bluuebunny/crossref_metadata_2025 huggingface dataset:
The influence of evaporation and drop‐interactions on a rainshaft
Authors: Brazier‐Smith P. R. , Jennings S. G. , Latham J.
Published: February 2007

Calculations have been made of the variations with fall‐depth z of the liquid water content, L, rainfall rate, p, radar reflectivity, Z, drop concentration, NT, and raindrop size distribution n(r), within a steady‐state rainshaft as a result of the evaporation and interaction of the drops. The interactions were described by a generalized stochastic equation which takes account, as described by Brazier‐Smith, Jennings and Latham (1973), of the variation of coalescence efficiency ε with drop radii R, γ and the fact that satellite drops are produced when drops separate after collision. The relative humidity is assumed to fall linearly from 100 per cent at cloud base (z = 0, T = 8°C) with a gradient β. The lapse rate within the rainshaft was taken to be 8°C km−1 and the initial size distribution was generally given by the Marshall‐Palmer equation. Calculations were made for 3 different situations in order to isolate the physical processes which most affect the rainfall: (1), evaporation without interaction; (2) interaction without evaporation; (3) both interaction and evaporation.

The calculations for Case (1) indicate that the numbers of drops in all size classes are diminished with increasing Z̄, with a preferential reduction at the small‐radius end of the spectrum. In Case (2) the interactions of the raindrops introduce considerable detail into n(r), with bimodal curves associated with satellite drop production, the highly efficient consumption of small drops by larger ones and the variations of ε and fall‐velocity V with drop‐radius γ.

The main feature of the calculations for Case (3) is the rǒle of coalescence in preserving within the spectrum a considerable amount of liquid water that would otherwise be lost by evaporation. For example, whereas the rainwater content at z = 1·5 km for β = 20 per cent km−1 and Lo = 1·0 gm m−3 is 0·28 gm m−3 if interactions are absent, their presence elevates L to 0·57 gm m−3 at this level. This retention effect is significant for all values considered of β, L, p, and the initial slope of the raindrop size distribution (varied from half to twice that of the Marshall‐Palmer equation). It is also found that the structure introduced into the size distributions as a result of interactions is partially smoothed out by evaporation, which replaces a substantial proportion of the smallest drops consumed by coalescence.
Stability of the Rossby‐Haurwitz wave
Authors: Hoskins B. J.
Published: February 2007

A spectral method is used to integrate the primitive equations for the motion on a sphere of a shallow layer of fluid with a free surface. A simple Rossby‐Haurwitz wave of zonal wavenumber 4 is found to change its form little over 24 days, whilst one of wavenumber 8 breaks down completely in 5 days. Inspired by the integrations, the barotropic stability of a wave to a perturbation composed of another wave and a zonal flow is considered. The theory suggests that waves of zonal wavenumber greater than 5 may be unstable because the feedback of the perturbation on itself is positive. Those of wavenumber 5 and less are stable, the feedback being negative. The theory and the general barotropic instability of Rossby waves is tested by integrations of the non‐divergent barotropic model, in which the wave is perturbed by a zonal flow. Some agreement with theory is found. Waves of zonal wavenumber 8 rapidly break down to produce jets in the same direction as the perturbation zonal flow. The stability of these jets is also examined.
A stochastic model of ice particle multiplication by drop splintering
Authors: Chisnell R. F. , Latham J.
Published: February 2007

A stochastic model is developed which describes the increase in ice particle concentrations resulting from the splintering of supercooled drops on freezing. Analysis of the experimental evidence suggests that significant splintering occurs at least for drops with diameters in the range 50 to 200 μm. At time t = 0 one such drop freezes and ejects r ice splinters. Each of these splinters may be captured by another water drop in the same size range at any subsequent time, which in turn will freeze and eject a further r splinters, and so on. Consideration of the probability that a particular capture will take place at a particular time yields an expression for m(t), the estimated number of particles existing at time t,

m(t) = (r2e(r‐1)t/τ − 1)/(r − 1),

Where τ is the mean lifetime of an ice splinter. It is stressed that by considering the probability of a collision occurring at any time after the birth of a splinter, far higher estimates of the population are obtained than from a non‐stochastic model.

Calculations show that the largest ice particle multiplication factors measured by Mossop et al. (1972), of order 104, can be produced in the available time if the value of r for the splintering size range is 5 or 6. Analysis of the experimental evidence suggests that such a value for r is possible.
The accuracy of radar‐derived rainfall measurements in hilly terrain
Authors: Harrold T. W. , English E. J. , Nicholass C. A.
Published: February 2007

A weather radar has been used to measure rainfall over hilly terrain in north Wales. The radar was a standard Plessey Type 43S, which has a wavelength of 10 cm and a beam width to half power points of 2°. Measurements were compared with those based on a raingauge network consisting of 62 tipping bucket gauges distributed over 1,000 km2. This density of gauges was inadequate to define the hourly areal rainfall sufficiently accurately on some occasions and in these circumstances the radar‐derived pattern was used to interpolate between the gauges so as to obtain an ‘optimum’ estimate of the actual sub‐catchment rainfall which was then used to evaluate the accuracy of the radar measurement.

