bluuebunny/crossref_metadata_2025 huggingface dataset:
Smooth quasi‐homogeneous gridding of the sphere
Authors: Purser R. J. , Rančcić M.
Published: February 2007

We describe a variational technique for generating smooth quasi‐homogeneous numerical grids covering the sphere. This work extends our earlier studies of non‐standard global grids by relaxing the requirement of conformality to admit a greater degree of homogeneity. The motivation for this extension is to increase the minimum grid distance and thus to enable a larger time step to be used within an Eulerian grid‐point model. The grids described here are specified by balancing terms in a variational principle that penalizes departures of grid‐point density from smoothness and homogeneity. Grids generated by the new procedure are tested in both the cubic and octagonal configurations we have described in our earlier studies. The time‐step restriction is successfully alleviated without significant loss of solution accuracy.
A first‐order closure for diturbed plant‐canopy flows, and its application to winds in a canopy on a ridge
Authors: Wilson John D. , Finnigan John J. , Raupach Michael R.
Published: February 2007

To calculate disturbed wind flows in plant canopies, we studied a variant of the K α λk1/2 first‐order closure (where K is the eddy viscosity, k is the turbulent kinetic energy calculated from its simplified governing differential equation, and λ is an algebraic length‐scale). We compare numerical solutions using this closure (which admits only three empirical constants) with earlier obervations of mean wind, shear stress, and turbulent kinetic energy in three, quite different, uniform canopy flows, and in disturbed canopy flow over a ridge. The closure performs as well as or better than the solutions of others based on modifications of the ‘k‐ϵ’ closure, where ϵ is the rate of dissipation of k.
A thermal oscillation under a restorative forcing
Authors: Cai Wenju , Chu Peter C.
Published: February 2007

The authors report an interdecadal oscillation in a wind‐ and thermally‐driven ocean general circulation model (OGCM). The oscillation is tantalizing in that it occurs under a relatively strong thermal damping (26.3 W m−2 K−1). Examinations involving a two‐dimensional OGCM, a simple thermal ‘flip–flop’ model, and a three‐dimensional OGCM with and without the nonlinear effect of temperature in the state equation of sea water demonstrate that the oscillation is not driven by mechanisms such as the so‐called convective oscillator, or the advective overshooting oscillator. Instead, the oscillation is associated with the propagation of modelled long Rossby‐waves with westward phase velocity. It is found that the north–south basin extent of the OGCM is an important factor; a larger north–south basin extent is conducive to the generation of thermal wind flows crucial for initiating the propagation of disturbances. The geophysical relevance of these results is discussed.
Boundary‐layer mean dynamic characteristics of cold fronts over Brittany
Authors: Lefloch Claire , Amory‐Mazaudier Christine
Published: February 2007

The European Mesoscale Frontal Dynamic Project/Fronts 87 lasted from 18 October 1987 to 13 January 1988. Its aim was to study the dynamics of activ cold fronts observed over the area centred on the English Channel between England and France.

In this paper the mean dynamic characteristics derived from the five intensive observation phases during which the acoustic sodar at Brest was operating are presented. Observations of wind at ground level, sodar wind in the surface and the boundary layers, sodar friction velocity, and radiosonde wind measurements are combined to determine the dynamics of a ‘mean cold front’ in the boundary layer. In spite of the important variability of the frontal phenomena, ‘mean’ frontal characteristics (over Brittany, in autumn) can be identified. The analysis shows that for all the cases, except for the dry cold front, the variations of the dynamical parameters at the frontal passage are closely related. Variations of the friction velocity and the horizontal wind in the surface and the boundary layers are linearly related.
Humidity in the ECMWF model: Monitoring of operational analyses and forecasts using SSM/I observations
Authors: Vesperini Michèle
Published: February 2007

Over oceans, total column water vapour (TCWV) estimates derived from brightness temperatures of the Special Sensor Microwave Imager (SSM/I) have been shown to give accuracies similar to radiosonde measurements. Data provided by the SSM/I on the DMSP satellites are not available in real‐time, and so they are not assimilated into the ECMWF model. They can thus be used for independent validation of this important aspect of the model's hydrological cycle. The quality of ECMWF operational analyses of humidity is assessed for the period between August 1992 and November 1994. At that time, assimilation was performed through an Optimal Interpolation scheme. For this work, 2‐week averages of TCWV have been computed from analysis humidity fields of the ECMWF model and compared with simultaneous SSM/I estimates. Discrepancies between model and SSM/I estimates associated with different meteorological regimes are discussed. According to the season, the underestimation of analysed humidity in the tropics can reach 30 to 50%, whereas oceanic subsidence areas located off western coasts of subtropical continents appear too wet by as much as 60%. Mid‐latitudes are generally too moist, but to a lesser extent. An example is also shown of the utilization of SSM/I TCWV to evaluate the impact of an experimental prognostic cloud scheme on analyses and forecasts.
Linearizing iterative processes for four‐dimensional data‐assimilation schemes
Authors: Polavarapu Saroja , Tanguay Monique
Published: February 2007

