bluuebunny/crossref_metadata_2025 huggingface dataset:
The ECMWF implementation of three‐dimensional variational assimilation (3D‐Var). III: Experimental results
Authors: Andersson Erik , Haseler Jan , Undén Per , Courtier Philippe , Kelly Graeme , Vasiljevic Drasko , Brankovic Cedo , Gaffard Catherine , Hollingsworth Anthony , Jakob Christian , Janssen Peter , Klinker Ernst , Lanzinger Andreas , Miller Martin , Rabier Florence , Simmons Adrian , Strauss Bernard , Viterbo Pedro , Cardinali Carla , Thépaut Jean‐Noël
Published: February 2007

In this third and final paper of a series, we assess the performance of the three‐dimensional variational data assimilation scheme, in the light of the results from the extensive pre‐operational programme of numerical experimentation. Its performance is compared with that of the previous operational scheme at the European Centre for Medium‐Range Weather Forecasts, which was based on Optimal Interpolation. The main features of the new scheme are illustrated, in particular the effects of non‐separable structure functions and the improved data usage. TIROS‐N Operational Vertical Sounder cloud‐cleared radiances, for example, are used directly without a separate retrieval step. Scatterometer data are assimilated in the form of ambiguous winds with the ambiguity removal taking place within the analysis itself. Problems encountered during the tests are discussed and the solutions implemented are explained.

The overall impact on forecast accuracy in the troposphere of the northern hemisphere extratropics is neutral for geopotential and positive for wind and temperature. The impact is neutral in the tropics, and significantly positive in the southern hemisphere. Analyses and forecasts for the stratosphere have improved in all regions. Other positive results include a clear improvement in analyses of near‐surface winds over oceans, particularly in the vicinity of tropical storms. This is predominantly because of the assimilation of scatterometer wind‐data.
Multiple‐truncation incremental approach for four‐dimensional variational data assimilation
Authors: Veerse F. , Thepaut J.‐N.
Published: February 2007

The incremental approach provides an approximate solution to four‐dimensional variational data assimilation (4D‐Var) at a reasonable CPU cost. An extension of this approach is studied, namely the multiple‐truncation incremental technique, and is compared to the standard and single‐truncation incremental implementations of 4D‐Var, using a two‐dimensional barotropic vorticity‐equation model.

The quasi‐continuous approach deals with the cut‐off problem and spreads the computer requirements over time. Quasi‐continuous incremental and quasi‐continuous multiple‐truncation incremental techniques are assessed.

The same number of iterations is specified in all the experiments. The multiple‐truncation incremental technique is found to produce a good approximate analysis at a lower CPU cost. Applying the quasi‐continuous approach to the incremental methods adds some benefit in terms of computer time.
Development of perturbations within growing baroclinic waves
Authors: Snyder C. , Joly A.
Published: February 2007

We explore the linear stability of a growing, three‐dimensional baroclinic wave by calculating the perturbation that grows most rapidly over various time intervals and at various stages in the development of the parent wave and its fronts. Three norms are used to measure growth: volume‐integrated energy, enstrophy and stream function variance. The flow is assumed adiabatic and quasi‐geostrophic for simplicity, and perturbations are required to have uniform potential vorticity.

These rapidly growing perturbations can produce realistic sub‐structures within the parent wave, such as upper‐level vorticity maxima that propagate relative to a synoptic‐scale parent wave or packets of synoptic‐scale waves within a planetary‐wave basic state. For a synoptic‐scale parent wave and the energy or enstrophy norms, however, the dominant characteristic of the fastest growing perturbations is that they rapidly evolve toward a final structure corresponding to a phase shift and slight change of shape of the original wave—in essence, the initial perturbation modifies the parent wave and the jet on which it propagates, which results in a modification, which grows in time, of the phase and amplitude of the parent wave. Amplifications in energy or enstrophy are also small compared to what would be estimated based on the locally large shears and baroclinicity within the parent wave. The fronts appear to be stabilized by the combined influences of synoptic‐scale horizontal deformation and the natural movement of perturbations relative to the parent wave.
The annual cycle of persistence in the El Nño/Southern Oscillation
Authors: Torrence Christopher , Webster Peter J.
Published: February 2007

A spring ‘predictability barrier’ exists in both data and models of the El Niño/Southern Oscillation (ENSO) phenomenon. In statistical analyses this barrier manifests itself as a drop‐off in monthly persistence (lagged correlation) while in coupled ocean‐atmosphere models it appears as a decrease in forecast skill.

