bluuebunny/crossref_metadata_2025 huggingface dataset:
The density and structure of ice formed by accretion
Authors: Macklin W. C.
Published: February 2007

Studies have been made of ice formed by accretion over a wide range of controlled conditions to ascertain the influence of the wind speed, ambient temperature, droplet diameter and liquid‐water concentration in the supercooled cloud, and the size of the accreting object. The density of the deposits ranged from 0·1 to 0·9 g cm−3, the density increasing with increases in the first four of the above parameters but decreasing with increasing object size. It was found that the density could be expressed as a function of − rv0/Ts, r being the median volume radius of the droplets comprising the cloud, v0 the speed with which the droplets struck the object and Ts the mean temperature of the accreting surface (in °C). The quantity rv0 is interpreted as being a measure of the force with which the droplets were packed together and Ts the temperature which governed their freezing rate. A microscopic examination of the deposits showed that at low values of − rv0/Ts (corresponding to low densities) the droplets tended to freeze together as spheres while at higher values they were distorted and spread considerably before freezing.
Model experiments relating to thermals with increasing buoyancy
Authors: Turner J. S.
Published: February 2007

It is pointed out that previous laboratory experiments which have been used to obtain information about atmospheric thermals have not dealt with the important case of an element whose total buoyancy is increasing. A new technique (based on the chemical release of small gas bubbles) is described here, which allows one to simulate in a liquid the increase of buoyancy due to the release of latent heat in clouds. It has been found possible to achieve experimentally two simple kinds of motion, namely, constant velocity or constant upward acceleration. These elements spread linearly with height in agreement with the predictions of a dimensional theory, at nearly the same angle as thermals in neutral surroundings.
Recent changes of east‐coast rainfall regimes
Authors: Kraus E. B.
Published: February 2007

New data are given which extend from 1941 to 1960 an earlier study (Kraus 1955). The analysis suggests a return to wetter conditions in low‐latitude east‐coast regions, after a drier period which persisted through nearly half a century. This change appears to be paralleled by the recurrence of lower mean temperatures in higher latitudes.
Secondary instability in steady gravity waves
Authors: Scorer R. S. , Wilson S. D. R.
Published: February 2007

A stratified stream may be stable when flowing horizontally and yet develop a Görtler type of instability in gravity waves when the flow is curved. The equation for the generation of secondary vorticity (rotation round the streamlines) may be used to study this, and we show that the curved flow in two‐dimensional gravity waves could become unstable in real cases. This is, therefore, one possible mechanism for the generation of ‘clear‐air turbulence’ in mountain waves.
Change of terrain roughness and the wind profile
Authors: Panofsky H. A. , Townsend A. A.
Published: February 2007

To describe the distribution of wind with height in hydrostatically‐neutral air following a change in terrain roughness, a theory is constructed by assuming that only the air below an internal boundary is affected by the change and that air above the boundary is moving with the speed and Reynolds stress that it had upwind of the change of roughness. Unlike in the closely related theory of Elliott, the assumed velocity distribution in the lower layer is consistent, not with stress independent of height, but with continuous variation of stress from the surface to the boundary where the stress is that in the original layer. the condition of overall conservation of momentum then leads to relations between surface stress, boundary height and fetch. the theory is compared with observations at a variety of locations, and the agreement with measurements is satisfactory. In particular, a fairly sharp boundary separates the masses of air influenced by the two types of terrain; for micrometeorological distances, the slope of this interface is of order 1/10.
Fall speeds of precipitation elements
Authors: Cornford S. G.
Published: February 2007

A basis is presented for the calculation of the terminal velocities, V, of thirteen types of precipitation element over a wide range of size and air density, ρ. It is found that changes in drag coefficient allowed for in the calculations are unimportant and that for the purpose of deducing rainfall rates from aircraft observations of precipitation particles they may be ignored; thus for particles of a given size and type.
The influence of friction on the growth and structure of baroclinic waves
Authors: Haltiner G. J. , Caverly D. E.
Published: February 2007

Analysis of a simple two‐level linearized baroclinic model shows that the effects of surface friction are quite significant with respect to the growth characteristics of harmonic waves. As expected, friction generally tends to reduce the amplification; however, some waves on the fringes of the unstable band which would otherwise be stable may amplify with friction. Increasing the drag coefficient has a similar effect on amplification as decreasing the thermal wind. A wave solution, comprised of two stable components travelling at different speeds, may amplify temporarily due to the resulting interference pattern which exhibits a periodic amplitude and propagation speed.