The tangent linear model is used in applications including Kalman filters and the growth of perturbations. It is also used to define adjoint models in applications such as four‐dimensional variational assimilation or sensitivity studies. The validity of the tangent linear model for all of these applications is determined by the period of time and initial amplitudes for which perturbation growth remains linear. In this work we examine the validity of various linearizations of a class of iterations known as fixed‐point iterations. The tangent linearization is found to converge more slowly than the nonlinear iteration, and also requires all iterates of the nonlinear process. The tangent linearization can also be invalid if too few iterations of the nonlinear process are taken. An approximate linearization which requires only the last iterate of the nonlinear process was found to be accurate when the correct linearization has converged. Application of these results to iterative processes occurring in a numerical weather‐prediction model reveals that the approximate linearization can be effective. Specifically, the linearization of the iteration for the mid‐trajectory position for semi‐Lagrangian advection was found to converge more slowly than the nonlinear iteration, for our choice of initial guess. For the iterative solution of an elliptic equation, the maximum number of iterations allowed by the model was sufficient for both the nonlinear and linearized iterations. Thus the approximate linearization could be used to save CPU time or memory space.
Baroclinic instability of semi‐geostrophic fronts with uniform potential vorticity. I: An analytic solution
Authors: Juckes M. N.
Published: February 2007

Baroclinic instability of uniform potential‐vorticity flow between solid upper and lower boundaries is analysed. The instability is driven by meridional temperature gradients on the boundaries. The classic Eady model of baroclinic instability uses just this system with the further idealization that the temperature gradients in the basic state are uniform. Here, this last idealization will be replaced with a complementary one in which the basic‐state temperature gradients are taken to be concentrated in a front. The analysis then takes advantage of the fact that the boundary temperature anomalies created by the growing baroclinic wave are localized at the front. The dependence of the growth rates and phase structure on wave number are remarkably similar to those of the Eady model. The wave number of maximum instability and the short‐wave cut‐off differ from those of the Eady model by less than 2% and 10% respectively. The solution is asymptotic in the limit of zero frontal width in geostrophic coordinates. For a physical flow this limit can never be achieved, but comparison with direct numerical solutions shows that the analytic solution is still accurate at physically relevant frontal widths. Part I develops the solution based on an equation for the evolution of the displacement of the surface and upper fronts. Part II will look at the three‐dimensional structure of the disturbance in more detail.
The detection of hurricane asymmetries from aircraft reconnaissance flight data: Some simulation experiments
Authors: Smith Roger K. , Glatz Arno
Published: February 2007

A series of observational system simulation experiments designed to investigate the data requirements for determining the flow asymmetry of a hurricane are described. Of particular interest is the azimuthal wave‐number‐one component of the asymmetry, which, together with the mean flow across the storm, is responsible for the storm motion. A principal finding is that the accuracy of retrieval of this component from wind data obtained from airborne omega dropwindsonde soundings depends on the orientation of aircraft flight patterns relative to that of the asymmetry. Since the orientation of the asymmetry is not known in advance, there will be an inherent uncertainty in the accuracy with which the asymmetry is determined. The simulation experiments enable one to estimate probable errors in the determination of the asymmetry and their implications for hurricane track errors.

The track errors are assessed by comparing the vortex track in a high‐resolution control experiment with those that result from initial analyses based on simulated data sets of varying data density and quality. In some of the experiments, an increase in resolution along the flight tracks, or additional data in the storm environment, does not lead to an improvement in the forecast track, in contrast to increased data coverage on a regular grid. This at first sight surprising result is likely to be a consequence of the constant influence radius used in the objective analysis scheme, a factor which is currently being investigated.

The addition of random wind‐measurement errors to the simulated data, comparable in magnitude to those in dropwindsonde measurements, does not lead to significantly inferior analyses or track forecasts based on them, suggesting that such errors are not the major cause of inaccuracy in diagnosing the vortex asymmetry. Moreover, the accuracy of the analysis is nearly independent of whether the data are taken at an instant of time, or over a typical nine‐hour flight duration and then corrected to a storm‐centred coordinate system.
The impact of the height of the model top on the simulation of tropospheric stationary waves
Authors: Ruosteenoja Kimmo
Published: February 2007

Wintertime stationary waves in the northern hemisphere were simulated with a linear model, employing an observational zonal‐mean state and recent estimates for thermal damping. the sensitivity of the simulations to the height of the model top, which was varied between 0.1 and 100 hPa, proved to be much smaller than in previous research. As long as the model top was located at 10 hPa or higher, the tropospheric stationary waves were well simulated, the correlation with the observed pattern at various levels being ∼ 0.8 or higher. Lowering the top to 30 or 100 hPa reduced the correlation by less than 0.1 units in the upper troposphere, and by even less near the surface. In an experiment with a local idealized high‐latitude forcing, the response was somewhat more sensitive to the placement of the top. Eliassen‐Palm fluxes proved an excellent tool in tracing the influence of the top on wave propagation. the distribution of the fluxes shows that the interaction with the model top takes place mainly in the extratropics, irrespective of the latitude of the forcing.