The ‘persistence barrier’ for ENSO indices is investigated using historical sea surface temperature and sealevel pressure data. Simple statistical models are used to show that the persistence barrier occurs because the boreal spring is the transition time from one climate state to another, when the ‘signal‐to‐noise’ of the system is lowest and the system is most susceptible to perturbations. The strength of the persistence barrier is shown to depend on the degree of phase locking of the ENSO to the annual cycle.

The phase locking of the ENSO to the annual cycle, as well as the ENSO variance, is shown to vary on interdecadal time‐scales. During 1871–1920 and 1960–90 the ENSO variance was high, while during 1920–50 it was low. Using wavelet analysis, this interdecadal variability in ENSO is shown to be correlated with changes in Indian summer monsoon strength. Finally, the change in persistence‐barrier strength between 1960–79 and 1980–95 is related to changes in the phase locking of ENSO to the annual cycle. These changes in persistence and phase locking appear to be related to the increased forecast skill seen from recent coupled ocean‐atmosphere models.
Radar estimates of rainfall rates at the ground in bright band and non‐bright band events
Authors: Smyth T. J. , Illingworth A. J.
Published: February 2007

The major error in estimating precipitation rates at the ground from radar arises from the change in radar reflectivity (Z) with height. In stratiform precipitation Z rises in the ‘bright band’, where the snow is melting, and then falls steeply in the dry ice above. In regions of vigorous convection containing graupel, the bright band is absent and the fall in Z with height is less marked. If accurate estimates of surface precipitation are to be made, based upon radar observations taken within or above the bright band, it is important to use a correction scheme which uses a different vertical profile of reflectivity (VPR) for the two precipitation types. Using a large radar data set, this work addresses the issue of VPR variability by constructing average vertical profiles and sorting them into snow and graupel categories using the value of the Linear Depolarization Ratio at the melting layer. These observations snow that convection is often embedded within stratiform precipitation, and some convective showers containing snow have bright bands. As a result, the use of an average profile over the whole region scanned by radar leads to only marginal improvement in rain estimates. Instead, this work proposes that a bright band correction should always be applied unless the value of Z at 1.5 km above the bright band exceeds 30 dBZ, in which case a ‘convective’ vertical profile of ‐2 dBZ km−1 should be used. This Z threshold indicates that graupel only forms when the ice water content exceeds 0.35 gm−3 which has implications for ice parametrization schemes in mesoscale models.
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

Results 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

Strong 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

The 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

A 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

Responses 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

The 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

The 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

A 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

A 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

The 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

It 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

Five 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

During 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

We 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.
The role of vertically varying cloud fraction in the parametrization of microphysical processes in the ECMWF model
Authors: Jakob Christian , Klein Stephen A.
Published: February 2007

General‐circulation models (GCMs) have generally treated solely the radiative impacts of vertically varying cloud fraction by using a cloud‐overlap assumption. In this study, the microphysical impacts of vertically varying cloud fraction are addressed by developing a subgrid‐scale precipitation model which resolves the vertical variation of cloud fraction. This subgrid model subdivides the grid boxes into homogeneous columns which are either clear or completely cloudy. By comparing the column‐averaged microphysical quantities from the subgrid‐scale precipitation model with the parametrization in the European Centre for Medium‐Range Forecasts (ECMWF) model, the ability of the ECMWF model to account for the subgrid nature of cloud and precipitation microphysics is assessed. It is found that the ECMWF model overestimates the evaporation of precipitation in the tropical mid‐troposphere. This results from (a) an incorrect parametrization of the area of the grid box covered by precipitation, and (b) the inadequacy of assuming a single value for the precipitation rate in the grid box.

