Analysis on a torrential rain storm caused by local weak convection in the front of Yunnan Myanmar pressure ridge

A local rainstorm that caused landslide and debris flows on 7 August 2005 was analyzed by synergically combing data from conventional observation,intensified automatic observation, the NCEP 1°×1° reanalysis and Doppler weather radar, using methods of scale separation and diagnosis of physical parameters. Results showed that a meso-cyclone developed in the shear line near Ailaoshan in the front of Yunnan Myanmar pressure ridge on 700 hPa was the main weather system causing the local heavy precipitation. Local rainstorm occurred in a weak convective thermal environment which has the characteristic of convergence at low-level and divergence at upper-level. Strong water vapor convergence at low-level provided water vapor supply which caused this local rainstorm. Before the onset of the severe rainstorm, vertical and horizontal helicities presented as spiral updraft at low-level and downdraft at mid-to-high level. Heavy precipitation happened during the weakening phase of the spiral updraft. Analysis of satellite images indicated that heavy rainfall happened in the place of convective clouds where temperature gradient increased rapidly. Doppler radar echo intensity and top height did not show the characteristics of strong convections. By analyzing Doppler radar radial velocity, it is found that an anti-wind region formed at local rainstorm area. The main cause of the local rainstorm was a meso-β scale convective system that has maintained for a long time because newly developed weak convective cells move southward continuously to heavy rain area.