Abstract:Based on the high resolution NCEP reanalysis data the helicity, moist potential vorticity (MPV) and some other essential physical quantities were computed, and then combined with Doppler Radar products, the associated thermodynamic and dynamic mechanisms for the occurrence and evolution of an extremely heavy rain in eastern Guangdong caused by the tropical storm (0903) Linfa were analyzed. The results suggested that the southwesterly jet in the periphery of typhoon trough provided abundant moisture and thermodynamic conditions. At mid low levels over the rainfall region, the equivalent potential temperature (θse) surfaces were steep and decreased with increasing altitudes, i.e. θse/p＞0. Meanwhile at low levels over the same area, the MPV was remarkably high negative. These two conditions indicated a strong convectional instability. Strong convergence at low levels and divergence at high levels, mesoscale circulation and intense rotational ascending motion provided the dynamic conditions. It was also shown that the location and time of high θse tongue and low MPV tongue overlapped agreed with those of the heavy rain, and the positive center at mid low levels and negative center at high levels of the vertical helicity were coincided with the rainfall center. Additionally, heavy precipitation tended to take place during the period when the rotational ascending motion rapidly intensified; and the echo characteristics also showed that the extremely heavy rain is produced by the high precipitation rate with long duration of precipitation.