bluuebunny/crossref_metadata_2025_split huggingface dataset:
Elliptical diagnostics of stratospheric polar vortices
Authors: Waugh Darry N. W. 
Published: February 2007

AbstractDiagnostics based on the spatial moments of isopleths of a long‐lived tracer, or of potential vorticity, are presented that enable the structure and evolution of stratospheric polar vortices to be concisely summarized and quantified. The area, centre, aspect ratio and orientation of the ‘equivalent ellipse’ of the vortex, on an isentropic surface, are defined using the second‐ and lower‐order spatial moments of contours within the vortex‐edge region. By examining the variations of these ‘elliptical’ diagnostics with time and altitude, the temporal evolution and vertical structure of the polar vortices can be quantified. The usefulness of the diagnostics is assessed by examining nitrous oxide data from the Geophysical Fluid Dynamics Laboratory ‘SKYHI’ general‐circulation model. The diagnostics show, and quantify, several differences between the Arctic and Antarctic vortices in the SKYHI model. The Arctic vortex moves further off the pole, is generally more elongated, and has a more complicated vertical structure than the Antarctic vortex (with larger variations of both the vortex centre and elongation with height). The elliptical diagnostics also identify the occurrence of large‐scale Rossby‐wave breaking events, both at the vortex edge and in the subtropics, in the model.
Modelling stably stratified boundary‐layer flow over low hills
Authors: Weng W. , Chan L. , Taylor P. A. , Xu D. 
Published: February 2007

AbstractThe mixed spectral finite‐difference model for neutrally stratified atmospheric boundary‐layer flow over complex terrain has been extended to include the effects of stable stratification with a simple turbulent‐kinetic‐energy and mixing‐length turbulence‐closure scheme. to allow internal gravity‐wave propagation, we have constructed a steady background flow such that wind velocity and buoyancy frequency are constants above the boundary layer with the assumption that there are momentum and heat sinks to balance the flux divergences in the governing equations. Model results are compared with inviscid theory. Special emphasis is placed on the pressure or form drag on topography.
Multi‐spectral calculations of the direct radiative forcing of tropospheric sulphate and soot aerosols using a column model
Authors: Haywood J. M. , Shine K. P. 
Published: February 2007

AbstractA newly developed multi‐spectral radiative‐transfer code is used to investigate the important features of the direct radiative forcing of sulphate and soot aerosol; the indirect effect of both aerosol species is not investigated in this study. the direct radiative forcing is presented for different surface albedos, solar zenith angles, relative humidities and aerosol vertical profiles together with effects upon the surface irradiance and long‐wave radiative forcing. the effect of subgrid‐scale variations in relative humidity are examined using idealized relative humidity distributions. the results show that subgrid‐scale variations in relative humidity and the spatial correlation between cloud and areas of high relative humidity should be considered in future general‐circulation model calculations of the direct forcing due to sulphate aerosol. A comparison of the direct forcing obtained by adjusting the surface albedo to that using the full multi‐spectral column calculation is performed; the results indicate that recent estimates of the climate response to the direct forcing of sulphate may be too large. the contribution to the direct forcing from cloudy‐sky regions appears to be negligible for sulphate aerosol but there is a considerable enhancement of the forcing due to soot aerosol if soot exists within or above clouds. These calculations show that a small amount of soot, relative to the sulphate mass loading, can cause a significant positive direct forcing and emphasize that the vertical profile of soot aerosol relative to cloud must be established to enable accurate assessment of the direct radiative effects of anthropogenic emissions of soot aerosol.
Convergence zones over the Greek peninsula and associated thunderstorm activity
Authors: Kotroni Vassiliki , Kallos George , Lagouvardos Konstantinos 
Published: February 2007

AbstractIn the frame of this study, the initiation of summer storm activity over the Greek peninsula during a prevailing weak synoptic flow is investigated using the Colorado State University‐Regional Atmospheric Modelling System. On 10 July 1994, thunderstorm activity was observed along a convergence zone which had developed following the main axis of the peninsula. the convergence zone first deepened the moist layer locally, providing a region potentially favourable to deep convection, while terrain variability and diurnal differential‐heating‐forced vertical motions. the cold air intrusion aloft, associated with the low‐level convergence, resulted in deep convection and significant storm activity. the origin of the moist air masses which fed this activity has been investigated through simulations with a Lagrangian particle dispersion model. Moist air masses, which originated from marine boundary‐layer of the Ionian Sea on the west, and north Aegean Sea on the east of the Greek peninsula, were transported over the land through the sea‐breeze mechanism and later, at the time of the storm activity, were deeply injected into the troposphere over the convergence line, at heights up to 5 km.
Vacillation cycles and blocking in a channel
Authors: Haines K. , Holland A. J. 
Published: February 2007

