bluuebunny/crossref_metadata_2025_split huggingface dataset:
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

AbstractMeasurements 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

AbstractA 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

AbstractTethered 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

AbstractHigh 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

AbstractThe 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

AbstractA 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

AbstractThe 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

AbstractA 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

AbstractA 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

AbstractLaboratory 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

AbstractA 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

AbstractA 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

AbstractA 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

AbstractLinear 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

AbstractMaximum 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

AbstractThe 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

AbstractThe 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

AbstractA 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

AbstractSimple 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.