bluuebunny/crossref_metadata_2025 huggingface dataset:
On the interaction of tropical‐cyclone‐scale vortices. III: Continuous barotropic vortices
Authors: Holland Greg J. , Dietachmayer Gary S.
Published: February 2007

The interaction of cyclonic vortices in spherical geometry is investigated using a shallow‐water model on an adaptive grid. It is found that the mutual approach and merger of binary cyclones support the compound vortex‐patch findings of Part II. As interaction commences, distortion of the weak outer vorticity fields leads to a change in the advecting flow over each vortex core, which leads to mutual approach or retreat depending on the shape of the vorticity fields. Raped merger occurs when the cyclone cores approach within the critical separation distance defined in Part II.

The core merger of cyclones seems to the largely independent of environment and the rotation of the earth. The initial approach can be quite sensitive, however. Non‐merging binary cyclones sweep equatorial parcels of air over long trajectories towards the pole to create exceptionally strong anticyclonic gyres. These can be of sufficient strength to capture the nearest cyclone and induce the escape mode described in Part I. Some sensitivity to latitude (and hence strength of the earth‐vorticity gradient), therefore, is found. A large anticyclone in a subtropical ridge location is found to be sheared and torn apart by the interacting cyclones, with little effect on merger.

The observations in Part I that interactions of three cyclones could be broken down into separate binary interaction sequences is supported by experiments with three vortices. When two of the vortices merge, the interaction evolves to a small/large cyclone situation. When no merger occurs, each pair of cyclones proceeds through a distinctive orbit/escape cycle.
Analysis of the surface hydrology in a regional climate model
Authors: Giorgi Filippo , Hostetler Steven W. , Brodeur Christine Shields
Published: February 2007

This paper discusses the surface hydrology of a multi‐year simulation of present day climate over the United States (US) conducted with a regional climate model (RegCM) nested within a general circulation model (GCM). The RegCM, which is run with a 60 km gridpoint spacing is interactively coupled with a state‐of‐the‐art surface physics package that includes full surface hydrology calculations (the Biosphere‐Atmosphere Transfer Scheme or BATS). The hydrologic budgets of ten regional drainage basins in the US are analysed. Model results are compared with available observations and with results from previous modelling experiments to evaluate the feasibility of using nested RegCM/GCM models for hydrology studies. In our experiment, the model captures the basic seasonality of the basin hydrologic budgets, although the simulated precipitation amounts are too high over the western US and too low over the eastern US. As a result, runoff, snow cover and soil water content are underestimated over the eastern US basins, while evaporation and runoff are overestimated in some of the western US basins. Topographically induced characteristics of precipitation, snow cover and runoff are well simulated over the mountainous western regions. Also well captured is the inter‐basin variation of hydrologic budgets which occurs in response to different climatic settings. The springtime snowmelt and peak runoff season generally occurs in the model earlier in the year than is observed. Although our work indicates that the coupled regional modelling system can be useful in applications to hydrological studies, results from this experiment indicate that better accuracy in the simulation of regional climatic variables and more detailed representation of some hydrologic processes would be required before the coupled modelling system could be used to provide accurate assessments of hydrologic responses to climate change.
A wide cold‐frontal rainband and its relationship to frontal topography
Authors: Locatelli John D. , Martin Jonathan E. , Hobbs Peter V.
Published: February 2007

A wide cold‐frontal rainband associated with a vigorous surface cold front was investigated using conventional meteorological data and the velocity and reflectivity data from high‐resolution Doppler radar. It was found that the movement of the rainband was closely related to the velocity of the frontal surface, and that the substructural features of the rainband corresponded closely with the topographical features of that surface. The evolution of the topography of the frontal surface is described, and some processes that might affect frontal topography, and therefore the structure and movement of rainbands, are examined.
Reply to comments by S. B. Feldstein and W. A. Robinson on ‘Spatial structure of ultra‐low frequency variability of the flow in a simple atmospheric circulation model’ (October 1992, 118, 1211–1233)
Authors: James I. N. , James P. M.
Published: February 2007

The note by Feldstein and Robinson (hereafter referred to as FR) raises two questions. The first is whether the EOF1 of zonal wind in the numerical integrations that we carried out is correlated with the atmospheric angular velocity or with the mean angular momentum. The second is whether the ultra‐low‐frequency fluctuations which we observed have any dynamical significance or are merely ‘climate noise’. We reply to each of these issues in turn, and are grateful to FR for raising some interesting questions about our results.
Water vapour continuum absorption in mid‐latitudes: Aircraft measurements and model comparisons
Authors: Rudman S. D. , Saunders R. W. , Kilsby C. G. , Minnett P. J.
Published: February 2007

