bluuebunny/crossref_metadata_2025 huggingface dataset:
How large must surface inhomogeneities be before they influence the convective boundary layer structure? A case study
Authors: Shen Shaohua , Leclerc Monique Y.
Published: February 2007

A high‐resolution large‐eddy simulation model has been used to investigate the effect of scale in surface inhomogeneities on the turbulence structure of the convective boundary layer (CBL). Surface heat fluxes were varied sinusoidally in two dimensions with wavelengths of 250, 500 and 1000 m, corresponding to about 0.27 zi, 0.55 zi and 1.1zi (zi is the boundary layer depth). the geostrophic wind speed was 1 ms−1. Horizontally‐averaged statistics of the CBL above surface heat‐flux inhomogeneities of different two‐dimensional scales were compared. the phase‐average method was used to study the horizontal variation of turbulence and the mean circulation in the CBL.

Results from horizontally‐averaged statistics show that surface inhomogeneities of small horizontal extent (x < zi, influence the horizontally‐averaged variances, covariances and third moments. the surface heterogeneities of larger scales produce more vigorous energy transport in the CBL and larger temperature variances near the ground than do those of smaller scales. There is more entrainment over the region of high heat flux than over the region of low heat flux. the magnitude of entrainment and the mean circulation both increase with increasing scale of heterogeneities present at the ground.

The effects on the turbulence structure of the two‐dimensional surface inhomogeneities differ from those in the one‐dimensional case reported by Hadfield et al. (1991, 1992). the most pronounced difference occurs when the scale of the surface heterogeneities is comparable to the height of the boundary layer; the v‐variance is larger than the u‐variance in the boundary layer. Although the scope of this case‐study is restricted to a limited number of typical scenarios, it does provide evidence in support of the fact that even small‐scale inhomogeneities in the surface thermal forcing can modify the CBL structure very differently than could an equivalent but uniform field. the sensitivity of the CBL turbulence structure to surface thermal perturbations with scales smaller than the boundary layer depth is also discussed.
A computational study of the relationships linking lightning frequency and other thundercloud parameters
Authors: Baker Marcia B. , Christian Hugh J. , Latham John
Published: February 2007

In an effort to optimize the value of global‐scale measurements obtained with the NASA/MSFC satelliteborne Lightning Imaging System (LIS), a simple computational model of thundercloud electrification has been developed, from which it is possible to derive crude relationships between lightning frequency f (which LIS will measure) and cloud parameters such as radar reflectivity Z, precipitation rate P, updraught speed w, cloud radius R, ice‐crystal concentration i and graupel‐pellet concentration Ng. Electric field‐growth is assumed to occur via the non‐inductive charging mechanism, for both Fletcher and Hallett‐Mossop types of glaciation mechanisms. A simple criterion is used to distinguish between cloud‐to‐ground and intracloud lightning discharges. f is found to be especially sensitive to w in situations where, as updraught speed increases, the temperature at balance level, Tbal, of the upper boundary of the charging zone falls. In these circumstances N1 and the sizes of the ice hydrometeors are significantly increased, with a corresponding enhancement of the effectiveness of charge transfer.

Over a wide range of conditions, f is found to be roughly proportional to the first power of the parameters R1 Ni Ng and Z and (in some circumstances) to at least the sixth power of w. the relationship between f and P depends critically on whether or not w and Tbal are strongly linked. Hallett‐Mossop glaciation is capable of producing inverted‐polarity lightning from thunderclouds; Fletcher glaciation is not.
Vortex axisymmetrization: Dependence on azimuthal wave‐number or asymmetric radial structure changes
Authors: Smith Gerald B. , Montgomery Michael T.
Published: February 2007

Intense vortices in the atmosphere and ocean exhibit a high degree of axisymmetry despite persistent asymmetric forcing from their environment. to further elucidate vortex axisymmetrization a variety of idealized initial‐value models for barotropic non‐divergent flow is considered. to ensure basic understanding, disturbance evolution is first examined in a rectilinear system of simple shear. Particular emphasis is placed on identifying how inviscid disturbance‐evolution depends on the zonal wave‐number and on the meridional structure of the initial conditions.

Insight acquired from the rectilinear problem is then applied to a bounded Rankine vortex. Here, the dependency of disturbance evolution on the azimuthal wave‐number is of special interest. Recent development of a low‐frequency balance theory for rapidly rotating (large Rossby number) vortices has provided observational evidence that the low‐azimuthal‐wave‐number asymmetries, especially wave number one, are dominant in the nearvortex region. the results of this work provide further theoretical evidence of an inviscid wave‐number‐selection mechanism that preferentially damps the high‐wave‐number asymmetries.

The radial structure and location of the initial conditions are found to be important factors in determining how rapidly a disturbance is compressed or elongated. This in turn controls the rate of disturbance growth or decay. For swirling flows, a definition of an effective shear is proposed that accounts for both the radial variations of the initial condition and the radial variation of the angular velocity. Using the reciprocal of this effective shear, time‐scales for a disturbance to decay to half its initial energy, the half‐life, are calculated for initial conditions and symmetric wind‐profiles that are found in hurricanes.

