The Characteristics And Physical Mechanisms Of Warm Season Convection Based On Long-term Radar Observations And Mesoscale Model

South China is a climatological rainfall center and a center of convective activities during warm season. In the region, convective precipitation is the main cause of meteorological disasters, including flash flood, urban waterlogging and mountain landslide. The warm season largescale circulation, weather systems and the horizontal distributions of the precipitation in South China have been extensively studied in the past, but there have few works focused on the spatial and temporal characteristics of convective precipitation in South China during warm season. Most of the related works have used long-term satellite observations, whose spatial and temporal resolutions are not good enough to investigate the detailed characteristics of the convection. This study examines the temporal and spatial characteristics and distributions of convection over South China coastal regions during the warm season and investigates the physical mechanisms related to the distribution, diurnal cycle and propagation of convection in the region, using, for the first time for such a purpose, long-term operational Doppler radar data and numerical model.Results show that, over Pearl River Delta, convective features occur most frequently along the southern coast and the windward slope of the eastern mountainous area of Pearl River Delta, with the highest frequency occurring in June and the lowest in September among the 5 months. The spatial frequency distribution pattern also roughly matches accumulated precipitation pattern. The occurrence of convection in this region also exhibits strong diurnal cycles. During May and June, the diurnal distribution is bimodal, with the maximum frequency occurring in the early afternoon, and a secondary peak occurring between midnight and early morning. The secondary peak is much weaker in July, August and September. Convection near the coast is found to occur preferentially on days when a southerly low-level jet (LLJ) exists, especially during the Meiyu season. Warm, moist and unstable air is transported from the ocean to land by LLJs on these days, and the lifting along the coast by convergence induced by differential surface friction between the land and ocean is believed to be the primary cause for the high frequency along the coast. In contrast, the high frequency over mountainous area is believed to be due to orographic lifting of generally southerly flows during the warm season.Using three years radar, reanalysis and surface data form 2007-2009, the study also investigates the reason why the diurnal cycle of convection has double peaks during pre-summer seasons. Observational results show that convection occurs most frequently during the afternoon over PRD due to solar heating. On the windward slope of the mountains, convection occurrence frequency exhibits two daily peaks, with the primary peak in the afternoon and the secondary peak from midnight to early morning. The nighttime peak is shown to be closely related to the nocturnal acceleration and enhanced lifting on the windward slope of southwesterly boundary layer flow, in the form of boundary layer low-level jet. Along the coastline, nighttime convection is induced by the convergence between the prevailing onshore wind and the thermally-induced land breeze in the early morning. Convection on the windward slope of the mountainous area is more or less stationary. Convection initiated near the coastline along the land breeze front tends to propagate inland from early morning to early afternoon when land breeze cedes to sea breeze and the prevailing onshore flow. Using ARW-WRF model, this study also studies the diurnal cycles of land-sea breeze circulation and its related precipitation over South China coastal region. The focus of the analyses is a 10-day simulation initialized with the average of the 0000-UTC gridded global analyses during the 2007-2009 Mei-Yu seasons (11 May-24 June) with diurnally varying cyclic lateral boundary conditions. Despite differences in the rainfall intensity and locations, the simulation verified well against averages of three years observations, and successfully simulated the diurnal variation and propagation of rainfall associated with the land-sea breeze over the South China coastal region. Simulated results show that thermodynamic forcing and coastal terrain have great impact on the diurnal cycle of lean-sea breeze and its related precipitation.In the last, we also have improved the 4-dimensional Variational Doppler Radar Analysis System (VDRAS) developed by NCAR. Observed surface temperature and winds are extrapolated to the lowest model level based on the dry lapse rate and wind profile power law in the surface layer. Then the modified surface observations are assimilated together with radar radial velocity and reflectivity into a convection-permitting model using the VDRAS four-dimensional variational (4DVar) data assimilation system. Using this improved VDRAS, this study examines a coastal convective rainfall process occurred over South China coastal region in 05 May 2008. This rainfall process occurred under weak synoptic force and no low level jet condition. Results show that the initiation and development of convection are closely related to the coastal terrain and sea breeze front. The rainfall line along coastline is produced by the combination of the convection induced by the lifting effect of coastal terrain and the convection on the sea breeze front.