bluuebunny/crossref_metadata_2025 huggingface dataset:
Identification of preferred areas of shower development by means of high power radar
Authors: Harrold T. W. , Browning K. A.
Published: February 2007

A high‐power, high‐gain radar has been used to investigate clear air echoes on days when surface heating produced low‐level convection. The echoes were from irregular refractive index gradients produced primarily by gradients of water vapour density of small scale (5 cm). It was observed that the height to which the convection extended was revealed by an almost continuous layer of echo. This layer was perturbed on a scale of kilometres by individual convective elements; in addition Areas of Deeper Convection (termed ADCs) some tens of kilometres across occurred. These features are described in this paper. Individual ADCs were identifiable as entities for several hours, and, on the one occasion when reliable movements could be determined, they travelled with the low‐level wind. When the vertical structure of the atmosphere was such that showers occurred, they developed only within ADCs. Evidently the showers occurred in clusters because of the pre‐existing structure of the convective layer and not because individual showers modified their near environment.

On two of the four occasions studied the ADCs could be related directly to topographical features. In one case of negligible wind an ADC developed and remained close to hills; on another occasion a plume‐like area of deeper convection over 100 km long appeared to stream away from high ground. More generally, however, advection of ADCs masked possible topographical influences.

It is suggested that by using a high power radar to identify ADCs it may be possible to forecast the location of outbreaks of showers and thunderstorms more precisely than is possible at present. Further lines of research are also proposed.
Mechanisms influencing the distribution of precipitation within baroclinic disturbances
Authors: Harrold T. W.
Published: February 2007

An analysis of the flow within baroclinic disturbances over and near the British Isles is made assuming that the wet bulb potential temperature (θw is a conserved property of the flow. Conservation of mass and, when the flow is dry, of mixing ratio, are also used as constraints in the analysis. A model of the flows significant for the production and distribution of precipitation is derived. It is shown that most of the precipitation reaching the surface initially forms within a single well‐defined flow, labelled the ‘conveyor‐belt’ which is typically a few hundred km wide and a few km deep and which flows parallel to and immediately ahead of the surface cold front. The speed of ascent above the warm frontal zone is of the order of 10 cm s−1. Ascent often begins within the warm sector so that there is no well‐marked discontinuity between the precipitation within the warm sector and that in advance of the surface warm front. This flow has been identified as the most significant flow producing widespread precipitation in a variety of synoptic situations, including some which would be analysed conventionally as non‐frontal.

Potential instability is continually generated by differential advection in advance of the cold front as a result of over‐running of the low level flow by mid‐tropospheric air of lower θw. The instability is gradually released as the low‐level flow ascends. The leading edge of the warm‐frontal precipitation at the surface is eroded by the evaporation within a descending flow beneath the warm‐frontal zone. When the melting level is low, or the air entering the system on its forward side is dry, the evaporation causes the width of the area of precipitation at the surface to be several hundred km less than that aloft. Some of the moisture which is evaporated is eventually re‐precipitated farther north as the air ascends close to the surface warm front.

The mesoscale variability of the airflow within the large‐scale flow modifies the distribution of precipitation. Various structures which have been identified in many studies are summarized and a model of the mesoscale airflow within the conveyor belt is deduced.
Measurements and interpretation of the polarization of radiation emerging from the atmosphere at an altitude of 28 km over South—Western New Mexico (USA)
Authors: Rao C. R. Nagaraja , Takashima T. , Toolin R. B.
Published: February 2007

The degree of linear polarization of the upward radiation emerging from the atmosphere at an altitude of 28 km was measured over south‐western New Mexico (USA) with a balloon‐borne photoelectric polarimeter. Measurements were made in four spectral intervals (bandwidth ∼ 15 nm) centred on λλ362, 401, 501 and 599 nm. The objective was to estimate the turbidity factors of simple, homogeneous models of turbid atmospheres from a comparison of the experimental results with computed values of the polarization of the radiation emerging from the top of the model atmospheres. The computations were based on the solution of the radiative transfer problem in a homogeneous turbid atmosphere. The ‘doubling’ method was used to obtain the solution. It is found that the optimal values of the turbidity factors required to establish good agreement between theory and experiment vary directly as the phase function asymmetry factors associated with the two types of aerosols used in the models. The uncertainty in the interpretation of the polarization data due to the similar effects that the atmospheric aerosols and the unpolarized part of the radiation reflected by the ground have on the polarization of the scattered radiation is briefly discussed.
Some comparisons between observed wind profiles at Risø and theoretical predictions for flow over inhomogeneous terrain
Authors: Petersen E. L. , Taylor P. A.
Published: February 2007

Velocity profiles observed in near neutral conditions at the Risø 125 m tower are presented. The upwind terrain contains, depending on wind direction, either one or two distinct changes in surface roughness.

