Instability Characteristics of Radar-Derived Mesoscale Organization Modes within Cool-Season Precipitation near Portland, Oregon*

Abstract

Abstract The instability characteristics associated with different radar-derived mesoscale organization modes are examined using six cool seasons of operational scanning radar data near Portland, Oregon, and operational sounding data from Salem, Oregon. Additionally, several years of Microwave Rain Radar Ka-band vertically pointing radar data from Portland and Merwin, Washington, are used to characterize the nature and occurrence of generating cells and fall streaks. The combination of a new metric, convective-stratiform intermittency, with the classification of radar reflectivity maps into convective and stratiform precipitation types was applied to periods when the freezing level was >1.4-km altitude. This method distinguishes periods with embedded convective within stratiform mesoscale organization from those that were mostly convective or mostly stratiform. Mesoscale organization occurs in a continuum of states with predominantly stratiform structure occurring most frequently. Generating cells in the snow layer are common in cool-season storms and are primarily associated with potential instability aloft. For mostly stratiform and embedded convective within stratiform 3-h periods, the vertically pointing radar data showed nearly ubiquitous fall streaks in the snow layer originating above 3-km altitude. Stronger generating cells enhanced reflectivity in the rain layer consistent with a seeder mechanism. Stronger generating cells were more common during embedded convection within stratiform than in mostly stratiform periods. Nearly all embedded periods have active or latent (potential) instability. Hydrostatic instability more typically occurred at higher altitudes for embedded convective within stratiform periods compared to mostly convective periods. The occurrence of vertical wind shear instability was primary below 2-km altitude and was not typically associated with levels with generating cells.