bluuebunny/crossref_metadata_2025 huggingface dataset:
Some observations of air flow over the sea
Authors: Hay J. S.
Published: February 2007

Wind speeds were measured over the sea at five heights up to 8 m. Though the fetch was limited, the wind profiles obtained are considered representative of flow over the sea.

The wind speed varies logarithmically with height when the temperature gradient is small. Assuming that laboratory laws of flow are applicable, values of the roughness parameter z0 are deduced. It is found that z0 increases from about 0·02 cm in a wind of 5 m/sec to 0·25 cm at 9 m/sec. The relationship between wind speed and z0 however is not unique for the state of the surface is determined by other factors beside wind speed. In inversion conditions the wind‐speed distribution departs from the logarithmic form a few metres above the surface. The results obtained are compared with those of some other workers.

Bi‐directional vanes were used to obtain gustiness measurements at the site. While the vertical gustiness is found to be of similar magnitude to that over land surfaces of comparable roughness, the cross‐wind gustiness appears to be greater over the sea.
Further studies in world climate
Authors: Simpson George C.
Published: February 2007

The effects of changes in solar radiation on the climate of the earth are discussed with special reference to glacial phases in high latitudes and to pluvial periods which are common to the Earth as a whole. The relationship between the radiation and the glacial and pluvial phases is expressed in tabular form. Adopting the chronology of the glacial and pluvial phases during the Pleistocene Period, as published by K. P. Oakley, a curve of solar radiation is constructed using only the glacial phases; this proves to be a periodic curve with three complete oscillations between the begining of the Pleistocene Period and the present day. Using this radiation curve the periods of heavy and light precipitation are determined and compared with the pluvial periods. Three periods of heavy precipitation during the Pleistocene are indicated and a correlation of these with the observed pluvial periods in Africa is suggested.
The collision efficiency of small drops
Authors: Hocking L. M.
Published: February 2007

The forces on two moving spheres are found from the Stokes equations of motion, full allowance being made for the mutual interference of the flows round the spheres, and the relative trajectories when they are falling under gravity are calculated so that the collision efficiency E can be found. The values of E are determined for drops of radius up to 30 μ colliding with droplets of any smaller size. The results differ widely from those found by Pearcey and Hill (1957) and it is suggested that the reason for the discrepancy is that the motion of the spheres when they are close together is of great importance in determining E. The results also differ from those found by Schotland and Kaplin (1956). Their experiments did not model the ratio of the density of the spheres to that of the medium through which they are falling and since it is shown that this factor is of importance, their results are not considered applicable to water drops falling in air.
Observations from aircraft of temperatures and humidities near stratocumulus clouds
Authors: James D. G.
Published: February 2007

Observations from aircraft of the Meteorological Research Flight flying near stratocumulus clouds showed steep inversions and hydrolapses directly above the tops of the clouds. Turbulence was encountered below and in the clouds and up to 300 ft above the cloud top. Above this level no turbulence was encountered. Analysis of the heat and water‐vapour budgets of the cloud and air below suggests that the cloud and the profiles of temperature and humidity can only be maintained if there is large‐scale subsidence. The effects of subsidence, outgoing radiation and turbulent transfer are all comparable.
Wind stress on a water surface: Measurements at low wind speeds with the aid of surface films
Authors: Vines R. G.
Published: February 2007

A monolayer on a water surface is affected by the action of wind, and retracts until the wind drag is counter‐balanced by the opposing surface pressure gradient. This effect has been used to determine the shearing stress on water at low wind speeds. Estimates of the drag coefficient have been obtained.
The small‐ion concentration and space charge near the ground
Authors: Adkins C. J.
Published: February 2007

A series of detailed measurements was made of the concentration of small ions near the ground. It was found that for values of the atmospheric potential gradient exceeding some ± 500 v m−1 the electrode effect in the small ions was readily observable, and that when rain fell in a large electric field, there took place a large production of ions of sign opposite to that of the field. The latter effect was examined in some laboratory experiments which showed that the release of charge in the form of ions was associated with splashing. The mechanism by which the ions were produced was found and a determination made of the dependence of the charge released by individual drops on the field, drop mass and drop speed at impact.

To determine the importance of the charge released by splashing at the ground, another series of observations was made including the measurement of space charge. It was found that in fine weather the field was sometimes controlled by clouds of charge near the ground. In disturbed weather natural point discharge gave rise to considerable values of the space charge, and in comparison with these, the charges produced by the splashing of rain only became important during heavy rain.
Temperature fluctuations and trends over the earth
Authors: Callendar G. S.
Published: February 2007

