Characteristics Of Rainfall And Convection And Synoptic Conditions Of Mesoscale Convective Systems At Yangtze-Huai River Basin: Contrasting Meiyu With Pre-and Post-Meiyu Periods

Previous studies investigated characteristics of precipitation before and after the onset of Asia summer monsoon at the south Asian monsoon region, South China Sea and South China, but not for the Yangtze-Huai River Basin (YHRB) which is also an important part of East Asian Monsoon region. The present study compares rainfall characteristics and convective intensity of precipitation systems over the YHRB among the pre-Meiyu, Meiyu and post-Meiyu periods, using surface station gauge observations and Tropical Rainfall Measuring Mission (TRMM) datasets during 1998-2008. Efforts are also made in this study to relate the structure and convective characteristics of mesoscale convective systems (MCSs) to synoptic conditions during the three periods. The study is not only necessary for investigating East Asian monsoon rainfall, but also helpful to evaluate atmospheric numerical models and improve physical schemes in the models.An impersonal analysis method is adopted to determine the starting and ending dates of the Meiyu period for each year. This method considers the intensity, continuity, and spatial coverage of Meiyu precipitation over YHRB, as well as the interseasonal variation of East Asia atmospheric circulation. A radar precipitation feature (RPF)-based analysis method is used to investigate rainfall characteristics and convective intensity of precipitation systems over the YHRB during the three periods. A RPF is defined as a contiguous area consisting of near surface raining pixels detected by TRMM PR. Based on their area and the existence of convective pixels, the RPFs are categorized into three types: MCS with area≧1000 km~2 and containing at least one convective pixel, sub-MCS with area﹤1000 km~2 and containing at least one convective pixel, and non-convective system (Other) without any convective pixel. The main conclusions are as follows:(1) The Meiyu rain bands extend nearly west-easterly over the YHRB, with an averaged lifetime of ~5 days. Total rainfall accumulation amount during the Meiyu period also distributes quasi-west-easterly over the YHRB, with several rainfall maxima being located near Wuling Mountain, Jiuling Mountain, Huangshan Mountain, Dabie Mountain and Huanghuai Plain. In contrast, the rainfall maxima appear mainly to the south of Yangtze River during the pre-Meiyu and are located at northwestern YHRB during the post-Meiyu period.(2) Non-convective RPFs contribute 63%-83% to total RPF populations, but contribute less than 6% to total near surface volumetric rain; while more than 80% of the rainfall during the three periods comes from the MCS-type RPFs that are only 2.3%-3.3% of the total RPF populations. Therefore, MCS-type RPFs are the most important contributors to precipitation over the YHRB during late-spring to midsummer. There is an increasing contribution of convective RPFs over the YHRB from pre-Meiyu to Meiyu and further to post-Meiyu. During the post-Meiyu period, the percentage of subMCS with area less than 1000 km2 is the largest (33.7%) among the three periods, which is probably in connection with the hot weather that favors formation of local thermal convection in the afternoon of post-Meiyu.(3) Compared among the three periods, the percentage of MCSs to total RPF population during pre-Meiyu is the lowest, and those during Meiyu and post-Meiyu period are about equal. By average, horizontal sizes of MCSs during pre-Meiyu and Meiyu are comparable, being nearly twice of the post-Meiyu MCSs. That is partially because MCSs during the pre-Meiyu and Meiyu periods are closely related to large-scale weather systems such as fronts, but are more connected with smaller-scale thermal convective systems in the afternoon during the post-Meiyu period.(4) Statistics of five parameters (the maximum radar reflectivity at 6 km, the maximum height of the 30 dBZ echo, the minimum polarization corrected brightness temperatures at 37 GHz and 85 GHz, respectively, and lightning rate) and the vertical profiles of maximum radar reflectivity consistently suggest that the convective activities in the precipitation systems over the YHRB intensify from pre-Meiyu to Meiyu and further strengthen to the post-Meiyu period. Compared to the other two periods, weaker convection during the pre-Meiyu period is due to both significantly drier atmosphere in this period as the summer monsoon hasn't reached the YHRB and smaller convective available potential energy during the pre-Meiyu period.(5) Among the three periods, percentage of RPFs with flashes to the total RPFs is the largest (3.8%) during post-Meiyu period and the lowest (1.1%) during pre-Meiyu period. During multiple thundercloud parameters, lightning rate has the best correlation (coefficients >0.8) with the area of >20 dBZ at -45℃and the area of >35 dBZ at -5℃, and has lower correlation coefficients with the parameters representing single pixel characteristics (minimum 85 GHz and 37 GHz Polarization Corrected Temperature, maximum radar reflectivity at -15℃), and has the lowest correlation coefficients with convective rainrate of precipitation.(6) By investigating synoptic conditions that favor presence of MCSs at the YHRB during the three periods, it is found that: (a) Compared with the condition of none MCS over YHRB, the synoptic conditions of MCSs located over YHRB during pre-Meiyu and Meiyu period exist many similarities: at lower troposphere over the YHRB, there is a cyclone-like perturbation and cold and warm air confront, suggesting enhancement of fronts, although the temperature difference between the north and south to the front during the pre-Meiyu period is larger than the Meiyu period; at middle troposphere, positive vorticity can be transported moe significantly to the YHRB from upstream by horizontal advection, the reason for which is the location change of trough relative to YHRB during pre-Meiyu and incontrast, the deepening of the trough to the west of YHRB during Meiyu; at higher troposphere, high pressure and anticyclone-like perturbation favor to enhancement of air flow divergence. (b) The synoptic conditions of MCS relative to non-MCS during post-Meiyu differ from those during pre-Meiyu and Meiyu: during the post-Meiyu period cold air at middle troposphere (500 hPa) is very strong because of deepening of trough located to the north of the YHRB, increasing convective unstable energy and being inductive to stronger convection; at high-level, the trough in north of the YHRB deepens and the geopotential height in south of the YHRB increases causing stronger outflow. (c) Among the three periods, the similarities of synoptic conditions that favor presence of MCSs at the YHRB are: stronger southwest wind at low-level, convergence of water vapor flux in the entire layer and vertical ascending motion, and more abundant precipitable water compared with the conditions without presence of MCS over the YHRB.