Applying a sponge layer with artificially enhanced damping near the top improved the solution significantly in most of the experiments in which the top was located at 30 hPa or lower.
A second‐order Lagrangian stochastic model for particle trajectories in inhomogeneous turbulence
Authors: Reynolds A. M.
Published: February 2007

The second‐order Lagrangian stochastic model of Du et al. for particle trajectories in homogeneous turbulence is extended to inhomogeneous turbulence. the model is shown to be in good agreement with experimental dispersion data for laboratory‐scale convective boundary layers and for a neutrally stable wind‐tunnel boundary layer. the results of numerical simulations suggest that finite Reynolds number effects are not important in determining the mean concentrations of tracer particles dispersing in these flows. Estimates for the value of the Lagrangian velocity structure function constant, obtained by optimizing model agreement with experimental dispersion data, are found to be flow‐dependent.
Entropy production and dynamical complexity in a low‐order atmospheric model
Authors: Nicolis C.
Published: February 2007

Lorenz's classical model of thermal convection is analysed from the standpoint of irreversible thermodynamics. the entropy production, describing how dissipative processes are operating within the system, is expressed in terms of the model variables. It is subsequently evaluated in different parts of the attractor and the values so obtained are compared with various indicators of the complexity of the dynamics, such as the local Lyapunov exponents and a local generalization of the Kolmogorov entropy. As it turns out, dissipative processes are most efficient when the number of locally stable directions increases. In particular, when the local Kolmogorov entropy is close to zero (three stable directions) the system can dissipate over the entire spectrum of values available. Finally, it is shown that the evolution on the attractor does not correspond to any extremal property of the entropy production.
Momentum budgets over idealized orography with a non‐hydrostatic anelastic model. II: Three‐dimensional flows
Authors: Héreil Philippe , Stein Joël
Published: February 2007

The study of the interaction between two‐dimensional nonlinear orographic effects and the synoptic flow presented in Part I of this paper is extended to three‐dimensional (3D) flows. Numerical simulations of idealized 3D orographic flows are carried out with a non‐hydrostatic anelastic model to test the hypothesis of current drag parametrizations through calculations of momentum budgets in different domains of variable size representative of a grid mesh of a general‐circulation model (GCM). Parametrizations are tested in three nonlinear regimes of 3D orographic flows. In the regime of quasi‐symmetrical lee vortices, the large‐scale flow is decelerated in a restricted region located below the maximum height of the obstacle. When the symmetry of the lee vortices is altered, by a long model integration or by introduction of a perturbation, the flow evolves towards the vortex‐shedding regime. A very long integration of the model numerically shows the saturation of the disturbance after 140 units of dimensionless time. Budgets of momentum indicate that vortex shedding significantly enhances the large‐scale flow deceleration, still located below the maximum height of the obstacle. For a case of non‐breaking 3D trapped lee waves, the mean flow is decelerated in the trapping region. Comparison of the predictions of two gravity‐wave drag parametrizations with the results of fine‐mesh simulations shows that the parametrization of 3D subgrid‐scale orographic effects in a GCM takes into account most of the physical phenomena existing in the set of idealized flows.
The sensitivity of domain‐averaged solar fluxes to assumptions about cloud geometry
Authors: Barker Howard W. , Stephens Graeme L. , Fu Qiang
Published: February 2007

The effects on domain‐averaged broad‐band solar fluxes due to assumptions about geometry of connective clouds are explored using a Monte Carlo algorithm and 3D distributions of water generated by a cloud‐resolving model (CRM). Domains are (400 km) with 2 km horizontal grid‐spacing, δx, and 35 layers of varying thickness. Optical properties are computed based on single‐scattering parametrizations for hydrometeors and the correlatedk‐distribution method for gaseous absorption. Benchmark fluxes are established using the CRM fields at δx= 2 km. Four plane‐parallel versions of these fields (affected by letting δx‐oo) are considered which mimic 1D algorithms: the independent column approximation (ICA) uses the full CRM fields; for the others, mixing ratios of cloudy cells are reset to associated layer‐mean values thus conserving water mass and cloud fraction in each layer.

