bluuebunny/crossref_metadata_2025_split huggingface dataset:
The radiative impact of a simple aerosol climatology on the Hadley Centre atmospheric GCM
Authors: Cusack S. , Slingo A. , Edwards J. M. , Wild M. 
Published: February 2007

AbstractResults from a new version of the Hadley Centre atmospheric general‐circulation model are presented in which the sensitivity of the model to the inclusion of a simple aerosol climatology is investigated. Without aerosols, comparisons with clear‐sky reflected short‐wave radiation from the Earth Radiation Budget Experiment (ERBE) and the Scanner for Radiation Budget (ScaRaB) satellite measurements indicate a missing scatterer in the clear‐sky atmosphere. In addition, recent estimates of global, annual mean absorbed short‐wave radiation at the surface, which used the Global Energy Balance Archive (GEBA) where possible for verification, indicate that the atmosphere did not absorb enough short‐wave radiation. The insertion of a simple aerosol climatology based on World Climate Research Programme recommendations into the model was found to be the only plausible means of reducing its significant short‐wave bias compared with ERBE and ScaRaB measurements at the top of the atmosphere, and with GEBA data at the surface. An increase in the global, annual mean atmospheric absorbed short‐wave radiation accompanied these changes.
Associations between China monsoon rainfall and tropospheric jets
Authors: Liang Xin‐Zhong , Wang Wei‐Chyung 
Published: February 2007

AbstractStrong associations, in both the annual cycle and interannual variations, between east China monsoon rainfall and tropospheric jets are established with the use of observations and general‐circulation model simulations. Two distinct systems dominate regional rainfall: the east Asian jet stream (EAJ) in the north and the Hadley cell in the south. The EAJ is associated with Mei‐Yu and polar fronts as well as vigorous jet‐transverse circulations, whereas the Hadley cell is allied to tropical upper‐level easterlies and intertropical convergence zone convection. An equatorward EAJ displacement causes precipitation to increase over south‐central (south) China during June‐August (January‐March). Conversely, a poleward shift of the summer (winter) EAJ brings heavier precipitation over north (central‐north) China. On the other hand, over the South China Sea the Hadley cell influence prevails and, consequently, increased rainfalls concur with enhanced lower‐level westerlies. Furthermore, the EAJ fluctuations are strongly coupled with southern oscillation variations. Their interactions tend to precede (follow) El Niño phenomena during October through May (summer). The EAJ related flow anomalies also have potential skill to predict China rainfall interannual variability. To conclude, a realistic China monsoon simulation requires accurate representation of the EAJ and Hadley cell. Both features link regional rainfall to tropical and extratropical planetary‐scale circulations and, in turn, to global surface characteristics.
The latitudinal structure of the quasi‐biennial oscillation
Authors: Haynes Peter H. 
Published: February 2007

AbstractThe theory of symmetric time‐dependent meridional circulations in a radiatively damped atmosphere shows that, when the frequency σ of the time variation of an applied body force is smaller than the radiative damping rate α, there are two distinct forms of response to the force. At high latitudes, the response appears primarily as a quasi‐steady meridional circulation, whilst at low latitudes it appears primarily as an acceleration. The transition from ‘low‐latitude’ to ‘high‐latitude’ response occurs at a distance from the equator
equation image
where N is the buoyancy frequency, D is the depth scale of the force and β is the horizontal gradient of the Coriolis parameter at the equator.It is argued here that this latitudinal scale may also be that of the quasi‐biennial oscillation (QBO), in that the width of the region where there is a substantial QBO in the zonal wind may be determined by the radiative damping rate and the frequency of the oscillation at the equator, rather than by the latitudinal scale of the wave‐momentum fluxes (provided that this latter scale is sufficiently broad). Numerical simulations of QBO‐like oscillations are carried out in a two‐dimensional (height and latitude) model with wave‐momentum fluxes imposed independently at each latitude. Variation of the background rotation rate and the thermal damping rate between these simulations helps to elucidate the effect of these two parameters on the latitudinal structure of the oscillations. In particular, it is shown that, provided that there is weak lateral transport of momentum, the width of the oscillation does indeed correspond to the low‐latitude/high‐latitude transition scale given above. For this mechanism of equatorial confinement, the depth scale D is, in effect, set by the vertical phase variation of the oscillation, which is highly unrealistic in many simple QBO models.
Inclusion of a prognostic cloud scheme with the relaxed Arakawa‐Schubert cumulus parametrization: Single‐column model studies
Authors: Das Someshwar , Sud Y. C. , Suarez Max J. 
Published: February 2007

