Abstract: Diagnosis is performed of the abrupt rainstorm on 4 —5 July, 2002 in Zichang County of north Shaanxi Province under the influence of a far-away typhoon.Result s suggest that the rainfall event happens due to the repeated occurrence of a meso-β intensely convective cluster moving over the study county.Obviously, the interactions between weather systems at low and middle latitudes are responsible for the moisture, instability, dynamic condition and a synoptic circulation background that favor the genesis, development and repetitive occurrence of the meso convective cloud cluster over the study county.Diagnostic study of moist potential vorticity (MPV)indicates that the typhoon traveling toward the northwest leads to the fact that 1)the low-level negativeMPV1 intensif ication is accompanied by the positiveMPV1 enhancement in the higher troposphere, thereby creating a condition for the accumulation of unstable energy in large amounts at lower levels ;2)the rainstorm area gives rise to the favorable dist ribution of MPV barotropic and baroclinic terms in vertical answerable for the generation and development of the cloud cluster ;3)the area of close-together MPV2 contours at the 850 hPa level is coupled with the MPV1=-2 PVU meso convectively unsteady co re, which is clearly indicative of the production and evolution of the cluster downstream of the coupling.Diagnosis also show s that w hen the typhoon moves north by east the instability occurs at lower and high levels over the rain zone ;the area ofMPV2 close-together contours at 850 hPa moves eastward and positiveMPV2 is intensified, resulting in dramatic release of accumulated convectively unsteady energy in the study county.