Errors in the radar estimates were excessive unless the radar was calibrated hourly using raingauge measurements from one site. The accuracy was further increased when the horizontal drift of the rain in the wind between the radar beam and the calibration site was allowed for. Using these procedures, the mean percentage difference regardless of sign, Y, between the radar and optimum estimates of three‐hourly rainfall over sub‐catchments of typically 50 km2 varied from about 15% close to the calibration site to about 20% at a distance of 20 km. Differences decreased as the period of integration was increased; Y was 13% for 6 hour periods compared with 20% for 2 hour periods. Y also varied with the area of comparison. For point measurements hourly estimates differed by 37% but over an area of 500 km2 around the calibration site they differed by about 13%.

The main causes of error in the radar measurements seem to be (i) variations in the drop size distribution relation, (ii) the spatial variation in the growth (or evaporation) which occurred between the beam and the ground and (iii) the horizontal drift of the rain in the wind. Probably all of these factors, but in particular (ii), are more important in the hilly terrain of this study than would be the case in flatter terrain. A further factor contributing to the differences between the radar and optimum estimates was the (unavoidable) error in each optimum estimate of the actual rainfall.

The accuracy of the radar estimates decreased markedly if the beam intersected the melting layer. The increased errors were reduced by introducing an empirical correction factor which is a function of the range of the sub‐catchment from the radar and the height of the melting layer, but the spatial and temporal variations of the melting layer were such that it was not possible to obtain the same accuracy as when the beam was entirely within rain.
A comparison of grid‐point and spectral methods in a meteorological problem
Authors: Doron E. , Hollingsworth A. , Hoskins B. J. , Simmons A. J.
Published: February 2007

Integrations of the shallow water equations on the sphere using a second order finite difference method and a spectral method are compared. By increasing the resolution, a good estimate of the exact solution may be made, thus allowing an estimate of the accuracy of each integration. The particular initial conditions used are a Rossby wave of zonal wavenumber 4 which moves with little change in shape or amplitude and a Rossby wave of zonal wavenumber 8 which undergoes large changes within 5 days. All the models perform reasonably for the wavenumber 4 integration. A 5° times; 3° grid for the finite difference simulation is insufficient to resolve the breakdown in wavenumber 8 despite there being 9 points per zonal wavelength. A spectral model with a truncation at wavenumber 16 uses less storage and takes the same computing time as the grid‐point model. However, it is able to predict quite accurately the breakdown.
The sizes of the stratospheric volcanic particles over south‐east Australia after Mt. Agung's eruption in 1963
Authors: Neumann J.
Published: February 2007

Assuming an air layer of geometric thickness L, a particle number density N, particles of uniform radius a and density ϱ, the optical depth of the particle layer is τ = πa2 QNL, Q being the extinction efficiency factor. The total mass M of the particles in a vertical air column of unit cross‐section is M = (4/3)πa3ρNL. The ratio of the two quantities is M/τ = 4aϱ/3Q.

The above ratio is applied to data of τ for the observed 20% reduction in direct solar radiation over south‐east Australia about mid‐1963, reported by Dyer and Hicks. For M, an estimate given by Deirmendjian is used. It is inferred from the above ratio that a particle radius of a few tenths of one micron leads to a Q value such that the values for the radius and for Q are mutually consistent with the Mie scattering theory. If we assume much smaller radii, we run into contradiction with the latter. We thus conclude that most of the extinction must have been effected by particles of radii of about the same magnitude as the wavelengths where the bulk of the solar radiation energy resides. We estimate that the particle concentration may have been about 5 cm−3 and that absorption by the particles was probably very small.
The production of sub‐micron ice fragments by water droplets freezing in free fall or on accretion upon an ice surface
Authors: Bader M. , Gloster J. , Brownscombe J. L. , Goldsmith P.
Published: February 2007

In an attempt to explain the excess concentration of ice crystals over ice nuclei in some maritime cumulus clouds, laboratory experiments were carried out to determine whether sub‐micron ice particles are produced when water droplets freeze in free fall or on riming. The results show that not enough ice fragments are produced when the droplets freeze in free fall but that the numbers of sub‐micron particles ejected during riming may go some way towards explaining the large concentrations of ice crystals in maritime cumulus clouds.
Polar lows: The development of baroclinic disturbances in cold air outbreaks
Authors: Mansfield D. A.
Published: February 2007

The formation of small disturbances in cold air outbreaks over the north‐east Atlantic is shown to be due to baroclinic instability. A case study of two such disturbances shows their growth rate, wavelength and phase speed to be very close to the predictions of the baroclinic theory. Surface friction is shown to have a very important effect; strong surface winds enhancing the friction and severely reducing the growth rate. Heating from the sea surface also reduces the growth rate. It is concluded that these lows will develop only when the surface winds are light and where the air flow is not parallel to sea‐surface isotherms, thus explaining their rarity compared to that of cold air outbreaks and providing more discriminating criteria for forecasting their development.
Simulation of three‐dimensional turbulence with given second‐order statistical structure
Authors: Lindberg William R. , Thompson Rory O. R. Y.
Published: February 2007

