bluuebunny/crossref_metadata_2025 huggingface dataset:
Total ozone and the middle stratosphere over arctic and sub‐arctic areas in winter and spring
Authors: Godson W. L.
Published: February 2007

It is well known that the arctic stratosphere customarily exhibits a rapid transition from winter to summer which has a permanent warming effect on stratospheric temperatures and causes a marked decrease in the intensity of the circulation at mid‐stratospheric levels. Prior to this final warming, temporary warmings occur with a marked effect only in sub‐arctic areas. These phenomena are closely related to the rapid increase in total ozone which takes place in the late winter and early spring in polar regions. The major ozone maximum occurs at the time of the final warming, with a magnitude dependent on the intensity of the rapid warming at high levels in the stratosphere. Earlier baroclinic waves during the winter cause closely correlated ozone changes, resulting in a stepwise increase in ozone prior to the final warming.

10‐day running means of total ozone and of 100 mb temperature are compared for stations in northern Europe and in Canada; these reveal the relations between long‐period changes and processes in total ozone and in the middle stratosphere. In particular, one may thereby explain the seasonal behaviour of total ozone, which departs so markedly from photochemical prediction, as well as the vital dynamical role played by active centres in the stratospheric warm belt (especially in the Aleutian area) in the production of warm ozone‐rich stratospheric air masses.
Some computations of the variation of vertical velocity with pressure on a synoptic scale
Authors: Knighting E.
Published: February 2007

The vertical velocity in pressure co‐ordinates is computed from synoptic data on two occasions for each 100 mb interval between 1,000 mb and 200 mb at the points, approximately 160 mi apart, of a mesh covering much of the Atlantic Ocean and Europe. The assumptions made are that the mean lapse rate is everywhere the same, that there is little movement of air through the upper and lower pressure surfaces and that the horizontal wind is approximately geostrophic. The results show that the vertical profile is approximately parabolic with respect to pressure in regions of great development, but that elsewhere the profiles are not of a simple shape.
A theoretical investigation of the sea breeze
Authors: Estoque M. A.
Published: February 2007

A primitive equation model is formulated and integrated numerically to study the sea breeze. The formulation is an extension of an atmospheric boundary‐layer model previously developed for a homogeneous terrain. External gravity‐wave type solutions are suppressed by a modelling assumption involving the equation of continuity. The results of an integration showing the evolution of the sea breeze under conditions of no large‐scale synoptic motion are presented.
An empirical method for estimating long‐wave radiation exchanges in the British Isles
Authors: Monteith J. L.
Published: February 2007

From radiation measurements at Kew Observatory (tabulated by Lönnqvist), the effective emissivity of the atmosphere ε, defined as the ratio of incoming long‐wave radiation to black‐body radiation at screen temperature, can be related to optical path m (cm) by

With standard deviation ± 0·018. The relation between m and surface vapour pressure e (mb) found from Belasco's (1952) air mass analysis is

giving

almost the same as Brunt's equation from Benson data.

Net long‐wave radiation L over short grass can be calculated from

where c is fractional cloudiness; σTA is black‐body radiation at screen temperature; ΔL1 is a correction for the difference between TA and cloud‐base temperature; and ΔL2 for the difference between TA and surface radiative temperature. From Roach's Kew data, ΔL1 = – 18 cal cm−2 day−1 and ΔL2 varies with season from – 12 cal cm−2 day−1 (December) to + 20 cal cm−2 day−1 (June). Throughout the British Isles, annual mean L with c = 0 is close to – 200 cal cm−2 day−1, and for real values of c

Estimated annual net (total) radiation is 29 k cal cm−2 with little variation over the British Isles.
The wake capture of water drops in air
Authors: Woods J. D. , Mason B. J.
Published: February 2007

A technique is described for evaluating directly from a single streak photograph the impact parameter for a collision between two falling water drops of similar size. Collision results from one member of a pair of drops being accelerated in the wake of the other. An analysis of many such photographs allows the collection efficiency to be determined. No coalescences were observed between droplets of radius R < 35 μ. Droplets of 40 μ < R < 100 μ had well‐developed wakes, and coalescences occurred relatively frequently, the collection efficiencies being close to unity. No evidence is found for the much larger values of collection efficiency reported by other workers. Collisions between droplets of this size appear always to be followed by coalescence.
Precipitation and cloud electricity
Authors: Reiter R.
Published: February 2007

Based on the evaluation of atmospheric electricity data recorded synoptically over a period of several years in a high‐mountain area between 600 and 3,000 metres above sea‐level, observations of the behaviour of several atmospheric electricity parameters are briefly presented. Besides providing information on charge separation occurring during drifting snow and on cloud charges, the results show the fundamental importance, with respect to atmospheric electricity phenomena, of the melting zone of falling precipitation and of the intensity of atmospheric turbulence and its dependence on altitude. A generally applicable mathematical formulation is given for the relation between the potential gradient and the precipitation current density. This is valid for both low and high altitudes above sea‐level and takes the differences between different types of precipitation into account.
The mass loss of water drops falling in electric fields
Authors: Latham J.
Published: February 2007

Experiments showed that water drops of radius 0.191 cm falling for 0.2 seconds through a horizontal electric field were disrupted and lost mass if the field strength exceeded 8,500 V cm−1. As the field strength was increased above this critical value the mass loss increased rapidly and in a field of 11,500 V cm−1 the drop lost about 35 per cent of its mass; for higher values of field strength the mass loss increased more slowly.