AbstractThe response to a low‐level high‐frequency wavemaker forcing in a two‐layer, β‐plane, quasi‐geostrophic channel model is examined. The wavemaker simulates regular baroclinic instability which then propagates to upper atmospheric levels to excite blocking. By altering the meridional shear in the upper layer, the large‐scale response can vary from a steady large‐amplitude split jet, very similar to observed blocks, to a weaker split with a low‐frequency vacillation cycle. The eddies will always resonantly excite the split flow, but a mixed instability process is responsible for the breakdown in cases which oscillate, and this is demonstrated using a simplified zonal stability analysis and energy‐tendency diagnostics. The re‐excitement by the eddies continues the cycle. This model provides a theory of how the meridional structure of the upper‐level winds may determine whether a large‐amplitude block can be excited or persist in the presence of similar high‐frequency eddy activity propagating up from below.
A cumulus parameterization with a prognostic closure
Authors: Pan Dzong‐Ming , Randall Davi D. A. 
Published: February 2007

AbstractThe paper describes the introduction of a prognostic cumulus kinetic energy (CKE) as a replacement for the quasi‐equilibrium closure hypothesis of Arakawa and Schubert (AS).In the original version of the AS parameterization, the cloud work function, a measure of the convective available potential energy, is assumed to be maintained at ‘small’ values through a quasi‐equilibrium between the cumulus convection and the ‘large‐scale forcing’. It is argued here, however, that the distinction between the convective and large‐scale processes is ambiguous and subjective. It is demonstrated that the need for such a distinction can be avoided by relaxing the quasi‐equilibrium assumption, through the introduction of a prognostic CKE; referred to as prognostic closure. A dimensional parameter, α, is introduced to relate the CKE to the square of the cloud‐base convective mass flux. It is shown that ‘adjustment time’ defined by AS is related to α, so that when the adjustment time approaches zero the prognostic closure reduces to quasi‐equilibrium closure. A second dimensional parameter, τD, is used to determine the rate at which the CKE is dissipated. In the limit of small α and τD, the convective mass flux is formally independent of both α τD if the environmental sounding is assumed to be given, but in reality the results of a prognostic model do depend on these two parameters because they affect the time‐dependent sounding.For simplicity, a single constant value of α is used for all cloud types in tests with a general‐circulation model, and this gives reasonably good results. Larger values of α lead to more frequent shallow cumulus convection and a cooler and more humid troposphere, in which stratiform condensation is more active and more large‐scale precipitation can reach the surface. A longer dissipation time‐scale leads to a warmer tropical troposphere. The interactions between stratiform cloudiness and convection prove to be quite important, leading to the conclusion that the convection parametrization really cannot be evaluated independently of the stratiform cloud parametrization with which it interacts.
Linear stability and single‐column analyses of several cumulus parametrization categories in a shallow‐water model
Authors: Yano Jun‐Ichi , Moncrieff Mitchell W. , McWilliams James C. 
Published: February 2007

AbstractUsing a mass‐flux based approach, the thermodynamic cumulus parametrization problem is reformulated in a simple atmospheric model, which is an analogue of the shallow‐water equations. The objective is to investigate basic effects of elementary representations of several parametrization categories. In particular, a linear stability analysis and a single‐column experiment are performed to infer the characteristics of each parametrization as regards its ability to simulate the large‐scale organization or coherence of tropical convection.The moisture‐convergence closure (MC) scheme, which assumes that the ensemble of cumulus convection is controlled by the low‐level moisture convergence as in Kuo‐type scheme, predicts the largest growth at the smallest scale. Hence, although it ensures the generation of a coherent propagating structure, its scale always corresponds to the grid size. Furthermore, the MC tends to produce a catastrophic positive feedback of moist convection to the large‐scale convergence.In contrast, the statistical equilibrium scheme, which assumes an instantaneous adjustment of the large‐scale environment to a quasi‐equilibrium state, such as Arakawa‐Schubert and moist convective adjustment schemes, asymptotes to a constant growth rate at small scales. Hence, this type of parametrization tends to generate a field like white noise with no large‐scale coherence. The lagged‐adjustment (LA) schemes, which have a short time‐lag for the cumulus growth, as in the Betts–Miller scheme, feature a finite scale selection in the linear growth rate. This ensures a smooth large‐scale coherence that is independent of the grid size, and is consistent with the scale‐separation principle.A new type of parametrization is also tested. This convective life‐cycle (CLC) scheme represnets the life cycle of a common type of convective system made up of deep precipitating convection and a subsequent mesoscale response. It uses a buoyancy‐based closure. The growth‐rate curve is similar to the other LA schemes, but the behaviour in the zero‐dimensional (single‐column) version of the model is qualitatively different. Although the CLC scheme does not automatically saisfy the scale‐separation principle, its grid‐size dependence can be treated by a re‐normalization principle.The result are used to interpret some reported general‐circulation‐model results regarding the impact of different parametrization schemes on the tropical atmosphere at large scales.
Vertical motion, diabatic heating, and rainfall characteristics in north Australia convective systems
Authors: Cifelli Robert , Rutledge Steven A. 
Published: February 2007

