bluuebunny/crossref_metadata_2025_split huggingface dataset:
MANDOP analysis and airborne Doppler radar for mesoscale studies
Authors: Dou Xiankang , Scialom Georges , Lemaǐtre Yvon 
Published: February 2007

AbstractThe development, these last ten years, of airborne Doppler radars able to follow precipitating systems during a significant part of their lifetime and, more recently, of radars equipped with dual‐beam antennas allowing the sampled area to be extended (avoiding the necessity of performing two consecutive flights over the same area in order to obtain the three‐dimensional wind field), has increased the need for new wind‐field analyses flexible enough to output wind fields at several scales of motion, in particular at mesoscale. Numerous retrieval techniques valid for ground‐based radars exist. Some of them are devoted to convective‐scale studies, others, such as the MANDOP analysis, are devoted to mesoscale meteorology. The application of these techniques to airborne Doppler‐radar data is subject to new problems linked to the influence of the flight parameter's accuracy onto the sensitivity of the retrieved wind field (in particular the vertical‐velocity field). The knowledge of the effect of this sensitivity is of crucial importance for mesoscale motions with very weak vertical velocities.In this context, the paper describes the application of the MANDOP analysis (a new analysis devoted to mesoscale meteorology and valid for any kind of Doppler‐radar data but until now only tested for ground‐based radar data) to the case of airborne dual‐beam radar data, with special attention brought to the accuracy of the wind‐field retrieval for mesoscale motions.The study of these problems, and of the expected accuracy on the retrieved wind fields, is performed by means of simulated data. An application of the method to real data extracted from the CaPE experiment data set is also made. This application is twofold. First, a comparison is made between fields retrieved from airborne radar data and several combinations of airborne and ground‐based radar data, illustrating the flexibility of the analysis and validating its application at mesoscale on dual‐beam radar data; then a comparison is made between MANDOP and an analysis devoted to convective‐scale studies, the COPLANE analysis, on the same airborne data set, illustrating the capabilities of the MANDOP analysis to be applied within convective areas and thus to be used to scrutinize a precipitating system at various scales with the same airborne Doppler‐radar data set.
Monsoons and the dynamics of deserts
Authors: Rodwell Mark J. , Hoskins Brian J. 
Published: February 2007

AbstractThe existence of subtropical deserts, such as the Sahara, has often been attributed to the annual‐mean, zonal‐mean Hadley circulation which shows strong descent in the subtropics. However, the zonal‐mean Hadley circulation shows considerable evolution over the course of the year with very strong subtropical descent during winter, but practically no zonal‐mean subtropical descent during summer when rainfall over the eastern Sahara and the Mediterranean is least. Charney (1975) proposed a biosphere‐albedo feedback mechanism whereby local anthropogenic effects related to over‐grazing could affect the radiative balance, enhancing summertime diabatic descent and leading to desertification of the subtropics in general. The present study, which uses an idealized model, suggests a monsoon‐desert mechanism for desertification whereby remote diabatic heating in the Asian monsoon region can induce a Rossby‐wave pattern to the west. Integral with the Rossby‐wave solution is a warm thermal structure that interacts with air on the southern flank of the mid‐latitude westerlies causing it to descend. This adiabatic descent is localized over the eastern Sahara and Mediterranean, and over the Kyzylkum desert to the south‐east of the Aral Sea, by the mountains of north Africa and south‐west Asia. Trajectories indicate that the monsoon‐desert mechanism does not represent a simple ‘Walker‐type’ overturning cell. Instead, the descending air is seen to be mainly of mid‐latitude origin. It is speculated that the monsoon‐forced adiabatic descent may result in clear air and, therefore, a local diabatic enhancement which effectively doubles the strength of descent. With this mechanism, desertification can be forced by remote changes in monsoon strength rather than by local effects. This conclusion is supported by the observed dramatic strengthening of descent over the Mediterranean and east Sahara during the onset of the Asian monsoon and, on the longer timescale, by relating prehistoric lake‐levels to Milankovitch‐monsoon forcing. The latter may help to explain the perceived discrepancies between the palaeoclimate of the eastern Sahara and the strength of a ‘tropic‐wide’ monsoon. The monsoon‐desert mechanism may not be confined to the Asian monsoon alone and the existence of other monsoon‐climate regions over the globe may, in a similar way, explain the observed summertime strengthening of the oceanic sub‐tropical anticyclones and the existence of western continental deserts and ‘Mediterranean‐type’ climate regions.
Diagnosing the global hydrological cycle from routine atmospheric analyses
Authors: Dodd James P. , James Ian N. 
Published: February 2007

