The formation of a vortex and shear line over the Qinghai‐Tibet Plateau (QTP) was investigated with numerical experiments using the ECMWF grid point model. The case chosen occurred between 24 and 26 July, 1982, when the south‐west monsoon invaded the QTP and a vortex developed. The correct simulation of the vortex in the model shows that it is possible to predict the atmospheric circulation over the QTP and the consequent weather over China. When the simulation was rerun with a dry model (i.e., without the release of latent heat), it was found that only a weak vortex formed. This shows that the release of latent heat is an extremely important factor in the development of the vortex over the QTP, and confirms earlier diagnostic and synoptic studies. The surface sensible heat fluxes, large near the northern and southern flanks of the QTP, appear to have a damping effect on the formation of the vortex; i.e., the vortex was greatly intensified in their absence. It was also found that the intensity of the vortex was dependent upon the height of the QTP: when the mountain height was reduced, the westerly trough invaded the QTP and no vortex formed at 500 mb; when it was increased, the south‐west monsoon travelled along the southern boundary of the QTP and intensified the low‐level jet south of the vortex area. The results of a 48‐h integration with a fine‐mesh model (resolution 0.5° of latitude and longitude) showed an improvement in forecasting the location of the vortex with respect to the coarse‐mesh model (1.875° of latitude and longitude). However, when compared with the coarse‐mesh analysis, the intensity of the vortex was overpredicted. This may be due to mesoscale orographic forcing and exaggerated latent heating.
SUMMARYThe influence of mechanical forcing on the formation of a mesoscale vortex which occurred over the eastern flank of the Qinghai-Tibetan Plateau (QTP) is investigated by running a series of numerical experiments. Different orography schemes are tested. It is shown that there are three branches of air flow converging towards the vortex: the northerlies associated with the middle latitude westerly currents, the southerlies with the lowlevel monsoon jet, and the easterlies with the west Pacific subtropical high. These flows are very sensitive to mechanical forcing and play very important roles in the formation and location of the vortex.The model with either the mean or the envelope orography is shown to be capable of predicting the formation of the vortex and the associated torrential rain. However, its location is not predicted precisely. A modified orography, simulating the 'step' structure of the eastern flank of the QTP, is then used in an attempt to improve the prediction. Results from this experiment suggest that the weakening of some of the steep slopes of the large-scale orography in a numerical model, due to excessive smoothing of mountain height and expanding of mountain area, is responsible for the inaccuracy in the forecast of such synoptic systems. Therefore further improvements in the orography scheme are discussed.
Some numerical simulations from real data were carried out to examine the response of the monsoon circulation to moist processes. Two cases were selected: one for the monsoon onset and the other for the active (fully established) period. The results show that latent heat is the main feed-back process in the monsoon circulation. The south-west monsoon current from the Arabian Sea to the South China Sea in the lower troposphere and the easterly jet stream over southern Asia in the upper troposphere are greatly enhanced by this process both in the onset and active periods. The surface latent heat flux (mostly over the ocean) is important in the maintenance of the circulation durhg the active period but is less important for the onset.
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