bluuebunny/crossref_metadata_2025_split huggingface dataset:
A comparison of model and observed network estimates of sulphur deposition across Great Britain for 1990 and its likely source attribution
Authors: Metcalfe S. E. , Whyatt J. D. , Derwent R. G. 
Published: February 2007

AbstractLong‐range transport models for acidic sulphur species are being used to develop emission‐control strategies to combat environmental acidification in the remote, sensitive regions of Europe. the incorporation of seeder‐feeder enhancement into a simple trajectory model for sulphur has markedly improved the representation of wet deposition across Great Britain, as shown by a detailed comparison of the model results with the observations from monitoring networks. an uncertainty analysis reveals that those input parameters which contribute most to uncertainty in the total sulphur deposition are the sulphur‐dioxide emissions, wind speeds and wet‐removal coefficients. the total Great Britain dry and wet depositions of sulphur are estimated to be 216 and 241 thousand tonnes per year, respectively, making a total sulphur deposition per year of 457 ± 110 thousand tonnes, in excellent agreement with the observed quantity of 433 thousand tonnes. the model was used to give an indication of the origin of the deposited sulphur across Great Britain. European land‐based sulphur sources contribute 34% of the deposition to Great Britain, with significant dry deposition in south‐east England and wet deposition in upland western Britain. UK power stations and oil refineries contribute 46.7% of total deposition, with shipping and natural sulphur sources from algal blooms, a mere 4.6%. Differences in source attribution between this UK‐scale study and those previously published for the long‐range transport scale are explored and it is concluded that national‐scale models have an important role to play in policy formulation.
Tropospheric dispersion: The first ten days after a puff release
Authors: Maryon R. H. , Buckland A. T. 
Published: February 2007

AbstractDispersion from a puff released in the atmospheric boundary layer in mid‐latitudes is simulated with a Lagrangian global multiple‐particle model using analysed three‐dimensional wind fields. Quantitative measures of the spread are computed and used to assess the influence of the synoptic system into which the puff is released, and the ways in which turbulent diffusion can contribute to the total spread. the nature of the dispersion over ten days is examined to elucidate the controlling meteorology and kinematics during the various phases which could be identified. the results are compared with measurements made from constant‐level balloons. A time index is proposed to characterize large‐scale dispersion from a point source.
Potential vorticity inversion and balanced equations of motion for rotating and stratified flows
Authors: Vallis Geoffrey K. 
Published: February 2007

AbstractBalanced equations of motion based on potential vorticity evolution and inversion for the shallow water and stratified primitive equations are derived and, in some shallow‐water cases, numerically tested. The schemes are based on asymptotic expansions in Rossby or Froude number, or rational scaling‐based truncations of the equations of motion, assuming that the dynamics are determined by the advection of potential vorticity. Thus, regimes of validity are rapidly rotating and/or highly stratified flow. Both new and familiar results are straightforwardly obtained, in a unified framework in both height and isentropic coordinates. For both shallow‐water and stratified equations, Rossby number expansions schemes give quasi‐geostrophy at lowest order. Both gradient‐wind balance and the nonlinear terms in the potential vorticity enter at next order. A low Froude number expansion for non‐rotating flow gives two‐dimensional flow, uncoupled in the vertical at lowest order. A single consistent inversion scheme can be derived that is valid at lowest order in Froude number for all Rossby numbers, for both shallow‐water and the stratified equations. It may be a particularly appropriate model for the atmospheric mesoscale and oceanic submesoscale, where rotation and stratification can both be important in defining balanced motion. A model is also proposed that is valid at both planetary and synoptic scales, combining the familiar planetary geostrophic and quasi‐geostrophic equations. Most of the models derived require the solution only of linear or near linear elliptic equations, possibly with varying coefficients.Numerical experiments indicate that a higher‐order inversion can be quantitatively better than quasigeostrophy, if Rossby number and divergence are sufficiently small. In some other cases, no noticeable improvement over quasi‐geostrophy is found, even when the Rossby number is quite small. However, the balanced model valid for both planetary and synoptic scales shows a significant qualitative and quantitative improvement over both planetary geostrophy and quasi‐geostrophy for large‐scale flows, and its evolution is in good agreement with a primitive equation model.
Aircraft measurements of potential vorticity and Richardson number in baroclinic zones
Authors: Vaughan G. , Pepler S. J. , McKenna D. S. 
Published: February 2007

