Relationship between Bell-Shaped Terrain Dynamic Forcing, Mountain Wave Propagation, and Orographic Clouds and Precipitation

Orographic clouds and precipitation have a critical role in regional water cycles, fresh water resources, ecosystems, the environment, and climate change. The relationship between terrain dynamic forcing, mountain waves, orographic clouds and precipitation, and the formation mechanism were investigated in a conditionally unstable moist atmosphere on May 1, 2009, by using the Weather Research and Forecasting (WRF) Model and the wet Froude number (F_w). The results show that at a topographic height of 2 km and half-width of 10 km, when unstable airflow gradually increases from 2.5 to 25 m/s, the wet Froude number increases from 0.19 to 1.81. The terrain block has a major role when F_w≤1; orographic clouds induced by terrain dynamic lifting form on the windward side, and mountain waves form mainly over the windward side and propagate toward the upstream region, which forms stratiform clouds at first and then evolves as quasi-stable shallow convective wave clouds. The maximum precipitation occurs at the terrain crest near the windward side. When F_w is too small, no precipitation is produced. When F_w>1, the orographic clouds induced by terrain dynamic lifting form primarily at the terrain crest, and mountain waves form mainly over the leeward side and propagate toward the region downstream of the crest, producing quasi-stable shallow convective wave clouds in the downstream region. In addition, in a weak and conditionally unstable moist atmosphere, orographic precipitation is produced mainly by warm microphysical processes induced by terrain lifting, and mountain wave clouds cannot produce precipitation.