Simple‐shear flow and the bounded Rankine vortex do not admit discrete modal solutions since there is no mean‐state vorticity‐gradient to support them. the unbounded Rankine vortex is examined to investigate how the presence of discrete modes (Rossby edge‐waves) associated with the radial vorticity‐gradient of the Rankine swirl modifies the continuous spectrum solutions presented here.
Entropy and climate. II: Simple models
Authors: O'brien D. M. , Stephens G. L.
Published: February 2007

The possibility that the climate of planet earth might be a state of maximum dissipation was explored by Paltridge through the development of a simple energy‐balance model of climate. In this paper we examine the assumptions in Paltridge's model and show that the model can be reduced analytically to a model involving trivial numerical computation. This step exposes the interplay of energy balance, dynamics, and extremum principles. In particular, we highlight the role of a secondary extremum‐principle, related to convective activity, which postulates that cloud cover and surface temperature conspire to maximize the sum of sensible‐and latent‐heat fluxes. We show that this convection hypothesis leads to simple algebraic relations (between cloud cover, surface temperature and horizontal convergence of energy) which could be tested against satellite data. We examine a single‐box version of Paltridge's model that exhibits remarkable temperature‐regulation through adjustment of cloud cover, and show that the regulation follows from the convection hypothesis rather than the maximum‐dissipation hypothesis. Next we investigate the relevance to climate of a theorem on maximum dissipation derived by Ziegler. We reconcile the maximum‐ and minimum‐dissipation theorems of Ziegler and Prigogine in the context of a simple model. Finally, we speculate how the principle of maximum dissipation might be applied in a climate model.
Summer thermal lows in the Iberian peninsula: A three‐dimensional simulation
Authors: Portela Adelaida , Castro Manuel
Published: February 2007

A summary of some of the most relevant climatological characteristics of summer thermal lows in the Iberian peninsula is given followed by a description of a high‐resolution numerical model which has been used to simulate the mesoscale system of a typical summer's day. The results from this model show the appearance of small low‐pressure centres before noon, one of which prevails over the others and intensifies during the afternoon. Other small‐size structures related to this system, and not evident in the analysis from the ECMWF (European Centre for Medium‐Range Weather Forecasts) forecast model, appear in the simulation, such as the existence of areas with strong convergence near the surface, and the deviation towards the interior of the peninsula of the surface winds as the depression intensifies. Also, the structures associated with certain variables, such as potential temperature, vertical velocity and potential vorticity, become more noticeable in the model's simulation compared with the ECMWF analysis. Finally, a detailed description of the dynamical development of the simulated surface low‐pressure system is presented, taking all these findings into consideration.
A shallow‐water semi‐geostrophic model on a sphere
Authors: Mawson Mark H.
Published: February 2007

The f‐plane semi‐geostrophic equations have provided much insight into atmospheric motions and we wish to see if more can be gained by removing the f‐plane restriction. As a first step we begin by considering the shallow‐water semi‐geostrophic equations on a sphere and describe a numerical solution procedure for them based on a semi‐Lagrangian predictor–corrector method. It is demonstrated, numerically, that the method appears to converge to a true solution of the semi‐geostrophic equations for fixed resolution, and this suggests that there is a unique solution to the continuous equations. The results from several idealized and real‐data test problems are presented and show that the scheme is stable and accurate. The results are compared with those obtained from a shallow‐water semi‐Lagrangian primitive‐equation model. The simulations are broadly similar but the semi‐geostrophic solution maintains stronger anticyclones than the primitive‐equation model. This is in agreement with results expected after considering f‐plane baroclinic wave simulations with the effects of spherical geometry taken into account.
Variational retrieval of humidity profile, wind speed and cloud liquid‐water path with the SSM/I: Potential for numerical weather prediction
Authors: Phalippou L.
Published: February 2007

Several regression algorithms have been proposed to retrieve geophysical parameters from the Special Sensor Microwave/Imager (SSM/I) radiances. Their performances are generally limited by a simplified handling of nonlinearities and/or by the poor quality of the a priori information. In this paper, a variational method is proposed for retrieving the atmospheric humidity profile, the wind speed and the cloud liquid‐water path from SSM/I observations over ocean. This method is based on nonlinear optimal estimation theory. The first guess is derived from a European Centre for Medium‐Range Weather Forecasts forecast, and the forecast‐error covariance is used as a constraint. The geophysical variable space is mapped into the radiance space through a radiative‐transfer model which permits an accurate representation of nonlinearities. This method has been applied to several orbits and the results for one of them are presented and discussed. It is argued that the variational approach is a simple optimal way of extracting information from SSM/I radiances, exploiting the high quality a priori information available from a numerical weather‐prediction model. The retrieved humidity profiles are found to compare well with the total precipitable water estimated from a regression algorithm, while avoiding local bias in very dry and very wet conditions. It is also shown that the potential wind speed‐cloud ambiguity is removed through the use of a high‐quality wind speed first guess.
Impact of orographically induced gravity‐wave drag in the GLA GCM
Authors: Zhou Jiayu , Sud Y. C. , Lau K.‐M.
Published: February 2007

The influence of gravity‐wave drag (GWD) parametrization has been investigated in the Goddard Laboratory for Atmospheres general‐circulation model with the resolution of 4° latitude × 5° longitude in the horizontal and 17 sigma layers in the vertical. The simulations showed several overall beneficial effects in the extended‐range forecast as well as the seasonal simulation for both winter and summer conditions. The key results include the following:

The mid‐latitude westerlies, which were shifted poleward (equatorward) in northern (southern) winter, have been corrected. Consequently, the systematic bias of low (high) sea level pressure over the north (south) polar region has been largely eliminated.

