Separation of the mesoscale flow field of an abrupt rainstorm in the central and southern parts of Shaanxi was conducted with three various mesoscale filtering methods. It is shown that there exist mesoscale vortexes or convergence lines in the lower atmosphere over the corresponding precipitation zone when an abrupt rainstorm happens. All the mesoscale systems from 200 to 1200 km can be separated from conventional radiosonde data. The selection filter is good for separating the low-level mesoscale systems;the station filter is simple,feasible and suitable for the middle-level;and the 9-point filter is applicable to both,whose filtered mesoscale convergence system is situated on the upstream side of the rainstorm zone.