bluuebunny/crossref_metadata_2025 huggingface dataset:
Flux‐gradient relationships for momentum and heat over a rough natural surface
Authors: Fazu Chen , Schwerdtfeger Peter
Published: February 2007

The flux‐gradient relationships for momentum and heat have been investigated over extensive flat bushland (h = 2.3 m, zo = 0.4 m) with observations being made at several levels extending up to 10 times the canopy height. It has been found that: (1) Eddy diffusivities for momentum and heat, KH and KM, are enhanced by factors of 1.65 and 2.5 respectively under near‐neutral conditions and within the layer close to the canopy. (2) These enhancements distinctly depend on instability, and their variations with height are not simply monotonic, as expected in earlier investigations. (3) Even a counter‐gradient flux of heat could occur under some circumstances. (4) The level of enhancement remains substantially unchanged over different surfaces for heat transfer but decreases as the canopy density increases in the case of momentum transfer. Possible mechanisms are discussed. (5) The length scale of atmospheric turbulence should be taken into account in studies of turbulent transfer over very rough surfaces, where the large‐scale eddies may have a key role.
The structure of radiatively driven convection in stratocumulus
Authors: Nicholls S.
Published: February 2007

Data from a series of five research flights in marine stratocumulus are used to investigate the structure of radiatively driven, free convective layers. Conditional sampling methods are used to determine the properties of the primary convective elements: negatively buoyant downdraughts. The turbulent velocity data are also subjected to spectral analysis and the turbulent kinetic energy (TKE) balance is evaluated for each case. Results from each case are found to be very similar if scaled using appropriate mixed layer quantities.

Downdraughts are selected using a criterion based on vertical velocity. The distributions of their intersected widths at cloud top, and the velocity spectra, are consistent with downdraughts occupying narrow regions (∼0.1‐0.15 h wide, h being the mean mixed layer depth) around the periphery of larger, regular (∼0.5 h‐0.75 h diameter) updraughts as suggested by cellular patterns observed in the cloud tops. Downdraughts are found to carry over half of the total turbulent fluxes of heat and water substance within cloud, and also transport variance down the mean gradients. Their thermodynamic properties are consistent with their containing a small fraction of air from above cloud top although their negative buoyancy is almost entirely due to the incorporation of radiatively cooled air. The properties of the downdraughts near cloud top suggest they are formed primarily as a result of the local horizontal convergence of upwelling motions constrained by the inversion. While being cooled radiatively, these quasi‐horizontal motions scour the base of the inversion, incorporating some drier air before being forced downwards in relatively narrow zones. Comparisons with other convective layers over land or sea suggest that convection in stratocumulus proceeds with a greater mass flux and correspondingly reduced differences between convective elements and the environment, possibly reflecting the improved ventilation possible at an inversion interface compared with much rougher surfaces.

The TKE balance divides into three main regions and is consistent with the interpretations given above. The pressure transport is implied to be the largest source term close to cloud top. Comparisons with model results reveal some important differences.
Numerical modelling of the offshore extent of sea breezes
Authors: Arritt Raymond W.
Published: February 2007

Two‐ and three‐dimensional versions of a nonlinear atmospheric model were used to investigate environmental controls on the offshore influence of sea‐breeze circulations. The temperature of the water surface relative to the atmospheric temperature had a small effect when the water was colder than the overlying air, and a greater effect when the water temperature was high enough to create convective mixing in the planetary boundary layer over the water. Ambient thermal stratification, including elevated inversions with thermal stratification similar to trade wind inversions, had a small influence. The predicted offshore extent of the sea breeze was substantially affected by latitude. The effects of both ambient stratification and latitude were generally consistent with predictions of linear theory. Prevailing synoptic winds were also found to have a significant effect on the sea breeze offshore, with offshore influence greatly suppressed by onshore large‐scale flow. Curvature of the coastline produces slightly stronger winds offshore for a concave coast than for a convex coast. The subsidence offshore also extends further over the water for a concave coast than for a convex coast.

The model predictions were compared with previous investigations which used linear theory. It was found that the present results agreed with the linear theory of R. Rotunno with regard to the effects of latitude and ambient thermal stability. Linear theory is less suitable for explaining the effects of water temperature, owing to the simplified treatment of turbulent exchange necessary for a tractable linear model.
Monthly‐mean diabatic circulations in the stratosphere
Authors: Pawson S. , Harwood R. S.
Published: February 2007

Three‐dimensional fields of net radiative heating rate have been calculated from temperature, ozone and water vapour data observed by the LIMS instrument. Monthly‐mean distributions of net heating rates are presented for the months November 1978–May 1979; particular attention is paid to the distribution of net heating rates at two isentropic levels in the northern hemisphere, one near 30 km in the mid‐stratosphere, the other near the stratopause. The seasonal evolution of the heating rate is discussed in relation to the vortex position. We also consider standard deviations of the daily radiative heating distributions from the monthly‐mean fields; large variability is found in the dynamically active months of the winter. An interhemispheric comparison of the net heating rate in November in the northern hemisphere with May in the southern hemisphere reveals similarities in the heating distribution at the lower level but differences at the higher level, where the southern hemispheric distribution is far more zonal than that of the northern hemisphere.