There is a significant improvement in the simulation of the winter hemisphere stationary waves; this can be attributed to the GWD induced Rossby‐wave propagation. Over the northern Pacific region, as the storm track is simulated better, precipitation distribution improves remarkably.

In cooperation with the improved regional zonal‐mean flow, an adjustment of the mean meridional circulation is found over the Asian summer‐monsoon region, leading to an improvement in the precipitation distribution.

Spurious absorption of gravity waves at the model top (10 mb) has been removed by applying an open upper‐boundary condition. The vertical propagation of the tropospherical gravity wave into the upper stratosphere and mesosphere over mid‐latitudes of the northern hemisphere in winter is consistent with observational studies as well as the middle‐atmospheric dynamics.
A comparison of ozone and thermal tropopause heights and the impact of tropopause definition on quantifying the ozone content of the troposphere
Authors: Bethan S. , Vaughan G. , Reid S. J.
Published: February 2007

A comparison has been conducted of the height and sharpness of the tropopause as revealed by temperature and ozone profiles. In the study, 628 ECC‐type ozonesonde profiles from four stations in northern Europe were used. Two tropopauses were defined for each profile: a thermal tropopause and an ozone tropopause defined in terms of both mixing ratio and vertical gradient of mixing ratio. On average, the ozone tropopause lay 800 m below the thermal. Large differences in tropopause height were associated with indefinite thermal tropopauses which were, in turn, often associated with cyclonic conditions (some corresponding to profiles taken within the stratospheric polar vortex). On almost all profiles the thermal tropopause was the higher of the two, and of the 15 profiles that did not fit this pattern, two‐thirds were associated with anticyclonic flow in the upper troposphere. It is also shown that the tropopause definition impacts greatly on the evaluation of the ozone content of the troposphere. Where the thermal tropopause is indefinite in character, on average 27% of the ozone found below the thermal tropopause lies above the ozone tropopause.
The Asian monsoon and predictability of the tropical ocean–atmosphere system
Authors: Lau K.‐M. , Yang Song
Published: February 2007

The influence of the Asian monsoon on the predictability of the tropical ocean–atmosphere is studied using long‐term records of monsoon rainfall, sea‐level pressure, sea surface temperature (SST) and surface wind data. Results indicate that the Asian monsoon plays an important role in mediating interannual variations of SST, surface pressure and surface wind in a quasi‐biennial oscillation of the tropical ocean–atmosphere as found in previous studies. During the boreal spring, monsoon‐related atmospheric and oceanic anomalies change most rapidly, while the east–west gradients in sea‐level pressure and SST across the equatorial Pacific are at a minimum. The so‐called ‘spring predictability barrier’ (SPB) is associated with the phase‐locking of interannual anomalies to the annual cycle variation of the tropical ocean–atmosphere.

Results suggest that the Asian monsoon defines two climatic states in the tropical ocean–atmosphere system, corresponding to the warm and cold phases of the tropical biennial oscillation. A weak Asian monsoon is associated with enhanced memory for predictions starting from late boreal spring through to summer and with a well‐defined SPB for those starting from boreal fall to winter. On the other hand, a strong monsoon is associated with a recovery of memory at long lags (> 4–6 months) from the boreal fall through to the following spring, indicating a partial breakdown or ‘tunnelling’ effect for the SPB. The above results may be exploited to improve seasonal‐to‐interannual predictions of the monsoon–ocean–atmosphere system.
Gravity‐wave drag on two mountains
Authors: Vosper Simon
Published: February 2007

The generation of linear internal gravity waves by the flow over two idealized mountains, whose summits are aligned in the same direction as the mean wind, is studied. In particular, the sensitivity of the wave drag to the downwind distance between the mountains is examined by using a three‐dimensional numerical model based on the linearized steady‐state Boussinesq equations of motion. Both two‐ and three‐dimensional orography is considered. When the upstream conditions are such that the wind and buoyancy‐frequency do not vary with height, and the mountains are wide enough for non‐hydrostatic effects to be unimportant, the model predicts the same drag dependence on two‐dimensional ridge spacing as was derived analytically by Grisogono et al. The corresponding three‐dimensional numerical result predicted by the model is qualitatively very similar to the two‐dimensional result. When the mountains are narrow and the waves are non‐hydrostatic the drag dependence on mountain spacing is more complicated; maxima and minima in the drag can occur when the distance between the two mountains is such that waves generated by the mountain furthest upstream are in phase (maxima in the drag), or out of phase (minima in the drag), with the waves generated by the downstream mountain. It is shown that this is an important effect when the waves are trapped by upstream profiles whose wind speed varies with height.
Infrared heating rates in the stratosphere due to volcanic sulphur dioxide
Authors: Zhong Wenyi , Haigh Joanna D. , Toumi Ralf , Bekki Slimane
Published: February 2007