Experiments also showed that the magnitude of the mass loss was dependent upon the time of exposure of the drop to the field. For exposure times less than about 2 × 10−2 sec drops falling in a field of 11,250 V cm−1 lost no mass but as the exposure time was increased above this value the mass loss increased rapidly and for an exposure time of about 0.1 sec the mass loss was 25 per cent; for longer exposure times the mass loss increased more slowly.

An assessment is made of the importance of this disruption process in modifying the concentration and size distribution of raindrops and cloud droplets inside a thundercloud.
Noctilucent clouds and thermospheric dust: Their diffusion and height distribution
Authors: Chapman S. , Kendall P. C.
Published: February 2007

Rocket sampling in latitude 66°N has shown that at heights around that of the mesopause, dust nuclei are far more abundant there when noctilucent clouds (hereinafter abbreviated to NLC) are present than when they are absent. The dust particles appear to be nickel‐based micrometeorites, and many of them, when NLC are present, appear to be ice‐coated. Hence the occurrence of NLC seems to require an adequate density of both water vapour and dust. The water vapour is supposed to rise to the mesopause by convection (in summer in latitudes above 45°); thence it diffuses into the thermosphere, where its upward penetration is limited by dissociation by ultra‐violet light, so that its effective scale height is estimated to be less than 2 km. Assuming that the air just above the mesopause is non‐turbulent, the scale height of the uncoated dust passing steadily downwards there is the same as that of the air, about 4.5 km. The estimated scale height of the NLC particles is about 1.5 km. Eddy diffusion below the mesopause causes the number density of both NLC and uncoated dust particles to decrease sharply just below the mesopause. Thus both the thinness and the location of NLC are explained.

The diffusion problems considered refer to both the dust and the water vapour; the recent study of dust diffusion by Yu and Klein (1964) is extended. Also the influence of upward dust convection is considered. Conclusions are drawn as to the form of the NLC particles from their estimated rate of descent.

The appearance of NLC is ascribed to the simultaneous occurrence (mainly, if not exclusively in the higher latitudes in summer) of a descent of the turbopause to the mesopause, a low mesopause temperature, and humid air below the mesopause.
Vertical structure of turbulence
Authors: Panofsky H. A. , Singer I. A.
Published: February 2007

Based on fluctuation statistics from the Brookhaven tower, statistical models are presented which permit estimation of cross‐spectra between wind components at various heights. It is shown that a similarity hypothesis, proposed by Davenport (1961), seems to be valid.

The upper portions of turbulent eddies are downwind of the lower parts, with slopes of order unity, probably dependent on wind shear.
Secular and other changes of atmospheric electrical potential gradient at Lerwick
Authors: Hamilton R. A.
Published: February 2007

The atmospheric electrical potential gradient at Lerwick is very much higher in stable airstreams from north‐west Europe than in other airstreams. The secular decrease in the general level of potential gradient began at the same time as at Eskdalemuir, and the gradient has now fallen to about one half of its value around 1950.
One‐dimensional spectra derived from an airborne hot‐wire anemometer
Authors: Payne F. R. , Lumley J. L.
Published: February 2007

Velocity fluctuations in the flight path direction were measured by a single hot‐wire mounted in the wing tip of a light aircraft. Magnetic tape recordings were later analysed by standard means to produce one‐dimensional spectra. These spectra exhibited the (– 5/3) power law behaviour over a range of several decades of wavenumber. Local isotropy was verified in the horizontal plane by flight paths into and perpendicular to the mean wind.
A 10‐level atmospheric model and frontal rain
Authors: Bushby F. H. , Timpson Margaret S.
Published: February 2007

A 10‐level primitive equation model suitable for studying the dynamics of fronts and frontal rainfall is described. The atmosphere is assumed to be hydrostatic and inviscid and the effects of friction and topography are ignored. Latent heat due to evaporation and condensation is incorporated in the thermodynamic equation. No distinction is made between the ice and water stage and the atmosphere is assumed to be dry above 300 mb. The horizontal grid length is 40 km. The results of one 24‐hr integration are described in detail.
Deposition of particulate matter by hydrometeors
Authors: Starr J. R.
Published: February 2007

