at the surfaces of hydrometeors where the ambient field is enhanced very locally due to the drop geometry. Laboratory experiments have indicated that colliding raindrops which coalesce to form elongated water filaments, can produce positive corona in ambient fields close to those measured in thunderclouds. We calculate the electric field distribution around a simulated coalesced drop, and use a numerical model to study the positive‐corona mechanisms in detail. Our results give good agreement with the laboratory observations. At the altitudes and reduced pressures at which lightning initiation is observed, our results show that positive corona can occur at observed in‐cloud electric fields strengths.
Do orographic gravity waves break in flows with uniform wind direction turning with height?
Authors: Broad Adrian S. 
Published: February 2007

AbstractThe properties and behaviour of orographically generated gravity waves which have a vertical component of propagation are examined by analysing the structure of stationary, hydrostatic waves embedded in a basic state wind field, U, whose direction and magnitude may vary linearly with height, z. From a ray‐tracing analysis it is evident that the wave energy density decays to zero approaching a three‐dimensional (3‐D) critical level in such an ambient flow field where U. k = 0 for the particular horizontal wave vector k = (k, l, 0). This decay of the wave energy density would appear to imply that wave breaking does not naturally occur in 3‐D flows where the wind turns uniformly with height (excepting that wave vectors which do not experience a 3‐D critical level can break due to the decrease of density with height). However, as a linear theory analysis of convective overturning demonstrates, the criterion for wave breaking relaxes as a 3‐D critical level is approached. the gravity wave amplitude is only required to exceed the inverse of the vertical wave number (which is proportional to U. k and decays to zero at the critical level) for overturning to occur. Consequently a linear estimate of whether convective instability will occur depends on the asymptotic limit of the ratio of the wave energy density to the square of the amplitude. It can be shown from the ray‐tracing analysis that for 3‐D flow fields with a mean wind which turns uniformly with height this ratio diverges, implying that the criterion for convective breaking of gravity waves will always be exceeded on approaching such a 3‐D critical level.
Critical levels and mixing layers induced by convectively generated gravity waves during CEPEX
Authors: Teitelbaum H. , Moustaou M. , Sadourny R. , Lott F. 
Published: February 2007

AbstractWe study data from the Central Equatorial Pacific Experiment, in particular the soundings providing temperature, pressure, wind and ozone profiles, and we link them to the large‐scale conditions deduced from the analyses of the European Centre for Medium‐Range Weather Forecasts. to the west of the date line, the mean wind shows a strong shear at the tropopause between the easterlies of the upper troposphere and the westerlies of the lower stratosphere. These are conditions propitious to the formation of critical levels for quasi‐stationary gravity waves generated by deep convection. the measured wind profiles show the main characteristics of the encounter of a wave with its critical level and, in the ozone and potential‐temperature profiles, the existence of a conspicuous mixing layer in the vicinity of this critical level. Our interpretation that the mixing layer is a consequence of the critical level is supported by numerical simulations with a nonlinear time‐dependent two‐dimensional model. the occurrence of a mixing layer in the tropopause structure seems to be characteristic of large‐scale equatorial convective areas during westerly phases of the quasi‐biennial oscillation.
Passage of a tracer through frontal zones: A model for the formation of forward‐sloping cold fronts
Authors: Parker D. J. 
Published: February 2007

AbstractIt is shown that a forward‐sloping cold front may form in a simple two‐dimensional model of an atmospheric frontal zone, through differential advection of dry‐ and wet‐bulb potential temperature. As such, two frontal zones may coexist, with opposing slopes. This behaviour is manifest in the neutral Eady edge wave, which exhibits kata‐cold front behaviour, while the unstable Eady wave, with an ana‐cold front, does not generate a forward‐sloping cold front.These results come about as a consequence of the wave‐like behaviour of the fronts in the model, that is, the ability of a front to propagate through the air locally. As such, they highlight the fact that a frontal zone need not be a barrier to airflow and that any air properties, from water vapour to atmospheric pollution, may pass through a front.
The dependence of clear‐sky outgoing long‐wave radiation on surface temperature and relative humidity
Authors: Allan R. P. , Shine K. P. , Slingo A. , Pamment J. A. 
Published: February 2007