The annual temperature deviations at over 400 meteorological stations are combined on a regional basis to give the integrated fluctuations over large areas and zones. These are shown in graphical form, and it is concluded that a solar or atmospheric dust hypothesis is necessary to explain the world‐wide fluctuations of a few years duration. An important change in the relationships of the zonal fluctuations has occurred since 1920. The overall temperature trends found from the data are considered in relation to the homogeneity of recording, and also to the evidence of glacial recession in different zones. It is concluded that the rising trend, shown by the instruments during recent decades, is significant from the Arctic to about 45°S lat., but quite small in most regions below 35°N. and not yet apparent in some. It is thought that the regional and zonal distribution of recent climatic trends is incompatible with the hypothesis of increased solar heating as the cause. On the other hand, the major features of this distribution are not incompatible with the hypothesis of increased carbon dioxide radiation, if the rate of atmospheric mixing between the hemispheres is a matter of decades rather than years.
The radiation balance of bare soil and vegetation
Authors: Monteith J. L. , Szeicz G.
Published: February 2007

Incoming short‐wave radiation S, reflected short‐wave radiation α S and net radiation R were measured over bare soil and crops from 1957 to 1959, and net long‐wave radiation (L) was deduced from

For grass, α increased from 0·23 at solar elevation 60° to 0·28 at 20° with daily mean 0·26. For bare soil, the corresponding increase was from 0·16 to 0·19 with mean 0·17. In mid‐June, L for bare soil decreased from – 0·1 cal cm−2 min−1 during the night to – 0·4 cal cm−2 min−1 in the early afternoon. For long grass, in August, the corresponding change was from – 0·05 to – 0·22 cal cm−2 min−1. Under clear skies the incoming long‐wave component varied much less than the outgoing component, and net flux L was closely related to surface temperature.

With a heating coefficient β = – dL/dR, the observed linear dependence of R on S in the absence of cloud may be expressed as

Where, formally, R = L0 when S = 0. For grass, sugar beet and potatoes, β lay between 0·15 and 0·22 with a variation which may depend on wind speed rather than on crop. The value for dry bare soil was higher (0·41) because there was greater surface heating.

Measurements under clear skies and over grass at Cambridge and Kew agree well with Rothamsted values (β = 0·22, L0 = – 5·9 cal cm−2 hr−1). Over Nebraska prairie, β = 0·25, L0 = – 4·5 cal cm−2 hr−1 from selected observations during Projects Great Plains and Prairie grass.
Electrification produced by the rupture of large water drops in an electric field
Authors: Matthews J. H. , Mason B. J.
Published: February 2007

The deformation and break‐up of large freely‐falling water drops, of diameter greater than 15 mm, has been studied by high‐speed photography. In confirmation of observations by earlier workers, the drops become distorted and flattened and, if their velocity relative to the air exceeds a critical value, they blow up to form a large bag supported on a toroidal ring of liquid. Eventually the bag bursts to produce a fine spray of droplets and the toroid breaks up into several large drops. the critical conditions for break up and the rate of expansion of the bag are studied experimentally and the observations are compared with available theory. When the drops break in a vertical electric field, a charge of one sign is induced on the bag and a charge of opposite sign appears on the toroidal ring. Disintegration of the drop causes a separation of charge. the charges carried by the large fragments of drops breaking in fields of 30 V/cm, 300 V/cm and 1,500 V/cm have been measured by an induction method. In a downwardly‐directed field of 300 V/cm, such as may be provided by the primary charging mechanism of a thunderstorm, the fragments carry charges of order 1 e.s.u./g., i.e., two orders of magnitude greater than those measured by Simpson in the absence of a field, and may therefore contribute significantly to the formation of the subsidiary positive charge that sometimes appears in the bases of thunderclouds.
Rainfall and moon phase
Authors: O'mahony G.
Published: February 2007

Daily rainfalls of an inch or more for 100 years at one station have been classified according to the phase of the moon with which they occurred. An apparent significant correlation between two 50‐year smoothed sub‐series is found to have been induced by the filtering process of taking running sums. When the effects of the ensuing serial dependence are eliminated, the results are not indicative of a significant overall relationship between the series of rainfalls and moon phase.

In both sub‐series, a persistent trough in frequency count of heavy rainfalls about full moon has been investigated further by examining data from three additional stations in different rainfall regimes. The trough was evident in the data from each of the frequency counts of rainfalls near full moon. However, when each series was viewed in isolation, statistical tests revealed that the observed numbers of rainfall occurrences were not significantly different from the expectations based on the assumption that the series were random. The consistency of the trough rather than its amplitude suggested a weak non‐random influence might be present.

An additional product of the statistical analysis of the single station data is an indication of the differing filtering effects of the smoothing process as the number of elements in the running sums is increased progressively from one to ten.
Critical layer instability in baroclinic flows
Authors: Bretherton F. P.
Published: February 2007

It is shown that for all inviscid, adiabatic, quasi‐geostrophic motions in a Boussinesq liquid or in a compressible atmosphere, subject to the usual boundary conditions, the integral over the fluid of the northward eddy flux of potential vorticity vanishes, provided a generalized interpretation of potential vorticity is adopted near rigid horizontal boundaries. For an infinitesimal disturbance in a normal mode to a general zonal flow, the presence of a critical layer within the fluid excludes the existence of stable modes, except in exceptional circumstances. A stable, or slowly growing mode, is associated with a down‐gradient flux of potential vorticity near the critical layer, which does not vanish as the growth rate tends to zero, and can only be balanced by the general growth in the disturbance.
Transvaal hailstones
Authors: Carte A. E. , Kidder R. E.
Published: February 2007

Characteristics of a large number of hailstones which have fallen in the Pretoria‐Johannesburg area in the past five years have been summarized under the following headings: size distributions, shapes, growth centres, layers and internal structures.

