A series of severe convective storms affected Guangdong Province and produced disastrous heavy rain in Guangzhou especially on August 11,2004.This paper introduces the elementary formation mechanisms for clear air-echo before severe convection and trys to extract the dissipation rate of turbulent energy ε from the spectral data of CINRAD/SA in Guangzhou.The results show that the turbulence energy dissipation rate in the low atmosphere layer obviously increased before the severe convective storms occurred,which reveals the precursory character of this weather phenomena.