The phase difference between the thermal and pressure waves is also markedly affected by the friction; for example, for the drag coefficients used, the limiting phase difference may differ by as much as 40° from the frictionless case.
Baroclinic instability and the short wavelength cut‐off in terms of potential vorticity
Authors: Bretherton F. P.
Published: February 2007

The stability is discussed of an infinitesimal quasi‐geostrophic perturbation to a baroclinic zonal wind which is independent of latitude, in terms of a two‐layer inviscid model with parallel but sloping upper and lower boundaries, so that the northwards gradient of potential vorticity in each layer is equal and opposite, but not a simple multiple of the difference between the basic eastward velocities. A long wavelength cut‐off to the instability is possible, as well as a short wavelength cut‐off. The mechanism for the instability is described qualitatively in terms of the balance between east‐west advection of perturbation potential vorticity, and generation by north‐south motions in the basic gradient, and the reasons for both cut‐offs are clarified. The short wavelength stable modes are described as advected Rossby waves in either the upper or the lower layer.

This qualitative description is then extended to apply to the short wavelength cut‐off found by Eady, and to the short wavelength Eady modes, which are always de‐stabilized by a small gradient of potential vorticity in the interior of the fluid, together with the action of a critical layer.
A spectral model for Cisk‐barotropic energy sources for tropical waves
Authors: Padro J.
Published: February 2007

A non‐linear, three‐dimensional spectral model on a β‐plane is developed to study the relative rǒles of important energy sources for tropical waves. Both the north‐south shear of the zonal wind and Cisk (conditional instability of the second kind) are included. Energy exchanges and wave growth rates are studied in the presence of various specified vertical profiles of latent heat. Radiation is parameterized by Newtonian cooling; it plays a minor rǒle. Latent heat is parameterized in terms of large‐scale boundary layer flow. Dissipation is also included.

The model includes three layers in the troposphere and one in the stratosphere. The top of the boundary layer is taken at 900 mb and its effect is described by frictionally induced vertical velocity and surface dissipation. The initial state is an idealized form of observations taken in the Marshall Islands.

Important results show that when latent heat release decreases with height and is large at low tropospheric levels, the eddies are initially generated and maintained barotropically and then Cisk assumes a dominating rǒle. When heat release increases with height, Cisk dominates at all stages of the wave development; the barotropic mechanism, however, is still necessary.

It is also found that when latent heat release is maximum at the low levels, the most dominant wave is 3,000 km with an initial growth rate of 3 days in e‐folding time. The longer waves assume greater dominance at the upper levels, especially when latent heat increases with height.
Scattering in liquid haze ‐ analytic approximations
Authors: Kuriyan J. G. , Sekera Z.
Published: February 2007

An analytic formula is derived which describes approximately the scattering of electromagnetic waves in a haze. This formula is compared, and found to agree well, with the exact results from Mie Theory that is available in the literature. The formula is used to arrive at a simple, non‐redundant but complete parameterization of the aerosol model.
On Penman's equation for estimating regional evaporation
Authors: Thom A. S. , Oliver H. R.
Published: February 2007

The form of Penman's evaporation equation most extensively applied in hydrometeorology is discussed. It is shown that this equation underemphasizes the importance of ventilation relative to radiation in maintaining regional evaporation. A generalized ventilation term is proposed and a modified equation for evaporation derived. This new equation is calibrated to give the same annual total for short vegetation as Penman's original version. It predicts both an enhancement of winter evaporation at the expense of summer evaporation and a significant increase in annual total evaporation with vegetation size, especially in regions of frequent substantial rainfall.
Coastally trapped waves in the atmosphere
Authors: Gill A. E.
Published: February 2007

It is suggested that the ‘coastal lows’ observed off southern Africa are similar in structure to ‘coastally trapped waves’ in the ocean. Inversion conditions typical of the area prevent escape of energy upwards and Coriolis effects keep energy trapped against the high escarpment which borders the continent. Coastal disturbances are produced because low‐level flow in the synoptic‐scale systems cannot cross the barrier provided by the escarpment. Nonlinear effects cause lows to be more prominent than highs. A simple nonlinear forced Kelvin wave model is found to reproduce many of the features observed.
Squall lines observed in GATE
Authors: Mansfield D. A.
Published: February 2007

A study of convection as portrayed by Oceanographer radar during phase III of GATE reveals four occasions on which an organized line of convection moved with a speed close to or greater than the wind speed at any level. They are shown to resemble closely the analytical model and numerical simulation of tropical line squalls developed by Moncrieff and Miller. This form of organization is associated with a lowlevel wind maximum in the direction of movement.
Probing tornadoes with a pulse doppler radar
Authors: Zrnic D. S. , Doviak R. J. , Burgess D. W. , Brown Rodger A.
Published: February 2007

Maximum tornado wind speeds and their radii are deduced by least square fitting of the vortex model velocity spectra to those measured with a pulse Doppler radar. Spectra are analysed from the Stillwater, Oklahoma, 13 June 1975, tornado. Simulated spectra are fitted to those measured for pulse volume gates nearest the vortex, and thus a tornado's position relative to those gates is estimated. Estimated maximum tangential velocity is 92ms−1 at an altitude of 630m and 85ms−1 at 1.5km; tornado diameter is 300m. the observed spectra have an asymmetry that suggests targets that are centrifuged outwards, with a velocity of 13ms−1. One spectrum analysed from the Union City, Oklahoma, 24 May 1974, tornado shows a bimodal shape similar to Stillwater spectra. Deduced maximum velocity is 72ms−1 and diameter 200 m.
Weakly nonlinear stability theory of stratified shear flows
Authors: Maslowe S. A. , Drazin P. G.
Published: February 2007

