Katabatic wind forcing of the antarctic circumpolar ...
as 10 meters per second. This difference is not a problem when aircraft are taking off to the south but could be a problem if aircraft take off to the north. The opposite is the problem when aircraft are landing. The possible vertical wind shear at the Pegasus Runway also will require further examination. The monitoring of the Pegasus Runway using AWS units is supported by National Science Foundation grant OPP 9015586.
References Holmes, R., C.R. Stearns, and G.A. Weidner. 1993. Antarctic automatic weather stations: Austral summer 1992-1993. Antarctic Journal of the U.S., 28(5).
Stearns, C.R., and G.A. Weidner. 1990. Wind speed events and wind direction at Pegasus site during 1989. Antarctic Journal of the U.S., 25(5),258-262.
Stearns, C.R., and G.A. Weidner. 1991. Wind speed, wind direction, and air temperature at Pegasus North during 1990. Antarctic Journal of the U.S., 26(5), 251-253.
Katabatic wind forcing of the antarctic circumpolar easterlies THOMAS R. PARISH, Department ofAtmospheric Science, University of Wyoming, Laramie, Wyoming 82071 DAVID H. BROMWICH, Byrd Polar Research Center, Ohio State University, Columbus, Ohio 43210
he marked climatic differences between the north and T south polar regions are in large part due to the contrast in landform and orography. The strong radiational cooling of the continental ice surface coupled with the strong sensible heat flux from the surrounding southern oceans to the north of the continent ensures that a large horizontal temperature gradient must persist near the continental margin throughout the nonsummer months. The land/sea contrast thus accentuates the preexisting meridional temperature gradient arising from solar geometry. A clearly defined thermally direct circulation is present in the high southern latitudes. Although not often discussed, katabatic winds may also be critical to the development of the sea-level pressure field about the continent (Schwerdtfeger 1984). Katabatic winds occur with great frequency in the lowest few hundred meters of the antarctic atmosphere. These downslope drainage winds occur as a result of the radiative cooling of the antarctic ice slopes. In a conceptual sense, the antarctic katabatic wind regime is the lower branch of the aforementioned high southern latitude direct thermal circulation. It is apparent that such a pronounced continentwide drainage circulation interacts with the large-scale ambient atmospheric environment. The attendant northward transport of mass must alter the horizontal pressure field some few hundreds of kilometers to the north of the continent. As the cold katabatic airstreams move northward away from the antarctic ice slopes and out over the oceanic region, Coriolis accelerations impart an anticyclonic curvature. This inertial turning of the katabatic airstream north of the continental periphery gives rise to a geostrophically balanced easterly flow in the lowest few hundred meters of the atmosphere to the north of the antarctic continent. Such circumpolar easterlies are a well-documented climatological feature of the lower troposphere around the coast of Antarctica. To examine the interaction between the katabatic wind regime and the large-scale horizontal pressure field around the antarctic continent, a series of two-dimensional numerical experiments has been conducted using a hydrostatic primitive
equation model. Details of the equation system, the horizontal grid structure, the boundary-layer parameterization, and the initial conditions can be found in Parish and Waight (1987). In this application, the horizontal grid consists of 80 points with a 20-kilometer (km) grid resolution; 15 vertical levels were employed with highest resolution in the lower boundary layer to capture details of the katabatic wind. The pressure at the top of the model is set at 250 hectopascals (hPa) and represents the tropopause. The lowest level corresponds to a height of approximately 10 meters (m) above the surface. Included in the model is an explicit longwave radiative transfer scheme KATABATIC WIND SPEED LEVEL =
I
20
16 I'n
E 12 W ( Cl)
4 0 -800
-4000 400 800 DISTANCE FROM COAST (km)
Figure 1. Katabatic wind speed at the lowest model level at 4, 8, 12, 16, 20, and 24 h (corresponding to curves A, B, C, D, E, and F, respectively) over model domain. (m S- 1 denotes meters per second; km denotes kilometers.)
