bluuebunny/crossref_metadata_2025 huggingface dataset:
On the dynamics of the general circulation
Authors: Fleagle Robert G.
Published: February 2007

The dominant features of large‐scale atmospheric motion emerge from a relatively simple treatment of the hydrodynamic equations, thus providing clear, intuitive understanding of the relation between cause and effect in the general circulation. The analysis is made possible by applying the perturbation equations to a sloping stream surface located in the middle troposphere. For reasonable values of the arbitrary parameters, the results show that baroclinic disturbances are unstable poleward of a critical latitude, roughly 35°, and that the preferred number of such unstable disturbances at any one time decreases from about 10 at 45°, to 2 at 80°. Greatest intensity occurs at about 60°. Jet streams must form close to the critical latitude and, in the case of long disturbances, at higher latitudes. Anticyclonic shear has a stabilizing influence, so that the normal velocity distribution in middle latitudes offers considerable resistance to penetration of disturbances to low latitudes. Below the critical latitude unstable disturbances are possible only in relatively small areas where the hydrostatic stability is nearly zero. Growth of these disturbances is favoured by anticyclonic shear, as well as by baroclinicity and by high latitude. Disturbances of maximum rate of growth are practically non‐dispersive so that their growth depends mostly on available potential energy. If sufficient energy is available, the identity of individual disturbances can be maintained over extended time periods. Wave speed relative to the zonal current exhibits a maximum positive value at a high latitude.
Anisotropic high‐level turbulence
Authors: Clodman J.
Published: February 2007

A study is made of several cases of high‐level turbulence in which the intensity varied with the direction of flight of the aircraft. The direction of maximum turbulence is found to coincide fairly well with the direction of the contour lines. A theory is developed which attributes this type of turbulence to quasi‐horizontal gusts associated with strong horizontal wind shear. Gravity waves are examined as a possible alternative explanation and tentatively rejected.
Preliminary discharge processes in lightning flashes to ground
Authors: Clarence N. D. , Malan D. J.
Published: February 2007

By using different techniques of recording the electrical effects of lightning discharges occurring at varying distances, it has been possible to sort out the major components of pre‐return stroke field changes. A comparison between these field changes has shown that the first return stroke of a lightning flash to ground is usually preceded by three successive and distinct discharge processes.

The first, or breakdown stage, is identified from the reversal of its field change with distance as a discharge between the main negative charge centre and the positive charge centre near the base of the cloud at heights of 3.6 and 1.4 km respectively, the discharge probably just as often starting from the positive as from the negative centre. This is followed by the second stage during which the highly conducting channel then existing in the cloud is being negatively charged. The charging continues until the field in front of the channel has become sufficiently strong to propagate a downward‐moving stepped leader breakdown streamer.

Stepped leaders have been observed to precede more than 75 per cent of all flashes to ground, though the radiation fields of most stepped leaders are so small that they can only be detected by using very high amplification.

The probable mechanisms involved in the preliminary stages are discussed.
Some further observations from aircraft of frost point and temperature up to 50,000 ft
Authors: Helliwell N. C. , Mackenzie J. K. , Kerley M. J.
Published: February 2007

During 1955, 46 measurements of frost point and temperature were made at varying altitudes up to about 50,000 ft using a Dobson‐Brewer pressurized frost‐point hygrometer and a flat‐plate thermometer mounted on the Canberra aircraft of the Meteorological Research Flight. The general character of these ascents in the lower stratosphere confirms the conclusions of earlier papers (Murgatroyd, Goldsmith and Hollings 1955). The mean frost point at 48,000 ft for the ascents made in 1954 and 1955 is ‐117°F and at 46,000 ft ‐115°F, with a standard deviation of about 3°F in each case.
On a distribution of weather cycles by length
Authors: Gabriel K. R. , Neumann J.
Published: February 2007

In the paper a weather cycle is defined as the combination of a wet (dry) spell with the successive dry (wet) spell. On the assumption that the lengths of wet and dry spells are independent and obey geometric distributions, the distribution function of the lengths of weather cycles is deduced and tested against observational data for Tel Aviv. The peak frequency of weather‐cycle lengths at Tel Aviv is found to be at about 4 days.
The travel and diffusion of the radioactive material emitted during the Windscale accident
Authors: Crabtree J.
Published: February 2007

The reactor accident at Windscale on 10‐11 October 1957 resulted in the release of radioactive fission products into the atmosphere over a period of several hours. Measurements were made of the fission products, particularly iodine‐131, collected on filters exposed for the study of atmospheric pollution in many places in the United Kingdom and Europe: measurements were also made of the iodine‐131 deposited on grass. These showed that the effluent had been widely dispersed, but that activities over England were about an order of magnitude greater than those found over the neighbouring areas of the continent. Meteorological trajectory analysis showed that the main portion of the cloud had remained over England until 15 October, and that the activities over continental Europe were due to the eastern edge of the cloud.