A derivation is given of the first‐order, nonlinear amplitude equation governing the temporal evolution of finite‐amplitude waves in stratified shear flows. the theory has been developed on an essentially inviscid basis by perturbing away from the linear neutral stability curve in Richardson number‐wavenumber space. However, viscosity and heat conduction are still required in order to eliminate the singularities that occur in the inviscid limit. Holmboe's mixing layer model has been studied by applying the present theory and the results show, surprisingly, that subcritical instability can occur, i.e. modes that would be stable on a linear basis (e.g. when the Richardson number is greater than 1/4) become unstable when the initial perturbation amplitude is greater than some critical value. an instability due to resonance which occurs at a Richardson number of 0·22 is also revealed by the analysis. These results have interesting implications in connection with clear air turbulence which are discussed herein.
Observation of inertial waves in the stratosphere
Authors: Thompson R. O. R. Y.
Published: February 2007

A series of radiosonde flights over Laverton, Victoria (38°S), is found to have sufficient vertical (100m) and temporal (3 h) resolution, and sufficient length (28 days) to detect inertial waves. Fourier analysis shows that in the stratosphere there is a strong anticyclonic rotation of the wind vector which has energy concentrated near the local inertial frequency. This rotation is associated with a vertical wavelength between 1 and 3 km, and a downward phase propagation. There is no sign of inertial waves in the troposphere. The waves appear to be generated below the stratosphere and spread upwards.
The turbulence kinetic energy budget in convective conditions
Authors: Caughey S. J. , Wyngaard J. C.
Published: February 2007

Data from the 1973 Minnesota atmospheric boundary layer experiment are used to investigate details of the turbulence kinetic energy budget in convective conditions. Surface layer results are presented in nondimensional form and compared with conclusions from earlier Kansas data. Above the surface layer the terms are nondimensionalized using mixed layer similarity length and velocity scales and compared with indications from other recent experiments.
Flow over an isolated hill of moderate slope
Authors: Mason P. J. , Sykes R. I.
Published: February 2007

A two‐dimensional theory of Jackson and Hunt for turbulent flow over a shallow ridge is extended to three‐dimensional topography. The results are compared both with theoretical results for a ridge and with surface wind observations from a nearly circular isolated hill. Agreement between theory and observations is encouraging.
Rainfall along the east coast of southern Africa, the southern oscillation, and the latitude of the subtropical high pressure belt
Authors: Dyer T. G. J.
Published: February 2007

Eighteen rainfall series within a latitude belt 28°08′S to 30°25′S are inspected for relationships with Pittock's L index for the latitude of the subtropical high pressure belt (eastern Australia), and the southern oscillation index. Principal components analysis is used to reduce the dimensions of each of the three data sets, and the components for the pressure systems are used as independent variables in a step‐wise regression analysis; rainfall components are taken as the dependent variables. Useful relationships exist between rainfall and Pittock's L, but there is no evidence to suggest that the same situation applies for the southern oscillation index. The results agree with those recently obtained for Mauritius by M. D. Dennett.
Mid‐ocean observations of atmospheric radiation
Authors: Simpson James J. , Paulson Clayton A.
Published: February 2007

Mid‐ocean (35°N 155°W) observations of the various components of radiative flux were made from the R/P FLIP during the period 2 through 13 February 1974. Cloud cover ranged from clear skies to overcast, and water vapour pressure varied between 9 and 18 mb, with sea surface temperature near 15.0°C.

The net longwave radiative fluxes reported here were obtained: (1) by taking the difference between simultaneous measurements of net all‐wave and net solar fluxes; and (2) by direct measurements with a net longwave radiometer designed by G. W. Paltridge. When observations during rain and fog are excluded, the difference between night‐time 15‐minute averages by the two methods is generally less than lmWcm−2. During day‐time, indirect measurements are often larger than direct by about 5mWcm−2; from internal evidence we prefer the direct values.

The albedo of the sea surface was calculated from simultaneous measurements of downward and upward solar fluxes. The observations were analysed to represent albedo as a function of solar altitude and atmospheric transmittance, following the work of R. E. Payne; our results suggest that Payne's smoothed representation is suitable for use over the open ocean. Albedo was observed to decrease with increasing wind speed for clear skies with solar altitude between 15 and 30 degrees but no variation was discernible at higher solar altitudes.