In addition to assessing the ability of the ECMWF model to parametrize the subgrid nature of cloud microphysics, the subgrid precipitation model is used to show that the cloud‐overlap assumption has a large impact on the evaporation of precipitation. In light of the current trend towards more sophisticated cloud and precipitation parametrizations in GCMs, more attention should be paid to the impact of vertical cloud‐fraction variations on the parametrized microphysics.
The generation of turbulence and entrainment by buoyancy reversal
Authors: Lock A. P. , Macvean M. K.
Published: February 2007

In the stratocumulus‐capped atmospheric boundary layer, the evaporation of cloud droplets in mixtures of cloudy and entrained air can, under certain circumstances, reduce their buoyancy to less than that of the surrounding unmixed cloudy air. With a negative buoyancy excess, these parcels will sink away from cloud‐top and therefore generate turbulence within the boundary layer. Dimensional arguments are used to formulate a scaling for the generation of turbulence by this mechanism, referred to as buoyancy reversal, which is found to agree remarkably well with results from idealized high‐resolution three‐dimensional large‐eddy simulations (LES). Furthermore, simple arguments relating the turbulence generation by buoyancy reversal to entrainment at cloud‐top give a simple parametrization for the entrainment rate which is also found to agree very well with the LES.
Fast and slow Kelvin waves in the Madden‐Julian oscillation of a GCM
Authors: Matthews Adrian J. , Slingo Julia M. , Hoskins Brian J. , Inness Peter M.
Published: February 2007

The structure of the Madden‐Julian Oscillation (MJO) in an 1800‐day integration of the Hadley Centre Unified Model was analysed, and interpreted within a Kelvin‐wave framework. the model was forced with constant equinoctial (March) boundary conditions so that a ‘clean’ MJO signal could be separated from the effects of the seasonal cycle and forced interannual variability. the simulated MJO was fairly realistic in terms of its large‐scale spatial structure and propagation characteristics, although its period of 30 days (corresponding to an average phase speed of 15 m s−1) was shorter than that observed. the signal in deep convection was less coherent than in observations, and appeared to move eastwards as a sequence of discrete convective anomalies, rather than by a smooth eastward propagation. Both ‘fast’ and ‘slow’ equatorial Kelvin waves appeared to play an important role in the eastward propagation of the simulated MJO. Enhanced convection over the Indian Ocean was associated with a ‘fast’ equatorial Kelvin wave that propagated eastwards at 55 m s−1 over the Pacific. On reaching the west coast of South America, a component of this Kelvin wave propagated northwards and southwards as a trapped wave along the mountain ranges of Central America and the Andes, in agreement with observations. the anomalous surface easterlies over the tropical eastern Pacific associated with this fast Kelvin wave enhanced the climatological mean easterlies and led to positive convective anomalies over the eastern Pacific consistent with the wind‐induced surface heat exchange (WISHE) mechanism. However, WISHE was not able to account for the eastward development of the convective anomalies over the Indian Ocean/western Pacific region. By splitting the equatorial divergence anomalies of the simulated MJO into their ϱu/ϱx and ϱv/ϱy components, the role of Kelvin‐wave dynamics in the ‘slow’ (15 m s−1) average eastward propagation of the simulated MJO was examined. Although the two components were of comparable magnitude, the ϱu/ϱx component exhibited a pronounced eastward propagation which tended to be disrupted by the ϱv/ϱy component, thus supporting the paradigm of an underlying, but strongly modified, Kelvin‐wave mechanism.
A microphysically based precipitation scheme for the UK meteorological office unified model
Authors: Wilson Damian R. , Ballard Susan P.
Published: February 2007

Many numerical weather prediction models are moving towards more prognostic schemes for the prediction of ice and liquid water contents within clouds. This paper describes a large‐scale cloud and precipitation scheme developed for the UK Meteorological Office's Unified Model. It uses physically based transfer equations to predict ice as a prognostic variable. We review similar schemes and then describe our new scheme, giving examples of its performance in mesoscale forecasts compared with the current operational scheme and with observations. the microphysical processes occurring in a frontal cloud are well modelled. the prediction of supercooled stratocumulus cloud is much improved.
The role of sound waves in sea‐breeze initiation
Authors: Tum A. B. C. , van Delden A. J.
Published: February 2007