ANTARCTIC JOURNAL - REVIEW 1993 294
(Cerni and Parish 1984) that drives the katabatic circulation. The terrain profile used in the model is representative of the ice topography of East Antarctica. The results of one 24-hour wintertime experiment will be presented; no solar radiation is considered in the numerical simulation and the ocean to the north of the model continent is assumed to be covered with
1000
VERTICAL PROFILE WIND (100 km FROM COAST)
800 E
600
I-
I
400
200 0 I -'-' 0 2 4 6 8 10 12 14 16 I
I
WIND SPEED (md) Figure 2. As in figure 1, except for vertical profile of wind speed for the grid point 100 km north of the coastline. (m s-1 denotes meters per second; m denotes meters; km denotes kilometers.)
a 0.c U) Z
4.0
SURFACE PRESSURE CHANGE
3.0
0 I> W
c
W
a:
C')
thick pack ice. The model simulation was carried out for a 24hour period, by which time a near-steady katabatic wind circulation had evolved. Emphasis is placed on the modification of the horizontal pressure field by the katabatic wind regime. Similar three-dimensional experiments are presented in Bromwich, Du, and Parish (in preparation). Radiative cooling of the sloping ice surface initiates the katabatic drainage over the continent; the slope flows develop quickly and reach a quasi-steady state within 12 hours (h). Figure 1 depicts the evolution of the katabatic wind at the 10-m level at 4-hour increments during the integration over the model domain (curve A represents winds at 4 h, curve B at 8 h, and so forth). The intensity of the katabatic wind is sensitive to the terrain slope; maximum katabatic wind speeds of approximately 20 meters per second (m s') are simulated at the steep coastal margin. A rapid decrease in wind speed occurs as the katabatic wind moves northward over the ice-covered ocean. The mass convergence resulting from the deceleration of the katabatic airstream forces the rising branch of the thermally direct meridional circulation alluded to above. Figure 2 illustrates the evolution of the vertical profile of wind speed at a grid point 100 km to the north of the coastline. The depth of the katabatic outflow increases with time such that by the 24-hour period, the thickness of the outflow exceeds 500 m. Because of the Coriolis deflection, the flow is primarily easterly. Significant modification of the horizontal pressure field to the north of the continent takes place during the integration period. Figure 3 illustrates deviations in the surface pressure from the initial pressure field at 4-hour increments during the model integration period. The mass transport by the katabatic winds acts to increase pressure just north of the coastal margin up to 4 hPa. In time, a south-to-north horizontal pressure gradient force becomes established, which supports the climatologically prevalent circumpolar easterly wind regime. Model simulation of this easterly wind regime (see figures 1 and 2) suggests near-surface speeds of 5 m s or so that extend a few hundred kilometers north of the continent. Such results are representative of observed features and suggest that the antarctic katabatic wind regime offers at least a supportive role in the development of the easterly wind regime about the continent. This work was supported in part by National Science Foundation grants OPP 91-17202 to Thomas R. Parish and OPP 89-16921 to David H. Bromwich.
U) W X
References
0 U-
Bromwich, D.H., Y. Du, and T.R. Parish. In preparation. Numerical simulation of winter katabatic winds crossing the Siple Coast area of West Antarctica and propagating across the Ross Ice Shelf.
aw 0.0 a:
C)
-1.0 -800 -400 0
Monthly Weather Review.
400 800
Cerni, T.A., and T.R. Parish. 1984. A radiative model of the stable nocturnal boundary layer with application to the polar night. Journal of Climatology and Applied Meteorology, 23, 1563-1572. Parish, T.R., and K.T. Waight. 1987. The forcing of antarctic katabatic winds. Monthly Weather Review, 115, 2214-2226. Schwerdtfeger, W. 1984. Weather and climate of the Antarctic. New York: Elsevier.