Most hailstorms did not produce large hailstones. Only 4 per cent of over 5,000 hail reports were of stones exceeding 3 cm diameter. With increase in size of the largest hailstones, broader size spectra tended to occur and the modal size of hailstones reported as being most common increased. Spheroidal shapes were encountered more often than others in practically all size groups but their relative frequency diminished with size. A high proportion of the hailstones had opaque growth centres. An investigation of layering revealed that many hailstones have more changes of opacity than of crystal structure. Two, three and four crystal layers were often found in medium, large and extra‐large stones. A simple up‐and‐down trajectory can account for the structure of many stones. Slushy ice may form on large hailstones but it is not an essential growth stage.
Albedos of natural surfaces in Barbados
Authors: Chia Lin‐Sien
Published: February 2007

In Barbados, albedo values for bare soil surfaces varied according to colour, the moisture condition of the surface layer, the presence of limestone concretions and the roughness of the surface. The albedo of sugar cane increased with crop height until the leaves reached complete cover. The albedo of vegetation, but not bare soil, depended on the zenith angle of the sun. The albedo of vegetation and cane trash was larger under clear skies than under overcast. The estimated mean albedo of Barbados was 16·5 per cent.
The effects of winds on acoustic‐gravity waves from explosions in the atmosphere
Authors: Mackinnon R. F.
Published: February 2007

Pressure fluctuations due to an explosive point‐source in an atmosphere containing steady winds may be described in terms of a fundamental mode of long period followed by modes of the acoustic and gravity type, with relative amplitudes, group speeds and periods dependent upon winds. Far‐field ground‐pressure contributions of various modes are presented for a particular model atmosphere. Inverse dispersion and the Airy phase are found to be important features of the dispersion. A study is made of barograms in the light of new results obtained. Some previously unexplained features of barograms are seen to be attributable to winds.
Experimental and field studies of precipitation particles formed by the freezing of supercooled water
Authors: Brownscombe J. L. , Hallett J.
Published: February 2007

Drops which freeze in isolation or by accretion on an ice particle are found to be single or polycrystalline depending on the drop supercooling and the particle temperature. Subsequent growth from the vapour depends on the number and orientation of these crystals. The airflow around the particle, characterized by the Reynolds Number, Re, is important in the initial accretion process. The particle falls steadily for small Re, but oscillates and eventually tumbles as Re approaches 500. The deformation of accreted drops depends both on the dendrite freezing velocity through the liquid compared with the impact velocity and on the drop impact kinetic energy compared with its surface energy. Viscous dissipation becomes important as the deformation, defined as the ratio of the base radius of the deformed drop to its height, exceeds 10. Deformations of about 10 have been measured for 40 μ radius supercooled drops impacting on an ice surface at 13.5 m sec−1. The heat economy of a freezing accreted drop is dominated by a collecting particle which is large compared with the drop, when complete freezing takes place from the contact area outwards. Symmetrical freezing, from top and bottom of the drop, only occurs for surface temperature approaching 0°C, or when the drop accretes on a narrow spike, when there is a possibility of drop shatter and separation of electric charge. Drops accreting on particles growing spongily may interact first with a liquid layer whose thickness may be of the same order as the drop size and produce splash droplets even at large supercoolings for critical values of layer thickness. Opacity is related to bubble size. Opaque ice forms when the particle is growing spongily or dry, with transparent ice forming when the growth is just wet. Just spongy growth at low temperatures is associated with small crystals and opaque ice.
A comparison between predictions of dimensional analysis for the constant‐flux layer and observations in unstable conditions
Authors: Swinbank W. C.
Published: February 2007

A number of relationships are derived by dimensional arguments which connect the vertical fluxes of heat and horizontal momentum in the ‘constant‐flux’ layer with the other relevant variables. Observations, all made in unstable conditions, are used to test the correctness of the predicted relationships and to determine their functional form. In particular, it is found that δθ/δz = −1.42θu (gz2)−1 (z/L)0.56 and KH/KM = 2·7 (z/L)1/4.

