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Qinglin Zheng and Kuo-Nan Liou

Abstract

The dynamic and thermodynamic influences of the Tibetan Plateau on the temperature geopotential height, horizontal wind, vertical velocity, cloud and precipitation patterns over eastern Asia am physically investigated employing a general circulation model specifically designed for medium range weather prediction. The model is a modification and improvement of the one recently presented by Liou and Zheng. Efficient and accurate numerical techniques have been devised for the computation of the pressure gradient force and the initial field in the vicinity of mountains in the σcoordinate. In addition, a cumulus convection scheme and a liquid water content prediction equation from which the large-scale precipitation may be evaluated have been developed. Utilizing the NMC objective analysis data covering the period from 13 May to 18 May 1979 and in light of the general circulation model experimentation, we find that incorporation of the mountain and the subsequent diabatic heating effects improves the overall modeling results for the temperature, geopotential height and wind fields, especially in low and middle levels. Without the consideration of the mountain effect, the prediction results from the model in the low level deteriorate and errors propagate into levels aloft. The thermodynamic effects involving radiation, condensation and the vertical transport of sensible heat are found to he very important in the maintenance of the position and intensity of the eastern Asia jet stream. Moreover, the presence of the orographic barrier strengthens the vertical velocity, and the effects of the facing and lee sides of the plateau on atmospheric flows significantly influence the prediction of temperature and geopotential height fields. Finally, it is observed that the formation of cloud and precipitation distributions east and south of the plateau is a direct consequence of the coupling of dynamic and diabatic heating effects caused by the plateau.

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Kuo-Nan Liou and Qinglin Zheng

Abstract

Physical interactions of radiation, cloud formation and dynamic processes are investigated utilizing a general circulation model designed for short and medium range prediction. The present model is a seven-level spectral model in the σ-coordinate system for the Northern Hemisphere with the generation of clouds in four σ layers through the predicted relative humidity field. Computations of the cloud liquid water content involving various cloud combinations for radiation transfer analyses are based on assumed particle size distributions assigned to cumulus, stratus, altostratus, cumulonimbus and cirrus. Parameterization of the thermal IR and solar flux transfer in a clear atmosphere utilizes the broadband emissivity (IR) and absorptivity (solar) concept for water vapor, ozone and carbon dioxide emission and absorption. Broadband solar flux reflection and transmission for various cloud types as functions of the liquid water content, solar zenith angle and average cloud temperature under a number of model conditions are parameterized in line with the gaseous broadband absorptivity. Moreover, high clouds are considered to be nonblack whose parameterized emissivity is expressed in terms of the vertical ice content. We have physically described and mathematically quantized the effects of radiative processes on the vertical velocity, temperature and cloud field (via the water vapor mixing ratio) by virtue of the fundamental equations governing these variables in the model. We demonstrate, through the predicted vertical velocity for the fifth day, that radiative flux exchanges in the tropics lead to a significant intensification of the Hadley circulation and that such processes also strengthen to some degree the meridional circulation. In addition, we show that the part of the vertical velocity produced by radiative processes represents a significant component in the tropical region, whereas it is only about 10–15% of the total in middle and high latitudes. Moreover, we also discover a consistent and characteristic pattern involving warm and cold advections of temperature associated with frontal activities and the part of the vertical velocity produced by radiative flux exchanges. In areas associated with high and low temperature advections radiative processes appear to enhance and suppress the upward motion, respectively. With verification from the satellite IR cloud picture we show that the present model performs quite realistically on the prediction of cloud cover. When radiative processes were not incorporated in the model, however, the cloud cover in the tropics is underestimated by as much as 50% due to the lack of the low cloud formation. Furthermore, we show that the present model with interactive radiative processes has improved the temperature prediction by about 1–2°C in middle latitudes and <1°C in the tropics. Statistical analyses show that this improvement is significant in low levels. Finally, we illustrate that the present model predicts accurately the mean 500 mb temperatures for nine consecutive days utilizing a data set covering the period from 7 to 15 June 1980.

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Elmar R. Reiter, J. D. Sheaffer, J. E. Bossert, Eric A. Smith, Greg Stone, Robert McBeth, and Qinglin Zheng

A long-planned field-measurement program to determine surface-energy budgets at two sites in Tibet was carried out during June 1986 in collaboration with scientists from the State Meteorological Administration, Academy of Meteorological Sciences, People's Republic of China. The data set obtained in Tibet is unique for this remote region of the world. The present report describes some of the experiences of the United States scientific team and its medical officer, M. Otteman of Ft. Collins, Colorado. The data are presently being archived on computer tapes. Preliminary analysis results are presented as typical examples of the conditions encountered at the two experimental sites near Lhasa (3635 m) and Nagqu (4500 m).

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