PARAMETERS CONTROLLING PRECIPITATION ASSOCIATED WITH A CONDITIONALLY UNSATURATED UNSTABLE FLOW OVER A TWO-DIMENSIONAL MESOSCALE MOUNTAIN

Idealized simulations for a conditionally unstable flow over a two-dimensional mountain ridge are performed to investigate the propagation and cloud types of precipitation systems controlled by the unsaturated moist Froude number (Fw) and the convective available potential energy (CAPE). In addition to the three flow regimes found in Chu and Lin (CL) and Chen and Lin, we have found a fourth flow regime (Regime IV), which is characterized by a stationary orographic stratiform precipitation system. We found that the orographic blocking or resistance and cold outflow associated with evaporative cooling are acting against the basic flow, which then control the propagation and cloud type of the precipitation systems. Briefly speaking, we may characterize the flow regimes for a conditionally unstable flow passing over a mesoscale mountain as: (1) Regime I (small Fw and large CAPE): flow with upstream propagating convective system and a transient convective system existing in the vicinity of the mountain at earlier time; (2) Regime II (small Fw and small CAPE): flow with stationary orographic convective system; (3) Regime III (large Fw and large CAPE): flow with downstream propagating convective system and stationary orographic convective or mixed convective and stratiform precipitation system; and (4) Regime IV (large Fw and small CAPE): flow with stationary orographic stratiform precipitation system. For moderate Fw or CAPE, the flow regime can be determined by the relative strength of Fw and Fd, where Fd is the forcing associated with cold-air outflow, which is related to CAPE.