The analysis emphasises the intimate connection between the transfers of the two quantities.
The structure of rainbands within a mid‐latitude depression
Authors: Browning K. A. , Hardman M. E. , Harrold T. W. , Pardoe C. W.
Published: February 2007

A case study is presented of a winter depression over the British Isles in which extensive banded structure was observed within precipitation ahead of the surface warm front. Measurements of the mesoscale airflow and precipitation structure of the rainbands were made using a variety of radar techniques together with multiple radiosonde and aircraft observations. The measurements were made over the sea to avoid the confusing effects of topography. The dominant rainbands were oriented parallel to the surface cold front and were typically 100 km wide. They moved with a velocity faster than the underlying warm front. For the most part the bands were characterized by clusters of weak small‐scale convective cells due to the release of potential instability produced where tongues of relatively dry air of low θw in the middle troposphere overran low‐level moist air undergoing slantwise ascent above the warm frontal zone. Although there was the usual large‐scale, and thermally‐direct, circulation associated with the active warm front, the air which ascended as small‐scale convection within the rainbands entered a region of weak cold frontal baroclinicity, whereupon it participated in a thermally direct circulation of its own. This led to each rainband having a rearward‐sloping anvil cloud canopy characterized by ascending air with colder drier air descending beneath. Precipitation falling from the canopy evaporated within the underlying drier air thereby probably intensifying the descending branch of the circulation. Very large ageostrophic winds were measured in association with these circulations.

The important ingredient responsible for the convective nature of the rainbands appears to have been the incursion of tongues of relatively dry air of low θw in the middle troposphere above the moist warm‐sector air in a region where the resulting instability could be realized by large‐scale ascent. Although the potential instability was very weak in the present case, the origin of the rainbands appears to have been similar to that of pre‐frontal squall lines. The intensity of the convection within rainbands depends on the stability but the very existence of any precipitation in the first place depends on other dynamical factors leading to widespread ascent.
The use of 500 mb anomalies in long‐range forecasting
Authors: Ratcliffe R. A. S.
Published: February 2007

An objective method of predicting the category of 15‐day and monthly rainfall and mean temperatures in terms of terciles and quintiles over England and Wales is developed. The method selects those areas in the Northern Hemisphere where anomalies on 500 mb monthly mean charts preceding warm months have been significantly different from the anomalies in the same areas preceding cold months at the same time of year. For this purpose data over the period 1945‐72 have been used and Welch's test has been applied to assess the significance of the difference between the two means.

Having found the areas which have been significantly different before warm and cold months, the anomalies at all grid points within these areas are added for each year of the data, taking due account of sign, and the years then ranked in terms of the total anomalies. The result is a list with most of the cold years at the top, the average ones in the middle and the warm ones at the bottom. If the same procedure is carried out on current data and the current month included in the ranking, a forecast of the probable temperature category can be made.

A similar procedure is used for rainfall, and for both elements on the 15‐day time scale. A test involving some 250 forecasts over a period of a year shows excellent results.
Calculations of electric field growth, field structure and charge distributions in thunderstorms
Authors: Illingworth A. J. , Latham J.
Published: February 2007

Calculations have been made of the electrical evolution of a thundercloud under the action of various precipitative charging mechanisms. the cloud is cylindrical, with a defined width and depth. the finite dimensions of the cloud permit the distributions of axial field and charge to be calculated. This was not possible with the infinite parallel‐plate cloud model employed by earlier workers, which in any event has been shown to provide greatly exaggerated rates of field growth.

The calculations take into account various criticisms of other precipitative treatments. These features include the important effects of the dependence of the charge transfer, under an inductive mechanism, on the charge carried by the larger of the two colliding particles and the angle defining their point of separation. Levitation, ionic dissipation and multiple collisions of cloud particles with precipitation are also discussed and shown to be unimportant in situations of interest.

It is found that the water‐water inductive charging mechanism is quite inadequate to explain thundercloud electrification. the available experimental evidence is insufficient to permit a conclusion to be drawn concerning the importance of the corresponding water‐ice mechanism.

Two established mechanisms which appear to be capable of producing breakdown fields in the available time ‐ although only marginally ‐ both involve collisions between hail pellets and ice crystals. These are the inductive mechanism and the thermoelectric mechanism. Although these two processes are about equally powerful, the details of the predicted field‐growth rates, the field structure and the charge distributions under these two mechanisms are quite different. Analysis of the available field evidence is consistent with the inductive theory in some cases and the thermoelectric theory in others. However, a combination of these two processes appears to be consistent with all the evidence, and to provide a tenable theory of thundercloud electrification.
Measurements of the deposition coefficient for ice, and its application to cirrus seeding
Authors: Choularton T. W. , Latham J.
Published: February 2007

Two techniques have been used to measure the deposition coefficient β for water vapour molecules onto ice formed from distilled water, from 0·01M HCl and from 0·1M HCl. One involved measurement of the evaporation rates of ice spheres at laboratory pressure. the other involved measurement of the equilibrium temperature achieved by a shell of ice surrounding a resistor within which electrical power was being dissipated at a known rate; this experiment was conducted at low pressures (< 10−2torr). Measurements of β were made over the temperature range −37 to −9°C.