A recent paper by Starr and Mason (1966) described experimental determinations of the collection efficiencies of water drops and of simulated snow crystals for small, roughly spherical particles in the form of spores and pollen grains. Using these data and additional information obtained from observations on the impaction of particles on sticky spheres in a wind tunnel, calculations are presented for the rates at which particles of these sizes would be removed from the atmosphere by impaction on raindrops and snow crystals as a function of precipitation intensity. The raindrop scavenging results are compared with those of earlier computations by Chamberlain (1953) who used the theoretically derived values of collection efficiency by Langmuir (1948) and applied an intuitive equation suggested by Langmuir to interpolate between viscous and potential flow solutions for the collection efficiency.
The energetics of the lower stratosphere including radiative effects
Authors: Dopplick T. G.
Published: February 2007

Approximate solutions of radiative heating are developed using monthly mean radiative parameters and are combined with the geopotential and temperature fields to study the energetics of the Northern Hemisphere lower stratosphere on a daily basis for 1964. Significant leakage of energy from the troposphere occurs throughout the year and absorption of this energy in the lower stratosphere contributes to the maintenance of the eddies for all months except January. In winter there is significant transmission of tropospheric energy into the upper stratosphere which suggests a close coupling of the upper and lower atmospheres. The seasonal circulations are examined and it is found that the spring warming of 1964 was due to a period of enhanced convergence of tropospheric energy flux. In the fall absorption of tropospheric energy intensifies the flow. The role of radiation is to destroy available potential energy for all months of the year.
The development of a dry inversion‐capped convectively unstable boundary layer
Authors: Carson D. J.
Published: February 2007

A model is proposed for the development of a dry convectively unstable boundary layer capped by a stable layer; included are the effects of time‐dependent surface heating, the stability of the capping layer, subsidence and any degree of turbulent interfacial mixing. A differential equation describing the evolution indicates quantitatively the relative importance of each factor and, in particular, indicates the importance of the entrainment process. The model is applied to the 1953 O'Neill boundary layer data in an attempt to assess to what degree interfacial mixing is realized in the atmosphere. Realistic boundary layer developments are obtained.
A study of the continuum absorption within the 8–13 μm atmospheric window
Authors: Lee A. C. L.
Published: February 2007

The spectral continuum within the 8–13 μm atmospheric window was studied using a tethered balloon borne radiometer, while simultaneous measurements of temperature and humidity profiles were made. Aerosol absorption was estimated as negligible as the measurements were made in conditions of high visual range. The results from a range of situations were tested separately against a line wing continuum model and a water dimer continuum model, both with appropriate spectral line band models. These results were found to agree well with laboratory data for the dimer model.
Stratospheric temperature anomalies in 1963 and 1966
Authors: Pittock A. B.
Published: February 2007

The existence, statistical significance, and possible causes of stratospheric temperature anomalies observed at the 100 mb level in the Australasian region in 1963 and 1966 are discussed with reference to the so‐called quasi‐biennial oscillation, volcanic dust, and the general circulation. This discussion is facilitated by the use of the observed correlation between stratospheric temperatures and the mean latitude of the surface high pressure belt, in order to increase the sensitivity of a statistical test for anomalous temperatures.
Planetary‐scale disturbances in the polar winter stratosphere
Authors: Simmons A. J.
Published: February 2007

Some models of steady planetary‐scale disturbances forced in the winter stratosphere by tropospheric pressure perturbations are presented for zonal flows with both vertical and meridional shear. The resulting structure of these disturbances is realistic. In particular, they are confined in the horizontal to regions of strong westerly wind, and do not exhibit a deficient vertical penetration, results in contrast with some earlier studies. Examples are included in which amplitudes are insensitive to the presence of either a strong New tonian cooling or singular‐line absorption.

The time‐dependent behaviour of stratospheric planetary waves is examined for a zonal wind with uniform vertical shear. Analytical solutions emphasize the importance of a low‐level conversion of zonal available potential energy in amplifying a disturbance forced at the tropopause. When allowance is made for the meridional structure of the stratospheric jet, a reasonable agreement is obtained between theory and observation, in particular with respect to perturbation geopotential heights observed in sudden warmings.
Electrical corona from ice hydrometeors
Authors: Griffiths R. F. , Latham J.
Published: February 2007

In contradiction to earlier work it has been found that substantial corona currents can be initiated from ice surfaces over a wide range of conditions. Measurements have been made of the critical fields required to initiate corona from ice particles a few millimetres in length suspended in a uniform electric field. The variables included size, shape, purity, orientation and the surface features of the particles, their initial charge, pressure and temperature. Currents of about 0.1 μA at onset were obtained and the details of the forms of discharge and associated currents were determined as a function of applied field. A particularly interesting finding was that substantial corona currents were obtained only at temperatures higher than −18°C. A semi‐quantitative explanation of these results is given in terms of the surface conductivity of ice. The characteristics of the streamers observed have been related to the Dawson‐Winn model as extended by Phelps. In view of these new results it is suggested that the role of corona from ice hydrometeors in a thunderstorm is of much greater importance than has previously been realized, particularly in connection with the initiation of lightning and the enhancement of ionic conductivity; the onset fields for corona emission from ice in the central regions of thunderclouds are probably in the range 400 to 500 kV/m.
Transmission function for infrared radiative transfer in an inhomogeneous atmosphere
Authors: Kuriyan Jacob G. , Shippony Zvi , Mitra Subir K. , Wurtele M. G.
Published: February 2007