The continuity equation may be used to determine the divergent horizontal flux of mass at isentropic levels. Over long periods the transient component is found to be dominated by the diabatic component, so that the net radiative heating distributions can be used to obtain this divergence. Near the stratopause we find an anti‐correlation between the northward components of divergent and geostrophic winds.

The zonal‐mean diabatic circulation has also been calculated and found to agree with the rather broad range of results of several other recent studies. Knowledge of the three‐dimensional heating rates and tracer distributions has been applied to the problem of tracer transport by mean and eddy motions across isentropic surfaces. The mean advection is found to be dominant, a consequence of the eddy motions being almost adiabatic. The implication of this result is that it is the zonal‐mean diabatic circulation which is responsible for any systematic cross‐isentropic tracer transport, the eddies playing only a minor role.
A general method for the linear stability analysis of stratified shear flows
Authors: Mobbs S. D. , Darby M. S.
Published: February 2007

A general method for testing the stability of a stratified, parallel shear flow is presented. The technique allows the routine determination of unstable modes from any high resolution radiosonde data and is a development of the procedure described by Lalas and Einaudi. Stability analyses of several idealized velocity profiles have been performed in order to validate the method. A particularly important feature is the ability to predict marginally unstable modes with more than one critical level; several examples are presented. An example using real radiosonde data is given.
The formation of internal bores in the atmosphere: A laboratory model
Authors: Rottman James W. , Simpson John E.
Published: February 2007

Recently several atmospheric observations have been interpreted as internal undular bores or internal solitary waves evolving from internal bores that propagate along low‐level temperature inversions. It has been speculated that such disturbances are generated by some type of gravity current (such as cold fronts, sea breeze fronts and thunderstorm outflows) interacting with an existing temperature inversion. In this paper we describe a systematic laboratory study in a water channel, of internal bores and their generation by the movement of gravity currents through a two‐layer model of the atmosphere. We compare the results of our laboratory experiments with previous theories and numerical simulations and with several detailed atmospheric observations of internal bores at different stages of development.
A simple tropical moist model applied to the ‘40‐day’ wave
Authors: Davey Michael K.
Published: February 2007

The ‘40‐day’ wave is a baroclinic eastward‐propagating disturbance commonly observed in the tropical atmosphere, with a period that actually lies in the band 30 to 60 days. Its structure resembles an equatorially‐trapped Kelvin wave, predominantly with zonal wavenumber 1. In this paper a simple model with marginally stable moist static stability is used to investigate one possible mechanism for its appearance: namely, as the response to variable thermal forcing amplified by the effect of interactive latent heating.

The model has just one vertical baroclinic mode, forced by prescribed heating and by interactively‐determined latent heating. Latent heating only occurs when and where there is low‐level convergence and moisture is tending to increase beyond some saturation level: in such regions the static stability takes its moist value, which is considerably less than the dry value acting in regions without precipitation.

Analytic solutions are first provided for cases with static stability everywhere constant. Waves propagate more slowly when the moist value is used, so the peak linear response to wave‐like thermal forcing (essentially a resonant response) occurs at longer periods than for dry static stability. For dry conditions the period of a free Kelvin wave with zonal wavenumber one is about 10 days, but for wet conditions the period increases toward the ‘40‐day’ range.

When static stability varies the model must be solved numerically. Solutions show that the increase in period predicted analytically is indeed realized, even when precipitation is patchy and dry static stability is acting almost everywhere. Effectively the amplified waves travel at a speed consistent with the maintenance of their latent‐heating energy source. Eastward waves are prominent near the equator. By analysing particular moisture‐dependent combinations of variables, these eastward waves can be dynamically identified as Kelvin waves.

Saturation moisture content is closely linked to sea surface temperature. The warm western Pacific and Indian Oceans are regions of high atmospheric moisture content, and hence of very low moist static stability. This ocean temperature effect is studied using the simple model, showing that intraseasonal waves are largest in such regions, as observed.
Modelling wind‐generated surface gravity waves using similarity in a coupled discrete wave model
Authors: Perrie W. , Günther H. , Rosenthal W. , Toulany B.
Published: February 2007

In deep water, there is a growing observational consensus that supports an ƒ4 variation for the Kitaigorodskii subrange of the high‐frequency energy spectra. We therefore re‐examine and extend the TMA (Texel Marsen Arsloe) similarity and concepts for shallow water waves, which were originally derived for an ƒ5 Phillips subrange. The similarity for an ƒ4 a subrange is found to closely resemble that of the ƒ5 subrange. The former exists when nonlinear transfer due to wave‐wave interactions dominates over other source terms, such as wind input, in the energy balance.