A narrow‐band radiative‐transfer model is used to assess potential infrared cooling rates due to SO2 injected into the stratosphere by volcanic eruptions. In addition to SO2 profiles representing the eruptions of El Chichón and Pinatubo, two hypothetical SO2 profiles following a massive volcanic eruption are used. The net radiative impact of the SO2 depends on the vertical distribution as well as the altitude and magnitude of its injection. For broader distributions the magnitudes of the infrared heating rates are comparable with those due to heating caused by the absorption of solar ultraviolet by SO2 tending to enhance the heating of the lower stratosphere and reduce it in the upper stratosphere. These results are explained in terms of the relative strengths of the SO2 bands and of those of H2O and O3 overlapping in the same spectral regions.
The spectral signature of global warming
Authors: Slingo A. , Webb M. J.
Published: February 2007

Simulations are presented of the change in the spectrum of the clear‐sky outgoing long‐wave radiation (OLR) associated with the global warming produced by increases in greenhouse‐gas concentrations. the input data for the present day and for the middle of the next century were taken from a recent climate‐prediction run of the Hadley Centre Climate Model. the simulations focus on the spectral signature of the warming as opposed to that of the forcing. Stratospheric cooling causes decreases of the OLR in the carbon‐dioxide and ozone bands, whilst surface warming increases the OLR in the region of the atmospheric window. the signal in the water‐vapour bands is more subtle, due to a near cancellation between the effects of changes in atmospheric temperatures and specific humidities that have little impact on the relative humidity. the residual signal is shown to be related to small changes in upper‐tropospheric relative humidity, although at some latitudes this relationship breaks down. It is suggested that satellite observations in the water‐vapour bands could be used to provide a quantitative measure of the water‐vapour feedback during global warming.
The global population of mesoscale convective complexes
Authors: Laing Arlene G. , Michael Fritsch J.
Published: February 2007

A global set of 714 mesoscale convective complexes is compiled and some of the common properties of the convective systems are identified and examined from a global perspective. the data set includes date of occurrence, time of first storms, initiation, maximum extent, termination, duration, cold‐cloud shield areas, and tracks from initiation to termination.

It is found that the typical convective complex is nocturnal, generates a cold‐cloud shield area of approximately 350 000 km2, and persists for about 10 h. the largest systems and most persistent systems tend to occur near the summer solstices. For the globe, about 400 systems occur each year, primarily over land areas. Most systems develop in favoured zones, although some activity occurs over every continent (except Antarctica) and all major oceans. the concentration of activity into favoured zones indicates that there must be special dynamic and/or thermodynamic conditions necessary for convection to organize into convective complexes.

Activity is strongly tied to the solar day, and shifts from 35°S in early January to about 50°N during the boreal summer and back to 35°S by December. Within the northern hemisphere there is a pronounced poleward migration as the jet stream shifts northward. Relatively little migration occurs in the ocean‐dominated southern hemisphere where the subtropical jet remains quasi‐stationary over the convective‐complex regions.

The nocturnal life cycles, copious rainfall, large cloud shields, and great frequency of mesoscale convective complexes suggest that they may be significant contributors to the global hydrologic cycle and earth‐system energy budget.
Organized convective systems in the tropical western pacific as a process in general circulation models: A toga coare case‐study
Authors: Moncrieff Mitchell W. , Klinker Ernst
Published: February 2007

We examine the large‐scale effects of organized convective systems in the tropical western Pacific observed during the Tropical‐Ocean Global‐Atmosphere Coupled Ocean‐Atmosphere Response Experiment (TOGA COARE). In a case‐study approach, we examine realizations of a supercluster, associated with the onset of the December 1992 westerly wind burst, in the T213 operational medium‐range weather forecasting model of the European Centre for Medium‐Range Weather Forecasts (ECMWF). We idealize a supercluster as a hierarchy of three interacting scales, namely organized cumulonimbu 𝒞1 mesoscale convective systems 𝒞2, and the supercluster component 𝒞3. It is shown that the ECMWF model represents this hierarchy as a 𝒞3‐like surrogate whose influence dominates the effect of parametrized convection. This causes over‐prediction of the model tendencies which, in the case of zonal momentum, is explained in elementary terms.

The structure of the resolved‐scale momentum flux is explained by Moncrieff's (1992) archetypal theory of organized convection which has been verified against observations and cloud‐resolving model data‐sets. the parametrization of subgrid‐scale convective momentum‐flux in the ECMWF model, based on a momentum mixing concept, produces subgrid‐scale tendencies that are physically different from transports associated with cumulonimbus convection in a shear flow.