AbstractVery‐high‐frequency wind‐profiler data are used to study the vertical draught structure within 13 tropical Mesoscale Convective Systems (MCSs) near Darwin, Australia during the 1989‐90 and 1990‐91 wet seasons. These studies are supported by single‐Doppler radar, soundings, and surface rainfall data to correlate radar reflectivity, thermal buoyancy, and surface rainfall patterns with vertical air‐motion structures. Because of Darwin's unique location at the southern tip of the Maritime Continent, vertical draughts in both the monsoon (maritime) and monsoon break (continental) convective regimes can be observed.The break‐regime MCSs (six in total) were all squall lines, characterized by a leading line of convection with heavy precipitation and trailing stratiform rainfall containing a characteristic radar bright band. These MCSs exhibited a pronounced life‐cycle pattern and were all sampled by the profiler in the mature to dissipating stages. In contrast, the monsoon systems (seven in total) were composed of regions of stratiform cloud with embedded convective bands which moved on‐shore in the monsoonal flow. Results from the Darwin rain‐gauge network indicated that the majority of the total rainfall in each MCS (break and monsoon) was associated with the convective portion of the system.The break‐regime MCSs were all characterized by a low‐level (4 km) updraught peak associated with convective cells on the leading edge of each squall line, trailed by deeper convective updraughts in the middle and upper troposphere. For the monsoon cases, the lower‐troposphere convective updraughts were typically less than those in the squall lines, yet were stronger in the upper troposphere. The low‐level differences in the convective updraughts were consistent with the smaller virtual‐temperature excess in the monsoon soundings, as well as the larger vertical radar‐reflectivity gradients observed in monsoon convection.Consistent with the differences in vertical air‐motion patterns, diabatic heating and moistening profiles for the monsoon MCSs were characterized by a higher‐level heating and drying peak compared with the break MCSs. The results have important implications for cumulus parametrizations in numerical models since the large‐scale circulation is sensitive to the vertical distribution of diabatic heating in tropical MCSs.
Stationary gravity‐wave structure in flows with directional wind shear
Authors: Shutts G. J. 
Published: February 2007

AbstractMost theoretical studies of flow over mountains assume that the wind direction is constant. For two‐dimensional orographic ridges this is not an issue, since only the component of the wind across the ridge forces internal gravity waves. A special case of rectilinear flow is when the wind component changes sign (i.e. the wind direction changes discontinuously through 180 degrees): such a point is a critical level for all stationary gravity waves. For isolated three‐dimensional mountains the gravity‐wave response comprises waves of all azimuthal orientations, and each of these may have a different critical‐level height‐depending on whether the wind backs or veers with height.Using a combination of ray‐tracing methods and stationary‐phase solutions, the structure of the stationary, hydrostatic gravity‐wave field generated by an isolated mountain is studied for a simple wind field which backs with height. In contrast to the behaviour for unidirectional flow, wave action does not accumulate indefinitely beneath the critical level but is continuously advected downwind by the component of the wind parallel to the phase lines. Instead of the wave energy becoming unbounded following a ray as in the 2D case, the wave energy decays to zero along a trailing wake zone downwind of the mountain along phase lines. This is just an extension of the ‘asymptotic wake’ found in the linear solutions of Smith for constant flow over an isolated mountain. The work described here underlines the desirability of including the effects of selective critical‐level absorption in the gravity‐wave drag parametrization schemes of numerical weather‐prediction models.
Sensitivity to potential vorticity
Authors: Arbogast P. 
Published: February 2007

AbstractA new method is proposed that computes the sensitivity to potential vorticity of any diagnosed aspect of the atmosphere. It does not require the computation of the adjoint of the full inversion procedure, but rather the tangent linear and the adjoint of the operator that relates the wind and mass fields to the distribution of potential vorticity. The formal approach employed helps in understanding the role of the balance condition. This method is applied to the study of the sensitivity of the boundary condition to the inner potential vorticity in a 2D atmosphere.
The ECMWF implementation of three‐dimensional variational assimilation (3D‐Var). I: Formulation
Authors: Courtier P. , Andersson E. , Heckley W. , Vasiljevic D. , Hamrud M. , Hollingsworth A. , Rabier F. , Fisher M. , Pailleux J. 
Published: February 2007

