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APRIL 2006 ALEC S. BOGDANOFF Cold Air Damming in the Appalachian Mountains ALEC S. BOGDANOFF Department of Meteorology, The Florida State University, Tallahassee, Florida (Manuscript received 30 March 2006, in final form 12 April 2006) ABSTRACT Cold air damming is most prevalent in the Appalachian Mountains in the United States. It can cause severe winter weather outbreaks and cause intense damage to the infrastructure of the affected cities. Cold air damming requires certain conditions to be a severe event and understanding the sources of the events will help forecast when and where generally, but not specifically where cold air damming will occur. _________________________ 1. Introduction Cold air damming occurs when cold air, from potentially several sources, becomes trapped by a mountain range (Whiteman 2000). In the United States, this is the most common around the Appalachian Mountains. Cold air from a high pressure system blows southward on the eastern side of the Appalachians, rises up the eastern slope further cooling adiabatically (Wesley 2001). Weather associated with cold air damming comes from warmer air masses west of the boundaries being lifted over the ranged and creating clouds above the cold air wedge. Eventually, if conditions allow, this wintertime phenomenon can cause ice storms and precipitation (Whiteman 2000). 2. Sources of Cold Air Cold air advection brings the most amount cold air into the regions affected by cold air damming. Generally from a high pressure system, cold air rushes down the eastern side of the boundary, and with colder air being denser, the mass rests on the surface (Bell and Bosart 1988). Adiabatic cooling always occurs with cold air damming events. Cold air driven south through cold air advection drifts west due to the pressure gradient and the coriolis force, and up the slope of the mountain ranges cooling further. The cold air that is driven up the slope of the mountain ranges is cooled by the adiabatic process. The final source for cold air comes from falling precipitation. Diabatic cooling from the melting and falling of precipitation causes the least significant change in the temperature of the air masses (Wesley 2001). 3. Types of Cold Air Damming Meteorologists have categorized three types of cold air damming. Wesley (2001) defines the types of cold air damming based on the sources of the cold air. The most frequent type of cold air damming, defined by the overwhelming reliance of cold air advection as the source for cold air, requires significant orographic features. Classic cold air damming situations rely on a strong synoptic scale feature, in most cases a high pressure system, located to the north of the topography. Evaporational (diabatic) cooling is not necessary in a classic cold air damming scenario, but can account for up to 30% of the cooling that occurs. Adiabatic cooling, also not necessary, can account for about 30% of the cooling (Bell and Bosart 1988). Diabatic cooling from the falling or melting precipitation of a storm defines In Situ cold air damming. The source of the precipitation is generally a short wave, a front, or warm advection. This can change the temperature profile and thus the type of precipitation that touches the ground. The anticyclone necessary in classic cold air damming events is not close enough to affect the area in the cold air damming event. Although there is no high pressure system need, some ridging east of the mountain boundary still occurs. Hybrid cold air damming combines cold air advection and diabatic cooling. Both play a considerable role, but cold air advection is not as strong as in classic cold air damming events. Neither source would create a significant cold air damming event alone, but together are strong enough to create one of significance. The surface high pressure located unfavorably for a classic scenario event, allows for enough cold air advection for a hybrid event. 4. Associated Weather Cold air damming events can cause noteworthy weather phenomenon. Continual low temperatures caused by the cold trapped air results from the stratification of air masses by temperature. The colder air mass sinks to the surface and settles for up to a week or two (Bell and Bosart 1988). In addition to cold temperature that will decide the type of precipitation, extensive cloud cover and dense fog APRIL 2006 ALEC S. BOGDANOFF are also associated with cold air damming events (Palmer 2005). The extensive cloud cover is caused by the warmer air stratifying over the colder air due to the density difference and the pressure gradient winds. The warm air raised over the mountains dry adiabatically creates clouds above the cold air wedge (Whiteman 2000). These are the clouds that can create precipitation. Thicker cold air wedges will cause different types of precipitation than thinner cold air wedges, referred to by meteorologists as the environmental temperature profile. As explained by Whiteman (2000), the type of precipitation depends mostly on the environmental temperature profile. There are four basic scenarios of temperature profiles. If the entire environment falls at or below freezing from the origin of the precipitation down the surface, snow will form and stay as snow all the way to the surface. If the temperature