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1993 Vol. 10, No. 3

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Three-Dimensional Numerical Simulations of the Effects of a Cold Water Surface on the Evolution and Propagation of Thunderstorms
Kong Fanyou, Huang Meiyuan, Xu Huaying
1993, 10(3): 261-272. doi: 10.1007/BF02658132
The influences of large areas of semi-unbounded cold water surface on the evolution, propagation and precipitation production of thunderstorms are simulated by using a fully elastic three-dimensional numerical hailstorm model. Real sounding profiles for temperature, humidity and wind are employed. The model has successfully simulated the significant modification of the propagation path of thunderstorms near the cold water area. The path change can be either ‘along-bank’ or ‘toward-bank’, depending on the position of the storm system relative to convergence zone of the water-land circulation. The simulations also show that thunderstorms developing or propagating within the convergence zone of local circulation will be intensified and produce much heavier hail, whereas those over cold water surface or the air that has been cooled by the water will be strongly inhibited.The influence of the cold water surface on thunderstorm characters is largely dependent upon the direction and intensity of the low-level wind.
Energetics Constraint for Linear Disturbances Development
Y.L. McHall
1993, 10(3): 273-286. doi: 10.1007/BF02658133
In development of baroclinic disturbances, baroclinity of basic temperature field varies with conversion of avail-able potential energy. The growth rate which depends on the baroclinity varies as well. However, in previous linear theories, the growth rate was considered constant, so development of disturbances was not constrained by energy sources in the linear theories, In terms of energy conservation and conversion in an isolated atmosphere, we may study the variations in the baroclinity and growth rate and draw the corresponding pictures of perturbation develop-ments in the varying environments. The amplification for the most unstable Eady wave is discussed as an example. It will be found that growth of baroclinic perturbations constrained by energy conservation is significantly different from the growth at the initial constant rate after mature stage.
The Propagation of Disturbances Excited by Low-Frequency Oscillations in the Tropics
Lu Peisheng
1993, 10(3): 287-295. doi: 10.1007/BF02658134
The propagation of disturbances excited by low-frequency oscillations in the tropics is investigated by applying the theory of wave packet dynamics. For simplicity, a linearized barotropic model is adopted and the zonal circulation is taken as basic current. Suppose that the disturbances or waves are superimposed on jet-like westerly basic cur-rent and excited by the forcing in the tropics. We have (1) only the eastward propagating (m>0, n>0 and σ>0) low-frequency disturbances and the stationary (σ = 0) waves can propagate into the middle and high latitudes in the Northern Hemisphere; the others, such as the westward propagating low-frequency wave (m>0, n0) due to the fact that the group velocity of stationary wave is larger; (3) there is a whole wave train excited by the forcing in the tropics and extended into the middle and high latitudes, if the amplitude of the source is independent on time, especially, the low-frequency wave (σ > 0) is of travelling type propagating along the ray; (4) if the source lasts only for an interval of time, namely, its amplitude also has the character of low-frequency oscillation, the excited wave train is not always a whole one, but is restricted in the vicinity of source region in the beginning, extended from the source region to the middle and high latitudes in its saturated stage, after that it gradually becomes weaker and weaker and is detectable only in some area at high latitude, and eventually disappears. Undoubtedly, case (4) is closer to the reality, even though case (3) gives a more impressive wavy pattern.
Some Effects of Rotation Rate on Planetary-Scale Wave Flows
Li Guoqing, Robin Kung, Richard L. Pfeffer
1993, 10(3): 296-306. doi: 10.1007/BF02658135
A series of experiments were performed in a rotating annulus of fluid to study effects of rotation rate on planeta-ry-scale baroclinic wave flows. The experiments reveal that change in rotation rate of fluid container causes variation in Rossby number and Taylor number in flows and leads to change in flow patterns and in phase and amplitude of quasi-stationary waves. For instance, with increasing rotation rate, amplitude of quasi-stationary waves increases and phase shifts upstream. On the contrary, with decreasing rotation rate, amplitude of quasi-stationary waves decreases and phase shifts downstream. In the case of the earth’s atmosphere, although magnitude of variation in earth’s rotation rate is very small, yet it causes a very big change in zonal velocity component of wind in the atmosphere and of currents in the ocean, and therefore causes a remarkable change in Rossby number and Taylor number deter-mining regimes in planetary-scale geophysical flows. The observation reveals that intensity and geographic location of subtropic anticyclones in both of the Northern and Southern Hemispheres change consistently with the variation in earth’s rotation rale. The results of fluid experiments are consistent, qualitatively, with observed phenomena in the atmospheric circulation.
