bluuebunny/crossref_metadata_2025 huggingface dataset:
On the link between cloud‐top radiative properties and sub‐cloud aerosol concentrations
Authors: Raga G. B. , Jonas P. R.
Published: February 2007

Microphysical 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

A 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

In 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

This 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

A 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

A 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

In 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

A 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

The 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

The 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

A 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

The 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.
Sensitivity of forecast errors to initial conditions
Authors: Rabier F. , Klinker E. , Courtier P. , Hollingsworth A.
Published: February 2007

The adjoint method has been used to calculate the sensitivity of short‐range forecast errors to the initial conditions. The gradient of the energy of the day 2 forecast error with respect to the initial conditions can be interpreted as a sum of rapidly growing components of the analysis error. An analysis modified by subtracting an appropriately scaled vector, proportional to the gradient, provides initial conditions for a ‘sensitivity integration’ that can be used to diagnose the effect of initial‐data errors on forecast errors.

Statistics of sensitivity calculations for the month of April 1994 characterize the sensitivity patterns as small‐scale, middle or lower tropospheric structures which are tilted in the vertical. The general pattern of these structures is known to be associated with the fastest possible growth of forecast error. When used as initial perturbations, they evolve rapidly into synoptic‐scale structures, propagating both downstream and to higher atmospheric levels.

On average, the sensitivity integration corrects for about a tenth of the day 2 forecast error, which indicates that indeed not all of the error is in the fastest‐amplifying modes. But the fraction of the error corrected at day 2 is important for an improvement in the medium‐range, as this fraction continues to grow substantially in the non‐linear regime. These results have proved that there is still scope for great improvement in the medium‐range forecast, particularly over Europe, by a better description of the initial conditions. The sensitivity experimentation suggests that many cases of major forecast‐errors may be explained by defects in the analysis. A small but well‐chosen change in the analysis can frequently improve the forecast quality.
The effect on regional and global climate of expansion of the world's deserts
Authors: Dirmeyer Paul A. , Shukla J.
Published: February 2007

An atmospheric general‐circulation model with realistic land‐surface properties is used to investigate the climatic effect of doubling the extent of the earth's deserts. Control and anomaly integrations are performed for 10 years. In the anomaly case, deserts are expanded over northern and southern Africa, Australia, south‐central Asia, south‐western North America, and parts of South America.

In the anomaly case, the troposphere is cooler across most of the tropics and subtropics, including all areas where desertification occurs. Remote effects in the winter circulation include a pronounced trough over northern Europe, and increased geopotential heights over the southern oceans. The response of climate in deforested areas is not uniform between the regions. The magnitude and seasonality of the changes, particularly in precipitation, seem to be functions of the monsoon regime. Northern Africa suffers a strong year‐round drought, suggesting this area is most sensitive to desertification, and southern Africa has a somewhat weaker year‐round drought. Rainfall over desertified parts of Asia, Australia and western South America decreases only during the summer, and not during the winter. North America and the Nordeste region of Brazil show the least sensitivity to desertification. Most regions show a notable correlation between decreases in evapotranspiration and resulting precipitation. However, regional land–sea distributions in the vicinity of each desertified area appear to determine the sensitivity and response of the local climate via induced changes in moisture flux convergence. Thus, different regions exhibit different responses of climate to desertification. Surface temperature decreases over most desertified areas, owing to reduced absorption of short‐wave radiation by the brighter surface. Some regions, particularly the Sahel, show an increase in surface temperature caused by decreased soil moisture and latent‐heat flux.
Dynamical structure functions in a four‐dimensional variational assimilation: A case study
Authors: Thépaut Jean‐Noël , Courtier Philippe , Belaud Gilles , Lemaǐtre Gwendal
Published: February 2007

This paper contributes to the understanding of the structure functions used implicitly in the four‐dimensional variational assimilation (4D‐Var) developed at the European Centre for Medium‐Range Weather Forecasts in the last few years. The theoretical equivalence between 4D‐Var and the Kalman filter allows us to interpret (after normalization by the error standard deviations) the analysis increments produced by one single observation as the structure functions used implicitly in 4D‐Var. The shape of the analysis increments provides a three‐dimensional picture of the covariances of the background errors, modified by the dynamics.

We study a baroclinic situation and observations have been regularly distributed along a latitude circle crossing the baroclinic wave. Eight standard pressure levels have been considered to sample the vertical.

