bluuebunny/crossref_metadata_2025_split huggingface dataset:
Zonal asymmetries in SAMS stratospheric methane and nitrous oxide
Authors: Ziemke J. R. , Stanford J. L. 
Published: February 2007

AbstractStratospheric and Mesospheric Sounder (SAMS) methane (CH4) and nitrous oxide (N2O) constituent measurements were taken a decade before the Upper Atmosphere Research Satellite (UARS) observations and are generally of lesser quality. However, SAMS data are important because of the limited lifetime of the UARS and because they provide a unique, historical data series for these gases involved in greenhouse and ozone‐depletion effects. While most prior SAMS analyses focused on zonal means, this paper assesses the strengths and weaknesses of analysing zonally asymmetric perturbations in SAMS data. It is shown that wave‐1 perturbations can be successfully investigated, provided sufficient care is taken and aliasing considered.At the lowest SAMS level, 20 hPa (∼ 28 km altitude) zonal‐wave‐1 CH4 and N2O data reveal similar features for latitudes 45°N–65°N during stratospheric warming events and break‐up of the polar vortex. Large wave‐1 anomalies in the upper stratosphere (2 and 0.6 hPa) were found to be out of phase with the corresponding anomalies at 20 hPa. In one episode in early 1981 (during stratospheric sudden warming) southward winds over North America transported air with low mixing ratios from polar latitudes, while northward winds over Siberia transported air with high mixing ratios from subtropical latitudes. the effect produced strong wave‐1 amplitudes in both CH4 and N2O mixing ratios.Cross‐correlations between wave‐1 CH4 and N2O are large and positive in middle and high latitudes (consistent with ideal tracer behaviour for both constituent gases) but weak over the tropics. the cause of the latter remains an open issue.
On the Hamiltonian formulation of the quasi‐hydrostatic equations
Authors: Roulstone I. , Brice S. J. 
Published: February 2007

AbstractA variational approach to the Euler‐Lagrange equations for the quasi‐hydrostatic model is presented together with an explicit Hamiltonian (Poisson bracket) formulation. the bracket acts on functionals of the zonal and meridional angular momentum. Features of the relationships between the Hamiltonian formulations of the equations for an ideal fluid, the hydrostatic primitive equations and the quasi‐hydrostatic equations are discussed.
Observations of the mesoscale sub‐structure in the cold air of a developing frontal cyclone
Authors: Browning K. A. , Clough S. A. , Davitt C. S. A. , Roberts N. M. , Hewson T. D. , Healey P. G. W. 
Published: February 2007

AbstractObservations from 58 dropwindsondes released in a mesoscale array during the FRONTS 92 experiment are interpreted in the context of satellite imagery to derive the mesoscale structure and evolution of parts of a frontal cyclone developing over the eastern North Atlantic. A conceptual model involving the intertwining of ‘dry intrusion’ and ‘cloud head’ flows is corroborated and is used to provide the framework for interpreting the detailed mesoscale behaviour. In the cold air, two distinct dry‐intrusions were responsible for two cold fronts, trailing south‐westwards from the tip of the cloud head. Both were surface features at the beginning of our study but the leading one evolved into an upper front with mid‐level convection as the dry intrusion responsible for it overran the warm sector. Areas of both high and low potential vorticity were indicated within the dry intrusions. Upon encountering a critical level of zero system‐relative velocity at the top of the moist boundary layer, the dry‐intrusions' arrival was associated with the development of multiple dry and moist laminae near the top of the boundary layer. the vertical wavelength of the laminae was about 1 km and they extended over 200 km in the front‐normal direction, with a slope of typically 1 in 60. Although most parts of the laminae were subsaturated, their circulations combined with the double structure of the dry intrusions to produce multiple shallow cloud‐lines within the boundary layer. These formed as an extension of the south‐western tip of the cloud head associated with the developing cyclone. Possible mechanisms for generating the observed structures are discussed.
Simulation of daily variability of surface temperature and precipitation over europe in the current and 2 × Co2 climates using the UKMO climate model
Authors: Gregory J. M. , Mitchell J. F. B. 
Published: February 2007

