Analysis Of Multi-scale Characters Of Two Heavy Rainfall Events In South China In May 2013 Before And After The South China Sea Summer Monsoon Onset

There happened two heavy rain processes in South China on 7–12 and 14–17 May in 2013 respectively, which are typical heavy rainy stages before and after the onset of SCS monsoon. Based on the field intensive observation of the South China Monsoon Rainfall Experiment(SCMREX) and other reanalysis data, a multi-scale analysis combined with the role of the South China Sea(SCS) summer monsoon of the two heavy rainfall events(7–12 and 14–17 May) in South China in 2013 was conducted. And the connection between the mesoscale and the large scale systems is discussed. Main results of the paper are showed below:1. The large-scale conditions of South China rainstorm before and after the outbreak of the SCS summer monsoon: During the heavy rainfall event(7–12 May) before the onset of SCS monsoon, forms two rain-bands which mainly influenced by the southward invasion of cold front and the convergence of warm moist airflow from tropical region in prefrontal warm area respectively. The precipitation in two rain-bands is less convective, its water vapor is mainly derived from the mid-latitude westerly transport, and its underlying physical mechanism is quasi-balanced thermal adaptation process. Meanwhile, the warm sector rainstorm during 7–12 May occurred under the condition of the atmospheric baroclinic instability induced by the strong low vertical wind shear. Differently, during the second process(14–17 May) after the establishment of SCS monsoon, one rain-band centers in prefrontal warm area, which nature is tropical monsoon rainfall and more convective. Due to the onset of monsoon, the southwester in the lower layers enhance, so that the water vapor entering from the southwest border in South China significantly increase, that results in the water vapor inpreciptation of 14-17 days mainly transported from the SCS. Therefore it creats more concentrated and convective precipitation than the former event, so that the underlying physical mechanismit is better reflected. Moreover, the strong coupling between the high-level and low-level jet streams cause the warm sector rainstorm during 14–17 May unlike the former one.2. The different features of mesoscale convective systems(MCSs) before and after the SCS monsoon onset: Firstly, the directly mesoscale influence systems during the first process(7-12 May) are MβCSs. They are local distribution notably and moving slowly. The large-scale atomospheric condition of MCSs show that lower water vapor is enough, convective available potential energy(CAPE) is strong in combination with moderate vertical wind shear in lower layers, but no low level jet(LLJ) exits. The strong convection activities are mainly affected by local terrain so that MCSs distribute at coastal areas of southwest area in Guangdong. While, the couterparts of the second process(14-17 May) are MαCSs composed of multiple MβCSs and eventually formed the mesoscale convective complexes(MCC), which are the most significant of mesoscale systems on the second events. MCC are centralized with wider range and form flaky covering the whole of Guangdong province with faster moving speed. The most significant large-scale circulation difference between the two processes is jet exit in both high and low levels during second event. Because the SCS monsoon, SCS becomes CAPE high storage center which enhanced the CAPE in Guangdong and atmospheric dynamic instability, then lead to more significant MCSs. In addition, the movement and development of strong convection is mainly caused by environmental factors.3. The impacts and differences of large scale circulation factors on MCSs evolution before and after the SCS summer monsoon: The conceptual models using the change and role of large scale circulation factors in MCSs evolution process, respectively outline the each stage of MCSs on 8May and the MCC development phase of 15-16 May. It shows that the dry intrusion, convective instability, low altitude wind field and water vapor convergence play important roles in the formation and development of MCSs before and after the monsoon. The dry intrusion from upper atmospheric turbulence will cause the local absolute vorticity positive change on lower levels and enhance the lower atmosphere convective instability obviously. Moreover, it is the leading cause of MCSs development that convergence enhancement of low-level wind field and moisture field at precipitation area. On account of no LLJ before the SCS summer monsoon, the evolution of MCSs at the pearl river delta on 8May closely is related to the position of low-level wind field convergence which provide the dynamic mechanism for MCSs generated and development. While, the LLJ is significant after monsoon onset that enhances the lower atmosphere convective instability. Therefore the dynamic mechanism of MCC development in 15-16 May is mainly created by the inflow of boundary layer and the cooperation of upper air divergence and lower convergence.4. The certain differences of internal cloud characteristics in the first and second process before and after the south China sea summer monsoon: Convections in the second process become more active affected by the onset of SCS summer monsoon, which makes the raindrop diameter at Sanshui in the second process is generally greater than in the event of precipitation for the first time. In addition, the temperature of cloud on each level is higher at second rainfall event after the outbreak of the SCS summer monsoon than before.