Comparisons are made between the observed profiles and those predicted using models based on mixing‐length theory, and, in the case of a single roughness change, the turbulent energy equation. Comparisons with the mixing‐length model are moderately good but some features of the observed profiles are missing from the theoretical predictions. The particular turbulent energy equation model used appears to give slightly less accurate predictions than the mixing‐length model.
Radiative properties of terrestrial clouds at visible and infra‐red thermal window wavelengths
Authors: Hunt G. E.
Published: February 2007

A detailed theoretical study of the radiative properties of water droplet and ice clouds at visible and infra‐red wavelengths of 2·3, 3·5, 3·8 and 8·5 and 11 μm is presented. The radiative transfer computations have used a model atmosphere in which the microphysical properties of the clouds have been accurately incorporated. A range of physically realistic size distributions for the cloud particles have been used. The results have been presented in the form of tables of the emissivity, reflectivity and transmissivity, and their equivalent fluxes, at a particular wavelength, as a function of the optical thickness of the cloud layer. A simple scaling relationship enables these extensive tables to be used for any water content of the cloud layers. The results provide a detailed understanding of the radiative properties of terrestrial clouds at thermal infra‐red window wavelengths.

The theoretical results are used to interpret infra‐red observations of stratocumulus and cirrus clouds.
Ozone trend and haze scattering
Authors: Kulkarni R. N.
Published: February 2007

Direct sun observations made with Dobson spectrophotometers at Darwin, Brisbane, Aspendale and Macquarie Island in Australia are used to assess the changing character of haze in the atmosphere using (XAD — XA) values which represent the large particle (larger than air molecules) scattering. It has been observed that at Darwin and Brisbane (XAD — XA) decreases from 1960 to 1970 implying that the smaller size particles are increasing in number in relation to larger size particles. An assessment of the error in the calculation of ozone by this gradual change of haze scattering in the last decade has been made. It is estimated that about 53 per cent of the observed change in XAD value at Brisbane is possibly due to changing haze pattern.
Global distribution of double cloud bands over tropical oceans
Authors: Saha K. R.
Published: February 2007

Available observational evidence regarding occurrence of double cloud bands, one in each hemisphere, over tropical oceans is reviewed. An examination of their locations appears to suggest that these may be related to maxima in ocean surface temperature. Significant asymmetry and discontinuity are noticed over areas which are affected by cold ocean currents and monsoons.
The turbulent transfer mechanisms: A time domain analysis
Authors: McBean G. A.
Published: February 2007

The turbulent transfer processes near the surface were investigated through time domain statistical analyses of the fluctuations of horizontal and vertical wind and temperature. The joint probability density functions for (w,u) and (w,T) and the probability densities, flux‐angle distributions and fluctuation‐magnitude‐and‐duration distributions for wu and wT were computed. The results showed that the transfer mechanisms for momentum and heat fluxes were different and these differences increased with instability. The effects of buoyancy are to lead to highly efficient heat transfers but to very inefficient momentum transfers. For z/L = −0.5 large amounts of momentum are transferred both up and down by large fluctuations in wu but the net transfer is relatively small and usually downward. The heat transfer process, on the other hand, is more uniform in time and the large fluctuations of wT are just about all upward.
Corona from colliding drops as a possible mechanism for the triggering of lightning
Authors: Crabb J. A. , Latham J.
Published: February 2007

Measurements have been made of the values of electric field Ec, required to produce electrical corona from a pair of water drops, of radii R = 2.7 mm and r = 0.65 mm colliding with a relative velocity of 5.8 ms−1. For central collisions the larger drop went into the ‘bag‐mode’ while for more glancing collisions a liquid filament, typically several times R in length, was drawn out between the separating drops, and eventually collapsed. The values of Ec decreased with increasing values of L ‐ the length of the combined drop‐pair in the direction of the field at the moment of separation ‐ which itself increased as the collisions became less central. Ec ranged from about 500 kV/m for head‐on collisions to about 250 kV/m for glancing collisions producing the longest filaments. Only a small number of corona pulses ‐ each carrying about 10−10 ‐ accompanied each collision producing a discharge.

Subsidiary experiments with model drops reinforced the conclusion that corona resulted initially from a point on the drop surface which had been disrupted by mechanical or electrical forces.

Calculations indicate that the emission of positive corona from colliding raindrops can readily occur in thunderclouds, and thereby possibly trigger lightning, in fields below about 350 kV/m; a value considerably lower than that resulting from any other natural triggering process so far studied.
The effect of model grid length and orographic rainfall ‘efficiency’ on computed surface rainfall
Authors: Collier C. G.
Published: February 2007

Surface rainfall distributions over North and South Wales are derived using a numerical parameterization model. Several different grid lengths are used which allow the dependence on grid length of areal rainfall produced by the model to be examined. It is found that as the grid length increases from one to ten kilometres areal rainfall totals predicted by the model decrease, in some cases quite rapidly. the areal rainfall totals also depend upon the amount of orographic enhancement operating in particular cases owing to cloud physics processes which are not parameterized in the model. A suggestion is offered as to how these processes may be taken into account in the model.
Laboratory studies of the effects of mixing on the evolution of cloud droplet spectra
Authors: Latham J. , Reed R. L.
Published: February 2007

Experiments were performed in which a cloud of droplets formed by condensation was drawn slowly down a cylindrical tube of length 5 m. the effect on the spectrum of mixing‐in humidified but undersaturated air was studied.

