bluuebunny/crossref_metadata_2025 huggingface dataset:
The development of tropical cyclones in the north‐west of Australia
Authors: Foster I. J. , Lyons T. J.
Published: February 2007

Case studies of developing and non‐developing tropical depressions in the north‐west Australian region during the 1979/80 and 1980/81 cyclone seasons have been constructed using conventional and satellite observations. These show that enhanced low‐level winds can occur during the lifetime of non‐developing disturbances and are not exclusively associated with cyclogenesis. Both types of system possessed lower‐level cyclonic circulations and there was an overlapping such that non‐developing storms could have stronger circulations than some developers. At upper levels, development was associated with the subtropical ridge being situated over the surface position of the disturbance. In contrast, the ridge tended to be located southwards of non‐developers.

These findings are supported by calculations of area‐averaged vorticity at 850 and 200 mb. Cross‐sections of zonal winds show that non‐developing systems could exhibit weaker vertical wind shears than some developing storms. Developers had only moderate shears at worst and maintained a well organized lower cyclonic circulation.

Such features are consistent with both classes of disturbance existing within the monsoon shearline. Non‐development was caused by interactions with large‐scale flows in the southern hemisphere that prevented the necessary conditions for cyclogenesis from being all fulfilled. This suggests a need for greater understanding of the influence of mid‐latitude events on depressions within the monsoon trough.
Wavenumber transition and wavenumber vacillation in Eady‐type baroclinic flows
Authors: Weng H.‐Y. , Barcilon A.
Published: February 2007

Wavenumber transition and wavenumber vacillation are studied numerically in a ‘maximum simplification’ model of the Eady type with uneven Ekman dissipation. The spectral wave solution contains three waves having adjacent zonal wavenumbers and the gravest meridional mode. The model only allows for wave‐mean‐flow interaction.

As the stratification parameter decreases away from the upper symmetric flow, we observe that baroclinic forcing increases faster than dissipation, implying the prevalence of a higher wave energy state. This state is realized by an increase in wave amplitude, number of temporal and/or spatial degrees of freedom, or dominant wavenumber. The first two occur first, but for small enough stratification, the flow transits to a higher wavenumber in order to realize the increased energy.

Wavenumber vacillation is a flow regime observed at some parameter settings when the flow has at least two spatial and three temporal degrees of freedom. This regime is usually found in the region of parameter space where wavenumber transitions occur as parameters change.

The balance between baroclinic forcing and damping adjusted by wave‐mean‐flow interaction seems to be a basic mechanism for wavenumber transition and wavenumber vacillation. Wave‐wave interaction is not necessary for such phenomena to exist.
Updraught characteristics of a Florida thunderstorm
Authors: Hill R. D.
Published: February 2007

Because of possible significance in understanding lightning, the characteristics of an updraught velocity enhancement observed in a typical Florida thunderstorm are investigated. The most probable explanation of the sudden velocity enhancement is increased buoyancy resulting from the release of latent heat of freezing of supercooled droplets on to rime in the updraught. From information on updraughts in two different cells in the same storm, it is tentatively concluded that enhanced electrical activity, in typical Florida coastal thunderstorm cells, is probably associated with the mechanism of the initial charging of ice particles.
Comments on ‘Objective quality control using Bayesian methods’ by A. C. Lorenc and O. Hammon (January B 1988, 114, 515–543)
Authors: Thiebaux H. Jean
Published: February 2007

The proposals for quality control (QC) with automatic updating of the QC system, which Lorenc and Hammon (LH) have presented are important and forward‐looking contributions to the future of operational numerical weather prediction. With expanding computer capabilities for data management, including the maintenance of archives which keep up‐to‐date records of data quality by observation type and location, it is possible to make use of highly relevant information whose assemblage previously was unpractical. Lorenc and Hammon have suggested mechanisms for doing this; and their proposals offer promise for significant improvements in objective quality control. My comments are designed to enhance practical development of the authors' ideas, by focusing on some implications of the underlying theory.