AbstractThis paper discusses the extent to which atmospheric analyses produced routinely by forecasting centres can be used to diagnose the global hydrological cycle. These analyses directly represent the storage and large‐scale advection of water vapour through a combination of observations and a short model forecast. In this work, the analyses produced by the European Centre for Medium‐Range Weather Forecasts are used. Due to changes in the model formulation, which have a profound effect on the analyses, only results from three years of these initialized analyses are presented. The equations representing the conservation of water and water vapour in the atmosphere are derived and discussed, including consideration of the approximations commonly made. Diagnostics of the storage and transport of water vapour have been calculated. The results are qualitatively similar to diagnostics from other sources. In addition, with regard to attempts to derive a global relationship between the surface specific humidity and the total column water vapour, special consideration is given to the variations in the vertical distribution of water vapour. The profile of water vapour is found to be very sensitive to the competing processes responsible for the water balance in the atmosphere. Although surface fluxes of water cannot be directly derived from these analyses, a technique is introduced which puts a lower bound on the evaporation and precipitation that would be consistent with the analyses. The results of this technique are rather encouraging, and comparisons with other climatologies of surface water fluxes could provide another way of validating these atmospheric analyses. A diagnostic characterizing the time‐scale of the the small‐scale processes responsible for surface evaporation and precipitation is also introduced. This time‐scale is found to vary widely but is of the order of one day where variability in the hydrological cycle is important.
Cold pools in shear
Authors: Parker Douglas J. 
Published: February 2007

AbstractGravity currents exist in many forms and develop within many types of flow. The dynamics of long‐lived cumulonimbus are thought to be strongly influenced by the interaction of a spreading pool of cold, low‐level air, which may act as a gravity current, and the ambient sheared flow in the boundary layer. In interpreting two‐ and three‐dimensional numerical models of such phenomena, the behaviour of the vorticity dynamics in the ambient flow and the spreading cold air is commonly discussed. In order to investigate these vorticity ideas from a gravitycurrent perspective, three‐dimensional numerical simulations have been performed in which a cold pool spreads from an isolated source in the presence of a horizontal flow that is sheared with height. It has been found that the shear of the incoming ambient flow does not qualitatively alter the horizontal spread of the cold pool, and that the location of the region of maximum horizontal convergence remains on the along‐wind axis: the actual strength of the ambient wind, which is shown analytically to be measured by the wind strength at the head height, has a greater bearing on the cold‐pool shape than the shear. Consequently, it must be the details of the moist convection which play a crucial role in determining the cold pool's development in sheared flows. As such, this model is a general indication of the robustness of gravity‐current theory, even in the presence of mean shear. The vortex structure on the flanks of the spreading cold air is found to be significantly modified, with a reversal in the dominant sign of vorticity and an increase in its intensity, when such mean shear exists.
Measurement of the pressure field on a mountain
Authors: Vosper S. B. , Mobbs S. D. 
Published: February 2007

AbstractFour microbarographs, accompanied by wind vanes and anemometers, were deployed on a mountain called Black Combe (height 600 m) in Cumbria during a field experiment that took place in Novemberer 1991. the aims of the experiment were to measure the small flow‐induced pressure differences across the mountain and relate these to the local wind. These pressure differences may be used to calculate directly the drag exerted by the atmosphere on Black Combe.The mean pressure difference between pairs of sites calculated over periods when the wind speed is small is due to the hydrostatic pressure difference caused by the height differences between the instruments. Removing this component reveals pressure differences across the mountain of up to 2 hPa. the main result of the experiment is the high correlation of the pressure differences with differences of the quantity pu2/2, where p is the air density and u is the wind speed. It is shown that this result holds for a stratified fluid when far upstream the streamlines originate from levels of similar wind speed.Although reliable data were only obtained at three of the four stations, making the assumption that the dynamic pressure varies linearly across the mountain surface enables an estimate of the drag to be made. the average drag exerted on Black Combe over the period of the experiment is estimated to be 3.6 Pa.
A yardstick for global entropy‐flux
Authors: O'brien D. M. 
Published: February 2007

AbstractThe flux of entropy carried from the planet by radiation was estimated, from Earth Radiation Budget Experiment data, by Stephens and O'Brien. the present paper provides the numerical context for Stephens and O'Brien's results by deriving: (1) the maximum possible flux of entropy, subject only to the constraint that the planet be in energy balance with incoming solar radiation; (2) the latitude profiles of albedo and brightness temperature that maximize the entropy flux for a planet without horizontal transport of energy; (3) the entropy flux for a planet with such efficient transport that the brightness temperature is uniform from equator to pole. In each case, the entropy fluxes are computed as functions of the planetary albedo. the entropy fluxes in models (2) and (3) are found to be almost indistinguishable numerically. the result is extended to a family of latitude profiles for albedo and brightness temperature which interpolate between the dynamical extremes represented by models (2) and (3), and it is shown that all members of the family have essentially the same entropy‐flux. This suggests that measurements of the planetary entropy‐flux derived from satellite radiances may not enable us to distinguish easily between different atmospheric and oceanic circulations. the entropy flux for planet earth reported by Stephens and O'Brien is observed to lie close to the maximum.
Effects of airmass type on the interaction between warm stratocumulus and underlying cumulus clouds in the marine boundary‐layer
Authors: Martin G. M. , Johnson D. W. , Jonas P. R. , Rogers D. P. , Brooks I. M. , Barlow R. W. 
Published: February 2007