The forecast error standard deviations and the structure functions implied in 4D‐Var may differ considerably from those used in the 3D‐Var analysis. Unlike 3D‐Var, the structure functions are flow dependent: the effective background error standard deviation can be four times larger and the correlation length scale twice as short in the vicinity of a low. A meridional extension of the experimentation at the surface shows that the effective background error standard deviations at 1000 hPa are largest in the areas of strong pressure gradient.

We quantify the link between the analysis increments produced by 4D‐Var and the fastest growing perturbations over the same time interval. In the depression, the explained variance of the analysis increments by the first 13 singular vectors reaches 30%.

The impact of the temporal dimension is assessed. A period of 24 hours seems a minimum for the increments to develop fully baroclinic structures.
Development of convection along the SPCZ within a Madden‐Julian oscillation
Authors: Matthews Adrian J. , Hoskins Brian J. , Slingo Julia M. , Blackburn Mike
Published: February 2007

A subtropical Rossby‐wave propagation mechanism is proposed to account for the poleward and eastward progression of intraseasonal convective anomalies along the South Pacific convergence zone (SPCZ) that is observed in a significant proportion of Madden–Julian oscillations (MJOs). Large‐scale convection, associated with an MJO, is assumed to be already established over the Indonesian region. The latent heating associated with this convection forces an equatorial Rossby‐wave response with an upper‐tropospheric anticyclone centred over, or slightly to the west of, the convection. Large potential‐vorticity (PV) gradients, associated with the subtropical jet and the tropopause, lie just poleward of the anticyclone, and large magnitude PV air is advected equatorwards on the eastern side of the anticyclone. This ‘high’ PV air, or upper‐tropospheric trough, is far enough off the equator that it has associated strong horizontal temperature gradients, and it induces deep ascent on its eastern side, at a latitude of about 15–30°. If this deep ascent is over a region susceptible to deep convection, such as the SPCZ, then convection may be forced or triggered. Hence convection develops along the SPCZ as a forced response to convection over Indonesia. The response mechanism is essentially one of subtropical Rossby‐wave propagation. This hypothesis is based on a case study of a particularly strong MJO in early 1988, and is tested by idealized modelling studies. The mechanism may also be relevant to the existence of the mean SPCZ, as a forced response to mean Indonesian convection.
Studies with a flexible new radiation code. II: Comparisons with aircraft short‐wave observations
Authors: Taylor J. P. , Edwards J. M. , Glew M. D. , Hignett P. , Slingo A.
Published: February 2007

Calculated irradiances from a new radiation code are compared with in situ observations of short‐wave irradiances from the UK Meteorological Office's C‐130 aircraft. Three cases of clear skies are studied and four where a liquid‐water boundary‐layer cloud was present. Under clear‐sky conditions the modelled and in situ observations agree to within 3%, which is the estimated accuracy of the observations. In the cloudy‐sky cases the albedo and transmittance agree to within ±0.1 but the absorption in the model is higher than that observed, sometimes by a factor of two; there is no evidence of anomalous absorption in the observations. The observed absorptions do not exceed 6% for the stratocumulus cases considered. The results clearly identify the problems of representing inhomogeneous clouds as plane parallel layers in radiation models. Analysis of the variability of the cloud microphysics provides some insight into the importance of regions of low optical depth within the clouds.
The generation of thermal oscillations in an ocean model
Authors: Cai Wenju
Published: February 2007

Recently, interdecadal oscillations induced by saline forcing have been extensively investigated using ocean models driven by a Haney relaxation‐condition for temperature and a flux condition for salinity (traditional mixed boundary‐conditions). By contrast, oscillations induced by thermal forcing remain largely unexplored. In this study, the generation of persistent thermal oscillations in the Bryan–Cox–Semtner model is described.

Initially the model is spun up by restoring surface temperature and salinity in the model to prescribed climatologies. When a steady state is reached, switching to mixed boundary‐conditions leads to a collapse in the modelled thermohaline circulation. Such a collapse suggests that the freshwater forcing is too strong. A reduction in the forcing of the freshwater flux then prevents the collapse, but no persistent oscillation occurs. When the thermal‐forcing condition is provided by a non‐traditional Schopf boundary‐condition, and when the reduced freshwater‐flux is applied, persistent thermal oscillations occur. Upon switching from the spin‐up state, the Schopf boundary‐condition allows a change only in the coupling strength, with the atmospheric heat transport remaining the same. This leads to a thermally dominant regime with a negative feedback which is crucial for the generation of thermal oscillations. Under mixed boundary‐conditions, such oscillations are less likely because the strong restoring thermal condition suppresses the negative feedback.

