Development of a Cold-air Damming Index for Nothern New England: A Comparison of Two Case Studies

In North America, cold-air damming has been studied extensively in the mid-Atlantic east of the Appalachians along with cold surges east of the Rockies. Bell and Bosart (1988) were the first to comprehensively examine  observations of cold-air damming in this region. However, there are few, if any, studies that have examined low-level cold-air trapped to the east and south of the northern Appalachians in New England. These cold-air damming episodes pose a signficant forecast challenge especially for high impact weather of low stratus, freezing rain, and sleet that affects the transportation industry. In order to better understand the life-cycle of cold-air damming events a 15-year climatology has been devleoped.  Events are identified using observed surface data to calculate a cold-air damming index (CADINX). An initial version of the CADINX compared the surface potential temperature of Plymouth, NH, located within the cold air to the east and south of the higher terrain in New England, to stations located in the warmer air north of the mountains (Sherbrooke, QC), west of the mountains (Glens Falls, NY), and along the southern New England coast (Providence, RI). The index represents the average potential temperature gradient between Plymouth and the surrouding stations.  Subsequently, the index was further refined to be calculated for a series of stations east of the mountains in New Hampshire and Maine and following previous studies, it includes checks for a sea-level pressure ridge associated with the cold-air in the cross-mountain direction along with synoptic scale higher pressures to the north and east in the along-mountain direction.

This new CADINX has been applied to two mid-winter cases of cold-air damming: 10-11 January 2014 and 18 January 2015. Both cases feature prolonged episodes of freezing rain and sleet. The magnitude of the potential temperature gradients calculated for each station are quite similar. However, the synoptic and mesoscale sea-level pressure patterns are signficantly different.  Consequently, the index behaves differently in each case. In the 10-11 January 2014 case, the typically observed mesoscale ridge of high pressure is found coincident with the low-level cold-air.  For the 18 January 2015 case, no mesoscale ridge is evident. Interestingly, the duration of both events is similar.  Subtle differences in the evolution of the upper-air flow patterns suggest that these play a role in the evolution of the differences in the sea-level pressure patterns. Future work will include examining the index for the entire 15-year period and investigating whether the differences in these two cases are prevalent in the longer data set.