We outline a strategy for parametrizing the momentum flux by the 𝒞1 component based on the archetypal model. the 𝒞2 component, which is part‐resolved and part‐parametrized, is at odds with the assumptions of scale separation underpinning parametrization. It is argued that this component should be represented as part of the prognostic treatment of convectively generated cirrus.

Finally, we suggest cloud‐resolving modelling studies to further quantify the structure and large‐scale impact of superclusters in a westerly‐wind‐burst environment, ranging from idealized models to models having data assimilation capability.
A multi‐thermal model of cumulus glaciation via the hallett‐Mossop process
Authors: Blyth Alan M. , Latham John
Published: February 2007

Analysis of field observations has yielded the conclusion that the Hallett‐Mossop process (H‐M) of secondary ice production plays a major role in the glaciation of summertime cumulus clouds over New Mexico. Other studies have revealed that these clouds possess a characteristic multi‐thermal structure.

In an effort to quantify more fully the role of H‐M in such clouds, and to establish which of the salient dynamical and microphysical parameters play important roles in the glaciation process, a model of ice‐particle growth and splinter production in a simple multi‐thermal framework is developed. the characteristics of the model are prescribed with values that are based on the above‐mentioned field studies. the model cloud possesses four distinct regions: the main updraught, a quiescent (debris) region, the cloud top, and a downdraught region.

The trajectories of all primary ice particles introduced into the cloud at t = O, together with those created as a consequence of the operation of H‐M, are followed as they grow and are transported around the cloud. the sensitivities of these trajectories and a multiplication factor f to variations in parameters such as updraught speed, liquid‐water content, L, thermal depth, inter‐thermal interval, and downdraught characteristics are examined.

These tests reveal that f is particularly sensitive to the values of L in the distinct regions of the cloud. Basically, combinations of parameter values which produce rapid growth of graupel pellets, large number of thermals, and efficient transport between cloud top and the Hallett‐Mossop temperature band yield the most rapid ice‐particle multiplication.
Balanced mesoscale motion and stratified turbulence forced by convection
Authors: Vallis G. K. , Shutts G. J. , Gray M. E. B.
Published: February 2007

Numerical experiments are carried out with a large‐eddy simulation model to investigate the production of stratified turbulence and gravity wave energy in the atmospheric mesoscale by deep convection. Relatively long integrations (typically about 2.5 days) are carried out, so that statistical equilibrium might be achieved for an atmosphere subject to fixed cooling and a ground surface maintained at constant temperature. The main simulations may be considered as representing convective activity in a cold airstream passing over warm seas. The transfer of updraught kinetic energy into quasi‐horizontal rotational energy, and its subsequent cascade to larger scales, is examined in the context of the production of balanced motion and stratified turbulence theory. Experiments are carried out to examine the effects of the Coriolis force, a wave‐damping stratosphere, boundary‐layer vertical wind shear, and precipitation.

An inverse mesoscale energy cascade is observed in experiments both with and without background rotation (the Coriolis effect), although weaker in the latter case. Mean boundary‐layer vorticity is found to be the principal source of the horizontal rotational energy created by deep convection, although the dipole character of the vorticity thereby produced frustrates the inverse energy cascade. At the level of convective detrainment, potential‐vorticity anomalies have a small monopolar element in the rotating case and this implies a direct forcing of energy at large scales. It also promotes a more efficient inverse energy cascade through long‐range influence. The inverse energy cascade arises primarily because of the quasi‐two‐dimensional, divergence‐free nature of the flow at scales larger than the convective forcing, which is maintained by the stable stratification. Even in the absence of background rotation balanced flow arises, especially in the upper troposphere and in the decay phase of the simulations when the surface fluxes and atmospheric cooling driving the convection are removed. In these cases potential‐vorticity inversion can be quite successful in determining the other field variables, with contributions from both geostrophic and cyclostrophic balance. Suppression of precipitation (i.e. removing the liquid‐water phase) increases the net convective mass transfer through the elimination of convective downdraughts, which leads to a higher efficiency of balanced‐flow energy production and upscale energy transfer. Intense ‘meso‐vortices’, with warm core structure, form spontaneously after about 2.5 days integration in these runs.
Secondary cyclogenesis: The initiation phase of a frontal wave observed over the eastern Atlantic
Authors: Rivals Hélène , Cammas Jean‐Pierre , Renfrew Ian A.
Published: February 2007

The initiation phase of a secondary frontal wave in the wake of a primary cyclogenesis event over the Atlantic is documented. North of the trailing surface cold front and south‐west of the primary cyclone, an intense upper‐level jet generates a direct and transverse ageostrophic circulation in its entrance region. Evidence is shown for two conflicting roles of this circulation: first, stabilization of the front through frontogenetic cross‐frontal low‐level convergence; and second, an increase of the potential for instability of the front through diabatic (e.g. latent‐heat release) potential‐vorticity (PV) redistribution in the upward branch of the circulation. Following saturation of the primary cyclogenesis and the associated along‐front north‐eastwards shift of the upper‐level jet, the low‐level branch of the ageostrophic circulation behaves like a zipper closing on the front, while in the upward branch an anomalous strip of PV builds along the cold front. Isentropic PV maps show that parcles of stratospheric origin embedded in the downward branch of the ageostrophic circulation (a tropopause fold) may have ended in the vicinity of the strip of anomalous low‐level PV, perhaps contributing to an increase in the potential for instability of the surface front.

