An investigation of cold-air cyclogenesis along the east coast of the United States

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Warren, Christopher J.
A nine-year (1997-2006) climatology of East Coast cold-air cyclogenesis was developed to determine significant atmospheric conditions associated with these rather unique events. In the literature, a few case studies have examined these small-scale synoptic low-pressure systems in the midlatitudes that form within arctic air masses as cold air moves offshore. Some of the characteristics of these storms have been shown to be similar to "polar lows." The goals of this study are to: 1) determine how frequently these systems occur; 2) examine conditions that are favorable for their development; and 3) find similarities or differences between these storms and other East Coast cyclones and polar lows. In order to identify cases of cold air cyclogenesis, all cyclone events along the East Coast (within a boxed region bounded at 45°N by 70°W and 62.5°W, and 30°N by 85°W and 72.5°W), were identified using the North American Regional Reanalysis (NARR). Cyclone events were counted when at least one closed sea-level pressure isobar (plotted at 2 hPa intervals) was present in the region of interest. Cyclogenesis was determined to have occurred when the sea-level pressure in the center of circulation decreased by at least2 hPa/6-hrs at least twice in a 24 hour period (not necessarily consecutively). Finally, cyclogenesis events were classified as cold-air events if the grid point values of the mean daily 1000-500 hPa thickness from the National Centers for Environmental Protection/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis near and immediately surrounding the storm were below -0.5 standard deviations from a 32 year (1975-2006) mean. During the nine-year period, twenty extratropical cyclones met the above cold air cyclogenesis criteria. Five of these cold-air cyclones developed when at least four out of five grid points near and immediately surrounding the storm were below -1.0 standard deviations from the 32 year mean. The East Coast boxed region was divided into Northern, Western, and Eastern sub-regions based on location of initial time of cyclogenesis and eventual storm tracks. Composites were constructed for each sector for cold air cases and for other East Coast cyclones during the 2000-2001 cool season. Comparisons were made between the cold air cyclones and 2000-2001 cool season cyclones composite results. Each cold air case was analyzed using a satellite-derived classification scheme and revealed that 65% were either comma cloud or instant occlusion types. Finally, two case studies are presented and reveal that the dynamical processes involved in cold air cyclogenesis are similar to that of other extratropical East Coast midlatitude systems.