摘要:

AbstractA quasi‐geostrophic β‐plane set using geometric height as vertical coordinate is derived without the conventional assumption that fractional changes of basic state potential temperature over a scale height of the atmosphere are small. Although the hydrostatic, continuity and vorticity equations contain terms not present in the usual set, and the rigid horizontal boundary condition is modified, the form of the implied potential vorticity equation in terms of the stream function is the same as before. Some of these conclusions are extensions to the nonlinear case of results obtained in 1968 by Lindzen in a linearized analysis. the modified equations have reasonable integral conservation properties. As regards accuracy they are closely comparable with the quasi‐geostrophic pressure coordinate equations, but since rigid horizontal boundary conditions are easily applied exactly in the modified set the most nearly equivalent pressure‐based system is evidently a (quasi‐geostrophic) σ‐coordinate set rather than thep‐coordinate set. the modified rigid horizontal boundary condition has the property of allowing for the change in the apparent vertical when a coordinate transformation to a frame moving with a uniform zonal velocity relative to the β‐plane is carried out. Application of the modified equations to anf‐plane baroclinic instability problem reveals a dependence of stability properties on the height‐averaged flowUoas well as the shear (as found also in 1947 by Charney and by Geisler and Dickinson in 1975 in β‐plane analyses), the dependence onUobeing most ma

ISSN:0035-9009    

DOI:10.1002/qj.49710343702

出版商:John Wiley&Sons, Ltd    

年代:1977

数据来源: WILEY

摘要:

AbstractThe dynamics of stationary ultra‐long waves in middle latitudes are studied using a set of approximate linear equations for small perturbations about a zonal flow. the equations are derived by an expansion procedure based on the assumptions of planetary scales of variation and small mean superrotation. the zero‐order equations reduce to a vertical structure equation containing no meridional derivatives. Analytical solutions are obtained for forced stationary waves in a model atmosphere consisting of three continuous layers, representing the troposphere, the lower stratosphere and the upper stratosphere.The waves are generated by thermal and topographic forcing in the model troposphere and propagate wave energy at zonal wavenumbers 1 and 2 into the upper stratosphere, where they are damped by Newtonian cooling. General relationships are found involving the zero‐order fluxes of sensible heat and wave energy. These greatly clarify the dynamics of the waves, particularly in the lower stratospheric layer which links the regions of generation and dissipation.Taking realistic values of the parameters for northern middle latitudes in winter, the solutions for wavenumbers 1 and 2 show good agreement with observation. an examination of the sensitivity of the solution to changes in various model parameters shows that the meridional flux of heat by ultra‐long waves in the troposphere is extremely sensitive to changes in the stratospheric wind profile and static stability. Since these waves are known observationally to account for a large fraction of the total poleward heat transport in northern middle latitudes in winter, a dynamical mechanism is suggested whereby variations in solar ultraviolet radiation or changes in the ozone content of the upper atmosphere could lead to climate

ISSN:0035-9009    

DOI:10.1002/qj.49710343703

出版商:John Wiley&Sons, Ltd    

年代:1977

数据来源: WILEY

摘要:

AbstractIt is suggested that the ‘coastal lows’ observed off southern Africa are similar in structure to ‘coastally trapped waves’ in the ocean. Inversion conditions typical of the area prevent escape of energy upwards and Coriolis effects keep energy trapped against the high escarpment which borders the continent. Coastal disturbances are produced because low‐level flow in the synoptic‐scale systems cannot cross the barrier provided by the escarpment. Nonlinear effects cause lows to be more prominent than highs. A simple nonlinear forced Kelvin wave model is found to reproduce many of the featur

ISSN:0035-9009    

DOI:10.1002/qj.49710343704

出版商:John Wiley&Sons, Ltd    

年代:1977

数据来源: WILEY

摘要:

AbstractThe rainfall distribution over Africa analysed for the period 1951–1975 shows a downward trend at 15°N not present at other latitudes. A widespread reduction in rainfall is observed during the drought years 1972 and 1973. Upper air analyses have been used to construct mean monthly vertical motion patterns and water vapour budgets but the results are not sufficiently precise to examine year‐to‐year differences. Changes in the August flow patterns show that low rainfall in the Sahel is associated with the virtual disappearance of the 850‐mb trough near 8°N and weakening of the easterly jet above it. an empirical orthogonal function analysis of the 850‐, 500‐ and 200‐mb temperatures from 150 stations shows that these changes are global in extent. the downward trend in the Sahel rainfall has evidently been paralleled by a weakening of the northern hemisphere circulation.These results suggest that local modification of surface conditions is not the principal cause of the decline in Sahel rainfall over the l

ISSN:0035-9009    

DOI:10.1002/qj.49710343705

出版商:John Wiley&Sons, Ltd    

年代:1977

数据来源: WILEY

摘要:

AbstractDiffuse radiation from cloudless skies varies with solar height and atmospheric turbidity but its angular distribution remains generally characteristic of single scattering. It was found that when sky radiance measurements,N, were normalized with respect to horizontal diffuse irradiance,Sd, the angular distributions ofN/Sdwere remarkably independent of atmospheric turbidity. This enabled standard distributions of normalized sky radiance, based on a long series of measurements on clear skies, to be drawn up for different solar zenith angles. Due to the difference between radiometric and photometric units these distributions are not comparable with previously published luminance distributions. Departures due to variation in atmospheric turbidity, of the radiance distributions from the proposed standards, were analysed and found to be small for the range of turbidities (ταfrom 0·1 to 0·5) normally encountered in Brit

ISSN:0035-9009    

DOI:10.1002/qj.49710343706

出版商:John Wiley&Sons, Ltd    

年代:1977

数据来源: WILEY

摘要:

AbstractAn empirical orthogonal function analysis has been performed on monthly mean sea surface temperature observations in the Atlantic Ocean between 70°N and 30°S for the years 1949–69. This method derives the most important ‘modes’ of variation, defined as those functions which explain the largest total variance in the data field. Such modes are calculated for two sets of temperature deviations in which the annual cycle is both included and excluded. For the seasonal case, only four functions explain more than 90% of the total variance, whereas in the nonseasonal case, ten functions explain less than 50% of the total. the most important seasonal mode indicates lower minimum temperatures in winter in the later years of study. the dominant nonseasonal function shows a cooling trend starting in early 1951. Nonseasonal functions of the tropical region alone suggest large variations, having a characteristic period of a few

ISSN:0035-9009    

DOI:10.1002/qj.49710343707

出版商:John Wiley&Sons, Ltd    

年代:1977

数据来源: WILEY

摘要:

AbstractIt has been shown by Starr that net absolute angular momentum cannot be advected by axisymmetric motions alone across a closed surface of constant absolute angular momentum. Hence it has been claimed that certain systems possessing differential rotation about a central axismustbe non‐axisymmetric. The theorem is analysed in detail and such a conclusion is shown to be invali

ISSN:0035-9009    

DOI:10.1002/qj.49710343708

出版商:John Wiley&Sons, Ltd    

年代:1977

数据来源: WILEY

摘要:

AbstractThe temperature field of the northern hemisphere mesosphere during the mid‐winter of 1975/1976 is described by the use of radiance observations made by the Pressure Modulator Radiometer (PMR) on the Nimbus 6 satellite.A month's average of the radiance field indicates that planetary waves of wavenumbers one and two are of significant amplitude in the winter hemisphere up to the highest level observed by the PMR (∼85 km). These waves show a westward tilt of wave axis with height, and the maximum amplitude is observed in the middle mesosphere to the north of the polar‐night westerly jet.Time series analysis of the zonal mean radiance during the period of a minor stratospheric warming in early 1976 indicates that the stratospheric and lower mesospheric temperature increases with time in the higher latitudes to the north of 30°N, associated with the amplification of wavenumber one, while the upper mesospheric temperature decreases with time at all latitudes.A rough estimate is also given of the time change of the mid‐latitude mean zonal wind during the period of the

ISSN:0035-9009    

DOI:10.1002/qj.49710343709

出版商:John Wiley&Sons, Ltd    

年代:1977

数据来源: WILEY

摘要:

AbstractThis paper is a sequel to the recent numerical study of thermally driven vortices by the authors. the model developed earlier is used as a basis for comparing a variety of radial boundary conditions and to assess their suitability for models of ‘tall‐thin’ atmospheric vortices, in particular dust devils. We have also determined those aspects of vortex dynamics which are least sensitive to the choice of radial boundary condition, and those which have a stronger dependence. the results confirm the suitability of the boundary conditions used in the earlier study, for the purposes for which that study was intended, but more realistic boundary conditions are obtained for a dust devil model.The discussion should provide useful guidance in the design of future observational studies of dust devils and their immediate environment, and in the formulation of models for other types of sub‐synoptic‐scale vortices such as tornadoes and wa

ISSN:0035-9009    

DOI:10.1002/qj.49710343710

出版商:John Wiley&Sons, Ltd    

年代:1977

数据来源: WILEY

摘要:

AbstractCalculations of the transmission of infrared radiation through the atmosphere must take into account inhomogeneities in the path due to variations of temperature, pressure and absorbing matter. For ozone absorption the method due to Curtis and Godson does not seem to yield results that are sufficiently precise. In this paper an improved approximation is developed that yields an analytic expression for the transmission function and this is compared with the exact results obtained from the Goody random model.

ISSN:0035-9009    

DOI:10.1002/qj.49710343711

出版商:John Wiley&Sons, Ltd    

年代:1977

数据来源: WILEY