AbstractIn the first of this set of three papers, the formulation of the European Centre for Medium‐Range Weather Forecasts (ECMWF) implementation of 3D‐Var is described. In the second, the specification of the structure function is presented, and the last is devoted to the results of the extensive numerical experimentation programme which was conducted. The 3D‐Var formulation uses a spherical‐harmonic expansion, much as the ECMWF optimal interpolation (OI) scheme used an expansion of Bessel functions. This formulation is introduced using a convolution algebra over the sphere expressed directly in spectral space. It is shown that all features of the OI statistical model can be implemented within 3D‐Var. Furthermore, a non‐separable statistical model is described. In the present formulation, geostrophy is accounted for through a Hough‐modes separation of the gravity and Rossby components of the analysis increments. As in OI, the tropical analysis remains essentially non‐divergent and with a weak mass‐wind coupling. The observations used, as well as their specified statistics of errors, are presented, together with some implementation details. In the light of the results, 3D‐Var was implemented operationally at the end of January 1996.
Impact of model resolution and ensemble size on the performance of an Ensemble Prediction System
Authors: Buizza R. , Petroliagis T. , Palmer T. , Barkmeijer J. , Hamrud M. , Hollingsworth A. , Simmons A. , Wedi N. 
Published: February 2007

AbstractEnsemble integrations for 14 cases are described. These integrations test the relative impact of increase in ensemble size and in the resolution of the model used to integrate the ensemble. The ensembles are evaluated using a variety of statistical tests. Some of these indicate a relative advantage of an increase in ensemble size, whilst most tests suggest a relative advantage of an increase in model resolution. However, overall, the best performance was obtained by combining enhancement in model resolution (from T63L19 to T106L31) with an increase in ensemble size (from 32 to 50 members).
Modelling the Asian summer monsoon rainfall and Eurasian winter/spring snow mass
Authors: Dong Buwen , Valdes Paul J. 
Published: February 2007

AbstractThe interannual variation of the south Asian summer monsoon is analysed based on sets of present day climate simulations using the UK Universities' Global Atmospheric Modelling Programme (UGAMP) generalcirculation model with different land surface parametrization schemes, different horizontal resolutions, and sea surface temperature variations. Generally, a negative relationship is found between south Eurasian winter/spring snow mass and the amount of summer monsoon rainfall over India in all simulations. However, the significance of this relationship is dependent on the land surface parametrization scheme. The simulations using the no‐flux boundary condition at the bottom of a three‐layer soil model give a strong negative correlation. This inverse relationship is the strongest over north India and the foothills of the Himalayas and is statistically significant. The model results are in good agreement with observational studies. Composite analyses suggest that less winter/spring snowfall over south Eurasia is associated with a strong Indian summer monsoon, characterized by strong south‐westerlies over the Arabian Sea in the lower troposphere in the June‐August season and heavy precipitation in early summer over north India and the foothills of the Himalayas. In contrast, heavy winter/spring snowfall delays the onset of the Indian summer monsoon through the feedback of snowmelt, soil moisture and evaporation processes, and is associated with weak summer precipitation over the two regions.Sensitivity studies confirm that the snow mass‐Indian monsoon relationship identified in the simulation at T42 is also robust in the simulation at T31. However, a negative relationship does not exist in the simulation at T21, indicating the importance of horizontal resolution in maintaining the snow‐monsoon relationship in the UGAMP model.
Time‐scales of adjustment to radiative‐convective equilibrium in the tropical atmosphere
Authors: Tompkins Adrian M. , Craig George C. 
Published: February 2007

AbstractIn the tropics the assumed existence of a balanced atmospheric state of radiative‐convective equilibrium is a useful and widely utilized concept. Given an atmospheric state of radiative‐convective equilibrium which is perturbed, this paper attempts to identify the mechanisms that determine the time‐scale for the restoration of the balanced state. The perturbation could arise from large‐scale atmospheric wave motions, local‐scale convective downdraughts or sea surface temperature perturbations for example. The resulting state immediately after the perturbation is applied can be one of suppressed or convective conditions, and it is the complex response under convective conditions that is explored in this paper.A three‐dimensional cloud resolving model is operated to a radiative‐convective equilibrium state to which sea surface temperature perturbations are then applied. It is found that the variability of the model state variables, such as temperature and total water vapour amount, can be divided by processes into an exponential adjustment to the new balanced state on a long time‐scale (15 days), superimposed by short time‐scale variability (<4 days) that is governed almost solely by the convective mass‐flux. The determination of the long time‐scale trend to equilibrium is then investigated with further numerical experiments, which demonstrate that radiation determines the adjustment time‐scale via its control of the subsidence velocities in the clear‐sky regions surrounding convection. Some implications of the results for cumulus parametrization are discussed.