bluuebunny/crossref_metadata_2025_split huggingface dataset:
The evolution of vortices in vertical shear. III: Baroclinic vortices
Authors: Jones Sarah C. 
Published: February 2007

AbstractThe evolution of baroclinic tropical‐eye lone‐like vortices in environmental vertical shear on an f‐plane is investigated. Idealized numerical calculations are performed using a primitive‐equation model. The vertical structure of the initial vortex is varied by changing the strength of the upper‐level tangential wind. The baroclinic vortices develop a vertical tilt and a cyclonic rotation of the mid‐level centre about the surface centre occurs. In most of the calculations the upper‐level part of the vortex is advected away from the surface centre. The motion of the upper‐level part of the vortex can be attributed to advection by the flow associated with large‐scale asymmetries in the potential vorticity and to the vertically penetrating flow associated with the potential vorticity of the lower portion of the tilted vortex. As in the case of initially barotropic vortices the cyclonic rotation is found to depend on the parameters in the Rossby penetration depth. The height at which the environmental flow is equal to the Speed of vortex motion is found to be higher for vortices with stronger flow at upper levels. This is consistent with observations of tropical cyclones.Significant changes in the potential‐vorticity structure of the vortex occur, especially at upper levels. The vortex becomes elliptic in shape and may extrude filaments of potential vorticily. These filaments thin, and sometimes break away from the main vortex, which as a result becomes more symmetric. The separation of the filaments from the main part of the vortex is often accompanied by a decrease in the vortex tilt. A: some levels no filaments form and the elliptic vortex becomes more symmetric with time. Calculations with a barotropic model show that both the potential‐vorticity structure of the initial vortex, and the horizontally sheared flow associated with the vertical projection of the tilted potential‐vorticity anomaly, play a role in the changes in the vortex Structure.The development of potential‐temperature asymmetries and an adiabatic vertical circulation are shown to be related to the direction of the vortex tilt. This occurs even when the direction of tilt changes with height. The changes in the low‐level static stability associated with the vortex tilt are investigated and the implications for tropical‐cyclone intensity change are discussed.
Parametrization of the effect of drizzle upon the droplet effective radius in stratocumulus clouds
Authors: Wood Robert 
Published: February 2007

AbstractA method is presented to parametrize the effects of drizzle upon the droplet effective radius in stratocumulus clouds. The cloud‐droplet size distribution in stratocumulus is represented by the sum of a modified Gamma distribution to represent the small (radius <20 μm) droplets and an exponential Marsh all‐Palmer‐type distribution to represent the large (drizzle) droplets. Using this approach a relationship is derived to account for the effect of drizzle upon k, the cube of the ratio between the volume and effective radius. Observational evidence from flights in a range of different air‐mass types is presented to validate the approach. The results suggest that the value of k pertaining to the small droplets is better parametrized as a function of volume radius rather than of droplet concentration. The results also suggest that, as the ratio of liquid‐water content contained in the large droplets to that in the small droplets increases beyond 0.05, the value of it decreases significantly. This results in an underprediction of the effective radius if commonly used parametrizations for k are used.
Estimation of entrainment rate in simple models of convective clouds
Authors: Gregory David 
Published: February 2007

AbstractA method for estimating lateral entrainment rate in clouds is suggested, linking its magnitude to buoyant production of vertical kinetic energy within a cloud updraught. In single‐column model studies the formulation captures the high values of entrainment seen in shallow convection and the values an order of magnitude lower in cases of deep convection, as suggested by observational studies and large‐eddy model simulations, together with a realistic vertical variation. Comparison is made with other recent attempts to parametrize entrainment.
The influence of the 1997–99 El Niňo Southern Oscillation on extratropical baroclinic life cycles over the eastern North Pacific
Authors: Shapiro M. A. , Wernli H. , Bond N. A. , Langland R. 
Published: February 2007

AbstractThe El Niňo Southern Oscillation (ENSO) strongly influences the interannual and seasonal atmospheric circulation over the North Pacific. The present study shows that the meridional displacement of the time‐mean upper‐level jet associated with ENSO modulates the time‐mean barotropic (meridional) wind shear over the central and eastern North Pacific storm track. Earlier theoretical and observational studies established the influence of barotropic wind shear on the life cycles of extratropical cyclones and their upper‐level potential vorticity (PV) waves. The present study suggests that differences in the time‐mean flow associated with the 1997–99 ENSO cycle had a similar impact on tropopause PV structure, meridional eddy fluxes of momentum and temperature, and predictability.
Ensemble prediction in a model with flow regimes
Authors: Trevisan Anna , Pancotti Francesco , Molteni Franco 
Published: February 2007