A method is presented for generating a turbulent, three‐dimensional flow field with correct mean flows, Reynolds' stresses, and energies, along with boundary layer structure. The details of the simulation of turbulent plane Poiseuille flow are given as an example. Plane Poiseuille flow was chosen because it is a useful model for many applications, and has available reliable and extensive measurements. All nine of the components of the spectral matrix are included, from actual measurements, along with incompressibility, a sheared mean flow, and measured boundary‐layer structure of the turbulence. The results appear to be realistic, and the statistical structure imposed is adequate for a variety of turbulence studies of three‐dimensional flows, in particular turbulent dispersion. Compared with finite‐difference methods for solving the dynamic flow equations, with comparable resolution, the method presented here requires some two orders of magnitude less storage, and some four orders of magnitude less computer time.
Comparison of aerodynamic and energy budget estimates of fluxes over a pine forest
Authors: Thom A. S. , Stewart J. B. , Oliver H. R. , Gash J. H. C.
Published: February 2007

Values of the total vertical flux of sensible and latent heat over a level forested region, obtained from aerodynamic (profile‐gradient) formulae appropriate to airflow over relatively smooth surfaces, are found to fall consistently short of independent energy‐balance estimates by a factor of 2 to 3 in unstable and near‐neutral conditions (Richardson number, Ri, in the range −0·4 to +0·01), whereas for Ri > + 0·02 no similar discrepancy is detected. These results, based on tangents drawn to (semi‐logarithmic) profiles at a height of about nine aerodynamic roughness parameters (z0) above the zero plane displacement level (d) of the forest, rely on the basic assumption that the value of d established in very nearly neutral conditions (|Ri| < 0·003), namely 0·76 mean tree heights, holds under all conditions of thermal stability.

Wake diffusion and thermal seeding effects are discussed as possible additional transfer mechanisms acting to reduce profile gradients immediately over aerodynamically rough surfaces. In terms of the former mechanism (assumed to operate below d + 25z0, or so), approximate empirical formulae are derived which attempt to quantify the observed discrepancy in terms of Ri and the proximity of the surface.

It is concluded that aerodynamic equations ought not to be used to give independent flux estimates close to aerodynamically rough surfaces.
Infrared emissivity of cirrus ‐ simultaneous satellite, lidar and radiometric observations
Authors: Platt C. M. R.
Published: February 2007

The infrared emissivity pattern of a large‐scale jet‐stream cirrus system has been mapped from Nimbus 4 satellite THIR (10·5‐12·5μm) data using simultaneous groundbased lidar (0·694μm) observations.

The average cirrus emissivity was 0·28. Dense cells in the cirrus canopy were evident in both the infrared map and lidar backscatter time‐height profiles. The cells were about 10km to 50km in horizontal diameter and 1km deep and had liquid water contents of 0·2gm m−3 or greater. On occasion, the cells were ‘black’ to infrared radiation.

Temporal variations of infrared emissivity (10–12μm) measured from the lidar site were in good agreement with spatial variations in the Nimbus THIR emissivity map. The frequency of occurrence, N, of a particular THIR cirrus emissivity ϵ followed the approximate distribution N = Nmax(1 − 1·1ϵ).
Dynamics of deserts and drought in the Sahel
Authors: Charney J. G.
Published: February 2007

It is suggested that the high albedo of a desert contributes to a net radiative heat loss relative to its surroundings and that the resultant horizontal temperature gradients induce a frictionally controlled circulation which imports heat aloft and maintains thermal equilibrium through sinking motion and adiabatic compression. In the subtropics this sinking motion is superimposed on the descending branch of the mean Hadley circulation but is more intense. Thus the desert feeds back upon itself in an important manner.

If one takes into account the biosphere, this feedback mechanism could conceivably lead to instabilities or metastabilities in desert border regions. It is argued that a reduction of vegetation, with consequent increase in albedo, in the Sahel region at the southern margin of the Sahara would cause sinking motion, additional drying, and would therefore perpetuate the arid conditions. Numerical integrations with the general circulation model of NASA's Goddard Institute for Space Studies appear to substantiate this hypothesis. Increasing the albedo north of the ITCZ from 14% to 35% had the effect of shifting the ITCZ several degrees of latitude south and decreasing the rainfall in the Sahel about 40% during the rainy season.
The opacity of accreted ice
Authors: Carras J. N. , Macklin W. C.
Published: February 2007

A technique which yields quantitative measurements of the opacity of accreted ice deposits is described. This consists of determining the attenuation of a light beam as it passes through a thin section of the deposit, using a photovoltaic cell as the detector. The transmittance of the deposits is related to the air bubble concentration, and the technique provides a useful supplementary method for analysing the air bubble structures of hailstones.
A measurement of the mixing ratio of water vapour from 15 to 45km
Authors: de Jonckheere C. G.
Published: February 2007

