摘要:

AbstractCase studies of developing and non‐developing tropical depressions in the north‐west Australian region during the 1979/80 and 1980/81 cyclone seasons have been constructed using conventional and satellite observations. These show that enhanced low‐level winds can occur during the lifetime of non‐developing disturbances and are not exclusively associated with cyclogenesis. Both types of system possessed lower‐level cyclonic circulations and there was an overlapping such that non‐developing storms could have stronger circulations than some developers. At upper levels, development was associated with the subtropical ridge being situated over the surface position of the disturbance. In contrast, the ridge tended to be located southwards of non‐developers.These findings are supported by calculations of area‐averaged vorticity at 850 and 200 mb. Cross‐sections of zonal winds show that non‐developing systems could exhibit weaker vertical wind shears than some developing storms. Developers had only moderate shears at worst and maintained a well organized lower cyclonic circulation.Such features are consistent with both classes of disturbance existing within the monsoon shearline. Non‐development was caused by interactions with large‐scale flows in the southern hemisphere that prevented the necessary conditions for cyclogenesis from being all fulfilled. This suggests a need for greater understanding of the influence of mid‐latitude events on depressions

ISSN:0035-9009    

DOI:10.1002/qj.49711448302

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractA variant of the classical deformation‐induced surface frontogenesis problem is studied in the semigeostrophic limit. The particular flow system examined is the response of a baroclinic, semi‐infinite, stratified, uniform potential vorticity atmosphere to an imposed deformation flow field.This flow system permits the derivation of: a class of simple analytical solutions in geostrophic space; an explicit functional solution of the Sawyer‐Eliassen ageostrophic circulation equation; and a self‐contained nonlinear equation in real space for the surface development of the along‐front wind component.The derived relationships are utilized to examine various frontogenetic aspects of this model system. These include the time scale of frontal development, the co‐occurrence of different frontal types, the slope and movement of frontal features, the structure and strength of the ageostrophic circulation, and the degree of validity of the semi‐geostrophic

ISSN:0035-9009    

DOI:10.1002/qj.49711448303

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractThree‐dimensional semigeostrophic moist baroclinic waves and frontogenesis are studied both analytically and numerically. It is found that a slantwise zone of small (either unconditional or quasi‐conditional) moist symmetric stability embedded in a dry and baroclinic basic state traps the energy of baroclinic waves, forming a strong meridional structure with maximum wave amplitude in the moist zone and exponential decay of the wave amplitude away from the moist zone. The unstable waves absorb energy from both the moist and dry baroclinic basic state, but the strongest energy conversion is in the moist zone, so growth rates decrease with the width of the moist zone. As the moist zone becomes infinitely narrow (wide) the model degenerates into a dry (moist) Eady model. The smaller the moist symmetric stability, the larger the growth rate and the wavenumber of the most unstable wave. The valid range of the model parameters depends on the semigeostrophic approximation, which sets a limit to the smallness of the moist symmetric stability.In semigeostrophic space, the moist zone changes shape due to geostrophic deformation. As the wave develops in real space, the surface pressure low (high) shifts northward (southward) due to geostrophic deformation, tightens (expands) due to ageostrophic convergence (divergence), and goes on to form a coupled warm‐cold frontal system.A numerical model of the semi‐Lagrangian finite element method is developed. The method is shown to be accurate in computing wave growths and efficient in resolving frontal structures, although the semigeostrophic approximation is found to become poor or even invalid locally in the narrow (meso‐β‐scale) cold fr

ISSN:0035-9009    

DOI:10.1002/qj.49711448304

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractWavenumber transition and wavenumber vacillation are studied numerically in a ‘maximum simplification’ model of the Eady type with uneven Ekman dissipation. The spectral wave solution contains three waves having adjacent zonal wavenumbers and the gravest meridional mode. The model only allows for wave‐mean‐flow interaction.As the stratification parameter decreases away from the upper symmetric flow, we observe that baroclinic forcing increases faster than dissipation, implying the prevalence of a higher wave energy state. This state is realized by an increase in wave amplitude, number of temporal and/or spatial degrees of freedom, or dominant wavenumber. The first two occur first, but for small enough stratification, the flow transits to a higher wavenumber in order to realize the increased energy.Wavenumber vacillation is a flow regime observed at some parameter settings when the flow has at least two spatial and three temporal degrees of freedom. This regime is usually found in the region of parameter space where wavenumber transitions occur as parameters change.The balance between baroclinic forcing and damping adjusted by wave‐mean‐flow interaction seems to be a basic mechanism for wavenumber transition and wavenumber vacillation. Wave‐wave interaction is not necessary for such pheno

