Numerical experiments using a general circulation model (GCM) were performed for the purpose of investigating the possibility of the generation of a long period (30-40 days) oscillation as a collective motion of cumulus activity (so-called wave-CISK) along the equator of an ocean covered globe ("aqua planet"). In our model, the SST distribution was symmetric about the equator and uniform in the longitudinal direction. The results of a 90 day integration exhibited the spontaneous appearance of a collective motion of convective activity together with an east-west wavenumber one circulation (the "30 day oscillation"). The characteristics of this east-west circulation resemble those of the observed 30-60 day oscillation in the actual atmosphere. The 30 day oscillation is characterized by a superposition of two different scales: the scale of precipitation patterns ("super clusters") which is nearly equal to the equatorial radius of deformation, and the scale observed as the modulation of precipitation patterns and the east-west circulation with a wavenumber one. The whole structure moves eastward at a slow phase speed (15m/s). The results also exhibited the spontaneous formation of double ITCZs around the equator. Thus the production of the double ITCZ structure does not necessarily require a minimum SST at the equator. In the equatorial region between the north and south ITCZs, active convection forms super clusters, while in the regions poleward of the ITCZs, active convection forms tropical cyclones. Another experiment without the moist processes resulted in the abrupt disintegration of the 30 day oscillation into Kelvin and Rossby waves. This indicates that strong mode coupling between the equatorial free waves is required in order to maintain the 30 day oscillation. The slow phase speed, the strong mode coupling and the double structure in scale indicate that the collective motionof convective activity along the equator cannot be explained simply as a Kelvin wave mode of the linear equatorial wave-CISK theory presented so far. The 30 day oscillation should be regarded as a new type solution of the equatorial wave-CISK problem.
Numerical simulations of two-dimensional tropical squall-line clusters (TSL) are performed using a nonhydrostatic, inelastic cloud model in the ambient winds and stratification, which are based on the sounding in GATE on Sept. 12, 1974. Water substance in the model is divided into 6 forms; water vapor, cloud water, rain, cloud ice, graupel and hail. The parameterization of cloud microphysics is mainly based on Lin et al. (1983). The grid sizes are stretched in both horizontal and vertical directions and the model domain is 241km and 14.45km in the horizontal and vertical direction, respectively. A cumulonimbus is initiated by an initial thermal under an imposed low-level large-scale convergence. Quasi-steady convection is attained after 2 hours of the model integration. The convective system is composed of convective and stratiform regions. In the convective region, a slant strong updraft is obtained and its angle from the horizontal is about 15 degrees. In the stratiform region, motions are nearly uniform in the horizontal direction and layer structures of strong horizontal winds are seen. There is weak updraft in the anvil cloud which is occupied by graupel and weak downdraft below the melting level where rain prevails. The sharp jumps of winds, temperature, water vapor mixing ratio and heat/moisture fluxes are obtained near the gust front on the surface. These features of the convective system simulated in the model are similar to those of the observed TSLs. However, fast propagation of the convective system is not well simulated. This may be due to that the simulated pressure rise on the surface is not sufficiently large to proceed the convective system fast.
Climate of East Asia marked significant fluctuations between winter 1983/84 and summer 1984: It was severely cold in winter and very warm and dry in summer. A diagnostic study on the features of atmospheric circulation and their association with the convective activity and SST (sea surface temperature) in the western tropical Pacific region is made in relation to these extremes of East Asian weather, with special emphasis on the midlatitudes-tropics interaction. The features are compared with those of climate 1982/83. In winter 1983/84 a deep trough persisted northeast of Japan, sustaining a significant meridional flow over East Asia. Zonal flow index was significantly low only second to winter 1967/68 in the last 40 years, and winter mean temperature in Central Japan was lowest since winter 1944/45, indicating a contrast to mild winter 1982/83. Meanwhile convective activities were significant around Indonesian maritime continent, and very weak around the date line. Thus winter 1983/84 showed a marked difference in distribution of convective cloud from the previous winter 1982/83. It is suggested that East Asian winter weather is closely associated with the interannual fluctuation of the convective activity in the western tropical Pacific Ocean. During summer 1984 the atmospheric circulation feature was quite different between early and later summer. During June and July a ridge northeast of Japan and a trough over China were almost stationary, and south-westerlies were prevalent over the eastern East Asia. In midsummer subtropical High (Sub High) developed north of the normal position and stayed for about a month near Japan, leading to hot and dry weather from south China to Japan. The formation of this Sub High followed the active convection around the Philippines from late July through August. In striking contrast to the continuation of lower SST during El Nino event of 1982/83 in the western tropical Pacific, it was above normal during winter 1983/84 through summer 1984. The convective activity in the region was also significant in phase with higher SST, suggesting that higher SST may have contributed to the active convection to a certain extent.