AbstractA prognostic cloud‐prediction scheme has been tested with the relaxed Arakawa‐Schubert cumulus‐parametrization scheme using the GATE phase III data set. Observed profiles of temperature, moisture and wind fields have been used as initial conditions for a semi‐prognostic as well as a fully prognostic study. The subsequent profiles of cloud liquid‐water/ice, cloud fraction and precipitation rates besides several other fields have been generated by using these schemes. The cloud water substance and the cloud mass fraction are allowed to evolve prognostically along with the temperature and moisture fields. Results indicate that errors in the temperature and moisture profiles are lowest when the cloud scheme is allowed to interact with radiation and downdraughts are included in the model. Sensitivity studies have been carried out to understand the role of cloud‐top entrainment instability. Results indicate that the cloud‐top entrainment instability leads to a reduction of the cloud mass fraction and liquid‐water content by 5–10% due to mixing with the environment.
Oceanic responses to gradual transitions of equator‐to‐pole temperature‐gradients
Authors: Cai W. , Chu P. C. 
Published: February 2007

AbstractResponses of an ocean model to gradual transitions of equator‐to‐pole gradients show that several quasi‐equilibrium states within the thermally driven regime are generated under identical forcing conditions. These different states are associated with different patterns (the location and the penetration depth) of oceanic convection. Small changes in the thermal forcing conditions can push the system into an oscillatory regime. Once in this regime, oscillations appear to proceed as internal processes. One important implication of these results is that the present‐day ocean climate may not recover fully if the atmospheric CO2 level, after doubling, is lowered to the present‐day level.
Inter‐hemispheric oceanic exchange
Authors: Nof Doron , Borisov Sergey 
Published: February 2007

AbstractThe exchange of water between hemispheres is examined theoretically by looking at the behaviour of continuous (double frontal) abyssal currents situated on the bottom of a (parabolic) meridional channel. We used a reduced‐gravity (fluid) model with an active lower layer on the bottom of the channel, and a passive infinitely deep upper layer on the top. The topography is the agent responsible for forcing the current across the equator. We first examined analytically the nonlinear, steady, frictionless case and, as a second step, we considered a nonlinear numerical simulation (of the Bleck and Boudra type). We then compared the results of these two (fluid) models to each other and to the solid‐balls model of Borisov and Nof which examined the migration of particles in the same cross‐equatorial channel.All three models show that, in general, the current (or a ‘cloud’ of solid particles) advances gradually toward the equator without much change to its structure. Upon reaching the vicinity of the equator, the current width (or the distance between neighbouring balls) decreases dramatically. The current (or cloud) then turns eastward, flows rapidly downslope, and rises on the eastern side. The flow (or cloud) ultimately splits into two parts, one progressing northward and the other recirculating and advancing southward.Remarkably good agreement is found between the detailed solid‐particles analysis and the new numerical fluid simulations, indicating that the inter‐hemispheric exchange is primarily an inertial process that depends mainly on the channel geometry. It is shown that the partition of mass flux between the two hemispheres depends on the way that the solid particles, or the current‐jet, impinge on the eastern flank of the channel. In contrast to the very good agreement between the solid balls and the fluid simulations, poor agreement was found between the detailed analytical prediction for the cross‐equatorial transport and the numerical solution (even when the viscosity was as small as we could use for numerical stability). However, the analytical solution does give the correct prediction for the position of the resulting currents and their general behaviour. It is argued that the disagreement between the analytically calculated fluid mass transports (which, in contrast to the balls, is subject to a potential‐vorticity constraint) and the numerical computations is due to the alteration of potential vorticity. Specifically, the friction alters the potential vorticity of the flow and prevents it from acting as a constraint to the crossing. Though crucial to the crossing, the role of friction is, therefore, passive. It merely allows the fluid to alter its potential vorticity in such a manner that the flow can adapt to the crossing pattern imposed by the geometry.It is suggested that the observed recirculation of Antarctic Bottom Water in the vicinity of the equatorial Atlantic may be a result of our splitting process.
Second differential of the entropy as a criterion for the stability in low‐dimensional climate models
Authors: Pujol T. , Llebot J. E. 
Published: February 2007

