Development of a Cold-Air Damming Index for Northern New England

Northern New England Cold Air Damming (CAD) is an important example of complex mesoscale flow interactions with orography that have an impact on low stratus, freezing rain, and sleet precipitation types. These wintry precipitation phenomena can have significant impact on the transportation industry if not forecasted and identified with high accuracy. While cold-air damming has been extensively studied in the U.S. Mid-Atlantic region east of the Appalachians, only very limited research has been conducted on events where cold air becomes entrenched east and south of the northern Appalachians of New England. In this study, cold-air damming events are identified using hourly surface station data to calculate a Cold-Air Damming Index (CADINX). The index is calculated as the average surface potential temperature gradient between five "central" stations located in the cold air along the eastern edge of the Appalachians (in Maine, New Hampshire, Massachusetts) and stations located in the warmer air west of the mountains (in Quebec, Vermont, New York) and stations located in the warmer air along the southern New England coast (in Maine, New Hampshire, Rhode Island). The index is set to zero if the central station is not colder than the surrounding stations. In addition, the index is only non-zero when higher sea level pressure is located to the north and east in the along mountain direction.

CAD events are objectively determined by applying the CADINX to hourly data for the five central stations. A CAD event is identified when three of the five stations have non-zero CADINX values for at least 6 consecutive hours. Once CAD events are identified, the following information is collected for each event: start/end time, peak time, average/max/min CADINX value. Initially, CAD events have been identified for a test month (Jan 2012) and for all of 2016. During these periods 28 northern New England CAD events have been identified. Examination of RUC analyses and observations show that these events exhibit “classic” CAD characteristics as described by Bell and Bosart (1988), where a thermal trough (ridge) of potential temperature exists east (west) of the mountain barrier with a surface anticyclone situated to the north of the damming region. While the synoptic configuration during these cases is similar and expected, the mesoscale features differ significantly from case to case. In approximately half of the CAD events examined thus far, the typical mesoscale pressure ridge associated with the low-level cold air is easily identified. The other half, however, indicates the presence of the low-level cold air with no sign of an associated mesoscale pressure ridge. Future work will extend the analysis to create a 15-year climatology of New England cold-air damming events using the CADINX, investigating the spectrum of CAD events, and exploring the dynamical and thermodynamical mechanisms of CAD life cycles.