bluuebunny/crossref_metadata_2025_split huggingface dataset:
The response of a turbulent boundary layer to arbitrarily distributed two‐dimensional roughness changes
Authors: Belcher S. E. , Xu D. P. , Hunt J. C. R. 
Published: February 2007

AbstractWe investigate small changes to a two‐dimensional turbulent boundary layer on a rough surface caused by arbitrary variations, in the streamwise direction, of the surface roughness length, z1(x). the linear changes to the flow are calculated as asymptotic sequences in the limit ϵ = u*/U0 → 0 (u* is the upwind friction velocity, and U0 is a typical value of the approach wind speed). the flow is divided into two regions, each of which is subdivided into two layers; the previous analyses may then be extended by calculating the second‐order effects. Throughout the bulk of the inner region, of thickness l, the Reynolds shear‐stress gradient balances the acceleration, but very close to the surface (in the inner surface layer), up to a height O (√lz0), the shear‐stress gradient is constant and matches with its surface value. the outer region (where the leading‐order perturbations are inviscid) also divides into two layers: the middle layer extending to a height hm, where the shear in the upwind profile determines the perturbations; and the upper layer, in which the perturbations decay in potential flow, and a weak perturbation pressure develops which produces second‐order changes in the inner region. At second order the normal Reynolds stresses also accelerate the mean‐flow perturbations, but their effect is numerically small. the solutions in the inner surface layer satisfy the full non‐linear equations, and by matching prudently, the layers above also contain a non‐linear correction. We show that this means that the theory is valid when Mu*/U0 ≪ 1 (so that it may be applied to atmospheric problems even when M = ln(z0/z1,) is of order one). the theory agrees well with experiments and numerical simulations (which use complex models for the turbulence closure) and improves on the previous analytic theories, especially in the prediction of the surface shear stress.
Observation and interpretation of wave clouds over Macquarie Island
Authors: Mitchell R. M. , Cechet R. P. , Turner P. J. , Elsum C. C. 
Published: February 2007

AbstractImages obtained from two NOAA/AVHRR overpasses of Macquarie Island in October 1985 show pronounced cloud bands associated with trapped lee waves. One image reveals striking evidence of an abrupt change in the propagating wave mode. the structure and evolution of the waves is interpreted using a linear wave‐model coupled with radiosonde data obtained from the island. the model successfully forecasts the mode change, and accurately predicts the associated wavelengths. an interpretation of the difference between the amplitudes of the two modes is also provided. Infrared channel measurement of the temperature of the cloud in the wave‐crests yields a useful constraint on the cloud height.
Some characteristics of thunderstorms over Bathurst and Melville Islands near Darwin, Australia
Authors: Keenan T. D. , Morton B. R. , Zhang Xu Shu , Nyguen K. 
Published: February 2007

AbstractA study defining the occurrence, structure and evolution of “maritime continent” island thunderstorms over Bathurst and Melville Islands near Darwin, Australia is presented. Satellite data for the November‐December pre‐monsoon transition season show unambiguous records of island thunderstorms on at least 65% of days over the period 1978‐1986. Storms develop most commonly between 1230LST and 1530LST and radar data indicate a mean lifetime of 2.5 h. Non‐occurrence was found on only 7% of the days examined and was associated with a return to dry flow typical of the early transition season with middle level subsidence or suppressed conditions in the wake of previous convective activity.Environmental sounding data indicated that these “maritime continent” thunderstorms develop in a regime with moderate CAPE, low to moderate shear and high moisture. the dynamic regime is characteristic of multicellular storms. Examination of the evolution of the storms indicates the existence of a typical lifecycle but with considerable variability in detail. Initially, the lifecycle is consistent with the multicellular storm concept, but the developing storms tend to aggregate to form one or more mesoscale systems which in turn may develop into propagating squall lines in the mature stage. the developing storms have preferred geographic regions of genesis and intensification over the islands; and while motion of the storms is variable, it is generally consistent with the mean surface‐to‐700mb pressure‐weighted flow.
The effect on the evolution of the drop spectrum in clouds of the preconditioning of air by successive convective elements
Authors: Roesner S. , Flossmann A. I. , Pruppacher H. R. 
Published: February 2007