Empirical formulae for calculating both shortwave and net longwave components of the radiative flux were compared with measurements. A formula due to F. E. Lumb for determining the incident solar flux given solar altitude, cloud amount and cloud type, consistently yields good agreement with the measurements, within about lmWcm−2. Daily averages of net longwave flux calculated from several empirical formulae using a linear correction for clouds are within 2mWcm−2 of the observations reported in this paper. Since daily radiation balance values were measured as only 5mWcm−2, the limitations of the best current empirical formulae are evident.
An experimental forecast using a non‐hydrostatic mesoscale model
Authors: Carpenter K. M.
Published: February 2007

A non‐hydrostatic mesoscale model formulated by Tapp and White has been extended to include the effects of orography and the variation of the large‐scale meteorological situation. Other changes to the model, particularly in the parametrization of boundary layer turbulence, are described and its application as a forecasting tool is illustrated by an extensive study of one situation.
An experimental study of the profiles of wind speed, shearing stress and turbulence at the crest of a large hill
Authors: Bradley E. F.
Published: February 2007

Measurements have been made, under neutral conditions, of the turbulent wind structure on a 100m tower at the crest of a hill of height h = 170m. A local velocity maximum, or ‘jet’, was observed at about h/5, below which the ratio of increase in wind speed to the upwind value at the same height was 1.07, and almost independent of height near the surface.

The r.m.s. turbulence components σw σv and σw were also approximately doubled near the surface, but whereas σu and σv both decreased with height, the surface increase in σw extended over the entire height of measurement. Reynolds stress was almost uniform above the ‘jet’, but increased sharply towards the surface, to 3–5 times the upwind value. Departures from neutrality affected the profile shape and the magnitude of turbulence fluctuations very markedly.

Although the size of hill lay outside the range of validity of an analytical theory of Jackson and Hunt, their expressions for velocity and horizontal pressure gradient were in good agreement with the observations. Numerical theories by Frost, Harper and Fichtl and by Taylor, for hills of different shape, but similar slope to the experimental hill, also produced values of surface stress and velocity increase of the magnitude observed.
Some simple solutions for heat‐induced tropical circulation
Authors: Gill A. E.
Published: February 2007

A simple analytic model is constructed to elucidate some basic features of the response of the tropical atmosphere to diabatic heating. In particular, there is considerable east‐west asymmetry which can be illustrated by solutions for heating concentrated in an area of finite extent. This is of more than academic interest because heating in practice tends to be concentrated in specific areas. For instance, a model with heating symmetric about the equator at Indonesian longitudes produces low‐level easterly flow over the Pacific through propagation of Kelvin waves into the region. It also produces low‐level westerly inflow over the Indian Ocean (but in a smaller region) because planetary waves propagate there. In the heating region itself the low‐level flow is away from the equator as required by the vorticity equation. The return flow toward the equator is farther west because of planetary wave propagation, and so cyclonic flow is obtained around lows which form on the western margins of the heating zone. Another model solution with the heating displaced north of the equator provides a flow similar to the monsoon circulation of July and a simple model solution can also be found for heating concentrated along an inter‐tropical convergence line.
A field study of nocturnal stratocumulus II Turbulence structure and entrainment
Authors: Caughey S. J. , Crease B. A. , Roach W. T.
Published: February 2007

Tethered balloon‐borne instrumentation has been used to study the small‐scale air motion and temperature variations occurring within a layer of nocturnal stratocumulus. The observations indicate a very sharp and intermittently turbulent entrainment interfacial layer with some evidence for a characteristic entrainment scale of about 10m. Within the cloud layer convective motions are apparent and these take the form of cold, downward moving plumes which increase in scale away from cloud top. They are generated by the large radiative loss from the upper part of the cloud. Some statistical features of the turbulence field in cloud are discussed and compared with other observations from stratocumulus and the daytime convective boundary layer.
A field study of nocturnal stratocumulus; III. High resolution radiative and microphysical observations
Authors: Slingo A. , Brown R. , Wrench C. L.
Published: February 2007

High resolution observations from a tethered balloon of nocturnal stratocumulus on three occasions are presented. Measurements of the microphysical properties of the clouds (drop‐size distributions, concentrations and liquid water contents) were obtained with an Axially Scattering Spectrometer Probe (ASSP). A ground based 95 GHz radiometer was used to infer the integrated liquid water path through the cloud. In the two thick clouds studied the drop‐size data show an almost monotonic increase of mean radius from cloud base to top. The liquid water content increases with height above cloud base at slightly less than the adiabatic rate and shows considerable variability towards cloud top. The observations support the concept of the inhomogeneous mixing of the cloudy air with the dry air entrained at the cloud top. Measurements were also made of infrared net radiative fluxes, and very good agreement is found with the theoretical fluxes from a high resolution radiative transfer scheme, using the ASSP drop spectra and liquid water contents, scaled by the 95 GHz radiometer data.
On the shortwave radiative properties of stratiform water clouds
Authors: Slingo A. , Schrecker H. M.
Published: February 2007