Meteorological data were used together with the air concentration data to deduce that about 22,000 curies of iodine‐131 were present in the cloud as it passed over south Lancashire and Yorkshire.
Synoptic and dynamic aspects of climatic change
Authors: Kraus E. B.
Published: February 2007

The changing pattern of world climate during the last hundred years is compared with the inferred pattern of the Pleistocene. Relatively cool climates at high latitudes appear to have been associated much of the time with reduced tropical sea‐surface temperatures, a lower snow‐line on tropical mountains, reduced aridity on the desert fringes and increased storm activity along continental east coasts in middle latitudes. Relatively warm climates at high latitudes tend to be associated with the opposite complex of conditions.

Considerations of the water balance suggest that a small temperature decrease would be necessary and sufficient for the re‐establishment of glacial conditions in many regions. Increased snowfall would help but is not essential. Synoptic considerations indicate that glaciation was associated necessarily with strong upper westerlies at relatively low latitudes over the continents and probably with vigorous anticyclones over the subtropical oceans.

An increased infra‐red cooling rate in the upper air, particularly over the tropics, could produce all the necessary conditions for the establishment of a glacial/pluvial phase. Possible causes for changes in the infra‐red cooling rate are discussed.
Observations of precipitation elements in cumulus clouds
Authors: Murgatroyd R. J. , Garrod M. P.
Published: February 2007

Cloud particles of diameters greater than about 100 μ can be sampled from an aircraft by exposing within the cloud a thin aluminium‐foil surface to the airstream and counting and sizing the indentations made on it. A series of flights has been made through cumulus clouds using this technique with the object of obtaining data on the concentrations of these larger particles in clouds of different size and temperature. A series of observations showing the measured concentrations is presented and discussed. Concentrations range from less than 1/m3 in the smaller cumulus clouds up to 1/litre or more in clouds about 1 km thick.
The flux of water vapour due to the mean winds and the convergence of this flux over the Northern Hemisphere in January and July
Authors: Bannon J. K. , Matthewman A. G. , Murray R.
Published: February 2007

Charts are presented showing the flux of water vapour due to the mean winds, based on the mean distribution of water vapour and the mean geostrophic winds, in January and July, over an area comprising most of the Northern Hemisphere.

There is evidence that this flux approximates fairly well to the total flux of water vapour, but the divergence of this flux does not always approximate so well to the divergence of the total flux. Nevertheless, comparison with independent estimates of evaporation minus precipitation over the oceans suggests that the two divergences are at least of the same order of magnitude, and further comparison with published charts of the divergence of the total annual flux of water vapour suggests some degree of detailed correspondence.
Statistics of lateral gustiness at 16 m above ground
Authors: Smith F. B. , Abbott P. F.
Published: February 2007

The horizontal lateral wind‐fluctuations at 16 metres above ground have been recorded over periods totalling 1,350 hours by a system described by Jones and Pasquill (1959). Hourly‐average values ⊝τ of the standard deviation of wind fluctuations sampled within times τ = 5, 30, and 180 sec are classified according to wind speed and stability s = ΔT/U2 °F/(m/sec)2 (where ΔT is the temperature difference between 7.1 m and 1·2 m, U is the wind speed at 15.5 m). A critical value of stability s = + 0·017 is indicated separating near‐neutral or unstable stabilities (s > 0·017), for which ⊝τ depends primarily on stability, from very stable conditions (s > 0·017), for which wind speed is the dominant parameter. Some support for this transition is also found in the variation of spectrum shape with stability. A second transition on the unstable side may correspond to Priestley's critical value separating forced‐ and free‐convection regimes.

The results are in a form suitable for application to particle diffusion in the atmosphere. However, the large variations in wind direction in very stable conditions with light winds cannot be interpreted, at present, in terms of diffusion with the same degree of certainty as in near‐neutral stabilities.
The introduction of topographic and frictional effects in a baroclinic model
Authors: Graystone P.
Published: February 2007

The effects of large‐scale topography and surface friction on short‐range numerical forecasts are investigated using a baroclinic two‐parameter prediction model, modified to include forced vertical velocities. Trial forecasts showed that the damping effect of surface friction is reproducible, but only moderate success was achieved in accounting for orographic influences.
Intensities and spectra of vertical wind fluctuations at heights between 100 and 500 ft in neutral and unstable conditions
Authors: Thompson N.
Published: February 2007

Records of vertical gustiness have been obtained using a sensitive vane attached to the cable of a captive balloon. Electrical filtering of the output signal from the vane reduced the labour of subsequent computation. Intensities of turbulence were found to decrease with increasing wind speed. Length scales generally showed no regular trend with height below 400 ft, because of (a) the dependence of scale on stability, and (b) the heterogeneity of the present data as regards stability. Most spectra exhibited two major peaks.
Attenuation of solar radiation: A climatological study
Authors: Monteith J. L.
Published: February 2007

