Sensitivity of Orographic Precipitation to Changing Ambient Conditions and Terrain Geometries: An Idealized Modeling Perspective

Abstract

This paper utilizes the fifth-generation Pennsylvania State University?National Center for Atmospheric Research (PSU?NCAR) mesoscale model (MM5) in a two-dimensional (2D) configuration at 4-km horizontal grid spacing in order to better understand the relationship between orographic precipitation and the height and width of a barrier, as well as the ambient flow, uniform moist static stability, and freezing level. The focus is on how these parameters affect the orographic precipitation by changing the circulation and microphysical structures over the barrier. As the low-level flow becomes blocked for moist nondimensional mountain heights greater than 3.0, there is a rapid upstream shift in the precipitation maximum and a reduction in precipitation over the upper windward slope. For the terrain geometries used in this study (500 to 3500 m high and 25- to 50-km half-width), the maximum precipitation is a strong function of barrier slope for relatively weak upstream flow (U = 10 m s?1). For moderate wind speeds (U = 20 m s?1) and a freezing level of 750 mb, melting effects lower the freezing level more along the windward slope as the mountain half-width and height increases for barrier slopes greater than 0.03. As a result, a low (1000 m) and narrow (25-km half-width) barrier has a greater surface precipitation maximum than a high (2000 m) and wide (50-km half-width) mountain of equivalent slope since the smaller barrier has more efficient warm rain processes occurring along the windward slope. For wind speeds greater than 20 m s?1, a wider and higher barrier has a greater precipitation maximum since it has a more extensive orographic cloud, while a narrower barrier has more precipitation advecting into the lee. The precipitation distribution is highly dependent on how the terrain-induced gravity wave modifies the circulation aloft. Even in the unblocked flow regime, the precipitation builds upstream of the crest for winds greater than 20 m s?1, since strong flow favors a large vertical wavelength of the mountain gravity wave, and therefore a deep layer of upward motion over the lower windward slope. Both a narrower barrier and weaker stability favor less tilt to the mountain wave, resulting in a more collapsed circulation above the crest and more precipitation spillover. Reverse shear above the crest favors low-level wave amplification and a windward shift in the precipitation, while forward shear favors a weaker mountain wave over the crest and more precipitation advection into the lee. Finally, a freezing level raised from 750 to 500 mb collapses the precipitation distribution over the windward slope with less leeside spillover, therefore the windward precipitation efficiency remains high (>90%) at strong (>20 m s?1) wind speeds.