AbstractAs the spatial scales simulated by general circulation models are refined and the parametrizations they employ are improved, greater accuracy can be achieved in their simulations of regional climate and events on short time‐scales, and greater confidence attached to their predictions of these for perturbed climates. In the present paper, we examine the daily (interdiurnal) variability of surface temperature and precipitation in Europe simulated by a version of the UK Meteorological Office (Hadley Centre) general circulation model, having 11 levels in the vertical and a horizontal resolution of 2.5°latitude × 3.75° longitude, coupled to a slab ocean, in the current climate and the equilibrium climate for a doubled atmospheric concentration of carbon dioxide. We evaluate the simulation of current conditions by comparison with climatologies and time‐series of observed data. In the modified climate, we find reductions in temperature variability in winter, related to a lessened land‐sea temperature contrast, and increases in summer, caused by changes in the surface heat balance. Precipitation in all seasons shows significant tendencies both to more dry days and to heavier events, with less frequent moderate daily amounts; this is connected with an increase in convective activity. We conclude that simulation of daily variability is strongly dependent on particular local physical processes and hence on the parametrizations used to represent them. While acknowledging deficiencies in the model's performance, we consider that our results are indicative of possible changes for a future climate.
Patterns of interannual variability defined by a cluster analysis and their relation with ENSO
Authors: Efimov V. V. , Prusov A. V. , Shokurov M. V. 
Published: February 2007

AbstractResults of the COADS data processing aimed at a classification of the structure of the large‐scale lowfrequency variability of the sea surface temperature, the sea‐level atmospheric pressure and the surface wind‐speed components are presented. Using cluster analysis, patterns of interannual variability in the temporal and spatial domains are obtained. Clusters consisting of points possessing similar temporal evolution are compared with the known dipole‐type patterns of interannual variability, and special attention is given to selection of the global ENSO‐related patterns. of particular interest are the teleconnection links between the equatorial and the extratropical regions in the Pacific Ocean. Clusters consisting of times with similar spatial patterns, as well as extended canonical correction analysis patterns, show the typical sequence of the spatial patterns during the ENSO cycle, especially for the ‘mature’ warm and cold phases. For these cases, typical features of the field anomalies, including the extratropical peculiarities in the Pacific Ocean, are considered and summarized.
Dependence of surface exchange coefficients on averaging scale and grid size
Authors: Mahrt L. , Sun Jielun 
Published: February 2007

AbstractThe value of the effective exchange coefficient for area‐averaged fluxes can depend significantly on the averaging scale. This dependence implies that the exchange coefficient in numerical models should depend on grid size. the main goal of this study is the assessment of the importance of such scale‐dependence. When the large‐scale flow is weak, the mesoscale motions on scales smaller than the averaging width (grid size) may generate significant turbulence which is not represented by the area‐averaged wind vector (resolved flow). In this case, the effective exchange coefficient must be larger than that predicted by similarity theory in order to predict the grid‐averaged turbulent flux.This study reviews different approaches for representation of spatially‐averaged surface fluxes over heterogeneous surfaces. the scale‐dependence of the effective exchange coefficient is posed in terms of spatially averaging the bulk aerodynamic relationship. This scale‐dependence is evaluated in terms of observed fluxes from three different field programs. the effective drag coefficient is found to be more scale‐dependent than the effective exchange coefficients for heat and momentum. the effective conductance is found to be less scale‐dependent than the effective exchange coefficient and effective resistance. Except for the case of weak flow over strong surface heterogeneity, the exchange coefficient is nearly independent of grid size.
Linear theory of momentum fluxes in 3‐D flows with turning of the mean wind with height
Authors: Broad Adrian S. 
Published: February 2007

AbstractCurrent orographically forced gravity‐wave‐drag parametrization schemes in numerical weather‐prediction models are based on two‐dimensional linear momentum‐flux theory. the vertical flux of horizontal‐momentum vector is considered parallel to the surface‐stress vector and independent of height unless the Richardson number falls below a quarter, or a critical level exists where the mean wind becomes zero in the direction of the flux vector. Non‐zero gravity‐wave drag is then taken to be parallel and opposite to the flux vector. Consideration of the full three‐dimensional linear problem reveals a number of fundamental differences: (i) the drag force is perpendicular to the mean wind where there is no wavebreaking, and (ii) the variation of the stress vector with height is dependent on the wave‐number decomposition of the underlying orography. For a general orographic profile having a continuous Fourier spectrum spanning an azimuthal angle of 180°: (a) critical levels exist at every height in the atmosphere for any wind profile, and (b) the momentum‐flux vector decreases in magnitude and turns with height.
An improved linear model of tropical surface wind variability
Authors: Saji N. H. , Goswami B. N. 
Published: February 2007

