bluuebunny/crossref_metadata_2025_split huggingface dataset:
Entrainment in cumulus clouds, II: Drop size variability
Authors: Telford James W. , Kim Kyung‐Eak , Keck Thomas S. , Hallett John 
Published: February 2007

AbstractA new analysis method for examining the effects of entrained subsaturated air on cloud drop sizes is described. This, independently, gives information about height changes following dilution with subsaturated air, and hence about whether the cloud parcels received the subsaturated air when higher or lower than their present level. The physical details of the entrainment mechanisms are first discussed to provide a basis for interpreting the data.Drop‐size distributions, measured by a Forward Scattering Spectrometer Probe and obtained from aircraft cloud samples at constant altitude in convective clouds in Montana, are used to analyse these cloud processes. The volume averaged drop radii for each parcel (i.e. each measured spectrum), in several clouds for several horizontal traverses in each, are plotted against concentration, given here in the form of drop numbers per unit mass of air. This concentration from parcel to parcel depends on the fraction of subsaturated air which has been entrained. The entity‐type entrainment mixing mechanism provides a satisfactory explanation for all the examples found in this study, where the average drop radius changes very little with drop‐number concentration from parcel to parcel. Alternative mechanisms are discussed and found to be insufficient.
On the dynamics of convection waves
Authors: Sang J. G. 
Published: February 2007

AbstractThe linearized system of atmospheric equations is solved analytically for a two‐layer model to study the characteristics of convection waves. The solutions show that convection waves induced by a disturbance are composed of trapped waves with selected wave numbers that are determined by the interaction between the convective boundary layer and the overlying stable layer. The explicit relation of wave‐number selection defines the pattern of the vortices in the convection waves, which may appear as cloud streets, cellular convections or convective thermals. The wave‐number selection explains the formation of longitudinal and cellular convections and the transition between them. The analytical solutions of the convection waves reveal, theoretially, the structure of the large vortices in the convective boundary layer, and bring to light the effects of the upper stable layer on these vortices.
The exchange of atmospheric ammonia with vegetated surfaces. I: Unfertilized vegetation
Authors: Sutton M. A. , Flower D. , Moncriefe J. B. 
Published: February 2007

AbstractMicrometeorological techinques have been used to measure the surface/atmosphere exchange of gaseous ammonia (NH3) over semi‐natural and unfertilized plant communities. A range of temperate ecosystems have been examined, including moorland, cut meadows and coniferous forest. Apart from a recently harrowed calcareous grassland, where deposition velocities, Vd (1 m), were in the range 1–11 mm s−1, ammonia was found to deposit rapidly (Vd typically 15–20 mm s−1 for short vegetation) with surface resistances, Rc, not significantly different from zero. Limited measurements over forest were consistent with a larger Vd (mean 66 mm s‐1) as a consequence of increased turbulence over the aerodynamically rough surface.These observations show that, except for the harrowed calcareous site, ammonia is efficiently deposited to leaf surfaces, and that the mean air concentration at the surfaces approaches zero. The results are used as a basis for estimating annual dry deposition of ammonia to semi‐natural and unfertilized vegetation. For example sites in the British Isles, subject to typical background air concentrations, the estimated annual dry deposition of ammonia is in range 2‐41 kg fixed nitrogen per hectare, showing that this is a major source of fixed atmospheric nitrogen.
Bayesian quality control using multivariate normal distributions
Authors: Ingleby N. Bruce , Lorenc Andrew C. 
Published: February 2007

AbstractAn expression for the probability density of any distribution of observed values (given background values of known accuracy) is derived from the properties of multivariate normal distributions. This is used in the quality control of observations—‘good’ and ‘bad’ observations are assumed to have errors from a normal distribution and from a distribution giving no useful information respectively.Three Methods of quality control are presented and compared; two of these are based on the probability density derived above, and the third is based on a related maximum probability analysis. They differ in the optimality principal used: Individual Quality Control finds the most likely quality (i.e. good or bad) for each observation, given information from all the others; Simultaneous quality Control finds the most likely combination of qualities; while Variational Quality Control is based on a variational analysis which finds the most likely true values. The quality control should be considered as part of the ‘analysis’ process of using the observations; these approaches to quality control are considered as approximations to a system giving the ‘best’ analysis, based on minimizing a Bayesian loss function. Approximation are also necessary in their practical implementations; the effect of these on various operational schemes is discussed.The multi‐observation framework used includes the ‘background’ check as a special case, and it is extended to deal with observations with common sources of gross error. Applications to multi‐level checks, bias checks and checks for known error patterns are sketched.As a by‐product the standard statistical interpolation formulae are derived from the properties of normal distributions, thus demonstrating the implicit dependence of statistical interpolation on the normal distribution.
On the link between cloud‐top radiative properties and sub‐cloud aerosol concentrations
Authors: Raga G. B. , Jonas P. R. 
Published: February 2007

