摘要:

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 hy

ISSN:0035-9009    

DOI:10.1002/qj.49710243202

出版商:John Wiley&Sons, Ltd    

年代:1976

数据来源: WILEY

摘要:

AbstractThe use of dynamic initialization is examined first for the case of a linearized barotropic model in which both the meteorological and gravity‐wave modes have non‐zero frequencies, and all variables are allowed to adjust mutually. It is shown that the gravity‐wave mode is rapidly damped, leaving the meteorological mode almost intact with appropriate values of divergence as well as balanced mass and vorticity fields. There are some problems associated with the slower rate of convergence at large scales; it is suggested how these might be overcome using spectral rather than finite‐difference methods. Mutual adjustment of the variables is shown to be generally more appropriate than forced adjustment.The arguments are extended to a multi‐level model by diagonalizing it into its vertical normal modes. The analysis indicates that the external gravity‐wave mode should be rapidly damped by dynamic initialization, giving appropriate values of the vertically‐meaned divergence, while low‐frequency internal modes will be relatively unaffected by the initialization procedure.A series of experiments is performed using a 5‐level hemispheric model, and the predictions of the linearized theory are confirmed. In particular it is found that the amount of computation required to reduce the amplitude of the external gravity‐wave mode to small levels is equivalent to that required for only 2–3 hours of forward integration. However, the quality of the forecast appears to be surprisingly insensitive to the presence or absence of gravity waves, and the need for such an initialization procedure is not conc

ISSN:0035-9009    

DOI:10.1002/qj.49710243203

出版商:John Wiley&Sons, Ltd    

年代:1976

数据来源: WILEY

摘要:

AbstractThe results of a field study of the evolution of radiation fog on three nights are presented. A parallel numerical model (Brown and Roach 1976 – referred to hereafter as II) was also developed to test ideas suggested by the principal features observed during the field study. These were:(i)Periods of significant fog development appeared to occur when wind speeds dropped below 0.5‐1ms−1.(ii)The liquid water content of the fog was a small fraction of the total condensed out by cooling. The balance of water appears to have been deposited on the ground.(iii)Radiative cooling (deduced from radiative flux divergence measurements) was in general greater than the actual cooling.(iv)As the surface became radiatively shielded by developing fog, the radiation inversion migrated to the fog top, accompanied by the establishment of a convective regime with a slight super‐adiabatic lapse rate in the lower part of the fog.(v)Ammonium sulphate was a principal constituent of condensation nuclei.It is shown that these features are consistent with the suggestion that the development of radiation fog is primarily controlled by a balance between radiative cooling, which encourages fog, and turbulence, which inhibits it. Gravitational settling of fog droplets and soil heat flux also emerge as important factors. The role of cloud microphysics is not passive, but is less clearly defined as yet. The numerical model (II) reproduces most of the observed features, but not the stepwise growth or sharpness of the fog top.Some of the practical implications of this work for forecasting and for fog modification are briefly di

ISSN:0035-9009    

DOI:10.1002/qj.49710243204

出版商:John Wiley&Sons, Ltd    

年代:1976

数据来源: WILEY

摘要:

AbstractStudies of a numerical model of fog are presented. These indicate that:(i)For given initial conditions, a reduction in turbulent diffusion leads to earlier and thicker fog formation.(ii)The formation and growth of fog on a realistic time scale is simulated by the inclusion of radiative cooling due to water vapour and carbon dioxide and (when formed) fog droplets.(iii)The inclusion of gravitational settling is necessary to simulate realistic liquid water contents.(iv)The nature of the underlying surface (through its thermal properties) influences the time of onset of fog.The effect of cloud cover and of solar insolation on dispersing fog was assessed.These results are broadly consistent with, and help to account for, the results of a parallel field study by Roach, Brown, Caughey, Garland and Readings (1976).

ISSN:0035-9009    

DOI:10.1002/qj.49710243205

出版商:John Wiley&Sons, Ltd    

年代:1976

数据来源: WILEY

摘要:

AbstractQuasi‐periodic oscillations of period 10–15min, observed in net flux of long‐wave radiation, surface temperature and wind during the development of a radiation fog on 7 December 1971, were found to have a significant phase relationship which persisted for about three hours. An interpretation of this phenomenon is suggested in terms of the evidence given by Roach, Brown, Caughey, Garland and Readings (1976) and Brown and Roach (1976) that the development of radiation fog is controlled by a (fluctuating) balance between radiative cooling and turbulent diff

ISSN:0035-9009    

DOI:10.1002/qj.49710243206

出版商:John Wiley&Sons, Ltd    

年代:1976

数据来源: WILEY

摘要:

AbstractThe effect of radiative heat transfer on droplet growth is assessed. It is estimated that radiative cooling is the principal agent of droplet growth in radiation fog, and that supersaturations are very small – slight undersaturation may even occur – and may be relatively unimportant for droplet growth in this environment. Gravitational settling is shown to be the principal limiting factor on droplet growth in radiation fog.It is also suggested that radiative transfer may have some relevance to the unexpectedly large numbers of 5μm radius droplets observed in cumulus clouds by Warner (1

ISSN:0035-9009    

DOI:10.1002/qj.49710243207

出版商:John Wiley&Sons, Ltd    

年代:1976

数据来源: WILEY

摘要:

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 ofR, withc≃ 0.3CAPE, but only possible ifR≳ 2.8 (small shear). This contrasts with a previous result for another regime of convection, obtained by Moncrieff and Green (1972), ifR⩽ 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 breakdo

ISSN:0035-9009    

DOI:10.1002/qj.49710243208

出版商:John Wiley&Sons, Ltd    

年代:1976

数据来源: WILEY

摘要:

AbstractThe characteristics of squall‐lines observed during the second Venezuelan International Meteorological and Hydrological Experiment (VIMHEX‐1972) are compared with the predictions of Moncrieff and Miller (1976). These squall‐lines are highly organized, propagate in a distinctive fashion, and effect a significant modification on both the thermodynamic and dynamic structure of the synoptic‐scale flow. The predicted propagation speeds and transport properties are similar to those observed, although some significant differences are also a

ISSN:0035-9009    

DOI:10.1002/qj.49710243209

出版商:John Wiley&Sons, Ltd    

年代:1976

数据来源: WILEY

摘要:

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 boundarie

ISSN:0035-9009    

DOI:10.1002/qj.49710243210

出版商:John Wiley&Sons, Ltd    

年代:1976

数据来源: WILEY

摘要:

AbstractAs a first step in the construction and testing of a theoretical model of lightning initiation, measurements have been made of the ambient electric fieldEo required to support the long‐range propagation of corona streamers as a function of dry air pressure pα and the partial pressure of water vapour pw. The experiments reveal thatEo falls more rapidly with pα than would be expected on the assumption thatEä/pα = const., which has been used in previous discussions of the problem of lightning initiation.Eo was found to increase linearly with pw. It appears that Eo is considerably less than the field required to initiate corona from any type of hydrometeor excepting colliding drop pairs. Arguments based on these results are presented to show that the high fields in thunderclouds are probably restricted to regions that are small in extent compared with the dimensions of the thunder

ISSN:0035-9009    

DOI:10.1002/qj.49710243211

出版商:John Wiley&Sons, Ltd    

年代:1976

数据来源: WILEY