The Climatic Features Of Spring Rainfall Over Regions South To Yangtze River And Its Relationship With Mechanical Effects Of The Tibetan Plateau

Based on the daily precipitation and temperature data of China from the National Meteorological Information Center during the period of1960-2004and the atmospheric data from National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP/NCAR) reanalysis daily datasets, the features of spring rainfall over regions south to the Yangtze river (SYR), the typical circulation of spring persistent rainfall (SPR) and the mechanical effect of the climbing and detouring flow around the Tibetan Plateau (TP) have been investigated. The conclusions are as follows:1. The climatic features of spring rainfall over SYRComparing different-persistent-days rainfalls in spring, the dominant process is4-and-more-days persistent rainfall event over SYR. Moreover, significant stations are55stations within the area of22.5°-33°N,105°-120°E, and time period is concentrated in the period of13th-27th pentad. Based on the definition of SPR over SYR, during45years,152SPR processes have been picked out, and two persistent heavy rainfall centers are located over the northeastern and southwestern part of SYR.For the linear trend distribution of days of SPR, significant decreasing trend is found over the whole region of SYR, especially after1995. For the intensity of SPR, the trend is not significant. The dominant patterns of SPR days and intensity variations show homogeneous anomalies. The second patterns of SPR days and intensity variations are characterized by north-south dipole pattern and north anomaly pattern. Both of time serials of SPR days and intensity indicate the loading of these patterns varies on decadal and interannual timescales.2. The circulation features of SPRAccording to the one-day rain process, when a cold trough goes across or the cold air suddenly outbreaks, the cold and warm air meets at middle layer over SYR, which results in the precipitation. But, this process is so quick that will not persist long. However, during the SPR occurred, the continuous stable low-level warm air and cold air meet at lower layer over SYR, which is one of the important reasons that the rainfall are persistent over SYR.To furtherly investigate the reason for the warm and cold air sustaining over SYR, from climatic and synoptic point of view, based on composite analysis on the persistent sunny and SPR, the typical circulation features have been investigated. The coupled configuration of vertical winds and temperature differences and the global potential height field have been analyzed. In the upper levels, continuous belt of westerly jet center have been found. In the lower levels, the strong westerly flow located over the southern part of TP and encounters with the Yunnan-Guizhou Plateau (YGP), then the flow bypass to the Indo-China Peninsula. So, the strong southerly winds along the western part of South China Sea Subtropical High (SCSSH) and westerly winds around Indo-China Peninsula will join together and flow to SYR, which gives robust and sustained warm southwest winds along the southern part of Nanling Mountains. Meanwhile, stronger cold air at low level is found along the northern part of Nanling Mountains. The warm and cold air nearly intersects in the Nanling Mountains. And, the warm southerly wind is stronger than the northerlywind that continues rising until it reaches nearly35°N, where it meets with northerly winds. In the global geopotential height fields, when the SPR occurs, significant patterns are with a weak polar vortex at100hPa and weaker trough in East Asian. Moreover, a weak ridge of high pressure is located over south to30°N, which maybe is the reason why the north winds sinks near35°N, causing the controlling movement over SYR to be the southerly upper flow. On the low levels, under the coupled controlling of the stronger SCSH and Siberian cold high, robust warm and cold air have steadily located over the northern and southern part of Nanling Mountains.3. The simulations on the mechanical effect of TP on spring rainfall over SYR Because of TP, the westerly flow is divided into climbing and detouring flow crossing the TibetanPlateau. Meanwhile, the Yunnan-Guizhou Plateau (YGP) locates over the southern part of China. As part of TP, the range and height is not as large as TP, but due to its location, it will largely affect the SPR. The model simulations have been used in this section for analyzing the effect of TP and YGP during SPR.Both of the climbing and detouring flow will increase the spring precipitation over eastern China, especially around Hunan province, while under the detouring flow, the impact regions are broader. The detouring flow has enhanced the cyclonic flow over eastern China, and the affected range is much larger than climbing flow. The detouring flow significantly increases low-level winds, especially over the southeastern part of TP, which is one of the key areas of water vapor transferring.Furtherly analyzing the local terrain effect on the spring rainfall over SYR, it is found that the Nanling Mountains will directly make the low-level warm and cold air intersect, which is benefit for the rising of warm air and rainfall. However, the effect of YGP is not due to topographic lifting, but forcing the flow at the south side of the Plateau bypass to the Indo-China Peninsula, increasing the southwesterly winds around the western part of South China Sea High, which brings more water vapor into SYR.