The influence of orographic Rossby and gravity waves on rainfall

Orography is known to influence winter precipitation in Asia and the Indo-Pacific Ocean by mechanically forcing winds, but the dynamics involved in the precipitation response are incompletely understood. This study investigates how two types of orographically forced oscillatory motions alter time-mean winds and precipitation. The quasi-geostrophic omega equation is used together with previous ideas of ` downward control' to develop a simple theory of the spatial distribution and amplitude of the vertical motion response to orographically forced stationary Rossby waves and gravity wave drag in a precipitating atmosphere. This theory is then used to understand the response of the boreal winter atmosphere to realistic Asian orography in a global numerical model that includes representations of moist processes and unresolved gravity wave drag. We isolate the effects of the orographically forced stationary Rossby wave and the gravity wave drag by incremental addition of wave sources in this model. The peak precipitation response to the forced Rossby wave is about a factor of three larger than that of the gravity wave drag, but the wave drag response has a distinct spatial structure that dominates in some regions. Both the Rossby wave and the gravity wave drag perturb precipitation in areas distant from orography, producing shifts in the equatorial intertropical convergence zone, an increase in precipitation over southern Asia, and drying in much of northern Asia.