The most striking observation was that the mixing process was highly inhomogeneous. Some droplets from all size categories were completely evaporated, while others were not significantly affected by the admixture of undersaturated air. the dispersion of the observable spectrum remained essentially constant. If this spectrum, with substantially reduced water content, was then allowed to grow by producing a supersaturation the measured dispersion increased appreciably. Calculations showed that the spectral evolution observed in a selected experiment was inexplicable in terms of homogeneous mixing, as envisaged by other workers. However, it could be explained by a rudimentary model of inhomogeneous mixing, which was found to offer a qualitative explanation for features of cloud droplet evolution observed by Warner, surrounding which there has been considerable recent discussion. This mixing process can be considered in terms of fluctuations in supersaturation not tied to fluctuations in vertical air velocity.
Studies of the ozone budget using a zonal mean circulation model and linearized photochemistry
Authors: Harwood R. S. , Pyle J. A.
Published: February 2007

A two‐dimensional, time‐dependent numerical model of the atmosphere has been used to investigate the ozone budget. the photochemistry includes catalytic destruction of ozone by the oxides of hydrogen and nitrogen with the source and sink terms linearized about the equilibrium values. the behaviour of the modelled ozone is quite satisfactory especially in low and mid‐latitudes. Many of the observed details have been reproduced ‐ including hemispheric asymmetries of the total ozone amounts.

The salient features of the ozone budget are described. There is photochemical production in the mid‐stratosphere in low latitudes, with destruction in the high latitudes of both hemispheres. the mean motion fluxes are the most important transport term in equatorial latitudes. Polewards, the eddies become relatively more important. A feature of the model is the near‐cancellation between mean and eddy flux divergences in middle and high latitudes. the vertical eddies play only a minor role.

The hemispheric asymmetry in the total ozone amount results in part from the high‐latitude mean circulation driven by the eddy fluxes of angular momentum. Tropospheric processes also are responsible for some differences between the hemispheres.
African rainfall and its relation to the upper air circulation
Authors: Kidson John W. , Newell R. E.
Published: February 2007

The rainfall distribution over Africa analysed for the period 1951–1975 shows a downward trend at 15°N not present at other latitudes. A widespread reduction in rainfall is observed during the drought years 1972 and 1973. Upper air analyses have been used to construct mean monthly vertical motion patterns and water vapour budgets but the results are not sufficiently precise to examine year‐to‐year differences. Changes in the August flow patterns show that low rainfall in the Sahel is associated with the virtual disappearance of the 850‐mb trough near 8°N and weakening of the easterly jet above it. an empirical orthogonal function analysis of the 850‐, 500‐ and 200‐mb temperatures from 150 stations shows that these changes are global in extent. the downward trend in the Sahel rainfall has evidently been paralleled by a weakening of the northern hemisphere circulation.

These results suggest that local modification of surface conditions is not the principal cause of the decline in Sahel rainfall over the last two decades.
Energy dispersion in a barotropic atmosphere
Authors: Hoskins B. J. , Simmons A. J. , Andrews D. G.
Published: February 2007

The propagation of energy on the sphere as described by the linear barotropic vorticity equation is studied. For simple flows, the concept of group velocity is examined. Planetary‐scale vorticity anomalies exhibit a surprising lack of dispersion in a hemispheric domain. On the sphere, there is more dispersion and a significant cross‐equatorial propagation of energy. Planetary‐scale vorticity sources tend to produce an orderly downstream train of waves which exhibit a strong tilt (NE‐SW for sources in the northern hemisphere) and cross the equator. Zonally elongated sources produce an almost north‐south train of waves. For realistic zonal flows with equatorial easterlies, large‐scale transients cross the equator, but the long‐term forced response in the opposite hemisphere is small.

Comparison of the results with observations of atmospheric wave behaviour is made, and some possible implications are presented.
Vertical heat flux in the convective boundary layer
Authors: Caughey S. J. , Kaimal J. C.
Published: February 2007

The variation of heat flux with height in inversion‐capped convective boundary layers is discussed. Cospectral shapes at levels above approximately one‐tenth the height of the lowest inversion base (z1) are shown to depart significantly from surface layer forms. the characteristic wavelength for heat flux increases rapidly in the first 150 m of the boundary layer but shows little variation between 150 and 300 metres. Around sunset, the transition to negative heat flux occurs first in the upper regions of the boundary layer and propagates downwards to the surface.
A new look at the ω‐equation
Authors: Hoskins B. J. , Draghici I. , Davies H. C.
Published: February 2007