The ways in which oscillations depend upon various parameters of the model, such as the strength of surface freshwater‐fluxes, vertical mixing coefficients, and timescales for damping thermal anomalies, are examined. It is found that the two most important requirements for the generation of thermal oscillations are a proper representation of the strength of thermal‐coupling and a not‐too‐strong flux of freshwater, and that the period of oscillation is rather insensitive to the choice of vertical mixing coefficient. The possible implication for coupled ocean–atmosphere modelling is discussed.
On the theory of cisk
Authors: Smith Roger K.
Published: February 2007

Theories for the cooperative interaction between cumulus convection and a developing cyclonic vortex proposed independently by Ooyama and by Charney and Eliassen in the early 1960s are re‐examined in the light of recent studies and recent criticisms. It is suggested that the criticisms have sidestepped the concept of cooperative intensification as envisaged by Ooyama. However, a later, but much neglected, contention by Ooyama is supported: this is that the linear theories worked out by Charney and Eliassen and others are, in themselves, physically illfounded. the relationship between Ooyama's cooperative intensification theory and the recent theory of Emanuel based on Wind‐Induced Surface Heat Exchange (WISHE) is also discussed. the main difference between these two theories appears to be the degree of emphasis placed by the respective authors on the relative importance of convection and surface fluxes in the intensification of hurricanes.
A Lagrangian‐based analysis of extratropical cyclones. I: The method and some applications
Authors: Wernli Heini , Davies Huw C.
Published: February 2007

A Lagrangian‐based method of analysis is introduced to examine the space‐time structure and dynamics of extratropical cyclogenesis. the three‐step method involves the calculation of extensive ensembles of trajectories, the evaluation of the Lagrangian time‐trace of physical variables along these paths, and the application of objective selection criteria to identify significant Lagrangian structures. the approach can provide a qualitative depiction of, quantitative information about, and dynamical insight into, key features of mid‐latitude cyclones. In this, the first of a two‐part study, examples are given of the application of the method. A case‐study approach has been adopted and used to identify coherent ensembles of trajectories (CETs) whose dynamics are seminal to an event of Atlantic cyclogenesis, to trace the origin of subsynoptic potential vorticity anomalies, and to analyse stratosphere‐troposphere exchange accompanying extratropical cyclogenesis. In addition, a statistical investigation has been made of the climatology of CETs in the northern hemisphere for one particular winter month. Some consideration has been given to the shortcomings and potential of the method, and its complementarity with other approaches.
Similarities of the deacon cell in the southern ocean and Ferrel cells in the atmosphere
Authors: Karoly D. J. , McIntosh P. C. , Berrisford P. , McDougall T. J. , Hirst A. C.
Published: February 2007

The meridional circulation in the ocean and the atmosphere, when averaged over longitude and time at constant height, shows a number of cells. Most of these appear as direct circulations, with ascent in response to forcing which reduces the density. There are several indirect circulations, particularly the Deacon cell in the Southern Ocean and the Ferrel cells in the mid‐latitude atmosphere, which appear to act against the mean density‐gradient in regions of no apparent mean density‐forcing. When the zonal‐mean circulation is calculated in density coordinates, both the Deacon cell and the Ferrel cells disappear. A transformation of the zonal‐mean circulation as a function of height is used to give the residual mean circulation, which is remarkably similar to the zonal‐mean circulation in density coordinates in both the Southern Ocean and the atmosphere. This shows that the existence of the Deacon and Ferrel cells is the result of correlations of zonal variations of density and meridional flow, and not of zonal‐mean density‐forcing. Zonal variations associated with the time‐mean eddies in the Southern Ocean are the main contributors to the Deacon cell, while correlations in transient weather systems are the major factor leading to Ferrel cells.
Numerical simulation of an extreme rainfall event in Catalonia: Role of orography and evaporation from the sea
Authors: Romero Romualdo , Ramis Clemente , Alonso Sergio
Published: February 2007

