bluuebunny/crossref_metadata_2025_split huggingface dataset:
On the general circulation of the atmosphere without clouds
Authors: Hunt B. G. 
Published: February 2007

AbstractAn experiment has been performed with an 18‐level, hemispheric, stereographic general circulation model to clarify the radiative role of clouds in the general circulation of the atmosphere. All cloud properties were removed from the radiation scheme in the model, and it was then integrated to generate a new set of quasi‐equilibrium atmospheric statistics which were compared with a control run.Apart from an expected warming of the surface and troposphere the overall changes to the general circulation characteristics following the removal of clouds were surprisingly small. For example, the zonal mean latitude‐height distributions of the zonal wind and eddy kinetic energy were almost identical to those of the control run, while synoptic distributions of most model variables revealed little change in character attributable to the removal of cloud. The largest modification occurred in the mean meridional stream function, with corresponding effects in surface pressure. This result was explicable in terms of the enhanced role of latent heat in the model which occurred at the expense of sensible heat. Despite the individual variations in latent and sensible heat, the poleward flux of total heat by large‐scale eddies was essentially unchanged in the two models. This surprising result indicated that the forcing function for the general circulations of the two models was also unchanged, and moreover that this forcing function was independent of cloud cover.Where possible, explanations are advanced to account for the limited response of the model to the removal of cloud. It appears that the major function of clouds in radiative calculations, at least in extended general circulation simulations, is to provide an obscuration or blocking effect, rather than a more subtle radiative‐dynamical coupling mechanism.
A wind tunnel investigation of the hydrodynamic behaviour of growing, freely falling graupel
Authors: Pflaum J. C. , Martin J. J. , Pruppacher H. R. 
Published: February 2007

AbstractThe hydrodynamic behaviour of riming frozen drops and riming simulated ice crystal plates was studied by means of a vertical wind tunnel. At all times the ice particles were freely suspended in the vertical airstream of the wind tunnel where temperature and liquid water content could be accurately controlled. The ice particles studied had initial radii between 100 and 300 μm and final radii between 400 and 600 μm. The supercooled cloud in which riming of the ice particles was observed had a temperature near −15°C, a liquid water content of 2g m−3, and consisted of drops with a modal radius of 7μm.Spinning, helical, and various oscillating motions, or simple straight fall with an almost fixed orientation of the ice particle, but no tumbling motions, were observed. These fall modes appeared to be closely related to the instantaneous shape, surface roughness, and mass loading of the ice particle. Frozen drops as well as ice crystal plates were found to develop into conically shaped graupel.During the initial stages of the observed dry growth riming, the terminal velocity of rimed frozen drops decreased with increasing ice particle size as a result of a less rapid increase of the mass of the ice particle compared with its cross‐sectional area perpendicular to the airstream, i.e. as a result of a less rapid increase in time of the force the ice particle experiences in the earth's gravitational field compared with the hydrodynamic drag acting on it. This behaviour may have some important bearing on the lifetime of graupel and hailstones in atmospheric clouds.
Tornadogenesis
Authors: Smith R. K. , Leslie L. M. 
Published: February 2007

AbstractIn this paper we use a simple numerical model to study vortex growth in a flow configuration which broadly simulates the principal characteristics of a severe tornadic storm system, i.e. strong vertical forcing by intense cumulus or cumulonimbus convection in the presence of an organized field of rotation on the cloud scale. The model is similar to one described by L. M. Leslie in which the updraught in the main cloud cell is modelled by an imposed body force, but differs in that the rotation field is determined by specifying the vertical profile of swirling velocity at the lateral boundary of the flow domain and air is allowed to enter or leave the computational region through its radial and upper boundaries. In particular, we compare situations in which the imposed swirl is concentrated aloft, primarily above cloud base, and when it extends to lower levels. In the former case, solutions exhibit genesis to a steady suspended vortex provided that the forcing strength lies within a certain range, depending on the level of rotation. However, if the imposed swirl extends sufficiently far below cloud base, the vortex continues its downward growth and establishes contact with the ground.We believe our results provide a plausible and consistent picture of the growth of a tornado beneath the main updraught of a severe thunderstorm and indicate why only a relatively small proportion of such clouds spawn pendant funnel clouds and why only a fraction of these develop into tornadoes. Moreover, they appear to be consistent with the observed development of circulation patterns deduced from single‐pulse Doppler radar measurements of a tornado‐producing storm system by Burgess, Lemon and Brown.
Quasi‐geostrophic planetary wave forcing
Authors: Shutts G. J. 
Published: February 2007