AbstractThe second differential of the entropy is used for analysing the stability of a thermodynamic climatic model. A delay time for the heat flux is introduced whereby it becomes an independent variable. Two different expressions for the second differential of the entropy are used: one follows classical irreversible thermodynamics theory; the second is related to the introduction of response time and is due to the extended irreversible thermodynamics theory. the second differential of the classical entropy leads to unstable solutions for high values of delay times. the extended expression always implies stable states for an ice‐free earth. When the ice‐albedo feedback is included, a discontinuous distribution of stable states is found for high response times. Following the thermodynamic analysis of the model, the maximum rates of entropy production at the steady state are obtained. A latitudinally isothermal earth produces the extremum in global entropy production. the material contribution to entropy production (by which we mean the production of entropy by material transport of heat) is a maximum when the latitudinal distribution of temperatures becomes less homogeneous than present values.
Monotonic cascade interpolation for semi‐lagrangian advection
Authors: Nair Ramachandran , Cǒté Jean , Stanforth Andrew 
Published: February 2007

AbstractA simpler, more efficient, and more robust form of cascade interpolation is proposed. It significantly reduces the considerable overhead required to determine the mesh points of the intermediate mesh, without degrading accuracy. A simple improvement of an existing monotonic filter is also proposed to address a deficiency and enhance robustness. These ideas are demonstrated using two standard test‐problems.
The parametrization of entrainment driven by surface heating and cloud‐top cooling
Authors: Lock A. P. , Macvean M. K. 
Published: February 2007

AbstractA numerical modelling study of entrainment in the convective atmospheric boundary layer has been undertaken. to isolate the effects of long‐wave radiative cooling at cloud‐top from those associated with phase changes of water, simulations of dust clouds have been performed. the radiative properties of these dust clouds were chosen to be broadly similar to those of stratocumulus. A wide range of interfacial and overlying stabilities were modelled, with turbulence driven by different combinations of surface heating and radiative cooling. the results of the simulations have been used to develop parametrizations both of the entrainment flux and the entrainment velocity.Where the boundary layer was driven by surface heating alone, the results are consistent with the widely accepted view that the magnitude of the entrainment buoyancy flux is a constant fraction (about 0.2) of the surface flux. When both driving mechanisms were active simultaneously, they appear to generate entrainment independently so that parametrizations based on a cubic average of ‘external’ velocity‐scales, characterizing the strength of the forcing, are found to be more successful than those based on measures of the actual strength of the turbulence, such as the generalized convective velocity‐scale. the assumption of a discontinuous inversion structure can lead to predictions of the entrainment rate which are much less accurate than those of the entrainment flux. In the worst cases, the predictions may be too low by a factor of two. the situation can be substantially improved, however, by allowing for a linear variation within an inversion of finite depth. Radiative cooling is then found to promote deepening of the boundary layer both directly (by cooling the air within the horizontally averaged inversion) and indirectly (through the buoyant production of turbulence).
The marine atmospheric boundary layer during semaphore. II: Turbulence profiles in the mixed layer
Authors: Lambert D. , Durand P. , Thoumieux F. , Bénech B. , Druilhet A. 
Published: February 2007

