bluuebunny/crossref_metadata_2025_split huggingface dataset:
Further development of a hybrid‐isentropic GCM
Authors: Webster Stuart , Thuburn John , Hoskins Brian , Rodwell Mark 
Published: February 2007

AbstractThe 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

AbstractAtmospheric 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

AbstractThe 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

AbstractNumerical 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

AbstractSome 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

AbstractA 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

AbstractKelvin 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

AbstractObservations 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

AbstractA 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

AbstractThe 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

AbstractObservations 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractIn 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.