AbstractThis paper describes a novel technique for deriving potential vorticity and Richardson number from aircraft cross‐sections of baroclinic zones. It exploits the fact that gravity waves distort isentropic surfaces, so that an aircraft flying horizontally can measure a vertical profile of temperature and wind under favourable conditions. The technique is applied to two flights of the UK Meteorological Research Flight's C‐130 Hercules aircraft, in April 1983 and June 1991. It is found to work reasonably well only when large‐amplitude gravity waves are present. Values of potential vorticity so derived agree with those calculated in the conventional manner from cross‐sections, and are found to correlate well with ozone mixing ratio.
The greenhouse Earth: A view from space
Authors: Harries J. E. 
Published: February 2007

AbstractThe natural greenhouse effect of the Earth is strongly influenced by the radiative effects of water vapour and clouds in the atmosphere, which control the energy absorbed from the sun, and that lost through thermal emission to space. Any perturbations to the climate balance, for example through so‐called ‘radiative forcing’ due to increasing CO2 amounts, variations in solar constant, or other causes, can be amplified by the feedback processes that involve water in its various phases. The radiative cooling of the Earth in the absence of clouds has recently been shown to be dominated by emission from upper‐tropospheric water vapour, in the far infrared portion of the spectrum, and this is illustrated: observations of this radiative flux, and of the distribution of water vapour in the upper troposphere, are urgently needed. The role of clouds is discussed, and it is noted that their response to global warming is not presently unambiguously determined with available models, due to the complexity of competing processes: again, as in the cloud‐free case, more accurate global observations are needed. The paper is illustrated by data from satellite experiments, most notably the Earth Radiation Budget Experiment sponsored by NASA.
Form and wave drag due to stably stratified turbulent flow over low ridges
Authors: Belcher S. E. , Wood N. 
Published: February 2007

AbstractWe investigate how stable stratification affects the aerodynamic force, or form drag, induced by turbulent boundary‐layer flow over two‐dimensional hills. Both analytical and numerical models are used to calculate the flow and thence the form drag on the hill. In the analytical model we use a two‐layer, truncated mixing‐length, turbulence model, which is consistent with scaling arguments and which produces reliable estimates of the form drag for the neutral flow. The form drag is also calculated analytically using an eddy‐viscosity model, and the results compare well with values computed with the nonlinear numerical model that uses a similar turbulence model.The leading‐order contribution to the form drag is from a non‐separated sheltering mechanism, which is similar to the mechanism in neutral flow. Stable stratification changes the magnitude of this mechanism through several effects. For weak stratification the predominant effect is an increase in shear in the upstream wind profile across a middle layer, which increases the form drag by a factor of two or more. There is indirect experimental evidence to support this finding. If the stratification is more stable, then the shear across the middle layer becomes limited because the boundary layer has a finite depth. Then the dynamical effect of buoyancy on the pressure perturbation becomes important and reduces the form drag, eventually to zero.For still more stable stratification, gravity waves and associated wave drag are produced. The analysis shows that the appropriate scaling velocity for wave drag is the approach‐flow wind speed evaluated at the middle‐layer height. The relationship between the form and wave drag components is investigated by evaluating the analytical formula for the drag on isolated hills of two idealized shapes. For weak stratification the form drag dominates, but as the stratification becomes more stable the wave drag increases and first equals the form drag, at a value that depends on the hill shape, and then dominates.
A global shallow‐water model using an expanded spherical cube: Gnomonic versus conformal coordinates
Authors: Rančić M. , Purser R. J. , Mesinger F. 
Published: February 2007

AbstractA model using shallow‐water equations with an Arakawa‐type scheme for momentum terms is tested on a quasi‐uniform geometry on the sphere, derived by a spherical expansion of the inscribed cube based on the gnomonic projection. Thereby, a quasi‐homogeneous distribution of grid points is achieved, and a global finite‐difference model is designed which does not require Fourier filtering or suffer from the burden of redundant computational points at high latitudes. Difficulties resulting from the directional discontinuity of the coordinate lines crossing the edges of the expanded cube are almost completely eliminated by using the Arakawa B‐grid, so that only scalar points are placed along the edges. An alternative approach is developed based on numerical orthogonalization of the grid whereby, inter alia, the directional discontinuity at the edges is avoided at the cost of some accumulation of points in the vicinity of the vertices of the cube. In the customary Rossby–Haurwitz wave‐4 tests, both approaches are shown to converge to a visually indistinguishable solution as the resolution is increased. However, with the orthogonalized, conformal grid, convergence towards the asymptotic solution was substantially faster.
A low‐order model of coherent structures in the convective atmospheric surface layer
Authors: Zhuang Yahui 
Published: February 2007