For the ICA, errors in reflected flux to space and surface irradiance rarely exceed 20 W m‐. Total atmospheric absorption and heating rates are almost always within 5 W m−2and ∼3%, respectively. This demonstrates that cloud sides and horizontal fluxes are unimportant for averages over large domains. However, when clouds are homogenized horizontally yet exact overlap is retained, errors increase by almost an order of magnitude. This demonstrates the importance of horizontal variability. When the same clouds are randomly overlapped, errors in boundary fluxes can exceed 250 W m‐ at high sun, and heating rates can be off by 50% to 100%. When these clouds follow maximal/random overlap, albedo is oftenunderestimatedbecause overlap of CRM liquid clouds falls between maximal and random. This demonstrates the importance of cloud overlap and ultimately the need for ID models to account equally well for both subgrid‐scale variability in cloud extinction and overlap.
On the Removal of the Effect of Horizontal Fluxes In Two‐Aircraft Measurements of Cloud Absorption
Authors: Marshak Alexander , Wiscombe Warren , Davis Anthony , Oreopoulos Lazaros , Cahalan Robert
Published: February 2007

Cloud absorption inferred from the difference between the net fluxes measured by stacked aircraft below and above clouds is strongly affected by the uncertainties due to cloud horizontal inhomogeneity. the simplest way to get rid of these uncertainties is to perform grand averages over flight legs; if flight legs are long enough, grand averaging may lead to a reliable estimate of cloud absorption. However, the amount of information on ‘true’ cloud absorption returned from such an expensive measurement program will be very limited‐often one number per flight leg.

This paper contains a discussion on how to enhance the harvest of true absorption data using two related methods: (a) subtraction and (b) conditional sampling. Both methods assume that, simultaneously with broadband measurements, some narrow non‐absorbing‐band net flux measurements are also available. Both methods are related to Ackerman‐Cox type corrections, where subtracting fluxes in a transparent spectral band from those in an absorbing band partially removes the radiative effects of horizontal inhomogeneity and allows the recovery of spatially resolved cloud absorption. the output of the two methods is different: while the subtraction method provides a contiguous record of recovered cloud absorption, the conditional sampling method yields a discrete set of data points where the vertical net flux divergence reliably estimates true cloud absorption.
Further development of a hybrid‐isentropic GCM
Authors: Webster Stuart , Thuburn John , Hoskins Brian , Rodwell Mark
Published: February 2007

The UK Universities' Global Atmospheric Modelling Programme hybrid‐isentropic general‐circulation model (HIGCM) uses a flexible σ‐θ;‐p vertical coordinate, where p is pressure, θ is potential temperature, and σ = p/p* where p* is surface pressure. Three major improvements to the HIGCM are presented. the first improvement is a modification to the vertical‐difference scheme so that spurious vertical motions in the isentropic domain are minimized. the second improvement is a modification to the implementation of the radiation scheme so that it is now able to damp, and does not itself create, noisy temperature profiles; this allows the model to be run without ad hoc extra vertical diffusion and so allows a cleaner comparison with σ‐p simulations. the third improvement is to extend the isentropic domain up to the top of the model thus allowing σ‐θ or σ‐θ‐p simulations to be performed.

Idealized baroclinic instability life‐cycle experiments are used to investigate the impact of the new vertical scheme on the dynamical core of the HIGCM. the reduction in spurious vertical velocities is found to be substantial whilst the impact on the global conservation properties and overall evolution is found to be very small. These simulations also show that the commonly used Δ2n form of scale‐selective dissipation can seriously compromise global energy conservation when model‐layer thicknesses have significant horizontal gradients.

The impact of the isentropic coordinate on the climate of the full GCM is investigated by performing perpetual January simulations using σ‐θ, σ‐θ‐p and σ‐p vertical coordinates. the most robust response to the isentropic coordinate is a warming of the southern hemisphere high‐latitude lower stratosphere. In the northern hemisphere the largest changes in zonal mean temperature are in the polar stratosphere. the possible mechanisms by which the isentropic coordinate may yield these changes are described and investigated. the results strongly suggest that many of the potential benefits of the isentropic coordinate are realized, to some extent at least, with the HIGCM.
Correlation functions for atmospheric data analysis
Authors: Gneiting Tilmann
Published: February 2007

Atmospheric data assimilation techniques rely on parametric models for spatial correlation functions. This article proposes and discusses various families of homogeneous and isotropic correlation models on Euclidean spaces and on the sphere. In particular, three simply parametrized classes of compactly supported, smooth, and analytically simple correlation functions are proposed. the first two classes approximate standard second‐ and third‐order autoregressive functions, and a member of the third family approximates the Gaussian function within a maximal error of 0.0056. Furthermore, correlation models suggested previously for meteorological applications are checked for permissibility, with both positive and negative results.
A pulsed coherent CO2 lidar for boundary‐layer meteorology
Authors: Pearson G. N. , Collier C. G.
Published: February 2007

The design and experimental characterization of a compact CO2 pulsed Doppler lidar for boundary‐layer meteorology are described. Examples of the predicted and demonstrated performance of the system with respect to signal‐to‐noise ratio and Doppler measurement accuracy are presented. an experimental dataset from the lidar is analysed with different signal processing parameters in order to elucidate the effects of accumulating different numbers of successive lidar pulses and varying the length of the range gate. Typical system parameters are shown to be a range capability of between 3 and 4 km (dependent upon atmospheric conditions) and a velocity accuracy of ≤ 0.5 m s−1 with 112 m range gates.
Numerical simulations of orographic gravity waves in flows which back with height
Authors: Shutts G. J. , Gadian A.
Published: February 2007

