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Polar Lows over the Gulf of Alaska in Conditions of Reverse Shear

Nicholas A. BondJoint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle, Washington

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M. A. ShapiroNational Oceanic and Atmospheric Administration, Wave Propagation Laboratory, Boulder, Colorado

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Abstract

The formation of two polar lows over the Gulf of Alaska are studied, using observations taken during the OCEAN STORMS field experiment. Synoptic-scale and mesoscale analyses were constructed using NOAA P-3 aircraft flight-level and radar data and drop windsonde profiles, in addition to conventional data sources. The synoptic-scale analyses show that polar low development occurred in a low-level mesoscale baroclinic zone near the center of a mature, occluded synoptic-scale low pressure system. The disturbances propagated along the zone in the reverse shear sense, i.e., in the direction opposite to the thermal wind within the zone. The mesoscale analysis reveals a boundary layer jet of 38 m s−1 on the cold side of the baroclinic zone accompanying the leading polar low. Indirect evidence suggests that a Sawyer-Eliassen secondary circulation was present; polar low development occurred in the region of a frontogenetical geostrophic deformation. Convective activity was not prominent during the growth phase of the polar lows, as determined from satellite imagery and radar reflectivity measurements. Extremely high ocean waves (∼13 m) occurred in response to intense wind forcing on the mesoscale on a sea state preconditioned by moderate forcing on the synoptic scale. The observed synoptic- scale and mesoscale structures are compared with results from previously studied polar lows. This case appears to represent an example of polar low development due primarily to moist baroclinic processes.

Abstract

The formation of two polar lows over the Gulf of Alaska are studied, using observations taken during the OCEAN STORMS field experiment. Synoptic-scale and mesoscale analyses were constructed using NOAA P-3 aircraft flight-level and radar data and drop windsonde profiles, in addition to conventional data sources. The synoptic-scale analyses show that polar low development occurred in a low-level mesoscale baroclinic zone near the center of a mature, occluded synoptic-scale low pressure system. The disturbances propagated along the zone in the reverse shear sense, i.e., in the direction opposite to the thermal wind within the zone. The mesoscale analysis reveals a boundary layer jet of 38 m s−1 on the cold side of the baroclinic zone accompanying the leading polar low. Indirect evidence suggests that a Sawyer-Eliassen secondary circulation was present; polar low development occurred in the region of a frontogenetical geostrophic deformation. Convective activity was not prominent during the growth phase of the polar lows, as determined from satellite imagery and radar reflectivity measurements. Extremely high ocean waves (∼13 m) occurred in response to intense wind forcing on the mesoscale on a sea state preconditioned by moderate forcing on the synoptic scale. The observed synoptic- scale and mesoscale structures are compared with results from previously studied polar lows. This case appears to represent an example of polar low development due primarily to moist baroclinic processes.

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