AbstractBased on a simple application of the dynamical systems theory and experimental evidence of coherent motions in the convective atmospheric surface layer, a low‐order model (15 ordinary differential equations) to study coherent flow dynamics is developed. The model is derived by performing a Galerkin projection of the Navier–Stokes equations on a set of 15 orthogonal functions ϕ (x, z), extracted from an ensemble of observed convective plumes (or coherent structures) with the Proper Orthogonal Decomposition method. These orthogonal functions, defined on a two‐dimensional domain (620 m × 150 m), are typical of large‐scale turbulent velocity and temperature fluctuations in the convective atmosphere, and are assumed to have captured enough physics to model the most important aspects of the coherent structures. The effects of the unresolved turbulence on the coherent structures are modelled by the Smagorinsky eddy viscosity closure. The model treats boundary conditions objectively, uses experimental data as the initial conditions, and provides a means to study the dynamics of the coherent structures.
The behaviour of the roughness length for temperature over heterogeneous surfaces
Authors: Malhi Yadvinder 
Published: February 2007

AbstractThe definitions of roughness length and zero‐plane displacement are examined, as well as the relationship of these quantities to the resistance of transfer processes close to a surface. It is demonstrated that, if molecular diffusion or free convection processes are dominant within the interfacial sub‐layer, then the roughness length for temperature z0T can be expected to decrease with increasing wind speed. Over homogeneous surfaces this sensitivity to wind speed is reduced by the relative efficiency of heat transfer, but over sparsely vegetated surfaces the sensitivity is greatly increased.The problem of correct measurement of surface temperature over a heterogeneous surface is outlined and examined; it is demonstrated that use of the area‐averaged radiometric temperature as a representative surface temperature can lead to consequences such as zero, negative or infinite apparent transfer resistances. Measurements of z0T are presented from two field experiments over sparsely vegetated surfaces; typical values of z0T are around 10−10 m, and vary by orders of magnitude over a quite moderate range of wind speed. These measurements are interpreted in terms of the behaviour of interfacial sub‐layer transfer processes; it is shown that a significant cause of the low measured value of z0T may be the use of radiometric surface temperature as a representative surface temperature.
A mechanism for the low‐frequency variability of the mid‐latitude troposphere
Authors: James I. N. , Dodd J. P. 
Published: February 2007

AbstractLow‐frequency variability of the zonal‐mean zonal wind in long runs of a simplified global circulation model is dominated by bands of wind anomaly which originate in the tropics and propagate slowly poleward with speeds of 0–2 m s−1. Such propagation could be the result of the interaction between the zonal‐wind anomaly and an equatorward flux of Rossby‐wave activity originating in middle or high latitude. A barotropic weakly nonlinear model of this process has been derived. It suggests propagation of zonal‐wind anomalies similar to those produced in the global circulation model, and leads to comparable propagation speeds.
The radiative properties of inhomogeneous boundary layer cloud: Observations and modelling
Authors: Hignett P. , Taylor J. P. 
Published: February 2007

AbstractThe importance of internal inhomogeneities in the microphysical structure of stratocumulus in relation to the radiative properties of various cloud fields is studied using aircraft measurements and a 3–D Monte Carlo model. This study is unique in that in situ microphysical data and observations of cloud thickness have been used in the initialization of a Monte Carlo model, and the resulting model predictions of reflectance and albedo have been compared with in situ aircraft radiometric observations of the same cloud. The model predicts a reduction in reflectance and albedo for the 3–D cloud field compared to a homogeneous cloud layer with the same average microphysical properties. The in situ observations confirm these simulations. The Monte Carlo model predicts that internal inhomogeneities lead to a reduction in cloud absorptance and an increase in cloud transmittance. A further study of the importance of the distribution of clouds within a domain was carried out using a Monte Carlo model where the cloud fraction is kept constant while the separation of the cloud elements is varied. These results reveal a large sensitivity of the radiative properties to the distribution of the cloud, and that this sensitivity increases as the separation of the cloud elements decreases.
Observation and simulation of barrier winds at the western margin of the Greenland ice sheet
Authors: van den Broeke Michiel R. , Gallée Hubert 
Published: February 2007