Recently reported measurements by the Meteorological Research Flight C‐130 aircraft over the tropical Atlantic have shown that existing models of infrared water vapour continuum absorption underestimate its magnitude by up to 30%. This paper presents additional results from more recent measurements made close to the south‐west approaches of the British Isles and over the eastern Mediterranean Sea. These measurements are compared with two models: GENLN2, a line‐by‐line model, and the widely used band model LOWTRAN7. The measured radiances suggest a consistent underestimate in both models of the water vapour continuum absorption. The modified parametrization for the water vapour continuum absorption, inferred from the tropical results, was also used here to check its validity for lower water vapour amounts and lower temperatures. As with the tropical results, increasing the self‐broadening coefficient by a constant factor, or increasing the temperature dependence, improves the agreement between the measurements and the model predictions.
The influence of the Indian Ocean/Indonesian SST gradient on the Australian winter rainfall and circulation in an atmospheric GCM
Authors: Frederiksen Carsten S. , Balgovind Ramesh C.
Published: February 2007

A number of atmospheric general circulation experiments have been conducted with the aim of studying the impact of an enhanced sea surface temperature (SST) gradient, between the Indonesian archipelago and the central Indian Ocean, on the Australian winter circulation and rainfall. Particular emphasis is on modelled synoptic events and the sensitivity of mean winter climate anomalies to changes in the strength of the anomalous SST gradient.

It is shown that an atmospheric general circulation model, forced by an anomalous SST gradient to produce mean winter circulation and precipitation anomalies qualitatively similar to the observed, simulates quite realistic synoptic events over the Australian continent. In particular, the model simulates an increased frequency of major Australian north‐west cloud bands. These events are associated with a triad of tropical systems in the Bay of Bengal, the Cocos Islands and the Indonesian archipelago, strong cross‐equatorial flow into a region north‐west of Australia, and interactions with short‐wave mid‐latitude disturbances.

The mean climate anomalies, over Australia, are shown to be largely independent of whether a fixed or evolving anomalous gradient is used, but very sensitive to the magnitude of the SST anomaly. This is seen in the greater variability of the results over Australia. It is also suggested that the presence and strength of the warm anomaly in the Indonesian region has a greater bearing on the results than the cold anomaly, in the Indian Ocean. Global tropospheric circulation and precipitation anomaly patterns, associated with an enhanced gradient, are also shown to be consistent with some observed patterns, and to be robust with respect to changes in the magnitude and distribution of the SST anomalies. This suggests that an enhanced SST gradient may also be associated with rainfall variability in a number of geographical locations.
Radiative damping of equatorial waves in the middle atmosphere
Authors: Vijayan Lekshmi , Reddy C. A.
Published: February 2007

Theoretical estimates are made of the radiative damping of various Kelvin and Rossby‐gravity (RG) wave modes in the 20–70 km region, for a period of 24 months (1979–80), using the observed monthly mean zonal‐wind profiles at 8.5°N latitude and realistic values of scale‐dependent cooling coefficients. The net changes in wave amplitudes at various height regions are calculated by combining the attenuation due to radiative cooling and the growth due to density decrease. The calculated height variations of wave amplitudes explain quite well several features of the observed height structures of Kelvin waves. They also explain the observed rapid decay of RG waves below about 35 km when their period exceeds 3 days.

The calculated momentum flux convergences of various waves are used to estimate the mean‐flow accelerations generated by the damped waves. The Kelvin waves are found to be capable of generating the accelerations needed to drive the westerly phase of the quasi‐biennial oscillation (QBO) and the semi‐annual oscillation (SAO) in the zonal wind. However, RG waves seem to be capable of driving the easterly phase of the QBO but not the SAO. The remarkable consistency of our theoretical results and the observed patterns of wave amplitudes and mean‐flow accelerations demonstrate that radiative damping is the dominant dissipative process for the equatorial waves in the 20–70 km region and our theoretical approach is capable of characterizing the equatorial wave behaviour and the associated flow accelerations with a good quantitative accuracy.
Impact of localized tropical and extratropical SST anomalies in ensembles of seasonal GCM integrations
Authors: Ferranti L. , Molteni F. , Palmer T. N.
Published: February 2007

A series of 120‐day ensemble integrations of a general circulation model, designed to assess the impact of geographically localized sea‐surface‐temperature (SST) anomalies in both the tropics and extratropics, are described. These experiments contribute firstly to an appraisal of the relative roles of tropical and extratropical SST anomalies on interannual variability of the large‐scale circulation in the northern extratropics, and secondly to an assessment of the role of quasi‐stationary diabatic‐heating anomalies on model systematic error, including blocking activity.

Overall it is found that SST anomalies associated with El Niño and La Niña have a larger and more reproducible impact on the extratropics than the chosen extratropical SST anomalies. These extratropical anomalies were localized to the north‐west Pacific, and north‐west Atlantic, with realistic amplitude. However, unlike earlier studies, a response to the extratropical North Pacific SST anomalies has been obtained over the North Pacific which is correlated with the sign of the imposed SST anomaly. The response to extratropical SST anomalies in the north‐west Atlantic are similar to the results obtained from an earlier study. The downstream responses to the extratropical Pacific and Atlantic SST anomalies are qualitatively similar to one another. Overall it is concluded that the northern large‐scale flow is influenced by such extratropical SST anomalies.

