A sea drag relation for hurricane wind speeds

Abstract

For the determination of the stress at the air‐sea interface and the near‐surface wind speed, numerical weather prediction (NWP) models commonly use a drag coefficient. Generally, the Charnock relation is used, which gives an increase in the drag coefficient for increasing near‐surface wind speeds. According to observations, however, the magnitude of the drag coefficient levels off at a wind speed of approximately 30 m s −1 , and decreases with a further increase of the wind speed. Consequently, the surface drag is overestimated in NWP models for hurricane wind speeds and the intensity of hurricane winds is underestimated in forecasts. In this study, a parameterization that gives a decrease in the surface drag is tested in an NWP model. Two hurricanes in the Caribbean are modeled: Ivan (2004) and Katrina (2005). The results show that this drag parameterization leads to much stronger hurricanes in forecasts, and are in good agreement with observations.