Wind Profiler Observations of the Meiyu/Baiu Precipitation in the downstream of the Yangtze River

Detailed observations of the Baiu/Meiyu frontal precipitation were acquired by several mobile platforms (three Doppler radars, a wind profiler system, three surface automatic weather stations) in the downstream of the Yangtze River for two campaigns of intensive observation (for about 50 days during June and July) period (IOP) in the years 2001 and 2001. For the first time, Frontier Observational Research System for Global Change (FORSGC) deployed a Lower Atmospheric Wind Profiler (LAWP) with Radio acoustic sounding System (RASS) at Dongshan (31°4'47" N; 120°26'3" E) in the Jiangsu province, about 120 km west of Shanghai, PR China. The two IOP data analysis suggested that the most of the time Meiyu/Baiu (heavy) precipitation tended to occur when the southwesterly low-level jet became strong under moist neutral stratification and strong gradient of equivalent potential temperature. During the heavy rainfall the LAWP can be used to provide clues for the forecasting of the maximum strength of winds and the arrival times of strong winds and gales. Observational results also indicate that the LAWP could help to improve the understanding of the atmospheric processes involved in severe weather during typhoon, clod front passage. The results suggest that convective boundary layer (CBL) height at Dongshan varies between 1 and 1.5 km and the CBL evolution depends on variety of factors and is not simply related to any local surface meteorological variables. The low boundary heights at Dongshan during July are probably related to low Bowen ratios (ratio of sensible to latent heat flux at the surface) and very high humidity. The CBL depth also indicates the prevailing synoptic situations during the Meiyu/Baiu season. We developed a simple algorithm to classify each profile into convective, transition (mixed convective-stratiform) and stratiform rain based on the wind profiler observations of the (Reflectivity, Doppler velocity and Spectral width) vertical structure of the precipitating clouds during Meiyu/baiu season. The highest percentage of occurrence appears in the evening and nighttime (18:00 to 20:00 BST). The peak of stratiform precipitating cloud has smaller value and comes later than the convective clouds. The time delay between the peak of the stratiform and convective precipitating cloud corresponds to the life cycle of the mesoscale convective system. In this presentation, we also discuss the similarities of the Meiyu/Baiu precipitating cloud systems observed during IOP 2001 and 2002 and also emphasize the characteristics difference of the frontal systems in the two IOP periods.