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Cold Air Damming, Orographic Lifting, and Precipitation Type in the Southern Appalachian Mountains

* Geographic and topographic maps created from the USGS/ESRI ArcGIS mapping software and database


An example of cold air damming

For a brief example of cold air damming click (here).

Some implications of cold air damming

Cold air damming produces orographic lift in the southern Appalachian Mountains. The Appalachian Cold Air Damming article, in reference to a climatological study done by Bailey, Lackmann, Hartfield, and Keeter (see reference section at end) states that even though winds blow from the northeast or north-northeast in the Foothills, Piedmont, and Coastal Plain, winds near the base of the mountains veer (make a right turn) to come from the east or southeast. That direction is nearly straight up the mountain slopes.

The lift causes adiabatic cooling of the air, dry adiabatic cooling when the air is unsaturated, moist adiabatic cooling once the air is saturated. The moist adiabatic cooling rate varies with temperature but is roughly one-half of the dry adiabatic lapse rate. The condensing of moisture releases heat into the air and balances some of the cooling. The added moisture intensifies precipitation. The cooling increases the potential for the mountains to get snow when the lower terrain to the east is getting freezing rain or rain. The cooling can also result in freezing rain verses rain, especially on the higher elevated eastward facing slopes. Several scenarios are presented to give examples of how precipitation type might be affected.

Description of the southern Appalachian Mountains

The portion of the Blue Ridge Escarpment that runs from southwest Virginia through western North Carolina and extreme northeastern South Carolina is a near-continuous wall of abruptly rising hills. Several higher ranges of the Blue Ridge Mountains of the Appalachians are positioned just west of the escarpment. Even major highways, such as Interstate 77, Highway 421, Interstate 40, and Interstate 26 heading into the east side of the mountains must traverse a steep rise. One of the best views of the mountains from the east occurs when traveling west on Interstate 40 heading toward Asheville, NC. Shortly before you head up the escarpment you see the escarpment with several higher ridges of mountains. The ridge rising above all the others is part of the Black Mountain range which is the highest range of the Blue Ridge Mountains.

Some scenarios

The following are some possibilities for differences in precipitation type between the mountains and the Piedmont to the east.

There are times that freezing rain falls in the Piedmont but just rain in mountains. This condition can occur when the cold air is very shallow, such as when the cold air damming event is weakening to allow warm air to erode the cold air from the top.

The difference in precipitation type between the mountains and the lower terrain to the east will depend on how cold the air is before it ascends the mountain slopes and the vertical temperature profile above the dome of cold the air. Increased precipitation intensity due to the orographic lift can help lower the snow levels as the melting snow cools the air. A layer of air above freezing that is above the cold air over the Piedmont may partially or completely melt the snow to result in sleet or freezing rain. When the cold air is lifted into the mountains, it also exerts an upward force on the warm layer above it. That warm layer may cool enough for snow to penetrate it without melting. In situations when the atmosphere is far too warm above the cold dome to support snow, the lifting of air up the mountains can still drop temperatures enough near the slope to produce freezing rain. The starting temperature of the air before lifting begins will determine whether the air can cool enough to drop below freezing.

References

A Climatology of Southern Appalachian Cold-Air Damming* - Jared A. Rackley and John A. Knox - Department of Geography, The University of Georgia, Athens, Georgia

A Comprehensive Climatology of Appalachian Cold Air Damming - Christopher M. Bailey and Gary M. Lackmann, North Carolina State University, Raleigh, NC; Gail Hartfield and Kermit Keeter, National Weather Service, Raleigh, NC

Cold Air Damming in the Appalachian Mountains - Alec S. Bogdanoff

Cold Air Damming discussion from theweatherprediction.com website - Article written by Robert Meyer, website created by Jeff Haby