Observations and modeling of banded orographic convection

 Shallow convective rain events are common in post-frontal flow over coastal mountain ranges of the Pacific northwest US.  Some of these events are characterized by banded, quasi-stationary precipitation patterns on small scales which can focus substantial amounts of rainfall over localized areas.  In this study three cases of convective orographic precipitation exhibiting varying degrees of organization are analyzed.  All events occur over a section of the Oregon Coastal Range located close to the Portland, OR NEXRAD site, allowing for high-resolution radar images of the convective formations.  During a six hour interval, the first case (9-10 Apr. 2002) developed weak convective cells which transitioned to organized bands for less than one hour and back into cells at later times.  The second case (12-13 Nov. 2002) had well-organized linearly-oriented bands that persisted over the mountains for over four hours without changing significantly in location, causing significant precipitation accumulations (over 69 mm in four hours) in preferred locations.  Convective structures were the most disorganized in the third case (12 Oct. 2003), which was characterized by slightly elongated cells that, once formed over the elevated terrain, advected downstream with the low-level flow.

Quasi-idealized numerical simulations of these three rainfall events qualitatively reproduce the varying behavior of the orographic convection.  Additional numerical simulations are used to isolate environmental and terrain-related factors responsible for the convective dynamics.  Three physical parameters that determine the convective structures in these three cases are found to be the vertical wind shear, the low-level upstream thermodynamic profile, and the presence of small-scale irregularities on the mountainous terrain.