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1991 Vol. 8, No. 1

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Criteria for the Nonlinear Stability of Three-Dimensional Quasi-Geostrophic Motions
Mu Mu, Zeng Qingcun
1991, 8(1): 1-10. doi: 10.1007/BF02657360
Some more proper criteria for the nonlinear stability of three-dimensional quasi-geostrophic motions are given by combining variational principle with a prior estimates method. The criteria are suitable for perturbations of initial condition as well as parameters in the model. The basic flow can be steady or unsteady. Particularly the difficulty due to the nonlinear boundary condition is completely overcome by the use of our method.
On Quasi-Biennial Oscillation in Air-Sea System
Shao Yongning, Chen Longxun
1991, 8(1): 11-22. doi: 10.1007/BF02657361
From the COADS (Comprehensive Ocean-Atmosphere Data Set) I and the COADS II, we got a monthly data set of sea surface temperature (SST). zonal and meridional wind components at sea level (U,V) and sea level pressure (SLP) with 4o × 4o grid system covering the period from Jan. 1950 to Dec. 1987 to study the evolutional features of the quasi-biennial oscillation (QBO) in the air-sea system. The analytic method of complex empirical orthogonal function (CEOF) is used to obtain the composite temporal sequences of amplitude (six phases for half a period) for the first and the second main components of SST, U, V and SLP. It is shown from the results that the main character-istics for different phases of the sea surface temperature anomaly’s (SSTA) QBO are warm water / cold water in the equator of the eastern Pacific (EEP). There are two warm or cold water centers of the SSTA in the EEP, which are lo-cated in the equator of the central Pacific (ECP) and the east part of the EEP. The features of the source propagation and the influence of these two centers on atmospheric circulation are discussed and it can be seen that in the forma-tion of these two centers, there are different features in oceanic and atmospheric circulations and air-sea coupled pro-cess.
The Effect of Topography on Quasi-Geostrophic Frontogenesis
Zhao Ming
1991, 8(1): 23-40. doi: 10.1007/BF02657362
This paper improves Bannon’s work on the quasi-geostrophic frontogenesis in a horizontal deformation field. By setting the lower boundary condition for the equation of potential temperature on the realistic topography instead of on z = 0, a general solution for the temperature field is derived after applying conformal mapping to the equation for the potential temperature, the vertical velocity and divergence field are also calculated. The general characteristics for the frontogenetic process still are frontolytic for warm front and frontogenetic for cold front in downstream of a mountain and the reverse is true upstream of a mountain, but more fine spatial structure of the temperature field and frontogenetic characteristics than Bannon’s are obtained near surface because of the treatment of lower boundary condition. II is concluded that the frontogenetic characteristics are related to the translating speed of the deformation field with respect to the topography.
Effect of Nonlinear Dynamic Process on Formation and Breakdown of Blocking
Zhang Pei, Ni Yunqi
1991, 8(1): 41-50. doi: 10.1007/BF02657363
With the L-P approximate method (variation of parameter method), a barotropic channel model in β-plane is used to study the effect of nonlinear interaction between two waves with different scales on the formation of blocking. The approximate analytical solution, which can describe the process of the blocking formation, maintenance and breakdown, has been obtained by using the method of approximate expansion. The importance of nonlinear interaction between two waves with different scales is stressed in the solution. The result suggests that the nonlinear interaction is the main dynamic process of the blocking formation. Some required conditions of blocking formation are also discussed.
Sensitivities of Numerical Model Forecasts of Extreme Cyclone Events
A.C.Yih, J. E. Walsh
1991, 8(1): 51-66. doi: 10.1007/BF02657364
A global forecast model is used to examine various sensitivities of numerical predictions of three extreme winter storms that occurred near the eastern continental margin of North America: the Ohio Valley blizzard of January 1978, the New England blizzard of February 1978, and the Mid-Atlantic cyclone of February 1979. While medium-resolution simulations capture much of the intensification, the forecasts of the precise timing and intensity levels suffer from various degrees of error. The coastal cyclones show a 5-10 hPa dependence on the western North Atlantic sea surface temperature, which is varied within a range (± 2.5℃) compatible with interannual fluctuations. The associated vertical velocities and precipitation rates show proportionately stronger dependences on the ocean temperature perturbations. The Ohio Valley blizzard, which intensified along a track 700-800 km from the coast, shows little sensitivity to ocean temperature. The effect of a shift of - 10?latitude in the position of the snow boundary is negligible in each case. The forecasts depend strongly on the model resolution, and the coarse-resolution forecasts are consistently inferior to the medium-resolution forecasts. Studies of the corresponding sensitivities of extreme cyclonic events over eastern Asia are encouraged in order to identify characteristics that are common to numerical forecasts for the two regions.
Some Analyses and Numerical Simulations of Meiyu in East Asia in 1983
Qian Zhengan, Rod. A. Bromley, Mike. A. Ayles
1991, 8(1): 67-78. doi: 10.1007/BF02657365
In this paper the differences between Meiyu and Baiu front in 1983 have firstly been analysed, the trajectories of air on and to the north side of Meiyu and Baiu fronts during the Meiyu period have then been traced, and the fore-casting and simulating of 4 sets of Meiyu onset of the year have finally been run utilizing the global model at UK Me-teorological Office. The results show: 1) Meiyu fronts are different from Baiu ones in temperature, humidity and stratification fields in lower atmosphere; and the possibly reasons for it are explained. 2) The Bay of Bengal is the main moisture source for Meiyu front, the South China Sea and the Pacific, for Baiu ones; and some existed argu-ments on it are also discussed. 3) The onset of Meiyu and its rainfall and rain belts are sensitive to the Tibetan Plateau, and the water vapour conditions over the Bay of Bengal and the South China Sea, but not sensitive to the SST over the equatorial area or to the East of Japan.