Starting with a standard second generation deep water coupled discrete spectral wave model, we implement the TMA similarity. To evaluate our modifications, we perform the real and synthetic shallow water tests of the recent SWIM study. Thus we intercompare our model with HYPAS, an analogously modified deep water coupled hybrid model, and with other wave models which were not based on the TMA similarity. Our model and HYPAS are found to behave quite similarly in the sloping and flat‐bottom SWIM tests, compared to the other wave models of the SWIM intercomparison. The more difficult and complex North Sea SWIM hindcast extends this property, in some regards.
Assessment of the practical utility of extended range ensemble forecasts
Authors: Murphy J. M.
Published: February 2007

Lagged‐average forecast experiments, produced with the Meteorological Office 11‐level general circulation model, are used to assess the benefits arising from the use of ensemble forecasts in practical monthly prediction.

As measured by anomaly correlation, forecast skill is consistently positive at days 6‐15, but very small on average beyond 20 days. For the monthly mean forecast, removal of an independent estimate of the model systematic error improves the average score significantly. Compared to the mean score amongst individual members, a modest increase in skill is achieved by ensemble averaging. Theoretically, however, there remains scope for a much greater impact in the future, through the use of models capable of more accurate forecasts.

Local forecast skill is correlated with a measure of the agreement between ensemble members, although the overall relationship is weak beyond the medium range. Nevertheless, for areas of high agreement, the probability of obtaining useful skill is considerably higher than average at days 6‐15 and 11‐20.

The experiments are also assessed as probability forecasts, using the Ranked Probability Score to measure skill. the ensemble probability forecast (EPF) is created by averaging the probability forecasts of the individual members. Relative to the average score for the latter, the EPF gives improved skill. For the medium‐range period of the forecast (days 1‐10), the use of an optimized EPF, calculated by weighting each ensemble member according to its age, leads to a small increase in skill compared to the individual forecast from the most recent analysis. This effect is not observed in the corresponding anomaly correlation results. the above results for local skill prediction are confirmed, and somewhat enhanced, in the probabilistic formulation, using the ensemble variance of forecast categories as the predictor. A simple method of issuing confidence‐based adjustments to the forecast probabilities is discussed.
An analytical model of the growth of a frontal discontinuity
Authors: Holt M. W. , Shutts G. J.
Published: February 2007

An analytic semi‐geostrophic model of frontogcnesis is described which exhibits the growth of a frontal discontinuity surface. the initial distribution of temperature on a plane horizontal boundary consists of a warm two‐dimensional strip embedded in an isothermal background; the potential vorticity in the atmosphere above is uniform and positive. A barotropic deformation field with axis of dilation along the warm strip forces a discontinuity in the velocity field as the warm air is occluded and becomes detached from the lower boundary.

The solution demonstrates some important general properties of the inviscid semi‐geostrophic equations when discontinuities are present. In particular, the vertical velocity on the lower boundary becomes non‐zero at the front. the maximum value of the vertical velocity occurs here when the front first forms, but then lifts off the boundary. the front itself constitutes a line of infinite potential vorticity, so that the global integral of mass‐weighted potential vorticity is conserved only if the boundary of the domain is redefined to exclude the front. the implications of these properties for numerical modelling are discussed.

For typical atmospheric rates of deformation forcing, the accelerations of air parcels extremely close to the front are large enough to violate the conditions of validity of semi‐geostrophic theory. the conditions of hydrostatic balance are, for all practical purposes, never violated.
Estimates of cyclone track predictability. II: Fractal analysis of mid‐latitude cyclones
Authors: Fraedrich Klaus , Grotjahn Richard , Leslie Lance M.
Published: February 2007

A nonlinear systems analysis is applied to cyclone tracks over the North Pacific area (based on surface pressure analyses and twelve‐hourly sampling time) and over the Australian region (based on 500mb heights and daily sampling) for the respective winter seasons. First estimates are obtained of the degree of their chaotic or irregular behaviour in phase space and of the self‐affinity of the planar tracks. This information is deduced from distance distributions (correlation integrals) between independent pairs of cyclone trajectories and from the structure function of individual cyclone paths.