AbstractA simulation of the earth's clear‐sky long‐wave radiation budget is used to examine the dependence of clear‐sky outgoing long‐wave radiation (OLR) on surface temperature and relative humidity. the simulation uses the European Centre for Medium‐Range Weather Forecasts global reanalysed fields to calculate clear‐sky OLR over the period from January 1979 to December 1993, thus allowing the seasonal and interannual time‐scales to be resolved. the clear‐sky OLR is shown to be primarily dependent on temperature changes at high latitudes and on changes in relative humidity at lower latitudes. Regions exhibiting a ‘super‐greenhouse’ effect are identified and are explained by considering the changes in the convective regime associated with the Hadley circulation over the seasonal cycle, and with the Walker circulation over the interannual time‐scale. the sensitivity of clear‐sky OLR to changes in relative humidity diminishes with increasing relative humidity. This is explained by the increasing saturation of the water‐vapour absorption bands with increased moisture. By allowing the relative humidity to vary in specified vertical slabs of the troposphere over an interannual time‐scale it is shown that changes in humidity in the mid troposphere (400 to 700 hPa) are of most importance in explaining clear‐sky OLR variations. Relative humidity variations do not appear to affect the positive thermodynamic water‐vapour feedback significantly in response to surface temperature changes.
Radiative‐dynamic effects of the antarctic ozone hole and chemical feedback
Authors: Mackenzie I. A. , Harwood R. S. , Stott P. A. , Watson G. C. 
Published: February 2007

AbstractA southern hemisphere winter/spring is simulated with the Coupled Stratosphere‐Mesosphere Interactive Chemistry model, a mechanistic model of the middle atmosphere with an interactive chemistry scheme. Simulations are initialized and forced at the lower boundary with dynamic fields taken from UK Meteorological Office assimilations for 1992. Radiative and dynamic effects of ozone depletion are investigated by comparing runs that include heterogeneous chemistry and hence develop an ozone hole, with otherwise identical control runs with no heterogeneous chemistry and no ozone hole. Temperatures in the ozone hole and control runs start to diverge from around mid‐September; by late October the lower stratosphere is up to ∼12 K cooler when an ozone hole is present. In the mid and upper stratosphere, a strengthened general circulation and an enhanced radiative heating rate in the ozone‐depleted runs lead to slightly higher temperatures at the high southern latitudes than in the control runs. the initially smooth evolution of the temperature difference at the upper levels is interrupted in mid‐October by more transient events associated with planetary‐wave activity.The feedback onto the chemistry of the physical changes wrought by using the depleted ozone in the radiative calculations is investigated by comparing runs of a chemical transport model forced by winds and temperatures calculated with and without an ozone hole. the ozone‐hole induced cooling does not change the spatial extent of the chlorine and bromine activation, and only slightly extends its duration. Consequently, the depth and size of the ozone hole are unchanged by chemical feedback. Local deficits in the ozone column of up to ∼30 Dobson units which arise in the cooler run in October are due to differences in horizontal transport. the increased diabatic descent resulting from using the depleted ozone for the radiation calculations does not significantly alter the vertical transport of polar ozone, the rate of which is low compared with the rate of chemical ozone destruction.
Predictive skill of the UKMO unified model in the lower stratosphere
Authors: Lahoz W. A. 
Published: February 2007

AbstractThis paper studies the short‐term (up to 10 days) predictive skill of the UK Meteorological Office Unified Model (UM) in the lower stratosphere by applying conventional verification statistics used in numerical weather prediction studies. Simulations from four versions of the UM, each of them having a different representation of the stratosphere, are used to assess the dependence of the model's skill on stratospheric vertical resolution, location of the highest model full‐level and location of the highest model half‐level. Results are shown for two winter periods: February 1994 (northern winter with a weak wave‐number two warming) and October 1994 (southern winter with a typical top‐down break‐up of the stratospheric polar vortex). the results of this paper indicate that the model's skill in the lower stratosphere depends on a number of factors, including flow structure, stratospheric vertical resolution and the location of the highest model full‐level. By contrast, the model forecast skill in the mid troposphere is insensitive to the stratospheric representation on the 10‐day time‐scale. the highest skill (10 days and above for temperature and geopotential height at 50 hPa) is associated with steady and deep stratospheric flows which span the upper troposphere. the poorest skill is associated with rapidly varying flows; flows with shallow vertical structure also tend to have relatively poor skill. In all cases the model's skill is a significant improvement over persistence. the model has higher forecast skill in the lower stratosphere than in the mid troposphere; it also has higher skill in northern winter than in southern winter: these differences in skill can be ascribed to the flow regime in the lower stratosphere being dominated by lower wave numbers than in the mid troposphere, and to larger initialization errors in the southern hemisphere. It is shown that higher stratospheric vertical resolution does not necessarily give higher skill in the lower stratosphere; this is particularly true in northern winter. the model results show evidence of links between tropospheric and stratospheric errors.