The stratospheric water vapour altitude profile has been derived from a measurement of the attenuation of solar infrared radiation at 2·61μ using a rocket‐borne sensor. The solar zenith angle during the flight, which was made from Woomera in April 1970, was 98°. The water vapour to air mixing ratio obtained was in the range 0·5 to 2μg/g in the lower stratosphere, with an upper limit of 4μg/g at 45km.
Two‐dimensional linear lee wave modes for models including a stratosphere
Authors: Berkshire F. H.
Published: February 2007

Lee waves in the lower stratosphere are estimated by a consideration of natural wave ‘modes’ which have complex wavenumber and hence decay with distance in the downstream direction. For simple two layer troposphere‐stratosphere models the ‘leaky modes’ can be evaluated explicitly and compared with computed waves. When there is a layer of neutral stability in the upper troposphere the natural complex wave‐numbers are responsible for some quasi‐resonance phenomena.
A numerical study of boundary effects on concentrated vortices with application to tornadoes and waterspouts
Authors: Bode L. , Leslie L. M. , Smith R. K.
Published: February 2007

This paper extends the numerical study of the structure and development of a concentrated vortex by Leslie (1971), in which a vortex is simulated by suddenly imposing an upwards body force along a section of the vertical axis of a contained rotating fluid, initially in a state of uniform rotation. Whereas the former paper was concerned primarily with demonstrating the prediction of Morton (1969) that a concentrated vortex may be generated only for a restricted range of the flow parameters, the present paper investigates the important role of boundaries on vortex behaviour.

Particular interest is focused on the boundary which is normal to the vortex core and ‘behind’ the body force. On this boundary the surface stress is related to the surface velocity by a drag coefficient CD and experiments are performed in which CD is infinite, unity and zero corresponding with a no‐slip, a partially yielding and a free‐slip boundary respectively. These calculations are motivated by the desire to assess what differences, if any, between tornadoes (which develop over land) and waterspouts (which develop over the sea) can be attributed to the different surface constraint. We also study the effect on a vortex due to an abrupt change in surface condition as this is relevant to the behaviour of a tornado which happens to cross a water surface, or even one which traverses ground with varying roughness characteristics, and conversely to the behaviour of a waterspout which moves over land.

It is shown that the strength of the meridional circulation associated with the vortex, and hence the strength of the upflow in the vortex itself, are increasing functions of the surface stress. On the other hand, the azimuthal kinetic energy, and in particular the strength of the vortex as measured by the maximum swirling velocity attained, decreases as the surface stress increases. Moreover, if the drag coefficient is suddenly increased, the meridional circulation increases, the azimuthal kinetic energy decreases and the vortex width (as measured by the radius of the maximum swirling velocity at a given height) increases. These effects are reversed if the drag coefficient is decreased and do not depend on the frictional effect of the container side wall. The role of the side wall itself is briefly explored.

The results accord with the behaviour of laboratory vortices formed in air over surfaces of different roughness as studied by Dessens (1972). They also appear consistent with the observation reported by Golden (1971, p. 146) concerning the behaviour of a waterspout whose circulation decreased rapidly and visible funnel expanded during a traverse of about a kilometre over land and which subsequently reformed on moving back over water.
The horizontal cross‐sectional shape of convective plumes
Authors: Davison Douglas S.
Published: February 2007

The horizontal cross‐sectional shape of the temperature field of convective plumes at a height of 3·5m in the atmospheric surface layer was observed to be very elongated in the downwind direction with a ratio of downstream to cross‐stream diameters of about 8:1. A dynamic explanation for this elongated shape is presented, based upon the minimization of the heat flux lost by a plume due to turbulent mixing around its boundary.
A comparison of spectral and finite‐difference simulations of a growing baroclinic wave
Authors: Simmons A. J. , Hoskins B. J.
Published: February 2007

A comparison is made of integrations of the primitive equations on the sphere using a second‐order finite‐difference model, and spectral models with both triangular and rhomboidal truncation. The initial conditions comprise a baroclinically unstable mid‐latitude jet, and a perturbation of small amplitude. Increasing the resolution in each model gives a good estimate of the exact solution, and thus of the errors involved in each integration.

No one method has a superiority in all respects. Spectral integrations with truncations at zonal wave‐number 16 give a more accurate description of amplitudes and phases in the growing wave than does a finite‐difference integration using a 5° × 3° grid, but a much poorer description of the fronts that form as the disturbance approaches maturity. Large‐scale changes to the zonal‐mean state are predicted to greater accuracy using the spectral models, but smaller‐scale changes are resolved better by the finite‐difference model. The spectral models require less computing time, and less storage. For these experiments, rhomboidal truncation is favoured.
Recent developments in dynamical oceanography
Authors: Bretherton Francis P.
Published: February 2007

The classical view of the ocean circulation posits steady anticyclonic subtropical gyres in the major ocean basins above nearly quiescent deep water. Recent evidence from water mass analysis and from mesoscale velocity measurements contradicts this picture, showing that the water circulates in a more complex manner and that the flow must be regarded as turbulent. Numerical simulations of the mesoscale eddies show good agreement with the observed statistical structure, suggesting that the latter is locally determined in mid‐ocean by nonlinear quasi‐geostrophic dynamics above topographic irregularities of the same scale. The energy source for the eddies and the implications for the general circulation are still undetermined.
Energy transformations in middle‐latitude disturbances
Authors: Kung E. C. , Baker W. E.
Published: February 2007

In an attempt to study the ensemble energetics properties of various types of the middle‐latitude synoptic‐scale disturbances, the kinetic energy balance is computed over a defined large area twice daily during a five‐year period. Prevailing flow patterns are examined with daily synoptic charts to group individual synoptic cases and evaluate the mean energy budgets according to the different types of disturbances. Eleven categories of transient disturbances, including various cyclonic, anticyclonic, and intermediate flow types, are considered.