Daily totals of direct and diffuse radiation transmitted by a cloudless atmosphere are calculated from the absorption and scattering coefficients given by Houghton (1954). Estimates of total (direct plus diffuse) radiation (T1) agree well with extreme maxima recorded at several British stations, but during spells of cloudless summer weather in south‐east England attenuation by aerosol decreases total radiation on average by 10 per cent, and increases the ratio of diffuse to total radiation by about 8 per cent of T1. The ratio of mean monthly radiation at country stations T2 to estimated T1 can be expressed as a function of cloud reflection (ρ), cloud absorption (ϕ), and surface reflection (α); and the values ρ = 0·50, ϕ = 0·16 are consistent with surface and aerial measurements. With α = 0·20, T2/T1 is approximately (1−0·61c) (0·6 < c < 0·9) where c is fractional cloudiness. The diffuse component beneath cloud can be estimated separately. Absorption by man‐made aerosol reaches maxima of 26 per cent at Kew and 30 per cent at Kingsway in January, and varies seasonally with measured smoke. In the country, absorption by pollution is negligible in summer but may reach 5 to 10 per cent in winter. At Kew, absorption by cloud and total back‐scattering (local planetary albedo) are about 9 per cent and 41 per cent of extra‐terrestrial flux respectively, with little seasonal variation.
Cloud nomenclature
Authors: Scorer R. S.
Published: February 2007

A very much simplified terminology in which cloud names are related to the physical and dynamical mechanisms producing the clouds is needed for the purposes of communication between cloud physicists, and for the purpose of teaching the subject in a scientific manner.

Such a terminology is outlined using a few of the best known Latin terms together with selected English words. There is no major departure from recent practice, but definitions are related to mechanisms, not appearances, and are made more exact.
The adjustment of profiles and eddy fluxes
Authors: Dyer A. J.
Published: February 2007

An assessment is made using simple diffusion theory of the rate of adjustment of profiles and eddy fluxes for (a), the leading‐edge case as a function of distance and (b), the horizontally uniform case as a function of time. The solutions of the appropriate diffusion equations obtained by Philip (1959) are used as the basis of the calculations. For the leading‐edge case the fetch‐height ratio for 90 per cent of the adjustment to be completed varies from 140 at a height of 0·5 metres to 530 at a height of 50 metres. The solution has application over a wide range of wind speed. For the case of horizontal uniformity and following an abrupt change in surface flux, 90 per cent of the adjustment is completed in 0·45 min at a height of 0·5 m increasing to 86 min at a height of 50 m.

It is suggested that these values indicate that the choice of site and conditions for micrometeorological research is critical, and that disparity between a number of experiments may be a consequence of failure to satisfy these basic requirements.
Measurements of lagrangian and eulerian properties of turbulence at a height of 2,500 ft
Authors: Angell J. K.
Published: February 2007

During the summer of 1962, super‐pressured, constant‐volume balloons (tetroons) were flown past wind‐vane instruments carried on a barrage‐balloon cable at Cardington, with the primary purpose of comparing the values of vertical velocity spectral density, S (n), obtained from the two sets of data. Based upon 25 comparisons at a mean height of 2,400 ft, on the average the maximum in the nS (n) spectrum derived from the radar‐positioned tetroons occurs at about one‐third the frequency of that derived from the wind vanes, i.e., on average the ratio, β, of Lagrangian to Eulerian scale is about three. There is a significant correlation of — 0·73 between turbulence intensity and log β, with the regression line indicating a β value of one at a turbulence intensity of 0·35 and a value of seven at a turbulence intensity of 0·05. Both factors in the turbulence intensity (mean wind speed and root‐mean‐square vertical velocity) contribute to this significant correlation, with the mean wind speed of greater influence. A comparison of five pairs of morning and afternoon tetroon flights for which lapse‐rate data are available shows that in all five cases β increases as the atmospheric stability increases.

At frequencies between 0·1 and 1·0 cycles per minute, three theodolite‐positioned flights suggest that the tetroon vertical velocity spectral density is proportional to between n−5/3 and n−2, where n is frequency. Since the latter value would be in agreement with the theoretical expectation for Lagrangian‐type data, this result is consistent with the assumption that, basically, the tetroons are following the vertical air motion.

Based upon 34 cases, there is evidence that surface frictional effects extend to at least 2,500 ft, with the tetroons moving across the surface isobars toward low pressure with a modal angle of 10–20 degrees and the tetroon speed being subgeostrophic to the extent of 1–2 m sec−1.
Comparison of bivane and sonic techniques for measuring the vertical wind component
Authors: Kaimal J. C. , Cramer H. E. , Record F. A. , Tillman J. E. , Businger J. A. , Miyake M.
Published: February 2007

A comparison test between a bivane and two sonic anemometers is described. Results of spectral computations for two runs show agreement between instruments in a frequency range between 0·007 and 0·375 cycles sec−1. Coherence plot shows good agreement over the entire range between the two sonic anemometers and up to 0·15 cycles sec−1 between the bivane and either sonic anemometer.
Some velocity and momentum‐flux statistics derived from transosonde flights
Authors: Angell J. K.
Published: February 2007