Methane Production, Emission and Possible Control Measures in the Rice Agriculture
Wang Mingxing, Shangguan Xingjian, Shen Renxing, Wassmann Reiner, Seiler Wolfgang
1993, 10(3): 307-314. doi: 10.1007/BF02658136
In the rice field methane is produced in the soil layer with depths of 2-25 cm. The vertical profile of methane production rate in the paddy soil during the water covering period differs from that in the paddy soil in dry phase. Only a small part, about 30%. of the produced methane is emitted to the atmosphere through rice plant, air bubbles, and molecular diffusion. Therefore, the methane emission rate from the rice field depends not only on the methane production rate in the soil, but also on the transport efficiency of the rice plant, air bubble formation that in turn depends on the production rate, and molecular diffusion.Field measurements show that methane emission rates from a particular rice field have very large diurnal, seasonal and interannual variations, which are related to soil characteristics, water regime, farming procedure, local climate, and rice growing activities. The relationship between the methane emission rate and the above mentioned factors is very complicated. The emission rates from different rice fields differ greatly not only in the absolute value, but also in the temporal variation patterns.Methane emission rate from the rice field may be significantly reduced by scientific management of fertilizer and irrigation. While the use of SO42- containing fertilizer and fermented organic fertilizer may reduce the methane emission significantly, the most promising measure for reducing methane emission from rice field is the frequent drainage irrigation procedure.
Constructions and Applied Examinations of a Kind of Square-Conservative Schemes in High Precision in the Time Direction
Ji Zhongzhen, Wang Bin
1993, 10(3): 315-324. doi: 10.1007/BF02658137
In order to meet the needs of work in numerical weather forecast and in numerical simulations for climate change and ocean current, a kind of difference scheme in high precision in the time direction developed from the completely square-conservative difference scheme in explicit way is built by means of the Taylor expansion. A numerical test with 4-wave Rossby-Haurwitz waves on them and an application of them on the monthly mean current the of South China Sea are carried out, from which, it is found that not only do the new schemes have high harmony and approximate precision but also can the time step of the schemes be lengthened and can much computational time be saved. Therefore, they are worth generalizing and applying.
Response of the Intensity of Subtropical High in the Northern Hemisphere to Solar Activity
Xu Qun, Yang Qiuming
1993, 10(3): 325-334. doi: 10.1007/BF02658138
Using the intensity data of each northern subtropical high measured by monthly 500 hPa height charts for the recent 38 years (1954-1991), we calculate their correlations with the monthly sunspot number and monthly solar radio flux at 10.7 cm wave length, respectively. Through strict test, we further confirm a series of high correlations. Next, using a method called the non-integer (year) wave, the significant response of each subtropical high’s intensity to so-lar activity at its main period of 10.9-year length is found. Special attention is paid to that of the eastern Pacific high, the possible mechanism of such sensible response is also analysed.
Numerical Simulation of Regional Short-Range Climate Anomalies
Yang Fanglin, Yuan Chongguang
1993, 10(3): 335-344. doi: 10.1007/BF02658139
With the high-speed development of numerical weather prediction, since the later 1980’s, the prediction of short-range climate anomalies has attracted worldwide meteorologists’ attention. What the so called short-range re-fers to the time scale from one month to one season or more. In dealing with the problem of short-range climate pre-diction, two points are needed noticing: one is the basic research to explore or investigate the mechanism of variability of the slow varying components which mainly include internal dynamics of extratropics, external forcings and tropical dynamics, and the other is the modeling efforts to simulate the process of the long-term evolution of the signal which include the improvement of model quality, stochastic prediction and the air-sea-coupled model (Miyakoda et al.,1986). Previous researches on the numerical prediction of short-term climate anomalies are mostly concentrated in the analysis of variables with global spatial scale, especially the global general atmospheric circulation analysis.As to the simulation or prediction of regional short-term climate anomalies, there exist many difficulties and problems. Though some meteorologists are devoting themself to this field, up to now, they have not reached satisfac-tory results. As a primary effort, by using the 2-level general atmospheric circulation model developed in the Institute of Atmospheric Physics, Chinese Academy of Sciences (IAP-AGCM) (Zeng et al., 1989), and taking the year of 1985 as a case, a numerical simulation of regional short-term climate change is completed. We pay high attention to the predictand of anomalous summer rainfall in the Yangtze River and Yellow River valleys, especially its month-to-month variation.
Improving Numerical Weather Prediction in Low Latitudes by Optimizing Diffusion Coefficients
Liu Jinda
1993, 10(3): 345-352. doi: 10.1007/BF02658140
The horizontal diffusion coefficients of the operational model (T42L9) in numerical weather prediction are optimized by the steepest descent search of multi-dimensional optimization. In order to improve prediction accuracy in low latitudes, the optimum horizontal diffusion coefficients are chosen, with changing variation of the basic diffu-sion coefficient with the passage of time, and later forecasts are also made better. In view of the averages of forecast verifications of 9 cases, the forecasts with optimum diffusion coefficients are an improvement on operational forecasts. It means that the forecasts are got much better with optimum values of some important parameters by optimization in numerical weather prediction.