The response to idealized tropical SST anomalies was also studied. In particular, a localized anomaly over Indonesia had a very substantial impact on the Hadley circulation, on zonal flow, and on blocking frequency over the North Pacific and Europe. This response was such as to reduce model systematic error: locally in the vicinity of the SST anomaly, remotely around the tropics, and remotely in the extratropics. A similar, though weaker, impact on Euro‐Atlantic blocking was obtained with an idealized Caribbean SST anomaly.

Further statistical and dynamical analyses suggested that the extratropical response to the Indonesian SST anomaly occurs through the creation of two distinct planetary‐scale regimes, in one of which the formation of blocks is much favoured by increased ridges on the north‐eastern side of the oceans.
Importance of microphysical processes in the dynamics of a CSI mesoscale frontal cloud band
Authors: Marecal Virginie , Lemaitre Yvon
Published: February 2007

Microphysical processes in a cloud band observed during the MFDP/FRONTS 87 experiment are studied. The importance of precipitation evaporation in the downdraught zone of the cloud‐band circulation, due to conditional symmetric instability (CSI), is clearly evidenced thanks to a two‐dimensional (2D) microphysical retrieval model applied to an observed 2D wind field. This study shows the crucial role of snow evaporation in maintaining the descent of the CSI circulation in a nearly saturated state. The evaporation creates a cooling which reduces the resistance of the atmosphere to subsiding motions, and thus reinforces the ascending ones. Comparisons with results on the effect of snow evaporation on mesoscale motions obtained by other authors with a one‐dimensional model clearly show the effect of the horizontal wind component.
Internal variability in a perpetual January integration of a troposphere‐stratosphere‐mesosphere GCM
Authors: Pawson Steven , Meyer Arnold , Leder Silke
Published: February 2007

A 1200‐day perpetual January integration of the Berlin troposphere‐stratosphere‐mesosphere general circulation model (GCM) has been performed with constant boundary conditions. The long‐term mean climatology represents the response to the different forcing terms in the zonal‐mean momentum budget: the winter stratospheric mean climatology is determined by the balance between the mean meridional circulation (m.m.c.), acting via the Coriolis force, and the Eliassen–Palm flux divergence. The winter stratospheric mean state oscillates between periods of strong wind (undisturbed conditions) and weaker winds (following minor warmings) on periods of several hundred days; this is the response to transient eddy forcing from the troposphere which has an almost‐red power spectrum, with maximum power on time‐scales of tens of days. As previously found in observations, there is a near cancellation between the wave‐ and the m.m.c.‐induced forcing of the mean flow, but the acceleration is well correlated with the wave forcing. The dominant spatial modes of variability of the GCM are isolated using principal‐component analysis. The dominant orthogonal modes of wind and temperature extend from the winter into the summer hemisphere. They represent structures close to thermal‐wind balance, with a dipolar structure in the lower stratospheric temperature field associated with variations in the strength of the polar‐night jet and weaker wind anomalies in the summer subtropics (22°S, 5hPa). There is some evidence of long‐period fluctuations in the vertical velocity in the lower tropical stratosphere associated with the high‐latitude velocity variations; these are consistent with the equatorial temperature anomalies. Time‐delayed point correlations with the zonal velocity at 62°N, 1 hPa reveal that the weak anticorrelation of velocity in the northern subtropics (28°N, 1 hPa), which is weak at zero‐lag, increases with time to about 58% at 20‐days lag. Singular‐spectrum analysis of the zonal velocity at the three reference points, which isolates orthogonal modes of temporal variability, reveals that the correlations are increased when only the low‐frequency velocity variations are considered.
Dynamically consistent, quasi‐hydrostatic equations for global models with a complete representation of the Coriolis force
Authors: White A. A. , Bromley R. A.
Published: February 2007

The spherical polar components of the Coriolis force consist of terms in sin ϕ and terms in cos ϕ, where ϕ is latitude (referred to the frame‐rotation vector as polar axis). The cos ϕ Coriolis terms are not retained in the usual hydrostatic primitive equations of numerical weather prediction and climate simulation, their neglect being consistent with the shallow‐atmosphere approximation and the simultaneous exclusion of various small metric terms. Scale analysis for diabatically driven, synoptic‐scale motion in the tropics, and for planetary‐scale motion, suggests that the cos ϕ Coriolis terms may attain magnitudes of order 10% of those of key terms in the hydrostatic primitive equations. It is argued that the cos ϕ Coriolis terms should be included in global simulation models.