The development of a comprehensive radiation scheme for computing shortwave fluxes and heating rates in a cloudy stratified atmosphere is described. The scheme uses the Delta‐Eddington multiple scattering method and treats Rayleigh scattering, absorption by water vapour and ozone, and scattering and absorption by the cloud drops. The molecular absorption data were obtained from the Lowtran 3B subroutine and incorporated using the exponential sum fitting of transmissions technique. Five versions of the scheme were used to examine the effect of spectral resolution on the results and comparisons are shown with previous work. The short‐wave properties of clouds were examined using 24 spectral bands between 0.25 and 4μm. A significant dependence of the properties on the drop size distribution was found, in agreement with previous work. A simple parametrization of the single scattering properties of the cloud drops in terms of liquid water content and equivalent radius was examined and shown to produce satisfactory results, thus circumventing the need for time‐consuming Mie theory computations. This allows the scheme to be used in comparisons with aircraft measurements of layer clouds, within which both liquid water content and drop size distribution are functions of height.
A numerical study of thunderstorm electrification using a three dimensional model incorporating the ice phase
Authors: Rawlins F.
Published: February 2007

A numerical model of cumculonimbus allowing a simulation of the growth of the electric field is described. The cloud model is three dimensional and includes a representation of the ice phase. The charge generating mechanism is assumed to arise from the collision of rimed hail particles with smaller ice crystals. Two possibilities are considered: an inductive method, in which hail particles are polarized in the local electric field, and a non‐inductive ice‐ice method, where charge transfer is attributed to a difference in surface potential of the ice particles in contact.

These mechanisms are compared for a test cloud displaying the rainfall characteristics of a typical, small maritime thunderstorm and their sensitivity to the assumed form of the hail size spectrum examined. A cloud of relatively small size is chosen to constitute a more severe criterion of the strength of the electrification mechanisms than is possible in large thunderstorms where weaker modes of charge transfer could also produce a lightning discharge. Hence the comparison of mechanisms presented is specifically for a small thunderstorm. The results indicate that the electric field can reach a breakdown threshold within half an hour of the appearance of precipitation with the inductive method, providing that the hail size spectrum represents relatively small particles and that the effect of multiple collisions of any ice crystal with more than one hail particle is discounted, i.e. when it is assumed that ice crystals are uncharged before impact. It is found that the non‐inductive ice‐ice mechanism can lead to lightning within a similar time providing that the product of ice particle concentration and charge separation per collision exceeds 7 pC per litre.
Entrainment and the droplet spectrum in cumulus clouds
Authors: Jonas P. R. , Mason B. J.
Published: February 2007

The model of the development of the droplet spectrum in cumulus clouds described by Mason and Jonas (1974) has been used to calculate the spectrum in penetrating downdraughts and the changes which occur when a region of dry air resulting from such a downdraught is subsequently forced to rise. It is shown that evaporation in the downdraught does not remove equal fractions of droplets of all sizes; indeed concentrations of some radii may be higher than those in the surrounding cloud. When such a dry parcel is forced to ascend again the droplet spectrum within this region is similar to that in the surrounding cloud and fresh activation of nuclei prevents the rapid growth of a small concentration of large droplets entrained into the region from the surrounding cloud. It is also shown that droplet growth in a subsequent thermal entraining material from the older cloud is little affected if the latter affected if the latter is considerably diluted by penetrating downdraughts.
Numerical forecasts of stratospheric warming events using a model with a hybrid vertical coordinate
Authors: Simmons A. J. , Strüfing R.
Published: February 2007

A numerical forecast model based on use of a hybrid vertical coordinate which is terrain‐following at low levels but which tends continuously to a pressure coordinate at upper levels is introduced. Results from this model are first presented for 10‐day forecasts (extending up to 10mb) of the major stratospheric circulation changes which occurred during the winter of 1979. A considerable degree of success is illustrated, not only in predicting a major wavenumber‐two warming up to 10 days ahead, but also in the forecasting of a preceding development of wavenumber one and a subsequent decay of the wavenumber two perturbation. Diagnosis using Eliassen‐Palm cross‐sections confirms previous observational diagnoses and aids understanding of the performance of the forecast model. Computations of the meridional gradient of zonal‐mean potential vorticity are also discussed.

The sensitivity of tropospheric forecasts to stratospheric resolution and the choice of vertical coordinate is also examined. These forecasts are in fact characterized by a marked insensitivity to such aspects. Thus lowering the top forecast level from 10mb to 50mb has little effect on objective forecast scores averaged over the extratropical troposphere of the Northern Hemisphere, although planetary‐wave structures in high latitudes are affected to a small degree. The hybrid coordinate gives results similar to those obtained using the usual sigma coordinate, but such differences as are found tend to favour the use of the hybrid representation. Idealized analyses of the stability of the semi‐implicit time scheme for hybrid coordinates are confirmed in practice.
Dispersion from a continuous ground‐level source investigated by a K model
Authors: Gryning Sven‐Erik , van Ulden P. , Larsen Ren E.
Published: February 2007