The objective of the work under discussion is to establish an algorithm whose application will result in the retention of observations which are within credible range of a background (or forecast) field, together with numerical assignment of their ‘credibilities’, and the rejection of observations which are not judged to be credible. Furthermore, part of its exercise will be renewal of the criterion of ‘within credible range of the background’, using information stored in quality control performance files.
A study of the stratospheric final warming of 1982 in the southern hemisphere
Authors: Mechoso C. R. , O'Neill A. , Pope V. D. , Farrara J. D.
Published: February 2007

The three‐dimensional evolution of the final warming that takes place in the stratosphere of the southern hemisphere during spring is studied using data from a satellite. The event of spring 1982 is discussed in detail, and other events from an 8‐year set are briefly surveyed. The zonal‐mean westerly jet moves poleward and downward in spring as strong, planetary‐scale disturbances develop which contribute to the weakening of the stratospheric westerly vortex. The processes governing this weakening are discussed by reference to isentropic maps of Ertel's potential vorticity and associated area diagnostics. The vortex breaks down first in the upper stratosphere and then later (and more slowly) in the middle stratosphere. This behaviour is broadly reproduced year after year. Repeating life‐cycles of growth, eastward movement and decay of anticyclones in the stratosphere are described and related to the behaviour of quasi‐stationary wave 1 and eastward‐travelling wave 2. Evidence that the topography of the southern hemisphere exerts a strong influence on the evolution of the final warming is presented. An association is found between the location of anticylones in the upper stratosphere, warm pools of air in the lower stratosphere and a climatological split of the westerly jet stream in the upper troposphere.
Humidity measurements in the free troposphere using Raman backscatter
Authors: Vaughan G. , Wareing D. P. , Thomas L. , Mitev V.
Published: February 2007

A lidar system has been developed to measure the mixing ratio of atmospheric water vapour using Raman scattering from water vapour and nitrogen. The system uses the third harmonic of a neodymium‐YAG laser at 354.7 nm, with interference filters to isolate the vibrational Raman bands. Humidity profiles extending from about 2 km to the tropopause have been obtained, with a resolution of 30 m vertically and 20 minutes in time below 5 km, and > 300m vertically and several hours in time above 10 km. Good agreement was found between the lidar profiles and those of coincident radiosondes. Distinctive layers in the humidity profiles are presented, and measurements in the vicinity of cirrus clouds indicate sharp transitions from supersaturation to saturation with respect to ice at the cloud boundary.
Aircraft measurements of the solar absorption by broken cloud fields: A case study
Authors: Rawlins F.
Published: February 2007

Measurements of broad‐band radiation at solar (0.3–3.0 μm) and near‐infrared (0.7–3.0 μm) wavelengths are presented for three flights of the Meteorological Research Flight C130 aircraft in broken cloud fields. The solar absorption is determined from observations made above cloud top and below cloud base, using data in the two spectral bands to minimize errors of cloud sampling between different runs and to reduce the scatter due to cloud edge effects. The method assumes that cloud layer absorption in the visible region is negligible compared with that in the near‐infrared. This allows average absorption to be calculated for small sections of the aircraft track, which will generally include both fully clear and totally overcast cases. The smallest absorption measured is about 2%, in agreement with model results for the clear atmosphere. The largest absorption was found to be 8–12% (for a maximum cloud depth of 1 km), which is similar to measurements reported for layer cloud. An additional finding is that absorption by the broken cloud field varies uniformly with albedo, indicating that the parametrization of absorption in numerical models can be adequately described as a simple function of the albedo in partially cloudy areas.
A field study of the oxidation of SO2 in a cap cloud at Great Dun Fell
Authors: Chandler A. S. , Choularton T. W. , Dollard G. J. , Gay M. J. , Gallagher M. W. , Hill T. A. , Jones B. M. R. , Penkett S. A. , Tyler B. J. , Bandy B.
Published: February 2007

A series of field experiments has been performed in natural cloud on Great Dun Fell to investigate the multiphase oxidation of sulphur dioxide by hydrogen peroxide and ozone. This has been done by observing the decay of hydrogen peroxide and the generation of sulphate ions in the cloud water. It is demonstrated that the apparent field reaction rate between SO2 and hydrogen peroxide is considerably faster than would be expected from recent laboratory studies. It was found that hydrogen peroxide was generally the most important oxidant for sulphate in the cloud. Significant quantities of this oxidant can be entrained from the free troposphere into the cloud and this may be rate‐determining for the production of sulphate over longer timescales. When the pH of the cloud water was maintained above about 4.8 (probably due to gas phase ammonia) ozone was found to be the major oxidant. It is also clear that these processes will also be important in free clouds remote from the ground with oxidation by hydrogen peroxide being dominant away from ground‐based ammonia sources.
On the influence of small‐scale topography on precipitation
Authors: Grabowski Wojciech W.
Published: February 2007

