Dropsonde Observations of Eyewall Boundary Layer Structure in Super-Typhoons Megi (2010) and Jangmi (2008

During the Impact of Typhoons on the Ocean in the Pacific (ITOP) and Tropical Cyclone Structure (TCS08) field programs conducted in the Western North Pacific in 2010 and 2008 respectively, unique dropsonde observations were obtained from WC-130J aircraft flights in and near the eyewall of two Super-Typhoons (STYs): Megi and Jangmi. These flights occurred during and following rapid intensification events resulting in peak SFMR surface wind speeds of 87- and 73 m/s and minimum surface pressures of 890 and 905 mb and allowed for unique insights into the eyewall boundary layer structure at extreme winds. Using a new generation GPS module, pioneered by NCAR Earth Observations Laboratory, and thermal/moisture observations at 4 Hz, rather than 2 Hz used previously, the Megi dropsonde data provided detailed wind and thermal observations all the way to the surface in 90% of the 16 sondes deployed in Megi's eyewall and along straight line flight legs tangent to, and just beyond, the radius of maximum winds. Similar data in the high wind boundary layer have been difficult to obtain in the past due to premature failure of wind observations well above the top of the frictional surface layer. One notable exception was the high-quality, extreme wind dropsonde observations in STY Jangmi's eyewall boundary layer, which are compared with those from Megi. These observations, together with Airborne eXpendable BathyThermograph (AXBT) profiles of sub-surface ocean temperature structure, were obtained from the WC-130J aircraft operated during these two projects by the dedicated crews of the U. S. Air Force Reserve Command's 53rd Weather Reconnaissance Squadron (Hurricane Hunters).

A unique aspect of the Megi observations was that most eyewall sondes were deployed in pairs closely spaced in time (typically one minute). The dropsonde pairs showed dramatically different boundary layer structures which were remarkably consistent in different storm quadrants with one sonde showing relatively weak boundary layer wind shear and its partner showing extreme wind shear, a result due in part, to very different sonde trajectories within the eyewall. The highest winds near the top of the apparently well mixed layer were in excess of 100 m/s, and were associated with some of the lowest eyewall surface winds (strong shear), while strongest surface winds in excess of 85 m/s were associated with the weakest boundary layer wind shear. A well-defined surface layer extending to 50-100 m exhibited constant potential temperature (theta) and radial and tangential winds with height resulting in higher near-surface winds than would be suggested by the boundary-layer log law. Unusual high-theta layers were observed above the surface layer and within the supposedly well-mixed layer. This paper will provide a thermodynamic and dynamic analysis of these dropsonde observations collected throughout the boundary under extreme wind conditions during STY Megi and STY Jangmi. Suggestions as to possible phenomena responsible for the unusual variations in the inner core boundary layer structure will be presented.