摘要:

AbstractIn agreement with previous published results, based on a 2‐layer model measure of baroclinicity, the extratropical northern hemisphere zonal mean atmosphere is found to be close to marginal stability in both winter and summer of the FGGE year. In winter, however, considerable zonal variations are present. Results of some numerical model runs, with both zonally symmetric and varying thermal forcing, are presented, and it is found that for sufficiently short forcing time scales a state close to marginal stability, and a storm‐tracklike region, can be produced. Large steady eddies do not seem to result so readily from the thermal forcing. Time‐lagged correlations provide evidence of forcing of the high frequency (periods less than 10 days) eddy heat flux by the baroclinicity, and it is suggested that the low frequency eddy heat flux may be a result of a nonlinear cascade from the baroclinic s

ISSN:0035-9009    

DOI:10.1002/qj.49711649202

出版商:John Wiley&Sons, Ltd    

年代:1990

数据来源: WILEY

摘要:

AbstractAn analytic semi‐geostrophic model of frontogcnesis is described which exhibits the growth of a frontal discontinuity surface. the initial distribution of temperature on a plane horizontal boundary consists of a warm two‐dimensional strip embedded in an isothermal background; the potential vorticity in the atmosphere above is uniform and positive. A barotropic deformation field with axis of dilation along the warm strip forces a discontinuity in the velocity field as the warm air is occluded and becomes detached from the lower boundary.The solution demonstrates some important general properties of the inviscid semi‐geostrophic equations when discontinuities are present. In particular, the vertical velocity on the lower boundary becomes non‐zero at the front. the maximum value of the vertical velocity occurs here when the front first forms, but then lifts off the boundary. the front itself constitutes a line of infinite potential vorticity, so that the global integral of mass‐weighted potential vorticity is conserved only if the boundary of the domain is redefined to exclude the front. the implications of these properties for numerical modelling are discussed.For typical atmospheric rates of deformation forcing, the accelerations of air parcels extremely close to the front are large enough to violate the conditions of validity of semi‐geostrophic theory. the conditions of hydrostatic balance are, for all practical purposes, nev

ISSN:0035-9009    

DOI:10.1002/qj.49711649203

出版商:John Wiley&Sons, Ltd    

年代:1990

数据来源: WILEY

摘要:

AbstractNumerical simulations of stratospheric major warmings with a multi‐level, primitive equation model of the stratosphere and mesosphere reveal several examples of processes giving rise to small‐scale structure in the simulated flow. For a simulation of the warming of 1984/85, the following examples are cited: (1) the maintenance of the Aleutian high by injections of air with low values of potential vorticity; (2) the erosion of the main cyclonic vortex as it is stripped of air with high values of potential vorticity; (3) the merger of a pair of anticyclones at the height of the warming; (4) differential advection of potential vorticity in a vertically sheared flow; and (5) the generation in the stratosphere of narrow baroclinic zones with strong temperature gradients in the horizontal and in the vertical. For a simulation of the warming of 1979, the example given is of the in situ generation in the stratosphere of inertio‐gravity waves near a developing baroclinic zone.The dynamics involved in the formation of narrow baroclinic zones in the simulated stratosphere is studied in some detail. It is shown that the ageostrophic flow (mainly the vertical motion) provides a positive feedback on the frontogenetic effect of the geostrophic (large‐scale) flow. Similarities and differences with frontogenesis at the ground and in the upper troposphere are enu

ISSN:0035-9009    

DOI:10.1002/qj.49711649204

出版商:John Wiley&Sons, Ltd    

年代:1990

数据来源: WILEY

摘要:

AbstractA nonlinear systems analysis is applied to cyclone tracks over the North Pacific area (based on surface pressure analyses and twelve‐hourly sampling time) and over the Australian region (based on 500mb heights and daily sampling) for the respective winter seasons. First estimates are obtained of the degree of their chaotic or irregular behaviour in phase space and of the self‐affinity of the planar tracks. This information is deduced from distance distributions (correlation integrals) between independent pairs of cyclone trajectories and from the structure function of individual cyclone paths.The correlation integrals reveal average structures of the substitute phase space (such as the dimension of the attractor, a measure of predictability) after stratifying the North Pacific tracks into a western domain of growing and an eastern domain of large amplitude cyclones. A further subdivision into large‐scale flow anomalies (the positive and negative Pacific/North America pattern in the eastern North Pacific) provides flowdependent estimates which indicate two subsystems differing in dimension and predictability. the time scaling analysis of the planar cyclone paths characterizes systems of no scale preference: the associated variance spectra decrease by a 2 to 3 power law with increasing freq

