Organization of oceanic convection during the onset of the 1998 East Asian summer monsoon

The South China Sea Monsoon Experiment (SCSMEX) was conducted during May and June of 1998 to study the onset of the East Asian summer monsoon. The BMRC C-POL and NASA/NOAA TOGA radars were deployed during the experiment in a dual-Doppler array near Dongsha Island in the northern part of the South China Sea to investigate the nature of convection during the monsoon onset. In addition to the radars, a special sounding network, wind profilers, surface meteorolgical stations, and oceanographic buoys were established during SCSMEX.

Prior to the monsoon onset on 15 May, convection over the northern South China Sea was generally shallow and scattered in nature. However, at the time of onset, a band of convection moved off the South China mainland and passed over Dongsha with an accompanying wind shift and temperature drop characteristic of a cold front passage. However, this continental influence was short-lived, as convection the next day passed over Dongsha from the southwest along with a return to warmer temperatures. During the five days following onset, convection in this region alternated several times between mesoscale convective systems (MCSs) moving off the China mainland and MCSs moving in from the south or southwest.

Soundings over the northern South China Sea indicate convection during monsoon onset existed in an environment of modest instability (CAPE < 1000 J/kg) and weak low-level shear. The MCSs were generally short-lived (< 3 h), except in instances where frontal forcing or other continental influences (e.g., land breezes) appeared to be present. The orientation of convection with respect to the shear will be examined in light of recent findings for tropical oceanic convection in TOGA COARE (LeMone et al. 1998).

Recent studies of midlatitude MCSs has revealed three distinct modes of organization: convection with trailing (TS), leading (LS), and parallel (PS) stratiform precipitation (Parker and Johnson 2000). Preliminary investigation of SCSMEX convective systems reveals MCS structures over the northern South China Sea of the TS, PS, and possibly even the LS modes. From the standpoint of heavy rainfall, the PS and LS modes are significant since they are often characterized by cells repeatedly moving (in the case of PS) or forming (in the case of LS) over the same location. An analysis of the dominant modes of MCSs over the northern South China Sea during the monsoon onset will be presented.

References

LeMone, M.A., E.J. Zipser, and S.B. Trier, 1998: The role of environmental shear and thermodynamic conditions in determining the structure and evolution of mesoscale convective systems during TOGA COARE. J. Atmos. Sci., 55, 3493-3518.

Parker, M.D., and R.H. Johnson, 2000: Organizational modes of midlatitude mesoscale convective systems. Mon. Wea. Rev., 128, 3413- 3436.