Using the domain‐independent attribution method of Bishop, it is shown that the frontal wave forms on a part of the cold front where: first, the along‐front stretching rate due to the environmental flow decays below the critical threshold value (0.6 × 10−5 s−1) prescribed by theoretical studies; and second, the across‐front frontogenetic component due to the ageostrophic circulation decays. This is consistent with semi‐geostrophic theory, where the cross‐frontal convergence and the stretching deformation are linked through the Sawyer‐Eliassen equation. Results add to other recent observational findings: that the potential for instability, the evolving environmental flow and the induced ageostrophic convergence each have a crucial role in differentiating between frontal waves that will grow and those that will be suppressed.
The frequency spectrum of mountain waves
Authors: Worthington R. M. , Thomas L.
Published: February 2007

Mountain waves are often assumed to remain steady with respect to time, but measurements of their vertical wind velocity by radar are known to show a clear time dependence. Specifically, the vertical wind W obeys a power law, where the spectral density varies as ω−5/3 over at least three orders of magnitude, and ω is the frequency. Power laws in the frequency and wave‐number spectra of parameters such as horizontal wind have been widely studied, but the frequency spectrum of mountain waves remains, by comparison, poorly understood. It is found that errors arising from the VHF radar measurement technique are not sufficient to explain the power law, and a previous model invoking vortical modes or two‐dimensional turbulence does not, on its own, explain entirely the W data. However, even a surprisingly simple model, in which the mountain‐wave pattern drifts upwind or downwind in a random walk, gives a similar time dependence to the observed W data. The radar data are contrasted with other observations and models, where W time variations have been interpreted in terms of lee‐wave trains with non‐zero phase speed; the relationship to other systems showing time‐scale invariant ωβ behaviour, such as 1/f noise, is also discussed.
The dynamics of Euro‐Atlantic blocking onsets
Authors: Michelangeli Paul‐Antoine , Vautard Robert
Published: February 2007

The mechanisms leading to Euro‐Atlantic blocking are investigated using 31 winters of 500 hPa geopotential height from NMC and ECMWF daily analyses, and a 30 000‐day perpetual winter run from a simple 3‐level quasi‐geostropic model. A composite analysis of the atmospheric flow during days preceding blocking events is carried out. It indicates that Euro‐Atlantic blocking events have two simultaneous main precursors: a high‐latitude retrograding wave‐number 1 pattern at planetary scale, and an enhanced baroclinic wavetrain travelling across the North Atlantic at synoptic scale. These phenomena are identified both in observed and in simulated data.

The nonlinear interactions between small scale, large scale and the mean flow, during the establishment of the blocking anomaly, are examined by a budget analysis of the quasi‐geostrophic potential‐vorticity equation. These diagnostics demonstrate that wave‐wave interactions contribute to about 50% of the local amplification of the blocking anomaly while wave/mean‐flow interactions contribute to propagation of the large‐ and small‐scale anomalies and their phase locking.

Finally, the question is addressed as to whether the two precursors are necessary and sufficient conditions for blocking onsets by performing various sets of initial‐value experiments performed with the quasi‐geostrophic model. These experiments show that the two precursors occur simultaneously but only the synoptic‐scale baroclinic wavetrain is necessary to trigger Euro‐Atlantic blocking.
Estimating downward clear sky long‐wave irradiance at the surface from screen temperature and precipitable water
Authors: Dilley A. C. , O'brien D. M.
Published: February 2007

This paper presents two parametrizations of the downward clear sky long‐wave irradiance at the surface of the earth in terms of screen temperature and precipitable water. The first parametrization is based on a simple physical model, while the second is an extension of Swinbank's model by the addition of a term involving precipitable water. The parametrizations achieve standard deviations of approximately 5 W m−2 when compared with detailed radiative‐transfer calculations for temperature and moisture profiles in the TOVS Initial Guess Retrieval data set. Both models are more accurate than Swinbank's model, and other published models that are expressed in terms of the screen temperature and surface vapour pressure.
Efficient methods to assimilate remotely sensed data based on information content
Authors: Joiner J. , Da Silva A. M.
Published: February 2007

Two basic approaches have evolved to utilize measurements of radiance (i.e. thermal or scattered solar radiation) by satellite‐borne instruments in data assimilation systems: radiances (raw or cloud‐corrected) may be assimilated directly, or they may be pre‐processed to retrieve geophysical parameters for subsequent assimilation. The retrieval process is often ill‐posed, and therefore requires the use of prior information to constrain the solution. For example, temperature and humidity profiles retrieved using radiances from nadir‐viewing infrared and microwave sounders often incorporate prior information in the form of climatology or forecasts. The use of prior information presents difficulties when assimilating retrievals. Here we present methods to remove prior information from retrievals in order to achieve a more consistent assimilation of the data. In addition, these methods can be used as a data compression device, which can reduce the amount of computation required by some analysis systems compared with radiance assimilation. The methods are implemented and compared in a one‐dimensional assimilation system using simulated data from current and future infrared‐temperature profiling instruments.
Singular vectors and estimates of the analysis‐error covariance metric
Authors: Barkmeijer Jan , Bouttier François , Van Gijzen Martin
Published: February 2007

