Dynamical and physical processes associated with orographic precipitation in a conditionally unstable uniform flow: Variation in basic wind speed

A series of systematic two- and three-dimensional (2D and 3D, respectively) idealized numerical experiments were conducted to investigate the combined effects of dynamical and physical processes on orographic precipitation with varying incoming basic-flow speed U in a conditionally unstable uniform flow. In addition to the three moist flow regimes found in Chu and Lin at lower wind speeds, a new flow regime, regime IV, is found for higher wind speeds (U > 36 m s-1) and is characterized by gravity waves and heavy precipitation and lack of upper-level wave breaking and turbulence over the lee slope. The transition from regime III to regime IV at 36 m s-1 is explained by the transition from upward-propagating gravity waves to evanescent flow, which can be predicted with a modified mountain wave theory. Although the basic features are captured well in low grid resolution (Δx = 1 km), high-resolution (Δx = 100 m) 2D and 3D simulations are required to resolve precipitation distribution and intensity at higher basic winds (U > 30 m s-1). Based on 3D simulations, gravity wave-induced severe downslope winds and turbulent mixing within hydraulic jump reduce orographic precipitation in regime III. A preliminary budget analysis indicated that, in regime IV, orographic precipitation further increases as a result of enhanced rain processes when the blocking effect of wave breaking vanishes.