The Southern Oscillation / Northern Oscillation Cycle Associated with Sea Surface Temperature in the Equatorial Pacific
Chen Lieting, Fan Zhen
1993, 10(3): 353-364. doi: 10.1007/BF02658141
This paper analyzed the time evolution of the global 1000 hPa height anomalies related to the sea surface tem-perature (SST) in the eastern equatorial Pacific by using ECMWF data in the period 1979-1988, in which two Pacific warm events, 1982/83 and 1986/87, are included. It is found that there are distinct evidences of eastward propaga-tion of alternate positive / negative height anomalies not only in the tropical South Pacific but also in the tropical North Pacific. The former is associated with the Southern Oscillation (SO) and the latter is associated with the so-called Northern Oscillation (NO).It is noteworthy that the alternate positive / negative anomaly centers associated with SO and NO can be traced back to the middle and higher latitudes of the South Indian Ocean and the East Asian continent respectively, which may be significant for the understanding of the causes and mechanism of SO and NO and for the monitoring of ENSO.Furthermore, these evolution processes have a strong symmetry about the Northern and the Southern Hemi-sphere. The whole SO / NO cycle takes about 3.5 years from extreme positive to negative index pattern and again from extreme negative to positive index pattern, It is suggested that the anomalous cold air activities over East Asia and Australia and the polar vortex activities over the northern and the southern Pacific seem to have dominant role on the SO / NO cycle.
Lidar Measurements of Aerosols in the Tropical Atmosphere
P.C.S. Devara, P. Ernest Raj
1993, 10(3): 365-378. doi: 10.1007/BF02658142
Measurements of atmospheric aerosols and trace gases using the Laser radar (lidar) techniques, have been in pro-gress since 1985 at the Indian Institute of Tropical Meteorology, Pune (18o32’N, 73o51’E, 559 m AMSL), India. These observations carried out during nighttime in the lower atmosphere (up to 5.5 km AGL), employing an Argon ion / Helium-Neon lidar provided information on the nature, size, concentration and other characteristics of the constituents present in the tropical atmosphere. The time-height variations in aerosol concentration and associated layer structure exhibit marked differences between the post-sunset and pre-sunrise periods besides their seasonal va-riation with maximum concentration during pre-monsoon / winter and minimum concentration during monsoon months. These observations also revealed the influence of the terrain of the experimental site and some selected me-teorological parameters on the aerosol vertical distributions. The special observations of aerosol vertical profiles ob-tained in the nighttime atmospheric boundary layer during October 1986 through September 1989 showed that the most probable occurrence of mixing depth lies between 450 and 550 m, and the multiple stably stratified aerosol lay-ers present above the mixing depth with maximum frequency of occurrence at around 750 m. This information on nighttime mixing depth / stable layer derived from lidar aerosol observations showed good agreement with the height of the ground-based shear layer / elevated layer observed by the simultaneously operated sodar at the lidar site.
Tropical Stratospheric Circulation and Monsoon Rainfall
A.B. Sikder, S.K. Patwardhan, H.N. Bhalme
1993, 10(3): 379-385. doi: 10.1007/BF02658143
Interannual variability of both SW monsoon (June-September) and NE monsoon (October-December) rainfall over subdivisions of Coastal Andhra Pradesh, Rayalaseema and Tamil Nadu have been examined in relation to monthly zonal wind anomaly for 10 hPa, 30 hPa and 50 hPa at Balboa (9oN, 80oW) for the 29 year period (1958-1986). Correlations of zonal wind anomalies to SW monsoon rainfall (r = 0.57, significant at 1% level) is highest with the longer lead time (August of the previous year) at 10 hPa level suggesting some predictive value for Coastal Andhra Pradesh. The probabilities estimated from the contingency table reveal non-occurrence of flood during east-erly wind anomalies and near non-occurrence of drought during westerly anomalies for August of the previous year at 10 hPa which provides information for forecasting of performance of SW monsoon over Coastal Andhra Pradesh. However. NE monsoon has a weak relationship with zonal wind anomalies of 10 hPa, 30 hPa and 50 hPa for Coastal Andhra Pradesh, Rayalaseema and Tamil Nadu.Tracks of the SW monsoon storms add depressions in association with the stratospheric wind were also exam?ined to couple with the fluctuations in SW monsoon rainfall. It is noted that easterly / westerly wind at 10 hPa, in some manner, suppresses / enhances monsoon storms and depressions activity affecting their tracks.