Numerical simulations are carried out to determine the steady‐state gravity‐wave structure above a circularly‐symmetric, bell‐shaped hill when the wind backs with height. Two types of idealized basic state flow are considered; one representing the advection of a uniform baroclinic shear flow, and the other being a flow of constant speed but with a wind vector that rotates uniformly in height. of particular interest is the nature of the critical level processes that must appear at all heights due to the three‐dimensionality of the wave field and the rotation of the wind vector. As suggested by linear theory, a critical level or asymptotic wake is found downwind of the hill at any height, where ‘downwind’ refers to the wind at that level only. For small hills (where the nondimensional mountain height ≤ unity), the simulated vertical momentum flux profiles are in very good agreement with linear hydrostatic wave theory if it is assumed that wave‐component critical‐level absorption is total (i.e. no wave transmission or reflections are permitted). the vertical momentum fluxes generated by a 1 km high hill show nonlinear enhancement by a factor of about 1.5. an interesting feature of the gravity‐wave field generated by this 1 km mountain is an asymptotic wake that is dominated by a single intense shear layer in which the Richardson number is order unity.
Numerical simulations of convective equilibrium under prescribed forcing
Authors: Shutts G. J. , Gray M. E. B.
Published: February 2007

Some characteristic properties of simulated moist convective equilibria are examined for different imposed cooling rates. These numerical simulations extend the work of Vallis et al. and were motivated by the need to show that their conclusions (concerning the upscale energy cascade) were not sensitive to vertical resolution. Although kinetic energy does cascade to large scales, much of the large‐scale motion in the model is associated with horizontally‐divergent winds, and the energy spectra may be better explained as a direct consequence of the generation of convective lines. These lines have a typical spacing of about 60 km which leads to a local maximum in the kinetic‐energy spectrum.

In addition, the design of our experiments was found to match that envisaged in recent idealized ‘heatengine theories’ of radiative‐convective equilibria, thereby providing an opportunity to evaluate their utility. It is shown that a variant of these scaling theories appears to fit the statistical properties of our simulations quite well and provides expressions for the convective available potential energy, cloud mass flux and the fractional area occupied by convective updraughts. Scaling arguments also suggest that the convective line separation is of the order of the convecting layer depth divided by the square root of the updraught fractional area—consistent with the wavelength of the local maximum in the energy spectrum.
Effects of cloud diabatic heating on the early development of the FASTEX IOP17 cyclone
Authors: Mallet Isabelle , Cammas Jean‐Pierre , Mascart Patrick , Bechtold Peter
Published: February 2007

A set of numerical simulations is used to study the first development phase of ‘The FASTEX cyclone’, a particularly well sampled frontal cyclone system that developed on the trailing front of a primary large‐scale baroclinic wave during Intensive Observation Period 17 (IOP17) of FASTEX (the Fronts and Atlantic Storm‐Track EXperiment). the role of diabatically induced vertical circulations is investigated with a combination of diagnostic tools, including a generalized omega equation and Keyser's psi‐vector technique, applied to the model outputs. the full‐physics simulation shows a good agreement with observed data, particularly the splitting of the upper‐level flow into a pair of jet streaks. Analysis of the contributions to the vertical ageostrophic transverse circulations near the jet‐streak‐pair structure, shows that a positive feedback exists between the storm's dynamical configuration and the diabatic processes. In the second part of the paper, this interpretation is confirmed by a sensitivity analysis excluding cloud processes. Results suggest that, after initial baroclinic growth, the storm evolves towards a purely diabatic deepening regime, leading to a ‘diabatic Rossby wave’ mechanism, as proposed by recent wave‐CISK (Conditional Instability of the Second Kind) theoretical studies.
Nonlinear equatorial Kelvin waves and CISK. II: Finite‐amplitude analysis and the leading edge of a cloud region
Authors: Cho Han‐Ru
Published: February 2007

Kelvin wave‐OSK (Conditional Instability of the Second Kind) is studied as a possible explanation for the intraseasonal oscillations of the equatorial troposphere. In this second part of the paper, the finite‐amplitude analysis is performed. It is shown that if one assumes that (he temperature field is separable for the vertical coordinate, then all variables are separable in terms of the vertical coordinate, in the cloud region or in the clear area.

Under the assumption that the temperature is separable, it was found that Kelvin waves with finite amplitude when coupled with cumulus heating are never unstable; this conclusion is the same as for the waves in the small‐amplitude approximation in part I. The rarefaction wave at the leading edge of a cloud region is analysed in detail. The analysis can be done only with the finite‐amplitude approximation because of the requirement of genuine nonlinearity.
Analytical and numerical modelling of jet streaks: Barotropic dynamics
Authors: Cunningham Philip , Keyser Daniel
Published: February 2007

Observations suggest that vortex dipoles of mesoscale dimensions may provide a simple yet realistic representation of the structure and dynamics of jet streaks in the extratropical upper troposphere. Moreover, the effects of horizontal divergence in the vicinity of jet streaks often may be of secondary dynamical importance, suggesting that a nondivergent barotropic framework may provide a logical starting point for an idealized investigation of jet streaks.