The dynamics of the pressure distribution associated with the sea‐breeze circulation is investigated. Analysis of hourly observations of surface pressure reveals that, on a typical sea‐breeze day, large surface‐pressure falls are recorded very far inland. Using a linear numerical model we show that this can only be understood when sound waves are taken into account. Sound waves are excited when air in the boundary layer over land expands due to diabatic heating. the numerical study reveals that vertically travelling sound waves induce a pressure increase through the entire atmosphere above the heated layer within a few minutes, while horizontally travelling sound waves induce a surface‐pressure decrease over land and a surface‐pressure increase over sea, with the resulting horizontal surface‐pressure gradient initiating the sea breeze. Because the signal travels inland at the speed of sound (about 300 m s‐1), points at a distance of more than 1000 km from the coast experience a surface‐pressure decrease within an hour after the initiation of diabatic heating over land. the pressure decrease near the earth's surface over land, as well as the pressure increase aloft, is in accord with the observations at mountain stations and the analyses of the observed daytime surface‐pressure changes in summer over the continent and adjacent seas. the implications of these findings for hydrostatic modelling of sea breezes are investigated. It appears that after one hour the hydrostatic sea breeze is 10% stronger than the non‐hydrostatic sea breeze, while the hydrostatic return current is about 20% weaker.
Feedbacks between sea ice and baroclinic waves using a linear quasi‐geostrophic model
Authors: Tansley C. E. , James I. N.
Published: February 2007

Both the high‐latitude atmospheric circulation and the oceanic sea‐ice cover exhibit significant synopticscale variability. Areas of open water within the ice cover, caused by divergence of the ice, may be generated by convergent, divergent or offshore wind fields. There is a large anomalous upward heat flux over such open water areas, leading to the possibility of complex feedbacks between the ice cover and the baroclinic weather systems that pass over it. the effect of this feedback on the development of baroclinic systems has been investigated in an idealized manner through normal‐mode analysis of a system of quasi‐geostrophic equations. Results indicate that the growth rate, scale, structure and propagation speed of high‐latitude systems may be strongly modified.
Near‐surface fluxes of cloud water evolve vertically
Authors: Kowalski A. S. , Vong R. J.
Published: February 2007

Occult deposition to vegetation, via mechanical interception of wind‐blown cloud water, can be a significant fraction of total ionic chemical deposition for some forests. Applying micrometeorological methods to the estimation of cloud water deposition requires particular consideration since cloud droplets are not conservative but are subject to material change (phase change); sedimentation also affects fluxes of droplets. the budget equation for liquid water (LW) in orographic cloud predicts that LW fluxes will diverge due to condensation during mean ascent. For hilltop measurements such as these, other factors can contribute to changes in the vertical flux with height above the surface.

Fluxes measured concurrently at two heights exhibit a persistent and significant divergence, while laterally separated measurements are found to agree. A LW budget equation is presented and simplified by scale analysis. Surface uptake is estimated by extrapolation of the measured fluxes. Estimated surface deposition is found to be substantially different from the flux measured at a reference height (10 m), often by a factor of two and occasionally with a different sign. This difference in estimated surface uptake extends to estimates of chemical as well as water deposition. the turbulent flux of LW is shown to be dependent on two criteria for describing ‘steadycloud’ conditions, thus presumably minimizing the effects of entrainment. an often used model relating droplet deposition to the ‘deposition velocity’ for momentum is found to be inappropriate for application in complex terrain. Deposition of LW is estimated to range from 5 to 50 mg m−2 s−1 during the third field campaign of the Cloud and Aerosol CHemistry Experiment (CACHE‐3). with an average of 19 mg m−2 s−1 (nearly 2 mm per in‐cloud day) in late summer at this Pacific coastal site in North America.
Targeted observations in FASTEX: Adjoint‐based targeting procedures and data impact experiments in IOP17 and IOP18
Authors: Langland R. H. , Gelaro R. , Rohaly G. D. , Shapiro M. A.
Published: February 2007

The Fronts and Atlantic Storm‐Track EXperiment (FASTEX) provided an opportunity for testing targeted‐observing procedures in a real‐time framework during January and February 1997. This study describes the use of singular vectors (SVs) for objective targeting during FASTEX, and the evaluation of the impact obtained from targeted dropsonde data, satellite wind data, and other observations on 1‐2 day forecast skill in intensive observation periods (IOPs) 17 and 18.

