Previous studies of Appalachian cold air damming (CAD) have generally emphasized strong "Classical" events. These damming events are characterized by a strong parent high to the north, usually 1035 mb or greater in strength, that is accompanied by ageostrophic northeasterly flow advecting cold air into the damming region. While these damming events are typically the strongest and most recognizable, weaker CAD events can have a pronounced affect on the weather. The National Weather Service in Raleigh has classified CAD into three categories: (1) Classical damming that is characterized by the strong parent high to the north, (2) "Hybrid" damming that is characterized by a weaker parent high and significant diabatic cooling, and (3) "In-situ" damming that is characterized by a weak parent high, usually located off the coast over the Atlantic Ocean, and reliant on diabatic processes to initiate damming. An objective CAD detection algorithm was developed with the ultimate goal of i) defining an objective climatology spanning the complete spectrum of CAD events, and ii) developing composites describing synoptic patterns accompanied each CAD sub-type, for use in research and forecasting.
In order to capture a broad spectrum of events, an objective CAD detection algorithm was developed. Three mountain normal lines were chosen each located in different part of the damming region. The first line is located in the northern part of the region centered at Lynchburg, VA. The second line is centered at Greensboro, NC and the third line is centered at Greer, SC. Along the three mountain normal lines, the Laplacian values of sea level pressure and potential temperature are computed for each hour. Negative sea level pressure Laplacian values in conjunction with positive potential temperature Laplacian values represents the damming region. A Fourth line was chosen parallel to the mountains using Greer, Greensboro, and Richmond, VA. The difference of pressure between the northern and southern points on the line represents the ageostrophic channeling along the mountain. Other requirements are used to eliminate non-CAD cases. All the events were then stratified using maximum Laplacian values, number of lines active during an event, duration, pressure difference between Richmond and Greer, and equivalent potential temperature advection parallel to the mountain.
The detection algorithm was run over a 12-year period from 1984 to 1995. Of the 374 events found, only fifty one percent of the events were cold season events, which is contrary to previous findings where the majority of CAD events occur during the winter months. However, after stratifying the cases by intensity, many more strong events in the cold season were found than in the warm season while weaker events in the summer months were more prevalent than in the winter months. More than just the three types of damming have emerged and composites of the many different types of damming events will be presented. Sea level pressure, surface temperature, upper level heights, temperature, and wind fields are some of the parameters that will be composited together. The composites will be presented for every six hours in animation loops. Each CAD type will be initialized at either the initiation time, end time, or at the peak of the event and show each parameter prior and after that time.