New Approach to Study the Evolution of Rossby Wave Packet
Ren Shuzhan
1991, 8(1): 79-86. doi: 10.1007/BF02657366
The average variational principle was employed in this paper to study the evolution of large-scale and slowly va-rying Rossby wave packet with basic flow both in barotropic and baroclinic atmospheres. The evolution of the struc-ture of Rossby wave packet with both time and space was studied. The results obtained in this paper are similar to the results of by WKBJ method. In addition, the dispersive process of the wave packet was analysed by taking Gaussian type wave packet as an initial disturbance. The valid time scale for application of wave packet theory in the atmos-phere was obtained.
The Effect of Spatial Structure Character of Heat Source on the Ray Path and the Evolution of Wave Energy of Meridional Wave Train
Xu Xiangde
1991, 8(1): 87-98. doi: 10.1007/BF02657367
This paper studies correlations between the spatial structure character of thermal forcing and deformation and the amplitude of rays of meridional wave train. It is shown that if thermal forcing appears a meridional linear varia-tion the rays of quasi-stationary planetary wave may propagate along oblique lines and if the meridional variability of heat source has second order term the rays show distinct deformation as a great circular route. Additionally, the inhomogeneous distribution may cause lower frequency oscillations in mid- and low- latitudes. The combination of zonal and meridional wave numbers and distributive character of heat source may form an inverse mechanism of variational trend of generized wave energy, reflecting in some degree the physical process of transition between meridional and zonal flow patterns.
Variations in the Atmospheric Electric Field at Tropical Station during 1930-1987
S. S. Kandalgaonkar, G. K. Manohar
1991, 8(1): 99-106. doi: 10.1007/BF02657368
The variations noticed in the atmospheric electric field recorded at Pune (18o32’N, 73o51’E, 559 m ASL), a tropi-cal inland station located in Deccan Plateau, India, during the period 1930-1987, have been examined in relation to the variations observed in the Angstrom turbidity coefficient (β) and selected meteorological parameters. The month-ly and annual mean values of the atmospheric electric field, Angstrom turbidity coefficient (β), rainfall, temperature and relative humidity for the years 1930-1938, 1957-1958, 1964-1965, 1973-1974 and 1987 were considered in the study.The results of the above study indicated gradual increases in the atmospheric electric field over the period of study (1930-1987) which is statistically significant at less than 5% level. The increases noticed during different periods varied from 30 to 109 %. The increase noticed during the period (1930-1938) and (1973-1974) was maximum (109%). The Angstrom turbidity coefficient also showed systematic increases during the period of study, which is consistent. The diurnal curve of the atmospheric electric field at the station by and large, showed a double oscillation, which is generally observed in the continental environments.
How much Numerical Products Affect Weather Forecasting
Xia Jianguo
1991, 8(1): 107-110. doi: 10.1007/BF02657369
The paper shows how much improvement can be achieved in weather forecasting by using NWP products. And for weather element forecasts, the types and number of NWP products highly impact on the quality of MOS forecasts and other utilities.
Some Physical Aspects of Summer Monsoon Clouds-Comparison of Cloud Model Results with Observations
A. Mary Selvam, R. Vijayakumar, A. S. R. Murty
1991, 8(1): 111-124. doi: 10.1007/BF02657370
The physical characteristics of the summer monsoon clouds were investigated. The results of a simple cloud mod-el were compared with the aircraft cloud physical observations collected during the summer monsoon seasons of 1973,1974,1976 and 1981 in the Deccan Plateau region.The model predicted profiles of cloud liquid water content (LWC) are in agreement with the observed profiles. There is reasonable agreement between the model predicted cloud vertical thickness and observed rainfall.The observed cloud-drop spectra were found to be narrow and the concentration of drops with diameter >20μm is either low or absent on many occasions. In such clouds the rain-formation cannot take place under natural atmos-pheric conditions due to the absence of collision-coalescence process. A comparison of the model predicted and ob-served rainfall suggested that the precipitation efficiency in cumulus clouds of small vertical thickness could be as low as 20 per cent.The clouds forming in the Deccan Plateau region during the summer monsoon are, by and large, cumulus and strato-cumulus type. The vertical thickness of the cumulus clouds is in the range of 1.0-2.0 km. The LWC is found to be more in the region between 1.6-1.9 km A. S. L., which corresponds to the level at almost 3 / 4 th of the total verti-cal thickness of the cloud and thereafter the LWC sharply decreased. Nearly 98 per cent of the tops of the low clouds in the region are below freezing level and the most frequent range of occurrence of these cloud-tops is in the range of 2.0-3.0 km A. S. L.. The dominant physical mechanism of rain-formation in these summer monsoon clouds it the col-lision-coalescence process.
Comments on Zeng’s Paper “Variational Principle of Instability of Atmospheric Motions”
Zeng Qingcun
1991, 8(1): 125-127. doi: 10.1007/BF02657371