Analytical solutions of barotropic vortex dipoles are shown to exhibit characteristic signatures similar to those identified in observational case‐studies of jet streaks. In addition to the dipole of relative vorticity, these signatures include: (i) a localized maximum in fluid speed (i.e. a jet streak), (ii) ageostrophic flow that is directed towards lower pressure in the entrance region and towards higher pressure in the exit region of the jet streak, (iii) a four‐cell pattern of ageostrophic vorticity that is cyclonic in the entrance and exit regions and anticyclonic on the flanks of the streak, and (iv) a translation speed that is significantly slower than the maximum fluid speed. On the basis of these similarities, it is suggested that vortex dipoles provide a plausible dynamical representation of the structure and motion of jet streaks.

Nevertheless, vortex dipoles in isolation are unable to account for certain observed features of jet streaks, such as the anisotropy of the wind field in the along‐stream direction and the asymmetry in the relative‐vorticity field, in which the cyclonic vortex typically is stronger than the anticyclonic vortex. Moreover, jet streaks generally are not isolated, but are embedded in a larger‐scale jet stream, which may be zonally varying or wavelike. Analytical and numerical solutions of barotropic vortex dipoles in the presence of a variety of non‐uniform background flows characteristic of the large‐scale extratropical circulation are shown to account for the above features absent from dipoles in isolation. These solutions are also shown to provide idealized depictions of the life cycles of jet streaks in the extratropical upper troposphere.
Generation of inertia‐gravity waves in a baroclinically unstable fluid
Authors: Lovegrove A. F. , Read P. L. , Richards C. J.
Published: February 2007

A novel technique is used to visualize and measure the baroclinically unstable flow in a two‐layer, rotating annulus experiment with very high resolution in space and time. It is found that small‐scale, high‐frequency waves are generated in certain preferred locations within the flow by a baroclinic wave undergoing amplitude vacillation. These fast waves are investigated in terms of their dispersion relation, and are shown to be consistent with inertia‐gravity waves, generated in association with a source moving with the large‐scale baroclinic wave. The occurrence of these waves is described and discussed in the context of spontaneous adjustment radiation and the meteorological slow manifold. Phase‐portrait analyses of the present experiments indicate that, despite the apparent generation of inertia‐gravity waves as spontaneous adjustment radiation, the resulting vacillatory behaviour remains consistent with low‐dimensional dynamics.
A generalization of the transformed Eulerian‐mean meridional circulation
Authors: Juckes Martin
Published: February 2007

The transformed Eulerian‐mean mass stream function gives an estimate of the mass flux across isentropic surfaces in terms of eddy fluxes evaluated on pressure surfaces. This is convenient both for data analysis and for the interpretation of theoretical results which are sometimes more easily expressed in pressure coordinates. A generalization is derived which describes the mass flux across an arbitrary iso‐surface, provided that the quantity defining the iso‐surface has a clear vertical gradient. The generalization is applied to iso‐surfaces of Ertel's potential vorticity, including the dynamical tropopause.
Rotational aspects of stratified gap flows and shallow föhn
Authors: Sprenger Michael , Schär Christoph
Published: February 2007

Observations of föhn in the Alps and other mountainous regions suggest that the underlying dynamics is often affected by gap‐like features in elongated ridge‐like topography. To assess the dynamics of these flows, idealized numerical experiments are conducted with a hydrostatic numerical model, using f‐plane geometry and a free‐slip lower boundary condition. The topography is taken to be a two‐dimensional ridge oriented in the west/east direction with a valley transect of depth ΔH across it. The upstream flow is westerly, with a constant wind speed U and constant Brunt‐Väisälä frequency N. The control parameters defined by this setting are a dimensionless gap depth NΔH/U, the ratio between ridge height and gap depth H/ΔH, a Rossby number describing the south‐north width of the ridge, and additional parameters associated with the shape of the gap. With intermediate Rossby numbers (Ro≈︁1) the setting resembles that of shallow Alpine south‐föhn cases, which are characterized by a cross‐Alpine flow essentially confined to valley transects. For small dimensionless gap depths and large Rossby numbers, the flow follows the predictions of linear theory and takes on an approximately symmetric pattern with respect to the ridge line. For NΔH/U ≳ 1, flow separation and splitting takes place upstream and downstream of the gap, respectively. The flow within the gap decouples from the flow aloft and is driven by the geostrophic south‐north pressure gradient to yield a föhn‐like flow. It is demonstrated that the limit f → 0 is singular (i.e. the flow solution does not converge towards the symmetric f = 0 solution), and that there exist multiple stationary solutions for f = 0 (two with northerly and southerly flow across the gap, respectively, and one with north/south symmetry). The existence of these multiple steady states is related to a wake instability, yet vortex shedding is suppressed by the presence of the ridge downstream of the gap. Additional simulations are presented which demonstrate that a transient external forcing can induce transitions between the multiple flow solutions. The relationship of the idealized setting to Alpine shallow föhn is discussed, and additional experiments are conducted to assess the effects of surface friction and of an inversion present to the south of the ridge.
Dynamical evolution of the error statistics with the SEEK filter to assimilate altimetric data in eddy‐resolving ocean models
Authors: Ballabrera‐Poy J. , Brasseur P. , Verron J.
Published: February 2007

