Diabatic Heating Characteristics In The East Peripheral Area Of Qinghai-Tibetan Plateau And Its Climate Effect During Spring-Summer Transition Season

Dali of Yunnan is located in the critical area of diabatic heating sensitive area and monsoon water vapor transport passage in the southern section at southeastern side of Qinghai-Tibet Plateau. Since various thermal and dynamic effects of atmosphere exerted by the plateau and peripheral areas on free air are made through boundary layers, it seems very important to observe, analyze and study the plateau and peripheral areas, especially the micrometeorological characteristics of atmospheric boundary layers and surface layers of diabatic heating sensitive area. Making use of the boundary layer comprehensive observation system built with the JICA Project of Sino-Japan Joint Research Center of Meteorological Disaster at Yunnan Dali National Climatic Station, focusing on the spring-summer transition in the diabatic heating sensitive area at the eastern side of the plateau as well as discussing the eddy covariance turbulence observation quality control and correction method, this paper analyzed the energy balance of turbulence observation system in Dali, observed the calculation results, soil data, gradient observational data, etc. with eddy covariance method and conducted a series of analyses on the micrometeorological characteristics of atmospheric surface layer, the radiation fraction variation characteristics of atmospheric surface layer, soil parameter variation characteristics as well as the variation characteristics of sensible heat flux, latent heat flux and momentum flux of atmospheric surface layer before and after the rainy season started in the spring-summer transition season in Dali; comparatively analyzed the differences of the variation characteristics of sensible heat flux and latent heat flux in Dali with those in the plateau core area—Changdu, Xilinhot featured with northern grassland and Shouxian dominated with paddy field over the Yangtze and Huaihe valleys; comparatively analyzed the differences between the diabatic heating at Dali observation station and the NCEP\NCAR reanalysis data with the observation results; analyzed and studied the possible reason for the regional diabatic heating abnormal variation with the mode simulation tool and on this basis, comparatively analyzed the possible effect of surface flux exchange abnormality in this sensitive area on the spring rainfall climate in south China. The present study has mainly drawn the conclusions as follows:1. Three methods for flux calculation are compared. The results of Dali data processing indicate that the results of Bowen method and eddy covariance method are more in agreement while the results of profile method and EC method are considerably different, but these differences are much smaller in dry season.2. The wind speed at atmospheric surface layer before rainy season in Dali is relatively faster, but it is gradually getting slower from May to June. The amplitude of diurnal temperature in Dali is gradually becoming smaller from April before rainy season starts to June in rainy season with the greatest in April but the smallest in June; the temperature gradient between each layer also shows the greatest in April but the smallest in June; the night temperature inversion is obvious and reaches the greatest in April, the temperature difference between 2m and 10m exceeds 3℃and the night temperature inversion strength is gradually becoming smaller from May to June. Vapor pressure is gradually increasing in three months—April, May and June just before and after rainy season in Dali. Water vapor is gradually increasing from the first ten-day period to the third ten-day period of May and inverse humidity at 10m and 20m exists in the three ten-day periods, but it is most obvious in the second ten-day period. The transient radiation has an average maximum daily peak value in April, a minimum daily peak value in May but a slightly increasing value in June. The monthly average daily peak value of atmospheric inverse radiation is monthly increasing from April to June and the average daily minimum value of each month changes in the same way. The monthly average daily peak value of reflection radiation is smaller in April, the daily peak value is the smallest in May and the average daily peak value of reflection radiation in June is the biggest in three months. The daily peak value of surface radiation is the greatest in the first ten-day period of May. The average daily value of net radiation is the smallest in continuous overcast and rainy weather in the period of rainy season. Surface temperature is high in the first ten-day period of May before rainy season starts, becoming lower in the beginning period of rainy season than that before rainy season starts and returning high after rainy season starts. Soil temperature at deep layers is not affected by rainfall weather but has an obvious feature of changing with seasons. Soil moisture content increases with depth increasing before rainy season starts and soil moisture content of shallow layer increases rapidly and even surpasses that of deeper layer after rainy season starts.3. Surface heat exchange in Dali just before and after rainy season starts is dominated by latent heat and latent heat flux is gradually decreasing with time while sensible heat flux is gradually increasing before rainy season starts and the difference between the two is decreasing; latent heat flux is gradually increasing while sensible heat flux is decreasing after rainy season starts and the difference is increasing.4. There is an apparent difference between the observed flux value at Dali station and the flux simulated value of NCEP reanalysis data and such difference is mainly found in April in spring-summer transition season. The sensible heat flux of reanalysis data is obviously overestimated against the observed value while the reanalysis value of latent heat flux is apparently underestimated against the observed value, to which the simulated result with WRF mode is similar.5. The sensitive run result of vegetation variation in the Hengduan Mountain area indicates that latent heat flux and sensible heat flux are more sensitive to vegetation variation and vegetation variation is an important factor of diabatic heating change.6. The results of the sensitivity test after vegetation variation suggest that the underlying surface vegetation variation in the Hengduan Mountain area may lead to the change of the circulation field, water vapor field and vertical velocity field in the region and the downstream area in the east. The simulation test with the flux partly revised by changing the vegetation type is improved to a certain degree in the rainfall simulation effect in spring in south China.