In the conventional quasi‐geostrophic form of the ‘omega’‐equation, the forcing of vertical velocity is usually expressed as the sum of two terms associated respectively with vorticity and temperature advection. Consideration of each term in isolation is misleading and there can be a large degree of cancellation. On the other hand, in Sutcliffe's development theory, this forcing is, in effect, represented by a single term. However, this is achieved at the expense of neglecting another term which is dominant in frontal regions. An investigation, based upon the governing equations, of the manner in which geostrophic balance tends to destroy itself, reveals a simple, concise, one‐term representation of the geostrophic forcing of ageostrophic motion. Many of the traditional synoptic rules are then simple deductions from this theory. An application of the theory in the case of a rapidly developing system is demonstrated using a 700mb chart.
Time‐resolved hailstone analyses and radar structure of Swiss storms
Authors: Federer Bruno , Waldvogel Albert
Published: February 2007

Time‐resolved, quenched, hailstone size distributions from five hailstorms simultaneously monitored by 10cm and 3cm radar (in the PPI and RHI mode respectively) have been measured. A total of 1220 hailstones have been sectioned and the embryo types (rimed embryos or frozen drop embryos) were classified according to the crystallographic and bubble structure of the centre unit and the initial growth layer. For three hailstones the trajectories deduced from crystallography have been checked by deuterium analyses.

The most important findings were: (1) In storms with relatively warm cloud bases a big‐drop zone containing large drops which freeze and become efficient hail embryos exists. (2) The time variation of the parameters of the hailstone spectra reveals the fundamental storm dynamics (multicell or steady state). (3) Variability of embryo types is smaller in steady‐state storms than in multicellular storms, implying that hail formation zones are regions of rather uniform temperature and liquid water content in steady‐state storms whereas in multicell storms growth occurs in a more heterogeneous environment, possibly in different updraught branches. (4) The two embryo growth mechanisms – riming, and freezing of supercooled drops – coexist in the storms. (5) A characteristic change of embryo type with time is observed consistently in the storms. (6) The size frequency distributions of those frozen drops whose diameters could be measured, showed a maximum at 2mm; this can be explained by ‘freezing’ an exponential spectrum of raindrops according to the stochastic hypothesis.
Quantitative bistatic acoustic sounding of the atmospheric boundary layer
Authors: Caughey S. J. , Crease B. A. , Asimakopoulos D. N. , Cole R. S.
Published: February 2007

In the bistatic configuration, acoustic echoes from the atmospheric boundary layer are determined by the magnitude of the structure parameters for wind and temperature, C and C respectively. Normally the contribution from C is relatively small and so this arrangement offers a remote method for determining C, and hence the rate of dissipation of turbulent energy, ϵ. A quantitative assessment of this technique is presented. Direct estimates of ϵ and C have been made using turbulence probes attached to the tethering cable of a large (1300m3) balloon. Sounder‐derived values were obtained using two carefully calibrated acoustic arrays. Results obtained in both convective and stable conditions are described and contrasted.
A laboratory study of charge transfer accompanying the collision of ice crystals with a simulated hailstone
Authors: Marshall B. J. P. , Latham J. , Saunders C. P. R.
Published: February 2007

In a series of experiments, conducted inside a cold‐room, clouds of small ice crystals of mean diameter 10μm were drawn at velocities between 1.5 and 18ms−1 past simulated hailstones. Two of these ice targets, side by side but under different conditions, were subjected to crystals from the same clouds simultaneously, so that the effects of various parameters on charge transfer could be isolated. The concentrations, sizes and shapes of the ice crystals were determined.

A target heated radiatively with respect to the crystals became charged more negatively than an unheated target. A colder target became charged more positively. However, the magnitude of the charging did not depend on the temperature difference between target and crystals, provided this was greater than about 1 K. Attempts to find evidence of discharge‐limited charging produced negative results. A strong velocity dependence of charging currents was found, but subsidiary experiments on the collection of crystals by the target suggested that this was largely due to the variation in the probability of separation of the crystals, which appeared to be close to zero at the lowest velocities employed. The effects of doping the targets with HF and NH3, and of surface roughness, were investigated and were found to be small.

These results were found to be in reasonable qualitative and quantitative agreement with the predictions of the charging mechanisms proposed by Auf der Maur and Buser, involving differences in the work functions of ice surfaces of different nature.

It was estimated that the charge, q', transferred in a rebounding collision between an ice crystal and a heated ice target is about 900 aC. It is shown that this is roughly compatible with the general pattern of results of other workers (with one exception) if the charging increases with the square of the diameter, d, of the small particles. There are good theoretical reasons for such a relationship, which may be described by the equation q'(aC)=5d2, for 10μm⩽d⩽200 μm. Charge transfer of this magnitude is estimated to be just sufficient to account for the electrification of thunderclouds.
Flux profile relations above tall vegetation
Authors: Garratt J. R.
Published: February 2007

Experimental data on the Monin and Obukhov functions, ΦM(ζ) and ΦH(ζ), obtained above an aerodynamically very rough surface (trees and shrubs, with zo, the aerodynamic roughness length, =0.4 m) in unstable conditions suggest they depend upon a nondimensional height ξ = z/z0.

