Numerical simulation of the Terra Nova Bay katabatic

References

Souders, C.G., and R.J. Renard. 1984. The visibility climatology of McMurdo! Williams Field, Antarctica. (Preprint volume, 10th American MeteDixon, W.J., et al. 1983. BMDP Statistical Software. (1983 Printing with orological Society Conference on Weather Forecasting and Analysis, June 1984, Clearwater Beach, Florida.) additions.) Berkeley: University of California Press. Panofsky, H.A., and G.W. Brier. 1958. Some applications of statistics to Stearns, C.R., and G. Weidner. 1985. Antarctic automatic weather stameteorology. (College of Earth and Mineral Sciences.) University Park: tions, austral summer 1984-1985. Antarctic Journal of the U.S., 20(5), 189-190. Pennsylvania State University.

Numerical simulation of the Terra Katabatic winds are commonplace over the coastal rim of The strong, downslope winds occur primarily beNova Bay katabatic wind regime Antarctica. cause of the radiational cooling of the sloping ice surface. ConTHOMAS R. PARISH

Department of Atmospheric Science University of Wyoming Laramie, Wyoming 82071

sequently, the most well-defined and strongest katabatic episodes are generally found during the winter months. It is clear from previous observations that antarctic surface winds are intimately coupled to the orientation and steepness of the Undenying ice terrain. Ball (1960) was one of the first to describe satisfactorily the dynamics of antarctic surface winds as a bal-

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Figure 1. Time-averaged near-surface pattern of cold air drainage winds off the antarctic continent. (After Parish and Bromwich in press.) 252

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ance of the horizontal pressure gradient force arising from the diabatic cooling of the ice surface, Coriolis force and friction. Recently, Parish and Bromwich (1987) have used the Ball (1960) model formulation to diagnose the time-averaged near-surface airflow pattern over the antarctic continent. The results are shown in figure 1. Over certain interior sections, flow from a vast area becomes channeled into a narrow zone which is focused on a restricted stretch of coastline. Such confluence zones represent regions of local enhancement of the supply of negatively buoyant air available to feed coastal katabatic winds (Parish 1984) and thereby allow an enhancement of the katabatic regime. Several confluence zones can be identified in figure 1 such as upslope of coastal Adélie Land, site of the infamous "Home of the Blizzard" station of Cape Denison, and inland of Terra Nova Bay (75°S 165°E). Currently underway is a joint study between the Ohio State University and the University of Wyoming aimed at documenting the katabatic wind regime at Terra Nova Bay and examining the dynamics of the confluence zone inland of the coast. The katabatic winds are thought to be a major factor in the development of a large, recurring polynya in Terra Nova Bay. Bromwich and Kurtz (1984) report that the intense winds effectively advect newly formed ice eastward and thereby prevent a solid ice cover. To understand the Terra Nova Bay katabatic regime better, a number of observational strategies have been incorporated including deployment of automatic weather stations (Aws), airphotography of the regional sastrugi fields, a series of LC-130 flights equipped with the newly designed data acquisitions and display system for recording meteorological state parameters, and high-resolution satellite imagery. During the 1986-1987 austral summer field season, a refurbished AWS was deployed at Inexpressible Island and airphotography of regional sastrugi fields was completed. Installation of two other AWS units could not be done because of logistical difficulties. Four additional AWS units are to be installed and several LC-130 flights will be conducted during the 1987-1988 field season. In addition, another regional mapping of the sastrugi fields will take place. To complement the observational data set, extensive numerical simulations of the katabatic regime are being conducted using a three-dimensional, primitive equation, hydrostatic model. A description of the model framework is given in Parish (1984); explicit representation of the longwave radiative processes and turbulence characteristics is incorporated into the model to provide a realistic numerical treatment of the relevant physics. Preliminary model simulations have been completed to provide confirmation of the large-scale drainage streamlines represented in figure 1 and to offer insight as to the gross features of the katabatic circulation. In addition, the numerical results provide suggestions as to the positioning of the AWS array in the Terra Nova Bay region. The numerical simulations start from a condition of rest to isolate the terrain-induced katabatic winds; the ice topography for the Terra Nova Bay region is digitized from the detailed and accurate antarctic contour map of Drewry (1983). Illustrated in figures 2 and 3 are results after a 24-hour simulation during which time the katabatic winds have developed and have nearly reached steady conditions. Figure 2 shows the resulting streamlines of the near-surface winds (50 meters above the ice terrain) after the 24hour model time integration. A marked confluence zone is present and is directed into the Reeves Glacier. This suggests that most of the negatively buoyant air is concentrated into one major glacier outlet; surrounding glaciers such as the David and Priestley Glaciers appear to be on the fringe of this confluence effect. The overall pattern revealed in figure 2 is very similar to 1987 REVIEW

