This paper reports on an investigation of the organization of mesoscale convective systems over the northern South China Sea during the onset of the 1998 East Asian summer monsoon using radar reflectivity data from the BMRC C-POL radar on Dongsha Island. Animations of the C-POL radar base-scan reflectivity within a 200 km radius of Dongsha Island are being used to determine the dominant modes of convective organization at six-hour intervals. Lines are classified as shear-parallel or shear-perpendicular if the line orientations are within 30 degrees of specific shear vectors or their perpendicular components for the low- and midtroposphere, following the definitions of LeMone et al. (1998) based on their study of Tropical Ocean Global Atmosphere (TOGA) Coupled Ocean-Atmosphere Response Experiment (COARE) convective systems.

It is found that lower and middle level vertical shears exert a dominant control over the structure and orientation of mesoscale convective systems in this region. The findings are consistent with those of LeMone et al. for TOGA COARE, except two new organizational modes have been identified: shear-parallel bands for strong low-level shear and weak midlevel shear when the air is dry aloft, and shear-parallel bands for strong shears in both layers when the shear vectors are in the same direction. Midlatitude influences likely contributed to these two additional modes by producing strong westerlies (in the latter case) during the passage of a strong upper-level trough and midtropospheric drying (in the former case) following passage of the trough. It is also found that the convective systems during onset in May have a low percentage (about 25%) of stratiform precipitation, quite different from the tropics as a whole but consistent with recent modeling results of Tao et al. Comparisons will be made of the characteristics of convective systems in June (three weeks following onset) with those in May.