A simple model of the influence of small‐scale topography on precipitation via the ‘seeder‐feeder’ mechanism is presented. In the model the large‐scale stratiform seeder cloud and the subcloud layer are modified by the airflow perturbations caused by small‐scale topography. Two separate scales of motion are considered; the large scale, with the updraught maintaining the seeder cloud, and the small scale tied to topography. In the diagnostic numerical model the interaction of these two scales is simulated. Two basic situations are considered: rain with a ground temperature of 12°C and snow with –5°C. Results of the model application to a bell‐shaped solitary hill are presented and compared with previous theoretical studies and field observations in the Welsh hills. The influence of orographic airflow perturbations on snow growth above the melting level may explain the observed strong dependence of rain enhancement on the horizontal wind speed. Simple considerations for the complex topography are also presented with reference to observations made near Uppsala, Sweden. It is suggested that the observed rain distribution may again result from influences on snow growth, with both the amplitude of airflow perturbations near the melting level and its phase relative to the topography beneath being very important.
Representativeness errors for radiosonde observations
Authors: Kitchen M.
Published: February 2007

Data from the United Kingdom operational upper air network, as well as from special radiosonde trials, have been analysed to provide statistics of spatial and temporal atmospheric variations. These statistics are applied to a study of the network performance and of representativeness errors associated with the use of the data in synoptic analysis. The results also have application to comparisons between measurements from radiosondes and those from other observing systems. For example, it is demonstrated that minimum collocation errors associated with radiosonde temperature measurements located at the centre of a satellite radiometer scan spot of radius 55 km are <1 degC for levels in the troposphere and lower stratosphere.
On the incorporation of atmospheric boundary layer effects into a balanced model
Authors: Cullen M. J. P.
Published: February 2007

A method of coupling the thermal and frictional forcing in the atmospheric boundary layer to a semi‐geostrophic model of the internal dynamics is developed. It is illustrated by two‐dimensional sea‐breeze simulations, which are compared with those of a two‐dimensional primitive equation model. It is demonstrated that the main features of the circulation on a horizontal scale larger than 20 km can be simulated with the semi‐geostrophic model, but the detailed local circulations cannot.
Ground truth confirmation and theoretical limits of an experimental VLF arrival time difference lightning flash locating system
Authors: Lee A. C. L.
Published: February 2007

During trials of a VLF Arrival Time Difference (ATD) technique for long range lightning flash location, an apparently well organized and isolated flash sequence was located. Internal ATD evidence suggests flashes were located at a thousand‐kilometre range to within 5 km. ‘Ground truth’ based on synoptic/mesoscale considerations, observer reports, and lightning damage claims confirms locations to 10–15 km. In addition, elimination of systematic effects from internal ATD evidence demonstrates relative flash location errors of 0.9‐1.8 km, and a ‘tight cluster’ of ATD fixes features scatter in location which is not inconsistent with these values. The nature of VLF lightning flash observation is examined, and shown to be consistent with errors even smaller than this.
Airflow over a two‐dimensional escarpment. II: Hydrostatic flow
Authors: Pitts R. O. , Lyons T. J.
Published: February 2007

Hydrostatically dominated forced flow over the asymmetric topography of the Darling Scarp, Western Australia, is modelled using a hydrostatic numerical mesoscale model and compared with observations. Simulations reproduce the essential features of wave overturning and the development of a shooting hydraulic flow. This response is dependent on the wind profile but not on the existence of a critical level. the high backsheared environment leads to a high ratio of the Brunt‐Väisälä frequency to wind speed throughout the profile, resulting in short wavelengths, high nonlinearity and overturning. With the ratio of the halfwidth of the topography to the boundary‐layer depth being small, the flow fields are shown to be sensitive to the boundary layer.
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

