P5.7 A COMPREHENSIVE CLIMATOLOGY OF APPALACHIAN COLD AIR DAMMING Christopher M. Bailey + Gary M. Lackmann + Gail Hartfield* Kermit Keeter* *National Weather Service, Raleigh, North Carolina + North Carolina State University 1. Introduction Cold air damming (CAD) is characterized by the trapping of cold air against the windward side of a mountain range (Richwien, 1980). During CAD, a dome of cold, stable air becomes established along the eastern slopes of the Appalachians and can be identified via characteristic “U- shaped” isobars in the sea-level pressure field. In general, whenever statically stable flow encounters an orographic barrier in a rotating system, upwind ridging and downwind troughing will develop, in part as a manifestation of the geostrophic adjustment process. For the Appalachian Mountains, the result of CAD is a dome of cool, stable air east of the mountains and surface flow that departs from geostrophy. The relative coldness of the CAD “wedge” is the result of (i) along-barrier cold advection, (ii) orographic ascent, and, when sufficient moisture and lift are present, (iii) evaporative cooling and sub-cloud sheltering from insolation. Fritsch et al. (1992) note that clouds and precipitation can play a significant role in the strengthening of CAD through evaporative cooling; near-surface evaporational cooling increases static stability, the degree of orographic blocking, and the strength of CAD. CAD can have a significant impact on the weather between the crest of the Appalachian Mountains and the coastal plain. The difference in temperature between the damming region (Fig. 1) and the coast can exceed 20°C during strong CAD events (Bell and Bosart, 1988). During the cold season, this can mean the difference between rain and freezing or frozen precipitation. Operational forecasting experience has demonstrated that CAD events can vary widely in terms of duration, forcing mechanisms, scale, and impact on sensible weather. Accordingly, National Weather Service forecasters in the damming region have identified three categories within an overall CAD spectrum (Hartfield et al., 1996; Kramer, 1997; Hartfield, 1998). The categories originally included: ___________________________________________ + Corresponding author address: Dr. Gary M. Lackmann, North Carolina State University, Dept. of Marine, Earth and Atmospheric Sciences, Raleigh, NC 27695. email: gary@ncsu.edu 1.) Classical damming: characterized by strong synoptic- scale forcing. A parent high with a central pressure greater than 1028 mb and located north of 40°N latitude is the predominant feature; 2.) Hybrid damming: associated with a weaker parent high to the north, and diabatic cooling plays a significant role in complementing the weaker synoptic forcing; 3.) In-situ damming: occurs with little or no synoptic-scale support. Diabatic processes are responsible for the development of CAD. The purpose of this study is to provide an objective climatology of CAD and CAD sub-types. Composites of various meteorological fields are being generated from the climatology, centered at the onset, peak, and demise of various CAD sub-types. It is hoped that these composites will assist forecasters in the recognition of features that are associated with the different CAD sub-types, and with different stages in the evolution of CAD events. For example, a particularly difficult forecasting problem is the demise of CAD; operational numerical models often scour the cold air too quickly. Composites may facilitate identification of the characteristic patterns associated with CAD decay, and allow the signatures common to various sensible weather phenomena to be elucidated. 2. Methodology An objective CAD detection algorithm was developed in order to capture a broad spectrum of events. Three mountain-normal lines and one mountain-parallel line were chosen, each consisting of three stations (Fig. 1). For the mountain-normal lines, the center station is located within the damming region while the other two are located on either side of the region. Along the three mountain-normal lines, the Laplacian values for sea level pressure and potential temperature are calculated for each hour. The Laplacian values measure the strength of the pressure ridge and the cold dome; negative values in the pressure Laplacian are usually associated with ridging in the center of the section, and a positive potential temperature Laplacian corresponds to colder central values. The fourth line (oriented parallel to the mountains) was chosen to represent the along-barrier pressure gradient.