The correlation integrals reveal average structures of the substitute phase space (such as the dimension of the attractor, a measure of predictability) after stratifying the North Pacific tracks into a western domain of growing and an eastern domain of large amplitude cyclones. A further subdivision into large‐scale flow anomalies (the positive and negative Pacific/North America pattern in the eastern North Pacific) provides flowdependent estimates which indicate two subsystems differing in dimension and predictability. the time scaling analysis of the planar cyclone paths characterizes systems of no scale preference: the associated variance spectra decrease by a 2 to 3 power law with increasing frequency.
Frontal instability generated by tropospheric potential vorticity anomalies
Authors: Joly Alain , Thorpe Alan J.
Published: February 2007

The development of secondary disturbances on a two‐dimensional front is described using a linear stability analysis. A two‐stage process is envisaged in which, first, strong frontogenesis produces condensation at the front. This leads to the generation of a lower tropospheric zone of high potential vorticity (PV) lying along the front. As frontogenesis weakens, the second stage is entered in which an essentially steady front is susceptible to the growth of linear normal modes along the front owing to the existence of the PV anomaly or, equivalently, the frontal rainband. Thus for a typical band of width 160 km a frontal wave of some 800 km is produced with a growth rate of about 1 day−1. the dynamics of these modes is examined by considering both the realistic frontal flow as well as idealized strips of PV anomalies. the idealized case is potentially applicable to a variety of meteorological situations such as upper jet streaks. Useful insight is gained from an analysis of the energetics of the modes. This shows that for narrow frontal PV anomalies the waves main energy source is the basic kinetic energy and further they have a negative vertical heat flux. Recent observations suggest that frontal waves are indeed characterized by such a heat flux. As most frontal zones involve significant condensation, and therefore are likely to exhibit a lower tropospheric PV maximum, it is stressed that studies of frontal waves need to include this aspect of the frontal structure.
Buoyancy reversal and cloud‐top entrainment instability
Authors: Siems Steven T. , Bretherton Christopher S. , Baker Marcia B. , Shy Shenqyang , Breidenthal Robert E.
Published: February 2007

In 1980, Randall suggested that cloud top entrainment instability, a runaway positive feedback between mixing, evaporative cooling and entrainment, may cause the transition from the subtropical stratocumulus layers, frequently observed over the cold water of the eastern oceans, to tradewind cumulus. Predicting how the mean position of this transition, and the associated large change in albedo, would vary with, and feed back on, a change in climate is important for any credible model of the climate system.

We have used numerical and laboratory simulations, as well as field observations, to study entrainment in buoyancy‐reversing systems. We consider a system in which an upper fluid stably overlies a second lower fluid; but some mixtures are less buoyant than either fluid. the laboratory results show that buoyancy reversal does not cause runaway ‘explosive entrainment’ unless D (the ratio of the density difference between the most cooled mixed parcel and the lower fluid to the density difference across the unmixed layers, a measure of CEI potential) exceeds 1.3, which is far greater than the range of D observed in stratocumulus. For D below this threshold, buoyancy reversal leads to weak circulations rather reminiscent of Rayleigh‐Bénard convection in the lower layer, but only slightly distorts the interface. For D > 1.3 an entrainment tongue of upper fluid penetrates a sizable fraction of the depth of the lower layer and entrainment is explosively rapid. Numerical experiments suggest that even when 0 < D < 1.3, entrainment can break up idealized subtropical marine stratocumulus within about an hour if radiation and surface fluxes are removed, but comparison of numerical simulations with the laboratory experiments suggests that the numerically simulated entrainment rate may be substantially too large in this regime owing to the inability to accurately produce and resolve the entraining eddies. Our observations of stratocumulus from FIRE typically had a strong stable layer above the cloud top inversions, leading to some ambiguity in defining D, but clearly show that stable stratocumulus layers often have 0 < D < 0.2. We suggest that sudden breakup of stratocumulus due to runaway entrainment is unlikely, but evaporatively enhanced entrainment may aid the transition from stratocumulus to tradewind cumulus in conjunction with some other mechanism, such as short‐wave absorption by liquid water, that can help dynamically decouple the cloud layer from the surface layer.
A perturbation analysis of turbulent flow through a porous barrier
Authors: Wilson J. D. , Swaters G. E. , Ustina F.
Published: February 2007

Earlier attempts to calculate the mean velocity field in flow about a thin, finite porous barrier (wall‐mounted or otherwise) are reviewed. By simplifying the governing mean vorticity equation and expanding the flow variables in powers of the pressure‐loss coefficient of the barrier (k,) we derive an analytical solution for the mean velocity field in unbounded and homogeneous turbulent flow through a finite barrier. This solution is compared with the earlier solution of Kaiser (1959) and with numerical solutions for windbreak flow (bounded, inhomogeneous turbulence), and it is shown that all these solutions demonstrate a rate of recovery of the mean velocity field which is slower than is observed. It is suggested that the weakness of all the solutions lies in the treatment of the Reynolds stresses. A further point of interest is that a useful approximation to the pressure field may be obtained by dropping the Reynolds stress terms and solving a simplified equation expressing a balance between streamwise advection, pressure gradient, and localized momentum removal at the barrier; this might prove useful for the solution of more general porous barrier flows.
A storm surge model for the Australian region
Authors: Hubbert G. D. , Leslie L. M. , Manton M. J.
Published: February 2007

