Wednesday, 7 August 2013: 1:30 PM
Multnomah (DoubleTree by Hilton Portland)
The mesoscale characteristics of cool season extratropical cyclones in the vicinity of Portland, OR in the Pacific Northwest are examined with emphasis on the precipitation and stability structures within the warm sector. Operational radar and upper air sounding data obtained over six cool seasons are analyzed. A new automated, objective radar reflectivity based method was developed for classifying precipitation spatial and temporal joint variability within 70 km radius of the Portland WSR-88D radar. The resulting continuum of states is described in terms of three broad categories: mostly convective, mostly stratiform, and embedded convection within stratiform precipitation. Over the 2002-2008 cool seasons, 60% of the 3-hour periods were categorized as mostly stratiform, 20% as embedded convective within stratiform precipitation, and 15% as isolated convective precipitation. As expected, embedded convection typically occurred within the warm sector near the surface cold front. Elevated hydrostatic instability and elevated convective cells associated with the seeder-feeder mechanism play a lead role in modulating precipitation structure within the warm sector. Hydrostatic instability more typically occurred at higher altitudes (> 2 km) for embedded convective within stratiform periods compared to during mostly convective periods. Vertically-pointing research radar data obtained over 3 years showed nearly ubiquitous fallstreaks in snow originating above 3 km for both stratiform and embedded periods. Fallstreaks in snow enhance the rainfall directly below and thus have a direct impact on precipitation intensity and variability in the rain layer. Our observations on embedded convection within stratiform precipitation in these storms are consistent with the forward-tilted cold front conceptual model.
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