bluuebunny/crossref_metadata_2025_split huggingface dataset:
Ozone trend and haze scattering
Authors: Kulkarni R. N. 
Published: February 2007

AbstractDirect 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.
The turbulent transfer mechanisms: A time domain analysis
Authors: McBean G. A. 
Published: February 2007

AbstractThe 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

AbstractMeasurements 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

AbstractSurface 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

AbstractExperiments 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

AbstractA 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractIn 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

AbstractTime‐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

AbstractIn 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

AbstractIn 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

AbstractExperimental 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

AbstractWeather 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

AbstractA 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

AbstractAn 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

AbstractThe 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

AbstractThe 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

AbstractThe 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.