A depth‐integrated ocean‐current model is developed to simulate currents and sea surface elevations on the Australian continental shelf. the nonlinear advective terms are included and the equations of motion are solved using an efficient split‐explicit finite difference scheme, which yields accurate and stable results. the model predictions are compared with observations over a four‐day period of both surface elevations and currents in Bass Strait during the passage of a typical cold front in April 1984.
A test of a semi‐implicit integration technique for a fully compressible non‐hydrostatic model
Authors: Cullen M. J. P.
Published: February 2007

A semi‐implicit scheme, in which the time‐step is limited only by the advective criterion, is analysed and successfully tested in a fully compressible non‐hydrostatic mesoscale model.
Entrainment instability and vertical motion as causes of stratocumulus breakup
Authors: Weaver C. J. , Pearson R.
Published: February 2007

Entrainment instability is thought to be a cause of stratocumulus breakup. At the interface between the cloud and the overlying air, mixtures may form which are negatively buoyant because of cloud droplet evaporation. Quantities devised to predict breakup, δ2, X and δm, are obtained from aircraft observations and are tested against cloud observations from satellite. Often, the parameters indicate that breakup should occur but the clouds remain, sometimes for several days. One possible explanation for breakup is vertical motion from passing synoptic cyclones. Several cases suggest that breakup is associated with the downward vertical motion from the cold air advected behind an eastward moving cyclone.
Simulating giant hailstone structure with a ballistic aggregation model
Authors: Lozowski E. P. , Brett M. , Tait N. , Smy T.
Published: February 2007

A ballistic aggregation model designed for thin film simulation in microelectronics has been adapted to simulate the rime growth behaviour of hailstones. The model is able to account for both lobe development and the occurrence of large air bubbles and voids. The microstructures predicted by the model and those observed in a thin section of a giant hailstone are seen to be both qualitatively and quantitatively similar, lending some support to the possibility that certain large hailstones may grow by the accretion of low density rime with subsequent soaking.
On the concept of exergy and available enthalpy: Application to atmospheric energetics
Authors: Marquet Pascal
Published: February 2007

The available enthalpy is an early form of the modern thermodynamic concept of exergy, which is the generic name for the amount of work obtainable when some matter is brought to a state of equilibrium with its surroundings by means of reversible processes.

It is shown in this paper that a study of the hydrodynamic properties of available enthalpy leads to a generalization of the global meteorological available energies previously introduced by Lorenz, Dutton and Pearce. A local energy cycle is derived without approximation. Moreover, static instabilities or topography do not prevent this theory from having practical applications. The concept of available enthalpy is also presented in terms of the potential change in total entropy. Using the hydrostatic assumption, limited‐area energetics is then rigorously defined, including new boundary fluxes and new energy components. This innovative approach is especially suitable for the study of energy conversions between isobaric layers of an open limited atmospheric domain.

Numerical evaluations of various energy components are presented for a hemispheric field of zonal‐average temperature. It is further shown that this new energetic scheme realizes a hierarchical partition of the components so that the smallest of those available enthalpy reservoirs are almost of the same magnitude as the kinetic energy. This is actually the fundamental property that induced Margules to define the primary concept of available kinetic energy in meteorology.
Regional air pollution, with special emphasis on Europe
Authors: Smith F. B.
Published: February 2007

The state of acidifying air pollution in Europe is reviewed, as is the ability and limitation of models to simulate concentrations and depositions. The lack of complete quality in emission data is often a significant problem in these modelling studies. Nevertheless, models, for all their uncertainties, can play an important, if not totally reliable, role in assessing the origins of acidic depositions on any area. Although emissions have changed over the years, generally having decreased over the last decade, the time trend in measured depositions appears to indicate no very simple response to these overall decreases: the unequal changes in different parts of the continent and year‐to‐year changes in weather preclude such simplicity. The total impact of acid deposition on the environment, which is reviewed, depends not only on the average magnitude but also on the range of magnitudes of the depositions, and in particular on the occurrence of severe episodes. Some discussion of the probability distribution function is given.
A diagnostic study of winter diabatic heating in the Mediterranean in relation to cyclones
Authors: Shay‐El Y. , Alpert P.
Published: February 2007

Diabatic heating and net condensation rates during winter are calculated over the Mediterranean and surrounding areas from the 1982‐88 ECMWF initialized analyses using the residual method for the heat and moisture budgets. The steady‐state contributions over the Mediterranean are found to be prominent as in the tropics, but contrary to the case of the tropics the transients contributions in the Mediterranean, attributed to cyclonic activity, are not at all negligible.