During the evening of 12 November 1988, heavy rainfall occurred in Catalonia (north‐east Spain); as a result there was flooding and heavy loss. In this paper we present a 12‐hour numerical simulation of that event by means of a meso‐β scale numerical model. the results show that the model is capable of giving the localization of observed precipitation satisfactorily, but it underestimates the amount of rainfall. Other numerical experiments were done to show the effects of orography and evaporation from the Mediterranean. the effect of orography is decisive as regards the spatial structure of precipitation over land, since it acts to concentrate the precipitation in the coastal zone. the effect of evaporation from the sea is very weak, probably on account of the shortness of the simulation.
Illuminance turbidity parameters and atmospheric extinction in the visible spectrum
Authors: Gueymard C. , Kambezidis H. D.
Published: February 2007

Expressions for the Linke, TLi, and Unsworth‐Monteith, TUi, turbidity parameters in the visible region of the solar spectrum are derived for application in illumination studies. They are based on new determinations of the visible transmittances of various atmospheric constituents, obtained by spectrally integrating the atmospheric transmittances calculated with a spectral irradiance research code. the influence of pressure, of circumsolar radiation, and of different atmospheric constituents (including both stratospheric and tropospheric nitrogen dioxide) on the interpretation of turbidity is discussed. TUi is shown to be virtually free of parasitic variations with air mass or other factors, and to approximate the aerosol optical thickness at 555 nm quite closely.

The experimental TUi has been obtained from measurements of direct illuminance for two days at Albany, New York, and compared to the aerosol optical thickness measured with a multiwavelength radiometer. the observed fair agreement between TUi and the experimental optical thickness at 555 nm suggests that the latter may be accurately approximated by the former, using equipment that can be easily calibrated in the laboratory—unlike sun photometers. the use of TUi in subsequent measurement campaigns is recommended rather than that of TLi.
Diurnal winds around lake Tanganyika
Authors: Savijärvi Hannu
Published: February 2007

Regular diurnal variations of coastal winds are observed around Lake Tanganyika during the dry season, when insolation is intense and the prevailing trade winds are weak. the long deep lake is at the bottom of the East African rift valley. A two‐dimensional mesoscale model across the schematic rift‐valley topography is used to study these local tropical winds. the full model gives a good simulation when compared with the observed winds on the east coast. It turns out that about half of the strong diurnal wind variation there, in the form of slope winds, is due to the sloping mountains. Roughly one quarter is due to the thermal lake effect (sea breeze), and one quarter is due to the existence of the south‐easterly trade wind, which enhances the sea‐breeze circulation component considerably during daytime, and adds on to the downslope winds during night‐time. the wind field across the lake is quite variable from hour to hour and from place to place, yet predictable to a good extent during the dry season, when the external conditions are rather similar from day to day.
The zonal momentum balance in an eddy‐resolving general‐circulation model of the southern ocean
Authors: Stevens David P. , Ivchenko Vladimir O.
Published: February 2007

The momentum balance in the zonally unbounded region of the Southern Ocean is examined using an eddy‐resolving ocean general‐circulation model (namely FRAM). Momentum, which is input at the surface and accelerates the Antarctic Circumpolar Current, is transferred down the water column and removed by topographic form stress. Bottom friction and lateral eddy viscosity are found to be negligible. the poleward flux of eastward momentum has a small effect in redistributing momentum. In spite of this, below the wind‐driven surface layer and above the level of topography, the poleward momentum‐flux divergence provides the main balance along with the ageostrophic flux of planetary vorticity (although the magnitude of these terms is an order of magnitude smaller than the wind stress). Below the Ekman layer, standing eddies produce a drag on the flow whilst transient eddies accelerate the flow. However, the impact of transient eddies is smaller. the downward transfer of momentum is achieved by interfacial form stress. This can be understood in terms of a poleward density (heat) flux. the main contribution comes from standing eddies, with a smaller contribution from transient eddies. Both contributions assist the transfer. the flux of density (heat) from the neighbouring oceans (to the north and south of the Drake Passage latitudes) influences the depth penetration of zonal momentum, particularly in the upper 1000 m. the Johnson‐Bryden theory is generalized to give an additional term which is proportional to the stream function for the residual circulation associated with Eliassen‐Palm cross‐sections.
A physically based scheme for the treatment of stratiform clouds and precipitation in large‐scale models. I: Description and evaluation of the microphysical processes
Authors: Rotstayn Leon D.
Published: February 2007

A stratiform‐cloud and precipitation scheme, incorporating prognostic variables for cloud liquid water and cloud ice, has been developed for the CSIRO global climate model (GCM). the scheme includes physically based treatments of key microphysical processes, turbulent mixing and semi‐Lagrangian advection of cloud‐water species and interactive cloud radiative properties. Objectives in the development of the scheme were to improve upon the physical realism of parametrizations used in earlier schemes, whilst also trying to provide a scheme with moderate computational overheads.