AbstractA multiple air‐parcel model is used to study the effect on the drop size spectrum inside convective clouds of preconditioning of air by successive convective elements. Numerical evaluation of this model shows that each rising air parcel establishes a new environment for the air parcel following it, thus substantially affecting its thermodynamic behaviour. This preconditioning of air allows successive air parcels to penetrate an inversion and produce precipitation‐sized drops under conditions which are not conducive to precipitation when only a single air parcel is considered.
Charge separation associated with frost growth
Authors: Rydock James P. , Williams Earle R. 
Published: February 2007

AbstractObservations have shown that negatively charged ice fragments are ejected from ice surfaces growing by water vapour deposition. Laboratory experiments with exaggerated temperature and vapour gradients were performed to quantify the charge transfer and its dependence on temperature and water vapour density. It was found that the temperature of the growing ice plays a strong role in determining the rate of charge transfer, with a pronounced maximum in the range −15 °C to −25 °C, but the growth history of the ice surface is also shown to be important. The temperature and vapour gradients employed in the study are unrepresentative of those encountered in the atmosphere, but the underlying physics of this phenomenon may be applicable to phenomena that do operate in thunderclouds. The physical processes responsible for the charge transfer remain unidentified.
Dynamics of an atmospheric blocking as deduced from its local energetics
Authors: Mak Mankin 
Published: February 2007

AbstractThis paper reports an analysis of the roles of temporal scale interactions (seasonal, intraseasonal and high‐frequency components) in the generation and maintenance of a pronounced block over the North Atlantic that lasted for three weeks in February 1983. This blocking disturbance had a distinct dipole structure initially oriented in an east‐west direction downstream from a strongly diffluent south‐west‐north‐east oriented seasonal jet. Although it has an equivalent barotropic structure, there is a well‐defined vertical velocity field with ascending (descending) motion on its western (eastern) flank. As it develops, matures and decays, it rotates systematically in a clockwise direction.The contributions from the various temporal scale interactions to the episodal average local energetics of this block are evaluated. There are five comparably important processes controlling the intensity, configuration, and evolution of the block. The synoptic eddy‐straining mechanism proposed by Shutts is manifested in three energetics terms, of which one is found to be particularly large. The blocking disturbance also barotropically extracts kinetic energy at a significant rate from the seasonal diffluent jet under the influence of the latter's strong deformation field. The pressure work process, the baroclinic conversion process, and above all, the nonlinear dynamics of the blocking disturbance itself are quantitatively important in redistributing the energy within the blocking region. The effects of the diabatic and subgridscale processes are found, as residues, to be substantially dissipative on the block.
Mass fluxes in New Mexico mountain thunderstorms from radar and aircraft measurements
Authors: Raymond D. J. , Solomon R. , Blyth A. M. 
Published: February 2007

AbstractWe have obtained the vertical and detrained mass fluxes through the life cycles of four thunderstorms using measurements from four Doppler radars and an instrumented aircraft. The storms formed over the Magdalena Mountains of central New Mexico, and remained near the mountains through their entire life.Updraughts appeared to entrain a certain amount of surrounding radar‐invisible air. Thermodynamic considerations suggest that this air came from neighbouring towering cumulus clouds rather than directly from the environment. Entrained air did not immediatly mix with cloud‐base air, but tended to maintain its own identity in ascent. Levels with decreasing parcel buoyancy induced significant detrainment from the updraught. Downdraughts were confined primarily to levels below 7–8 km above mean sea level.The net mass flux entering or leaving the underside of the thunderstorms equalled that supplied by the mountain upslope flow. Since this net flux is a combination of updraughts and downdraughts, unstable air at low levels is gradually replaced by stable downdraught air, thus ensuring the demise of the storm.
Variation with altitude of the drop‐size distribution in steady light rain
Authors: Rogers R. R. , Zawadzki I. I. , Gossard E. E. 
Published: February 2007