Figure 2. Streamline pattern of near-surface windfield after 24-hour model integration. Height contours in dashed and thin solid lines. ("km" denotes "kilometers:')

Figure 3. Wind speed (in meters per second) of near-surface windfield after 24-hour model integration. ("km" denotes "kilometers:')

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that seen in the large-scale streamline map of figure 1, providing at least tentative support for the viability of the simple Ball (1960) model. The resulting wind speeds of the katabatic regime after the 24-hour simulation are shown in figure 2. The intensity of the katabatic regime is clearly enhanced downslope of the confluence zone along the Reeves Glacier. The maximum wind speeds of greater than 20 meters per second are consistent with data from the AWS on Inexpressible Island downwind of Reeves Glacier (Kurtz and Bromwich 1985). The wind speeds appear to decrease significantly away from the confluence zone to values representative of more normal coastal katabatic winds. Such simulations confirm the importance of the interior confluence zone and the katabatic potential of the Reeves Glacier and vicinity. This research has been supported by the National Science Foundation through grant DPP 85-21176. I was in the Antarctic from 12 January to 3 February 1987.

A case study of mesoscale cyclogenesis over the southwestern Ross Sea

References Ball, F.K. 1960. Winds on the ice slopes of Antarctica. In Antarctic meteorology. New York: Pergamon. Bromwich, D.H., and D.D. Kurtz. 1984. Katabatic wind forcing of the Terra Nova Bay polynya. Journal of Geophysical Research 89, 3561-3572. Drewry, D.J. 1983. The surface of the Antarctic ice sheet. In D.J. Drewry (Ed.), Antarctica: Glaciological and geophysical folio, (Sheet 2). Cambridge: Scott Polar Research Institute. Kurtz, D.D., and D.H. Bromwich. 1985. A recurring, atmospherically forced polynya in Terra Nova Bay. In S. S. Jacobs (Ed.), Oceanology of the Antarctic Continental Shelf. (Antarctic Research Series, Vol. 43.) Washington, D.C.: American Geophysical Union. Parish, T.R. 1984. A numerical study of strong katabatic winds over antarctica. Monthly Weather Review, 112, 545-554. Parish, T.R., and D.H. Bromwich. 1987. The surface windfield over the Antarctic ice sheets. Nature, 327, 51-54.

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DAVID H. BROMWICH Byrd Polar Research Center Ohio State University Columbus, Ohio 43210

Bromwich (1986a) and Kurtz and Bromwich (1985) have proposed that the frequent west and northwest winds recorded at Franklin Island are primarily due to katabatic winds from Terra Nova Bay which propagate at least 190 kilometers across the western Ross Sea without the assistance of regional atmospheric pressure gradients. This paper analyzes the birth, evolution, and dissipation of a mesoscale storm complex near Franklin Island which produced prolonged adverse weather conditions at McMurdo Station in February 1984. The intense katabatic outflow from Terra Nova Bay (Bromwich 1986b) appeared to play an important role in the development and maintenance of this cyclone. The primary database is composed of observations from the mesoscale array of automatic weather stations (Aws) around Ross Island (Savage et al. 1985). The cyclone was first detected on regional AWS analyses around 1200 GMT (Greenwich mean time, about 12 hours behind local time) on 20 February 1984. Figure 1 shows the cyclone forming (called cyclogenesis) near the coast of Victoria Land just to the south of the Drygalski Ice Tongue. The cyclone presence is most clearly revealed by the slightly lower pressure at Inexpressible Island. The katabatic wind speed at Inexpressible Island is very strong (28 meters per second) and the air temperature contrast between Inexpressible Island and Franklin Island is only 2°C. The wind direction at Franklin Island is north-northwest which may be due to katabatic winds from 254

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60'E 'L1700E 80' 170°W 60'W Figure 1. Sea-level pressure analysis for 1200 GMT 20 February 1984. Circles with numbers attached identify automatic weather stations whose data were used to construct the chart. Wind direction is shown by the orientation of the line drawn to each Aws; wind speed is indicated by the symbols attached to this line. No symbol denotes a speed less than 1.3 meters per second; half a barb signifies 2.5 meters per second; a full barb equals 5 meters per second; and a flag represents 25 meters per second. Solid lines are sea-level isobars (contours of constant pressure) and are labelled with the hundreds digit omitted; 90 equals 990 hectopascals (hPa). The dashed lines represent constant air temperatures (isotherms) in degrees Celsius. ANTARCTIC JOURNAL