| Based on multiple meteorlogical data, including conventional surface data, sounding data, NCEP reanalysis data, Automatic Weather Station (AWS) precipitation data, integrated satellite precipitation data CMORPH, Japan satellite MTSAT TBB data, Doppler radar data from Guangzhou and Lightning Position Indicator data from Guangdong province, a mesoscale analysis on a warm sector heavy rainfall event occurred on 26 May 2007 over pearl river delta and its northen neighbour is performed. By investigating the activities of the heavy rainfall associated MesoβScale Convective Systems (MβCSs), a better understanding of this warm sector heavy rainfall event with different characteristics from the usual rainstorm is given. Based on these analysis, by applying LAPS (Local Analysis Prediction System) to synthetically analyze various conventional and unconventional data and with diagnostic analysis methods, output data from this system are used to study the environmental background of the MβCSs and possible trigging mechanism for their initation and development. Dynamical and thermal features of the investigated warm sector heavy rainfall event are also discussed. The results show that:(1) The heavy rain event demonstrates distinct convective features, with strong intensity and high efficiency and most of the precipitation fallen in 3 hours. The rainfall area shows a feature of moving from north to south, which is related directly to activities of three consecutive development of MβCSs.(2) The associated convective cloud clusters mainly initiate within a surface inverted low trough and developed stronger in convergent flows between the surface low trough and rear of the transformed surface cold high ridge. As happening in a background of affecting by concurrent southwesterly flows, showing no striking baroclinic zone around the rainfall areas, this rainfall event bear most feature of the so-called"refluence heavy rain", which presents a good example for the warm sector heavy rainfall event over South China.(3) The heavy rain formed near the convergence region over which at 200 hPa it was dominated by divergent airflows. However, the divergent airflow pattern is different from that of the typical airflow pattern usually found during the first rainy season of South China. Instead of exposing to the east of South Asian High, the upper level divergent zones of this warm sector event are located west to the over sea South Asian High, and are composed by the southeast and south flows; The divergent flow at high level through pumping action caused ascending movement and ultimately reinforced the heavy rain. (4) The MβCSs manifest a backward propagation and feature with new convections form downstream from the existing one, which is distinguished from the forward propagation, another typical MβCSs propagation mode usually found over South China; Besides the favorable large-scale environment condition, the interaction between local circulation and low-level flow from the preceding MβCS may play an important role in the development and evolution of the new MβCSs.(5) The heavy rainfall is also associated with the activities of a Low Level Jet (LLJ). The LLJ not only transport abundant warm and moist air to the heavy rainfall area, providing an advantage for the accumulation of unstable energy and generation of the MβCSs, the corresponding low level warm advection is considered playing a more directly effect in the development of convection. The area of convection development corresponds well to the positive center of temperature differential advection, which found to be a key indicator for the convective system. As a triggering mechanism for the organized convective system, the effect of the low level warm advection deserves our attention.(6) Diagnostic analysis of helicity in environment field shows that heavy rain forms in a position where low-level mean helicity ASRH1 positive center value corresponds to high-level negative mean helicity ASRH2. The movement of total helicity high center value reflects the tendency of heavy rain and its associated MβCSs and has a better indicative significance for the heavy rainfall area. This also suggests that the formation of vertical rotation mechanism is important for the development and evolution of warm sector heavy rain.(7) Analysis of moist potential vorticity shows that severe precipitation area basically located at the position where negative center of MPV1 and positive center of MPV2 coordinates with each other on 850 hPa. This configuration, with convective instability in atmosphere, benefits the development of vertical vorticity. The convective instability of environment is of great importance to formation of MβCSs. Furthermore, as the magnitude of MPV1 is larger than that of MPV2 over 1 order, it reveals again the fact that warm sector heavy rain forms in a weak baroclinic environment. |
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