At the lowest height (ξ = 20), in neutral conditions, the inferred values of ΦM and ΦH(0) are 0.58 and 0.61 respectively, generally increasing as ξ increases to values not significantly different from unity at the greatest height (ξ = 85). Such values compare with the value unity found over ‘smooth’ surfaces at much greater values of ξ. This indicates that the depth of the transition layer for momentum transfer above this surface (=Zmin, the minimum height of validity of the neutral logarithmic law) is given by Zmins ≈ 4.5h, where h is the height of the main roughness elements; and for heat transfer Zmin ≈3h.

The results suggest that eddy diffusivities are considerably greater just above tall vegetation than at corresponding heights above ‘smooth’ surfaces.
Variability in the frequency of unusual weather over approximately the last century
Authors: Ratcliffe R. A. S. , Weller J. , Collison P.
Published: February 2007

Weather varies on many time scales. In this paper an attempt is made to answer the question: has the variability become greater in recent years? The matter is considered, over a period of about the last 100 years, from the point of view of: (a) surface pressure anomalies over much of the northern hemisphere and over a more limited area near Britain; (b) surface temperature for central England, and at three individual British stations; and (c) rainfall for England and Wales, and at three individual British stations. Time scales considered range from a pentad to a year. No trend towards increased variability of any of these elements is apparent over the period considered; the analysis does, however, highlight the unusual nature of the 1975–76 drought in southern England, and draws attention to exceptional changes of temperature from one 3‐month period to the next, embracing the winter of 1974–75 and the summers of 1975 and 1976.
Cyclogenesis in the lee of the Alps: A case study
Authors: Buzzi A. , Tibaldi S.
Published: February 2007

A case of deep and rapid cyclogenesis in the lee of the Alps is analysed by means of cross‐sections, isentropic maps and trajectories based on synoptic data, to investigate the three‐dimensional structure of the phenomenon and the nature of the processes that are responsible for it. The results of the analysis are also compared with available theoretical models. Considerations of the various scales involved in the development lead to a description of the phenomenon in terms of two distinct phases: a very rapid ‘trigger’ phase due to interaction between the frontal layer and the Alps, and a more usual ‘baroclinic development’ phase. Through a selection process typical of baroclinic instability, the lee cyclone acquires the observed horizontal and vertical scales and undergoes the normal life of a mid‐latitude depression. The insufficiency of the present synoptic network for a satisfactory analysis of such a meteorological phenomenon is also stressed.
Strongly stratified flow past three‐dimensional obstacles
Authors: Brighton P. W. M.
Published: February 2007

An experimental study has been made of the shear flow of stratified fluid of characteristic buoyancy frequency 𝒩 with characteristic speed 𝒰 past a three‐dimensional obstacle of height 𝒽 when the Froude number, Fr = 𝒰𝒩 𝒽, is much smaller than one. It is already known that in the limit Fr→O the inviscid equations of motion may be expanded in powers of Fr2 and that the lowest‐order solution is a flow confined to horizontal planes, passing around the obstacle rather than over it. This theory breaks down within a distance 𝒰/𝒩 from the level of the top of the obstacle.

The experiments were carried out in a closed‐circuit stratified water channel with a hemisphere, a cone and a truncated cylinder for Fr between 0.03 and 0.3 at Reynolds numbers between 100 and 1000. The qualitative features have been determined by flow visualization with dye. The flows are found to be nearly in horizontal planes except near the tops of the obstacles. Also there are revealed two other prominent features which the theory cannot predict. In the lee of the obstacles at the level of the top, a cowhorn‐shaped eddy with horizontal axis is observed for high enough Fr; it combines the characteristics of rotors and of horseshoe vortices. Below this level, there is a separated wake flow in each horizontal plane. Vortices are shed provided the Reynolds number is large enough and provided Fr is less than about 0.15. The shedding frequency is the same at all heights.

Attention is drawn to atmospheric situations of which these features may be ingredients.
Change of surface roughness and the planetary boundary layer
Authors: Jensen Niels Otto
Published: February 2007

The ratio between upstream and far downstream surface friction velocities relative to a change in surface roughness is given on the basis of results from surface Rossby number similarity theory. By simple theories for the internal boundary layer, which are found here to compare quite well with recent numerical results from higher‐order closure models, it is found that even at a downwind distance such that the internal boundary layer has grown to the full height of the planetary boundary layer, the surface stress still considerably exceeds the equilibrium value.
Calculation of the neutral wind profile following a large step change in surface roughness
Authors: Wood D. H.
Published: February 2007

The response of the boundary layer wind profile and the surface shear to a large step change in surface roughness is predicted by three calculation methods and the predictions are compared with measurements by Bradley. The first method, which assumes that local equilibrium exists everywhere in the adjusting region, performs less well than the second and third methods, which both employ a transport equation for the shear stress. It is also shown that the extra sophistication of the third method, in allowing for the non‐local effects on the dissipation length scale, does not give consistently better results than the second method.
The stability of ultra‐long waves on a mid‐latitude β–plane
Authors: Clark John H. E.
Published: February 2007