ISSN:0035-9009    

DOI:10.1002/qj.49711448305

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractAirborne electric field measurements in two small thunderstorms in New Mexico show the existence of a narrow region of charge in each storm during the early stage of electrification. In one case a net negative region of charge was observed at about 7 km (‐ 12°C) about 1.5 km below the radar cloud top, when the electric field was only 600 V m−1, and could be accounted for by total charge of −0–01 C with a maximum net space charge density of −0.15 nCm−3, assuming spherical symmetry. This region of charge was about 500 m across and appears to have been associated with an updraught‐downdraught transition zone. In the other cloud, a region of net positive charge, also about 500m across, was detected at 7.7 km (‐20°C) about 500 m below the radar cloud top, when the electric field was about 2000 Vm−1. In both regions of charge, supercooled liquid water and ice particles including graupel were present, and ice particle concentrations, sizes, and collision rates were at a relative maximum, suggesting that the charge generation occurred via a precipitat

ISSN:0035-9009    

DOI:10.1002/qj.49711448306

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractMeasurements have been made of the deposition of cloud droplets and dissolved chemical species to moorland vegetation. Measurements of liquid water fluxes have been made by the gradient technique using Knollenberg FSS probes and by a weighing lysimeter. Deposition velocities for cloud liquid water in the range 0.1 to 0.24 g m−3at a height of 1.0m were close to those for momentum. The FSS probes enabled the determination of deposition velocities as a function of droplet size. It was found that the deposition velocity increased with droplet radius from 2.5 μm to 6.5 μm radius. Between 6.5 and 12.5 μm radius, deposition velocities exceeded those for momentum, but decreased sharply between 12.5 and 15.5 μm.In periods of thin cloud (〈0.1 g m−3〉), evaporation from the surface occurred despite continued

ISSN:0035-9009    

DOI:10.1002/qj.49711448307

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractThis paper describes an extension of conventional multichannel s.s.t. algorithms for NOAA‐7 AVHRR/2 data to include data from the Meteosat‐2 IR1 (11 μm) channel, generally viewing through a different zenith angle. This provides a combination of multichannel and dual‐angle techniques for determining the atmospheric correction. These algorithms, derived statistically from the results of transmission calculations on a number of temperature/humidity profiles, were tested under ‘pseudo‐operational’ conditions against ship measurements of s.s.t., and the results compared with the AVHRR‐only algorithms of other workers. The conclusion is that the technique is feasible but, due to the uncertainties in the Meteosat radiances caused by residual cloud contamination, the additional data are given only a low weight in the algorithms and the influence on retrieved s.s.t. is small. There were also opportunities to test the Rutherford‐Appleton Laboratory transmission model used in deriving the algorithms, and the results suggest that the model underestimates the absorption by the water vapour continuum in

ISSN:0035-9009    

DOI:10.1002/qj.49711448308

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractWe follow the approach suggested by F. Schmidt to implement a spectral global shallow‐water model with variable resolution. A conformal mapping is built between the earth and a computational sphere and the equations are discretized on the latter using the standard spectral technique associated with a collocation (Gaussian) grid. We prove that the only non‐trivial conformal mapping which exists between the two spheres is based on the transformation introduced by Schmidt, but the pole of the collocation grid has no longer to coincide with the pole of dilatation. We implement the technique in an explicit model, where only minor modifications to a uniform resolution model are needed. The semi‐implicit scheme and the nonlinear normal mode initialization are proved to work satisfactorily. 24‐hour forecasts show that the method is successful in dealing with the shallow‐water equations and allow us to discuss the potential of the

ISSN:0035-9009    

DOI:10.1002/qj.49711448309

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractThe horizontal momentum equation of the shallow‐water equations on the sphere is written in 3‐D vector form using the undetermined Lagrange multiplier method. The direct time discretization of this equation is proposed as an alternative to the usual approach, i.e. time discretizationafterexplicit removal of the constraint by the use of generalized coordinates. The proposed procedure is applicable to any coordinate system and eliminates an instability associated with the metric term in a spherical grid spectral semi‐Lagrangian model. For a polar‐stereographic grid finite‐element model, equivalent results and a gain in efficiency are obtained. The formalism can be extended to multilev

ISSN:0035-9009    

DOI:10.1002/qj.49711448310

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractBecause of possible significance in understanding lightning, the characteristics of an updraught velocity enhancement observed in a typical Florida thunderstorm are investigated. The most probable explanation of the sudden velocity enhancement is increased buoyancy resulting from the release of latent heat of freezing of supercooled droplets on to rime in the updraught. From information on updraughts in two different cells in the same storm, it is tentatively concluded that enhanced electrical activity, in typical Florida coastal thunderstorm cells, is probably associated with the mechanism of the initial charging of ice particles.

ISSN:0035-9009    

DOI:10.1002/qj.49711448311

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY