bluuebunny/crossref_metadata_2025_split huggingface dataset:
Sensitivity of forecast errors to initial conditions
Authors: Rabier F. , Klinker E. , Courtier P. , Hollingsworth A. 
Published: February 2007

AbstractThe 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

AbstractAn 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

AbstractThis 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

AbstractA 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

AbstractCalculated 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

AbstractRecently, 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

AbstractTheories 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

AbstractA 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

AbstractThe 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

AbstractDuring 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

AbstractExpressions 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

AbstractRegular 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

AbstractThe 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

AbstractA 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

AbstractThis 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.