We 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

Images 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

A 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

A 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

Observations 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

This 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

We 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.
A conservative split‐explicit integration scheme with fourth‐order horizontal advection
Authors: Cullen M. J. P. , Davies T.
Published: February 2007

A split‐explicit finite difference scheme is developed which combines the accuracy and economy required for numerical weather prediction with the conservation properties required for climate‐change experiments. Results are presented to demonstrate the scheme working in practice.
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

By 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

Squall 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

A 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

We 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

An 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

Recent 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

A 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

A 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

An 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

An 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

Wind 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.
On the relationship between northern hemispheric weather regimes in wintertime and spring precipitation over China
Authors: Liu Hui , Tosi Ennio , Tibaldi Stefano
Published: February 2007

In this study, a possible relationship between global wintertime weather regimes in the northern hemisphere (NH) mid latitudes and the total precipitation over mainland China in the season following is investigated. Weather regimes are defined, as by Molteni et al. (1990), as clusters in the low‐dimensional phase space of the leading rotated EOFs of 500 hPa geopotential height, while the total precipitation over continental China is cumulated over different periods in the following spring (representative of a pre‐Mey‐Yu rainfall). Some distinct NH winter weather regimes are found to be highly correlated with the subsequent spring rainfall and so constitute a good basis for a method of quantitative prediction based on simple linear regression techniques.
The adjustment of a rotating, stratified fluid subject to localized sources of mass
Authors: Shutts Glenn
Published: February 2007

The time‐dependent response of a rotating, stratified fluid to localized, impulsive forcing is examined under the hydrostatic approximation and the assumption of two‐dimensionality. Analytic solutions of the linearized quasi‐Boussinesq equations are derived for quiescent and uniformly sheared flows when the axis of the forcing function lies in the direction of the mean wind shear. The mass forcing function, it is imagined, represents the effect of deep, penetrative convection at the cloud top.

In the unsheared case, the phase lines of gravity waves emanating from the forcing region adopt a fanlike configuration symmetric about the vertical. The slopes of the phase lines become more horizontal as time progresses resulting in a vertical ‘fine structure’. As a consequence of this the value of the wave Richardson number may fall below 1/4 in the vicinity of the forcing function.

When the basic‐state shear is chosen to be equal to the buoyancy frequency, the flow has neutral symmetric stability. In this case the response to impulsive heat and momentum forcing is biased towards modes with phase lines of orientation similar to that of the absolute momentum (or equivalently, potential temperature surfaces). A permanent, ageostrophic residual flow along these surfaces is revealed in the long‐time limit. In contrast, the response to mass forcing shows a far greater degree of symmetry about the vertical, with no residual ageostrophic flow.

In general, for symmetrically stable flows, a geostrophic and hydrostatically balanced flow remains after the gravity‐wave energy has dispersed, consistent with the potential vorticity change caused by the mass forcing. However, the potential vorticity anomaly created is proportional in size to the basic state potential vorticity. For slantwise neutral flows therefore, mass forcing does not create a potential vorticity anomaly, and no balanced flow remains.
Dependence of the energy balance of the Greenland ice sheet on climate change: Influence of katabatic wind and tundra
Authors: Meesters Antoon
Published: February 2007

The summer‐time atmospheric circulation at the margin of a large ice sheet, with an extended adjacent tundra, is investigated with a two‐dimensional atmospheric model. The assumed topography corresponds to the section through west Greenland for which the Greenland Ice Margin Experiment (GIMEX) was performed. With this model, the local sensitivity of the ablation of the ice sheet to changes in the external parameters (which pertain to climate) is explored. The main focus is the sensitivity which is related to atmospheric dynamics. A comparison is made between the results with an exposed tundra and results with a snow‐covered tundra. In the latter case, the katabatic wind is strongly reduced close to the ice margin, so that the downward sensible‐heat flux for the ablation zone is more than halved. This shows that the influence of the exposed tundra on the energy balance of the ice sheet is important. Increasing the initial air temperatures leads to a stronger katabatic wind. As a consequence, the downward sensible‐heat flux at the ablation zone is strongly enhanced. Neglecting the change of the katabatic wind would lead to a much smaller growth of this flux, which shows that the presence of an exposed tundra increases the sensitivity of the energy balance of the ice sheet to climate change.