Calculations of the transmission of infrared radiation through the atmosphere must take into account inhomogeneities in the path due to variations of temperature, pressure and absorbing matter. For ozone absorption the method due to Curtis and Godson does not seem to yield results that are sufficiently precise. In this paper an improved approximation is developed that yields an analytic expression for the transmission function and this is compared with the exact results obtained from the Goody random model.
A case study of the measurement of snowfall by radar: An assessment of accuracy
Authors: Collier C. G. , Larke P. R.
Published: February 2007

Radar measurements during a period of snowfall in a region of variable terrain are described. The accuracy of estimates of areal snow depth, using a calibrated radar, is shown to be similar to that achieved for areal rainfall using the same technique. This detailed case study supports similar conclusions made by other workers, who used more cases but fewer validating measurements. It is also shown that provided the variations in terrain height are not very large, compensation for the effects of melting over low terrain can be made in deriving the snow depth field by using two independent snow depth calibration measurements ‐ one representative of upland areas, and the other of lowland areas.
Persistence and orographic modulation of mesoscale precipitation areas in a potentially unstable warm sector
Authors: Hill F. F. , Browning K. A.
Published: February 2007

In this paper we use data from radars and raingauges in England, Wales and southern Ireland to study the motion and changes in intensity of mesoscale precipitation areas (MPAs) some tens of kilometres across as they traversed two regions of high land and an intervening stretch of sea. The MPAs studied were associated with an area of middle‐level potential instability within a windy and moist wintertime warm sector, a situation known to produce major orographic effects. Despite the very large modulation of the rainfall intensity by orography, it was found that most of the MPAs could be tracked for 6h over a distance of 600 km, all the way from the west coast of Ireland to southern England. The implications of this are discussed with regard to the use of radar and satellite data in short‐period forecasting.
Effects of coronae on electric fields beneath thunderstorms
Authors: Standler Ronald B. , Winn William P.
Published: February 2007

Intense electric fields beneath thunderstorms produce electrical discharges (coronae) at the tips of trees, bushes and other sharp objects attached to the surface of the earth. We find typical corona current densities of about 1 nA m−2 in an 8 kV m−1 field at the ground. The ions released into the air limit the magnitude of the field at the ground to about 10kV m−1. Our measurements beneath thunderstorms with a balloon‐borne electric field meter show that the magnitude of the field a hundred metres above the ground is several times larger than at the ground; in one case the field 300 metres above ground was 6 times that at the ground. The substantial thickness of the space charge layer and the speed with which it vanishes when the electric field strength declines imply that the charge carriers have substantial velocities (0.4m s−1) either because their mobilities are high or because they are carried by air motions.

Coronae also influence the time behaviour of the electric field at the ground. The field at the ground often changes very rapidly after a lightning flash. The rate of change decreases as the field approaches the value it had prior to the flash. In contrast, the field a hundred metres above the ground, which is often above most of the influence of space charge produced by coronae, increases more uniformly (linearly) during the time interval between lightning flashes. This behaviour is similar to that of the field farther aloft in the interior of the cloud. Our numerical simulations of the shapes of recovery curves indicate that the corona current density is more accurately described by a cubic function than by a quadratic function of the electric field strength at the ground.

Despite strong influences of coronae, three properties of the field at the ground accurately reflect what happens above the space charge layer. First, the rapid changes in electric field during a lightning flash are not usually affected by corona space charge. Second, when the field at the ground is nearly constant it usually has the same polarity as the field above the space charge layer. And third, when the field strength at the ground is nearly zero, and when certain other conditions are met, the time rate of change of the field at the ground is the same as that above the space charge layer.
A Lagrangian statistical analysis of diffusion from a ground‐level source in a turbulent boundary layer
Authors: Hunt J. C. R. , Weber A. H.
Published: February 2007

A Lagrangian statistical analysis is proposed for the motions of a single particle and the growth of a small cloud released from the ground in a turbulent boundary layer, essentially by considering the fluctuating as well as the mean vertical Lagrangian velocity, by noting that its integral time scale is much smaller than the travel time of a particle from the source and by estimating (quite conventionally) the Lagrangian statistics in terms of the measured Eulerian statistics of atmospheric surface layer turbulence, in neutrally stable conditions.

Comparing the results with a 1968 analysis by P. C. Chatwin based on a diffusion equation, the same value is obtained for the dispersion of the vertical displacement of a particle as a function of its mean travel time; however, a small but physically significant difference is predicted for the growth of a cloud released at ground level.