The Singular Evolutive Extended Kalman (SEEK) filter introduced by Pham et al. is applied to a primitive‐equation model in order to reconstruct the mesoscale circulation typical of the mid‐latitude ocean from altimetric data. The SEEK filter is a variant of the Kalman‐filter algorithm based on two concepts: the order reduction of the initial‐error covariance matrix, and the dynamical evolution of the reduced‐order basis. This makes the method potentially suitable for problems with a high number of degrees of freedom.

Previous work has shown the ability of a steady version of the filter to improve the vertical structure of the ocean thermocline in the case of the quasi‐linear dynamics associated with the equatorial tropical Pacific Ocean, and the need to combine the dynamical evolution of the basis with an adaptive scheme in a mid‐latitude ocean model of the Gulf Stream region.

This work examines the potential advantages of the dynamical evolution of the basis functions with simple assimilation experiments. It demonstrates the ability of the method to propagate in time the statistical properties of the system when the filter is initialized properly. However, the lack of robustness of the filter is investigated theoretically and experimentally, showing the need to consider variants of the method when the filter is not properly initialized.
Large‐scale organization of tropical convection in two‐dimensional explicit numerical simulations
Authors: Grabowski Wojciech W. , Moncrieff Mitchell W.
Published: February 2007

The large‐scale organization of tropical deep convection is investigated in idealized two‐dimensional cloud‐resolving simulations. A 20 000 km periodic computational domain allows interactions among moist convection, mesoscale organization, and surface exchange on a wide range of scales. A uniform 10 m s−1 easterly wind and a uniform sea surface temperature of 30°C are assumed. A prescribed temperature tendency mimicking the mean radiative cooling of the tropical troposphere is used in this pilot study. The large‐scale organization of convection resembles the observed tropical atmosphere as well as results from some previous numerical studies using parametrized convection. The simulation highlights mesoscale convective systems with horizontal scales of several hundred kilometres moving east to west with speeds similar to the mean flow, and an envelope of convection with a horizontal extent of a few thousand kilometres and propagating west to east. The propagation speed of the large‐scale envelope compares well with the phase velocitites of convectively coupled Kelvin waves observed in the equatorial waveguide. Convective momentum transport and the impact of convective systems on temperature and moisture near the surface are key processes responsible for the large‐scale organization of convection. These results are discussed in the context of recent observations and numerical studies of convection organization in the tropics.
Ozone deposition to coastal waters
Authors: Gallagher M. W. , Beswick K. M. , Coe H.
Published: February 2007

The air‐sea exchange rate of ozone to a coastal region on the North Sea was measured using the eddy correlation technique at the UK Weybourne Atmospheric Observatory. The average measured surface resistance to ozone uptake,rs[O3], was found to be 950 ± 70 s m−1. This exchange rate for coastal waters is very close to that recommended for open ocean conditions by Galbally and Roy of 1000 s m−1. The results are discussed in the light of currently accepted exchange rates and the rather small micrometeorological measurement database available for trace gas exchange to sea surfaces. Although more data are needed to confirm the observations, a tentative dependence on the surface uptake with friction speed,u*, was found, given byrs[O3] ≈︁ –574 In(u*) + 282 s m−1, possibly explaining the variation in previous measurements presented in the literature which were mostly obtained using chamber techniques.
Optimality of variational data assimilation and its relationship with the Kalman filter and smoother
Authors: Li Zhijin , Navon I. M.
Published: February 2007

The known properties of equivalence between four‐dimensional variational (4D‐Var) data assimilation and the Kalman filter as well as the fixed‐interval Kalman smoother point to particular optimal properties of 4D‐Var. In the linear context, the 4D‐Var solution is optimal, not only with respect to the model trajectory segment over the assimilation time interval, but also with respect to any model state at a single observation time level; in the batch processing (cycling 4D‐Var) method, the information in 4D‐Var is fully transferred from one batch to the next by the background term; 4D‐Var allows the processing of observations in subsets, while the final solution is optimal as all observations are processed simultaneously. These properties hold even for models that are imperfect, as well as not invertible. Various properties of equivalence of 4D‐Var to the Kalman filter and smoother result from these optimality properties of 4D‐Var. Further, we show that the fixed‐lag Kalman smoother may also be constructed in an optimal way using a multiple batch‐processing 4D‐Var approach. While error covariances are crucial for the equivalence, practical techniques for evaluating error covariances in the framework of cycling 4D‐Var are discussed.
The influence of transient upper‐level barotropic growth on the development of baroclinic waves
Authors: Kucharski Fred , Thorpe Alan J.
Published: February 2007