For those runs the large‐scale wind is zero, but in the presence of an inland‐directed large‐scale wind a strong surface wind parallel to the glacier front is found to develop above the ablation zone, by which the sensible‐heat flux is increased. The advection of heated air from the tundra to the ice sheet plays a lesser role.
An observational and theoretical study of the radiative properties of cirrus: Some results from ICE'89
Authors: Francis P. N. , Jones A. , Saunders R. W. , Shine K. P. , Slingo A. , Sun Zhian
Published: February 2007

Observations of the microphysical and radiative properties of two cirrus clouds are reported. The measurements were taken from the C‐130 aircraft of the UK Meteorological Research Flight as part of the field campaign of the International Cirrus Experiment, ICE'89.

Cirrus microphysical properties are analysed in terms of ice water content and a suitably defined ice crystal ‘effective radius’. Estimates of the ice water content are derived from both a standard Particle Measuring Systems 2D‐C cloud probe and a Lyman‐α total water content (TWC) hygrometer. With both flights large discrepancies are seen to occur between the two instruments near the cloud base and top, with the ice water contents derived from the TWC probe being consistently greater than those from the 2D‐C probe. Evidence is presented which suggests that the differences near the cloud base are due to a significant number of large (>800 μm) crystals, which the 2D‐C probe is unable to sample. Near the cloud top the differences are assumed to be due to small (<100μm) crystals, which the 2D‐C probe is again unable to sample correctly. Thus more weight has been given to the TWC probe measurements of ice water content, with the measurements of ice crystal effective radius being adjusted accordingly.

Model clouds, constructed from the microphysical measurements, are used as input to a number of radiation parametrizations, both in the long‐wave and short‐wave regions of the spectrum. In the long wave, model agreement with both narrow‐band and broad‐band measurements is reasonably good, although discrepancies exist near the cloud tops. Some evidence is presented which suggests that the measured effective radii from one of the flights are too large to explain the radiative characteristics of the cloud. The effect of long‐wave scattering is shown to be small. The inferred long‐wave broad‐band mass‐absorption coefficient varies between 0.002 m2g‐1 and 0.087 m2g−1, and it shows a marked inverse dependence on the ice crystal effective radius, as anticipated by simple theoretical calculations. In the short wave, agreement between the measured irradiances and those computed using several parametrizations is again reasonably good, although limited by the natural variability of the clouds. The net radiation budgets of both cloud layers are analysed, and the possibility of the existence of a relationship between ice water content and effective radius is considered.
Wave‐zonal‐flow interaction and ultra‐low‐frequency variability in a simplified global circulation model
Authors: James P. M. , Fraedrich K. , James I. N.
Published: February 2007

The interactions between baroclinic waves and the zonal‐mean mid‐latitude flow are investigated using a multilevel primitive‐equation model of moderate resolution with idealized surface friction and diabatic heating terms. Integrations corresponding to 100 years were performed under perpetual solstice conditions. Empirical Orthogonal Function (EOF) analyses were carried out on the zonal‐mean zonal wind, [u]. Adjustments to the winter hemisphere zonal jet due to the growth and decay of baroclinic waves can be described in terms of the first two EOF patterns, which exhibit substantial interannual variability. For a single lifecycle, such adjustments take the form of a circuit of a phase‐space whose axes are defined by the amplitudes of the two EOFs. The cumulative effects of multiple lifecycles lead to a low‐frequency circulation in the phase‐space, with a timescale of about 150 days. We suggest that this effect may be intrinsically related to the internal generation of ultra‐low‐frequency variability in the model. Normal‐mode lifecycle calculations, including dissipation, reveal similar phase‐space properties. The growth rates of normal‐mode disturbances are shown to be highly sensitive to the phase of the first EOF.
Short baroclinic waves of small amplitude in a moist environment
Authors: Fantini Maurizio , Buzzi Andrea
Published: February 2007

Baroclinic instability at short wavelengths is investigated using a primitive‐equation non‐hydrostatic two‐dimensional numerical model in which the vertical shear is uniform and in which moisture is present in the lower part of the troposphere. The height of the transition between saturated and unsaturated air determines the vertical and horizontal scales of the most unstable modes. As the transition region is raised the most unstable wavelength increases, until the large‐scale (moist) Eady wave, extending to the tropopause, prevails.