AbstractMicrophysical observations obtained in cumulus clouds over the sea are presented and related to background pollution levels. The sub‐cloud aerosol concentrations vary from 50 to 5000 cm−3, which can hardly be considered ‘maritime’. Observed droplet size distributions are used to determine radiative properties using Mie theory. Functional expressions are derived for the extinction cross‐section and the single scattering albedo as functions of the sub‐cloud aerosol concentrations.
Parametrization of the depth of the entrainment zone above the daytime mixed layer
Authors: Gryning Sven‐Erik , Batchvarova Ekaterina 
Published: February 2007

AbstractA new parametrization for the depth of the daytime entrainment zone is presented. It is derived from a simple energy budget, in which the eddy velocity in the mixed layer penetrating the overlying stable entrainment zone is taken to be proportional to the characteristic top‐down velocity scale. It gives a better ordering of the data of two earlier studies by Deardorff et al. and Boers and Eloranta as compared with traditional parcel theory based on the bottom‐up velocity scale. Furthermore, it can be seen that the data from the two studies belong to different regimes of the characteristic scaling parameter.
Aircraft measurements of water vapour continuum absorption at millimetre wavelengths
Authors: English S. J. , Guillou C. , Prigent C. , Jones D. C. 
Published: February 2007

AbstractIn preparation for the future AMSU‐B mission, measurements of clear‐air radiative transfer at 89 and 157 GHz were made using a radiometer operating on the C‐130 aircraft of the UK Meteorological Research Flight. Observations of water vapour and oxygen absorption in arctic, middle latitude and tropical atmospheres were obtained. Four different empirical models of continuum absorption are compared and tested; three are found to be more representative of the observations in middle‐latitude conditions. In the tropical Atlantic, large model deficits are found for these models. The fourth is more representative of observations in the tropics but over‐estimates absorption at middle and arctic latitudes. The results confirm observations taken during other atmospheric field experiments, and laboratory measurements, that a model absorption deficit at high humidity exists in the microwave and millimetre wavelength regions.
Convective processes and tropical atmospheric circulations
Authors: Raymond David J. 
Published: February 2007

AbstractThis paper presents calculations of an idealized large‐scale tropical circulation driven solely by sea‐surface‐temperature anomalies. Cumulus clouds are represented by a minimal convective parametrization that includes a highly simplified representation of cloud microphysical effects. A grey‐atmosphere model is used for longwave radiation, while a fixed heating profile is imposed for the short‐wave component. Variations in radiation due to fluctuating humidity and cloud amount are suppressed so that other processes can be examined without complicating side‐effects. Surface fluxes are handled by the bulk aerodynamic formula modified for weak winds.The diabatic parametrizations are embedded in a simple, hydrostatic, two‐dimensional, non‐rotating model of the atmosphere. The resulting flow resembles the Walker circulation, but lacks the meridional transports of angular momentum present in the real phenomenon. The strength and structure of this ‘mock Walker circulation’ is sensitive to the strength of the sea‐surface‐temperature anomaly and to variations in physical processes in clouds.A qualitative change in the structure of the circulation occurs when the sea‐surface‐temperature anomaly exceeds a certain threshold. This result may explain the systematic differences seen in convective‐heating profiles between the eastern Atlantic and the western Pacific. Variations in the ratio of convective to stratiform precipitation, the fraction of rain evaporated, and the strength of convective wind gusts at the surface also affect the strength and form of the circulation. The dependence of tropical flows on the formation and evaporation of precipitation provides a way for cloud physical processes to affect global circulations. The importance of this mechanism relative to others such as the interaction of clouds with radiation needs to be assessed.
Large‐eddy simulation of a neutrally stratified boundary layer: A comparison of four computer codes
Authors: Andren A. , Brown A. R. , Mason P. J. , Graf J. , Schumann U. , Moeng C.‐H. , Nieuwstadt F. T. M. 
Published: February 2007

AbstractA neutrally stratified Ekman layer limited in depth by a stress‐free rigid lid has been simulated using four different large‐eddy simulation computer codes. This is the second large‐eddy simulation intercomparison study in a series of simulations of different flows aimed at a systematic analysis of large‐eddy simulation sensitivity to subgrid scale model assumptions and numerical methods. The resolution was chosen to be coarse so that the results might be sensitive to the details of the various subgrid scale model formulations. Stochastic backscatter of subgrid kinetic energy and scalar variance was included in one subgrid scale model.Profiles of first and second moments, budgets of resolved‐scale second moments, and spectra have been analyzed. It is found that significant differences in the lowest third of the boundary layer result from moderate changes in subgrid scale eddy diffusivities or from the inclusion of backscatter.To isolate effects of the various numerical methods, a set of tests with a fixed subgrid scale K‐profile were made. When run with a common subgrid scale model the results from the four large‐eddy simulation codes converge. This test indicates that the large‐eddy simulation results are more sensitive to the subgrid scale model formulations than to the various numerics.
The impact of changing the horizontal diffusion scheme on the northern winter climatology of a general circulation model
Authors: Stephenson David B. 
Published: February 2007

