The development of mesoscale convective systems during the onset of South China Sea summer monsoon

The South China Sea Monsoon Experiment (SCSMEX) was conducted in South China Sea (SCS) and surrounding areas during MayæJune 1998. The primary goal of the experiment is to provide a better understanding of the key physical processes for the onset, maintenance and variability of the monsoon over the Southeast Asia and southern China leading to improved monsoon predictions. As the first comprehensive field experiment aiming on the SCS summer monsoon, the goals of the mesoscale program associated with the primary objective are: 1) to define the initiation, structure, evolution and dynamics of precipitation processes associated with the onset and mature phase of the SCS monsoon, and 2) to obtain quantitative rainfall estimates, vertical air motion and inferences on microphysical structure over a tropical oceanic site. Prior to the monsoon onset in the northern SCS on 15 May 1998, depressed conditions were observed for the most of time. Convection was primarily in the form of scatted precipitating congestus, with isolated cumulonimbus cells. The first strong and deep convection during SCSMEX IOPs appeared in the northern SCS in the morning of 15 May 1998 when the low-level airflow started to shift from easterly to southwesterly flow. The onset of southwesterly at lowest levels atopped by a layer of weak southeasterly flow also generated a mild low-level wind shear. The broad mesoscale convection observed on 15 May originated from the southern coastal area of mainland China. When moving southward to the radar observing area, the mesoscale convective systems generally maintained its size and intensity. After passing the TOGA radar, the convective systems weakened gradually on its way further south to the southern SCS. In the next few days of transient period, the convective systems in the northern SCS exhibit a complexity of organization, orientation, and propagation. On 16 May, a series of "wave type" squall lines moved northeastward in the northern SCS in the direction similar to the low-level southwesterly flow. The squall lines intensified in the Doppler radar observing domain. New "secondary" bands generated perpendicularly between the original squall lines to form a circular convective bands with a radius of 70-100 km. Severe convective events such as waterspout were also observed. During 17-20 May 1998, more extensive and stronger convective systems including many squall lines appeared in a larger portion of the SCS.