The parametrized microphysical processes are evaluated in relation to observations and theory, and are compared to treatments used in earlier schemes in a series of short GCM experiments. It is argued that the treatment of precipitation formation in warm, and mixed‐phase, stratiform clouds is more realistic in the present scheme than in earlier schemes, which used crude methods for the parametrization of autoconversion, and did not treat key ice‐processes in a consistent way. In the present scheme, accretion processes are more important, whereas autoconversion is less important than in earlier schemes.

To determine whether the cloud scheme requires the use of a reduced (split) time‐step, the sensitivity of the various terms to the time‐step is evaluated in another series of short GCM experiments. It is shown that the various terms are not very sensitive to the time‐step, so the scheme can be efficiently implemented without the use of a split time‐step. Overall, analytical or time‐centred treatments perform better than implicit or explicit schemes, especially in the calculation of the precipitation of cloud ice, where only an accurate analytical treatment is found to perform satisfactorily at large time‐steps.

As a preliminary validation of the scheme, zonal‐mean fields from a six‐year model‐run are presented for the month of July. the results generally agree well with observations; in particular, the modelled cloudiness and long‐wave cloud‐forcing fields are more realistic than those obtained with the standard version of the CSIRO GCM.
Optimality of empirical Z‐R relations
Authors: Haddad Ziad S. , Rosenfeld Daniel
Published: February 2007

This paper attempts to justify mathematically the two empirical approaches to the problem of deriving Z‐R relations from (Z, R) measurements, namely the power‐law regression and the ‘probability matching method’. the basic mathematical assumptions that apply in each case are explicitly identified. In the first case, the appropriate assumption is that the scatter in the (Z, R) measurements reflects exactly the randomness in the connection between Z and R due to a lack of sufficient a priori information about either of them. In the second case, the assumption is that the measurements have been classified into categories a priori, in a way that allows one to expect a nearly one‐to‐one correspondence between Z and R in each catergory, the scatter in the measurements being due to residual noise. the paper then shows how the assmuptions naturally lead, in the first case, to a ‘conditional‐mean’ Z‐R relation of which the power laws are regression‐based approximations, and, in the second case, to a probability‐matchedr relation.
A Lagrangian‐based analysis of extratropical cyclones. II: A detailed case‐study
Authors: Wernli Heini
Published: February 2007

A detailed case‐study analysis is undertaken of the occurrence of significant coherent ensembles of trajectories (CETs) that accompany Atlantic cyclogenesis. The approach is based upon the Lagrangian‐based analysis scheme introduced in Part I.

The analysis reveals the existence of several distinct moist ascending (and dry descending) CETs. Two types of moist ascending CETs associated with strong condensational processes ascend from the boundary layer of the warm sector, either in the close vicinity of the cyclone centre, or in the trailing cold‐frontal region. The latter is the main contributor to the precipitation along the cold front during the early phase of the development. Another type of moist CETs—distinguished by maximum water vapour flux—travels at low levels through the warm sector and ascends to mid‐tropospheric levels in the warm‐frontal region, where it produces strong precipitation around the time of maximum storm intensity.

CETs with an upper‐level origin are linked to the classical dry intrusion. Different CETs correspond to: strong descent of upper‐level tropospheric air that penetrated to the lower troposphere, the movement of stratospheric air toward the centre of the maturing cyclone, and an event of stratosphere‐to‐troposphere exchange upstream of the upper‐level trough.

Two different visualizations of time sequences of the moist ascending CETs indicate that they represent well‐defined ‘flexible tubes’ or airstreams that exist over a time period comparable with the duration of the main cyclone intensification. They provide a translating, flexible but coherent ‘river bed’ for the quasi‐continuous ascent of moist boundary‐layer air to the upper troposphere. These ‘tubes’ are akin to the concept of warm conveyor belts, and the present analysis method objectively diagnoses their geometrical and physical characteristics and potential‐vorticity pattern.