AbstractDecoupling of the marine boundary‐layer beneath stratocumulus clouds and the formation of cumulus clouds at the top of a surface‐based mixed layer (SML) have frequently been observed and modelled. Observations of the cumulus‐stratocumulus interaction during a Lagrangian study as part of the Atlantic Stratocumulus Transition Experiment (ASTEX), detailed in Martin et al. (1995), were made in an airmass which, although it was over the sea, was highly polluted, since it had recently come from industrial Europe. These observations suggested that the interaction was associated with significant changes in the stratocumulus thickness, microphysics and radiative properties. However, it was suggested that such changes may vary according to the type of airmass in which the cumulus—stratocumulus interaction was taking place. In this paper, two further case studies of the interaction in airmasses from different locations, exhibiting different thermodynamic properties and different levels of pollution, but being generally cleaner than that in the ASTEX Lagrangian study, are analysed to try to assess the influence of airmass type on the interaction. Although the cloud liquid‐water content in the stratocumulus generally increases locally in a region of cumulus cloud penetration, the changes in the droplet spectrum which result from mixing between the cumulus and stratocumulus droplets depend on the individual droplet spectra in the two cloud types. This, in turn, is influenced by the aerosol characteristics in the boundary layer, the updraught velocities associated particularly with the cumulus clouds, and the actual and relative cloud thickness of the cumulus and stratocumulus. the effects of penetrating cumulus clouds on the droplet‐effective‐radius in the stratocumulus, and hence on the cloud radiative properties, may therefore differ significantly between boundary layers in which the interaction is occurring.
Climatic variability of cloud radiative forcing
Authors: Weare Bryan C. 
Published: February 2007

AbstractMultiple‐regression models have been developed relating interannual departures of the Earth Radiation Budget Satellite net, long‐wave, and short‐wave cloud radiative forcing primarily to variations in the International Satellite Cloud Climatology Project cloud amounts for low, middle and high clouds, and cloud water. These models are used to evaluate the effects on cloud radiative forcing of specified and observed changes in cloud properties.The calculated changes of cloud forcing due to 16.5% increases in low‐, middleand high‐cloud amounts and 25% increases in cloud water are compared with those of a radiative‐transfer model. the two methods have results which agree with respect to the signs of the responses and the order of the significance of the independent variables for net cloud forcing. Overall, variations in cloud water have the largest effect on net cloud forcing; those in high cloud have the largest effect on long‐wave cloud forcing; those in cloud water make the largest contributions to short‐wave cloud forcing.Using the statistical models forced by one‐standard‐deviation variations in cloud properties, variations in high cloud are shown to have increased importance relative to the results for the 16.5 and 25% perturbations. In addition, variations in clear‐sky planetary albedo and long‐wave flux are also found to be important at higher latitudes.In a small sample of observed short‐term climate‐change scenarios, compensations amongst the effects of the variables alter the magnitude, and in two cases the sign, of the change in net cloud forcing relative to those resulting from uniform variations of the cloud properties.
A simple‐model study of the interaction between cumulus and stratocumulus clouds in the marine boundary layer
Authors: Martin G. M. , Jonas P. R. 
Published: February 2007

AbstractDecoupling of the marine boundary layer beneath stratocumulus clouds and the formation of cumulus clouds at the top of a surface‐based mixed layer have frequently been observed and modelled. However, little is known of the details of the interaction between the two cloud types, how this affects the microphysical and radiative properties of the stratocumulus, and how the cloud‐topped marine boundary layer evolves in such cases. Recent observational studies suggest that the microphysical and geometrical characteristics of the stratocumulus layer can be modified significantly by interaction with cumulus clouds. In this paper, a preliminary assessment of the relative contribution of different processes and initial boundary‐layer conditions, as suggested by the observations, to the outcome of the interaction between cumulus and stratocumulus clouds is carried out using a one‐dimensional entraining parcel model, which has been modified to simulate a cumulus cloud which penetrates a stratocumulus layer. the results suggest that the change in droplet size at the stratocumulus top as a result of mixing with the penetrating cumulus clouds is particularly sensitive both to the amount of pollution in the air mass and to the vertical structure of the boundary layer, and that these may be more important in determining the outcome of the interaction than the local temperature. A simple coalescence calculation is then used to show that the introduction of cumulus cloud droplets into a stratocumulus layer may alter the potential for drizzle formation in the cloud layer by introducing droplets which are of a different size from those already existing in the stratocumulus. However, it is clear that more detailed dynamical, microphysical and radiative‐transfer models are required, in conjunction with further observations, if these effects are to be parametrized for use in large‐scale numerical models.
A finite‐volume integration method for computing pressure gradient force in general vertical coordinates
Authors: Lin Shian‐Jiann 
Published: February 2007