AbstractA general circulation model was used in perpetual January mode, to study the wintertime climatological sensitivity to changes in the horizontal hyper‐Laplacian diffusion scheme at both spectral T21 and T42 horizontal resolutions.Analysis of the global energy cycle revealed a large impact especially on the conversion of eddy to zonal kinetic energy. Less diffusion resulted in less zonal available potential energy, weaker baroclinic conversions and caused the barotropic conversion of eddy to zonal kinetic energy to become significantly weaker. It was found that by weakening the diffusion in the T21 model, the eddy kinetic energy increased and became more realistic yet at the same time the conversion of eddy to zonal kinetic energy decreased and became less realistic.Weakening the horizontal diffusion caused a northern hemisphere tropospheric response similar to that obtained by increasing the upper‐level orographic gravity wave drag in the model. The zonal‐mean zonal winds weakened poleward of the subtropical jets, the Ferrel cells weakened and the troposphere warmed up at high latitudes. The weakening of the zonal wind and the Ferrel cells was associated with a weakening of the upper‐level convergence of the poleward eddy momentum flux and the wave‐mean flow interaction was demonstrated using Eliassen–Palm diagnostics. A substantial part of the change in the poleward eddy momentum flux was due to large changes in the poleward momentum flux of the transient eddies in the mid‐latitude stormtracks and changes in their horizontal phase tilt. A large northern hemisphere stationary‐wave change appeared in these regions and had a similar structure in the Atlantic sector to that seen when strengthening upper‐level gravity wave drag.
Boundary‐layer parametrization of drag over small‐scale topography
Authors: Xu Dapeng , Taylor Peter A. 
Published: February 2007

AbstractIn regional and global numerical weather prediction and climate models, sub‐grid scale effects are often accounted for as part of the surface and boundary‐layer parametrizations. There are several proposals (Wood and Mason 1993; Belcher et al. 1993; Taylor et al. 1989) regarding the parametrization of drag over small‐scale topography. We review these proposals and make comparisons between them, utilizing results from an efficient numerical model with second‐order turbulence closure (Non‐linear mixed spectral finite difference model—Xu et al. 1994). Model results cover an extensive range of parameters, such as hill slope, roughness length, horizontal scale and hill shape. We initially confine ourselves to the neutrally stratified surface boundary‐layer but extensions to the planetary boundary‐layer and for stable stratification are in progress.
A melting‐layer model and its use in correcting for the bright band in single‐polarization radar echoes
Authors: Hardaker P. J. , Holt A. R. , Collier C. G. 
Published: February 2007

AbstractA new dynamic microphysical model of the melting layer is presented, developed from a previous model. This new model is applied to obtain vertical reflectivity profiles (VRPs) and vertical attenuation profiles (VAPs) through the melting layer which both allow for and neglect propagation effects. The model profiles are compared with measured profiles at a range of different frequencies; in particular, a detailed comparison was made for frequencies in the X‐band. The VRPs are used to generate relations between bright‐band peak‐intensity and surface rainfall‐rate, R0, at frequencies in the S‐, C‐ and X‐bands. Relations between other model bright‐band parameters and R0 are found to be more complicated functions of model‐input parameters. The model profiles are also applied to the problem of correcting for the radar bright‐band in the Meteorological Office's radar data. Early results, from three detailed case studies, seem promising; however, the technique has yet to be tested in an operational environment.
The retrieval of cloud radiative and microphysical properties using combined near‐infrared and microwave radiometry
Authors: Taylor J. P. , English S. J. 
Published: February 2007

AbstractThe optical depth of a cloud is an important parameter in describing its radiative properties. Many schemes have been developed to retrieve optical depth from measurements of reflectance in the visible or near‐infrared. In this paper the impact of cloud inhomogeneity and vertical structure on the retrieval of optical depth is discussed. Retrievals of cloud optical depth and effective radius using the multi‐channel radiometer flown on the UK Meteorological Research Flight C130 are presented for clouds studied during the Atlantic Stratocumulus Transition Experiment.A new technique for retrieving cloud optical depth, which is less susceptible to cloud inhomogeneity, using combined microwave and near‐infrared radiometry, is presented. This technique is tested for three flights both in the Azores and the South Atlantic and shows good agreement with in situ measurements. Finally the use of these retrieved parameters in radiation codes is considered.
Potential vorticity and the electrostatics analogy: Ertel—Rossby formulation
Authors: Thorpe Alan J. , Bishop Craig H. 
Published: February 2007

