Weather and Climate Systems

Moisture and thermal characteristics of Southern Plains Ice Storms: Insights from a Regional Synoptic Climatology and High-Resolution WRF-ARW Sensitivity Study

Esther Mullens
OU School of Meteorology

09 April 2014, 3:00 PM

National Weather Center, Room 5600
120 David L. Boren Blvd.
University of Oklahoma
Norman, OK

Winter storms, including snowstorms and ice storms, are an infrequent yet significant hazard for the Southern United States. This work examined characteristics of freezing precipitation events for the Southern Plains (SGP) by developing a regional spatial and synoptic climatology between 1993-2011 from a combination of sounding analysis, EOF and composite techniques. From this work and past literature, a hypothesis was proposed suggesting the Gulf of Mexico (GOM) Sea Surface Temperature (SST), as the proximal basin and major moisture source, impacts the severity of ice storms by modulation of the melting layer profile and moisture potential. This assertion was tested using high-resolution nested WRF-ARW sensitivity studies for two synoptically distinct winter storms (December 9-11 2007, January 28-30 2010) and six representations of the GOM SST field. These included the 1981-2010 climatology, a uniform 2 K perturbation to the control (control=real SST), and a physical upper and lower limit SST field for the warmest and coolest event date basin-average anomalies observed during 1981-2011. The control WRF simulation was evaluated against a suite of observations, including Oklahoma Mesonet, Stage IV precipitation, and ARM-ACRF to determine model efficacy to the observed evolution of key features.
The sensitivity simulations revealed discernable influence of SST on freezing precipitation. For the December 2007 event, SST impacts on melting layer intensity were weak in comparison to its existing magnitude, however negative SST departures produced stabilization of the profile by cooling the lower melting layer, whilst the upper melting layer was unperturbed due to its different air-mass source, reducing convection on the first day of the ice storm. The second day response was a mixture of thermal and dynamic changes forced by SST. The January 2010 case study showed more robust changes to freezing precipitation accumulation, associated with melting layer formation being tied to return flow diabatic heating and moistening of air parcels in the 48-72 hours prior to the winter storm. Both thermal and dynamical changes to the event were observed. Key results, applications and extensions for this work will be discussed.

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