AbstractMeteorological observations performed in the melting zone of the Greenland ice sheet during GIMEX‐90/91 show the regular occurrence of moderately strong barrier‐winds. The barrier wind is a thermally generated jet that develops when the large‐scale flow advects warm tundra air towards the cold, melting ice sheet, creating a large local horizontal gradient of temperature. As a result, the geostrophic wind in the atmospheric boundary‐layer acquires a component perpendicular to the temperature gradient. In the melting zone of the ice sheet, barrier winds interact with persistent surface‐based katabatic winds. The forcing mechanisms and detailed two‐dimensional structure of the barrier wind have been investigated with a mesoscale meteorological model. The model results confirm the important role of the warm tundra during the development of barrier winds. Barrier winds cause a pronounced rise in temperature and a strong increase of turbulent exchange in the lower melting zone: friction velocity and sensible‐heat flux in the melting zone showed peak values of 1.3 m s−1 and 300 W m−2 respectively. In combination with large positive net radiation, the daily melt rate increased to 10 cm water equivalent, which is twice the rate in typical katabatic wind conditions. This shows that barrier winds could have a significant impact on the melt regime of areas where the ice sheet ends in the tundra. Although barrier winds in this part of Greenland may occur less frequently than the observations during GIMEX‐90 suggest, they are of great importance for the climate of this and of other polar regions, such as the Antarctic, and so merit careful study.
In situwater vapour measurement, and a case study of the drying mechanism of the tropical lower stratosphere
Authors: Ovarlez Joëlle , Ovarlez Henri , Teitelbaum Hector 
Published: February 2007

AbstractDuring an experiment performed in tropical regions, water vapour content was measured in the lower stratosphere, using long duration balloons. A set of profiles show a vertical structure consistent with what has been named the tape‐recorder effect. One of the results, when the balloon descends to 100 hPa, shows a value of the water‐vapour mixing ratio as low as 2.8 parts per million by volume, in a layer approximately 1 km thick at about 80 hPa. This layer is delimited by two minima of air temperature, suggesting that the drying is a consequence of a vigorous convective process. This kind of process can be so strong that the convective cloud overshoots the tropopause and forms an anvil of ice crystals which grow and finally fall under gravity. Some of the characteristics of the METEOSAT cloud imagery and of the ECMWF analyses could be interpreted as the signature of this type of convective process. Nevertheless one cannot discard other mechanisms, such as the generation of stratospheric buoyancy waves by tropical convection. These waves can cross the tropopause and form thin ice clouds in the lower stratosphere. In this hypothesis, also, the ice falls and transports moisture downwards.
Predicting cloud‐droplet effective radius and indirect sulphate aerosol forcing using a general circulation model
Authors: Jones A. , Slingo A. 
Published: February 2007

AbstractVarious methods for predicting cloud‐droplet effective radius in the Hadley Centre general circulation model are compared with aircraft and satellite retrievals, and are used to estimate the indirect radiative forcing by anthropogenic sulphate aerosols since the beginning of the industrial era. The effects both of different parametrization approaches and of different input sulphate data sets are examined; however, there is no clear evidence to prefer either of the two sulphate data sets used in the study. Two of the parametrizations generate distributions of present‐day effective radius which are similar to each other and compare favourably with observations, yet provide very different estimates of the indirect effect, ranging from −0.5 to −1.5 W m−2 in the global annual mean. A sensitivity experiment in which it is assumed that droplet concentrations are not determined by sulphate concentrations in continental air reduces this global‐mean forcing to −0.3 to −0.8 W m−2. This sensitivity demonstrates the need for a much better understanding of the link between sulphate aerosol mass concentrations, cloud condensation nuclei, and cloud‐droplet number concentrations.
Towards improved radar estimates of surface precipitation rate at long range
Authors: Kitchen M. 
Published: February 2007

AbstractA method has been developed for estimating surface precipitation rates using reflectivities measured with a radar scanning at several different angles of elevation. In each pixel of a radar image, an idealized profile of the reflectivity factor is constructed. Each profile is defined in terms of three unknowns: the reflectivity in the rain beneath the melting layer, and the slope of the profile of the reflectivity in each of two layers above the melting layer. In these layers above the melting layer, it is necessary to invoke assumptions of horizontal homogeneity in the shape of the profile. Simple parametrizations are used for low‐level orographic growth and for the bright band, and the profile is diagnosed with the help of non‐radar information. the difference between the idealized and observed profiles is expressed as a single value for an entire radar‐image; the difference is minimized by iteration and this avoids using complicated methods of inversion. A simulation experiment was carried out for 15 different cases; in most of them, the incorporation of data from several elevation angles reduced bias errors at long range.The new method was then used on real radar‐data. In typical frontal rainfall, it showed no consistent improvement. Because it avoids using corrections which can otherwise become detrimental at long ranges, however, the new method gave improved results when reflectivity profiles were atypical.Unresolved spatial variability of the reflectivity profile continues to be a problem.