The validated diffusion equation is then used to relate the concentration of a cloud to that of a continuous plume. It is shown that the index, s, used in the exponential formula exp(–Azs) to describe observations of the concentration in a plume, varies slowly with distance downwind of a source. Diffusion measurements in the atmospheric boundary layer and in wind tunnels are shown by the theory to be consistent with turbulence measurements. The theory suggests the kind of further turbulence and diffusion measurements that ought to be made.
Turbulent transfer of sulphur dioxide to a wheat crop
Authors: Fowler D. , Unsworth M. H.
Published: February 2007

Fluxes of sulphur dioxide to wheat were measured over two growing seasons. The deposition velocity, vg, depended primarily on affinities of surfaces for SO2 and varied from less than 0.1 to about 1.5cms−1. Stomata were an important sink, and diurnal changes in stomatal resistance accounted for most of the variation in vg when leaves were green and dry. When leaves were wet with dew, vg was generally large, irrespective of the physiological state of vegetation. A resistance analogue of transfer to the canopy agrees well with published work and is used to estimate seasonal uptake of SO2 by the crop, showing that 30% of the sulphur content of the crop at harvest was probably supplied by absorption of SO2 through stomata.
Mesoscale energy generated in the boundary layer
Authors: Green J. S. A. , Dalu G. A.
Published: February 2007

We examine the kinetic energy that may be generated from surface heating. Cumulonimbus, cumulus, mesoscale and large‐scale conversion of potential into kinetic energy can be identified in an idealized way, as distinct processes. Each has a characteristic thermodynamic efficiency.
Shade‐ring corrections for pyranometer measurements of diffuse solar radiation from cloudless skies
Authors: Steven M. D. , Unsworth M. H.
Published: February 2007

When a shade ring is used to shield a pyranometer from direct solar radiation, a correction to the measured diffuse radiation is necessary to account for diffuse radiation intercepted by the ring. A general analysis is developed to relate shade‐ring corrections to the radiance distribution of diffuse radiation. The corrections are split into two components: a geometric component based on an isotropic sky and varying with shade‐ring dimensions; and an anisotropy component, relatively independent of ring dimensions. Shade‐ring corrections are calculated using mean distributions of the radiance of cloudless skies. Tables and figures are given for calculating these corrections as functions of latitude, ring dimensions and time of year. Comparisons with published measurements at a variety of sites reveal generally good agreement with calculations. Some of the discrepancies may be due to differences in aerosol scattering.
Long‐term budget of zonal momentum in the free atmosphere over Europe in winter
Authors: Holopainen Eero
Published: February 2007

Two sets of aerological data are used to study the time‐averaged budget of the zonal momentum in the free atmosphere over Europe in winter. In many respects the budget appears to be different from that reported by Holopainen et al. (1980) for North America.

Over Europe the coriolis force associated with the southward‐directed ageostrophic meridional flow tends to decrease the westerly momentum. A compensating effect is caused by horizontal momentum flux convergence which takes place in the zonal as well as in the meridional plane and is due mainly to transient flow systems (large‐scale turbulence).

The residual term in the momentum budget indicates that over Europe the sub grid‐scale processes on the average cause a small accelerating effect on the large‐scale zonal flow in the upper troposphere. The residual force over the mountainous region of Europe (5 °E ‐ 30 °E, 45 °N ‐ 50 °N) is a decelerating one in agreement with the results for the corresponding area of North America.
Measurements of the mean, solar‐fixed temperature and cloud structure of the middle atmosphere of Venus
Authors: Schofield J. T. , Taylor F. W.
Published: February 2007

Data from the orbiter infrared radiometer (OIR) of the Pioneer Venus mission have allowed the global structure of the middle atmosphere of Venus to be studied in detail for the first time. Between 4 December 1978 and 14 February 1979 this instrument made over 300 000 soundings in ten spectral channels, covering most of the northern and some of the southern hemisphere in the altitude range 65–100 km (100–0·01 mb). Preliminary analyses indicate that mean atmospheric structure in this region is strongly dependent on latitude and local time of day, although day‐to‐day variations are seen. This paper presents the mean temperature field and cloud top structure retrieved from the data of five OIR channels averaged in a solar‐fixed coordinate system. The retrieval scheme and the derivation of the weighting functions that characterize the vertical response of each channel are also described.

The middle atmosphere can be divided into two distinct regions separated by a low‐latitude temperature minimum of less than 170K at 95km in the retrieved zonal‐mean temperature field. Below 95km, day‐night temperature contrasts are small but above 70km pole‐equator contrasts are positive reaching a maximum of 20–25 K at 85km. Fourier analysis shows that within 45° of the equator the longitudinal variation of temperature is dominated by wavenumber‐2 tidal structure with a phase that moves eastwards with increasing altitude. Above 95km, the pole‐equator temperature gradient is reversed, day‐night contrasts become appreciable and wavenumber‐1 longitudinal structure dominates. At equatorial latitudes mean cloud optical depth at 11·5 μn is unity at 100mb (66·5 km), and the cloud top has a scale height of 0·85 times the atmospheric value. The cloud top falls slowly with increasing latitude and has a wavenumber‐1 longitudinal dependence of £20mb (£1 km), with the highest cloud found just before the evening terminator. Retrievals are insensitive to cloud structure in the polar regions, but it is clear that the cool collar that surrounds the warm polar region is not a high‐cloud feature. It is in fact a deep temperature inversion in which temperatures can be more than 30 K less than equatorial values at the same level.
Air motion and precipitation growth in a major snowstorm
Authors: Browning K. A.
Published: February 2007