The wind speeds derived from 300 and 250 mb constant‐level balloon (transosonde) flights from Japan are found to be significantly greater than the wind speeds obtained from rawins and the geostrophic or gradient winds obtained from synoptic maps. This is partly due to a bias in the conventional data and perhaps partly due to horizontal entrainment of the transosondes into the jet‐stream core. Products of 2‐hour average zonal and meridional, geostrophic and ageostrophic velocity components obtained from transosonde trajectories indicate a northward ageostrophic flux of angular momentum in middle latitudes equal to about one‐fourth of the geostrophic flux. the product of 12‐hour average vertical and zonal velocity components indicates an upward flux of angular momentum in middle latitudes at 300 and 250 mb, thus implying that the downward flux of angular momentum in these latitudes is maintained by relatively small‐scale eddies.

Temperature changes along transosonde trajectories suggest that on the average there is upward air motion on the ridges and downward air motion in the troughs. In conjunction with the mean position in the ridge‐trough pattern of the maximum ageostrophic flow to north and south, an indirect circulation in the meridional plane (Ferrel Cell) is postulated for middle latitudes.
A further note on horizontal dispersion from an instantaneous ground source
Authors: Gee J. H. , Davies D. R.
Published: February 2007

This paper is written as a sequel to a previous note by Gee and Davies (1963), where it was shown that the introduction of a shearing advection term into the diffusion equation is significantn for an instantaneous source of the type discussed by Saffman (1962). A second additional term, suggested by Matsuoka (1961), has been introduced into the diffusion equation and the analysis shows that this term also has a significant effect on the properties of the cloud.
Stratospheric dynamics north of 40°N
Authors: Hare F. K.
Published: February 2007

Kinetic energy in middle and high latitudes, at levels above 20 km, increases throughout winter until the final warming, which supports the view that frictional dissipation is dynamically unimportant at these levels. A climatology of wave disturbances is presented for the years 1958‐61, little support being found for a special role for bipolarity in instability. Comments are made on the pronounced layer structure shown in the wind profiles and the regular ozone‐sonde records. It is suggested that the next stages of research in stratospheric meteorology should be directed towards the layer structure.
Concerning the similarity theory of A. S. Monin and A. M. Obukhov for the turbulent structure of the thermally stratified surface layer of the atmosphere
Authors: Calder K. L.
Published: February 2007

This paper considers the theoretical basis of the Monin‐Obukhov similarity theory for the turbulent, thermally stratified, surface layer of the atmosphere. It is shown that the classical form of the similarity theory cannot be applied legitimately to the variances of the horizontal components of the wind velocity fluctuation. This limitation of the theory has recently been suspected on the basis of observational data, although its theoretical demonstration emphasizes the need for caution when attempting to apply similarity theory to other characteristics of the surface layer turbulence that are currently of interest.
Soil moisture deficit and water‐table depth in London clay beneath neutral grassland
Authors: Coleman J. D. , Farrar D. M.
Published: February 2007

During the period 1954 to 1964 the amount of water was estimated and the depth of the water‐table was measured in a soil profile beneath meadow grass at a site on London clay at Uxbridge, Middlesex.

Each winter the soil profile was saturated and draining freely and the water‐table was within six inches of the surface. During the spring and summer the soil profile dried out from the surface downwards with a corresponding lowering of the water‐table. By late autumn the water‐table reached a depth of six feet, and the moisture deficit in the soil profile reached a value of from five to six inches. Re‐charge of the soil profile to the winter conditions occurred in late autumn or early winter.

Measurements of humidity, sunshine, wind, temperature and other data were used to calculate the monthly potential evaporation, and this information, combined with rainfall data, permitted the calculation of the total deficit of soil moisture at any time from that held at field capacity.

For the particular site investigated, the observed depth of the water‐table at any season varied linearly with the calculated moisture deficit in the soil profile beneath the grass, thus providing a simple method for continuously logging the moisture deficit in the soil. The result also provides a basis for correlating weather data with moisture conditions in the soil beneath roads; the moisture conditions are largely controlled by the depth of the water‐table.
On the critical liquid water concentrations of large hailstones
Authors: Macklin W. C. , Bailey I. H.
Published: February 2007

Considerations of the collection efficiencies of large hailstones, the local increase in the rate of heat transfer due to surface irregularities, and possible increases in critical liquid water concentrations at high Reynolds numbers give some support to the conclusion of Browning (1966) that large hailstones grow closer to the wet‐growth limit than formerly supposed.
The propagation of groups of internal gravity waves in a shear flow
Authors: Bretherton F. P.
Published: February 2007

The propagation in a shear flow of travelling internal gravity waves of short wavelength is investigated by the W.K.B. approximation. This implies that the Richardson number is large. It is shown that a wave group moves with the local group velocity defined in terms of the dominant wavenumber, the absolute frequency and horizontal components of wavenumber being constant. The wave energy carried by a wave packet varies, being proportional to the frequency relative to a frame moving with the local mean wind. A wave packet, on this approximation, can never pass through a critical level at which its horizontal phase velocity is equal to the mean wind, nor is it reflected there.

