Characterization of the variability in 10 meter GPS dropwinsonde winds in extremely high wind environments

The introduction of GPS dropwindsondes into aircraft reconnaissance has enabled much to be learned regarding the wind structure of the boundary layer just above the ocean surface underneath tropical cyclones and intense baroclinic lows. In the most intense hurricanes and North Atlantic baroclinic "bombs" a great deal of variability exists in the 10 meter winds reported from one dropwindsonde to another in the same storm and in seemingly similar storm relative locations and environmental settings.

This study attempts to characterize the variability of these 10 meter winds along with advancing hypotheses as to the reasons behind the observed variability. A total of 87 soundings were examined. This included 67 sondes launched into the eyewall of several of the most intense hurricanes in recent years. Sonde data from both the NOAA Aircraft Operations Center WP-3D as well as the USAF Reserve 53rd Weather Reconnaissance Squadron WC-130J was collected from missions into Category 4 and 5 Hurricanes Ivan (2004), Katrina (2005), Rita (2005), Dean (2007), Felix (2007), Bill (2009) and Earl (2010). In addition, 20 NOAA WP-3D dropwindsondes launched into intense North Atlantic baroclinic lows in January and February of 2011 (during the NESDIS sponsored Ocean Winds 2011 Field Experiment) were analyzed.

Comparisons will be made between the 10 meter winds (or lowest winds available when a 10 meter wind was not measured (ie: a 16 meter wind)) and the mean wind in the lowest 20, 30, 40, 50, 150 and 500 meters of the sounding. For example, on 28 August 2005, a WP-3D mission into Category 5 Hurricane Katrina included eyewall wind measurements taken at nearly identical radii from center and similar storm relative locations at 10 meters, 16 meters and 22 meters of 104 knots, 139 knots, and 114 knots respectively. The WL150 winds (the mean wind speed of the lowest 150 meters) in each case were 127 knots, 147 knots and 133 knots respectively while the Mean Boundary Layer winds (the mean winds in the lowest 500 meters) for each sonde were 135 knots, 149 knots and 143 knots respectively.

Many factors that may influence the variability of 10 meter winds will be examined. Among these is the thermodynamic instability of the boundary layer. The possible effects of "shadowing" of 10 meter winds by gigantic ocean waves along with the emulsive nature of the air-sea interface in extremely high wind settings will also be discussed.