A theoretical discussion is given of the structure of observed 30-50 day atmospheric oscillation in the tropics by the use of a simple linear diagnostic model based on primitive, shallow water, β-plane equations with the long wave approximation. It is assumed that this oscillation is forced by a localized heat source pulsating with a 40-day period. The response of the model shows that the zonal wind oscillation consists of a standing wave with a phase jump in the region of heating and traveling waves with slow zonal phase variations elsewhere. The pressure field is, however, not associated with a phase jump in agreement with observations.
We compare solutions of the equations for three-dimensional inviscid homogeneous flow to those of the shallow water equations and the nonlinear and linear barotropic vorticity equation for flow over and around obstacles which resemble the Tibetan Plateau. The homogeneous flow splits in front of the obstacle in two branches even at levels well above the plateau height, the northern branch being slightly more intense. The flow above the plateau is relatively weak as compared to the flow around it. It is found that both the nonlinear vorticity equation and the shallow water equations are qualitatively correct in predicting this splitting but tend to produce too strong flow above the plateau. However, the results obtained from the linear vorticity equation are not even qualitatively correct. The reasons for this failure are discussed.
A series of numerical forecasts were carried out to investigate a heavy rainfall event over the monsoon region of Eastern Asia which was caused by a vortex formed over the Tibet Plateau during Summer. The ECMWF global grid point model was capable of predicting the vortex four days in advance. The importance of the moist processes in the development of the vortex was investigated by comparing the results from a moist and a dry (without latent heat) versions of the model. It was found that the low level jet associated with the vortex and the upper level easterlies are more active when the latent heat is included in the model. The ECMWF limited area model with resolution 0.5° of latitude and longitude (-37km at 45°N) was used in order to obtain a more accurate and detailed forecast. Some intermediate scale disturbances were predicted over the Tibet Plateau and verified with satellite pictures and observations. This result is significant for the problem of numerical weather prediction over the Asian Continent.
Utilizing FGGE Level IIIb data, the moisture balance over an extensive region (30°S- 30°N, 60°E-120°W) during the 1978-79 Southern Hemisphere summer monsoon is analyzed. In this study, the onset and break periods are defined by the changes in the moisture budget over the northeast Australia-western South Pacific region (7.5°-22.5°S, 135°-165°E). The analysis shows that the major moisture source regions for the monsoon rainfall are the Indonesian Seas (0°-12°S, 95°-130°E) and the subtropical South Pacific off the east coast of Australia. The contribution of the Northern Hemisphere moisture flux is less than these Southern Hemisphere sources. The monsoon activity shifts eastward and southward from pre-to post-onset. During the break period, the monsoon activity shifts still further eastward and poleward. The moisture convergence pattern over Australia illustrates some of the similarities between the break conditions for the Southern Hemisphere summer monsoon and the Indian monsoon. The strong moisture convergence center over northeast Australia (about 15°S, 140°E) during the active period splits into two centers during the break period. The stronger center is situated around 20°S, 140°E and is about 5° poleward of the previous active period position. The other center is more equatorward at around 7.5°S, 140°E. During the break period, a strong interhemispheric moisture exchange exists over the central Pacific between 150°E and the dateline. During the same period, a strong northerly moisture flux around 150°W transports a large amount of moisture into the mid-latitudes of the South Pacific.
TIROS-N moisture data and Level III-b humidity fields from European Center for Medium Range Weather Forecasting (ECMWF) have been analyzed during the 1979 summer (May 1-September 15) to obtain fields of Precipitable water in three layers (surface-700mb, 700-500 mb and 500-300mb) at 00 and 12GMT over Africa and the eastern and central Atlantic Ocean (50°W-40°E; 30°N-30°S). The accuracy of the final product has been estimated from a comparison with dropsonde and upper air sounding humidity data. The relative accuracy is about 10%. The seasonal northward migration of the zone of maximum PW is analyzed from the study of mean fields. May and June are characterizedd by a rapid shift of the zone of maximum PW. A large decrease of the atmospheric water vapor content takes place during early July. In September the zone of maximum PW shifts southward in association with the southward retreat of the monsoon trough. Time series of water vapor content over different areas of the subsaharan region shows the existence of fluctuations related to rainfall variations. The main periodicities of PW lie in the range 3-4 days and 6-9 days. The first type of fluctuations is related to the African easterly waves whereas the second type corresponds to another mode of fluctuation of the tropical atmosphere over Africa and the Atlantic ocean.
The growth rate and maximum value of electric field inside the thundercloud of finite dimensions are calculated incorporating the effect of positive space charge produced by corona at the ground surface. Our theoretical findings reveal that the transport of corona space charge increases significantly electric dipole moment already existing in thunderclouds of moderate heights and is partly responsible for their electrification.
Meteorological network planning involves the study of multivariate phenomenon in both space and time domains. In order to intensify the existing meteorological network it is essential to consider all possible multivariate combinations of the existing stations. In a study area with a large number of stations, it is computationally infeasible to consider all possible multivariate combinations. To resolve such complexities, Shannon's information theory is used such that the information transmitted by a set of stations in one subregion about the other subregion is minimized. The total information at a point is decomposed into internal information common throughout the subregion and the information contributed by other stations at that point. To gain a better understanding of the significance of the partition results, and to improve the spatial resolution of the methodology, the partitioning of three-station subgroup is investigated.
We investigate the stability of a baroclinic flow in which a centrifugal force acting on the zonal flow cannot be neglected compared to the Coriolis force. It is found that in the governing equations of this problem, the coefficient which corresponds to the Coriolis parameter in the usual baroclinic instability theory becomes a function of height, and the wave number in the longitudinal direction (k) becomes an independent external parameter. When the value of the Rossby number (R0=k/_??_ ΔH, here Δ is a vertical shear of the basic zonal flow and H is a depth of the layer) is small, the dynamic balance of the basic flow is always in the vicinity of pure geostrophic balance, so that the departure of eigenvalues obtained from pure geostrophic case remains small. For R0 larger than about 3.0, with an increase of ΔH, the dynamic balance of the basic flow is continuously changed from geostrophic balance to cyclostrophic one. It is found that in the balance close to the latter, there appear unstable waves which have fairly larger growth rate than unstable waves appearing in the usual baroclinic instability.