These results imply restrictions on the propagation of internal gravity waves from the troposphere to the upper atmosphere.
Rainfall evaporation in thunderstorm downdraughts
Authors: Kamburova Petia L. , Ludlam F. H.
Published: February 2007

It is explained that in air descending at speeds of some m sec−1 raindrop evaporation must proceed at a large subsaturation. Under some simplifying steady‐state assumptions, the vertical temperature profiles in thunderstorm downdraughts of a range of speeds are estimated, and it is shown that the most favourable conditions for the production of strong draughts are a small raindrop size, and, especially, an intense rainfall rate and a general lapse‐rate close to the dry adiabatic. The two latter conditions are often associated in severe storm situations, and the downdraught then arrives near the ground with a rather low relative humidity, and a temperature little lower than found generally at the same level.
On the role of mean meridional motions in the biennial wind oscillation
Authors: Wallace J. M.
Published: February 2007

Detailed time‐height sections and meridional cross‐sections of zonal wind are presented for the zonally averaged conditions between 32°N and 20°S. The patterns which appear in the sections vary slowly and systematically with latitude and time. The biennial wind oscillation is evident in the sections, but since 1963, the two year periodicity, as such, has vanished and the wind fluctuations, though still similar in form to the previous ones, have become much less predictable. The sections suggest that the lower edges of westerly regimes propagate downwards more rapidly than those of easterly regimes.

The momentum budget of the region is then examined, term by term, and it is concluded that a mean downward motion is necessary over the Equator to account for the downward propagation of the wind regimes there. It is suggested that the enhanced vertical motion during the descent of westerly regimes at the Equator might account for the observed downward propagation at sub‐tropical latitudes. The heat budget is then examined with respect to the vertical motions suggested from momentum considerations. It is found that a heat sink in the equatorial stratosphere would be required to support the vertical motion field. It is shown that enhanced downward motion during the descent of westerly regimes (which the zonal wind data suggest) is necessary to keep the temperature field in geostrophic equilibrium with the wind field.
Temperature and vertical velocity fluctuations in strong convection
Authors: Myrup L. O.
Published: February 2007

An experimental study of the structure of thermal convection in the lower atmosphere is presented. The measurements were undertaken with the primary purpose of comparing the statistics of convective motion occurring in the lower atmosphere under conditions of light winds and strong surface heating with the laboratory heated plate results of Townsend and also with the predictions of the similarity theory of free convection. Temperature and vertical velocity fluctuations were measured above a desert dry lake bed in Southern California using a light aircraft as an instrument platform. Modern data acquisition methods were used which allowed high speed data sampling and rapid computer processing of several million data points. Negative Richardson numbers at 1·5 m of the order of 2 or more were typical of the midday hours. The temperature traces obtained during horizontal aircraft traverses had the typical intermittent structure previously noted both in the atmosphere and in the laboratory by other investigators. Individual warm pulses often had an asymmetric appearance in that the upwind edge tended to be the warmest. This was noted to hold even for the lightest wind conditions included in these data (1 ms−1 at 4 m). Although the observed intermittency of the temperature field agreed qualitatively with Townsend's description of the laboratory case, the variation with height of the r.m.s. temperature and vertical velocity fluctuations agreed with the predictions of the similarity theory of free convection. It is concluded that the light wind case in the atmosphere differs considerably from the laboratory case with no mean motion. Since winds appreciably lighter than those occurring during this study are quite rare for lapse conditions in the atmosphere, it is suggested that there may always be a fundamental difference between atmospheric measurements and those made for the classical heated plate experiments. These data are also used to construct a preliminary kinetic energy budget for various heights above the lake bed. The results indicate that the flux divergence of turbulent kinetic energy is a large term in the budget under the conditions of these measurements.
Temperature persistence at Bombay during the South‐West Monsoon
Authors: Sarker R. P. , Mhaiskar P. R.
Published: February 2007

In this note three‐day mean temperature values observed at Colaba (Bombay) during the years 1871 to 1960 have been examined by means of contingency tables for evidence of any pattern of persistence during the South‐West Monsoon. It appears that the tendency for the existing temperature character to persist from one triad to the next at least is strong during the entire monsoon, being strongest in the month of August, when it may persist over a period of 4 triads or so.
An improved Millipore filter technique for measuring the concentrations of freezing nuclei in the atmosphere
Authors: Stevenson Catherine M.
Published: February 2007

A technique to measure the concentration of freezing nuclei in the atmosphere is described. It is a development of the Millipore filter method described by Bigg, Mossop and Heffernan (1961), from which many of the difficulties experienced by them and other workers in the field have been eliminated.