A mesoscale analysis of the snowstorm of 8–9 January 1982 has been carried out using data from the U.K. weather radar network, the geostationary satellite Meteosat and other more conventional sources. More than 60cm of snow accumulated over S.E. Wales and neighbouring parts of England in association with an intense frontal zone. Much of the snow occurred during a period when synoptic forcing and orographic modulation were essentially steady. The depth of cloud and moist air was not great in the region of heavy snowfall in south Wales, but an orographically enhanced circulation on the mesoscale nevertheless led to efficient growth of precipitation through the seeder‐feeder mechanism. The seeder cloud was generated within a shallow current of air which ascended above the front and which originated from low levels, probably mostly from the boundary layer, on the warm side of the front. Precipitation was initiated within this current repeatedly along a well‐defined and remarkably stationary line just upwind of the south Wales coast. As the rising current travelled inland towards the E.N.E., convective generating cells embedded in it produced precipitation particles which fell into the underlying cold easterly flow. Considerable growth occurred as these particles descended through orographically enhanced feeder cloud, much of the growth occurring in the lowest kilometre or so.
The effect of Gaussian particle‐pair distribution functions in the statistical theory of concentration fluctuations in homogeneous turbulence
Authors: Sawford B. L.
Published: February 2007

The consequences for the theory of concentration fluctuations of different proposals by L. F. Richardson and G. K. Batchelor about the rate of separation of pairs of marked particles (giving rise to non‐Gaussian and Gaussian particle separation probability density functions respectively) are explored via two Lagrangian Monte Carlo models.

The models are applied to an instantaneous one‐dimensional cloud source (which is approximately equivalent to a continuous line source) and in many respects give similar results for concentration fluctuations. The crucial difference is that whereas the non‐Gaussian model predicts, in agreement with observation, that at large time the fluctuations remain the same order of magnitude as the mean field (the ratio depending only on the source size), the Gaussian model incorrectly predicts that fluctuations ultimately vanish compared with the mean field.

The reason for the failure of the Gaussian model is explored by partitioning the total fluctuations into contributions due to the variation of the distribution of material within the cloud (i.e. in coordinates relative to the centre‐of‐mass) and due to motions of the cloud as a whole (meandering). It is shown that at large time the Gaussian model accounts only for the meandering contribution to fluctuations, and by smoothing out all internal structure of the doud eliminates relative fluctuations.
A model of the feeder–seeder mechanism of orographic rain including stratification and wind‐drift effects
Authors: Carruthers D. J. , Choularton T. W.
Published: February 2007

Orographic enhancement of rain via the ‘feeder‐seeder’ mechanism has been calculated using a stratified airflow model. The effect of wind drift on precipitation has been included. Comparisons have been made using potential flow models both in two and in three dimensions and in the model of M. J. Bader and W. T. Roach.

For high windspeeds (≥ 15ms−1), when large enhancements occur, the potential flow treatment is sufficient. Inclusion of stratification does not significantly affect the total washout, although in the case of long hills (half‐length ≥ 10km) the precipitation maximum is moved upwind. For long hills the results are also in close agreement with the model of Bader and Roach. Over shorter hills both wind drift and non‐hydrostatic effects are important. Wind drift decreases both maximum and total enhancements. The Bader and Roach airflow formulation is inaccurate since it overestimates streamline displacements over the hill. Three‐dimensional effects are small except over short steep hills.
A weather radar correction procedure for real‐time estimation of surface rainfall
Authors: Collier C. G. , Larke P. R. , May B. R.
Published: February 2007

Several sources of error have been identified in measuring rainfall by radar. In this paper we discuss these sources of error as they affect the calibration of a radar system using raingauge data. Present calibration techniques are considered, and the need for procedures based upon a knowledge of the structure of precipitation systems is noted. Finally, a new procedure is described, and its performance assessed for a number of cases of frontal and convective rainfall.
On sympathetic fluctuations of cloud water content and cloud top height in anticyclonic stratocumulus
Authors: Roach W. T.
Published: February 2007

Evidence is produced which suggests that sympathetic fluctuations in cloud top height and total cloud water content observed during two case studies of nocturnal anticyclonic stratocumulus at Cardington (52°06′N, 0°24′W) may be due to vertical adiabatic displacements of the cloud layer as a whole, perhaps forced by local topography. Simultaneous observations from more than one site will be needed to test this tentative hypothesis, and these are defined.
The evolution of planetary‐scale 200 mb divergent flow during the FGGE year
Authors: Lorenc A. C.
Published: February 2007

A descriptive study of the large‐scale horizontally divergent wind field during the FGGE year is presented, using empirical orthogonal functions (EOFs) of the 200 mb velocity potential. The mean flow and annual cycle are shown to be realistically related to climatological features. These and large‐scale patterns represented by the EOFs are consistent with very similar patterns of opposite sign seen at 850 mb.