AbstractBy application of the stochastic coalescence equation to evolving populations of cloud droplets and raindrops, we show that in light rain there is a range of drop sizes for which, to a good approximation, raindrops grow only by cloud accretion and are neither created by autoconversion nor destroyed by breakup. In these conditions, the differential equation that describes the one‐dimensional steady‐state form of the drop‐size distribution may be solved analytically, using plausible assumptions about the cloud water content and the raindrop collection efficiency. The solution gives the drop‐size distribution as a function of distance below a reference level, from which we can calculate radar‐measurable quantities such as the reflectivity factor and the Doppler spectrum.Recent radar data from the Hawaiian Rainband Project provide the opportunity for a close comparison of the steady‐state accretion theory with observations. An example is presented of a steady situation in which, over a limited range of altitude, the observed vertical profiles of reflectivity and Doppler velocity do agree reasonably well with the theory. Other observations in equally steady‐appearing conditions, however, are not in accord with the theoretical predictions. If the parameters of the model are chosen to fit the velocity profile, the theory then tends to predict reflectivity gradients stronger than those actually observed. Similarly, if the parameters are chosen to fit the reflectivity profile, the predicted increase of Doppler velocity with distance fallen is less than what is observed. Effects of evaporation can be included in the steady‐state model, and are shown to explain the sense of the discrepancy but not its magnitude. The conclusion is that the one‐dimensional model, possibly because of its inability to account for horizontal advection, is usually inadequate to explain the observations.
The Development of Organized Convection In A Simplified Squall‐Line Model
Authors: Garner S. T. , Thorpe A. J. 
Published: February 2007

AbstractSquall lines stand out from ordinary cumulonimbus convection because of a special structure that dramatically extends their duration, precipitation output and area of influence. Time‐dependent models have identified surface outflow boundaries and low‐level environmental wind shear as the crucial elements in squall‐line formation and/or maintenance. Steady‐state models have shown how properties of the downdraught reservoir, or ‘cold pool’, must be matched to the far‐field wind and thermodynamic profiles to sustain the convection. However, the dynamical role of the environmental shear and the nature of the interaction between the positively and negatively buoyant air have remained uncertain. For help with these issues, a time‐dependent numerical model is developed with all moist processes either parametrized or based on ad hoc assumptions. the simplifications make it possible to distinguish more clearly than in the past between the formation and maintenance of squall lines and between the initial and disturbed far‐field environments.The convective updraught forms within an expanding region of disturbed flow separated from the undisturbed environment by ‘storm fronts’. Forced (or neutral) ascent occurs at a surface ‘gust front’ on the upwind side of the cold pool. We distinguish squall lines from ordinary convection by looking at the coherence between the forced and convective updraughts. It is found that subsidence over the cold pool disorganizes the updraughts, whereas a deep overturning circulation downshear from the gust front has an organizing effect. Hence, factors which allow the initial subsidence to occur ahead of the gust front, namely contrasting lower‐and upper‐level winds and large potential buoyancy, favour squall‐line development. It is argued that, in cases with large convective potential energy, the deep mesoscale circulation is more important in the formation and maintenance of the line than either the vertical shear of the air reaching the cold pool or the strength of the forced updraught.
Probabilistic Aspects of Error Growth In Atmospheric Dynamics
Authors: Nicolis C. 
Published: February 2007

