Extreme weather events provide substantial socio-economic impacts over large urban areas. During the cool season urban areas are strongly impacted by temperature extremes, heavy rain events and snow/ice storms. In particular, temperature extremes such as cold air outbreaks significantly impact energy consumption and human safety (via exposure). Such socio-economic impacts are accentuated by the presence of wind which helps to whisk heat away from point sources. Interestingly, however, cold air outbreaks have been historically defined in terms of temperature anomalies without consideration of the role of winds.

We explore a new means of characterizing cool season extreme temperature events in terms of a wind chill parameter. Our pilot study concentrates upon identifying cold air outbreaks over three distinct regions: the Midwest, Northeast Megalopolis, and Deep South, all of which include major urban areas. Analyses of regional statistical characteristics reveal that although a wind chill approach identifies many of the same events as a temperature-only criterion, there is a substantial re-ordering of the ranking among cold air outbreaks, particularly over the northeast US. We also perform synoptic and dynamic analyses of selected events from January 2004, taking advantage of the new NASA-GMAO MERRA reanalysis effort (which emphasizes accuracy in the hydrological cycle and provides high spatial and temporal resolution). In particular, we perform comparative thermodynamic and hydrodynamic analyses of these events, contrasting results derived from MERRA and NCEP-NCAR reanalyses with archived operational analyses. The new MERRA product provides an improved representation of the synoptic features associated with the events studied, especially the local thermal and moisture gradients associated with smaller scale frontal features. We further apply a modern diagnostic approach (based upon a potential vorticity dynamical framework) to infer the physical mechanisms leading to cool season extreme temperature events. Our diagnostic approach serves to efficiently isolate the key dynamical features responsible for the southward surge of cold air that occurs during cold air outbreaks.