AbstractThe marine atmospheric boundary layer (MABL) analysed over the midlatitude ocean during SEMAPHORE (Structure des Echanges Mer‐Atmosphère, Propriétés des Hétérogénéités Océaniques: Recherche Expérimentale) was topped by a broken stratocumulus layer. Around midday, this layer was decoupled from the surface. the mixed layer which develops under the influence of surface buoyancy flux therefore stays well below the cloud layer. In this paper, we present a parametrization of turbulence moments and spectral characteristics in this mixed layer. the data were gathered in autumn 1993 by two instrumented aircraft in the Azores region. We began by analysing a large number of runs, each over 100 km in length, in order to determine what scales must be taken into account for estimating turbulence moments. the turbulence characteristics were thus calculated on 241 straight and level runs performed at various altitudes in the mixed layer during 17 flights over a period of 10 days, with various winds and surface fluxes. the turbulence profiles were normalized using the convective‐scales, computed from the mixed‐layer thickness (not the whole MABL thickness), and the surface fluxes. the contribution to buoyancy of temperature and moisture fluctuations are of the same order of magnitude, but they are of the same sign in the lower part of the mixed layer and of the opposite sign in the upper part. the result is that buoyancy flux vanishes at the top of the mixed layer, and entrainment is weak. These features are significantly different from what is observed in continental convective boundary layers. We also examine the usefulness of skewness profiles (mainly of moisture skewness) in understanding mixed‐layer dynamics (mainly of the respective contributions of the surface and top processes).
The role of the inertia of cloud drops in the evolution of the spectra during drop growth by diffusion
Authors: Pinsky Mark B. , Khain Alexander P. , Levin Zev 
Published: February 2007

AbstractIt is shown that the inertia of small droplets leads to the formation of drop velocity flux divergence, with the maximum of the spatial spectrum at scales from 1 cm to 2.5 cm for different values of the dissipation rate. the space‐length of correlation of the divergence field, calculated using the Batchelor model of isotropic and homogeneous turbulence, is also of centimetre scale. These results are interpreted in such a way that the drop inertia leads to the formation of the centimetre‐scale structure of a cloud with ‘spots’ of enhanced and decreased drop concentration. Because of temporal and spatial changes of the turbulent flow structure, droplets appear alternately within the areas of positive or negative drop‐flux velocity divergence and tend to leave the areas of the positive and enter the areas of negative drop‐flux velocity divergence. Drop motions through the interface between two cloudy, or cloudy and clear‐air, volumes lead to drop exchange between these volumes (inertial drop mixing). the characteristic time of this process was shown to be of the same order as the characteristic time‐scales of molecular diffusion, droplet evaporation and gravitational sedimentation. Possible effects of the inertial mixing are illustrated using a simple model, in which a comparatively large air volume consists of many centimetre‐scale volumes of different drop‐flux divergence and ascent velocity within a cloud updraught. Drop exchange between the centimetre‐scale volumes, together with the generation of supersaturation in the ascending volumes, leads to the evolution of droplet size spectra. the inertial mixing ‘itself’ leads to a weak droplet spectrum broadening in the case of uniform initial droplet concentration. the droplet spectrum broadening appears to be much more pronounced in cases of cloudy and clear‐air volumes mixing, especially when fresh nucleation is assumed. It is shown that inertial mixing leads to the homogenization of the drop spectrum, i.e. to the formation of the same drop spectrum shape in all centimetre‐scale volumes. Thus, inertial mixing may be important for the formation of local droplet spectra. At the same time, the inertial mixing may lead to significant fluctuations of drop concentration in these volumes.
Diagnosing extratropical synoptic‐scale stratosphere‐troposphere exchange: A case study
Authors: Wirth V. , Egger J. 
Published: February 2007