AbstractA probabilistic approach accounting for the variability of error growth in the atmosphere is developed and applied to an idealized low‐order atmospheric circulation model. Error growth in a single‐realization, ensemble‐averaged error versus time and the time‐dependent as well as the asymptotic probability distribution of errors are determined. A wide dispersion around the mean is found, showing clearly the inadequacy of a description that is limited to averaged properties only. A self‐consistent one‐variable, two‐parameter model of error dynamics is constructed in the form of a stochastically driven logistic equation, and the results are shown to be in good quantitative agreement with the simulation of the full equations. A generalization to more realistic atmospheric models is also outlined.
Treatment of Gross Errors Using Maximum Probability Theory
Authors: Dharssi I. , Lorenc A. C. , Ingleby N. B. 
Published: February 2007

AbstractWe describe a nonlinear analysis scheme capable of directly treating observations which have a non‐Gaussian error distribution. This non‐Gaussian analysis scheme is shown to be a generalization of the familiar optimum interpolation scheme and must be solved iteratively. the approach to the treatment of gross errors is compared with a more traditional approach in which a quality‐control step precedes the analysis. Simulation experiments are performed in which the observations have many of the characteristics of observations from the proposed space‐borne atmospheric laser Doppler instrument and a 50% probability of gross error. Both approaches are shown to perform well, with the non‐Gaussian analysis giving the better results.
Mountain wave drag over double bell‐shaped orography
Authors: Grisogono Branko , Pryor Sara C. , Keislar Robert E. 
Published: February 2007

AbstractAn integral expression is developed for the mountain wave drag (MWD) associated with linearized, hydrostatic flow over two infinitely long, bell‐shaped ridges. This analysis extends previous studies of airflow over a single ridge. The MWD is expressed in canonical form with three terms: the sole effect of each ridge and a cross‐term. While all three terms tend to zero with the appropriate inverse Rossby number, the cross‐term also tends to zero with large ridge‐ridge separation normalized by the sum of the half‐widths. Asymptotic expansions of the dimensionless integral forms arising from the three terms in the MWD expression are presented.
Vortex motion in relation to the absolute vorticity gradient of the vortex environment
Authors: Smith Roger K. , Ulrich Wolfgang 
Published: February 2007

AbstractRecent attempts to establish a relationship between tropical cyclone motion and the absolute vorticity gradient of the cyclone environment are reviewed. Evidence is presented, both from analytic and numerical calculations, to show that for barotropic vortices there is no unique relationship between vortex motion, c. and the local absolute vorticity gradient of the (imposed) environment, ∇ξa. Our results suggest that it is also unlikely that any similar relationship holds for tropical cyclones.It is shown that conventional methods of averaging over an annular region about tropical cyclone centre to determine the absolute vorticity gradient of the cyclone environment may be very sensitive to the size of the region chosen. For this reason as well as from theoretical considerations our results would indicate that there are certain intrinsic limitations for tropical cyclone track forecasting arising from statistically‐derived relationships between c and ∇ξa.
Using an ocean model to validate ECMWF heat fluxes
Authors: Carrington David J. , Anderson David L. T. 
Published: February 2007

AbstractA general circulation model of the Indian Ocean has been used as one method of assessing the quality of surface heat fluxes over the ocean. The fluxes of momentum and heat obtained from the numerical‐weather‐prediction model of the European Centre for Medium‐range Weather Forecasts (ECMWF) for the period from 85 to 1988. The near‐surface temperatures simulated by the ocean model provide an indicator of the accuracy o these fluxes. From this study it was estimated that the net surface heat flux in the annual average was better than 20 W m−2, though this could shroud larger errors in shorter averages. The interannual differences in sea surface temperature simulated by the model showed little similarity to those observed, however, raising the question about the quality for representing such processes and indicating a need for continued improvement.One potential improvement is in the parametrization of evaporation at low wind speed. The impact on the fluxes of such a change was investigated. The fluxes in this sensitivity study were also obtained from the ECMWF model, but from climate‐mode simulations rather than from analyses. The parametrization change was designed to raise the latent‐heat flux in areas of light winds, such as in the equatorial region. The ocean‐model intergration, however, suggest that the effect may have been overdone, with insufficient heat being put into the ocean. The change in evaporation not only changes the latent‐heat flux but also leads to substantial changes in the solar flux and in the surface wind and stress fields.
Turbulent transfer relationships over an urban surface. I. Spectral characteristics
Authors: Roth M. , Oke T. R. 
Published: February 2007