The balance of source and sink terms in the cyclonic and intermediate types of disturbances is basically similar with maximum generation and dissipation in the lower troposphere and at the jet stream level. However, in the anticyclonic disturbances there is adiabatic destruction of kinetic energy in the upper troposphere through cross‐isobaric flow with a negative dissipation which may indicate a required supply of kinetic energy from other scales of motion in the maintenance of the anticyclonic flow. Different degrees of cyclonic and anticyclonic developments are also reflected in the intensity of the energy transformation rate and the manifestation of the transport terms. A number of individual synoptic cases are also examined.
The effect of a warm mid‐latitude sea surface temperature anomaly on a numerical simulation of the general circulation of the southern hemisphere
Authors: Simpson R. W. , Downey W. K.
Published: February 2007

A mid‐latitude Sea Surface Temperature Anomaly (SSTA) is introduced into a 9‐level polar stereographic southern hemisphere model. Two 50‐day anomaly integrations (SSTAR1 and SSTAR2) are compared with their respective control integrations. In SSTAR2 the anomaly is introduced 10 days later than in SSTAR1. Comparison of the two integrations reveals systematic effects in some regions of the Antarctic trough and the mid‐Atlantic and Indian Oceans, but no systematic effect in the immediate area of the SSTA. The latter region shows a weak variation in SSTAR2 while SSTAR 1 is characterized by a strong variation and a blocking sequence some 20 days into the integration. The region of enhanced cyclogenesis in SSTAR1 occurs on the equatorial side of the anomaly and not the region of enhanced surface baroclinicity. One such large cyclone is a major feature of the blocking sequence. Enhanced latent heat release appears to be an important factor in these developments.
An example of rainbands associated with stationary longitudinal circulations in the planetary boundary layer
Authors: Browning K. A. , Bryant G. W.
Published: February 2007

Doppler radar and rawinsonde observations are presented showing the structure of some longitudinal circulations which extended downwind of hills and gave rise to stationary rainbands. The circulations occurred within a low‐level jet situated in a deep adiabatic boundary layer ahead of a moving cold front. They were several kilometres wide and up to 100km long with updraughts of about 30cm s−1. The layer of cloud was shallow in the present study so that the circulations gave rise to only light rain; however, in the presence of precipitation generated in the middle troposphere the airflow at low levels would be moistened and seeded and it is suggested that stationary bands of heavy rain might then occur as a result of boundary layer circulations of the kind described in this study.
Lunar tides in meteorological data
Authors: Palumbo A.
Published: February 2007

Long series of observations of atmospheric pressure and temperature at Naples and other Italian stations are analysed by various methods to determine the variations due to the moon's tidal influence. It is shown that the main lunar tide in atmospheric pressure varies also with solar time, having a larger amplitude by day than by night. This tide is also shown to be dependent on local meteorological conditions, indicating a contribution of lower origin than has generally been considered. New determinations are made of the main declination and elliptical tides, and their amplitudes are shown to be in reasonable accord with equilibrium theory, though the phase of the declination tide is wild. A significant lunar semidiurnal term is detected in the air temperature, which is too large to be accounted for by the adiabatic heating associated with the corresponding pressure tide. It is suggested that this term might result from the ebb and flow of warm water with the sea tide.
Atmospheric attenuation of solar radiation at Adelaide
Authors: Lyons T. J. , Forgan B. W.
Published: February 2007

Attenuation of solar radiation at Adelaide is estimated by a comparison of simultaneous readings taken at an urban and semi‐rural site. These show that the city contributes over one‐third of the total turbidity and the role of the aerosol layer is to scatter rather than absorb solar radiation.
Production of secondary ice particles during the growth of graupel by riming
Authors: Mossop S. C.
Published: February 2007

Experiments indicate that secondary ice crystals are produced when rime grows in a supercooled cloud containing drops ≥ 24μm in diameter. This occurs between temperatures of −3 and −8°C, the production rate being greatest at −5°C. These temperatures are constant to about ±0.5 deg C for velocities of the riming body ranging from 1.4 to 3.0m s−1. On average, one ice splinter is thrown off for every 250 drops of diameter ≥ 24μm accreted, at cloud temperature −5°C, and this value does not change much with target velocity. Various possible splintering mechanisms are discussed.
On the use of artificial smoothing in Galerkin and finite difference solutions of the primitive equations
Authors: Cullen M. J. P.
Published: February 2007