AbstractThe problem of stationary planetary wave forcing by orography and longitudinal asymmetry of heating is re‐examined in the light of recent observations and theories of stratospheric motion. It is deduced that the region between 30 and 60km is a sink of planetary wave energy for zonal wavenumbers one and two in winter and that, in contrast to many theoretical descriptions, the transmission of wave energy to these levels is unimpeded.Quasi‐geostrophic theory expressed in spherical polar geometry suggests that in winter, wave energy should be able to propagate freely into the upper stratosphere for the largest scales of motion. Uncertainty in the absorption mechanism is avoided by the use of an energy‐transmitting upper boundary condition at a suitably chosen level and attention is concentrated on the structure of the wave motion below. Features of the January mean contour charts such as the Siberian and Aleutian anticyclones are shown to be consistent with the theory of untrapped, thermally forced motion. Close agreement of the structure of wavenumber one in theoretical solutions with existing observations is obtained for all heights below 30km, vindicating the use of energy‐transmitting boundary conditions.An important consequence of the ability of forced waves to propagate energy into the stratosphere is the poleward transport of heat, which is particularly strong for thermally forced motion near the level of non‐adiabatic heating. These calculations suggest that the contribution of stationary, thermally generated waves to the total poleward transport of heat is a major one in the lower troposphere.Some attempt to describe the monsoonal circulation of the northern hemisphere in summer is made with the inclusion of a stratosphere dominated by easterly mean zonal winds. The presence of a critical layer (where the mean zonal wind vanishes) introduces some interesting problems as regards the ‘realism’ of the linearized solution. Observations in July show that the phases of low wavenumber disturbances tilt rapidly eastwards with height in marked contrast to the westward tilt in winter. Solutions allowing critical‐level absorption are in poor agreement with observation in the troposphere with westward‐sloping phase lines, though the rapid attenuation of all waves in the stratospheric easterlies is well represented.
Transfer characteristics for a heterogeneous surface of large aerodynamic roughness
Authors: Garratt J. R. 
Published: February 2007

AbstractValues of zo and ZT, roughness scaling lengths for momentum and heat, are found to be 0.4±0.2m and 0.035±0.02m, respectively, for a heterogeneous surface comprising 8m high trees with transpiring foliage, 1 m high dry grass, and sandy soil. Determination of ZT utilizes aircraft and ground‐based radiometric observations of surface temperature.The ratio Zo/ZT ≈ 12 was found essentially constant over the observed roughness Reynolds number range 5000–25000, consistent with values published for homogeneous natural vegetated surfaces.
The dynamical structure of two‐dimensional steady convection in constant vertical shear
Authors: Moncrieff M. W. 
Published: February 2007