A composite Cyprus low was constituted from 67 typical cyclones in the eastern Mediterranean. The diabatic heating and net condensation distribution indicate vigorous sea fluxes especially in the lee of the Turkish mountains over the Aegean Sea where the air‐sea contrast is maximized. In contrast with other regions, the maximum in the vertical profile of diabatic heating due to latent heat release is found at a lower altitude of about 850 hPa. It is suggested that this fact has important implications in modelling Mediterranean cyclones.
Morphology and density of ice accreted on cylindrical collectors at low values of impaction parameter. II: Rotating depositsxs
Authors: Prodi F. , Levi L. , Nasello O. B. , Lubart L.
Published: February 2007

The morphology and density of deposits grown on rotating cylinders of 0.5 cm radius, collecting droplets of median volume radius Δ = 10 μm, are studied. The wind speed and temperature are regulated so that Macklin's parameter and Stokes number are 0.5 < X < 7 μm m s−1 K−1 and 0.8 < K < 3.2 respectively. The knobby structure typical of rotating deposits is shown to form after the formation of the first 0.5–1 mm rime layer. Pronounced lobes, separated by wide entrances are observed at K < 3. Entrances change into air gaps for larger K values, as is shown also by comparison with previous results obtained for K > 4.

Rime density measurements are made by the X‐ray micrography technique. The first rime layer and the lobe interior density is represented by a ρ(X) curve located between those of Prodi and Levi and Macklin. The former is approached by the experimental point at X = 6.5, K = 3.2. The mean deposit density is evaluated by averaging the results obtained by scanning the X‐ray micrographs, at different distances d from the point of deposit initiation. The mean density is shown to decrease markedly with d for K ± 3, as a consequence of the knobby structure, so that the average density of deposits 3–4 mm thick approaches Macklin's curve. This effect is very much reduced for K > 4, where lobes are separated by thin air gaps.

These results are discussed by taking into account the shadow effect, and its dependence on K. Formulae for ρ(X) for different values of K and deposit thickness are proposed.
The structure of turbulence in a coastal atmospheric boundary layer
Authors: Shao Yaping , Hacker Jörg M. , Schwerdtfeger Peter
Published: February 2007

This study, based on airborne observations obtained during the Upper Spencer Gulf experiments in South Australia, investigates the structure of turbulence in a coastal boundary layer, which represents an extreme situation of horizontal inhomogeneity. The development of the coastal boundary layer under mainly onshore wind conditions, which is characterized by the formation of a convective thermal internal boundary layer (TIBL), is illustrated using cross‐sections of potential temperature, sensible‐heat fluxes and turbulent kinetic energy. The similarity of turbulence within the TIBL and the effectiveness of local scaling for inhomogeneous conditions are examined. It is suggested that, even in advective conditions, turbulence can be considered as being in local equilibrium. By introducing a simple TIBL model, local similarity relationships can be linked to external parameters. Some turbulence properties, such as the variance of vertical velocity, are found to be determined by three external parameters: the surface heat flux over land, the onshore wind speed and the thermal stability of the approaching airflow.
An Idealized Simulation of the Indian Monsoon Using Primitive‐Equation and Quasi‐Equilibrium Models
Authors: Mawson M. H. , Cullen M. J. P.
Published: February 2007

An idealized simulation of the Indian monsoon is performed using a prescribed heat source representative of the time‐averaged latent heating due to convection over India, and a simple representation of the east African orography. A quasi‐equilibrium model is used to solve this problem in three dimensions on a sphere. the model is shown to be able to simulate the cross‐equatorial flow and monsoon circulations driven by this problem and to be in good agreement with parallel integrations carried out with a hydrostatic primitive‐equation model except in areas affected by orographically forced gravity waves. the solutions given by these time‐dependent models are in good agreement with those found by using linearized steady‐state equations.
Potential Vorticity Index Vacillation In the 1978/79 Winter: Its Relation to Teleconnection Patterns
Authors: Weng Heng‐Yi
Published: February 2007

A potential vorticity (PV) index is defined as a measure of the zonally averaged, raid‐latitude PV gradient on the 300 K isentropic surface in the northern hemisphere, and use is made of the 1978/79 wińter ECMWF FGGE Hl‐b data set to study the evolution of that index and its relation to teleconnection patterns of 500 mb geopotential‐height anomaly. This index vacillates with a dominant period of about two weeks. For some time lags, the isoline patterns of the cross‐correlation function between the PV index and 500mb geopotential‐height anomalies are similar to some teleconnection patterns studied by several investigators using large climatological data sets. Scandinavia, Greenland and the Pacific are important centres of ‘action’ in the northern hemisphere in the 1978/79 winter. the formation of blocking highs at these locations occurs at rather specific phases during the vacillatory evolution of the PV index, especially as the flow transits from a low to a high PV index.