A global, quasi‐hydrostatic model having a complete representation of the Coriolis force is proposed. Conservation of axial angular momentum and potential vorticity, as well as energy, is achieved by a formulation in which all metric terms are retained and the shallow‐atmosphere approximation is relaxed. Distance from the centre of the earth is replaced by a pseudo‐radius which is a function of pressure only. This model is put forward as a more accurate alternative to the traditional hydrostatic primitive equations; it preserves the desired conservation laws and may be integrated by broadly similar grid‐point methods.
Cisk: A theory for the response of tropical convective complexes to variations in sea surface temperature
Authors: McBride John L. , Fraedrich Klaus
Published: February 2007

A set of balanced and unbalanced slab‐symmetric linear 2‐layer models is analysed. the stability properties depend on a parametrized moisture budget dominated by the value of the underlying sea surface temperature. Convective heating occurs within an inner region characterizing the size of the convective disturbance. the heating is equal to the sum of two terms, representing the mid‐level and lower‐level upward mass‐fluxes.

A fast and a slow mode of instability can be distinguished. the slow mode shows finite maximum growth for infinitesimally small disturbances and is the instability traditionally associated with Conditional Instability of the Second Kind (CISK). the fast mode reveals infinitely large positive growth‐rates for finite‐size disturbances.

The transition of the model solution from slow to fast mode occurs at a threshold value of sea surface temperature. For the parameters chosen in this study, the threshold occurs at approximately 25.5°C.
The Asian summer monsoon and ENSO
Authors: Ju Jianhua , Slingo Julia
Published: February 2007

The relationship between the evolution of the Asian summer monsoon and equatorial sea‐surface‐temperature anomalies has been studied using results from an integration with the UK Universities' Global Atmospheric Modelling Programme (UGAMP) General Circulation Model (UGCM). the integration was performed as part of the Atmospheric Model Intercomparison Project and thus used the observed sea surface temperatures (SSTs) for the decade January 1979 to December 1988. the mean evolution of the Asian summer monsoon has been successfully simulated in terms of many aspects of the rapid transition of the large‐scale circulation during the boreal spring and summer. However, the results for individual years showed considerable interannual variability, both in the strength of the monsoon and in the time of onset. A relationship has been identified between the evolution of the monsoon flow and the phase of the El Niño Southern Oscillation (ENSO). In agreement with observed results, years with warm SST anomalies in the equatorial central and east Pacific Ocean (El Niño) have a weaker monsoon circulation and a delayed onset. an opposite behaviour is noted for those years with cold Pacific SST anomalies (La Niña). Diagnostics from analyses from the National Meteorological Center and the European Centre for Medium‐range Weather Forecasts, and from data on the outgoing long‐wave radiation observed by the Advanced Very High Resolution Radiometer, have been used to verify the model results. A description of the mechanism by which the phase of ENSO remotely influences the dynamics of the Asian summer monsoon has been developed involving changes in the heating patterns over Indonesia and the west Pacific in the preceding spring.
Meso‐β‐scale circulations in realistic fronts. I: Steady basic state
Authors: Fischer Claude , Lalaurette François
Published: February 2007

This paper investigates the occurrence of conditional symmetric instability during MFDP/Fronts87IOP2. A saturated negative equivalent‐potential‐vorticity area is detected by the mesoscale analysis in the upper troposphere, and this is confirmed by dropsounding data. Two‐dimensional hydrostatic simulations are performed with the Météo‐France PERIDOT model which give rise to the onset of symmetric instability. A complete life cycle is obtained as well as the growth rate of the perturbation (about 2.5 × 10−5 s−1 ≈ (11 hours)−1). Particular emphasis is put on the meanings and results of the kinetic‐energy conversions. Most of the perturbation energy comes from the basic state through vertical momentum fluxes, which is consistent with previous theoretical or numerical works. the decay of the perturbation is also shown to be due to its own dynamics, the symmetric rolls advecting positive equivalent potential vorticity into the unstable region. Deformations of equivalent potential temperature (θe) surfaces and m‐surfaces give an insight into the parcel trajectories: the ascending parcels follow θe‐surfaces. leading to a buckling of the m‐surfaces, and the descending parcels follow θ‐surfaces. the small value of the growth rate is stressed as well as of the maximum intensity (w ≈ 5 cm s−1). Thus, interactions with the frontogenetical forcing are expected owing to the small growth rate obtained. This is the subject of Part II of this paper.
A comparison of model and observed network estimates of sulphur deposition across Great Britain for 1990 and its likely source attribution
Authors: Metcalfe S. E. , Whyatt J. D. , Derwent R. G.
Published: February 2007