of the environment moves above freezing above the surface and continues to stay above freezing all the way to the surface, the precipitation will have a chance to melt and the type of precipitation will be rain. The most dangerous winter precipitations, sleet and freezing rain, share a similar temperature profiles. Both sleet and freezing rain start with a temperature profile, near the source of the precipitation, at or below freezing, then melt in an area of the atmosphere above 0°C. The difference between sleet and freezing rain comes from the depth of the layer of freezing temperatures close to the surface. If given enough time to freeze, thus having a deeper frozen layer, the melted precipitation will refreeze into sleet. Sleet is ice that falls as precipitation. If the layer is not deep enough to allow refreezing, it will fall as freezing rain, meaning it will freeze upon contact with a surface. Both sleet and freezing rain are extremely dangerous. Sleet causes severe damage to crops and material possessions, while freezing rain freezes to trees and power lines and will cause them to fall. After the North Carolina storm of 2002, parts of the Carolinas were without power for over two weeks because of all the power poles taken down by freezing rain (Blaes et. al. 2002). 5. Classic Appalachian Cold Air Damming Cold Air Damming in the Appalachians is the most frequent and dangerous in the nation due to the population that live in the areas generally affected. Classic Appalachian cold air damming occurs between three to five times monthly from December to March (Bell and Bosart 1988). The classic scenario occurs when a deep trough over the Mississippi Valley corresponds to ridging in New England. For a strong cold air damming event, it is important for the system to be slow moving to allow the cold air to situate without too much motion. With a large high pressure system over northern New England dominating the synoptic pattern of the eastern seaboard, strong ridging east of the mountain range raises sea level pressure, because cold air is denser than warmer air. A low pressure generally develops due to the trough, and the large high pressure system will move in after a large “cyclogenesis event” (Wesley 2001). This allows the cold air to flow down the eastern seaboard, and classic cold air damming develops. The December 4th and 5th storm of 2002 caused by classic cold air damming covered the MidAtlantic States in snow, sleet and ice (2001). The National Weather Service, Raleigh Office, issued a winter storm warning for 22 counties. The following surface analysis map from Wednesday morning, 4 December 2002, shows the strong 1032 hectopascal high pressure system to the north of the Appalachians and the cold air being wedged into the Carolinas and south. This is a clear example of a classic cold air damming event in the Appalachian mountain rage. The following surface map prepared by the Raleigh Office of the National Weather Service from Wednesday night, 5 December 2002, shows a deep wedge of cold air east of the Appalachians. There is, again, a clear and strong 1032 hectopascal high APRIL 2006 ALEC S. BOGDANOFF pressure system north of the Appalachian. The two low pressures systems over Mississippi and off the South Carolina coast are joined by a single frontal boundary and are indicative of cold air damming events. The following map of North Carolina shows the snow and sleet accumulations from the December event. In the mountains, greater than six inches of accumulations were seen, but it was mainly snow. In the Raleigh area, there was equal accumulation of snow and sleet due to the difference in the temperature profiles. This map shows a clear pattern that within the mountains, mainly snow will fall, but further away from the mountains, sleet and snow mixes will precipitate. The 12Z sounding below from Greensboro, South Carolina on 05 December 2002 shows a shallow layer of air with temperatures below freezing with a decently deep layer of air above freezing, allowing ample time for melting and refreezing for sleet or freezing rain. All the previous maps exemplify the December 4 to 5th event in North Carolina as a Classic Cold air damming scenario. 6. Forecasting Cold Air Damming Because so much of the effects of cold air damming depend on the temperature profile of the environment, forecasting cold air damming proves to be very tough. The topography of the Appalachian Mountains changes very rapidly with distance thus it is hard to forecast the temperature profiles of every location, which would be necessary to find where sleet, snow, or freezing rain will occur. Furthermore, the position and depth of the cold air depends upon time, and therefore is ever changing and very difficult to forecast (Whiteman 2000). RESOURCES Bell, G. D., and L. F. Bosart, 1988: Appalachian cold-air damming. Mon. Wea. Rev., 116, 137-161. Blaes, J., P. Badgett, G. Hartfield, and K. Keeter, 2002: December 4-5, 2002 Winter Storm. Retrieved February 13, 2006, http://www4.ncsu.edu/~nwsfo/storage/cases/ 20021204/ Palmer, C., 20 May 2005: How East Coast ice storms form. USA Today. Wesley, D., et. al., 2001: Cold Air Damming. Retrieved February 8, 2006, http://meted.ucar.edu/mesoprim/cad/ Whiteman, C. D., 2000: Mountain Meteorology: Fundamentals and Applications. Oxford University Press, 355