A crosswind‐integrated K model with wind profiles and K profiles described by Monin‐Obukhov similarity relations is solved numerically for the case of the release of a passive substance from a point source at ground level. An extensive analysis is undertaken to compare the numerical model results with experimental results from the Prairie Grass dispersion experiments. Simulations of these experiments are carried out with and without deposition. It is shown that the numerical solution of the diffusion equation yields a good approximation to both the vertical concentration profile and the crosswind‐integrated ground‐level concentrations when consideration is given to the effect of deposition of the tracer. Despite considerable scatter, the measurements are seen to support the use of the eddy diffusivity of heat rather than the modified expression of that of momentum. For comparison Dyer's (1974) expressions for the eddy diffusivity of heat and momentum are also used; only minor differences are found in the performance of the two sets of expressions.
A laboratory study of the scavenging of sub‐micron aerosol by charged raindrops
Authors: Barlow A. K. , Latham J.
Published: February 2007

Laboratory experiments were conducted in order to measure the collection efficiencies, E, with which small water drops (radii varying from 270 to 600μm) carrying electric charges, Q, of magnitude similar to those carried on natural raindrops (10−14 to 10−11 C) collect spherical aerosol particles of radii ranging from about 0·2 to 1 μm. The values of E were found to increase with Q, were characteristically in the range 1 to 10%, and were independent of the sign of Q. They agree reasonably well with the results of earlier workers and are about two orders of magnitude greater than charge‐free (Q = 0) values.

A simple analysis indicates that the observed rates at which atmospheric visibility increases as a consequence of scavenging of particulates by rainfall are consistent with the measured values of E.
The sea breeze with mean flow
Authors: Pearson R. A. , Carboni G. , Brusasca G.
Published: February 2007

A quasi‐geostrophic mean flow can form the basic state upon which a sea breeze is superimposed; a model is described. With a mean flow perpendicular to the coast the speed of the sea breeze front is shown to be a linear function of the onshore component of the mean flow. When a constant mean flow is introduced the general structure of the sea breeze is unchanged. In particular, for a constant energy input the intensity is also unaltered. In contrast, a shear changes the general qualitative structure but leaves the speed of the front and the intensity of the sea breeze unchanged. These features are consistent with the available potential energy of the system and a constant transfer, per unit area, of this into kinetic energy. Model results and other studies are discussed.
An optical device for the measurement of liquid water content in clouds
Authors: Blyth A. M. , Chittenden A. M. I. , Latham J.
Published: February 2007

A passive optical device has been developed for the ground‐based in situ measurement of liquid water content, L, in clouds. Light scattered at five, computationally selected, small forward angles is collected and ‐ with the application of appropriate weighting factors ‐ combined to give a response which is linearly proportional to L over the droplet diameter range 1 to 80 μm.

L can be measured to within ±0.05 gm−3 at a frequency of up to 100 Hz, thus permitting fluctuations in the liquid water content of clouds to be examined on spatial scales of less than one metre. A miniaturized version of the device ‐ for airborne usage ‐ is under development.
Some wave characteristics of the middle atmosphere simulated in a general circulation model extending from the surface to 100 km
Authors: Hunt B. G.
Published: February 2007

A zonal Fourier analysis has been made of selected meteorological fields produced in a general circulation model extending up to 100 km. The model was set up for non‐diurnal, fixed January conditions, included land‐sea contrast but omitted mountains. The results largely conform to the predictions of the Charney‐Drazin criterion, at least in that only low wavenumbers in the winter are able to penetrate to the lower thermosphere. Higher wavenumbers are rapidly attenuated above the tropopause, but, very interestingly, they are regenerated with increasing amplitudes from 50 to 100 km. The occurrence of such small‐scale features is consistent with deduced dissipation rates and aeronomical characteristics in the lower thermosphere. The energy source for these small waves appears to be partially attributable to barotropic energy transfer, responding to a wave spectrum excited by baroclinic instability and internal gravity waves. A further unquantifiable energy source is associated with upwards‐propagating gravity waves. The combination of baroclinic and inertial instabilities together with barotropic processes, upwards‐propagating planetary and gravity waves indicates the wealth of dynamical processes occurring in the middle atmosphere and reproduced in the model.
The importance of nonlinear wave processes in a quiescent winter stratosphere
Authors: Austin John , Palmer T. N.
Published: February 2007

Linear wave modelling about a zonally symmetric state is often assumed to be able to simulate accurately time‐averaged planetary wave geopotential heights in the stratosphere during quiescent winter periods. We test this linear assumption by using a time‐dependent three‐dimensional primitive equation model of the stratosphere and mesosphere to simulate the quiescent winter stratospheric circulation during December 1980. The results from this simulation, together with two other idealized integrations, indicate that nonlinear wave interactions were crucial in accounting for the observed monthly mean wavenumber‐1 planetary wave amplitude, particularly in the upper stratosphere. Our results suggest that, in general, such interactions cannot be ignored for quantitatively accurate simulations of the middle atmosphere.
More realistic distributions for extreme wind gusts
Authors: Revfeim K. J. A. , Hessell J. W. D.
Published: February 2007

Maximum wind gusts are commonly analysed by fitting the Type I (Gumbel) extreme value distribution. No physical meaning is given to the usual parametric form of the distribution. The stochastic nature of wind gusts provides an alternative distribution form with physically meaningful parameters. For wind gusts the implication of this alternative form is that the most frequent gust size is infinitesimally small. Other stochastically derived extreme value distributions allow for non‐zero modality in the distribution of wind gust sizes underlying the maximum.