This paper presents the results of several global experiments that were carried out to study the formation of a surge vortex. This vortex formed over the South China Sea during the Winter Monsoon season. The numerical experiments explore the sensitivity of the prediction to horizontal and vertical resolution, to the use of high resolution cloud winds, and to two different representations of orography. The results of these studies show that the simulation of the surge vortex was only possible at a minimal horizontal resolution of 42 waves (triangular), 11 vertical levels and the use of envelope orography. The results did not appear to be sensitive to the inclusion of high resolution cloud winds. A major synoptic aspect of this study deals with the interaction of westward propagating easterly waves and the quasi-stationary cold surge over the South China Sea. That aspect of the cyclogenesis was shown observationally as well as via the numerical weather prediction experiment. That interaction appears to be quite sensitve to the inclusion of steep orography.
In order to investigate whether extratropical cyclones may have almost negligibly short warm fronts in some area of the northern hemisphere, we examined statistically the ratio of lengths of warm fronts to those of cold fronts accompanied by extratropical cyclones. The FGGE IIIb data was used and the period analyzed was between 1 December 1978 and 28 February 1979 and a parameter GGθ (the directional derivative of the gradient θ along its gradient) proposed by Renard and Clarke (1965) was used to draw fronts. It was shown that warm fronts are generally shorter than cold fronts around Japan and the east coast of the North America and that warm fronts are longer than cold fronts over Europe and the west coast of the North America. This geographical difference of the lengths of fronts are closely related with developing rates of cyclones. Around Japan and the east coast of the North America a number of cyclones develop with large pressure decreases. Most of them have shorter warm fronts. On the other hand, most cyclones over the Europe and the west coast of the North America develop weakly and often decay there and tend to have longex warm fronts and shorter cold fronts. It suggests that cyclones have different structures in different areas of the globe and that they should be classified into various types other than the Bjerknes model.
Global simultaneity of the abrupt seasonal changes in precipitation during May and June of 1979 has been studied by using the FGGE level II-c precipitation and snow data set. Two stages of abrupt change in 10-day precipitation are identified around the first 10-day periods of May and June. The first stage is around the first 10-day period of May. The heavy rainfall area (above 50mm/ 10-day) shifts northwards from Indonesia to the Philippines and around the Bay of Bengal (the east coast of India and the west coast of Burma) in the second 10-day period of May. Simultaneously, non-precipitation area extends northwards in the mid-latitudes of mid- and western Eurasia, and western North America. Precipitation increases over equatorial East Africa. One 10-day period earlier (in the first 10-day period of May), it increases over equatorial West Africa, coincident with the northward shift of the non-precipitation area along the west coast of South America. The second stage is around the first 10-day period of June. Precipitation increases between 20°N and 60°N along the east coast of Eurasia in the first 10-day period of June. Simultaneously, the main rainbelt shifts northwards from eastern South America to around the Caribbean Sea, and from the equator to 15°N over West Africa. One 10-day period later (in the second 10-day period of June), monsoon rain begins along the west coast of India. At the same time, precipitation decreases over equatorial East Africa, while it increases along the east coasts of Madagascar and South Africa. Based on the periods of the two stages of abrupt change, we can divide the whole hemisphere into four sectors (Sector I (north-east), Sector II (north-west), Sector III (south-west) and Sector IV (south-east)) in terms of two boundary lines which roughly coincide with the meridians of 70°E and 90°E and the parallels of 20°N and 35°N, and cross at the eastern Tibetan Plateau. In Sector I (over eastern Eurasia), abrupt change occurs in the first 10-day period of June. In Sector II (in the mid-latitudes of mid- and western Eurasia, and western North America), abrupt changes occur in the second 10-day period of May. In Sector IV (over Indonesia and the Philippines, and around the Bay of Bengal), abrupt changesoccur in the second 10-day period of May. In Sector III, abrupt changes occur around the first 10-day periods of May and June. In detail, abrupt changes occur in the western Sector III (around the Atlantic Ocean) in the first 10-day periods of May and June, and in the eastern Sector III (around the western Indian Ocean) in the second 10day periods of May and June.