AbstractA nonlinear regime of two‐dimensional steady convection in unidirectional flow of constant vertical shear is examined theoretically, the main objective being to determine the organization of the updraught and downdraught system. The asymptotic solution for the travel speed of the system and the remote flow obtained by Moncrieff and Green is used as lateral inflow and outflow boundary conditions. The internal flow and the shape of the interface separating the updraught and downdraught branches of the circulation are determined as solutions of a free‐boundary problem. The orientation of the flow is basically determined by the ratio of convective available potential energy (CAPE) to the square of the cloud layer shear expressed as R=CAPE/½ (Δ U)2. In all realistic ranges of R the interface slopes downshear, the slope being little affected by compressibility, by the negative vorticity generated in an interfacial boundary layer and by different values of R in the updraught and downdraught branches. Quasi‐steady storms are thus not adequately described by two‐dimensional dynamics. Certain three‐dimensional effects appear essential to significantly advance the theory of severe storms, and these are discussed with reference to existing numerical and observational models. Steady convection can exist only if the values of R in the updraught and downdraught branches lie within a limited range, a result of considerable significance in cloud physical terms.
A split explicit integration scheme for numerical weather prediction
Authors: Gadd A. J. 
Published: February 2007

AbstractAn economical explicit integration scheme for numerical weather prediction models is described. A splitting technique is used, in which the horizontal advection terms in the governing equations are integrated with a timestep limited by the wind speed, whilst the terms which describe gravity‐inertia oscillations are integrated in a succession of shorter steps. A two‐level numerical scheme with small phase speed errors is used for the advection stage and a forward‐backward method for the gravity‐inertia terms.The split explicit scheme has been applied to the Meteorological Office operational 10‐level model, and to a similar sigma coordinate model, to compute forecasts for periods up to six days ahead. The quality of the numerical forecasts obtained is not reduced when the timesteps used are close to the limits set by linear computational stability criteria. The use of such timesteps leads to a substantial computational economy relative to previously available integration schemes. For the northern hemisphere version of the 10‐level model the computing time required is one‐third of that for a split semi‐implicit scheme and one‐sixth of that for the original explicit Lax‐Wendroff scheme. Verification statistics for split explicit forecasts indicate improved accuracy when compared with those for earlier operational models.
Some effects of meridional shear and spherical geometry on long stratospheric waves
Authors: Simmons A. J. 
Published: February 2007

AbstractForced, low‐wavenumber disturbances of normal‐mode form are determined for zonal flows with meridional shear, but no vertical shear, using the linearized primitive equations with spherical geometry. Results are presented both for stationary waves in the winter westerlies and for travelling, equatorially trapped modes.For flow profiles representative of the winter lower stratosphere results are in agreement with some earlier studies, curvature effects giving a maximum geopotential perturbation close to the axis of the polar night jet. Wave structure is more variable for upper stratospheric profiles of larger meridional scale, and more sensitive to zonal wavenumber, spherical geometry and ageostrophy. Geopotential maxima are typically located polewards of mean flow maxima, and meridional structures are in general agreement with observation. The vertical penetration of disturbances to these broader flows is significantly reduced by singular‐line absorption.Equatorial wave solutions show the vertical wavelength of the Rossby–gravity wave to be sensitive to the meridional scale of the mean flow, and for and asymmetric jet the wave structure is approximately symmetric about the latitude of zero zonal mean absolute vorticity. The Kelvin wave is much less sensitive to meridional shear. A simple expression is given for the zonal mean acceleration due to waves dissipated by Newtonian cooling, and results confirm a suggestion that asymmetries in the zonal mean flow profile will be enhanced by the presence of such waves.
Aircraft measurements of ice in Florida cumuli
Authors: Hallett John , Sax Robert I. , Lamb Dennis , Murty A. S. Ramachandra 
Published: February 2007