Various techniques for smoothing numerical forecast integrations are compared for both finite difference and finite element Galerkin methods. The difference in the accuracy of finite element and finite difference methods is analysed to illustrate the removal of ‘aliasing’ by the Galerkin approach. This is then related to the appropriate choice of smoothing technique. The extra accuracy of finite elements allows the use of more selective methods. The use of filtering is shown to be suspect when significant energy is present in high wavenumbers and the Sadourny gravity‐wave smoothing is also discussed.
Effects of aerosol on the local heat budget of the lower atmosphere
Authors: Glazier J. , Monteith J. L. , Unsworth M. H.
Published: February 2007

The convergence of heat below subsidence inversions was estimated from radiosonde ascents on fine summer days. Sensible heat input from the ground was estimated from measurements of heat fluxes and net radiation over a wheat field. Absorption and back scattering of solar radiation by aerosol throughout the atmosphere were calculated from measurements of solar radiation at the ground. Radiation absorbed by aerosol in the thermal boundary layer heated the lower atmosphere at an average rate of 3.3 degC day−1 (60Wm−2), about twice as fast as estimated for gaseous constituents.

An equation is derived to show how the net effect of aerosol on the lower atmosphere depends on the reflection coefficient of the surface and on the aerodynamic and surface resistances to vapour transfer. Over the wheat, the net effect was almost zero when the crop was transpiring fast, but when the crop was mature, aerosol caused net cooling.
A semi‐implicit forecast model using the flux form of the primitive equations
Authors: Gauntlett D. J. , Leslie L. M. , Hincksman D. R.
Published: February 2007

As part of an attempt to implement more efficient integration procedures, a semi‐implicit time differencing procedure has been evaluated within the context of a southern hemisphere, six‐level primitive equations model. The model employs the flux form of the primitive equations and includes processes such as convection, vertical and horizontal diffusion, and orography.

Comparative explicit/semi‐implicit integrations show only marginal meteorological differences. However, the semi‐implicit scheme allows a timestep about five times as large. This represents a net saving factor of approximately 2.5 for integrations involving no orography. In instances where high orography is included net efficiency advantages may be degraded significantly when using the flux form of the primitive equations unless special numerical procedures are adopted during the solution of key model equations.

On the basis of these successful results, the semi‐implicit scheme is now being used in the separate development of a limited‐area prediction model to provide routine forecasts for the Australasian region.
A non‐hydrostatic mesoscale model
Authors: Tapp M. C. , White P. W.
Published: February 2007

A 3‐dimensional non‐hydrostatic compressible model designed for mesoscale studies is discussed. A semi‐implicit scheme for integrating the equations has been developed where the terms which involve the horizontal and vertical propagation of sound waves are treated implicitly and all other terms are represented explicitly. In an atmosphere with a neutrally stable lapse rate the sound waves are then unconditionally stable while the stability of advection is governed by a criterion similar to the usual Courant‐Freidrichs‐Lewy condition. When the atmosphere is stratified the stability criterion for sound waves, which depends on the Brunt‐Väisalä frequency but is independent of the grid length, is also sufficient for the stability of gravity waves. Tests conducted to simulate a sea‐breeze in Florida agree qualitatively with published results for a hydrostatic model.
The dynamics and simulation of tropical cumulonimbus and squall lines
Authors: Moncrieff M. W. , Miller M. J.
Published: February 2007

Techniques of theoretical analysis and numerical simulation are combined to produce a dynamical model of tropical cumulonimbus convection which features a close cooperation between the updraught and downdraught circulations. The cloud‐scale dynamics determine the structure and transfer properties. Sub‐cloud‐scale transfer is unimportant. A steady‐state dynamical model shows that the upshear or down‐shear propagation speed, c, of a cumulonimbus cell relative to the mid‐level flow is determined as a function of the convective available potential energy, CAPE, and weakly influenced by the windshear through a non‐dimensional number, R, of the large‐scale flow. This propagation speed is almost constant for a wide range of R, with c≃ 0.3CAPE, but only possible if R ≳ 2.8 (small shear). This contrasts with a previous result for another regime of convection, obtained by Moncrieff and Green (1972), if R ⩽ 1 (high shear). The transfer of momentum is distinctive and of large magnitude. The initiation and growth of the convective circulation represent an essentially nonlinear, finite‐amplitude process, whose properties are closely related to the wind profile in the tropical atmosphere. The numerical simulations attain a quasi‐steady state of a complex, three dimensional nature, basic features of which can be represented in terms of the steady‐state analysis. Moreover, the outflow of the downdraught air is closely related to the attainment of a steady circulation and suggests a mechanism both for the maintenance and the eventual breakdown of the convective regime.
A lateral boundary formulation for multi‐level prediction models
Authors: Davies H. C.
Published: February 2007

An expedient method is proposed for the lateral boundary treatment of a limited‐area prediction model. The method involves the relaxation of the interior flow in the vicinity of the boundary to the external fully prescribed flow. A systematic study of the method is undertaken with an (x, z), linear, primitive equation model. Analytical considerations of the method for the continuous equations demonstrate the manner in which the method consumes gravity wave energy, error and fine spatial scale potential vorticity near the lateral boundaries. Numerical experiments are also undertaken to assess the usefulness of the method. The results indicate that the method gives an adequate representation of outgoing gravity waves with and without an ambient shear flow, and also allows the substantially undistorted transmission of geostrophically balanced flow out of the interior of the limited domain.