AbstractFive methods of diagnosing the synoptic‐scale mass exchange across the tropopause are investigated and compared with each other. Three of them are derived from Wei's general formula using pressure, potential temperature, and potential vorticity as vertical coordinates. the fourth method involves the computation of a large number of trajectories. the fifth method evaluates the flux directly as the difference between the motion of the air and the motion of the tropopause. the methods are tested in the context of a case study. an episode with strong convective heating in a decaying cut‐off cyclone was selected. A consistent data set was obtained for this event through a special run of the European Centre for Medium‐Range Weather Forecasts model with enhanced output storage.There was a large cross‐tropopause mass exchange during the episode. It is found that the different diagnostic methods yield quite different results. Those versions of Wei's formula which predominantly draw on standard information like wind, temperature and pressure turn out to be unreliable as they suffer from strong cancellation of large terms. Tests with the direct technique reveal that the problem lies in the lack of consistency between the model data and the diagnosed location of the tropopause. the amount of cancellation is substantially reduced when full use is made of model information about the non‐conservative fields in Wei's general formula with potential vorticity as vertical coordinate. This formulation also allows a straightforward interpretation of the net flux in terms of individual physical processes. the trajectory method, which does not require information about the non‐conservative fields, also yields reasonable estimates for the cross‐tropopause mass flux. the problems with the different methods and their differences show up especially clearly in a second model run in which all non‐conservative processes are switched off.
On the possible mechanisms of the evolution of a mini‐warm pool during the pre‐summer monsoon season and the genesis of onset vortex in the South‐Eastern Arabian Sea
Authors: Rao R. R. , Sivakumar R. 
Published: February 2007

AbstractDuring the pre‐summer monsoon season (February‐May), the near‐surface waters in the Arabian Sea progressively warm up and a mini‐warm pool with a core >30°C is manifested in the south‐eastem region. the possible mechanisms for the observed seasonal build‐up of this mini‐warm pool are examined, utilizing all the available monthly mean climatologies of surface wind field, surface heat fluxes, near‐surface thermohaline fields, near‐surface circulation, and mean sea level as monitored by satellites and by some of the recent model solutions on the Arabian Sea circulation.During winter (November‐February), the equatorward‐flowing East India Coastal Current in the western Bay of Bengal and westward‐flowing North Equatorial Current in the southern Bay bring low‐saline waters into the south‐eastern Arabian Sea, causing a haline stratification within the near‐surface isothermal layer. During December‐April, the positive surface‐wind‐stress curl and the associated Ekman divergence shoals the pycnocline. A south‐westward propagating mode‐2 Rossby wave from off south‐west India seen in satellite‐derived mean sea level and model solutions also modulates the underlying pycnocline. During the pre‐summer monsoon season, under clear skies and light wind conditions, the radiative heat input overwhelms turbulent heat losses at the air‐sea interface, and the net surplus heat energy is absorbed in a shallow haline stratified near‐surface layer, resulting in the formation of the observed mini‐warm pool.An examination of historical data on the genesis of monsoon onset vortices reveals that on most occasions the genesis has occurred over this mini‐warm pool region. Evidence for the geographic coincidence in the occurrence of the genesis of onset vortex and the sea surface temperature maxima during individual years of a three decade period (1961‐90) is presented.
A simple radiative‐convective model with a hydrological cycle and interactive clouds
Authors: Kelly Michael A. , Randall David A. , Stephens Graeme L. 
Published: February 2007

AbstractWe have developed a simple, analytically tractable radiative‐convective model of the tropical climate system that includes an explicit moisture budget, a simple convection parametrization, a simple but physically based radiation parametrization, and interactive clouds. the underlying surface is assumed to be ocean. the model includes prognostic equations for the sea surface temperature and the vertically integrated water vapour content. A stratosphere in radiative equilibrium limits the depth of the convective layer. the lower‐tropospheric lapse rate, surface evaporation rate, and clear‐sky long‐wave and short‐wave radiative fluxes at the surface and the top of the atmosphere are determined as functions of the sea surface temperature and precipitable water only. the radiative‐convective equilibria of the model atmosphere resemble the observed tropical climate, if realistic sea surface temperatures are prescribed. However, cloud‐free radiative‐convective equilibria of the tropical atmosphere‐ocean system do not occur for realistic values of the surface albedo. When cloud radiative effects are included, the model produces radiative‐convective equilibria that are unrealistically warm. With prescribed realistic lateral energy and moisture transports, however, the equilibria of the model are realistic.