Vertical Structures of Precipitation in Cyclones Crossing the Oregon Cascades

Abstract

The vertical structure of radar echoes in extratropical cyclones moving over the Oregon Cascade Mountains from the Pacific Ocean indicates characteristic precipitation processes in three basic storm sectors. In the early sector of a cyclone, a leading edge echo (LEE) appears aloft and descends toward the surface. Updraft cells inferred from the vertically pointing Doppler radial velocity are often absent or weak. In the middle sector the radar echo consists of a thick, vertically continuous layer extending from the mountainside up to a height of approximately 5?6 km that lasts for several hours. When the middle sector passes over the windward slope of the Cascades, the vertical structure of the precipitation exhibits a double maximum echo (DME). One maximum is associated with the radar reflectivity bright band. The second reflectivity maximum is located approximately 1?2.5 km above the bright band. The secondary reflectivity maximum aloft does not appear until the middle sector passes over the windward slope of the Cascades, suggesting that this feature results from or is enhanced by the interaction of the baroclinic system with the terrain. In the intervening region between the two reflectivity maxima there is a turbulent layer with updraft cells (>0.5 m s?1), spaced 1?3 km apart. This turbulent layer is thought to be crucial for enhancing the growth of precipitation particles and thus speeding up their fallout over the windward slope of the Cascades. In the late sector of the storm, the precipitation consists of generally isolated shallow convection echoes (SCEs), with low echo tops and, in some cases, upward motion near the tops of the cells. The SCEs become broader upon interacting with the windward slope of the Cascade Range, suggesting that orographic uplift enhances the convective cells. In the SCE period the precipitation decreases very sharply on the lee slope of the Cascades.