Long‐range transport models for acidic sulphur species are being used to develop emission‐control strategies to combat environmental acidification in the remote, sensitive regions of Europe. the incorporation of seeder‐feeder enhancement into a simple trajectory model for sulphur has markedly improved the representation of wet deposition across Great Britain, as shown by a detailed comparison of the model results with the observations from monitoring networks. an uncertainty analysis reveals that those input parameters which contribute most to uncertainty in the total sulphur deposition are the sulphur‐dioxide emissions, wind speeds and wet‐removal coefficients. the total Great Britain dry and wet depositions of sulphur are estimated to be 216 and 241 thousand tonnes per year, respectively, making a total sulphur deposition per year of 457 ± 110 thousand tonnes, in excellent agreement with the observed quantity of 433 thousand tonnes. the model was used to give an indication of the origin of the deposited sulphur across Great Britain. European land‐based sulphur sources contribute 34% of the deposition to Great Britain, with significant dry deposition in south‐east England and wet deposition in upland western Britain. UK power stations and oil refineries contribute 46.7% of total deposition, with shipping and natural sulphur sources from algal blooms, a mere 4.6%. Differences in source attribution between this UK‐scale study and those previously published for the long‐range transport scale are explored and it is concluded that national‐scale models have an important role to play in policy formulation.
Tropospheric dispersion: The first ten days after a puff release
Authors: Maryon R. H. , Buckland A. T.
Published: February 2007

Dispersion from a puff released in the atmospheric boundary layer in mid‐latitudes is simulated with a Lagrangian global multiple‐particle model using analysed three‐dimensional wind fields. Quantitative measures of the spread are computed and used to assess the influence of the synoptic system into which the puff is released, and the ways in which turbulent diffusion can contribute to the total spread. the nature of the dispersion over ten days is examined to elucidate the controlling meteorology and kinematics during the various phases which could be identified. the results are compared with measurements made from constant‐level balloons. A time index is proposed to characterize large‐scale dispersion from a point source.
Potential vorticity inversion and balanced equations of motion for rotating and stratified flows
Authors: Vallis Geoffrey K.
Published: February 2007

Balanced equations of motion based on potential vorticity evolution and inversion for the shallow water and stratified primitive equations are derived and, in some shallow‐water cases, numerically tested. The schemes are based on asymptotic expansions in Rossby or Froude number, or rational scaling‐based truncations of the equations of motion, assuming that the dynamics are determined by the advection of potential vorticity. Thus, regimes of validity are rapidly rotating and/or highly stratified flow. Both new and familiar results are straightforwardly obtained, in a unified framework in both height and isentropic coordinates. For both shallow‐water and stratified equations, Rossby number expansions schemes give quasi‐geostrophy at lowest order. Both gradient‐wind balance and the nonlinear terms in the potential vorticity enter at next order. A low Froude number expansion for non‐rotating flow gives two‐dimensional flow, uncoupled in the vertical at lowest order. A single consistent inversion scheme can be derived that is valid at lowest order in Froude number for all Rossby numbers, for both shallow‐water and the stratified equations. It may be a particularly appropriate model for the atmospheric mesoscale and oceanic submesoscale, where rotation and stratification can both be important in defining balanced motion. A model is also proposed that is valid at both planetary and synoptic scales, combining the familiar planetary geostrophic and quasi‐geostrophic equations. Most of the models derived require the solution only of linear or near linear elliptic equations, possibly with varying coefficients.

Numerical experiments indicate that a higher‐order inversion can be quantitatively better than quasigeostrophy, if Rossby number and divergence are sufficiently small. In some other cases, no noticeable improvement over quasi‐geostrophy is found, even when the Rossby number is quite small. However, the balanced model valid for both planetary and synoptic scales shows a significant qualitative and quantitative improvement over both planetary geostrophy and quasi‐geostrophy for large‐scale flows, and its evolution is in good agreement with a primitive equation model.
Aircraft measurements of potential vorticity and Richardson number in baroclinic zones
Authors: Vaughan G. , Pepler S. J. , McKenna D. S.
Published: February 2007

This paper describes a novel technique for deriving potential vorticity and Richardson number from aircraft cross‐sections of baroclinic zones. It exploits the fact that gravity waves distort isentropic surfaces, so that an aircraft flying horizontally can measure a vertical profile of temperature and wind under favourable conditions. The technique is applied to two flights of the UK Meteorological Research Flight's C‐130 Hercules aircraft, in April 1983 and June 1991. It is found to work reasonably well only when large‐amplitude gravity waves are present. Values of potential vorticity so derived agree with those calculated in the conventional manner from cross‐sections, and are found to correlate well with ozone mixing ratio.
The greenhouse Earth: A view from space
Authors: Harries J. E.
Published: February 2007