AbstractA comprehensive study of atmospheric turbulence over an urban surface has been carried out. In this Part I of the study the spectral characteristics are analysed. Results involving all the important atmospheric variables, Measured at two heights (z'/z0=21 and 37) under unstable conditions, are presented, normalized by the respective variances/covariances, and within the Monin‐Obukhov similarity framework. Although the shapes and location of the peaks of the urban spectra/cospectra are in good agreement with homogeneous surface layer data, the analysis of the normalized dissipation rates and spectral correlation coefficients reveals differences which can be attributed to the rough surface. For example, the dissipation of turbulent kinetic energy is relatively small, perhaps owing to an increase in the transport and export of locally produced turbulent energy; there is a very efficient transfer of Momentum which it is suggested may be a result of wake production associated with bluff bodies, and it is found that the transfer of heat and moisture are dissimilar.
An extended analytic theory of tropical‐cyclone motion in a barotropic shear flow
Authors: Smith Roger K. , Weber Harry C. 
Published: February 2007

AbstractAn earlier analytic theory for the motion of a barotoropic vortex in a zonally varying basic shear flow on a beta‐plane is consolidated and extended. It is shown that the theory can be derived from a power‐series expansion of variables representing the vortex asymmetry in terms of a single nondimensional parameter ϵ = BL2/U, where B is the Meridional gradient of basis state absolute vorticity and L and U are the lenght and velocity scales, respectively, for the outer part of the vortex. The derivation requires inter alia that ϵ ≪ 1 and that the magnitude of the basis flow scales with ϵ. Insight is provided into the dynamics of vortex motion when the latter condition is not fulfilled, as in cases of moderate and large shear.For vortices that decay appreciably more rapidly with radius than those studies in the earlier work it is necessary to include additional terms when calculating the asymmetric vorticity to obtain an accurate prediction of the vortex track. Analytic expressions have been obtained for these terms that lead to a theory that is valid for a broad range of vortex profiles.Comparisons are made between the present theory and two recent analyses for the case of zero basis flow, contrasting the important differences in approach. The present theory gives results that are very close to one of these, a semi‐spectral numerical Method.The paper concludes with a brief discussion of aspects of vortex stability.
An isentropic two‐dimensional model with an interactive parameterization of dynamical and chemical planetary‐wave fluxes
Authors: Kinnersley Jonathan S. , Harwood Robert S. 
Published: February 2007

AbstractAn interactive chemical‐dynamical isentropic two‐dimensional model is described, and its behaviour analysed when a parameterization of the three longest Rossby wave is included in the model. Eddy fluxes of chemicals and of Ertel's potential vorticity are all calculated from the modelled waves, and are therefore consistent with each other. The chemical fluxes arise from dissipation of zonal asymmetries, both by photo‐chemistry (‘chemical eddy’ fluxes) and by ‘planetary wave breaking’. The chemical eddy fluxes for ozone and Noy are much larger than the wave‐breaking ones, and thus the diffusion matrix (which has only one non‐zero element) is far from being independent of chemical species. The waves have a large immediate effect on the ozone distribution, but the effect persists throughout the year only when the wave fluxes induced a persistent change in the photochemical equalibrium state for ozone (mainly by changing NOx).
Wind profiles in the stable boundary layer, and the effect of low relief
Authors: Grant A. L. M. 
Published: February 2007

AbstractWind profiles through the stable boundary layer are presented. The data were collected at a site located in a shallow river valley and show evidence of upstream effects near the edge of the valley. The wind‐speed profiles are consistent with a non‐dimensional shear Øm = 1 + 3z/L for z/L < 0.5. Changes in wind speed and direction are found to occur over a greater depth than the depth of the turbulent nocturnal boundary layer estimated from various diagnostic formulae.