This project is targeted to the following four broad scientific questions focusing on the prefrontal and/or warm-sector linear convective lines and nonlinear cloud clusters in central Guangdong province during mid-April to mid-June.
(1) The roles of boundary layer processes and underlying surface on the initiation and evolution of the heavy-rain-producing MCS
The topography of Guangdong province is very complicated which involves land-sea, mountain-valley, and city-rural contrasts. The nonlinear interaction between underlying surface and large-scale environmental flow may lead to the development of mesoscale disturbance such as the convergent lines and/or vortex to trigger convection initiation. Land-sea contrast may produce apparent convergence in coastal area during land breeze phase from mid-night to early morning and apparent convergence in inner land area during sea breeze phase from afternoon to early evening (Bao 1986). The trumpet shape mountain-valley distribution around Zhujiang River delta area is favorable for the convergence between westerly or southwesterly flow and southeasterly flow. Heating differences between city and rural areas in the central part of Guangdong province may trigger local mesoscale circulation and thus mesoscale disturbance. Mesoscale boundaries such as convergent lines may form due to the complex interaction between above mentioned various local mesoscale circulations forced by topography and PBL processes or between the local mesoscale circulations and background flow.
This project will use the obtained comprehensive data set and numerical simulation and dynamic diagnoses to examine the roles of boundary layer processes and underlying surface (such as sea breeze, mountain-valley wind, urban heat island, PBL rolls, convergent lines, and mesoscale convective vortex) on the initiation and evolution of the heavy-rain-producing MCS.
(2) The role of low level jet (LLJ) on the development/maintenance of the warm-sector heavy-rain-producing MCS
LLJ is one of the main weather components that may trigger the heavy rainfall in south China in early summer rainy season (Bao 1986; Ding et al. 2011). About 75-80% of the heavy rainfall events (>50 mm/day) are associated with LLJ (Zhao and Wang 2009). Statistic shows that 16 out of 19 strong LLJ processes in Guangdong during May and June of 1970-1973 were followed by heavy rainfall of > 100 mm/day (Bao 1986). About 94% of heavy rainfall events in North Taiwan are also accompanied by LLJ at 850hPa (Chen et al. 2005). Studies show that the appearance of LLJ above 2 km is usually a couple of hours ahead of the occurrence of severe heavy rain (Liu et al. 2003; Cao et al. 2006). The appearance of severe heavy rain is better associated with the intensification of LLJ and downward expansion to below 1.5-2 km. It is hypothesized that the intensification and downward expansion of LLJ play important roles in triggering the severe heavy rain likely through providing moisture, heat, energy and disturbance and/or trigger inertial gravity wave.
This project will use the obtained comprehensive data set and numerical simulation and dynamic diagnoses to examine the evolution features of the LLJ, the interaction between LLJ and upper tropospheric systems (such as south Asia high) and its impact on the initiation and development of warm-sector heavy-rain-producing MCS.
(3) The microphysical processes in the heavy-rain-producing MCS
Microphysical processes in the heavy-rain-producing MCS are closely related to the precipitation properties such as the intensity, efficiency, lightening, and more importantly the accuracy of microphysics parameterization scheme of NWP. The rain rate in the warm sector is generally much larger than that at front which may be closely related to microphysical properties. Observations on the microphysical properties of the MCSs in early summer rainy season in South China were very limited in previous field experiments due to the unavailability of polarimetric radars. Numerical studies indicated that both ice cloud and warm cloud could be the main microphysical process of the MCSs in early summer rainy season in South China (e.g., Wang et al. 2002).
This project is aimed to examine the key microphysics and the distribution of hydrometers in warm-sector heavy-rainfall-producing MCS in South China based on the dual-polarimetric radar, micro rain radar, millimeter wave cloud radar, raindrop disdrometer in combination with satellite products such as TRMM and CloudSat.
(4) Differences of the warm sector and frontal heavy rainfall before and after the onset of SCS monsoon
Heavy rains in warm sector and at front are the two main rainfall categories in early summer in South China. Previous case studies show that these two kinds of heavy rain have apparent differences in the rainfall features and their formation environment (Zhao et al. 2008; Zhang and Ni 2009). Heavy rains in warm sector are more convective and intensive than at front. Warm-sector heavy rain has more direct influence from monsoon, less direct influence from cold air, stronger low-level vertical wind shear, smaller convective inhibition, higher column precipitable water relative to that at front. The two kinds of rainfall may also have different microphysical processes and different initiation and maintenance mechanisms partly due to possible different impacts from mountain-valley and land-sea contrasts.
This project will use the obtained comprehensive data set and numerical simulation and dynamic diagnoses to examine the different convection-initiation mechanism and microphysical properties of the warm sector rainfalls before and after the onset of monsoon under the apparently different common flow and moisture supply, as well as the differences in the evolution and microphysical processes of warm-sector and frontal rainfall, the isolated and linearly organized convections.
Chinese Academy of
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