The main result of this work is the clear description of an eastward propagating wave in the 200 mb divergences with zonal wavenumber 1 and period 30–50 days. This mode has been deduced in other studies, and related to variability in the Asian summer monsoon. These results show it to exist throughout the FGGE year, and imply that a relationship exists between the planetary‐scale eastward propagating mode and smaller‐scale divergence bands apparently propagating northward over the monsoon region.
Convective and boundary layer parametrizations in a diagnostic model of atmospheric fronts
Authors: Thorpe A. J. , Nash C. A.
Published: February 2007

Numerical integrations of initial value problems and observations have shown that along‐front geostrophy is a good approximation in frontal regions, even in the presence of strong frontogenetic forcing. Consequently the cross‐frontal circulation equation described by A. Eliassen in 1961 is a powerful diagnostic tool. Extra terms have been added to represent parametrized convective and boundary layer physics. The inclusion of cumulus heating modelled on that used in CISK theories produces bands of ascent whose scale decreases as the heating amplitude increases. For realistic cold fronts bands are produced just ahead of and behind the surface temperature contrast depending on the type of frontogenetic forcing.

The application of this convective parametrization to finite amplitude circulations about a front is rather different from previous uses in theories of CISK. The solution method makes it simple to examine a much larger class of representations, hence providing a fresh appraisal of the parametrizations.

The integrated frontogenetic tendency is calculated in the frame of reference moving with the front for the various cases considered. This allows discussion of the notion of a steady state front and indicates the crucial role of convective and boundary layer processes.
On the stability of forced non‐zonal flows
Authors: Andrews David G.
Published: February 2007

Attention is drawn to the fact that the stability of forced, zonally asymmetric atmospheric flows depends not only on the flow configuration but also on the nature of the forcing. This fundamental point is illustrated by consideration of topographically induced flows in a barotropic non‐divergent model. It is shown that a steady barotropic basic flow which is not a free solution of the equations of motion can be forced in this model by any one of an infinite number of topographic configurations. Since the topography also influences the disturbances to the basic flow, the stability of this flow depends crucially on which topography is present.

Other mechanisms which may contribute to the forcing of atmospheric climatological mean states (such as diabatic effects and the time‐averaged effects of transient eddies) are also mentioned, together with the implications of the present study for numerical investigations of the instabilities of such states.
Radiation budget components and their parametrization in JASIN
Authors: Lind Richard J. , Katsaros Kristina B. , Gube Marianne
Published: February 2007

Components of the radiation balance over the sea surface were measured from RV Meteor and MV Endurer during July, August and September in the eastern North Atlantic (59°N 12°W) during the 1978 Joint Air Sea Interaction (JASIN) Experiment. Each component was modelled using routine hourly surface weather observations and compared to measurements. These models were used to estimate radiation components for RV John Murray and HMS Hecla. Daily net radiation at the surface ranged from 50 to 190 Wm−2 during JASIN. Standard deviations of daily mean net radiation averaged 45 Wm−2 for phase I of JASIN (23 July through 8 August), and 23 Wm−2 for phase II (22 August through 3 September). Shortwave irradiance estimates made with an empirical formula due to F. E. Lumb, using solar zenith angle and cloud amount and type, gave results to within 10 Wm−2 for most 5‐day periods, confirming results of other investigators. Shortwave exitance calculations, based upon smoothed observations of albedo by R. E. Payne, were made using both measured and model‐derived shortwave transmittances. Our results showed high correlation between model results and measurements. A recent model of longwave irradiance by two of the authors, RJL and KBK, gave results to within 10 Wm−2 when compared to daily mean measurements. The accuracy of these models, over periods of a few days, was found to approach what is obtainable with direct measurements.
Radiative cooling near the top of a cloudy mixed layer
Authors: Nieuwstadt F. T. M. , Businger J. A.
Published: February 2007

The role of radiation above a cloudy mixed layer is examined. We find that whereas turbulent entrainment can be identified with a very sharp temperature jump at the cloud top, the effect of radiative flux divergence above the cloud leads to a much more gradual temperature increase. With adequate observations the two can easily be distinguished. The development of the boundary layer can be described in terms of equations in which above‐cloud radiation plays no role.
Modelling tracer budgets in the stratosphere
Authors: Pyle J. A. , Rogers C. F.
Published: February 2007

The ozone budget in the stratosphere of a modified diabatic circulation model is compared with traditional Eulerian calculations. The importance of the chemical contribution in comparison with net dynamical contributions, even in the lower stratosphere, is stressed. Furthermore the chemical dependence of the eddy transports is shown to introduce significant differences in the meridional transport and points to a potentially important source of error in two‐dimensional simulations of tracer distributions in the stratosphere. Other aspects arising from photochemistry, the photochemical eddy term and the eddy coupling of different species, are also considered.
Finite amplitude, steady Rossby waves and mean flows: Analytical illustrations of the Charney‐Drazin non‐acceleration theorem
Authors: White A. A.
Published: February 2007

