Download Cold Air Damming in the Appalachian Mountains

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.
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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