The stability of a small but finite amplitude stationary baroclinic ultra‐long wave associated with a constant zonal flow is considered. A trapped primary is found to be stable, but a propagating primary can be unstable with minimum e‐folding times of two to four days for wavenumbers one and two. The amplifying disturbance is constrained to have the same north‐south structure as the primary. It consists of a wave with the same east‐west wavenumber as the primary and a zonal mean component. It is shown to grow directly at the expense of the primary by resonantly interacting with it. The wave component of the amplifying disturbance has a very large vertical wavelength such that it is almost barotropic. It propagates with a phase speed that is close to the basic westerly wind speed. The zonal mean disturbance component decays rather rapidly with height.

The theoretical growth rates are found to compare favourably with those of growing ultra‐long waves at lower stratospheric levels during November 1970.

A crucial assumption in the analysis concerns the nature of the singular region for the potential vorticity equation at large enough heights. The region is assumed to be totally absorbing to upward‐moving wave energy.
Aircraft measurements of humidity in the low stratosphere over southern England 1972–1976
Authors: Cluley A. P. , Oliver M. J.
Published: February 2007

A frost point hygrometer installed in a Canberra aircraft of the Meteorological Research Flight has been used to make measurements of humidity in the high troposphere and low stratosphere. The results from 60 flights over southern England in the period 1972 to 1976 are reported. It is shown that above some level in the stratosphere the humidity mixing ratio (h.m.r.) becomes independent of height; the average h.m.r., by mass, in this region was (3.1±0.3)10−6. An annual oscillation in stratospheric h.m.r. of amplitude 0.4 × 10−6 and significant at the 5% level is found. The measurements are compared with an average h.m.r. of (1.9±0.2)10−6 derived from previous MRF measurements made in 1954 and 1955.
A parameterization scheme for non‐convective condensation including prediction of cloud water content
Authors: Sundqvist Hilding
Published: February 2007

A model for non‐convective condensation processes has been developed. The model allows condensation to begin before relative humidity reaches 100%. The liquid water content of clouds is a prognostic variable of the model. The rate of condensation is a function of relative humidity and moisture flux convergence. The micro‐physical processes involved in the formation of clouds and precipitation are parameterized by assuming that the rate of precipitation formation is a function of the amount of cloud water. Evaporation from falling rain is taken into account.

Quantitative tests with the model indicated that it yields reasonable evolution times and water content of clouds, and gives reasonable precipitation amounts.
Spectral distribution of solar radiation. I: Direct radiation
Authors: McCartney H. A. , Unsworth M. H.
Published: February 2007

Measurements of the spectral distribution of direct solar radiation, made in both wide and narrow wavebands, are reported. The measurements suggest that the wavelength dependence of aerosol attenuation in central England, in summer, can generally be described by a power law with wavelength exponent of about 1.7. Attenuation of visible radiation is reported in terms of an integral turbidity coefficient, τv, and is shown to depend to some extent on air mass origin, air masses from the continent being more turbid than maritime air masses. The mean value of τv a is 0.31 with a range of 0.05 to 0.6. The ratio of visible to total direct irradiance is found to be a linear function of τv, the constants of linearity being functions of zenith angle. From these relationships a semi‐empirical model is developed enabling spectral irradiance, visible irradiance and total solar irradiance to be estimated from arbitrary values of turbidity and zenith angle.
On the escarpment wind profile
Authors: Jensen Niels Otto , Peterson Ernest W.
Published: February 2007

It is shown that miscellaneous theories for flow over low ridges give results consistent with each other and that these results can be used to quantify certain observed features of the wind profile downwind from an escarpment.
The application of the Rayleigh–Gans approximation to scattering by polydispersions
Authors: Box M. A. , Lo S. Y. , McKellar B. H. J. , Reich M.
Published: February 2007

The possibility of analysing data on light scattering by polydisperse aerosols using the Rayleigh–Gans approximation is considered. We show that this approximation may be used to determine the mean radius of a polydispersion from measurements of light scattered in the near‐forward direction by correcting for the systematic errors involved. However, neither the absolute normalization of the phase function nor the multispectral extinction data may be usefully analysed by this method to obtain the mass loading.
A pilot study of concomitant changes in total ozone and atmospheric general circulation
Authors: Newell Reginald E. , Wu Mao‐Fou
Published: February 2007

Correlation between monthly total ozone anomalies from long‐term monthly means shows a pattern with latitude that is consistent with the concept of Hadley cell circulation modulation. The anomalies are positively correlated with temperature at middle latitudes. Long‐term trends appear to be related to changes in the overall intensity of the Hadley cell circulation and in tropospheric meridional temperature gradients.
A numerical study of tornadogenesis in a rotating thunderstorm
Authors: Smith R. K. , Leslie L. M.
Published: February 2007

The structure and dynamics of tornadoes and the mechanisms leading to their formation are explored in the context of an axi‐symmetric numerical model of a tornado cyclone. The paper extends a recent study of tornadogenesis by the authors to include explicit representation of moisture effects.

