The randomized statistical experiments of precipitation enhancement performed in the Gutian Reservoir for 12 years (1975-1986) has gained achievements for scientific research,and have had international influence in the world.For the continuous study of precipitation enhancement in the Gutian Reservoir,the cloud echo characteristics and operational conditions of precipitation enhancement in the Gutian experiment area are analyzed based on the weather analysis and data of Doppler weather radar,sounding,ground rainfall and other observations from April to June of 2008-2012.The cloud echo characteristics in this study include echo intensity(Z),echo area(S),echo top(H),vertical integrated liquid water(VIL) and echo moving direction by Doppler radar,located in Jianyang,on the northwest side of the Gutian experiment area.Combining the sounding data and echo characteristics,the depth of the negative temperature layer is calculated,and the cloud echo types are also confirmed by the echo characteristics.The results show that the weather systems influencing the Gutian experiment area include the low vortex shear,warm-sector convergence,upper trough,continental high,subtropical high pressure,and typhoon or intertropical convergence zones.There are evident differences between the cloud echo characteristics and precipitation features in the different weather systems.Under low vortex shear,the stratus-cumulus mixed clouds are more numerous than cumulus,the rainfall intensity is greater,and the cloud cover time is longer.The convection clouds are more active under warm-sector convergence,upper trough and continental high than the others,indicating local precipitation and short lifecycle.Unlike warm-sector convergence and upper trough,the rainfall intensity and cloud cover time of stratus-cumulus mixed clouds are nearly half the size as under continental high.The weather systems in the above-mentioned influenced the Gutian experiment area mainly from April to June of 2008-2012.Then the subtropical high pressure zone and typhoon or intertropical convergence zone occurred rarely in the Gutian experiment area,but they all had much higher rainfall intensity and more cloud cover time than the others.Overall,the precipitation cloud systems in the Gutian experiment area are mainly stratus-cumulus mixed clouds,followed by cumulus.The echo intensity of the cumulus clouds are higher than the stratus-cumulus mixed clouds,whose center intensity are mostly higher than 40 dBz,presented as block structures with vigorously vertical development.The horizontal scales of the cumulus clouds in the Gutian experiment area are nearly equal to the vertical scales.The stratus represents many small cumulus clouds embedded in the large area of stratus.The echo center intensity of the stratus can also be higher than 40 dBz partially.The structure of the stratus-cumulus mixed clouds is favorable to rainfall.Its strong echo area S_{>25 dBZ} (S_{>25 dBZ} referring to the area where the echo intensity is larger than 25 dBz) is is clearly larger than that of the cumuliform clouds,while its average echo height and echo top are both lower than those of the cumuliform clouds.The vertical integrated liquid water in the cumuliform clouds is much greater than that in the stratus-cumulus mixed clouds,and the depth of the negative temperature layer of both types of clouds exceeds 2 km.The maximum echo intensity,strong echo area S_{>25 dBz} and depth of negative temperature layer of the stratus-cumulus mixed clouds all correspond well to the regional mean daily rainfall.The operational indexes of the stratus-cumulus mixed clouds in the Gutian experiment area are as follows:the echo intensity should be larger than 25 dBz,S_{>25 dBz} larger than 400 km²,echo top higher than 5.5 km,depth of negative temperature layer larger than 1.5 km,and vertical integrated liquid water more than 1 kg/m².