ISSN:0035-9009    

DOI:10.1002/qj.49711649205

出版商:John Wiley&Sons, Ltd    

年代:1990

数据来源: WILEY

摘要:

AbstractThe motion of an initially symmetric vortex on a beta plane and the motion of initially asymmetric vortices on a beta planes are studied using a nondivergent. barotropic model. It is assumed that there is no basic flow, the effects of a basic flow will be addressed in part II. Parameter values such as vortex size and strength and computational domain size are chosen for their relevance to tropical cyclones. the simultaneous evolution of the asymmetric vorticity and streamfunction fields is investigated in terms of a partitioning of the flow introduced by Kasahara and Platzman in which all asymmetries are regarded as part of the vortex environment. It is shown that in this partitioning the asymmetric streamflow provides a 'steering current' for the vortex to a very close approximation.For early times, typically 24 h for the parameters chosen, the development of the asymmetries in the vortices studied can be largely understood in terms of a simple analytical theory in which the vortex centre is fixed and the initial absolute vorticity distribution is rearranged by advection by the (initial) symmetric circulation.The shearing effect of this mechanism alone would appear to preclude the establishment of a true steady state in the flows considered. the numerical calculation for the initially symmetric vortex on a beta plane is used to assess averaging procedures for computing the environmental wind field of a tropical cyclone from observed wind data. the results show that the averaging regions that have been commonly employed in observational studies are too large to properly characterize the environmental flow at the cyclone centre and they suggest caution in making inferences from these studies in relation to physical mechanisms such as beta drift.The calculations for initially asymmetric vortices on anfplane show that the effects of the asymmetry on motion depend strongly on the scale of the asymmetry compared with that of the vortex. When the scale of the asymmetry is relatively small, the asymmetry is rapidly degraded by tangential shear and its effect on motion is not sustained. In contrast, larger asymmetries have a more persistent effect on motion. When the motion takes place on a beta plane, asymmetries induced by the advection of planetary vorticity ultimately dominate those present initially and, irrespective of the scale of the initial asymmetry, the vortex track finally turns to the north‐west as in the case of the initially symmetric vortex. Finally, as a case of extreme asymmetry we study the mutual interaction of a strong vortex and a weak one. the behaviour is similar to that just described, the weak vortex merging with the stronger one. the analytical theory also proves useful in understanding the evolution of initial vortex asymmetries.The interpretations of the calculations add insight into the dynamics of tropical cyclone motion, complementing the findings of recent studies by Chan and Williams and Fiorino and Elsberr

ISSN:0035-9009    

DOI:10.1002/qj.49711649206

出版商:John Wiley&Sons, Ltd    

年代:1990

数据来源: WILEY

摘要:

AbstractHydrostatically dominated forced flow over the asymmetric topography of the Darling Scarp, Western Australia, is modelled using a hydrostatic numerical mesoscale model and compared with observations. Simulations reproduce the essential features of wave overturning and the development of a shooting hydraulic flow. This response is dependent on the wind profile but not on the existence of a critical level. the high backsheared environment leads to a high ratio of the Brunt‐Väisälä frequency to wind speed throughout the profile, resulting in short wavelengths, high nonlinearity and overturning. With the ratio of the halfwidth of the topography to the boundary‐layer depth being small, the flow fields are shown to be sensitive to the boundary

ISSN:0035-9009    

DOI:10.1002/qj.49711649207

出版商:John Wiley&Sons, Ltd    

年代:1990

数据来源: WILEY

摘要:

AbstractProfiles of wind speed, temperature and turbulent fluxes of heat and momentum in the lowest 32 m have been measured at a station on an antarctic ice shelf. During the antarctic winter the surface layer often shows strong static stability, with temperature gradients as large as 1 Km−1in the lowest few metres. the surface inversion is destroyed during periods of high wind speed but the wind profile shows significant deviation from the expected logarithmic form under such conditions. Measurements of stress at 5 m indicate that the roughness length of the snow surface is about lO−4m.At 5 m height, the variations of the dimensionless wind shear, ϕm, and potential temperature gradient, ϕT, agree with previously determined forms of the Monin‐Obukhov similarity functions. Above 5 m, the behaviour of ϕmand ϕTis only qualitatively similar and surface‐layer similarity theory does not provide a good description of the profiles.Turbulence length scales have been deduced from vertical velocity power spectra. Under near‐neutral conditions, the ratio of turbulence length scale to measurement height is observed to decrease with increasing height of measurement. This observation is consistent with the variation of turbulence length scale with height implied by the measurements of ϕm. It is suggested that the stability of the overlying atmosphere restricts the depth of the turbulent boundary layer and hence the length scales of turbulence within this layer. Increasing stability causes a decrease of turbulence length scales at all levels.The ratios of turbulence kinetic energy to stress and temperature variance to heat flux are examined. Measurements are somewhat scattered, but the distribution of values varies little with height or stability. the form of the distribution suggests that large‐scale motions, possibly internal gravity waves, may be playing an important role in boundary

ISSN:0035-9009    

DOI:10.1002/qj.49711649208

出版商:John Wiley&Sons, Ltd    

年代:1990

数据来源: WILEY

摘要:

AbstractThe potential of the TIROS Operational Vertical Sounder (TOVS) and the Advanced TOYS (ATOVS) for providing information on the vertical profiles of atmospheric temperature and humidity is investigated. the theoretical performances of these systems are examined primarily in terms of their ability to contribute information to a numerical weather prediction (NWP) system. the covariance of retrieval error expected from an optimal nonlinear inversion scheme, which retrieves simultaneously all the atmospheric and surface variables which significantly affect the measured radiances, is calculated theoretically and compared with the covariance of error assumed in the prior information which constrains the inversion. Performance is assessed for different combinations of radiometers, corresponding to present and future operational systems, and for different sets of assumed conditions: in the atmospheric profile, the cloud conditions, the surface characteristics and the assumed prior knowledge of these variables.The potential performance of ATOVS is shown to be a considerable improvement on the present TOVS in many respects. the value of retaining a combined infrared and microwave system is confirmed. It is demonstrated that, although the ability of these sounding systems to improve on an NWP model's analyses of temperature and humidity is rather limited, the potential for improving on the analysis of layer quantities is much greater.Some new ideas are presented for extending current retrieval methods to accommodate the additional problems presented by ATOVS data. Methods are proposed for treating the effects of microwave surface emissivity and cloud liquid water in a simultaneous inversion.

ISSN:0035-9009    

DOI:10.1002/qj.49711649209

出版商:John Wiley&Sons, Ltd    

年代:1990

数据来源: WILEY

摘要:

AbstractA cloud water variable has been included in the Meteorological Office atmospheric general circulation model. the cloud amount and water content are generated by a scheme which assumes a distribution of thermodynamic and water content variables about their grid‐box‐mean values. A parametrization of the rate of depletion of cloud water by precipitation is included. the model's boundary layer mixing is reformulated in terms of variables conserved during changes of state of water, allowing a more realistic calculation of atmospheric stability in cloudy regions.The scheme produces realistic cloud distributions and the predicted cloud water content verifies well against microwave radiometer data in all regions except the mid‐latitudes of the summer hemisphere. the parametrization of cloud optical properties in terms of the cloud water content will be an important future development of the

ISSN:0035-9009    

DOI:10.1002/qj.49711649210

出版商:John Wiley&Sons, Ltd    

年代:1990

数据来源: WILEY

摘要:

AbstractAn empirical investigation of the effect of the stability of the atmosphere on near‐surface maritime aerosol concentrations is presented. the data base consists of an extensive series of aerosol experiments conducted on the western coast of the Outer Hebrides.In the absence of any well‐defined capping inversion over the Atlantic, stability is characterized in terms of convective and mechanical mixing heights, derived from radiosonde ascent data and from friction velocity, respectively. After wind‐speed‐production effects have been removed from the data, a simple relationship of the form\documentclass{article}\pagestyle{empty}\begin{document}$ {\rm concentration}\, = \,A\, + \,B/({\rm mixing}\,{\rm height})\,\,\,(A,B\,{\rm constants}) $\end{document}was found to adequately describe most data subsets. A discussion of the errors involved in such a model

ISSN:0035-9009    

DOI:10.1002/qj.49711649211

出版商:John Wiley&Sons, Ltd    

年代:1990

数据来源: WILEY