Precipitation over concave terrain

Most barriers consist of concave or convex terrain on a wide variety of scales. Climatological data indicates that the windward side of concave terrain is usually wetter. There have been speculations regarding the possible role of concave terrain in funneling moist air and therefore enhancing precipitation.

In this model-based idealized study, the enhancement of precipitation by concave terrain is examined systematically. The concave terrain is idealized as a pair of left-right symmetric tilting ridges. The enhancement of the vertical motion and precipitation by concave terrain is evaluated by comparing simulations with the concave terrain and ones with reference terrain, a straight ridge with identical intercept area relative to the upstream wind. The relevant dynamics and physics are investigated based on diagnosis of mesoscale simulations and theoretical formulations.

It is demonstrated that for low terrain (i.e., linear regime), direct mountain lift dominates and the vertical motion induced by the concave terrain is weaker than that induced by the corresponding reference terrain. Associated with blocking and flow deflection, the funneling effect and enhancement of vertical motion by the concave terrain increases with the increasing non-dimensional mountain height. For higher terrain, reversed flow appears over the windward slope associated with severe blocking. The precipitation maximum moves upstream associated with low-level flow convergence. The distribution and enhancement of precipitation are sensitive to horizontal scales of terrain, terrain concavity, the earth's rotation, and the stability of the low-level air.