The natural greenhouse effect of the Earth is strongly influenced by the radiative effects of water vapour and clouds in the atmosphere, which control the energy absorbed from the sun, and that lost through thermal emission to space. Any perturbations to the climate balance, for example through so‐called ‘radiative forcing’ due to increasing CO2 amounts, variations in solar constant, or other causes, can be amplified by the feedback processes that involve water in its various phases. The radiative cooling of the Earth in the absence of clouds has recently been shown to be dominated by emission from upper‐tropospheric water vapour, in the far infrared portion of the spectrum, and this is illustrated: observations of this radiative flux, and of the distribution of water vapour in the upper troposphere, are urgently needed. The role of clouds is discussed, and it is noted that their response to global warming is not presently unambiguously determined with available models, due to the complexity of competing processes: again, as in the cloud‐free case, more accurate global observations are needed. The paper is illustrated by data from satellite experiments, most notably the Earth Radiation Budget Experiment sponsored by NASA.
Form and wave drag due to stably stratified turbulent flow over low ridges
Authors: Belcher S. E. , Wood N.
Published: February 2007

We investigate how stable stratification affects the aerodynamic force, or form drag, induced by turbulent boundary‐layer flow over two‐dimensional hills. Both analytical and numerical models are used to calculate the flow and thence the form drag on the hill. In the analytical model we use a two‐layer, truncated mixing‐length, turbulence model, which is consistent with scaling arguments and which produces reliable estimates of the form drag for the neutral flow. The form drag is also calculated analytically using an eddy‐viscosity model, and the results compare well with values computed with the nonlinear numerical model that uses a similar turbulence model.

The leading‐order contribution to the form drag is from a non‐separated sheltering mechanism, which is similar to the mechanism in neutral flow. Stable stratification changes the magnitude of this mechanism through several effects. For weak stratification the predominant effect is an increase in shear in the upstream wind profile across a middle layer, which increases the form drag by a factor of two or more. There is indirect experimental evidence to support this finding. If the stratification is more stable, then the shear across the middle layer becomes limited because the boundary layer has a finite depth. Then the dynamical effect of buoyancy on the pressure perturbation becomes important and reduces the form drag, eventually to zero.

For still more stable stratification, gravity waves and associated wave drag are produced. The analysis shows that the appropriate scaling velocity for wave drag is the approach‐flow wind speed evaluated at the middle‐layer height. The relationship between the form and wave drag components is investigated by evaluating the analytical formula for the drag on isolated hills of two idealized shapes. For weak stratification the form drag dominates, but as the stratification becomes more stable the wave drag increases and first equals the form drag, at a value that depends on the hill shape, and then dominates.
A global shallow‐water model using an expanded spherical cube: Gnomonic versus conformal coordinates
Authors: Rančić M. , Purser R. J. , Mesinger F.
Published: February 2007

A model using shallow‐water equations with an Arakawa‐type scheme for momentum terms is tested on a quasi‐uniform geometry on the sphere, derived by a spherical expansion of the inscribed cube based on the gnomonic projection. Thereby, a quasi‐homogeneous distribution of grid points is achieved, and a global finite‐difference model is designed which does not require Fourier filtering or suffer from the burden of redundant computational points at high latitudes. Difficulties resulting from the directional discontinuity of the coordinate lines crossing the edges of the expanded cube are almost completely eliminated by using the Arakawa B‐grid, so that only scalar points are placed along the edges. An alternative approach is developed based on numerical orthogonalization of the grid whereby, inter alia, the directional discontinuity at the edges is avoided at the cost of some accumulation of points in the vicinity of the vertices of the cube. In the customary Rossby–Haurwitz wave‐4 tests, both approaches are shown to converge to a visually indistinguishable solution as the resolution is increased. However, with the orthogonalized, conformal grid, convergence towards the asymptotic solution was substantially faster.
A low‐order model of coherent structures in the convective atmospheric surface layer
Authors: Zhuang Yahui
Published: February 2007

Based on a simple application of the dynamical systems theory and experimental evidence of coherent motions in the convective atmospheric surface layer, a low‐order model (15 ordinary differential equations) to study coherent flow dynamics is developed. The model is derived by performing a Galerkin projection of the Navier–Stokes equations on a set of 15 orthogonal functions ϕ (x, z), extracted from an ensemble of observed convective plumes (or coherent structures) with the Proper Orthogonal Decomposition method. These orthogonal functions, defined on a two‐dimensional domain (620 m × 150 m), are typical of large‐scale turbulent velocity and temperature fluctuations in the convective atmosphere, and are assumed to have captured enough physics to model the most important aspects of the coherent structures. The effects of the unresolved turbulence on the coherent structures are modelled by the Smagorinsky eddy viscosity closure. The model treats boundary conditions objectively, uses experimental data as the initial conditions, and provides a means to study the dynamics of the coherent structures.
The behaviour of the roughness length for temperature over heterogeneous surfaces
Authors: Malhi Yadvinder
Published: February 2007

The definitions of roughness length and zero‐plane displacement are examined, as well as the relationship of these quantities to the resistance of transfer processes close to a surface. It is demonstrated that, if molecular diffusion or free convection processes are dominant within the interfacial sub‐layer, then the roughness length for temperature z0T can be expected to decrease with increasing wind speed. Over homogeneous surfaces this sensitivity to wind speed is reduced by the relative efficiency of heat transfer, but over sparsely vegetated surfaces the sensitivity is greatly increased.