It was found that for pure ice β increased from 10−3 at −37°C to 5 × 10−3 at −9°C. Similar temperature dependency was found for the ice specimens formed from HCl solutions, but the values of β were lower at all temperatures. This reduction was greater for the more contaminated ice.

The measured values of β were employed in calculations of the distance which cirrus crystals will fall through undersaturated air below cloud base. Various humidity structures and temperature distributions were considered. For a particularly well‐documented case it was calculated that crystals of dimensions 80 and 300 μm will survive a fall of about 0·5 and 2·3 km respectively; these predictions are consistent with observation. These two values of survival depth are greater than the Maxwellian values (β = 1) by factors of about 3 and 1·2, respectively. It is concluded that in most situations the persistence of a stream of cirrus crystals is not profoundly affected by these new values of β but that in conditions conducive to the formation of middle‐level cloud the survival depths may be substantially increased.
Standard distributions of clear sky radiance
Authors: Steven Michael D. , Unsworth M. H.
Published: February 2007

Diffuse radiation from cloudless skies varies with solar height and atmospheric turbidity but its angular distribution remains generally characteristic of single scattering. It was found that when sky radiance measurements, N, were normalized with respect to horizontal diffuse irradiance, Sd, the angular distributions of N/Sd were remarkably independent of atmospheric turbidity. This enabled standard distributions of normalized sky radiance, based on a long series of measurements on clear skies, to be drawn up for different solar zenith angles. Due to the difference between radiometric and photometric units these distributions are not comparable with previously published luminance distributions. Departures due to variation in atmospheric turbidity, of the radiance distributions from the proposed standards, were analysed and found to be small for the range of turbidities (τα from 0·1 to 0·5) normally encountered in Britain.
Evaluation of the aerodynamic method of determining fluxes over natural grassland
Authors: Saugier B. , Ripley E. A.
Published: February 2007

Profiles of wind speed, temperature, humidity, and CO2 content up to a height of 6.5 m were measured over a period of several months above natural grassland in an area with an average fetch of 2km. Profile shapes have been analysed in relation to atmospheric stability.

In unstable conditions, temperature and CO2 profiles exhibit similar shapes and depart more from logarithmic than does the wind profile. All profiles are relatively well described by existing empirical stability corrections.

In stable conditions all profiles have approximately the same shape: they depart from the logarithmic profile at moderate stabilities and return to it at strong stabilities. However, the oft‐quoted log‐linear profile does not provide a very good fit.

Empirical stability corrections have been used to develop a generalized aerodynamic method for flux computation. This method tends to underestimate (by up to 20%) fluxes during unstable conditions and to overestimate (by up to 40%) in moderately stable conditions. These results stem from a comparison of aerodynamic and Bowen ratio estimates of sensible heat flux. They are in essential agreement with the analysis of profile shapes.

The drag coefficient of vegetation may be predicted from the leaf area index. This allows the use of a simplified aerodynamic method, based on measurements of wind speed at one level and of temperature (or H2O or CO2) at two levels.
The influence of drop size distribution on the production of secondary ice particles during graupel growth
Authors: Mossop S. C.
Published: February 2007

The production of secondary ice particles when a moving body gathers rime in a supercooled cloud at −5°C has been studied for various drop size distributions. It is found that the rate of production of ice ‘splinters’ depends not only upon the concentration of large drops (⩾about 24 μm diameter) but also upon the concentration of small drops (⩽about 13μm) in the cloud.
Spectral distribution of solar radiation. II: Global and diffuse
Authors: McCartney H. A.
Published: February 2007

Measurements of the spectral distribution of global and diffuse solar radiation made in both wide and narrow wavebands are reported. Relationships between the ratio of diffuse to global irradiance, α, in both the visible and total solar wavebands, and atmospheric turbidity and zenith angle are developed. Narrowband measurements are analysed to yield relationships between α and spectral turbidity in the waveband 400–800 nm. The measurements indicate that atmospheric turbidity has a greater influence on both the magnitude and shape of diffuse irradiance than of global. The biological implications of the results are briefly discussed.
Three‐dimensional lee‐wave pattern
Authors: Gjevik B. , Marthinsen T.
Published: February 2007

Satellite photographs are analysed to study the lee‐wave patterns generated by isolated islands in the Norwegian Sea and the Barents Sea.

In situations where the wave motion is confined to the lower atmosphere (trapped waves) the waves are located within a wedge‐shaped wake behind the islands. Both the diverging wave type, where the crests are orientated outwards from the centre of the wake, and the transverse wave type, where the crests are nearly perpendicular to the wind direction, are observed. The former wave type is, however, the more common.