AbstractThe isomorphism between the theory of electrostatics and the quasi‐geostrophic potential vorticity is extended to the Ertel‐Rossby potential vorticity. Anomalies of mass‐weighted potential vorticity are defined relative to an arbitrary zonal‐mean or horizontal‐average flow and given in terms of the divergence of a vector field. the vector is the sum of linear and non‐linear contributions and can be written as a dielectric tensor acting on the geopotential gradient. the linear components of the tensor differ from those for the quasi‐geostrophic potential vorticity only if there exists a vertical variation of background potential vorticity, such as occurs at the tropopause, or if there is shear of the assumed background flow. the non‐linear components are absent in the quasi‐geostrophic case.The forms of free, bound and total charge are defined for accurate non‐linear forms of the potential vorticity. the free‐space Green's function for the operator defining the total charge is identical to that for quasi‐geostrophic theory and provides a scheme whereby the field attributed to each potential vorticity element is an invariant quantity. One of the most important results arising from this formulation is that the non‐linearities in the definition of potential vorticity can be neglected when considering the far‐field effect of potential vorticity anomalies. an analytical example of these ideas is given for a uniform anomaly of semi‐geostrophic potential vorticity embedded in an otherwise uniform background potential vorticity. the dielectric constant and bound charge are calculated and give a clear insight into the differences between this and the quasi‐geostrophic solution.
A global shallow‐water numerical model based on the semi‐lagrangian advection of potential vorticity
Authors: Bates J. R. , Li Y. , Brandt A. , McCormick S. F. , Ruge J. 
Published: February 2007

AbstractA global shallow‐water primitive equation model based on the semi‐Lagrangian advection of potential vorticity is presented. A modification of the basic advection scheme needed to stabilize Rossby waves is introduced. the divergence and continuity equations are the remaining governing equations. A two‐time‐level semi‐Lagrangian semi‐implicit numerical scheme that avoids forward extrapolation of non‐linear terms is used. This leads to a set of non‐linear implicit equations to be solved at each time‐step. the wind field is expressed in terms of a streamfunction and velocity potential, and a spatial discretization based on second‐order finite differences on an unstaggered grid is used. the implicit equations are solved simultaneously using a non‐linear multigrid method.The model is integrated for periods of up to 50 days at various resolutions, using a variety of initial conditions including real data. Comparisons with an existing semi‐Lagrangian finite‐difference shallow‐water model and an Eulerian spectral shallow‐water model are carried out. the new model is found to integrate stably and efficiently, and to require no noise suppressors other than the inherent diffusivity associated with the interpolations. the model gives results that are, in general, very close to those of the comparison models, but a case of highly non‐linear flow (where the true solution is unknown) is presented in which it gives results that stand notably apart.
The ECMWF Ensemble Prediction System: Methodology and validation
Authors: Molteni F. , Buizza R. , Palmer T. N. , Petroliagis T. 
Published: February 2007

AbstractThe European Centre for Medium‐Range Weather Forecasts (ECMWF) Ensemble Prediction System (EPS) is described. In addition to an unperturbed (control) forecast, each ensemble comprises 32 10‐day forecasts starting from initial conditions in which dynamically defined perturbations have been added to the operational analysis. The perturbations are constructed from singular vectors of a time‐evolution operator linearized around the short‐range‐forecast trajectory. These singular vectors approximately determine the most unstable phase‐space directions in the early part of the forecast period, and are estimated using a forward and adjoint linear version of the ECMWF numerical weather‐prediction model. An appropriate norm is chosen, and relationships between the structures of these singular vectors at initial time and patterns showing the sensitivity of short‐range forecast error to changes in the analysis are discussed. A methodology to perform a phase‐space rotation of the singular vectors is described, which generates hemispheric‐wide perturbations and renormalizes them according to analysis‐error estimates from the data‐assimilation system.The validation of the ensembles is given firstly in terms of scatter diagrams and contingency tables of ensemble spread and control‐forecast skill. The contingency tables are compared with those from a perfect‐model ensemble system; no significant differences are found in some cases. Brier scores for the probability of European flow clusters are presented, which indicate predictive skill up to forecast‐day 8 with respect to climatological probabilities. The dependence of these scores on flow‐dependent model errors is also discussed. Finally, ensemble‐member skill‐score distributions are presented, which confirm the overall satisfactory performance of the EPS, particularly in summer and autumn 1993. In winter, cases of poor performance over Europe were associated with the occurrence of a split westerly flow with a blocking high and/or a cut‐off low in the verifying analysis.Two cases are studied in detail, one having large ensemble dispersion, the other corresponding to a more predictable situation. The case studies are used to illustrate the range of ensemble products routinely disseminated to ECMWF Member States. These products include clusters of flow types, and probability fields of weather elements.