AbstractThe primary goals of ensemble prediction are the identification of particularly unpredictable situations, and the prediction of forecast error. In the medium range, major forecast failures are associated with regime transitions. The predictions of linear theories on error dynamics are inadequate to describe these situations, characterized by the rapid growth of errors which are far from being infinitesimal. Little is known about the efficiency of ensemble forecasting methods when applied to highly nonlinear situations like those associated with a change of regime. This problem is examined in an atmospheric model with two regimes and a small number of degrees of freedom by comparing current ensemble forecasting methods with ensemble forecasts that span all the degrees of freedom of the system. The study, performed in a perfect model environment, shows how the statistical properties of errors are affected by regime transitions. Results obtained selecting initial perturbations in the direction of the leading Lyapunov vectors (LVs) and singular vectors (SVs) are compared with those obtained with initially random vectors (RVs) spanning the whole phase space. It is found that the LV ensembles do not necessarily capture those particular perturbations that eventually lead to a regime transition, but closely reproduce the average RV distribution. On the other hand, the SV ensembles reproduce very closely the distribution obtained by selecting the maximum amplitude of random perturbations, although their consistency with the distribution of the full RV ensemble is poorer than in the LV ensembles.When the spread of individual RV ensembles is analysed by an empirical orthogonal function (EOF) decomposition, one finds that the variations in the amplitude of the first EOF (which carries the bimodal signature of regime transitions) is better estimated by the SV than by the LV ensembles. In verification‐based contingency tables, incorrect categorical forecasts of transition probability are more frequent in the LV than in the SV ensembles.
A physically based parametrization for surface flux enhancement by gustiness effects in dry and precipitating convection
Authors: Williams Alastair G. 
Published: February 2007

AbstractEnhancement of bulk surface fluxes by subgrid variability is reviewed, with special attention to conditions ranging from fair‐weather free convection to precipitating deep convection over uniform surfaces. Aircraft measurements in the Tropical Ocean/Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment subcloud layer indicate that the predominant mechanism for flux enhancement is wind variability associated with both boundary‐layer‐scale coherent eddies (dry convective gustiness) and density currents beneath precipitating convection (moist convective gustiness). A new evaluation of the dry convective gustiness parameter, β, supports recent indications that its effective value is less than previously thought (0.60–0.80, rather than 1.00–1.25), due both to a discrepancy in the technique often used for its estimation from field data, and to contamination of datasets by meso‐γ‐scale (∼10 km) motions. A strong relationship is found in the aircraft time series between the moist convective gustiness velocity and the theoretical speed of density‐current gust fronts driven by the observed meso‐γ‐scale temperature variability. This prompts development of a simple theoretical model for the latter, based upon a known convective rainfall rate and ambient thermodynamic structure for the lower atmosphere. Parameters in the model are estimated using data from simultaneous aircraft and shipboard radar measurements, and model behaviour is compared with a simpler scheme developed by Météo‐France. It is concluded that an explicit representation of the rain evaporation process in the generation of density perturbations is crucial for accurate determination of the strength of moist convective gustiness in the model, and through it the degree of surface flux enhancement.
Second‐order accuracy of two‐time‐level semi‐Lagrangian schemes
Authors: Gospodinov I. G. , Spiridonov V. G. , Geleyn J.‐F. 
Published: February 2007

AbstractThe accuracy of the two‐time‐level semi‐Lagrangian semi‐implicit time integration scheme is analysed. Two relatively independent problems are identified–the discretization of the implicit trajectory equation and the treatment of the nonlinear residual. Two theorems giving conditions for second‐order accuracy of the two‐time‐level semi‐Lagrangian semi‐implicit scheme are stated. As a result, a new scheme for the trajectory equation is proposed and a class of schemes for the nonlinear residual, which are second‐order accurate in time, are introduced. A simplified one‐dimensional ‘shallow water’ model is developed in order to test the proposed scheme in comparison with other, previously known, ones. Some experiments proving stability, accuracy and conservation ability are presented.
Baroclinic developments in jet entrances and exits. I: Linear normal modes
Authors: Hare Sylvia H. E. , James I. N. 
Published: February 2007

