Observational and modeling studies of the katabatic winds at Terra ...
Ball, F.K. 1960. Winds on the ice slopes of Antarctica. In Antarctic inetr'orologt!. Proceedings of the S y mposium, Melbourne, 1959. New York: Pergamon Press. Bromwich, D.H. 1986. Surface winds in West Antarctica. Antarctic Journal of the U.S., 21(5), 235-237. Bromwich, D.H. 1989. Satellite analyses of Antarctic katabatic wind behavior. Bulletin of the American Meteorological Society, 70, 738-749. Parish, T.R., and D.H. Bromwich. 1986. The inversion wind pattern over West Antarctica. Monthly Weather Review, 114, 849-860. Parish, T.R., and D.H. Bromwich. 1987. The surface windfield over the Antarctic ice sheets. Nature, 328, 51-54.
Parish, T.R., and D.H. Bromwich. 1989. Instrumented aircraft observations of the katabatic wind regime near Terra Nova Bay. Monthly Weather Review, 117, 1,570-1,585. Schwerdtfeger, W. 1970. The climate of the Antarctic. In Climates of the Polar Regions, World Survey of Climatology (Vol. 14). New York: Elsevier Publishing Company. Zwally, N.J., J.C. Comiso, and A.L. Gordon. 1985. Antarctic offshore leads and polynyas and oceanographic effects. In S.S. Jacobs (Ed.), Oceanology of the Antarctic Continental Shelf. (Antarctic Research Series, Vol. 43.) Washington, D.C.: American Geophysical Union.
Observational and modeling studies of the katabatic winds at Terra Nova Bay
As part of the study, numerical simulations of the Terra Nova Bay katabatic wind regime have been conducted. The model used is a six-level, bulk-layer version of the three-dimensional, hydrostatic, primitive equation model described in Parish (1987). Details of the model equations, finite difference forms, and boundary conditions can be seen in Parish and Waight (1987). The lowest layer is approximately 115 meters in depth and corresponds to a well-mixed katabatic wind layer. Boundary-layer radiative and turbulent flux parameterizations
THOMAS R. PAI.usII
Depart mnent of Atmospheric Science University of Wttomning Laranue, Wiomning 82071
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DAVID H. BROMWICII
Bt,rd Polar Research Center Ohio State University Columbus, Ohio 43210
( Priestley Neve
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One of the most commonplace meteorological features in the lower atmosphere over Antarctica is the gravity-induced slope or katabatic wind. Such winds are best developed during the austral winter months above the steeply sloping coastal periphery of the continent. Intense winds in excess of 30 meters per second are frequently encountered during these periods. Currently underway is a comprehensive study of the katabatic wind regime near Terra Nova Bay. This site was selected because previous studies (Bromwich and Kurtz 1982; Kurtz and Bromwich 1985) have shown the area to be prone to intense katabatic winds for nearly the entire winter. Parish and Bromwich (1987) have shown that significant channeling of the cold air in the interior of the continent acts to make the Terra Nova Bay region one of the windiest in all of Antarctica. Automatic weather stations have been deployed at Inexpressible Island, some 30 kilometers downwind from the mouth of the Reeves Glacier, since 1984. More recently, an array of five additional automatic weather stations have been deployed to sample the spatial variation of the katabatic wind. In addition, four automatic weather stations have been set up in support of ongoing, cooperative meteorological studies of the Italian Antarctic Expedition. Data for the winter months of 1988 are illustrated in figure 1. The strongest katabatic winds (winter resultant winds in excess of 15 meters per second are seen at Inexpressible Island; other stations situated further in the interior experience somewhat weaker winds generally between 8 and 10 meters per second. The wind directions at the stations reflect the general streamline pattern shown in Parish and Bromwich (1987). 1989 REVIEW
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Figure 1. Winter surface winds in the Terra Nova Bay area, February to September 1988. The data with adjacent bold numbers denote automatic weather station sites. The following variables are listed vertically near each automatic weather station: directional constancy, mean 3-meter wind speed in meters per second, and average potential temperature in celsius. The wind vectors plotted for each site give the vector-average wind and follow conventional plotting notation. Automatic weather stations 50, 52, and 53 belong to the Italian Antarctic Expedition and the remainder are U.S. deployments. 221
are based on results obtained from the high-resolution mesoscale numerical model (Pickett 1989). The bulk model offers a convenient means of providing results comparable with the more refined approach without excessive computational demands. The bulk model uses a 50-by-50 grid of antarctic terrain heights spaced 20 kilometers apart. The height values were obtained from the digitized height map of Drewry (1983). The model atmosphere was initialized about a state of rest; the initial temperature profile is representative of average atmospheric conditions over Antarctica during the non-summer months (see figure 6.9 in Schwerdtfeger 1984). The model
equations have been integrated for a period of 12 hours by which time near-steady solutions prevail. To focus in on Terra Nova Bay, results from a 21-by-21 grid nested within the larger 50-by-50 domain are presented. Figure 2 shows the 20-kilometer topographic representation of the Terra Nova Bay region as well as results from the 12-hour simulation for wind vectors, streamlines, and wind speeds. Some of the detailed topographic features are smoothed somewhat; the terrain surrounding the Reeves and David glaciers is extremely complex with mountain peaks interspersed throughout the area of interest. The 12-hour field of velocity
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Figure 2. Results from 12-hour numerical simulation of katabatic winds over Terra Nova Bay; clockwise from top left: terrain features, katabatic wind vectors, katabatic wind speeds, and streamlines of cold air drainage. (km denotes kilometer.)