On the basis of these results, it is suggested that the method constitutes a promising utilitarian treatment of the lateral boundaries.
Moisture features of the convective boundary layer in Oklahoma
Authors: Schaefer J. T.
Published: February 2007

Data from an instrumented 461m tower and rawinsonde balloons are analysed to determine the characteristics of the moisture field in the planetary boundary layer over central Oklahoma. In the mean, the lowest 94% of the sensibly adiabatic layer exhibits an effectively constant mixing ratio. Inferences as to the effectiveness of downward mixing through the top of the boundary layer are drawn.
Medium term fluctuations of rainfall in southeastern England
Authors: Gray Barbara M.
Published: February 2007

An areal average data series based on 18 stations scattered throughout south and east England was set up. The incomplete gamma function was used as a model for the frequency distribution of the data, since it gave good agreement with the parameters of the longest record in the area, Kew (1697–1971). The power spectrum of the areal average series shows that non‐linear processes on a time scale of months or years may be involved in rainfall fluctuations. Decade average values of the total monthly rainfall of the area series were used to forecast decade mean monthly rainfall totals for 1970–1979, 1980–1989 and 1990–1999, using a mixed time series model.
An additional model test of positive feedback from high desert albedo
Authors: Ellsaesser H. W. , Maccracken M. C. , Potter G. L. , Luther F. M.
Published: February 2007

Charney's (1975) suggestion that high desert albedo initiates a biogeophysical positive feedback response by the atmosphere has been tested in the 2‐D zonal atmospheric model, ZAM2. The results agree with the earlier test in the NASA GISS 3‐D GCM model (Charney et al. 1975) in confirming a positive feedback through precipitation but indicate a negative feedback in terms of temperature. The latter suggests that tropical deserts may play a role in air conditioning or placing an upper‐bound on the temperature of our planet.
Heat and moisture flux in the stratified boundary layer
Authors: Warhaft Z.
Published: February 2007

Using simple models for the form of the pressure strain terms in the heat and moisture flux transport equations, it is shown that the magnitude of the heat and moisture flux depends on the cross‐correlation coefficient of the temperature and humidity fluctuations. When the temperature and humidity fluctuations are weakly correlated it is shown that the coefficient of eddy conductivity may be significantly different from the coefficient of moisture eddy diffusivity.
The relation of the poleward flux of sensible heat during the winter half year to some zonal and meridional indices of the circulation
Authors: Miles M. K.
Published: February 2007

Simple indices of the zonal and meridional surface circulation have been obtained for the months of the winter half year. The strength of the zonal circulation appears to be little related to the poleward flux of heat in this period. An index of the meridional circulation, measuring the strength of the main northerly and southerly currents, is much more closely related to the seasonal variation of heat flux and better represents the few year‐to‐year differences available. This result may be helpful in interpreting the secular changes in the zonal circulation.
Seasonal variation of planetary waves in the stratosphere observed by the Nimbus 5 SCR
Authors: Hirota I.
Published: February 2007

The structure and behaviour of planetary‐scale temperature waves in the stratosphere are described by the use of observations from the Selective Chopper Radiometer on the Nimbus 5 satellite during the two years from January 1973 to December 1974.

From a latitude‐height diagram of monthly root‐mean‐square wave amplitudes, it is found that the planetary wave of wavenumber one is predominant in the upper stratosphere at middle and high latitudes throughout the year for both hemispheres. Waves of higher wavenumbers are rather weak in the summer hemisphere.

Power spectral analysis of monthly r.m.s. wave amplitude for 24 months shows that the 12‐month period oscillation is prominent in middle and higher latitudes, while a 6‐month periodicity is observed in tropical latitudes for wavenumber one. The phase of maximum of the 6‐month cycle is about one month after the equinoxes, suggesting that planetary wave activity in lower latitudes is closely related to the semiannual oscillation of the equatorial zonal current in the upper stratosphere.

Further analyses are made of seasonal characteristics of ‘transient’ waves to find the relationship between the wave motion and the mean zonal wind velocity. The predominant period (or phase velocity) is determined by a power spectral analysis using the Doppler effect due to the east‐west motion of waves. With wavenumber one in the upper stratosphere, a 15‐ to 20‐day period standing oscillation is observed in wintertime, while a 10‐day period westward moving wave is found in the summer hemisphere. During the spring and autumn, a standing oscillation with a period of about 3 weeks can be seen together with an 8‐ to 10‐day period westward motion. Higher wavenumbers show eastward motion in winter and no significant periodicity is found in other seasons as the wave activity is weak.