Some consideration is also given to the relationship of CETs and ‘tropospheric rivers’, and to possible shortcomings of the approach.
The contribution of cloud inhomogeneities and droplet concentration to the albedo of broken‐cloud fields
Authors: Coley P. F. , Jonas P. R.
Published: February 2007

Cloud fields from a Large‐Eddy Simulation (LES) combined with a Monte Carlo radiative‐transfer model have been used to examine the behaviour of clouds with regard to the monochromatic albedo of short‐wave solar radiation. the LES cloud fields have an inhomogeneous structure in two dimensions and contain highly irregular concentrations of liquid water. the calculations show that the albedo sensitivity to the cloud‐field orientation becomes notably important for solar zenith angles above 30° for the LES clouds used. Replacing each cloud point with the in‐cloud mean water content, increases the cloud‐field albedo, compared to the original cloud‐field albedo, in many cases by more than 30%. Calculations have also been made with differing droplet concentrations throughout the cloud fields. It is found that changes in the cloud‐field albedo with droplet concentration are influenced by the general degree of inhomogeneity in the cloud structure. the original irregular LES cloud fields are more sensitive to changes in the droplet concentration than their equivalent plane‐parallel clouds with the same total water content and mean vertical water‐content profiles.
On the maintenance of potential vorticity in isentropic coordinates
Authors: Edouard Sandrine , Vautard Robert , Brunet Gilbert
Published: February 2007

We present a diagnostic study of the maintenance of the potential vorticity (PV) on isentropic surfaces, in the troposphere to the lower stratosphere, using ten years of analyses produced by the European Centre for Medium‐Range Weather Forecasts. After a brief three‐dimensional description of the general circulation in isentropic coordinates, we examine the budgets of the PV evolution equation. By decomposing the flow into its mean, high‐frequency transient and low‐frequency transient parts, we assess the role of these various scales of motion in the maintenance of the mean PV distribution. the contribution of vortical and thickness fluxes is also investigated. One interesting result is the key role of the divergent part of the transient flow along the tropopause in the midlatitudes, which creates a large PV sink region. We give here a tentative explanation.

The PV maps are also used to generate a climatology of a wave activity called pseudomomentum, that indicates four centres of wave activity over the Atlantic and Pacific. the first two are located in the northern hemisphere storm tracks and the two others in the upper tropospheric subtropical areas between the 310 K and the 350 K levels.
Seasonal forecasting
Authors: Carson D. J.
Published: February 2007

Examples are used to provide an appreciation of the most recent developments in real‐time experimental and operational seasonal forecasting and also of some of the wide issues surrounding the provision and use of such forecasts. The stimulus provided by the study of the El Niño Southern Oscillation phenomenon is discussed, especially with respect to operational seasonal forecasting in the US National Weather Service. The Meteorological Office's seasonal‐forecasting activities and objectives are then reviewed briefly, leading to a description of, and some tentative results from, a major European collaborative project on Prediction Of Climate Variations On Seasonal to Interannual Time‐scales. A wider international perspective is given by considering the relevant projects of the Word Climate Programme and also the proposal for an international research institute for seasonal‐to‐international climate prediction. In conclusion, some further general issues are raised for consideration.
Aircraft observations of the surface energy balance in TOGA‐COARE
Authors: Grant A. L. M. , Hignett P.
Published: February 2007

The TOGA‐COARE (Tropical Ocean/Global Atmosphare Coupled Ocean‐Atomspahere Response Experiment) was in an international observational campaign designed to study the processes that occur in the Pacific warm pool region. Data from the Meteorologiacal Research Flight C130 aircraft are presented to illustrate some of the characteristics of the surface energy balance in this region. The dominant terms in the balance are the turbluent latent‐heat flux and the solar flux. The C130 moisture flux data are compared with a parametrization based on data from the RV Moana Wave, one of the ships that took part in the COARE. It is found that there is no evidence for a systematic difference between the aircraft and ship flux estimates. In addition to the comparison with the COARE parametrization the effect of gustiness due to convective clouds on the surface latent‐heat flux is estimated for three flights, and the thermal balance of the mixed layer in undisturbed conditions described. The data suggest that direct solar absorption is an important part of the thermal balance, and the implications for diurnal variations in the mixed‐layer temperature are considered.

The variation in the downwelling solar flux with cloud cover is compared with simple parametrizations, which are found to agree reasonably well with the present data. The correlation between the long‐wave and short‐wave cloud forcings at the surface are described, to illustrate the different effects of different cloud types on the surface radiative balance.