The problem of correct measurement of surface temperature over a heterogeneous surface is outlined and examined; it is demonstrated that use of the area‐averaged radiometric temperature as a representative surface temperature can lead to consequences such as zero, negative or infinite apparent transfer resistances. Measurements of z0T are presented from two field experiments over sparsely vegetated surfaces; typical values of z0T are around 10−10 m, and vary by orders of magnitude over a quite moderate range of wind speed. These measurements are interpreted in terms of the behaviour of interfacial sub‐layer transfer processes; it is shown that a significant cause of the low measured value of z0T may be the use of radiometric surface temperature as a representative surface temperature.
A mechanism for the low‐frequency variability of the mid‐latitude troposphere
Authors: James I. N. , Dodd J. P.
Published: February 2007

Low‐frequency variability of the zonal‐mean zonal wind in long runs of a simplified global circulation model is dominated by bands of wind anomaly which originate in the tropics and propagate slowly poleward with speeds of 0–2 m s−1. Such propagation could be the result of the interaction between the zonal‐wind anomaly and an equatorward flux of Rossby‐wave activity originating in middle or high latitude. A barotropic weakly nonlinear model of this process has been derived. It suggests propagation of zonal‐wind anomalies similar to those produced in the global circulation model, and leads to comparable propagation speeds.
The radiative properties of inhomogeneous boundary layer cloud: Observations and modelling
Authors: Hignett P. , Taylor J. P.
Published: February 2007

The importance of internal inhomogeneities in the microphysical structure of stratocumulus in relation to the radiative properties of various cloud fields is studied using aircraft measurements and a 3–D Monte Carlo model. This study is unique in that in situ microphysical data and observations of cloud thickness have been used in the initialization of a Monte Carlo model, and the resulting model predictions of reflectance and albedo have been compared with in situ aircraft radiometric observations of the same cloud. The model predicts a reduction in reflectance and albedo for the 3–D cloud field compared to a homogeneous cloud layer with the same average microphysical properties. The in situ observations confirm these simulations. The Monte Carlo model predicts that internal inhomogeneities lead to a reduction in cloud absorptance and an increase in cloud transmittance. A further study of the importance of the distribution of clouds within a domain was carried out using a Monte Carlo model where the cloud fraction is kept constant while the separation of the cloud elements is varied. These results reveal a large sensitivity of the radiative properties to the distribution of the cloud, and that this sensitivity increases as the separation of the cloud elements decreases.
Observation and simulation of barrier winds at the western margin of the Greenland ice sheet
Authors: van den Broeke Michiel R. , Gallée Hubert
Published: February 2007

Meteorological observations performed in the melting zone of the Greenland ice sheet during GIMEX‐90/91 show the regular occurrence of moderately strong barrier‐winds. The barrier wind is a thermally generated jet that develops when the large‐scale flow advects warm tundra air towards the cold, melting ice sheet, creating a large local horizontal gradient of temperature. As a result, the geostrophic wind in the atmospheric boundary‐layer acquires a component perpendicular to the temperature gradient. In the melting zone of the ice sheet, barrier winds interact with persistent surface‐based katabatic winds. The forcing mechanisms and detailed two‐dimensional structure of the barrier wind have been investigated with a mesoscale meteorological model. The model results confirm the important role of the warm tundra during the development of barrier winds. Barrier winds cause a pronounced rise in temperature and a strong increase of turbulent exchange in the lower melting zone: friction velocity and sensible‐heat flux in the melting zone showed peak values of 1.3 m s−1 and 300 W m−2 respectively. In combination with large positive net radiation, the daily melt rate increased to 10 cm water equivalent, which is twice the rate in typical katabatic wind conditions. This shows that barrier winds could have a significant impact on the melt regime of areas where the ice sheet ends in the tundra. Although barrier winds in this part of Greenland may occur less frequently than the observations during GIMEX‐90 suggest, they are of great importance for the climate of this and of other polar regions, such as the Antarctic, and so merit careful study.
In situwater vapour measurement, and a case study of the drying mechanism of the tropical lower stratosphere
Authors: Ovarlez Joëlle , Ovarlez Henri , Teitelbaum Hector
Published: February 2007

