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Cold Air Damming along the Appalachian Mountains

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

Introduction

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

For a more detailed example click (here).

For an example of a typical case transitioning into an "in situ" case click (here).

The information presented in this discussion is most applicable to the southern Appalachian Mountains, especially North Carolina. Cold air damming is a critical process for the production of snow, sleet and freezing rain along and east of the Appalachian Mountains of the Middle Atlantic and the Southeast United States. Cold air damming helps supply cold air into the first few thousand feet of air above the ground. Cold air damming is called such because cold air is pushed up against the eastern slopes of the mountains which act as a dam to block the cold air from moving west. Sometimes cold air spills into the valley locations like Asheville, NC, and Roanoke, VA. The depth of the cold air and the strength of the cold air's advancement will help determine how well the cold air gets into the valleys.


Physical description

Typically the surface high pressure system supplying the cold air is centered over the Northeast or northern Middle Atlantic states. Rackley and Knox in "A Climatology of Southern Appalachian Cold-Air Damming*", indicated that the central pressure of the high pressure system is generally around 1030 millibars (mb) or higher. A ridge of high pressure develops southward with the axis to the east of the Appalachian Mountains. Winds around the base of the high pressure ridge push the dense low-level cold air west into the mountains. The air upstream is then forced to move southwest along the Foothills, Piedmont, and Coastal Plain. As noted by Bailey, Lackmann, Hartfield, and Keeter in "A Comprehensive Climatology of Appalachian Cold Air Damming", cold air damming is identifiable on surface maps as a narrow "U-shaped" pressure ridge in the isobar analysis. This "U" shape is especially prominent when the center of a low pressure system, tracking south to north, passes west of the mountains. The wind direction at the surface is from the north-northeast or northeast over the Coastal Plain, Piedmont, and Foothills, then near the mountains veers to east or southeast to head directly up the mountain slopes. The winds in the Piedmont and Foothills are nearly ageostophic, meaning that they cut almost directly across the base of the "U" in the isobar pattern. In contrast, a nearly geostophic wind would be one that blows almost parallel to the isobars.

Sometimes the high pressure system to the north or northeast is weaker, ill-defined, or has moved off the East Coast out over the Atlantic Ocean. In these cases, a high pressure ridge is "implied" to the west of a low pressure trough or warm front near the Atlantic Coast. A low pressure system moving east toward the mountains will further define the ridge. The cold air that already exists is held in place more so than additional cold air flowing in from the north or northeast. The meteorological term for this process is "in situ" cold air damming. "In situ" means "in place".

Because of cold air damming the Western Piedmont and Foothills of South Carolina, North Carolina, and Virginia are particularly prone to major ice storms compared to other parts of the Southeast and Middle Atlantic states. A few of the cities included in this "Piedmont Ice Storm Zone" are Danville, VA, Winston-Salem, NC, Greensboro, NC, and Hickory, NC. To a lesser extent, Charlotte, NC, and Greenville, SC can also be included. When the cold air spills over the initial rise into the mountains, known as the Blue Ridge Escarpment, and fills in the valleys, places like Asheville, NC, and Roanoke, VA can also get extended periods of freezing rain.

The Asheville Regional Airport about 10 miles south of downtown Asheville, NC is a particularly interesting location to watch when cold air "backdoors" into that location during a cold air damming event. Asheville is sandwiched between the Blue Ridge Mountains to the northeast and the Great Smokey Mountains to the west and west-southwest. To the east is the Blue Ridge Escarpment. To the south and southwest are more Blue Ridge mountains including Mount Pisgah. Winds at the airport frequently shift to come from the south-southeast as cold air starts to flow into the area. As stated earlier, winds already tend to move into the mountains from the southeast during a cold air damming event. The topography surrounding the airport appears to contribute to the wind's south component. Also, bear in mind that when winds above the shallow cold pool are from the south or southwest, the winds at the surface may be embedded within a veering layer. The veering layer is where the winds turn from the southeast to the south or southwest, giving winds at the airport a more south component.

Contribution to winter precipitation

Cold air damming aids in the formation of sleet and freezing rain by helping to sustain cold air near the surface. If the vertical temperature profile of the atmosphere is cold enough to support snow except in the lowest several hundred to several thousand feet, cold air damming can supply enough colder air at the surface for the snow to reach the ground without completely melting. Orographic lifting that occurs as air moves up the slopes of the Appalachian Mountains may cool the air enough to get snow or freezing rain when temperatures are still warm enough for rain in the lower terrain to the east. Sometimes, freezing rain will occur east of the mountains with rain in the mountains when the cold air damming event is more shallow or as a deeper stronger event weakens and warm air erodes the cold dome from the top.

Sometimes cold air damming pushes dry air to the south more so than cold air. If temperatures are only a few degrees above freezing, the melting and evaporation processes can cool the air enough to get freezing rain, sleet, or snow, in particular along the leading edge of precipitation moving into the area. Any location that remains underneath the leading edge of the precipitation for an extended period of time is in danger of getting much more icing than other locations only a few tens of miles away. Good luck forecasters. You wanted the job!

Influence on precipitation production

Cold air damming can contribute to at least light precipitation production as warmer, higher dew point air from the southwest, south, or east is lifted over the dense colder air. The cold air acts as "effective terrain" such that the less dense warm is forced to rise up over the barrier of colder air.

Another lifting mechanism called "frontogenetic forcing" also contributes to precipitation formation in some circumstances. Frontogenesis is the process of increasing the temperature (or density) gradient by pushing opposing warm and cold air masses together. Air is forced upward toward the cold air since the boundary slopes toward the cold air. The process increases the steepness of the boundary causing the air to rise in a more abrupt fashion. The strength of the lift is influenced by additional factors such as the stability of the air above the frontogenetic zone. If the air above is more stable resistance to rising is greater. Lower stability allows for stronger upward motion. Forcing will likely be strongest along the periphery of the cold air. Orographic lifting due to the Appalachian Mountains also provides lift for precipitation enhancement.

Duration and strength of cold air damming events

The duration of an event depends in part on the strength and depth of the cold air, the efficiency to which the cold air is maintained, and various mechanisms acting to warm the air. Melting and evaporation can cool the air if precipitation occurs. Adiabatic cooling results from upslope as air moves from the Coastal Plain and lower Piedmont up into the higher Piedmont, the Foothills, and the Appalachian Mountains. In cases associated with extensive cloud cover, the clouds protect the cold air by reducing the sun's radiation. Precipitation produced by storm systems moving through the southeast United States can also balance the sun's heating by cooling the air through evaporation, sublimation, or melting.

The development of a warm front, and its associated trough of low pressure, along the Atlantic Coast, helps to exaggerate the sharpness of the high pressure ridge to the west. The sharper ridge shape forces the wind to blow from the north-northeast as opposed to blowing from the east-northeast. The stronger north component to the wind helps funnel cold air from the north. The development of a coastal warm front is frequently aided by the approach of a surface low pressure system from the southwest which increases the flow of warm air from the Atlantic Ocean. The warm air increases the temperature gradient between the cold air over the land.

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 (view)