DISTANCE FROM COAST (km) Figure 3. As in figure 1, except for surface-pressure deviations from the start of the model integration. (hPa denotes hectopascals; km denotes kilometers.)
ANTARCTIC JOURNAL - REVIEW 1993 295
References Holmes, R., C.R. Stearns, and G.A. Weidner. 1993. Antarctic automatic weather stations: Austral summer 1992-1993. Antarctic Journal of the U.S., 28(5).
Stearns, C.R., and G.A. Weidner. 1990. Wind speed events and wind direction at Pegasus site during 1989. Antarctic Journal of the U.S., 25(5),258-262.
Stearns, C.R., and G.A. Weidner. 1991. Wind speed, wind direction, and air temperature at Pegasus North during 1990. Antarctic Journal of the U.S., 26(5), 251-253.
Katabatic wind forcing of the antarctic circumpolar easterlies THOMAS R. PARISH, Department ofAtmospheric Science, University of Wyoming, Laramie, Wyoming 82071 DAVID H. BROMWICH, Byrd Polar Research Center, Ohio State University, Columbus, Ohio 43210
he marked climatic differences between the north and T south polar regions are in large part due to the contrast in landform and orography. The strong radiational cooling of the continental ice surface coupled with the strong sensible heat flux from the surrounding southern oceans to the north of the continent ensures that a large horizontal temperature gradient must persist near the continental margin throughout the nonsummer months. The land/sea contrast thus accentuates the preexisting meridional temperature gradient arising from solar geometry. A clearly defined thermally direct circulation is present in the high southern latitudes. Although not often discussed, katabatic winds may also be critical to the development of the sea-level pressure field about the continent (Schwerdtfeger 1984). Katabatic winds occur with great frequency in the lowest few hundred meters of the antarctic atmosphere. These downslope drainage winds occur as a result of the radiative cooling of the antarctic ice slopes. In a conceptual sense, the antarctic katabatic wind regime is the lower branch of the aforementioned high southern latitude direct thermal circulation. It is apparent that such a pronounced continentwide drainage circulation interacts with the large-scale ambient atmospheric environment. The attendant northward transport of mass must alter the horizontal pressure field some few hundreds of kilometers to the north of the continent. As the cold katabatic airstreams move northward away from the antarctic ice slopes and out over the oceanic region, Coriolis accelerations impart an anticyclonic curvature. This inertial turning of the katabatic airstream north of the continental periphery gives rise to a geostrophically balanced easterly flow in the lowest few hundred meters of the atmosphere to the north of the antarctic continent. Such circumpolar easterlies are a well-documented climatological feature of the lower troposphere around the coast of Antarctica. To examine the interaction between the katabatic wind regime and the large-scale horizontal pressure field around the antarctic continent, a series of two-dimensional numerical experiments has been conducted using a hydrostatic primitive
equation model. Details of the equation system, the horizontal grid structure, the boundary-layer parameterization, and the initial conditions can be found in Parish and Waight (1987). In this application, the horizontal grid consists of 80 points with a 20-kilometer (km) grid resolution; 15 vertical levels were employed with highest resolution in the lower boundary layer to capture details of the katabatic wind. The pressure at the top of the model is set at 250 hectopascals (hPa) and represents the tropopause. The lowest level corresponds to a height of approximately 10 meters (m) above the surface. Included in the model is an explicit longwave radiative transfer scheme KATABATIC WIND SPEED LEVEL =
I
20
16 I'n
E 12 W ( Cl)
4 0 -800
-4000 400 800 DISTANCE FROM COAST (km)
Figure 1. Katabatic wind speed at the lowest model level at 4, 8, 12, 16, 20, and 24 h (corresponding to curves A, B, C, D, E, and F, respectively) over model domain. (m S- 1 denotes meters per second; km denotes kilometers.)