The main advantages which this technique has over others using Millipore filters are as follows:

the ability to work at relatively low and controlled supersaturations;

the ability to activate large numbers of nuclei on the filter without the crystals interfering;

the finding of a linear relationship between the number of nuclei detected and the volume sampled, except in very dusty conditions;

an improved reproducibility.
In clarification of the equations of shallow‐layer thermal convection for a compressible fluid based on the Boussinesq approximation
Authors: Calder K. L.
Published: February 2007

This paper discusses the reference equilibrium atmosphere for the approximate equations of shallow layer thermal convection.
Some extensions and applications of the new random model for molecular band transmission
Authors: Rodgers C. D.
Published: February 2007

The general line shape transmission is derived for the new random model described by Malkmus (1967) for the representation of molecular band transmission. The model is applied to two absorbing paths in series, where an expression is found which is simpler than the corresponding expression for the Goody random model. The new model is also used to give an empirical fit to the ozone transmission measurements of Walshaw (1957).
The effect of vertical line averaging on the spectra of temperature and heat‐flux
Authors: Kaimal J. C.
Published: February 2007

The paper describes an attempt to evaluate the effects of vertical line averaging on temperature statistics by using simultaneous measurements from sensors of different lengths. Results show that the transfer function for vertical line averaging derived by Gurvich for the vertical velocity component applies also to temperature spectra. Heat‐flux, when computed with vertical velocity measurements from a sonic anemometer, is relatively insensitive to line averaging (at least up to 80 cm at 9 m height). But the coherence between the line average and its mid‐point value shows consistent behaviour only for short line lengths of the order of 20 cm. With longer lengths the coherence is likely to vary from one run to another depending on mean wind speed.
Justification for neglecting the third moment in the steady state expression for the turbulent heat‐flux near the ground
Authors: Businger J. A. , Miyake M.
Published: February 2007

Observations indicate that the third moment term ρ′ ω′ T′ which appears in an expression for the turbulent heat‐flux is negligible.
Measurements of atmospheric infra‐red emission with a balloon‐borne multifilter radiometer
Authors: Pick D. R. , Houghton J. T.
Published: February 2007

A radiometer sonde has been constructed to observe radiation from the atmosphere above the radiometer in the spectral region 5–7 μ with a spectral resolution of about 0·3 μ. The results from a flight of the instrument at night up to a height of 25 km are presented. Emission from water‐vapour indicates a dry stratosphere with a mass mixing ratio of about 3 × 10−6. An energy flux originating above 25 km amounting to about 2 μW cm−2 has been observed near 5·7 μ. Various possible sources for this additional radiation are discussed.
Some average wave lengths on short‐crested seas
Authors: Scott J. R.
Published: February 2007

Twelve average sea spectra are truncated at 1·6 rad s−1 and fitted with tails in which energy density varies inversely with the fifth power of frequency. By use of a spread parameter measured in typical rough weather these spectra are converted into symmetrical two‐dimensional wave number equivalents, which are, inter alia, numerically integrated along normals to the dominant wave direction, yielding the wave number spectra of instantaneous wave profiles along that direction. These spectra provide unbiassed estimates of average wave lengths, while the originals provide similar estimates of average periods. Zero‐crossing averages are shown to be preferable to those measured from crest‐to‐crest.

It is found that, for the estimation of average zero‐crossing wave lengths, the original frequency tail can be replaced by one in which the density varies inversely with frequency raised to the power of 5·5, and that the fourth moments of the main (truncated) spectra require reduction by about a quarter, to allow for the effect of spreading. These findings also appear to be applicable in individual situations where the dominant direction is well defined.

The variations of zero‐crossing periods and wave lengths with wave height are studied for two locations.
The temperature profile near the ground on calm clear nights
Authors: Oke T. R.
Published: February 2007

An observational study of the temperature profile over grass, snow and bare soil surfaces in strong stability is presented. It was found that in nearly calm, clear conditions the minimum temperature occurred 1 to 50 cm above the surface. The height and intensity of the minimum is strongly correlated with wind speed, surface roughness and stability. A discussion of existing hypotheses suggests that the most likely explanation for the phenomenon should involve both turbulence and infra‐red flux divergence.
Richardson number limited shear zones in the free atmosphere
Authors: Browning K. A. , Harrold T. W. , Starr J. R.
Published: February 2007

Two case studies are presented of the detailed structure of frontal zones with strong vertical shear. Temperature structure has been obtained from sequential radiosonde ascents. Wind structure, including the fields of horizontal divergence and deformation, has been obtained on scales of 1 to 10 km by means of Doppler radar techniques. The results are consistent with the frontal zones being at least partly controlled by shearing instability, in so far as there was a balance between a wind field tending to diminish the Richardson number (Ri) and shearing instability tending to impose a lower limit to Ri. One of the main factors tending to diminish Ri was stretching deformation in a direction normal to the local shear vector. A typical rate of decrease of Ri which this would have produced in the absence of shearing instability was 10−4 s−1. The Richardson number itself within the frontal zones was mainly between 0.2 and 0.4 when averaged vertically over 200 m and horizontally over circles of diameter 10 km; however, Ri diminished to about half of these values locally on horizontal scales of about 1 km. As a result shearing instability is thought to have led to turbulent breakdown in only isolated patches rather than in a sustained fashion over the entire depth of the frontal zones.
An observation of gravity waves in shear flow in the lower stratosphere
Authors: Axford D. N.
Published: February 2007