AbstractThis study examines the effect of the ageostrophic circulations associated with jet entrance and exit regions on the structure and growth rates of baroclinic normal modes. A linearized GCM is used to find the most unstable normal‐mode disturbances for a zonally symmetric basic state consisting of a zonal jet and meridional circulation. Normal‐mode growth rates are reduced by up to 20% for realistic strengths of meridional overturning, and normal‐mode structures are significantly affected. For example, the normal‐mode poleward tilt with height varies, affecting the conversion of basic‐state available energy. A mechanism for the change in poleward tilt is proposed. Ageostrophic meridional circulations may play a significant role in determining the magnitude and length of mid‐latitude storm tracks.
Deterministic prediction of the ensemble variance for a barotropic vorticity‐equation model
Authors: Riishøjgaard Lars Peter 
Published: February 2007

AbstractThe problem of estimating the error variance of a meteorological forecast is important both for data assimilation purposes and in order to increase the value of the forecast to the end user. Current data assimilation systems have no–or at best a severely limited–capability to estimate this quantity. In principle, the prognostic equation for the forecast error covariance is known from the Kalman filter. However, in its full form this equation is prohibitively expensive to implement and solve. In this article, the possibility of evolving a variance‐only estimate is explored. It is shown that for a barotropic vorticity‐equation model, an approximate equation for the error variance evolution can be written in a closed form under certain assumptions. The cost of solving this equation is comparable to that of solving the model equation itself. Through a demonstration experiment, it is shown that the approximate equation tracks the actual variance of a model run–represented by the variance over a randomly perturbed ensemble of forecasts generated by the model–remarkably well, qualitatively as well as quantitatively.
Estimation of the representativeness error caused by the incremental formulation of variational data assimilation
Authors: Desroziers Gerald , Brachemi Omar , Hamadache Bachir 
Published: February 2007

AbstractA formalization of the representativeness error caused by the use of a simplified increment in certain implementations of the variational formalism is introduced. The incremental representativeness‐error can be determined by summing up the part of the background‐error power‐spectrum that is beyond the truncation of the simplified increment. Thus, this error will be more important for fields, such as humidity, whose forecast errors have large components at small scales. The length scale and the oscillating structure of this incremental representativeness‐error are explained as the consequences of the application of a square window in spectral space to the corresponding background‐error covariances. A computation of the covariances of this error has been performed in a realistic framework given by the French numerical weather prediction system. In particular, it shows that this error is not negligible, and that its size is of the same order as that of observational instrument error which occurs when using a T63 spectral resolution of the analysis increment.
Aspects of interannual and intraseasonal variability of the tropopause and lower stratosphere
Authors: Kiladis G. N. , Straub K. H. , Reid G. C. , Gage K. S. 
Published: February 2007

AbstractRadiosonde and National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis data are utilized to consider aspects of large‐scale variability in tropopause height, temperature and pressure. This variability is related to coherent dynamical fluctuations in the troposphere and lower stratosphere through the use of linear correlation and regression. On interannual time‐scales, significant global‐scale tropopause fluctuations are tied to variability in sea surface temperature (SST) associated with the El Niňo/Southern Oscillation phenomenon. When SST is anomalously high in the central tropical Pacific, tropopause height (pressure) is high (low) throughout the Tropics, with largest perturbation amplitudes in the subtropical Pacific. At the same time, the tropopause is cold over the tropical and subtropical Pacific sector but warm elsewhere in the Tropics. Over the extratropics, wave‐like perturbations in the tropopause are seen, with anomalous cyclonic flow corresponding to a lower tropopause height and higher tropopause temperature and pressure, and vice versa. The sign of the temperature anomalies in the lower stratosphere tends to match that at the tropopause over much of the globe, with opposite‐signed anomalies in the upper troposphere. The vertical structure of these perturbations is consistent with the expected potential‐vorticity anomalies induced by quasi‐stationary Rossby waves and vertically propagating gravity waves forced by displacements of tropical convection. Similar relationships are associated with the eastward propprotion of tropical convection due to the Madden‐Julian Oscillation on intraseasonal time‐scales.
A data study of the influence of the equatorial upper stratosphere on northern‐hemisphere stratospheric sudden warmings
Authors: Gray L. J. , Phipps S. J. , Dunkerton T. J. , Baldwin M. P. , Drysdale E. F. , Allen M. R. 
Published: February 2007

