Lower atmosphere studies

Nonhydrostatic numerical simulation of antarctic katabatic wind events

THOMAS R. PARISH, LARRY D. OOLMAN, and JOHN J. CASSANO, Department of Atmospheric Science, University of Wyoming, Laramie, Wyoming 82071

A significant body of antarctic meteorology literature has been dedicated to the persistent, near-surface slope (katabatic) flows ( see Bromwich and Parish in press and references contained therein). Katabatic winds are forced partly by the radiative cooling of the sloping ice surface (e.g., Parish and Waight 1987; Parish and Wendler 1991) and partly by the large-scale horizontal pressure gradients associated with transient synoptic disturbances (e.g., Parish, Pettré, and Wendler 1993; Yasunari and Kodama 1993).

During the past few decades, significant advances have been made in understanding the meteorology of the lower antarctic atmosphere. Much of the progress can be attributed to the collection of data sets, notably automatic weather stations (AWSs), and subsequent analyses (Bromwich et al. 1993; Wendler et al. 1993). Mathematical simulation of katabatic flows has also been a useful tool in understanding antarctic katabatic winds. Briefly, such modeling involves grid-point solutions of finite difference forms of the hydrodynamical equations governing atmospheric motion. Currently, a number of numerical models exist, and they have been used to simulate the low-level drainage flow regime (Parish 1984; Gallée and Schayes 1992; Hines, Bromwich, and Parish 1995; Seefeldt 1996; Cassano in press). One of the widely used meteorological models is the Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model (MM5). The MM5 equations are written in terrain-following sigma coordinates for use in either a hydrostatic or nonhydrostatic mode. The horizontal and vertical resolution in the model is variable, allowing application to a variety of atmospheric problems. This model has elaborate physics parameterizations to simulate effects such as solar and terrestrial radiation, cloud physics, and turbulence. The model uses standard meteorological data sets for initial conditions and so can be used in a real-time forecast mode.

Assessment of the application of MM5 to the Antarctic is currently underway at the University of Wyoming. This model is being used in both two-dimensional (Cassano in press) and three-dimensional versions. Application of any numerical model to antarctic meteorology presents several challenges.

A number of physical parameterization schemes exist. Some are better adapted for use in the antarctic environment than others.
The temperatures and wind fields in the lower antarctic atmosphere meteorology are often strongly influenced by the local topography. As seen with earlier modeling work, it is essential to specify terrain representative for the scale of phenomena of interest. Thus, use of a large-grid spacing [greater than 100 kilometers (km)] such as used in most operational weather forecast models is inadequate for simulation of local phenomena.
It is necessary to begin model simulations with appropriate initial conditions for the underlying ice terrain. This re-quirement often necessitates the use of nonstandard techniques to initialize MM5 and means the user must acquire familiarity with the model.

Figures 1 and 2 illustrate results from one MM5 simulation for Antarctica. The case study in question is from 25 March 1993. During this period, an intense katabatic wind storm damaged wind sensors on AWS units at Port Martin (66.8°S 141.4°E) and Penguin Point (67.2°S 146.0°E; Keller et al. 1995). A nonhydrostatic MM5 simulation was run with an inner and outer domain, each consisting of a 52 x 52 grid. The grid spacing was set at 60 km for the outer domain and 20 km for the inner domain. The model simulation was for 12 hours commencing at 0000 UTC 25 March 1993. Results at the end of the simulation (1200 UTC) are presented here. Figure 1 illustrates the 12-hour simulated surface pressures (in hectopascals) and streamlines of wind approximately 5 meters above the surface from the outer mesh. An intense cyclone is seen to the north of the continent; streamlines of the airflow suggest a strong control of airflow off the continent by the large-scale horizontal pressure field. The streamline pattern over the continent also suggests high-pressure ridging over the high plateau of East Antarctica. Surface winds appear to display an upslope component across a broad portion of the antarctic continent on the eastern side of the anticyclonic (counterclockwise in the Southern Hemisphere) circulation. Results from the 12-hour simulation for the inner mesh, centered on the Adélie Land terrain, are shown in figure 2. The cyclone and terrain forcing accelerate the surface katabatic winds such that winds in excess of 15 meters per second are seen along the near-coastal margin near 140°E. The simulated winds are weaker than observed, however. AWS units record winds in excess of 30 meters per second at this time. Reasons for this underestimation are currently being investigated.

Research is continuing to improve the utility of such modeling efforts. It is hoped that real-time simulations can be routinely provided to enhance forecasting capabilities in the Antarctic. Results of the modeling work will be of use for both research and operational activities.

This research has been supported in part by the National Science Foundation grant OPP 92-18544.

References

Bromwich, D.H., and T.R. Parish. In press. Meteorology of the Antarctic. In D. Karoly and D. Vincent (Eds.), Meteorology of the Southern Hemisphere . Boston: American Meteorological Society.

Bromwich, D.H., T.R. Parish, A. Pellegrini, C.R. Stearns, and G.A. Weidner. 1993. Spatial and temporal characteristics of the intense katabatic winds at Terra Nova Bay, Antarctica. In D.H. Bromwich and C.R. Stearns (Eds.), Antarctic meteorology and climatology: Studies based on automatic weather stations (Antarctic Research Series, Vol. 61). Washington, D.C.: American Geophysical Union.

Cassano, J.J. In press. Analysis of east antarctic katabatic winds using two numerical models. (Ph.D. thesis, Department of Atmospheric Science, University of Wyoming, Laramie, Wyoming.)

Gallée, H., and G. Schayes. 1992. Dynamical aspects of katabatic wind evolution in the antarctic coastal zone. Boundary-Layer Meteorology , 59, 141-161.

Hines, K.M., D.H. Bromwich, and T.R. Parish. 1995. A mesoscale modeling study of the atmospheric circulation of high southern latitudes. Monthly Weather Review , 123, 1146-1165.

Keller, L.M., G.A. Weidner, C.R. Stearns, and M.T. Whittaker. 1995. Antarctic automatic weather station data for the calendar year 1993. Madison, Wisconsin: Department of Atmospheric and Oceanic Sciences, University of Wisconsin.

Parish, T.R. 1984. A numerical study of strong katabatic winds over Antarctica. Monthly Weather Review , 112, 545-554.

Parish, T.R., P. Pettré, and G. Wendler. 1993. The influence of large scale forcing on the katabatic wind regime of Adélie Land, Antarctica. Meteorology and Atmospheric Physics , 51, 165-176.

Parish, T.R., and K.T. Waight. 1987. The forcing of antarctic katabatic winds. Monthly Weather Review , 115, 2214-2226.

Parish, T.R., and G. Wendler. 1991. The katabatic wind regime at Adélie Land. International Journal of Climatology , 11, 97-107.

Seefeldt, M.W. 1996. Wind flow in the Ross Island region, Antarctica, based on the UW-NMS with a comparison to automatic weather station data. (M.S. thesis, Department of Atmospheric and Oceanic Sciences, University of Wisconsin, Madison, Wisconsin.)

Wendler, G., J.C. André, P. Pettré, J. Gosink, and T. Parish. 1993. Katabatic winds in Adélie Land. In D.H. Brom-wich and C.R. Stearns (Eds.), Antarctic meteorology and climatology: Studies based on automatic weather stations (Antarctic Research Series, Vol. 61). Washington, D.C.: American Geophysical Union.

Yasunari, T., and S. Kodama. 1993. Intraseasonal variability of katabatic wind over East Antarctica and planetary flow regime in the Southern Hemisphere. Journal of Geophysical Research , 98, 13063-13070.