Case 14: 17 - 18 October 1996

Case 14: 17 - 18 October 1996

Geostrophic and observed winds

Objectives

Understand when geostrophic wind is a good approximation to the observed wind.
Understand the variety of influences on the observed surface wind.

Exercises
Plots

Clear screen.
Create a loop of the mean sea level pressure, 1000 hPa geostrophic wind using the ETA model output every six hours for the period 00 UTC 17 October through 00 UTC 18 October using the F000 and F006 times.
Select the surface reports button (second icon) and select the surface reports for the same time as the model times.

Questions

1. Zoom in over the surface low at 12 UTC (upper midwest). Compare the observed winds to the 1000 hPa geostrophic winds. Why are the geostrophic winds so much stronger over Wisconsin and Illinois than the observed winds? In the vicinity of the surface low at this time, is the difference in wind direction between the observed and geostrophic winds what you expect?

2. At 06 UTC there is a strong pressure gradient across southern Nevada and Utah with strong geostrophic winds (at about 40 kts from the east). What are the observed winds in this region and why are they so different. (Hint: you may want to plot the terrain).

Plots

Reset screen.
Create a single map of 300 hPa geopotential height, geostrophic wind and observed winds for 12 UTC 18 October using the F000 time.
Zoom in on the region including Montana and east to western Pennsylvania.
Use the Upper Air button (4th button) for the observed winds and make sure and select "Plan" for type and change the level to 300 hPa! Also, to make the plot easier to read, edit the station model in the Upper Air button to include ONLY the station identifier and the wind barb.

Questions

3. What are the observed and geostrophic winds at the following locations: Springfield MO (SGF), Little Rock AR (LZK), Glasgow MT (GGW), Bismark ND (BTS), and Amarillo TX (AMA).

4. Explain why the observed wind is less than the geostrophic wind at some locations and greater at others.

Main Points

Winds are supergeostrophic in ridges and subgeostrophic in troughs due to the gradient wind balance.
In regions of complex terrain, the winds tend to blow from high to low pressure. (However, note that a comparison between 1000 hPa geostrophic winds and surface winds may not be "fair" since the 1000 hPa surface may be above the ground).

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