The assumptions of the stochastic model are qualified in their application to wind gusts. The maximum‐likelihood method may not find parameter estimates of the Type I distribution for a given sample of maximum gusts. However, for any data set a family of size distributions, with non‐zero modes, does provide parameter estimates having greater mean likelihood than the Type I distribution. Seasonal patterns in the parameters are estimated as harmonic functions and expected long term maxima are given.
Eddy‐mean‐flow interaction in a barotropic ocean model
Authors: Marshall J. C.
Published: February 2007

The role of eddies in the maintenance of idealized, antisymmetrically forced double gyres is studied using a barotropic ocean circulation model. A diagnosis in terms of q, the quasi‐conserved absolute vorticity, enables an understanding of the effect of the eddies on the time‐mean flow. The unstable model Gulf Stream, partitioning counter‐rotating gyres, is the confluence region where q contours from widely differing latitudes become concentrated, interwoven and irreversibly deformed. Here there is a strong enstrophy cascade, large eddy fluxes and eddy flux divergences, and significant driving of mean flow by the eddies. This barotropic instability of the internal jet results in a lateral transfer of vorticity sufficient to balance the net forcing of the subtropical gyre by the wind‐stress curl. The separation of the eddy flux into non‐divergent ‘advection’ and divergent ‘conversion’ contributions, using the eddy enstrophy equation as a dynamical reference, makes transparent the sense of the local eddy q flux and its systematic effect on time‐mean flow.
Satellite multichannel infrared measurements of sea surface temperature of the N.E. Atlantic Ocean using AVHRR/2
Authors: Llewellyn‐Jones D. T. , Minnett P. J. , Saunders R. W. , Zavody A. M.
Published: February 2007

The accuracy with which sea surface temperature (s.s.t.) can be measured using satellite infrared radiometers is limited primarily by uncertainties in the correction for atmospheric effects upon the measured radiance. This paper reports an investigation of the accuracy with which this correction can be made over the north‐eastern and tropical Atlantic Ocean using data at 3.7, 11 and 12 μm wavelengths from the Advanced Very High Resolution Radiometer (AVHRR/2) on the NOAA‐7 satellite, and the results are used as a test of atmospheric transmittance models. Simulations of atmospheric transmittances based on line‐by‐line calculations, using published line listings and experimental data on the water vapour ‘continuum’ absorption, provide regression relationships which permit the s.s.t. to be calculated from the brightness temperatures measured in each channel. New algorithms for both the ‘split window’ (11 and 12 μm) and the ‘triple window’ (3.7, 11 and 12 μm) have been derived for a range of airmasses from 1 to 2 to enable the s.s.t. to be retrieved from a 2280 km wide swath centred on the sub‐satellite track. To test the validity of the simulations, the s.s.t. values derived from the satellite measurements were compared with near‐coincident in situ measurements from oceanographic research ships. The absence of significant bias (˜0.1 K) in the comparison constitutes an important experimental verification of the atmospheric transmittance model used in the simulations, and the r.m.s. difference between ship and satellite values indicates that daytime measurement of s.s.t. using the ‘split‐window’ can be made to an accuracy of about ±0.6 K, in the eastern North Atlantic. Some possible sources of this uncertainty are discussed.
Generation and propagation of African squall lines
Authors: Bolton David
Published: February 2007

A study of squall lines affecting Minna, Nigeria during 1974–1976 emphasizes two particular aspects of their dynamics: that a strong jet around 650 mb is essential for their development, and that the forward edge of the squall line moves slightly faster than the maximum jet speed. A linear model of the squall line as a superposition of normal modes of an unstably stratified sheared flow sheds light on these observations. If the flow profile has a mid‐level maximum, constructive interference of normal modes gives rise to an expanding circle of convection with an arc front moving slightly faster than the maximum mean flow speed, followed by individual cells. If, however, shear is unidirectional, there is no arc front, and a line of cells is formed moving at the middle level mean flow speed. These two cases are suggested as models for tropical and mid‐latitude squall lines respectively, while a further case provides a possible model for storm‐splitting.
Temperature dependence of secondary ice crystal production during soft hail growth by riming
Authors: Heymsfield Andrew J. , Mossop S. C.
Published: February 2007

Simple experiments confirm that the production of secondary ice particles during the growth of rime depends upon the surface temperature of the riming particle rather than upon the cloud temperature. The required surface temperature of −2.5 to −7.5 °C may be produced over a wide range of cloud temperatures depending on the liquid water content of the cloud and the nature of the riming particle. A particle of high fall velocity in a cloud of high liquid water content may produce secondary ice particles at cloud temperatures considerably below −8 °C.
Structure and evolution of a mesoscale convective system near the British Isles
Authors: Browning K. A. , Hill F. F.
Published: February 2007