This paper investigates the African monsoon circulations which determine main rainfall areas in the 1979 northern summer, and focuses on the different causes of rain between East and West Africa, by using the FGGE level II-c precipitation and snow data set and the FGGE level III-b data set. When the rainbelt reaches its most northerly position over tropical Africa, the main rainfall areas exist within the surface southwesterlies. In the zone of 7°-17°N, the non-rainfall area appears between Lake Chad and Mt. Marra even in the wet air mass (i.e., equatorial westerlies). The main rainbelt is divided by this non-rainfall area into the rainfall areas in West Africa and west of the Ethio- pian Plateau. The main rainbelt in West Africa roughly corresponds to the mid-tropospheric easterly jet. The two maximum axes of the 850mb υ power spectra in the 3.1-4.4 day period are found along approx. 8°N and 20°N. West of 5°E, the main rainbelt exists between them, while east of it, the southern axis is obscure and the main rainbelt is concentrated along approx. 13°N, about 5 degrees south of the northern axis. This axis disappears between Lake Chad and Mt. Marra where the non-rainfall area appears. The main rainfall area west of the Ethiopian Plateau corresponds to the thickest zone of equatorial westerlies. The 850mb υ power spectra in the 3.1-4.4 day period is unclear in East Africa. Orographic disturbances in the equatorial westerlies are therefore responsible for the rain. In the zone of 2°-7°N, the main rainfall area south of the mountains in Cameroun corresponds to the southern one of the two thicker equatorial westerlies in their latitudinal profile.
Nocturnal cooling processes in simple 2-dimensional small basin were investigated by a numerical method, in which Cartesian co-ordinate system was used and hydro-static apprximation was not used. When radiative heat transfer by the long wave was not considered, no slope wind occurred on the surrounding slope, whose angle was 5.7°. In the calculation cooling of the air by radiative heat transfer near the surface was a trigger which gave rise to nocturnal down slope flow. There were 3 stages in the slope flow during 6 hours after sun set. Strong down slope flow appeared at only early stage. The temperature profiles were changed especially between 20m level above the bottom of the basin and the ridge of mountain, that is, the air between these levels was more cooled when slope flow existed. The total heat loss in the atmosphere in excess of that by radiative cooling was roughly balanced with the sensible heat absorbed by the wall slope. Judging from the Lagrangean flows in this small basin, pollutants emitted continually at the center of the slope diffused in whole basin in 6 hours in spite of very weak turbulent diffusion. And this means small ascending flow and weak circulation over the center of the basin cooled the air in the whole basin.
Aircraft observations of warm rain in Hawaii were analysed concerning the effect of wind shear on the drop growth processes in different cloud types. Drizzle formation is slowed near the cloud top when a strong wind shear exists in an isolated cumulus clouds or a band clouds, while drizzle formation is accelerated with presence of a strong wind shear in cloud clusters. A moderate updraft is needed for raindrops to grow near the cloud top. As drops fall through a cloud, rainwater is accumulated best in cloud clusters and least in isolated cumulus clouds. Rain duration becomes shorter with a strong wind shear in isolated cumulus clouds, but rain lasts for a long period of time from band clouds where the wind is parabolic in form and a strong wind blows at the middle of the trade wind layer.
A statistical forecasting method for the surface wind system was developed for the purpose of predicting the behavior and evaluating the effects of pollutants released to the atmosphere near the surface. The surface wind system represented by data from many stations can be considered to be subject to larger scale atmospheric motion, even if the observed wind data show some discrepancies with each other. The wind system is forecasted by the following method. First, the principal components of surface wind system fluctuation are extracted using principal component analysis. Next, the future principal components are forecasted from time series of these principal components using autoregressive equations by applying the maximum entropy method. The future values of the wind system are then obtained. This method was applied to the Kanto Plain using AMeDAS wind data. Forecast time of 1, 3, 6 and 12 hours were selected. A comparison of this method with persistence forecasting method shows that this method gives better results for each forecasting period.
Exact solutions of the system of equations for steady two-dimensional flow of an inviscid fluid with Coriolis term have been obtained by the symmetry method as developed by Steinberg. Solution for different forms of the Coriolis parameter have been indicated. Present study shows the usefulness of the symmetry method in generating the similarity solutions of a physical problem governed by the system of nonlinear partial differential equations. Though the present approach is quite involved, it is simple in concept and straightforward in application and helps in generating new solutions of the given problem.