AbstractFor coupled modelling studies, the atmospheric component is required to simulate the large‐scale part of the observed surface winds accurately, as only this part is responsible for driving low‐frequency interannual variability in the oceans. A simple linear model of the tropical atmosphere is developed in which surface winds are viewed as the response of the atmospheric boundary layer to pressure perturbations produced by deep convection and sea surface temperature (SST) gradients. An empirical parametrization of total convection as a nonlinear function of total SST is adopted. The large‐scale variability of the simulated surface winds is examined for the period 1974–1991. The model not only captures the large‐scale low‐frequency part of the surface winds given by the first two empirical orthogonal functions (EOFs), it also successfully simulates the equatorial surface wind anomalies and their evolution during the entire period 1974–1991.Certain interesting differences between precipitation forcing and SST induced forcing of surface winds is brought out in this study. The precipitation forcing was found to be dominant over the central and western Pacific, while the SST forcing was found to be dominant over the eastern Pacific. The resultant wind response was also found to reflect this behaviour. The most interesting result is the change in balance of the precipitation‐related and SST‐gradient‐related terms in the forcing of anomalous and climatological winds. Our results indicate that convective heating predominates over SST‐gradient‐induced effects in forcing anomalous winds, but the balance reverses in the case of the climatological winds. We also find that SST‐gradient effects are critical in the simulation of the climatological wind field.
Sensitivity of a multi‐layer quasi‐geostrophic β‐channel to the vertical structure of the equilibrium meridional temperature gradient
Authors: Pavan Valentina 
Published: February 2007

AbstractA multi‐layer, quasi‐geostrophic, Boussinesq model on a β‐channel is used to study the sensitivity of the mid‐latitude storm‐track to changes in the vertical structure of the zonal mean equilibrium meridional temperature gradient. Motivation for this study is taken from the observation that the doubled CO2 climatology of general‐circulation models presents a weaker gradient in the lower troposphere and a stronger gradient in the upper troposphere than in a ‘control’ climatology.It is observed that the dynamics of the quasi‐geostrophic model are generally more sensitive to the lower than to the upper tropospheric temperature gradient as a consequence of the importance of shallow eddies. In order to give a good representation of the eddy dynamics it is necessary to use a sufficiently high resolution, which, in the case chosen, proved to be between 10 and 20 tropospheric layers. If only three layers are used the general features of the response are still reproduced, but the model does not represent the sensitivities of the eddy meridional momentum‐flux convergence adequately. It is argued that this is connected with the particular choice taken when parametrizing the surface Ekman friction. When a higher‐resolution model is used, the eddy momentum‐flux convergence proves to be more sensitive to changes in the upper than in the lower meridional equilibrium temperature gradient but is generally much less responsive to these changes than other quantities like the surface eddy potential‐vorticity flux or eddy kinetic energy.For each variable the two sensitivities become close to each other as the baroclinicity of the equilibrium flow is increased because of the prevalence of deep eddies in the dynamics, sensitive only to the total radiative equilibrium wind vertical gradient. For very high values of the baroclinicity of the equilibrium flow the sensitivities approach a constant value in accordance with a simple scaling argument.
On the relationship between the quasi‐biennial oscillation, total chlorine and the severity of the antarctic ozone hole
Authors: Butchart Neal , Austin John 
Published: February 2007

AbstractA 3‐dimensional model of the dynamics and radiation of the stratosphere and mesosphere is used to simulate austral spring conditions. An idealized representation of the southern hemisphere planetary waves is prescribed at the 316 mb model lower boundary, and the tropical zonal winds are relaxed towards an idealized time‐dependent representation of the quasi‐biennial oscillation (QBO). As the initial phase of the QBO is changed, the October mean lower‐stratospheric temperatures polewards of 75°S respond systematically, with the two extreme responses found for phases only 1/4 cycle apart rather than for opposite phases. The integrations are repeated for these two phases but with fully interactive photochemistry and different levels of chlorine, corresponding to conditions in about 1980, the mid to late 1980s, and those anticipated in about the year 2010. Increasing the chlorine amount produced systematically deeper ozone holes with areas significantly larger for conditions in the year 2010 than for those in the late 1980s, broadly in agreement with the secular trend observed by the Total Ozone Mapping Spectrometer. For the QBO phase giving lowest temperatures the simulated ozone holes are consistently deeper but with reduced sensitivity for high chlorine amounts. Differences in polar ozone amounts between the two phases are comparable with the observed interannual variability. In the model the results show that the QBO modulates the severity of the antarctic ozone hole directly through the wave‐induced transporting circulation rather than indirectly through the temperature‐dependent heterogeneous chemistry. Further, it is argued that the phases of the QBO producing the two extreme responses in the springtime ozone amounts over Antarctica are likely to depend, in reality, on details of the tropospheric planetary‐wave flux. The model results are also shown to be consistent with the observed downward trend in October antarctic temperatures since 1979.
The climatology of the stratospheric ‘THIN AIR’ model
Authors: Kinnersley Jonathan S. 
Published: February 2007

