Variabilities of Drag Coefficient under Five Tropical Cyclones

Tropical cyclones as devastating mesoscale storms can induce significant upper ocean physics via surface wind stress. The surface wind stress plays an important role in the tropical cyclone-ocean interaction, often parameterized by the drag coefficient, which is critical to the forecast of tropical cyclones’ intensity in the numerical models. Using the measurements of ocean current velocity taken by 19 EM-APEX floats deployed in five tropical cyclones (Hurricane Frances 2004, Hurricane Gustav 2008, Hurricane Ike 2008, Typhoon Fanapi 2010 and Typhoon Megi 2010), the integration of storm-induced momentum in the ocean can be used to estimate the surface wind stress at the ocean surface, and thereby the drag coefficient. The parameterizations of drag coefficient vary in different sectors under tropical cyclones, presumably due to other factors, such as surface waves. Significant orientation offset between the surface wind stress and wind at 10-m height above the sea surface is also found, can be up to 30 degree under tropical cyclones.