AbstractEquatorial winds in the stratosphere are known to influence the frequency of stratospheric midwinter sudden warmings. Sudden warmings, in turn, influence the Earth's climate both through their direct influence on polar temperatures and through the temperature dependence of ozone depletion in the lower stratosphere. The conventional (Holton‐Tan) explanation for the equatorial influence on sudden warmings is in terms of the equatorial winds in the lower stratosphere (∼20‐30 km) acting as a waveguide for midlatitude planetarywave propagation. This study employs stratospheric‐temperature analyses and equatorial rocketsonde wind data extending to 58 km to diagnose the relationship between the northern‐hemisphere polar temperatures and equatorial zonal winds at all height levels in the stratosphere. In addition to the recognized Holton‐Tan relationship linking the polar temperatures to the quasi‐biennial oscillation in equatorial winds in the lower stratosphere, a strong correlation of polar temperatures with equatorial winds in the upper stratosphere is found. We suggest that this may be associated with the strength and vertical extent of the westerly phase of the semi‐annual oscillation in the upper stratosphere, although the observations alone cannot provide a conclusive, causal relationship. The main diagnostic tools employed are correlation studies and composite analysis. The results underline the need for continued high quality, equatorial wind measurements at all stratospheric levels.
Estimation of the error distributions of precipitation produced by convective parametrization schemes
Authors: Errico Ronald M. , Stensrud David J. , Raeder Kevin D. 
Published: February 2007

AbstractIf a parametrization scheme for convective precipitation is to be used for assimilating observations of precipitation using a statistically based technique, then statistics of the errors produced by that scheme are required. These are the errors produced by the scheme's formulation itself, not counting any errors in the scheme's input. Such errors are extremely difficult to estimate, but examination of differences produced by various suitable schemes can yield qualitative descriptions of such errors. Here, hourly accumulated convective precipitation fields produced from six different versions of a short‐term forecast model are compared. The versions have identical initial and boundary conditions, but vary in the schemes used for either the convection or the planetary boundary layer, or both. The distribution of differences, or differences in logarithms of accumulations, between corresponding precipitating grid points for pairs of forecasts are examined using a simple binning technique. When the convection schemes differ, results reveal that if either a log‐normal or normal distribution is a better characterization of the distributions, it is the log‐normal one. The standard deviations of these logarithmic distributions correspond to different schemes at identical grid points producing values differing by factors of 2 or more. A large proportion of grid points that have non‐zero hourly accumulations using one model version may have no accumulation using another version. For most pairs of forecasts examined, however, grid points having larger values of accumulation for one scheme tend to have a smaller fraction of values having no accumulation in the other scheme. These results suggest that the finite probability that the model produces no precipitation when the corresponding, true atmospheric state does, should be considered in the statistical description of the model errors and that, because of the large standard deviation of model errors as well as large possible errors of hourly precipitation observations, the quantitative usefulness of assimilating such observations may be very limited.
Sensitivity of a heavy‐rain‐producing western Mediterranean cyclone to embedded potential‐vorticity anomalies
Authors: Romero R. 
Published: February 2007

AbstractThe development and track of a surface cyclone that produced heavy precipitation in the western Mediterranean region on 28–29 September 1994 is numerically simulated using a mesoscale model. Diagnostic calculations reveal that surface‐pressure falls‐and upward motion‐occur in response to a well‐defined pattern of upward quasi‐geostrophic forcing at all tropospheric levels. In addition, convective instability and low‐tropospheric water vapour flux convergence are significant over the western Mediterranean, and therefore the environment is highly supportive for convection development.The appreciable dynamical forcing at upper levels appears to be associated with two positive potential‐vorticity (PV) anomalies that are embedded within the large‐scale trough and rotate about each other. Motivated by the fact that the small‐scale features of the PV field are more prone to analysis or forecast error than the large‐scale components, a sensitivity study is conducted in order to analyse the dependence of the mesoscale forecast on the initial intensity and position of the two embedded PV centres. This is accomplished by first applying a piecewise PV‐inversion technique which allows the balanced flow associated with each PV centre to be calculated, and then the inverted mass and wind fields are used to modify the model initial conditions. Eight simulations are run after doubling or removing one or both anomalies (sensitivity to intensity), and eight other simulations after displacing the anomalies towards or away from the Iberian peninsula (sensitivity to position).The results exhibit a clear dependence of the track and shape of the surface cyclone and its associated pattern of low‐tropospheric warm air advection on the characteristics of the PV anomalies. Therefore, spatial details of the mesoscale forecast are shown to be highly sensitive to the precise structure of the upper‐level dynamic forcing. Nevertheless, for the full set of arbitrarily defined initial conditions, one or several low pressure systems develop in the western Mediterranean area and heavy precipitation always occurs. On the other hand, there is an overall tendency of the simulations to favour cyclone development leeward of the Atlas mountain range and local rainfall enhancements over the exposed mountains of eastern Spain. The relative roles of the orography and sea surface latent‐heat flux versus the action of the upper‐level PV centres are examined by means of additional simulations.