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Re: Wind anomaly over Twin Cities metro

Posted By: John M. Brown
Date: 13-September-04 1857Z

In Response To: Wind anomaly over Twin Cities metro (Thomas Lowe)

Thomas,
Thanks for your astute observation. The fundamental reason why you see the reduced surface winds over the Cities derives from our use of an assumed large ground roughness (in this case due to buildings) over these areas. This has the effect of locally reducing the wind at the surface as predicted in the model, since the surrounding areas, which are mainly under cultivation, are assumed to be much smoother. Similar effects can be seen in RUC analyses and forecasts at Des Moines, Omaha, and other locations in the Midwest that are surrounded by land predominantly under cultivation.

Since we don't have access to observations in your area except MSP airport, we can't quantify how much reduction there is (airports, of course, are of necessity open, so observations from airports will show less reduction in wind speed than elsewhere). The reduction will typically be highly variable, depending on proximity to large buildings. Shadowing of the surface wind to the lee of buildings and trees typically extends over 10 times as far downwind as the buildings or trees are high. We recommend you also look at our "Surface wind gust speed" graphics, which take into account vertical mixing of the wind from above the surface, where the wind speed is less affected by buildings. Also, you may want to look at the wind speed predicted away from urban areas, particularly over larger lakes.

In addition to the RUC20 products on our web site (http://ruc.fsl.noaa.gov) you can see forecasts from a new experimental version of RUC running at 13km (identified as "13km dev RUC" on the index on the left side of the home page). Again, thanks for your observations; if you see other problematic behavior let us know.

Messages In This Thread

Wind anomaly over Twin Cities metro -- Thomas Lowe -- 13-September-04 1231Z
Re: Wind anomaly over Twin Cities metro -- John M. Brown -- 13-September-04 1857Z

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