Frequent digital cloud imagery from Meteosat and precipitation patterns from radar have been analysed together with surface observations and conventional and satellite soundings, to develop a conceptual model of a small mesoscale convective system. The system, observed to the south‐west of England, had many of the features previously found in the United States, where the term mesoscale convective system or complex is applied to a cluster of thunderstorms that generates a common cirrus shield of mesoscale dimensions. A mesoscale region of fairly uniform, essentially stratiform, rain developed beneath the cirrus shield downwind of the active convection. Thunder was widespread throughout the precipitation area which was also characterized by a surface mesohigh. The mesoscale convective system, and another similar one before it, lasted for about half a day each, during which time they produced large falls of rain over an extensive area. The mesoscale convective systems developed in a region of locally strong baroclinicity on the eastern flank of a cut‐off low. They happened to form over the sea and were decoupled from the surface by a cool easterly boundary layer flow. Their development could be traced to the onset of advection from France of warm moist air with a wet‐bulb potential temperature in excess of 18 °C at 850 mb. The arrival of this air coincided with the intensification of the SE‐ly flow around the approaching cut‐off low.
Characteristics of southern hemisphere blocking as determined from a time series of observational data
Authors: Lejenäs Harald
Published: February 2007

A time series of 500 mb geopotential heights has been used to assess characteristics of southern hemisphere blocking. A zonal index similar to the index used in an analogous study of northern hemisphere blocking has been used. Characteristics of blocking situations have been computed with a simple algorithm, which has been programmed, and are presented as statistics.

We found one preferred region for blocking action: the Australian‐New Zealand region. Two other regions where blocks occur are the Atlantic east of South America, and the Indian Ocean south‐east of South Africa. The number of days with blocked flow is considerably less over the Atlantic and the Indian Ocean than in the Australian‐New Zealand region. The annual variation shows two maxima: one peak value of 19% in April and one extended maximum of c. 20% during June–August in the Australian‐New Zealand region. There is no clear indication of a seasonal variation in the Atlantic and the Indian Ocean region.

Investigation of individual blocking episodes reveals that the majority of the blocks have a mean eastward velocity, although the longer they last the more of them are westward‐moving. Longlasting episodes are almost exclusively found in the Australian region.
Errors in atmospheric tidal determination from surface pressure observations
Authors: Cooper Neill S.
Published: February 2007

It has previously been assumed that by taking a sufficient length of data the mean atmospheric tide at any tropical station could be determined to an accuracy of a few percentage points by using surface pressure observations. However, evidence is herein presented which shows that the inferred amplitude of the tide can be changed by over 20% just by changing the instrument used to measure atmospheric pressure. This has occurred both when one barometer has replaced another and when one barograph has replaced another at the same station. It is suggested that the main reason for the inaccuracy is that diurnal temperature variations cause systematic instrumental errors. There are important implications for the accuracy of all tidal determinations that have been made and for the reliability of tidal theory.
The response of the upper ocean to solar heating II: The wind‐driven current
Authors: Woods J. D. , Strass V.
Published: February 2007

The current profile generated by a steady wind stress is disturbed by the diurnal variation of mixed layer depth forced by solar heating. Momentum diffused deep at night is abandoned to rotate inertially during the day when the mixed layer is shallow and then re‐entrained next night when it deepens. The resulting variation of current profile has been calculated with a one‐dimensional model in which power supply to turbulence determines the profile of eddy viscosity. The resulting variations of current velocity at fixed depths are so complicated that it is not surprising that current meter measurements have seldom yielded the classical Ekman solution. However, the progressive vector diagrams do exhibit an Ekman‐like response (albeit with superimposed inertial disturbances) suggesting that the model might be tested by tracking drifters designed to follow the flow at fixed depths. The inertial rotation of the current in the diurnal thermocline leads to a diurnal jet, the dynamical equivalent of the nocturnal jet in the atmospheric boundary layer over land. The role of inertial currents in deepening the mixed layer is clarified, leading to proposals for improving the turbulence parametrizations used in models of the upper ocean. The model predicts that the diurnal thermocline contains two layers of persistent vigorous turbulence separated by a thicker band of patchy turbulence in otherwise laminar flow.
A laboratory study of secondary ice particle production by the fragmentation of rime and vapour‐grown ice crystals
Authors: Griggs D. J. , Choularton T. W.
Published: February 2007

Laboratory experiments have been performed to study the strength of rime and vapour‐grown ice crystals to investigate the possibility that fragmentation may occur following collisions in cloud. The results show that fragmentation of rime is very unlikely to occur in natural clouds. Vapour‐grown crystals are found to be considerably more fragile, with dendrites the weakest crystal habit. In the experiments one or two microscopic fragments were produced. Collisions between dendritic crystals and graupel particles may be able substantially to enhance ice crystal concentrations in some clouds.