An important ingredient of ensemble forecasting is the computation of initial perturbations. Various techniques exist to generate initial perturbations. All these aim to produce an ensemble that, at initial time, reflects the uncertainty in the initial condition. In this paper a method for computing singular vectors consistent with current estimates of the analysis‐error statistics is proposed and studied. The singular‐vector computation is constrained at initial time by the Hessian of the three‐dimensional variational assimilation (3D‐Var) cost function in a way which is consistent with the operational analysis procedure. The Hessian is affected by the approximations made in the implementation of 3D‐Var; however, it provides a more objective representation of the analysis‐error covariances than other metrics previously used to constrain singular vectors.

Experiments are performed with a T21L5 Primitive‐Equation model. To compute the singular vectors we solve a generalized eigenvalue problem using a recently developed algorithm. It is shown that use of the Hessian of the cost function can significantly influence such properties of singular vectors as horizontal location, vertical structure and growth rate. The impact of using statistics of observational errors is clearly visible in that the amplitude of the singular vectors reduces in data‐rich areas. Finally, the use of an approximation to the Hessian is discussed.
Radiative–convective equilibrium in a three‐dimensional cloud‐ensemble model
Authors: Tompkins Adrian M. , Craig George C.
Published: February 2007

A knowledge of radiative convective interactions is key to an understanding of the tropical climate. In an attempt to address this a cloud‐resolving model has been run to a radiative‐convective equilibrium state in three dimensions. The model includes a three‐phase bulk microphysical scheme and a fully interactive two‐stream broadband radiative‐transfer scheme for both the infrared and solar radiation. The simulation is performed using a fixed sea surface temperature, and cyclic lateral boundary conditions. No ‘large‐scale’ convergence, mean wind shear or background vorticity was imposed.

The total integration lasted 70 days, and a statistical equilibrium state was reached at all heights after 30 days of simulation in all model variables. It is seen that some variables, such as vertical mass flux, adjust quickly to their equilibrium values while others, such as column‐integrated water amount, domain‐mean temperature and convective available potential energy (CAPE) display variation on a longer 30‐day time‐scale. The equilibrium state had a column‐integrated vapour amount of 42.3 kg m−2, a mean temperature of 258.7 K and a pseudo‐adiabatic CAPE value of 1900 J kg−1. The equilibrium‐state statistics are consistent with tropical observations.

The convection does not remain randomly distributed but instead becomes organized, aligning in a band structure associated with high moisture values in the boundary layer. This organization seems to result from interactions between radiation, convection and surface fluxes. The surface‐flux feedback is due to higher boundary‐layer winds, associated with convection, increasing surface fluxes of moisture locally. Horizontally inhomogeneous radiation can act to make clouds longer lasting and also increase convergence into cloudy region. Replacing the wind‐sensitive surface‐flux calculation with a linear relaxation to surface values appeared to largely destroy this organization, as did the use of an imposed horizontally uniform radiative‐heating rate.
Predictive skill of an NWP system in the southern lower stratosphere
Authors: Waugh D. W. , Sisson J. M. , Karoly D. J.
Published: February 2007

The predictive skill of the Australian Bureau of Meteorology's Global Assimilation and Prediction (GASP) system in the southern lower stratosphere is examined using two different sets of diagnostics: (i) conventional verification statistics used in numerical weather‐prediction studies (namely, root‐mean‐square (RMS) error, anomaly correlation, and bias), and (ii) elliptical diagnostics of the polar vortex (defined using potential vorticity on isen‐tropic surfaces). Both sets of diagnostics indicate the same variation in predictive skill for forecasts during October 1994. The stratospheric forecasts are a large improvement over persistence even at seven days, with the performance at seven days being comparable to that in the troposphere of three‐day forecasts. There is large daily variability in the forecast scores for seven‐day forecasts, and the days with below‐average scores occur when the flow (vortex) is rapidly changing. Examination of the differences in the elliptical diagnostics show that the forecast vortex is weaker, less disturbed (i.e. closer to the pole and less elongated), and rotates faster than the analysed vortex. Consistent with a weaker forecast vortex, the minimum polar temperature and maximum zonal wind are underpredicted in the forecasts. The verification statistics in the stratosphere have a large seasonal variation, although the variation is different for different statistics. The GASP RMS errors are largest (smallest) in late‐spring (summer) whereas both the ratio of GASP to persistence RMS error and the anomaly correlation indicate that the performance relative to persistence is best (worst) in late‐spring (summer).
Analysis of an ex‐tropical cyclone after its reintensification as a warm‐core extratropical cyclone
Authors: Browning K. A. , Panagi P. , Vaughan G.
Published: February 2007