During a routine instrumentation flight check in the lower stratosphere periodic variations of temperature and winds were observed. It is deduced that these variations are gravity waves which appear to be associated with a narrow frontal zone separating regions with different potential temperatures. Vertical and horizontal wind changes of ± 1 m s−1 and potential temperature variations of up to ± 2°C are measured with a wavelength of 2.3 km. The breakdown of waves such as these may be associated with observations of clear air turbulence in the stratosphere along shear layers separating weak flows of different origin.
On the influence of synoptic development on the production of high level turbulence
Authors: Roach W. T.
Published: February 2007

This paper considers free shear layer turbulence in the atmosphere (clear air turbulence) as a mechanism for relieving the formation of discontinuities of wind and temperature in the vertical plane by shearing and stretching deformation.

An expression for the rate of change of the logarithm of the gradient Richardson number following the air motion is derived and the assumption is made that turbulence counteracts the dynamical processes which are reducing the Richardson number, maintaining it at a limiting value. The rate of working required to do this is equated to the turbulent energy dissipation rate and in the model used is given by Φ (ΔV)2/24 (where the logarithmic rate of reduction of Ri which would take place in the absence of turbulence, and ΔV is the velocity difference across the turbulent layer).

Two preliminary tests of the theory as a forecasting tool using the Bushby‐Timpson 10‐level numerical model show that the dynamical processes changing Ri are largest in areas where clear air turbulence might be expected from synoptic experience.
Buildings in the wind
Authors: Owen P. R.
Published: February 2007

Some of the problems arising from the flow of the wind about buildings are selectively illustrated in terms of the aerodynamics of vortex shedding and the interaction between a bluff body and turbulence in the approaching stream. Examples chosen draw attention to the spanwise incoherence of vortex shedding from a rigid body and vortex‐capture by an oscillating body, the effects of geometrical taper and shear in the undisturbed stream and the pecularities exhibited by the wakes behind porous windbreaks. Galloping oscillations of cables together with the large amplitude motions that can be engaged in by stranded electrical conductors are also briefly mentioned.

As a preliminary to discussing the effect of turbulence on the flow past a body, as well as the distortion of the turbulence produced by the body, observations are offered on the relation between the known properties of the micro‐spectrum of atmospheric turbulence and wind tunnel measurements made in turbulent boundary layers. An interpretation of the latter in terms of the ‘inactive’ component of the unsteady motion near a surface, contributed by eddies in the outer parts of the boundary layer, suggests that further meteorological data, so frequently demanded by architects and aerodynamicists, may fail to add systematic information about the behaviour of the u‐component of the turbulent velocity other than is already available.

Modern techniques of wind tunnel testing, which attempt to represent mean and turbulent properties of the wind, are described and cautionary remarks are made about the prospect for significant further refinement.
Drag coefficients of water droplets decelerating in air
Authors: Ogden T. L. , Jayaweera K. O. L. F.
Published: February 2007

The relation of drag coefficient to Reynolds number for decelerating water droplets was tested by releasing into a 50 m s−1 airstream droplets up to 230 μm in diameter. After travelling for 1.83 m in the airstream, the droplets passed through a small hole into still air, and the distances they subsequently travelled could be predicted fairly well by assuming that the effective drag coefficients were about 20 per cent below the steady‐velocity values.
The structure and intensity of turbulence in air over the sea
Authors: Warner J.
Published: February 2007

Measurements of atmospheric turbulence have been made from an aircraft at heights in excess of 30 m over the open sea in convective situations. They are related to height, wind speed, heat flux and stability. The variation of the r.m.s. vertical velocity with height is examined for two different scaling systems. From calculations of the power spectra a length scale is derived for the vertical velocity fluctuations which is found to increase slowly with height.
Non‐Boussinesq effects and further development in a model of upper tropospheric frontogenesis
Authors: Hoskins B. J.
Published: February 2007

In previous papers, upper tropospheric frontogenesis has been exhibited in a model in which a horizontal deformation field acts on a temperature distribution in a Boussinesq two fluid system. The basic approximation made is that the flow along the incipient front is geostrophic. This approximation allows the reduction of the problem to that of solving a non‐linear second order partial differential equation. Here this equation is solved by a slightly different technique. Non‐Boussinesq effects are included and are shown to produce a strengthening of the upper‐tropospheric front and its associated jet stream. Further deformation is allowed and the development of a more realistic front described.
The shedding of accreted water during hailstone growth
Authors: Carras J. N. , MacKlin W. C.
Published: February 2007

Icing tunnel experiments have shown that loss of accreted water occurs in both the dry and wet growth régimes at ambient temperatures above about −12°C. The loss in the dry growth régime is probably due to splashing while that in the wet growth régime is undoubtedly due to shedding of water from the liquid layer which covers the surface of the accreting object. The latter can affect appreciably the growth rates of hail stones. These data, together with calculations of the collection efficiency, have shown that the structure of the hailstone analysed by Macklin, Merlivat and Stevenson (1970) is consistent with growth in an adiabatic updraught.
Precipitation characteristics during the Venezuela rainy season
Authors: Riehl H. , Cruz L. , Mata M. , Muster C.
Published: February 2007

An extensive program of observations designed to determine the structure of tropical mesoscale rain areas over land and their relation to synoptic envelopes was carried out during the rainy season of 1969 (June to September) in eastern Venezuela.