In certain situations a long single‐crested wave is observed at Jan Mayen. The wave appears on photographs as a straight lane in the cloud layer. In one case it extends sideways from the island to a distance of about 350km. Wave kinematics is used to obtain the phase lines for a steady wave pattern for different atmospheric models. The theory is found to explain some important features of observed wave forms.
Penetrative convection at the top of a growing boundary layer
Authors: Mahrt L.
Published: February 2007

Motions at an entraining interface of a growing mixed layer are ideally partitioned into large eddies, such as plumes and thermals which act to distort the interface, and smaller scale turbulence responsible for entrainment through the interface.

Influences of horizontal entrainment and pressure effects on the depth of penetration of large eddies into the stratified free flow are examined. This penetration plays an important role in the initiation of cumulus at the mixed layer top and modifies the turbulence energy balance and entrainment rate there. For example, previous models predict the entrainment rate to depend on the cube of the fundamental velocity scale at a stratified interface and linearly on this velocity scale with vanishing stratification (free entrainment). The present development, which includes specific roles of penetrating elements, suggests a possible intermediate quadratic regime corresponding to weak inversion strength.
Microphysics and dynamics of clouds associated with mesoscale rainbands in extratropical cyclones
Authors: Matejka Thomas J. , Houze Robert A. , Hobbs Peter V.
Published: February 2007

Measurements obtained with research aircraft in clouds associated with various types of mesoscale rainbands in extratropical cyclones have been examined together with radar data and other detailed measurements. The analyses have yielded information on the structure and microphysical characteristics of the clouds and the nature of the vertical air motions in the clouds associated with each type of mesoscale rainband, and schematic models of the different mesoscale rainbands have been constructed.

In clouds accompanying warm frontal rainbands, ice particles formed above the warm front, probably in shallow convective cells, and fell into the stable air mass below, where they grew by aggregation and helped glaciate stratiform clouds.

Warm sector rainbands resembled squall lines. A younger, convectively active rainband occurred just ahead of an older, less convectively active one. The younger rainband contained both cloud liquid water and ice particles, and ice particles were growing by riming. The older rainband was nearly glaciated, with ice particles growing by aggregation.

Narrow cold frontal rainbands were located along the convergence lines at the advancing edges of cold frontal zones. Each was associated with a convective updraught, 1 to 5 km wide and 1 to 3ms−1 in magnitude, and a similarly narrow downdraught. The core of the updraught contained young, developing cloud, with much higher cloud liquid water content and lower ice particle concentration than in its immediate surroundings. Ice particles grew by aggregation and were heavily rimed. Wide cold frontal rainbands (several tens of kilometres in width) occurred when lifting above a cold front was enhanced to several tens of centimetres per second. Clouds in this region consisted of supercooled water and ice and contained embedded convective elements. Below the cold front, high concentrations of ice particles grew by aggregation.

Prefrontal surges of cool and dry air occurred above the warm front and ahead of the cold front in occluded cyclones. A surge rainband, resembling a wide cold frontal rainband, occurred at the leading edge of the surge and was followed by a field of convective elements. These elements, sometimes arranged in lines, were in various stages of development.

Postfrontal rainbands were lines of convective clouds located well behind a cold front. They may have been associated with secondary fronts and sometimes appeared to behave as organized convective systems.
Correction of flux measurements for density effects due to heat and water vapour transfer
Authors: Webb E. K. , Pearman G. I. , Leuning R.
Published: February 2007

When the atmospheric turbulent flux of a minor constituent such as CO2 (or of water vapour as a special case) is measured by either the eddy covariance or the mean gradient technique, account may need to be taken of variations of the constituent's density due to the presence of a flux of heat and/or water vapour. In this paper the basic relationships are discussed in the context of vertical transfer in the lower atmosphere, and the required corrections to the measured flux are derived.

If the measurement involves sensing of the fluctuations or mean gradient of the constituent's mixing ratio relative to the dry air component, then no correction is required; while with sensing of the constituent's specific mass content relative to the total moist air, a correction arising from the water vapour flux only is required. Correspondingly, if in mean gradient measurements the constituent's density is measured in air from different heights which has been pre‐dried and brought to a common temperature, then again no correction is required; while if the original (moist) air itself is brought to a common temperature, then only a correction arising from the water vapour flux is required.

If the constituent's density fluctuations or mean gradients are measured directly in the air in situ, then corrections arising from both heat and water vapour fluxes are required.

These corrections will often be very important. That due to the heat flux is about five times as great as that due to an equal latent heat (water vapour) flux. In CO2 flux measurements the magnitude of the correction will commonly exceed that of the flux itself. The correction to measurements of water vapour flux will often be only a few per cent but will sometimes exceed 10 per cent.
The transport of minor atmospheric constituents between troposphere and stratosphere
Authors: Robinson G. D.
Published: February 2007