These characteristic features of transient waves are explained by the theory of critical line absorption for vertical wave propagation through the mean zonal wind.
The effect of spherical geometry on momentum transports in simple baroclinic flows
Authors: Hollingsworth A. , Simmons A. J. , Hoskins B. J.
Published: February 2007

We consider the baroclinic instability of flows on the sphere. An approximate expression is derived for the momentum fluxes associated with exponentially growing modes. It is shown that the occurrence of these momentum fluxes depends on the Rossby beta‐effect in conjunction with the variation with latitude of the Coriolis parameter in the thermal wind relation and in the vortex stretching term. A physical interpretation of the results is presented.
The effect of soil moisture on circulation and rainfall in a tropical model
Authors: Walker Julia , Rowntree P. R.
Published: February 2007

The sensitivity of a tropical model to soil moisture content has been investigated using a simplified version of the UK Meteorological Office 11‐layer model. the model area represented a zonally‐symmetric version of West Africa. the results of one experiment (D) with a desert in the same latitudes as the Sahara were compared with those of another experiment (W) in which the desert was replaced by moist land. the initial fields contained an idealized easterly wave which developed during the experiments to form depressions whose movement, structure and associated rainfall were strongly influenced by the dryness of the underlying surface. the energetics of these disturbances were analysed and two distinct mechanisms for maintaining their eddy kinetic energy are proposed, depending on the soil moisture.

Results from W suggest that once the land was moist it maintained itself in this state for at least several weeks. In D, however, the initial aridity north of 14°N was sustained, suggesting that ground dryness alone can cause deserts to persist. an analysis of observed rainfall data for the northern Sahel provides much stronger evidence for the persistence of rainfall anomalies from year to year than do similar analyses for the southern Sahel. the possibility that soil moisture anomalies could contribute to this persistence is briefly discussed.
The surface flow in a statistical climate model ‐ a test of a parameterization of large‐scale momentum fluxes
Authors: White A. A.
Published: February 2007

Green's parameterization of the latitudinal flux of zonal momentum by large‐scale eddies is investigated. In a steady‐state, β‐plane model of the zonal mean troposphere a simple basis for estimating transfer coefficients (eddy diffusivities) leads to a satisfactory representation of this large‐scale momentum flux, as indicated by the surface zonal flow. the simple basis appears inappropriate for systems in which the β‐effect is not dominant. In a spherical polar, quasi‐geostrophic model of the zonal mean troposphere an adaptation of the method leads to less satisfactory results. It is clear that the spatial variation of the transfer coefficients is more important than in the β‐plane case; and a more precise way of estimating this variation is desirable. Barotropic/baroclinic stability analyses seem likely to provide the most reliable basis, but their use would amount to an essentially different parameterization technique: less elaborate methods may be adequate.
Some aerodynamic features of a cotton canopy
Authors: Bache D. H. , Unsworth M. H.
Published: February 2007

Analysis of profiles measured over irrigated cotton in the Sudan Gezira showed that the crop boundary layer remained stable throughout most of the day. The shape of wind profiles measured within the canopy suggested that momentum was absorbed mostly in the upper layers of the canopy, with the lower regions remaining isolated from the microclimate above. A similarity analysis based on dynamic scaling factors yielded a generalized wind profile from which momentum diffusivities and mixing lengths were calculated. The analysis showed that aerodynamic features of the upper layers of the canopy were characterized by the friction velocity and the height of the zero‐plane displacement.
Inertial and frictional effects on rotating and stratified flow over topography
Authors: Buzzi A. , Tibaldi S.
Published: February 2007

The separate effects of fluid inertia and friction at the lower boundary on the flow of a rotating, stably stratified, incompressible fluid over a three‐dimensional shallow isolated obstacle are investigated.

Analytic solutions of the perturbation equations are obtained as a power series expansion in the Rossby number, and some examples are presented. Possible meteorological applications to the flow over the Alps are discussed.
An observed relation between the macroscale local eddy flux of heat and the mean horizontal temperature gradient
Authors: Tucker G. B.
Published: February 2007

Frequent and detailed upper air observations at Laverton (38°S) for 30 days during September/October 1966 are analysed to determine the local horizontal eddy flux of heat and the mean horizontal potential temperature gradient. Components in the meridional and zonal directions are studied, but a more meaningful relation emerges when the vector eddy flux is considered in relation to the vector temperature gradient. the angle, δ, between these two vectors, and the down‐gradient and cross‐gradient exchange coefficients, K and R, are derived. A 3‐tiered structure between the surface and 30 km emerges, with down‐gradient flux (K positive) in most of the troposphere and mid‐stratosphere, and counter‐gradient flux (K negative) in the lower stratosphere. the cross‐gradient flux is in the direction of the thermal wind (R positive) at nearly all heights except in the lowest layers and between 20 and 24 km. δ exhibits a fairly steady and systematic change throughout the height range analysed (2–28 km). Computations using standard upper air observations at selected Australian stations for 1970 show a mean annual pattern which is remarkably coherent with latitude (10–55°) and height (0–22km), except in the tropical troposphere. Typical derived mid‐latitude values for 1970 are:

Only part of the vertical variation in the lowest layers can be accounted for by surface frictional influences.