When the above study is duplicated using a zonal mean wind gradient at 500mb as a zonal index, the results do not reflect the teleconnection patterns as crisply as the PV index does for the FGGE year.
Reply to Comments By C. B. Moore and B. Vonnegut: Further Analysis of Two Regions of Charge During Initial Thunderstorm Electrification
Authors: Dye J. E. , Jones J. J. , Weinheimer A. J. , Winn W. P.
Published: February 2007

In response to the comments of Moore and Vonnegut we have thoroughly re‐examined the aircraft and radar measurements made in two small thunderstorms in New Mexico originally reported by Dye et al. (1988). Our conclusions remain the same. Although pressure altitude was used instead of geometric altitude in one case and distance labels were interchanged for a vertical section used to illustrate the other case, improved calculations for the particle image measurements and the ice particle collision rate show that the two regions of charge were in or near locations with appreciable supercooled liquid water, ice particle concentrations and graupel, and also near the top of reflectivity cores as originally stated. As before, we conclude that, given this spatial correlation of the inferred charge with ice particles and liquid water, the observations are consistent with what is expected from the non‐inductive ice‐collision charge separation mechanism of thunderstorm electrification. However, because the data are incomplete we cannot preclude the possibility that one of the regions of charge (on 15 August 1984) was the result of positive ions being attracted to the cloud by a downward‐directed field, as suggested by Moore and Vonnegut.
An Integrated View of the 1987 Australian Monsoon and Its Mesoscale Convective Systems. I: Horizontal Structure
Authors: Mapes Brian , Houze Robert A.
Published: February 2007

This observational study is organized around three issues: the forcing of the monsoon flow by embedded deep convection, the thermodynamic conditions that support the convection, and the structures of the mesoscale convective systems (MCSs) that constitute much of the deep convection.

The monsoon convective cloudiness occurred predominantly offshore, over the earth's warmest waters. Week‐long periods of widespread deep convection caused cycles of monsoon spin‐up, culminating in tropical‐cyclone formation. Momentum transports by the convection also created smaller‐scale vortex pairs in the upper troposphere.

Sounding data suggest that the convection was modulated by low‐level processes, not by large‐scale deep forced ascent. Triggering by mesoscale boundary‐layer cold‐pool boundaries and coastlines determined specifically where convection occurred, while several positive feedbacks acted to keep regional thermodynamic conditions favourable. Hence monsoon convection, once initiated, persisted as self‐exciting ‘superclusters’, composed at any instant of many distinct MCSs.

The observed MCSs all contained areas of deep convection and stratiform precipitation areas. Stratiform precipitation usually evolved in place from convective cells, although precipitation also occasionally fell from overhanging anvils created by upper‐level shear. the MCSs within similar synoptic wind environments tended to have similar mesoscale structures. This fact reflects the effects, illustrated herein, of environmental winds upon the relative motions of cold pools (which trigger new convective cells) and stratiform precipitation areas (which evolve from old convective cells).
On the behaviour of baroclinic waves undergoing horizontal deformation. I: The ‘RT’ phase diagram
Authors: Bishop Craig H.
Published: February 2007

Non‐modal baroclinic wave growth in an idealizsed flow comprising baroclinic shear and horizontal confluence is considered. It is shown how a general semi‐analytic solution may be derived for the evolution of non‐modal waves with uniform potential vorticity in this ‘mixed’ basic state. The solution is new. It represents a synthesis of baroclinic waves with pure deformation solutions.

For a given initial wavelength the solution is capable of describing an infinite number of distinct non‐modal developments. To overview this large variety, phase diagrams, dubbed ‘RT’ diagrams, have been developed. The diagrams depict the evolution of vertical structure for a wide range of initial conditions. Some specific solutions with relevance to type B cyclogenesis and the potentially destablizing effect of deformation are discussed.
A case study of lee waves over the Lake District in northern England
Authors: Shutts Glenn , Broad Adrian
Published: February 2007

The structure and intensity of stationary, orographically‐forced over the Lake District and northern Pennines on he afternoon of 26 November 1991 is determined through a combination of field observations and high‐resolution numerical modelling. Observations were provided by the Meteorological Office's C130 aircraft at four different levels and by five radisondes (of differing ascent rates) released in rapid succession from an observing station located on the Cumbrian coast. Vertical velocity was inferred from both of these data sources and, with the help of satellite imagery, it was possible to build up a comprehensive picture of the lee wave structure.

In addition, a non‐hydrostatic primitive equation model was run with a 2‐km grid, covering a 180‐km square region of the northern Pennines, in an attempt to simulate the gravity wave motion observed. The model was initialized with an upstream profile formed by merging a descent profile obtained from the aircraft with one from the sondes. The model orography was defined using a terrain height data‐set with 500‐m resolution and with some smoothing to the model grid. After about four hours of integration the model achieves a quasi‐steady state with strong lower‐tropospheric lee waves.