AbstractAircraft observations of cloud and precipitation particles just below the tops of Florida cumuli at temperatures near −4 and −9°C show that fresh, vigorously growing towers (updraughts of order 10 ms−1) tend to contain low concentrations (<0.11−1) of graupel, whereas older towers with lower updraught speeds contain much higher concentrations (>101−1). The ice phase, mostly as graupel, was often found to appear within a period of five minutes. Cloud drops of diameter >25μm were always found to be present in concentrations of 10 to 100cm−3 between −4 and −6°C, a condition, according to criteria established by laboratory studies, necessary for secondary ice crystal production by the riming of graupel particles. Vapour‐grown columns in concentrations as high as 501−1 were sometimes observed at the colder level, an observation consistent with the concept of a copious production zone below. Supercooled raindrops in concentrations > 11−1 occur in many regions of the clouds, often concurrently with graupel and crystalline ice. It is shown by means of a simple analytical model that aerodynamic capture of secondary ice particles by the supercooled raindrops and the subsequent transformation of the raindrops to new splinter‐producing graupel may be the main mechanism responsible for the rapid increase in observed graupel concentrations. The coalescence processes are therefore seen to be instrumental in the evolution of the ice phase. This process is favoured in lower updraught regions where the particles can be retained in the −4 to −6°C production zone long enough (≃100s) for the entire feedback process of splinter production, capture, drop freezing and renewed splinter production to take place.
Longwave radiation at the ground: IV. Comparison of measurement and calculation of radiation from cloudless skies
Authors: Dalrymple G. J. , Unsworth M. H. 
Published: February 2007

AbstractOn cloudless days in summer, longwave irradiance was measured with a radiometer and was calculated from a radiation chart using records from radiosondes released at regular intervals. The results show that the chart underestimated the longwave flux density at the ground by up to 40 Wm−2, corresponding to an extra atmospheric emissivity of up to 0.12. The extra emissivity was correlated with turbidity during the day, and on turbid days it showed a diurnal variation with a maximum at noon. It is suggested that emission in the atmospheric window from dust, pollen and spores may account for some of the observed excess.
An analytical model of rainfall interception by forests
Authors: Gash J. H. C. 
Published: February 2007

AbstractThe description of the evaporation of rainfall intercepted by forests in terms of a regression of evaporation loss on incident rainfall is discussed and some of the assumptions implicit in that method are re‐examined. The two major factors which control the evaporation of intercepted rainfall are identified. These are: (i) the amount of time that the canopy spends saturated during rainfall and the evaporation rate applicable under these conditions; and (ii) the canopy saturation capacity and the number of times this store is emptied, by drying out after the cessation of rainfall. A model is then constructed which is conceptually similar to the Rutter model, but which replaces that model's numerical approach with an analysis by storm events. The evaporation from a saturated canopy during rainfall is estimated from the Penman‐Monteith equation; the evaporation after rain has ceased, the effect of small storms insufficient to saturate the canopy, wetting‐up the canopy and evaporation from the trunks are added as separate terms. The model has been tested against data from Thetford Forest in East Anglia, with satisfactory agreement between observation and estimation. It is suggested that the model may be capable of making useful estimates of the evaporation of intercepted rainfall, solely from rainfall measurements.
A simple air‐sea interaction model
Authors: Nicholls N. 
Published: February 2007

AbstractThe way in which equatorial sea surface temperature (s.s.t.) anomalies may interact with the atmosphere is discussed. A simple model is developed to simulate this air‐sea interaction in the Australian region. It is shown that the model can correctly simulate many interesting aspects of the observed statistical behaviour of the ocean‐atmosphere system, including the seasonal variation in persistence of pressure anomalies, the seasonal variation in pressure v. s.s.t correlation, a biennial oscillation in the atmosphere and ocean, and certain lag correlations between pressure, temperature and rainfall. It is concluded that the air‐sea interaction simulated by the model is an important control on the behaviour of the atmosphere and ocean.
Aspects of the energetics of the upper stratosphere during the January‐February 1973 major sudden warming
Authors: Crane A. J. 
Published: February 2007