222
ANTARCTIC JOURNAL.
vectors within the bulk katabatic layer shows that the flow is strongly channeled at the head of the Reeves Glacier and accelerates down along the neck of the glacier. The topographically forced drainage convergence leads to a highly asymmetric katabatic wind pattern across the glacier making the flow on the north side significantly stronger than that depicted on the south side. Thermal infrared satellite images reveal north-south temperature gradients across Nansen Ice Sheet which are probably caused by this meridional speed distribution (Bromwich in press). The corresponding flow streamlines illustrate a marked confluence zone upwind of the Reeves Glacier, in agreement with the streamlines proposed by Parish and Bromwich (1987). In addition, a minor confluence feature can be seen approximately 40 kilometers south of the main confluence zone. Bromwich, Parish, and Zorman (in press) have noted that airborne sastrugi observations upstream from the David Glacier offer strong support for a second significant confluence zone in the vicinity of Terra Nova Bay. The model-produced 12-hour katabatic wind speeds in the lowest layer suggest strong katabatic winds are confined to the steep slopes in the vicinity of Reeves Glacier. A maximum wind speed in excess of 25 meters per second is seen within the Reeves Glacier; this is well within the limits observed from the automatic weather station platforms situated along Reeves Glacier as well as from data collected during the instrumented flights of November 1987 (Parish and Bromwich in press). The strong katabatic winds issuing from Reeves Glacier can be seen to persist for extended distances. Note that a 15-meter-per-second flow can be traced well over 100 kilometers from the base of the ice slope. Again, this facet of the numerical simulations is in at least qualitative agreement with the data collected during airborne study as well as inferred from automatic weather station data and satellite imagery (Bromwich in press). We wish to thank Charles Stearns and George Weidner of the University of Wisconsin at Madison for their efforts in the construction, deployment, and data collection of the automatic weather stations. The Italian automatic weather station data
A strong katabatic wind event at Terra Nova Bay DAVID
H.
BIoMwIcH
Byrd Polar Research Center Ohio State University Columbus, Ohio 43210 THOMAS R. PARISH
Department of Atnosphcric Science University of Wyoming Laramie, Wyoming 82071
Surface winds over the sloping ice fields of Antarctica appear to be highly irregular with marked areas of confluence and difluence inland from the steep coastal ice slopes (Parish and 1989 REVIEW
presented here were made available to David H. Bromwich under the Data Exchange Agreement between the Italian National Antarctic Research Program and the Byrd Polar Research Center. This research has been supported by the National Science Foundation through grants DPP 87-16127 (to Thomas R. Parish) and DPP 87-16076 (to David H. Bromwich).
References Bromwich, D.H. In press. Satellite analyses of Antarctic katabatic wind behavior. Bulletin of the American Meteorological Society.
Bromwich, D.H., and D.D. Kurtz. 1982. Experiences of Scott's Northern Party: Evidence for a relationship between winter katabatic winds and the Terra Nova Bay polynya. Polar Record, 21, 137-146. Bromwich, D.H., T.R. Parish, and C.A. Zorman, In press. Dynamics of the confluence zone of the intense katabatic winds at Terra Nova Bay, Antarctica as derived from airborne sastrugi surveys and mesoscale numerical modeling. Journal of Geophysical Research.
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. 1987. Numerical simulation of the Terra Nova Bay katabatic wind regime. Antarctic Journal of the U.S., 22(5), 252-254. Parish, T.R., and D.H. Bromwich. 1987. The surface windfield over the Antarctic ice sheets. Nature, 328, 51-54. Parish, T.R., and D.H. Bromwich. 1989. Instrumented aircraft observations of the katabatic wind regime near Terra Nova Bay. Monthly Weather Review, 117, 1,570-1,585. Parish, T.R., and K.T. Waight. 1987. The forcing of Antarctic katabatic winds. Monthly Weather Review, 115, 2,214-2,226. Pickett, J.L. 1989. A mixed-layer model of katabatic winds. (MS. thesis, Department of Atmospheric Science, University of Wyoming.) Schwerdtfeger, W. 1984. Weather and climate of the Antarctic. New York: Elsevier.
Bromwich 1987). Where air from a large section of the ice sheet becomes focused (a "confluence zone"), the supply of cold air to downwind coastal sectors is considerably enhanced, and the resulting katabatic winds are intensified and more persistent. Confluence zones seem to be the dominant features of the antarctic surface windfield and may be responsible for the majority of the boundary-layer transport of air across the antarctic periphery (Parish and Bromwich 1986). Terra Nova Bay is a region with intense katabatic winds that are sustained by an inland confluence zone and is viewed as a prototype of this climatically important coupling. We are conducting an in-depth investigation to describe the kinematics and dynamics of this representative wind regime. Completed analyses include establishment of the time-averaged characteristics of the confluence zone from aerial photography of the snow surface and from mesoscale numerical modeling (Bromwich, Parish, and Zorman in press). An important finding was that within about 180 kilometers of the coast, the broadscale confluence zone becomes organized into two smaller confluence zones focused on the Reeves and David 223