During an experiment performed in tropical regions, water vapour content was measured in the lower stratosphere, using long duration balloons. A set of profiles show a vertical structure consistent with what has been named the tape‐recorder effect. One of the results, when the balloon descends to 100 hPa, shows a value of the water‐vapour mixing ratio as low as 2.8 parts per million by volume, in a layer approximately 1 km thick at about 80 hPa. This layer is delimited by two minima of air temperature, suggesting that the drying is a consequence of a vigorous convective process. This kind of process can be so strong that the convective cloud overshoots the tropopause and forms an anvil of ice crystals which grow and finally fall under gravity. Some of the characteristics of the METEOSAT cloud imagery and of the ECMWF analyses could be interpreted as the signature of this type of convective process. Nevertheless one cannot discard other mechanisms, such as the generation of stratospheric buoyancy waves by tropical convection. These waves can cross the tropopause and form thin ice clouds in the lower stratosphere. In this hypothesis, also, the ice falls and transports moisture downwards.
Predicting cloud‐droplet effective radius and indirect sulphate aerosol forcing using a general circulation model
Authors: Jones A. , Slingo A.
Published: February 2007

Various methods for predicting cloud‐droplet effective radius in the Hadley Centre general circulation model are compared with aircraft and satellite retrievals, and are used to estimate the indirect radiative forcing by anthropogenic sulphate aerosols since the beginning of the industrial era. The effects both of different parametrization approaches and of different input sulphate data sets are examined; however, there is no clear evidence to prefer either of the two sulphate data sets used in the study. Two of the parametrizations generate distributions of present‐day effective radius which are similar to each other and compare favourably with observations, yet provide very different estimates of the indirect effect, ranging from −0.5 to −1.5 W m−2 in the global annual mean. A sensitivity experiment in which it is assumed that droplet concentrations are not determined by sulphate concentrations in continental air reduces this global‐mean forcing to −0.3 to −0.8 W m−2. This sensitivity demonstrates the need for a much better understanding of the link between sulphate aerosol mass concentrations, cloud condensation nuclei, and cloud‐droplet number concentrations.
Towards improved radar estimates of surface precipitation rate at long range
Authors: Kitchen M.
Published: February 2007

A method has been developed for estimating surface precipitation rates using reflectivities measured with a radar scanning at several different angles of elevation. In each pixel of a radar image, an idealized profile of the reflectivity factor is constructed. Each profile is defined in terms of three unknowns: the reflectivity in the rain beneath the melting layer, and the slope of the profile of the reflectivity in each of two layers above the melting layer. In these layers above the melting layer, it is necessary to invoke assumptions of horizontal homogeneity in the shape of the profile. Simple parametrizations are used for low‐level orographic growth and for the bright band, and the profile is diagnosed with the help of non‐radar information. the difference between the idealized and observed profiles is expressed as a single value for an entire radar‐image; the difference is minimized by iteration and this avoids using complicated methods of inversion. A simulation experiment was carried out for 15 different cases; in most of them, the incorporation of data from several elevation angles reduced bias errors at long range.

The new method was then used on real radar‐data. In typical frontal rainfall, it showed no consistent improvement. Because it avoids using corrections which can otherwise become detrimental at long ranges, however, the new method gave improved results when reflectivity profiles were atypical.

Unresolved spatial variability of the reflectivity profile continues to be a problem.
Formation of inhomogeneity in drop concentration induced by the inertia of drops falling in a turbulent flow, and the influence of the inhomogeneity on the drop‐spectrum broadening
Authors: Pinsky M. , Khain A.
Published: February 2007

The mechanisms of drop‐concentration inhomogeneity formation are studied using both a numerical simulation with a model of isotropic and homogeneous turbulence, and analytical methods. It is shown that atmospheric turbulence can create a significant drop‐concentration inhomogeneity due to the effects of drop inertia. Two types of area in the turbulent flow are revealed. Drops tend to leave the areas of ‘drop vortices’ and collect within the zones out of the vortices. As a result, the zones of drop‐track collection turn out to be the zones of enhanced drop concentration. the rate of concentration enhancement is studied for drops of different sizes using the Monte Carlo method. It is shown that the drop flux velocity divergence and droplet‐concentration variations reach their maximum at 100 μm drop radius. Zones of enhanced drop concentrations are stretched along the drop tracks. Characteristic scales of drop‐concentration fluctuations along the drop tracks are of the order of several metres or even a few tens of metres. Across the drop tracks, the characteristic scale of concentration pulsations is of the order of a few centimetres.

The existence of the areas of drop‐concentration enhancement and the areas of decreased concentration means that drop collisions are not distributed uniformly, but are concentrated within the areas with greater drop concentration. the possible effects of droplet inhomogeneity is simulated by solving a stochastic coalescence equation for the water drop‐size‐distribution function in areas of enhanced and decreased droplet concentration with the subsequent mixing of the corresponding spectra at each 10 s. It is shown that due to the nonlinear nature of collision processes the effects of the increase of collision frequency in the areas of enhanced concentration dominate over those of its decrease. the effects of the drop‐concentration inhomogeneity are accumulated with time and lead to substantial acceleration of droplet spectrum broadening. It is supposed that the inhomogeneity of rain rates with time and space can be attributed to these turbulence effects determining drop‐track collection and drop‐concentration inhomogeneity. These turbulence effects can contribute to the formation of inhomogeneity of ice and aerosol particles as well.