Ex‐hurricane Lili reintensified as an extratropical cyclone before travelling across the data‐rich region of the British Isles on 28 October 1996. The cyclone centre passed close to a Mesosphere‐Stratosphere‐Troposphere (MST) radar, providing continuous profiles of wind etc. which were used to evaluate diagnostics from the mesoscale version of the operational UK Meteorological Office Unified Model. The paper presents a mesoanalysis of the mature extratropical cyclone using model output together with radar and satellite observations. The combined analysis reveals a vertically extensive warm core three‐quarters surrounded by a low‐level jet reaching over 40 m s−1. There was an associated eye, relatively free of cloud and partly surrounded by a hook cloud producing extensive heavy rain, which was itself encircled by cooler dry‐intrusion air. The stratospheric part of the dry intrusion (and its potential‐vorticity (PV) anomaly) descended within a tropopause fold around the cloud hook generally to below 400 hPa, with small pockets penetrating significantly lower. The cyclone's reintensification as an extratropical cyclone was related to its interaction with the stratospheric PV anomaly. This interaction commenced immediately after the decay of the strong moist ascent and associated deep column of diabatically generated positive PV that had characterized the earlier tropical‐cyclone phase. Following reintensification, the dry‐intrusion air entered the eye region of the extratropical cyclone over a deep layer. The mesoscale model represented many aspects of the cyclone structure well but it underestimated the dryness of the dry‐intrusion air entering the eye. The MST radar vividly depicted the region of moist boundary‐layer air responsible for the hook cloud rising up into the region of the lowered tropopause.
Remotely and Regionally Forced Pre‐Monsoon Signals Over Northern India and Neighbourhood
Authors: Krishnan R. , Mujumdar M.
Published: February 2007

A detailed diagnostic examination of the National Centre for Environmental PredictiodNational Centre for Atmospheric Research Reanalysis data supplemented by a theoretical study is undertaken with the goal of investigating the dynamics of pre‐monsoon stationary wave anomalies occurring over northern India and adjoining region prior to weak Indian summer monsoons. It is seen from the analysis that the rotational wind anomalies preceding weak monsoon years are primarily forced by tropical divergent wind anomalies located over the tropical central‐eastern Pacific ocean, south Indian ocean and locally over north‐west India. the divergent wind anomalies over the central‐eastern Pacific ocean can be linked to anomalous changes in the tropical Pacific sea surface temperature arising due to El Nioñ/Southern Oscillation (ENSO). the upper level divergent wind forcing overlying the south Indian ocean seems to be locally connected to the warm sea surface temperature anomalies over that region.

Results of simple experiments using a forced divergent barotropic vorticity model establish that the maintenance of rotational wind anomalies over north India prior to weak monsoons is attributable to the existence of anomalous divergent wind forcing over the aforementioned regions in the tropics. A normal mode experiment shows that the climatological basic flow alone is not sufficient to support these pre‐monsoon vorticity perturbations. Furthermore, calculation of vorticity generation terms suggests that the divergent wind advection and Rossby source terms make significant contributions to the production of rotational anomalies over north India during the pre‐monsoon months. Based on the above results, it is conjectured that energy injection into the subtropics via Rossby wave dispersion could be a foremost dynamical mechanism which efficiently sustains rotational wind perturbations over north India and neighbourhood. an assessment of the relative importance of the ENSO and non‐ENS0 divergent forcings, shows that the latter tend to exert a larger influence on the pre‐monsoon signals over north‐west India.

The implications of using the pre‐monsoon signals for long range forecasting of Indian summer monsoon activity are determined by two factors viz., (i) the robustness and intensity of the pre‐monsoon signals and (ii) the persistence of the signals during the following months. the first factor in turn depends on the manner in which the different forcing anomalies co‐ordinate. the amplitude of the pre‐monsoon signals can be large (small) depending on whether the ENSO and non‐ENSO boundary forcings act in a mutually co‐operative (competitive) manner. the second factor, which is related to the maintenance of anomalous quasi‐stationary patterns during the following months, depends crucially on how the different boundary forcings evolve during the summer monsoon season.
Quasi‐lagrangian energetics of an intense mediterranean cyclone
Authors: Michaelides Silas C. , Prezerakos Nicholas G. , Flocas Helena A.
Published: February 2007

Studies of the energetics of synoptic‐scale systems and similar kinds of investigation have traditionally used a Eulerian framework. In this study, the energetics of a synoptic‐scale system have been considered using a quasi‐Lagrangian method, in order to isolate the disturbance under consideration within a volume which moves together with the system at each stage of its development.

Applying a Lagrangian framework implies that the dimensions of the computational area can be modified on the basis of predetermined criteria. In this study, an area surrounding a depression as shown on the surface analysis, has been selected. This area moves together with the centre of the depression. the energetics results obtained using such a quasi‐Lagrangian scheme are compared to those obtained by using a Eulerian framework.

The synoptic‐scale system studied here is a wintertime frontal depression, the greatest development of which occurred in the central Mediterranean on 7 December 1991. This depression moved east accompanied by significant temperature changes, heavy precipitation and gale force winds.