The calculations show that the distribution of buoyancy in the updraught of a severe ‘supercell’ thunderstorm can account for the generation and maintenance of an intense tornado when the level of rotation in the storm is within the observed range of values. The simulated vortex extends to high levels in the cloud but maximum swirling velocities occur within the lowest kilometre or two above the ground in concordance with recent Doppler radar observations.

For a fixed vertical distribution of ambient temperature and moisture, the vortex strength is sensitive to the imposed circulation: if the latter is too small, the cloud updraught rotates weakly but there is no evidence of a vortex or a funnel cloud; if it is too strong, a vortex and its associated funnel cloud may form but terminate aloft. This explains why only a few apparently suitable thunderstorms spawn damaging tornadoes.

The effects of surface friction on vortex strength and structure are studied in some detail and the results suggest that the terminating inflow layer at the ground may have a less important control on the vortex dynamics than has hitherto been supposed.
Catastrophes and resilience of a zero‐dimensional climate system with ice‐albedo and greenhouse feedback
Authors: Fraedrich Klaus
Published: February 2007

An ‘almost trivial’ climate system of geometrical dimension zero is analysed. It consists of the global energy storage which is balanced by the globally averaged net radiation (solar input minus infrared emission) with ice‐albedo and greenhouse feedbacks included. This nonlinear and time‐dependent climate system is formulated as a gradient system of a potential. It can be analysed without explicit time integration according to catastrophe theory, so that the resilience of the system, due to changes of the external parameters and due to variations of the state variable, can be determined. For the ice‐albedo or the greenhouse feedback alone, two equilibrium solutions appear: the stable ones are attractors, characterizing the present day interglacial; the unstable ones define a lower or an upper bound, respectively, for temperature changes to be absorbed by the systems. Beyond thresholds of external parameters (fold catastrophes) no equilibria exist, so that the systems attain the states of ‘deep freeze’ and ‘desert heat’, respectively. The two feedbacks combined lead to three equilibria, the stable interglacial being bounded by two unstable solutions, beyond which initial temperature values tend towards the ‘deep freeze’ or ‘desert heat’. External parameter changes can lead to structural instability occurring at fold lines, which meet at the cusp catastrophe. Outside the fold and cusp catastrophes only unstable equilibria appear, from which initial temperature values tend towards ‘desert heat’ or ‘deep freeze’ situations.
Dew and thermal lag: A model for cocoa pods
Authors: Monteith J. L. , Butler D. R.
Published: February 2007

Dew forms on cocoa pods after sunrise when the dew‐point temperature of air in the canopy rises faster than the surface temperature of the pods. Exchanges of heat and water vapour are estimated by treating a pod as an isothermal body with homogeneous thermal properties. Two treatments of this simple model are discussed. First, the heat balance equation is simplified to a form which allows the environmental limits for condensation to be derived explicitly. Second, the equation is solved numerically to find the duration and mean depth of wetness as a function of environmental variables. Consistent with observation, the thermal lag of the pod is about 1 to 3K; condensation to a depth of 10 to μm occurs when air at sunrise is almost saturated and windspeed is light.

Condensation on cocoa pods is likely to be significant in the spread of Black Pod, a serious fungal disease.
A coupled ocean‐atmosphere experiment: The ocean response
Authors: Wells N. C.
Published: February 2007

A mixed layer ocean model, which includes the effects of advection and salinity, is coupled to an atmospheric general circulation model of the southern hemisphere. The response of the ocean model is compared with observations and shown to be successful in simulating the pattern of heating occurring during the summer season, in subtropical and middle latitudes.

The sensitivity of the model to changes in atmospheric circulation, advection and salinity is assessed.
Observations of downwind development of wind speed and variance profiles at Bognaes and comparison with theory
Authors: Peterson Ernest W. , Jensen Niels Otto , Højstrup Jørgen
Published: February 2007

Observations of atmospheric flow over a change in surface roughness are reported. Both wind speed and turbulence characteristics were measured. Although the observation site departed from the ideal assumed in roughness change models, it was found that the predictions of ‘second‐order closure’ models are consistent with the observed flow.
On the use of an analytic solution in estimating eddy viscosity distribution and water vapour flux in a mature hurricane
Authors: Evans H. P. , Davies D. R.
Published: February 2007

