bluuebunny/crossref_metadata_2025_split huggingface dataset:
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

AbstractValues 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

AbstractThe 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

AbstractIt 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

AbstractA 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

AbstractThe 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

AbstractLee 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

AbstractThis 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

AbstractThe 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

AbstractA 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

AbstractThe 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

AbstractIn 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

AbstractA 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

AbstractDoppler 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

AbstractLong 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

AbstractAttenuation 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

AbstractExperiments 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

AbstractVarious 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

AbstractThe 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

AbstractAs 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

AbstractA 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

AbstractTechniques 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

AbstractAn 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

AbstractData 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.