In IOP17, targeted dropsondes improve a 42 h forecast of L41 (Low 41; cyclones were numbered in sequence throughout FASTEX) in terms of sea‐level pressure, but the forecast skill is degraded in the upper troposphere. It is suggested that the degraded forecast may be caused by an incomplete survey of the SV target area, that improved the analysis in one region, but made the analysis less accurate in an adjacent part of the target area where no dropsonde data were provided. In a series of experiments, the best 42 h forecast of L41 is obtained by the addition of a few radiosonde profiles provided specially for FASTEX at off‐times, that provide observational data in the most sensitive part of the SV target area. the analysis differences introduced by the radiosonde profiles are much smaller in magnitude than those from the dropsonde data, but have a larger forecast impact, because they occur in an area that has larger error growth rates in this forecast.

In a series of experiments for IOP18, the best 24 h forecast of L44 is obtained using a combination of targeted‐dropsonde data and satellite wind data. Both data types can also be used separately to improve this forecast. the assimilation of satellite wind data and ship‐based soundings in areas of weak initial‐condition sensitivity (‘null’ areas) is shown to have minimal impact on the forecast error. the target areas identified by SVs in these two IOPs occur in strongly baroclinic regions, tending to favour the right‐entrance and left‐exit regions of the upper‐level jet, but with greatest sensitivity near 600 hPa.
Dynamical effects of ice sublimation in a frontal wave
Authors: Clough Sid A. , Lean Humphrey W. , Roberts Nigel M. , Forbes Richard M.
Published: February 2007

The dynamical role of ice sublimation in weather systems is briefly reviewed. Observations are presented from the Fronts and Atlantic Storm‐Track Experiment (FASTEX) Intensive Observation Period 16 that show variations of static stability, humidity and mesoscale circulation corresponding to those associated theoretically with sublimation of ice precipitation. It is thus suggested that the observations display the mechanism proposed by Clough and Franks in which forward mesoscale flows are associated with moist adiabatic descent supported by the sublimation cooling. This mechanism was suggested as an important stage in the evolution of many mesoscale rain bands.

A set of three model simulations of the event has been made with versions of The Mel. Office's Unified Model. Of these a mesoscale model integration with 11 km resolution and 45 levels clearly displays the symptoms, and is diagnosed to demonstrate its consistency with the Clough‐Franks mechanism. An integration omitting the cooling due to sublimation differs significantly from the full model experiment in the structure of low‐level wind fields, frontal troughs and mesoscale precipitation distribution. Il is also demonstrated that the static‐stability transition, mesoscale circulation and mid‐tropospheric potential‐vorticity perturbations are substantially weakened in this integration, thus confirming that the Clough‐Franks mechanism is also operating in the numerical weather prediction (NWP) model.

We deduce from these studies that ice precipitation and its sublimation has a major role in determining mesoscale circulation and structure in mid‐latitude weather systems, affecting stratification and the formation of features such as fronts and rain bands. These are substantially affected by the fall and evaporation of ice crystals, which are both important to temperature and moisture transports and the behaviour of NWP models on time‐scales of hours to days. In our integrations dynamical feedback due to sublimation cooling coincided with extreme negative potential‐vorticity values and potentially conditional symmetric instability, hence the anticyclonic motion occurring in the cloud head or deep cloud of the moist warm sector as in this case. In moist warm sectors a substantial role for sublimation may be anticipated more generally, particularly for air trajectories receiving most ice precipitation.

We suggest that the described phenomenon be referred to as sublimation enhanced descent or SED.

It is concluded that in view of tins demonstrated sensitivity substantial attention should be given to refining microphysical parametrizations in NWP models, and that radar and sounding observations from the FASTEX experiment provide a suitable basis for validating these schemes.