AbstractData from the Nimbus 5 selective chopper radiometer have been used to study certain features of the dynamics and energetics of the January‐February 1973 sudden warming. Daily values of the energetics of the upper stratosphere during a sudden warming have been calculated. Analyses of the rates of change of temperature and geopotential wave amplitude give no support to the suggestion that the disturbance suffers a time lag with increasing height. Maximum rate of warming is shown to descend from the upper stratosphere. Interaction between travelling and quasi‐stationary waves did not appear to be important in the development of the warming. Two main results arise from an analysis of the latitudinal distributions of the energetics. Firstly there is good evidence to suggest that the amplification of the temperature wave between 10 and 2mb results from a transfer of eddy potential energy from the 50–10mb layer, and not from the lower stratosphere or troposphere below 50mb. However, energy propagation above 2mb is initiated by a large increase in energy flux through the 50mb level. Secondly, despite considerable uncertainty in the kinetic energy budgets, it appears that eddy kinetic energy increases in high latitudes as a result of convergence of the vertical energy flux but increases in middle latitudes as a result of barotropic conversion from the zonal flow. The westerly zonal flow is decelerated in mid‐latitudes and initially accelerated in high latitudes by divergence and convergence of large poleward eddy momentum transport. Eventually the effect of the induced mean meridional circulation leads to easterly acceleration in high as well as middle latitudes. Some strong resemblances are found in the numerically simulated zonal flow changes of some model warmings which show a meridional lag in the onset of easterly acceleration.
Occurrence and structure of the equatorial trough zone in Venezuela
Authors: Riehl Herbert 
Published: February 2007

AbstractAircraft traverses were made from north to south across Venezuela during the rainy season of 1972 to determine whether a convectively active equatorial low pressure trough occurred and, if so, its structure, average properties and mechanics. The trough could indeed be located regularly near latitudes 5−7°N flying along the Orinoco River. Near cloud base (920mb) there was a wind shift from east to west and from north to south components. Farther south, winds from southeast were encountered. The low troposphere was warm with a maximum equivalent potential temperature at the axis. At 700mb neither the warm layer nor any wind shift could be found. The structure closely resembled that of the equatorial trough observed in the Garp Atlantic Tropical Experiment of 1974.The traverses largely indicated negative correlation between thermodynamic energy, mainly controlled by water vapour, and buoyancy, given by virtual temperature, on mesoscale distances of 30–50km. Occasional ‘hot tower’ penetrations were encountered. Surface air reached cloud base undiluted. Absolute vorticity was highest at cloud base just north of and at the trough, decreasing sharply to near zero to its south, just where heaviest convection was concentrated. Here also the largest vertical shear of the zonal wind component was located, again in agreement with GATE. The pressure‐height field was calculated from the wind data and indeed yielded a minimum value at the trough axis. To the north, the zonal wind increased with height though temperature decreased northwards. Calculations on the balance of forces with frictional retardation and on the origin of the low‐level warm layer give indications of the mechanics of the structure; the latter appears to be suggestive of a widely applicable equatorial trough model.
Stratospheric temperatures and geostrophic winds during 1973–1974
Authors: McGregor James , Chapman W. A. 
Published: February 2007

AbstractRadiance measurements derived from the Nimbus 5 selective chopper radiometer during the two years 1973–1974 are utilized to produce estimates of the global stratospheric temperature structure. Results are presented as polar stereographic maps of equivalent temperature for various of the instrument channels, and as meridional cross‐sections. Seasonal, monthly and daily averages, as well as monthly and daily averages of zonal wavenumbers one and two in temperature, are analysed. Monthly mean zonal winds are also computed, using the radiance data together with conventional 200mb height fields.The seasonal analysis confirms that the southern hemisphere experiences greater extremes of temperature than does the northern. In the monthly analysis considerable differences in temperature and zonal wind are found between the two hemispheres. Maximum wavenumber‐one amplitudes are generally found in regions of weak westerly winds. Wavenumber‐two amplitudes are usually less than half those of wave‐number one. A peak in the wavenumber‐two amplitude at the equator during January 1974 suggests that cross‐equatorial wave propagation occurs. Daily variations of zonal mean temperatures show that equatorial coolings occur at times of sudden warmings in the winter hemisphere and the daily variations of wavenumbers one and two reveal the more regular nature of wave activity in the southern hemisphere compared to the northern hemisphere.