AbstractA finite‐volume integration method is proposed for computing the pressure gradient force in general vertical coordinates. It is based on fundamental physical principles in the discrete physical space, rather than on the common approach of transforming analytically the pressure gradient terms in differential form from the vertical physical (i.e., height or pressure) coordinate to one following the bottom topography. The finite‐volume discretization is compact, involving only the four vertices of the finite volume. The accuracy of the method is evaluated statically in a two‐dimensional environment and dynamically in three‐dimensional dynamical cores for general circulation models. The errors generated by the proposed method are demonstrated to be very low in these tests.
Pollution transport by convective clouds in a mesocale model
Authors: Gimson N. R. 
Published: February 2007

AbstractThe effects of cumulus convection on the dispersion of passive tracers are considered in the framework of a mesoscale model, where the subgrid‐scale clouds are parametrized. the parametrization uses a ‘mass‐flux’ scheme, where guidance on vertical fluxes and cloud cover is obtained from a cloud‐resolving large‐eddy model run, and applied to the one‐dimensional time‐dependent vertical transport model. A set of three one‐dimensional schemes is presented, the simplest of which contains approximations which are appropriate to, and commonly used in large‐scale models. On the mesoscale, however, it is postulated that these approximations are no longer valid and a less idealised scheme is necessary.The schemes are compared using chosen initial tracer profiles, and their differences demonstrated in terms of the tracer evolution. It is found that the schemes diverge as the mass flux or grid‐box area are decreased. There is some sensitivity to mixing between the clouds and their surrounding environment, where differences between the schemes are decreased if entrainment dominates and increased if detrainment dominates at in‐cloud levels.The results imply a necessity for the scheme with fewest approximations when the host code is at mesoscale resolution, and this is implemented in the UK Meteorological Office non‐hydrostatic model.A case is studied, 23 October 1992, in which convective activity is present in the cold air over the United Kingdom behind a frontal system which passed through on the previous day. the dispersion from hypothetical emissions of conserved tracers is modelled, and results compared both with and without the convective transport parametrization scheme. the convection is widespread over the Atlantic Ocean, the Irish Sea and the North Sea, and more intense during the afternoon. the intersection of a mid‐level pollutant plume with the convection over the Irish Sea leads to a rapid vertical spread, both to the surface and cloud‐top levels, which are mainly between 4 km and 5 km. It is then transported in this redistributed form over the UK mainland, and over the North Sea where it encounters more maritime convection. It is also found that pollutants initially below the convective‐cloud‐base level are, through the effects of boundary‐layer turbulent diffusion, lifted to cloud‐top levels.
The information content of clear sky IASI radiances and their potential for numerical weather prediction
Authors: Prunet Pascal , Thépaut Jean‐Noël , Cassé Vincent 
Published: February 2007

AbstractThe estimation‐error covariance matrix associated with the analysis of the atmospheric state is used in one particular meteorological situation to examine the potential benefit of radiance data for numerical weather prediction. The gain of information content obtained from simulated Infrared Atmospheric Sounding Interferometer (IASI) data is studied and compared with the current information present in the TIROS Operational Vertical Sounder (TOVS) radiances.Nineteen independent items of information on a typical temperature/humidity profile are available from the IASI data, compared to six from the TOVS data. In terms of temperature, fine‐scale structures associated with a vertical resolution of 1 km are estimated with a 0.7 K error standard deviation. The gain of information for specific humidity is of the same order of magnitude as for temperature. Typically, humidity structures associated with 1 km vertical resolution are estimated with a relative error of 16%.The projection of the analysis‐error covariance matrix on atmospheric‐error structures (relevant for numerical weather prediction) gives a measure of the impact of the use of radiances on the observability of such structures. The results indicate that IASI data would be a decisive source of information for the analysis of such structures.Finally, a preliminary sensitivity study suggests that the degradation due to radiance noise associated with possible modifications of the IASI instrument, hardly affects the quality of the analysis.
On modified rotational and divergent eddy fluxes and their application to blocking diagnoses
Authors: Nakamura Mototaka 
Published: February 2007

AbstractRotational and divergent eddy fluxes, introduced by Marshall and Shutts in the early 1980s, are modified to account for significant divergence in the mean flow and self advection by large‐amplitude eddies. Use of the modified rotational and divergent eddy fluxes is demonstrated by diagnosing two blocking episodes with potential vorticity.