Verification on the model response was sought by comparing the vertical velocity in the model along the paths of the aircraft and sondes with that actually deduced from the observations. The overall agreement of the simulated amplitude and phase of the lee waves with that observed is very encouraging—as found in a different case study (Shutts 1992). The success of the model in predicting lee wave motion gives strong support for the use of these models for the development of gravity‐wave‐drag parametrization schemes.

The domain‐averaged surface frictional stress was found to be much larger than the wave drag, and consistent with the high roughness lengths likely in the case of mountainous terrain.
Effective resistance to sensible‐ and latent‐heat flux in heterogeneous terrain
Authors: Blyth E. M. , Dolman A. J. , Wood N.
Published: February 2007

Numerical‐weather‐prediction models resolve mean variable at the scale of their grid length only. Effective surface parameters must, therefore, be used to relate the mean properties of humidity, temperature and wind speed to the mean surface fluxes. The effective parameters can be calculated using a model with a finer grid. In this paper effective aerodynamic and surface resistances to sensible‐and latent‐heat flux for a surface which varies over a horizontal scale of one kilometre are calculated from a two‐dimensional model with a grid size of 30m, and compared with simpler averaging techniques. The averaging techniques considered include taking the mean of the aerodynamic and surface resistance in parallel or in series, the mean of the solutions given by these two, and a heuristic model that treats the different surface covers independently but assumes that they are exposed to the same mean meteorological variables at a blending height. The heuristic model consistently estimates the effective resistances to within 25% but, in comparison to the other methods, requires more information. For heterogeneous surfaces, the effective parameters for momentum and sensible‐ and latent‐heat transfer do not bear the same relationship to each other as they do for homogenous surfaces. Effective parameters for each type of flux, therefore, have to be calculated separately. In situations where negative sensible‐heat fluxes occur over rough ground, the effective resistance to sensible‐heat flux can be very large, becoming underfined in the extreme case where the area‐average sensible‐heat flux is zero.
On the response of the tropical atmosphere tio large‐scale deforestation
Authors: Eltahir E. A. B. , Bras R. L.
Published: February 2007

Recent studies on the Amazon deforestation problem predict that removal of the forest will result in a higher surface temperature, a significant reduction in evaporation and precipitation, and possibly significant changes in the tropical circulation. Here, we discuss the basic mechanisms contributing to the response of the tropical atmosphere to deforestation. A simple linear model of the tropical atmosphere is used in studying the effects of deforestation on climate. It is suggested that the impact of large‐scale deforestation on the circulation of the tropical atmosphere consists of two components: the response of the tropical circulation to the negative change in precipitation (heating), and the response of the same circulation to the positive change in surface temperature. Owing to their different signs, the changes in predicted temperature and precipitation excite competing responses working in opposite directions.

The predicted changes in tropical circulation determines the change, if any, in atmospheric moisture convergence, which is equivalent to the change in run‐off. The dependence of run‐off predictions on the relative magnitudes of the predicted changes in precipitation and surface temperature implies that the predictions about run‐off are highly sensitive, which explains, at least partly, the disagreement between the different models concerning the sign of the predicted change in Amazonian run‐off.
The exchange of atmospheric ammonia with vegetated surfaces. II: Fertilized vegetation
Authors: Sutton M. A. , Fowler D. , Moncrieft J. B. , Storeton‐West R. L.
Published: February 2007

Measurements of the exchange of gaseous ammonia (NH3) have been made over fertilized plant canopies using micrometeorological techniques. The NH3 exchange over ungrazed grassland and arable crops was examined in a range of environmental conditions: a bidirectional exchange of ammonia was observed, with warm dry conditions favouring emission (NH3 fluxes up to 24 ng m−2s−1) and cool wet conditions favouring deposition (fluxes in the range 1–35 ng m−2s−1). Limited measurements made when the ground was frozen suggest an increased surface resistance may occur in these conditions (Measured Rc up to 80 s m−1), resulting in reduced rates of deposition.

The fluxes are contrasted with measurements over unfertilized vegetation, and it is suggested that the net canopy exchange is the combined result of leaf surface, stomatal and soil exchange processes; the increased tendency of fertilized vegetation to emit ammonia is attributed to the larger fixed nitrogen concentrations in these ecosystems. The bidirectional fluxes may be related to the existence of ammonia compensation points. Estimates of the stomatal compensation point for fertilized croplands were in the range 1‐7 μg NH3m−3, though because of deposition to leaf surfaces, this frequently does not control net fluxes. The Mean surface concentration of ammonia X(z'o), or canopy compensation point, was therefore also estimated and found to vary, with environmental conditions, in the range 0‐3 μg m−3. A simplified procedure is used to estimate the net annual exchange of ammonia over an example fertilized crop in the British Isles. This predicts an emission of fixed nitrogen of less than 1 kg per hectare, which is the net result of emission and deposition throughout the year.