The frequency distribution of precipitation in the area seen by radar (radius 100 km), as given by the average of 30 stations, was highly skewed: 14 per cent of the days in the experiment delivered half of the precipitation; 80 per cent could be attributed to rain episodes related to mobile synoptic systems. On the hourly basis, 0·45 per cent of all hours (or 10 hours) produced half of the precipitation.

Daily precipitation in the radar circle was not correlated with radar echo height but markedly with echo area. Heavy rainfall occurs when a few very large clouds appear.

A cloud distribution in terms of area and rain productiveness was determined for rainy and for dry days. Rain productivity increases with cloud area; however, since the number of clouds also decreases as their size increases, water yield to the ground is almost independent of cloud scale. No clear difference in the 200 mb flow could be found between cloud ensemble.

On rainy days the temperature is lower and the specific humidity higher than on dry days confirming that cold‐core precipitation takes place also over Venezuela. A mean ascending velocity appears to exist in and out of clouds in the convection cores, but could not be found prior to onset of afternoon convection.
Baroclinic instability at the winter stratopause
Authors: Simmons A. J.
Published: February 2007

Baroclinic instability is examined in an idealized model of the winter stratopause. The upper stratosphere is more stably stratified than the lower mesosphere, and instability can occur when the zonal flow has westerly shear., Using representative parameter values, a maximum doubling time of 2 days is found for a disturbance with zonal wavelength 4000 km and vertical scale 10 km, moving with a velocity within 2ms−1 of that of the zonal wind at the stratopause. Newtonian cooling reduces growth‐rates by some 80% of the damping rate, and becomes important for relaxation times of less than 7 days.
Crystal size in accreted ice
Authors: Rye P. J. , Macklin W. C.
Published: February 2007

Studies have been made of the crystallographic orientation of 100μm‐radius supercooled droplets frozen on ice substrates whose c‐axis orientations varied from 0 to 90° to the surface normal. For a given droplet temperature and substrate orientation, there is a critical substrate temperature below which a frozen droplet has a high ( > 0·8) probability of having an orientation which differs from that of the substrate. Above this critical substrate temperature there is a small ( < 0·2) residual probability of reorientation which is dependent on the droplet temperature and substrate orientation but independent of the substrate temperature.

The lengths and maximum widths of ice crystals in accreted ice deposits have also been determined. In the dry growth regime there is a general decrease in the mean length from ∼ 8 to 0·25mm, and in the mean maximum width from ∼ 1 to 0·2mm, as the ambient temperature decreases from − 5 to − 30°C. At ambient temperatures above − 15°C there is no dependence of the crystal dimensions on the temperature of the deposit. At ambient temperatures below this value, the crystal dimensions vary with deposit temperature in a manner consistent with the individual droplet studies. Consequently, measurements of the crystal dimensions in hailstone layers may give useful indications of their growth conditions.
The effects of vertical wind shear on the evolution of convective clouds
Authors: Pastushkov R. S.
Published: February 2007

The effects of ambient wind shear Uo on the evolution of an isolated three‐dimensional convective cloud are studied. Sixteen cases are considered which differ from one another both in the value of U′0 in the initial value of the energy of atmospheric instability per unit height of the unstable layer, E0. It is found that convective clouds may be grouped into two types, ‘weak’ and ‘strong’: if E0/H < 0−65 × 102cm s−2 then convection is ‘weak’; if E0 0−65 × 10−2cm s−2 then convection is ‘strong’. For ‘weak’ convective clouds there is a critical value of shear, U′0cr |U′0| < U′0crthere is an inhibiting effect on the development of convection; and for |U′0| U′0 its evolution is completely suppressed. The essential new result is that ‘strong’ convective clouds are intensified under the effect of U′0. In this case there is a resonance value of shear, U′0res at which the degree of the intensification of convection has a maximum (30–40%).
Budget studies of heat flux profiles in the convective boundary layer over land
Authors: Cattle H. , Weston K. J.
Published: February 2007

Observations of the convective boundary layer over southern England using an instrumented light aircraft and radio‐sondes enable its structure and development to be described for two periods of active, shallow convection. Budgeting is used to deduce the sensible heat flux as a function of height, showing a downward flux in the upper part of the boundary layer associated with entrainment of warmer air from above. For the two occasions studied results indicate that the entrained sensible heat flux is about 0·3 of the sensible heat flux at the ground. For one of the occasions a moisture budget was possible yielding a value of the mean latent heat flux at the ground.