2005年“5.31”湖南大暴雨中尺度模拟和发生机制

Study on Numerical Simulation and Occurring Mechanism of "5.31" Heavy Rainstorm in Hunan in 2005

利用NCEP 1°×1°的6 h再分析资料和MM5模式15 km水平分辨率模拟的逐时资料,对2005年初夏湖南大暴雨的中尺度系统及其触发、维持机制进行了诊断分析研究。结果表明:高低空急流最佳配置,动力耦合关系建立为暴雨发生提供了有利的环境条件;中尺度系统在渝南、黔西生成后移入湖南停滞发展,加强了湖南上空动力不稳定条件,促进对流发展;偏南暖湿气流整层突然增强对暴雨的对流系统爆发性发展起十分重要的作用,它在中高层凝结释放潜热加热高层大气,对上升气流起到正反馈作用;高层大气强“抽吸”效应和前期降水产生地表潜热和感热的强迫作用亦是对流爆发性发展的重要因子。湖南复杂地形在暴雨启动机制中起重要作用。但敏感性试验表明,地形抬升不是这次暴雨唯一必需的触发条件,削平南方山地不仅能增强水汽输送,低层偏南气流增强还能加强地面中尺度系统的辐合作用,辐合抬升是触发机制之一。降雨过程发生发展主要受天气系统控制,地形效应可明显改变降雨落区和强度,迎(背)风坡度的增加(减小)通常使降雨加强,反之使降雨减弱。

Meso-scale system and its triggering,maintaining mechanisms of ＂5.31＂ heavy rainfall in Hunan in 2005 are studied and diagnosed by used of NCEP 6 h reanalysis data with resolution 1°×1° and simulation data from MM5.The results show that the best collocation of high-low layer jet and establishment of their coupling pattern provide favorable environment conditions for the rainstorm occurring.The meso-scale systems occur in south of Chongqing and west of Hunan Province to move to Hunan with its developing,which strengthen unstable dynamical field so as to boost convection.Southerly warm and moisture airflow abruptly strengthen is key for onset of heavy rainstorm.Pump from high layer and both latent heat and sensible heat from weak rainfall that occurred before the heavy rainstorm are very important for convection start and development.Additionally,the complicated terrain in Hunan is also crucial for startup of heavy rainstorm.South hill slope lead south wind slide to Hunan along low layer,west mountain mostly uplift airflow,which is one of reasons that heavy rainstorm happened.Whereas,the sensitivity experiment shows that the uplift of terrain is not necessary for the rainstorm startup,it is propitious to enhance vapor transport and surface meso-scale system which converge became mainly trigger mechanism when southerly mountain are removed.The rainstorm event is lied on the system development,terrain may impact rain shape and its amount,windward（leeward） hillside gradient increase lead to rainfall enhancing（weakening）,contrarily,windward（leeward） hillside gradient decrease lead to rainfall weakening（enhancing）.