Exact steady‐state solutions of the nonlinear quasi‐geostrophic equations are used to illustrate some important repercussions of the Charney—Drazin theorem on the non‐acceleration of zonal mean flows by steady conservative waves. The solutions, which are similar to those discussed by Kuo in 1959, consist of spatially non‐uniform zonal flows and barotropic and baroclinic Rossby waves each having the same three‐dimensional wavenumber; a uniform zonal flow is also allowed. Two examples, both consisting of an internal jet zonal flow, a barotropic Rossby wave and a baroclinic Rossby wave, are considered. The absence of wave/mean interaction in these steady‐state solutions is clearly reflected in the absence of poleward eddy potential vorticity fluxes, but is disguised by the existence of non‐zero poleward eddy heat and zonal momentum fluxes and active Lorenz energy cycles. The solutions thus exhibit the same behaviour as the linearized examples discussed previously by various workers, and emphasize the danger of inferring from incomplete vorticity, energy or heat budget analyses that wave forcing (or maintenance) of Eulerian mean flows by eddy motion is taking place.

One of the solutions (which is valid in a laterally open domain) is examined as a possible theoretical model of the steady waves seen in rotating annulus experiments. The spatial variations of the zonal mean flow and the eddy heat and momentum fluxes are in reasonable agreement with the results of a numerical simulation of a steady annulus wave, but the analytical solution is unsatisfactory in some other respects. It is clear that dissipative effects must be represented in any complete theoretical model of these steady waves.
Zonal wind components in the stratosphere and troposphere over India
Authors: Reddy C. A. , Reddi C. Raghava , Mohankumar K. G.
Published: February 2007

Using rocket and balloon data for 1979 and 1980, the stratospheric and tropospheric zonal wind components are quantified for three rocket ranges in India: (a) Thumba (8.5°N 76.9°E); (b) Sriharikota (13.7°N 80.2°E); and (c) Balasore (21.5°N 86.9°E). Values of the mean zonal wind and the amplitudes and phases of the quasibiennial oscillation (QBO), the annual oscillation (AO), and the semi‐annual oscillation (SAO) in the altitude range 2–60 km are presented. By subtracting the combined contribution of these long‐period components from the measured winds, the shorter‐period fluctuations of zonal wind, attributable to wave disturbances, are obtained. These fluctuations show a significant decrease in amplitude in the height region where the QBO is dominant, signifying a loss of wave momentum in that region. The fluctuations also indicate that some wave modes propagate into the mesosphere without significant attenuation. Significant inter‐annual variability in the AO and the SAO is present. The significance of the observational results is discussed in relation to earlier observations and current theoretical ideas.
Rain from clouds with tops warmer than – 10°C in Israel
Authors: Rangno Arthur L.
Published: February 2007

Rawinsonde observations combined with surface synoptic reports and satellite data, indicate that in Israel rain often falls from clouds with top temperatures ≥− 10° C. This suggests the presence of relatively large cloud droplets and/or high concentrations of ice particles at these temperatures, and a relatively efficient natural precipitation‐forming process.
Sensitivity of medium‐range weather forecasts to the use of an envelope orography
Authors: Jarraud M. , Simmons A. J. , Kanamitsu M.
Published: February 2007

The performances of grid‐square mean and enhanced ‘envelope’ orographies are compared in a set of 10‐day forecasts performed using the ECMWF spectral model at horizontal resolutions T21, T42, T63 and T106. Twenty‐four cases chosen at monthly intervals from May 1983 to April 1985 are examined, and for diagnosis of results for the northern hemisphere these are divided into 12 ‘winter’ (November to April) and 12 ‘summer’ (May to October) situations.

The winter results in many respects confirm those of previous studies. Apart from T21 resolution, use of the envelope orography in this season is clearly beneficial in general, and synoptic assessment illustrates how mountain barrier effects are better represented by the envelope. Conversely, in summer the envelope has a detrimental effect (according to some, though not all measures) at T42 and T63 resolutions, though not at T106. This appears to be at least in part due to the more northerly position of the (northern hemisphere) jet in summer, which interacts with mountains that are less of a barrier, with T106 giving a better separation of the localized orographic features of these regions. Some problems are seen at all seasons with the envelope representation of the Asian relief.

Results for the southern hemisphere are, not surprisingly, generally less sensitive to the representation of mountains, the principal impact coming from the southern Andes and Antarctic Peninsula to the south. A small beneficial impact of the envelope orography is found in the tropics.

In individual cases, the use of envelope rather than mean orographv gives rise to local forecast differences near mountains which tend to propagate and amplify (principally on synoptic scales), following the upper‐level flow. Much of the hemisphere can be influenced in 7–10 days. Ensemble‐ and time‐averaged errors are generally reduced by use of the envelope, though not dramatically, and not at all locations.