Measurements of the water vapour content of air in the upper troposphere and lower stratosphere are reviewed. They are interpreted to indicate that significant upward movement of air through a pressure level of about 100 mb, by processes of any scale, occurs only in equatorial latitudes, but they leave uncertain the mechanism of the upward transfer. This implies that contaminants emitted into the atmosphere below this level can reach the upper stratosphere only by way of the equatorial tropopause and after passage through precipitating cloud. The minimum hemispheric exchange of mass through this level is about 1017kg per year. The mass balance of several minor atmospheric constituents, naturally or artificially introduced in the surface layers or in the high stratosphere, is shown to be consistent with this minimum mass exchange.
Dew and thermal lag: Measurements and an estimate of wetness duration on cocoa pods
Authors: Butler D. R.
Published: February 2007

A model to estimate the duration of condensed water on the surface of cocoa pods after sunrise is modified to use readily available meteorological data. Changes in wind speed, air temperature and humidity with height and time in a cocoa plantation are presented. Pod temperature and the amount of condensation calculated from the maximum and minimum temperatures and humidity at 0900h agree well with measured values. The effects on pod wetness of changing pod size, short‐wave radiation and wind speed are investigated.
The albedo of a tropical evergreen forest
Authors: Pinker R. T. , Thompson O. E. , Eck T. F.
Published: February 2007

The diurnal variation of the average albedo, during 1970, over a tropical dry evergreen forest and a nearby clearing was investigated. The study is based on a most comprehensive set of solar radiation data in Southeast Asia. A strong dependence of the albedo on the zenith angle is evident. This dependence is subdued during cloudy days. The average midday albedo for the winter monsoon season is 10.6 for the forest and 13.4 for the clearing; for the summer monsoon it is 12.0 for the forest and 14.6 for the clearing. Generally, results show great uniformity. The largest standard deviations are 0.05 with a more representative value in the range 0.01–0.02.
The diagnosis of middle latitude synoptic development
Authors: Hoskins B. J. , Pedder M. A.
Published: February 2007

The use of diagnostics based on different forms for the forcing term in the omega equation is explored. These forms are the two‐level and continuous versions of the approximation used by Sutcliffe (1947) in his development theory, the usual dynamical meteorology version involving vorticity and thermal advection, and that involving the so‐called Q‐vectors which was introduced by Hoskins et al. (1978). The diagnostics are applied to a model baroclinic wave and to a subjectively analysed real data case. The Sutcliffe form is simplest and gives a global view of the system movement and development, but details such as active frontal regions are missed. The vorticity and thermal advection form has few advantages. It is demonstrated that the Q‐vector analysis can provide more information than the Sutcliffe form in describing details of system development, particularly with respect to (a) a vectorial view of the horizontal ageostrophic motion field, and (b) some indication of the intensity of frontal circulations. A case is presented for including Q‐vector fields in low‐ and mid‐tropospheric forecast charts.
Cluster analysis of rainfall stations of the Indian peninsula
Authors: Gadgil Sulochana , Narayana Iyengar R.
Published: February 2007

Principal component analysis is applied to derive patterns of temporal variation of the rainfall at fifty‐three stations in peninsular India. The location of the stations in the coordinate space determined by the amplitudes of the two leading eigenvectors is used to delineate them into eight clusters. The clusters obtained seem to be stable with respect to variations in the grid of stations used. Stations within any cluster occur in geographically contiguous areas.
The relevance of conditional symmetric instability to the prediction of mesoscale frontal rainbands
Authors: Bennetts D. A. , Sharp J. C.
Published: February 2007

This paper examines the theory of Conditional Symmetric Instability (Bennetts and Hoskins 1979) and considers its relevance to meso‐scale frontal rainbands. The theoretical growth rate of the instability is evaluated from synoptic scale data obtained from a numerical forecast model and compared with observations of the horizontal structure of rainfall observed by radar. One case study is presented in detail and a further 44 cases summarized. Rainbands were observed on 80% of the occasions on which they were predicted.
Two‐dimensional convection in non‐constant shear: A model of mid‐latitude squall lines
Authors: Thorpe A. J. , Miller M. J. , Moncrieff M. W.
Published: February 2007

Numerical simulations of two‐dimensional deep convection are analysed using analytical models extended to include shallow downdraughts and non‐constant shear. The cumulonimbus are initiated by low‐level convergence created by a finite amplitude downdraught. These experiments have constant low‐level shear and differ only in the profile of mid‐and upper‐level winds. Quasi‐steady convenction is produced if the mid‐ and upper‐level flow has small shear and the low‐level shear is large. The surface precipitation ismaximized for no intial relative relative flow aloft, if stationary, this storm (P(O)) can give prodigious locilized rainfall; P(O) is the two‐dimentisonal equivalent of the supercell. These results are placed in context with previous two‐dimensional simulations. Attention is drawn to the similiarity with previous two‐dimensional simulations. Attention is drawn to the similarity with squall lines in central and eastern U.S.A.

Storm P(O) is analysed by construction of time‐averaged fields of streamfunction, vorticity, teperature, and height deviation. The smoothness of these fields suggests a conceptual model of the storm dynamics which involves cooperation between distinct charcteristic flows; an overturning updraught, a jump type updraught, a shallow downdraught, a low‐level rotor, and a boundary layer.