AbstractAn improved two‐and‐a‐half dimensional chemical–dynamical–radiative model shows remarkably realistic behaviour in a number of respects. The stratospheric/mesospheric westerly jet slopes equatorwards with height and is split at mid‐latitudes in the southern hemisphere. The parametrized orographic gravity waves exert a mid‐latitude drag and are important in splitting the jet, and are possibly important in pre‐conditioning the stratosphere for a sudden warming. A large easterly drag is required in the winter high‐latitude mesosphere in order to obtain a tropical mesospheric westerly jet. The summer easterly jet maximum lies above the tropospheric jet, due to gravity‐wave absorption in the lower stratosphere. Very realistic annual and semi‐annual cycles are produced in the model, with a mesospheric semi‐annual oscillation forced by tidal drag and gravity waves. Planetary‐wave temperatures are also modelled well, and wave amplitudes are correlated with the westerly jets. In the lower stratosphere, zonalmean Ertel's potential vorticity gradients strongly influence the wave activity, horizontal convergence and heating rates, with descent being strongest equatorwards of the vortex edge. This forms the basis for an explanation of the inter‐hemispheric asymmetry in the ozone column. Nitrous oxide distributions indicate too much equatorial ascent but show a ‘double peak’ connected with the tropical circulation, and steep horizontal gradients near the winter vortex edge.
Vortex dynamics and the evolution of water vapour in the stratosphere of the southern hemisphere
Authors: Lahoz W. A. , O'Neill A. , Heaps A. , Pope V. D. , Swinbank R. , Harwood R. S. , Froidevaux L. , Read W. G. , Waters J. W. , Peckham G. E. 
Published: February 2007

AbstractThe seasonal evolution of water vapour in the stratosphere of the southern hemisphere is studied by using water vapour measurements made by the Microwave Limb Sounder on the Upper Atmosphere Research Satellite. This evolution is interpreted with the aid of meteorological fields produced at the UK Meteorological Office by data assimilation. The processes governing the distribution of water vapour are clarified by focusing on the physical and dynamical conditions in and around the stratospheric polar vortex. Sustained diabatic descent in the vortex causes isopleths of water vapour mixing ratio to dip down markedly in the polar vortex, strengthening radial gradients of water vapour in the westerly jet, while stretching and folding of material lines in anticyclones adjacent to the polar vortex leads to awidening zone of weak horizontal gradients of water vapour. The circulation is discussed in terms of the dynamics of interacting vortices. Two different flow regimes are identified: (a) mid and late southern winter, with a strong polar vortex and one or more eastward‐travelling anticyclone; and (b) spring, with a relatively weak polar vortex and a quasi‐stationary anticyclone. The phenomenon of merger of anticyclones, already observed in the stratosphere of the northern hemisphere, is documented for the southern hemisphere. A detailed study of the tracer transport during merger is made by computing the isentropic advection of many thousands of particles. A comparison is made of the seasonal evolution of water vapour in the stratosphere of the southern and northern hemispheres.
A non‐hydrostatic version of the NMC's regional Eta model
Authors: Gallus William A. , Rančić Miodrag 
Published: February 2007

AbstractA non‐hydrostatic version of the regional Eta model used operationally at the National Meteorological Center (NMC) has been developed by implementing the ideas of Juang (1992) and Laprise (1992), who independently recommended a hydrostatically based, vertical coordinate for a fully compressible set of equations. The grid‐point model dynamics is based on perturbation equations in the η‐vertical coordinate; the base state may be taken from the operational NMC model, and thus updated with time.The non‐hydrostatic model uses a stepwise treatment of terrain present in the operational version, eliminating the pressure‐gradient‐term error associated with sigma‐coordinate models over steep topography. The compressible equations are written in a form that allows conservation of energy in a horizontally closed domain with appropriate advective schemes.A two‐dimensional version of the model, without parametrizations of physical processes, has been used successfully to simulate ascending warm bubbles and collapsing cold bubbles at high resolutions.
A statistical model of drop‐size spectra for stratocumulus clouds
Authors: Considine Geoffrey , Curry Judith A. 
Published: February 2007

AbstractThe purpose of this work is to present a simplified model of cloud‐droplet spectra that can be used as a tool for interpreting cloud microphysical observations in boundary‐layer clouds, and as a way to begin quantifying the couplings between cloud microphysics, dynamics and radiative properties for eventual use in cloud parametrizations. The model is steady‐state, ignores nucleation effects, and is formulated to produce horizontally averaged statistics. The major difference between this model and previous work lies in the model assumption that droplet spectra at a given level within a cloud are horizontal averages over a large number of air parcels, each of which can have a different lifting condensation level (LCL). Vertical motions are driven by buoyancy, so that liquid water and vertical velocity are simple functions of height above the LCL. This relationship, treated statistically over a large number of parcels, relates turbulent kinetic energy to horizontally averaged statistics of the droplet spectra. The broadening of droplet spectra is thus directly related to the turbulent kinetic energy within a cloud. In validating the model, results are compared with observations. It is shown that the basic droplet spectra predicted by the model are quite realistic for stratus and stratocumulus. The modelled droplet spectra broaden from cloud base to cloud top as is frequently observed, and the relationship between drop spectrum width and mean radius agrees quite well with observations.