Abstract: The torrential rain process on July 23, 1992 over Beijing-Tianjin-Hebei region is simulated by using the model MM4. With the model products of high temporal and spatial resolution, some kinds of numerical experiments employing numerical models Mass & MM4 are designed here to demonstrate the effects of underlying surface mechanical and thermal forcing on the torrential rain process. It is pointed out that the strong upward motion associated with surface convergence forced by underlying surface is the trigger mechanism of the torrential rain process.