ANTARCTIC JOURNAL - REVIEW 1993 294
(Cerni and Parish 1984) that drives the katabatic circulation. The terrain profile used in the model is representative of the ice topography of East Antarctica. The results of one 24-hour wintertime experiment will be presented; no solar radiation is considered in the numerical simulation and the ocean to the north of the model continent is assumed to be covered with
1000
VERTICAL PROFILE WIND (100 km FROM COAST)
800 E
600
I-
I
400
200 0 I -'-' 0 2 4 6 8 10 12 14 16 I
I
WIND SPEED (md) Figure 2. As in figure 1, except for vertical profile of wind speed for the grid point 100 km north of the coastline. (m s-1 denotes meters per second; m denotes meters; km denotes kilometers.)
a 0.c U) Z
4.0
SURFACE PRESSURE CHANGE
3.0
0 I> W
c
W
a:
C')
thick pack ice. The model simulation was carried out for a 24hour period, by which time a near-steady katabatic wind circulation had evolved. Emphasis is placed on the modification of the horizontal pressure field by the katabatic wind regime. Similar three-dimensional experiments are presented in Bromwich, Du, and Parish (in preparation). Radiative cooling of the sloping ice surface initiates the katabatic drainage over the continent; the slope flows develop quickly and reach a quasi-steady state within 12 hours (h). Figure 1 depicts the evolution of the katabatic wind at the 10-m level at 4-hour increments during the integration over the model domain (curve A represents winds at 4 h, curve B at 8 h, and so forth). The intensity of the katabatic wind is sensitive to the terrain slope; maximum katabatic wind speeds of approximately 20 meters per second (m s') are simulated at the steep coastal margin. A rapid decrease in wind speed occurs as the katabatic wind moves northward over the ice-covered ocean. The mass convergence resulting from the deceleration of the katabatic airstream forces the rising branch of the thermally direct meridional circulation alluded to above. Figure 2 illustrates the evolution of the vertical profile of wind speed at a grid point 100 km to the north of the coastline. The depth of the katabatic outflow increases with time such that by the 24-hour period, the thickness of the outflow exceeds 500 m. Because of the Coriolis deflection, the flow is primarily easterly. Significant modification of the horizontal pressure field to the north of the continent takes place during the integration period. Figure 3 illustrates deviations in the surface pressure from the initial pressure field at 4-hour increments during the model integration period. The mass transport by the katabatic winds acts to increase pressure just north of the coastal margin up to 4 hPa. In time, a south-to-north horizontal pressure gradient force becomes established, which supports the climatologically prevalent circumpolar easterly wind regime. Model simulation of this easterly wind regime (see figures 1 and 2) suggests near-surface speeds of 5 m s or so that extend a few hundred kilometers north of the continent. Such results are representative of observed features and suggest that the antarctic katabatic wind regime offers at least a supportive role in the development of the easterly wind regime about the continent. This work was supported in part by National Science Foundation grants OPP 91-17202 to Thomas R. Parish and OPP 89-16921 to David H. Bromwich.
U) W X
References
0 U-
Bromwich, D.H., Y. Du, and T.R. Parish. In preparation. Numerical simulation of winter katabatic winds crossing the Siple Coast area of West Antarctica and propagating across the Ross Ice Shelf.
aw 0.0 a:
C)
-1.0 -800 -400 0
Monthly Weather Review.
400 800
Cerni, T.A., and T.R. Parish. 1984. A radiative model of the stable nocturnal boundary layer with application to the polar night. Journal of Climatology and Applied Meteorology, 23, 1563-1572. Parish, T.R., and K.T. Waight. 1987. The forcing of antarctic katabatic winds. Monthly Weather Review, 115, 2214-2226. Schwerdtfeger, W. 1984. Weather and climate of the Antarctic. New York: Elsevier.
DISTANCE FROM COAST (km) Figure 3. As in figure 1, except for surface-pressure deviations from the start of the model integration. (hPa denotes hectopascals; km denotes kilometers.)
ANTARCTIC JOURNAL - REVIEW 1993 295