An idealized analytical model is described by solution of the equations for steady convection. These solutions, for the remote flow, are derived from energy conversation, mass continuity and a momentum budget, and they give relationships between the non‐dimensional parameteres of the problem. It is apparent that the convection is a high Froude (or low Richardson) number flow demanding the existence of a cross‐storm pressure gradient. Inherent in this idealized model is a vortex sheet between updraught and down‐draught and it is considered that the dynamical instability of this sheet is related to complexities in the numerical simulation. Furthermore, these results show that in two‐dimensions both non‐constant shear and a shallow downdraught are necessary to maintain steady convection.
Angular momentum transports in tropical cyclones
Authors: Holland Greg J.
Published: February 2007

Angular momentum budget equations are developed for Eulerian and Lagrangian cylindrical coordinate systems centred on a tropical cyclone. All non‐friction terms are reduced to boundary transports and the effects of cyclone motion in the Eulerian coordinate system are isolated. A number of intensifying and weakening composite northwest Pacific cyclones are used to illustrate the physical role and relative importance of each process in the maintenance or destruction of tropical cyclones at various intensity stages.
Freezing modes of riming droplets with application to ice splinter production
Authors: Griggs D. J. , Choularton T. W.
Published: February 2007

Laboratory studies of riming have shown that, provided the heat loss is fairly symmetrical, the internal freezing pattern of an accreted drop shows two distinct modes in the temperature range −3°C to −9°C and a transition to a third below this temperature. (1) At high temperatures the freezing is dominated by an ice front growing outwards from the rimer with a somewhat enhanced growth rate around the drop edge. (2) At temperatures below about −5°C two ice fronts are evident, one from the substrate, one from the remote edge of the drop. (3) At temperatures of −9°C and below the freezing is essentially symmetrically inwards. Freezing modes (1) and (2) sometimes give rise to protuberances which may be associated with the Hallett‐Mossop ice splinter production. These are suppressed by the addition of ammonia to the drop. The three modes of freezing are accounted for in terms of the pattern of dendritic growth during the first stage of freezing, and it is suggested that this explains the low temperature cut‐off in secondary ice particle production.
A laboratory experiment on momentum and heat transfer in the stratified surface layer
Authors: Fukui Keisuke , Nakajima Masamoto , Ueda Hiromasa
Published: February 2007

Stratification effects on transport processes in the atmospheric surface layer or in the wall region are investigated by a laboratory experiment. A fully developed stratified condition was attained in turbulent flow between horizontal parallel plates kept at different temperatures. The experiment was performed both for stable and unstable conditions and provided conclusive data on stratification effects. A local gradient model is proposed, based on the concept that stratification effects may be evaluated at the same non‐dimensional distance from the wall z+ (= zu*w/vw u*, vw being friction velocity and kinematic viscosity, respectively, at the wall), the effects depending only on ε (ε z/L; L being the Monin‐Obukhov length). A comparison of the shear functions with previous field and laboratory experiments shows excellent agreement. By use of this concept, the measured intensities of velocity and temperature fluctuations near the wall are also well correlated with ε.
The propagation of eddies in diffluent jetstreams: Eddy vorticity forcing of ‘blocking’ flow fields
Authors: Shutts G. J.
Published: February 2007

Numerical experiments are performed with barotropic models to test a hypothesized mechanism by which barotropic eddies can, through Reynolds stresses, reinforce blocking flow patterns. Eddies propagating into a split Jetstream suffer an east‐west compression and north‐south extension of their vorticity fields and this enhanced, local enstrophy cascade is associated with energy transmission to the straining flow (i.e. the blocking flow field) and a characteristic pattern of vorticity forcing by transient motion.

As a first step in demonstrating this mechanism, the barotropic vorticity equation is integrated in a form linearized about a state chosen to represent a split Jetstream with a prescribed eddy forcing function upstream. After thirty days of model integration, the time‐mean eddy kinetic energy, enstrophy and vorticity flux divergence are calculated, together with mean eddy vorticity flux vectors. The resulting eddy vorticity forcing field can be used to determine the second‐order induced flow.

Experiments with the nonlinear barotropic vorticity equation show that dipole blocking patterns can be created simply by the introduction of an eddy generator into sufficiently weak, uniform westerly flow. Eddies alone are responsible for the block since the time‐mean vorticity equation has no external forcing functions such as an orographic term. Monthly mean statistics are again calculated and terms in the time‐mean vorticity equation are compared. Anticyclonic eddy vorticity forcing appears just upstream of the ‘blocking high’ in agreement with data analysed by Illari (1982) for the anomalous circulation over Europe in July 1976. Model flow fields fluctuate in a manner highly reminiscent of blocking episodes as observed in 500mb contour maps with strong bursts of southerly winds on the western flank of blocking highs during the approach and subsequent stagnation of each depression. The proposed straining mechanism and model simulations are in agreement with early observational studies of blocking.