An analytic solution, in p coordinates, of the dynamical equations of a mature hurricane system is used to determine the p dependence of eddy viscosity coefficients for an assumed quasi‐stationary and axisymmetric phase. The solution is based on: (i) eddy viscosity coefficients K1 and K2, describing respectively horizontal and vertical transfers of momentum, expressed as general functions of p; and (ii) the premise that the radial variation of the magnitudes of the tangential and radial components of velocity are of similar form. This leads to an integral equation for the tangential velocity. Explicit expressions for K1 and K2 are finally obtained by choosing power law forms, in the p variable, which lead to good agreement of the tangential velocity solution with observed distributions. By using this (inverse) method of calculating K1 and K2, the whole eddy system, ranging from eddies created by the very strong velocity shear close to the sea surface to those connected with violent cloud convection, is described by continuous mathematical functions. The associated temperature and condensation heating distributions are then calculated from the hydrostatic relation and the thermodynamic equation. To complete the model an integral water vapour flux condition relates the sea surface temperature to prescribed humidity conditions at the outer boundary of the model.
Eddy flux measurements using an instrumented powered glider
Authors: Milford J. R. , Abdulla S. , Mansfield D. A.
Published: February 2007

An instrumented powered glider was used to measure eddy fluxes of heat and water vapour within the planetary boundary layer in summer over southern England. The errors involved in the method are analysed; it appears that individual 25km runs can give heat fluxes to±15W m−2 and latent heat fluxes to ±25W m−2.

The significance of individual runs was tested for internal consistency, against budget methods and, for heat fluxes, against tethered balloon measurements. With a well‐marked inversion and winds more than 5m s−1, heating rates agreed within 0.2K h−1. With light winds the discrepancies were larger; these are attributed to sampling problems. Latent heat fluxes could not be compared with budgets usefully, mainly because of the great variability encountered early in the day. The humidity measurements show the gradual establishment of the mixed layer. They show that sampling of the mean state of the boundary layer by single ascents, or of fluxes by 25km runs, may give misleading results.

In light winds and deep boundary layers the heat flux carried by scales less than 3.5km was reduced, whereas the larger‐scale latent heat flux was increased, in each case by a factor of 2.
Structure parameters in the atmospheric surface layer
Authors: Bradley E. F. , Antonia R. A.
Published: February 2007

The behaviour of structure parameters, relating turbulence properties to other turbulence properties, has been determined from atmospheric surface layer data obtained over both land and ocean. The ratio of momentum flux to turbulence energy is found to decrease significantly as the instability increases. Structure parameters relating heat and moisture fluxes to variances of temperature and humidity fluctuations, respectively, also exhibit a strong stability dependence, under unstable conditions. Monin‐Obukhov similarity for these parameters seems to be satisfied by only part of the data examined. Neutral values of these parameters are smaller than corresponding values obtained in the laboratory boundary layer. Possible reasons for this difference are discussed.
Wind direction statistics and lateral dispersion
Authors: Sawford B. L.
Published: February 2007

Eulerian observations of wind direction fluctuations in slightly unstable conditions are analysed in terms of Taylor's statistical theory of dispersion which, with the Hay‐Pasquill assumption of similar shapes for Lagrangian and Eulerian spectra, relates the standard deviation of the crosswind material distribution, σy, to the standard deviation of fluctuations in wind direction, σθ. The dependence of the predicted cross‐wind dispersion parameter, S=σy/(σθx), where × is the distance downwind from the source, on travel time, T, is examined, and it is concluded that the data are well represented by an empirical form originally proposed by Draxler to fit observed crosswind dispersion.

In contrast to direct dispersion observations, the present data show little scatter and unambiguously demonstrate that the common assumption of an exponentially decaying correlogram results in a less accurate representation. In particular, the exponential form approaches the large‐time limit, S2 α 1/T, too rapidly, and thus may be expected to perform poorly in extrapolating from a limited range of data.

The consistency of the Draxler form in representing both the Eulerian measurements presented here, and direct Lagrangian dispersion observations, serves to support not only the form itself, but also, indirectly, the Hay‐Pasquill hypothesis.

Evidence of a correlation between the intensity and scale of turbulence is also presented, which, if confirmed over a larger range of atmospheric stability conditions than considered here, will be very useful for estimating dispersion from wind statistics.
A numerical simulation of BOMEX data using a turbulence closure model coupled with ensemble cloud relations
Authors: Yamada Tetsuji , Mellor George L.
Published: February 2007

A one‐dimensional version of a simplified second‐moment turbulence closure model, coupled with a recently developed cloud model, is used to simulate BOMEX (Barbados Oceanographic and Meteorological Experiment) data. Partial differential equations for the turbulence energy and a master length scale are solved. Simulated mixing ratios of water vapour, virtual temperatures and horizontal wind speeds are compared with observations. Horizontal wind speeds agree quite well; however, simulated temperature and mixing ratio of water vapour at the end of the fourth day are about 2K and 1.5 g kg−1 higher, respectively, than corresponding observations; possibly this is due to the fact that the surface temperature used in the simulation is too high. Mean liquid water, cloud volume, liquid water variance, turbulence energy and eddy viscosity coefficients are presented, but data for these variables are not available for comparison. Surface momentum, heat and moisture fluxes are also presented and are compared with data. Sensitivity studies indicate that the simulated mixing ratios of water vapour agree best with observations when both vertical wind and horizontal advection obtained from the data are included.

The present study is encouraging, although further research is required to improve the model and to develop confidence in its predictive capability.