| Using observational snow depth data in China, the distribution feature of winter and spring snow depth over Tibetan Plateau was analyzed and the "strong signal" key area of snow in the central-east part of Tibetan Plateau was selected. The spatial distribution of snow depth over the key area was relatively stable, the values and amplitude of it was significantly higher than other parts of the Tibetan Plateau, and it also showed a significant seasonal variation. By analyzing the abnormal index, samples for anomalous heavier and lighter snow cover years were respectively chosen. Then differences of winter and spring land-air temperature difference (ground surface temperature minus surface temperature), sensible heat flux of NCEP reanalysis data, Q1 (apparent heat source) and Q2 (apparent water vapor sink) calculated from NCEP reanalysis data over the Tibetan Plateau key area between heavier and lighter snow years were calculated and respectively analyzed from time evolution and spatial structure, then the effect of Tibetan Plateau snow depth on the various thermal factors was summarized. The results showed that the difference of Q1 over the Tibetan Plateau key area between heavier and lighter snow years was significantly negative, and the snow cover over the Tibetan Plateau key area resulted in a vertical temperature structure characterized of a "cold source column" from the ground surface to the 100 hPa level.By dynamic statistics and correlation analysis, precursor features of winter snow depth over the Tibetan Plateau key area and its impact on summer precipitation in eastern China were comprehensively analyzed. Differences of seasonal 500 hPa temperature and geopotential height of NCEP reanalysis data, winter and spring surface Q1 and Q2, land-air temperature difference, sensible heat flux, summer full column water vapor flux and TBB between heavier and lighter snow years were also calculated. The results showed that when the Tibetan Plateau key area was a distinctive negative value center area on the winter snow depth difference field between heavier and lighter snow years, the eastern China was a significant negative value area on the spring surface Q1 and Q2, land-air temperature and sensible heat flux difference field between spring heavier and lighter years (Figure omitted). The above computational results accord with the basic empirical rule that a heavier snow winter of Tibetan Plateau follows a reduced spring land-sea thermal contrast in eastern China and would have an impact on the East Asian monsoon. The computations and analysis on the cold source (external thermal forcing) effect of winter snow depth over the Tibetan Plateau key area showed that with shift of season from winter to spring and summer, negative extreme difference value areas of the 500 hPa temperatures and heights of winter heavier snow years minus lighter snow years exhibited an eastward movement feature from the Tibetan Plateau downstream to the Japanese Sea and western Pacific. It was also found from the difference field of summer water vapor fluxes of the winter heavier minus lighter snow years that two moisture flows from the south and north sides of Tibetan Plateau converged in the mid-lower reaches of the Yangtze River, and the difference field of TBB there was a significant negative value center (convection center). The study suggests that the anomaly of winter snow depth over the Tibetan Plateau key area might persistently affect the downstream monsoon circulation system, especially leading to the south-north position oscillation of the west Pacific subtropical high. The heavier (lighter) winter snow over the Tibetan Plateau could persistently affect the downstream monsoon system, resulting in reduced (increased) temperatures, especially in the downstream area in the mid-latitudes of 30°N-45°N, reduced (increased) geopotential heights east of 105°E, and a subtropical high south (north) of normal, thereby forming a similar "north-low-south-high pattern" of geopotential heights and a stream field structure of the confluence of southward and northward moist flows over the low-mid reaches of the Yangtze River. The winter snow depth over the Tibetan Plateau key area was also distinctively related with the summer precipitation in the eastern of Mainland China, revealing the significant correlation of winter TPSC status with subsequent summer Meiyu (Baiu) moisture transfer structure.Adopting the statistical correlation model, including anomaly index and the correlation vector calculation method, the regional characteristics of spring drought in Southwest China and the "strong signal" of snow cover and heat source on the Tibetan Plateau were analyzed in this paper. This paper researched the typical region of spring drought in Southwest China and then got the key area of atmospheric apparent heat source over Tibetan Plateau snow cover which in its later stage influenced the spring precipitation over the typical region of spring drought in Southwest China. Through the comprehensive analysis of the correlation between the snow depth and the atmospheric apparent heat source in all levels over the "strong signal" key area of the Tibetan Plateau snow cover, we find that February is the precursor "strong signal" typical month of the snow cold source over the key area of the Tibetan Plateau snow cover. Through the anomaly index and the correlation vector analysis, we focus on the correlation of the annual change of anomaly index between the Q1 in "strong signal" typical month (February) over the key area of Tibetan Plateau snow cover and spring precipitation in the later period over the area of spring drought in Southwest China. Then, we find that the relation between the two anomaly index above have a character of "anti-phase" in annual change. The analysis showed that when the snow depth on the Tibetan Plateau was shallow (deep), Q1 over the key area of Tibetan Plateau snow cover was high (low). And this was beneficial for relative dry (wet) northerly (southerly) flow over Southwest China in spring. This also lead to a abnormal low (abnormal high) precipitation in spring over Southwest China.The atmospheric apparent heat source in all levels of the air over the key area of Tibetan Plateau snow cover has a obviously precursory effect to predict the spring drought in Southwest China.
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