OROGRAPHICALLY MODIFIED WINDS OBSERVED BY THE NASA SCATTEROMETER

Michael H. Freilich

College of Oceanic and Atmospheric Sciences

Oregon State University


From mid-September, 1996 through June, 1997, the spaceborne NASA Scatterometer (NSCAT) provided researchers and operational meteorologists with all-weather surface wind measurements having unprecedented accuracy, resolution, and coverage. The accompanying figures, showing data acquired in the very first days of the NSCAT mission and surface analyses from an operational global weather forecast model, illustrate the dramatic influence of a mountainous island on the ocean wind field and demonstrate NSCAT's ability to measure wind speed and direction under harsh conditions at remote locations. The colors in the figures show the wind speed while the arrows indicate the wind direction at the NSCAT measurement locations or model grid points.

The NSCAT wind measurements were acquired at 1245 UTC (0845 local time) on September 13, 1996 in the vicinity of South Georgia Island, approximately 1300 km east of the Falklands/Malvinas in the Scotia Sea. South Georgia Island is only 170 km long and less than 30 km wide, yet it contains 12 peaks between 2000 and 3000 m in height. At the time of the data pass, NSCAT was programmed to make measurements having 12.5 km resolution in a 300 km wide swath. Subsequent analyses comparing NSCAT wind measurements with data obtained from open-ocean meteorological buoys demonstrates that the basic NSCAT wind measurements are accurate to better than 1.7 m/s in speed and 20 degrees in direction.

 !Click on the thumbnail to see a bigger version of the figure!

 

Figure 1.

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Figure 2.

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Figure 3.

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Figure 1 shows the overall synoptic near-surface wind conditions predicted by the operational National Center for Environmental Prediction (NCEP) global forecast/analysis system for 1200 UTC on September 13, 1996. An elongated high pressure ridge caused anticyclonic flow in the Scotia Sea. The operational analysis suggested that the near-surface winds in the vicinity of South Georgia Island would be blowing from the north-northwest with 19.5 m neutral stability speeds of ~ 18 m/s. Figure 2 shows the NCEP analysis speeds interpolated to the NSCAT wind measurement swath, based on the 6 model grid points (locations of the vectors in the figure) in the vicinity of South Georgia Island.

In contrast to the small wind speed and direction variations predicted by the global NCEP model (Figure 2), the NSCAT data (Figure 3) show five distinct wind regimes near South Georgia Island. Several hundred kilometers north (upwind) of the island (at the top of the figure), the winds are seen to be blowing generally southward at ~ 17 m/s. Within a 20-50 km wide band on the upwind side of the island, the upstream blocking effects of the mountains cause a substantial decrease in wind speed and a change in direction so that the winds are blowing parallel to the island. The winds accelerate to more than 25 m/s and again turn southward in a broad region off the eastern tip of the island. South (downwind) of the western portion of the island, the winds blow strongly (20 m/s) in a generally south-eastward direction. Directly downwind of the largest peaks, the NSCAT measurements reveal a dramatic wind shadow extending southward for several hundred kilometers and having minimum wind speeds less than 4 m/s.

Although it is well known that mountainous islands can perturb the oceanic surface wind field over large areas, the lack of high resolution, spatially extensive measurements has greatly hindered our ability to accurately model such orographic influences. Scale analyses (e.g., Overland and Bond, 1995) based on the known topography of South Georgia Island and the far-upwind synoptic conditions from the NCEP analysis (and confirmed by the NSCAT wind data far north of the island) suggest that the island topography should completely block the boundary layer winds, resulting in an upwind blocking region characterized by low wind speeds and nearly shore-parallel wind direction. The NSCAT measurements clearly show such an upwind blocking feature with a width of 50 km that compares favorably with the 60 km characteristic dimension calculated from the scale analysis.


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