The Calculation of Low-Level Winds from the Archieved Data of a Regional Primitive Equation Forecast Model

Abstract

Meteorological data fields from NOAA's Nested Grid Model (NGM) were archived at 2-hour intervals from initialization to the 12-hour forecast time, two forecasts per day, for January, February, and March 1987. The NGM predictions of the winds at the lowest level (175 m) and predictions of surface momentum and sensible heat flux were used in a separate boundary layer model (BLM) to derive values of wind speed and direction between the surface and the lowest NGM level. BLM results were compared with measurements made on Savannah River Laboratory's meteorologically instrumented 366 m TV tower on Beech Island, South Carolina, and the nearby Augusta, Georgia, weather station. The comparison between predicted and measured wind speeds and directions was quite favorable in that the forecast model was able to reproduce the diurnal variations at all levels of the tower. However, the BLM underpredicted the mean tower wind speed profile by about 20% (1 m s?1) at most levels of the tower. Diurnal variations of wind speed of 4 m s?1 at the top of the tower to less than 0.5 m s?1 at the bottom were well predicted at all levels, with the predicted maximums and minimums occurring at the appropriate times. Wind direction changes of 25 deg at night and less than 10 deg during the day were similarly predicted; however, predicted wind directions were biased by about +10 deg compared with the tower measured wind directions.