In a previous study of two cyclones within the Fronts and Atlantic Storm Track EXperiment (IOP11 and IOP17), it was shown that upper‐level barotropic growth dominated the initial phase of the development. The cyclones later developed due to baroclinic energy conversion. These cyclones resemble Petterssen's Type B paradigm. This contribution presents the dynamics of archetypical Type B cyclogenesis by addressing the consequences of upper‐level barotropic growth before baroclinic cyclone development within the framework of idealized primitive‐equation simulations. Localized and finite‐amplitude upper‐level pressure perturbations have been designed to represent a variety of amplitudes and horizontal tilts of a localized barotropic trough structure. These are added to a zonal jet basic state, and it is shown that favourable initial situations, similar to the observed cyclones, lead to much stronger and faster cyclonic development compared with unfavourable ones. The most effective barotropic initial situation (without a vertical tilt and considering growth over a time‐scale of 48 hours) is one with a horizontal ‘tilt against the shear’ of an upper‐level trough on the cyclonic side of a jet associated with a baroclinic zone.
A mass‐flux convection scheme for regional and global models
Authors: Bechtold P. , Bazile E. , Guichard F. , Mascart P. , Richard E.
Published: February 2007

A bulk mass‐flux convection parametrization for deep and shallow convection is presented that includes an efficient and straightforward treatment of numerics, moist thermodynamics and convective downdraughts. The scheme is evaluated in a single‐column model context for a tropical deep‐convective period and a trade‐wind cumulus case. Preliminary applications in a global numerical weather‐prediction model and a mesoscale model are also discussed.

The results suggest that the present scheme provides reasonable solutions in terms of predicted rainfall, and tropical temperature and moisture structures. The application of the scheme to various scales is supported by the use of a convective available potential energy convective closure that assures a smooth interaction with the large‐scale environment and efficiently suppresses conditional instability of the second kind‐like spin‐up processes on the grid‐scale.

Finally, the theoretical and practical limits of the present approach are discussed together with possible future developments.
The correlation between tropical total ozone and outgoing long‐wave radiation
Authors: Williams Victoria , Toumi Ralf
Published: February 2007

Interannual variations in tropical column ozone, as observed by the Total Ozone Mapping Spectrometer, are analysed for the period 1979–94 using linear regression to extract signals related to the solar cycle, quasi‐biennial oscillation, and El Niño Southern Oscillation (ENSO). Using time series of outgoing long‐wave radiation (OLR) as a proxy for ENSO variations, rather than the widely used Southern Oscillation Index is found to improve the fit to observed ozone. The variability in the ozone and OLR datasets is investigated using empirical orthogonal function analysis. The most strongly correlated OLR and ozone modes exhibit ENSO‐like variability.
Filtered non‐hydrostatic models in pressure‐related coordinates
Authors: Rõõm Rein , Miranda Pedro M. A. , Thorpe Alan J.
Published: February 2007

The anelastic sigma‐coordinate version of the pseudo‐anelastic Miller‐Pearce model is developed. The pseudo‐anelastic model is characterized by the presence of external‐mode acoustic perturbations which propagate in the horizontal, compromising the model performance by the need to use smaller time steps than would otherwise be required. The approach developed in this study allows for the filtering of acoustic waves and adjustment of surface pressure perturbations. The approach is tested in a new numerical implementation of an adiabatic sigma‐coordinate non‐hydrostatic model and results are found to be comparable to the unadjusted version, and to theory when applicable. The new code is found to allow for significantly larger time steps whilst still retaining numerical stability, showing the possibility of further model improvements.
Influence of the β‐effect on non‐modal baroclinic instability
Authors: Rivière G. , Hua B. L. , Klein P.
Published: February 2007

We focus on the influence of the β‐effect on the singular modes of baroclinic instability in the Phillips model. An analytical intercomparison between normal and singular modes for the Eady problem on an f‐plane has already been performed, showing that the amplification rate of the singular mode for the kinetic‐energy norm after a dimensional time of three days is about 1.2 times larger than for the normal mode. We show here that, in the presence of the β‐effect, the maximum amplification rate of the singular mode can be 1.5 times larger than for the normal‐mode case. This difference is due to the lesser stabilizing effect of β on the singular modes compared with the normal modes. This result is obtained for the case of equal layer depths which is relevant for the atmospheric situation. For the oceanic situation with unequal layer depths the amplification due to β is further increased leading to a factor of 1.8 in amplification rates between the two types of modes after a dimensional time of about 20 days. This behaviour is explained by the fact that β only intervenes in the anti‐Hermitian part of the linear dynamical operator, the main effect of which is to alter the vertical phase tilt of the perturbations. As a consequence, the structure of the bi‐orthogonal mode becomes more inclined to the vertical in the presence of β while, on the contrary, the structure of the unstable normal mode becomes less inclined.