Boundary Layer Structure within the Eyewall of 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, the Megi dropsonde data provided detailed wind and thermal observations all the way to the surface in 14 of 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.

A unique aspect of the Megi observations was that most eyewall sondes were deployed in pairs closely spaced in time (< 1-min time separation). 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 high wind-shear partner sonde exhibited highest winds near the top of the mixed layer on order of 90-100 m/s, and were associated with surface winds on order of 60-70 m/s. The weak wind shear partner exhibited top of the mixed layer winds of order of 90-95 m/s while strongest surface winds were on order of 80-85 m/s. A well-defined surface layer extending to 50-100 m exhibited constant potential temperature (theta) as well as constant radial and tangential winds. This constant wind with height layer resulted 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. We speculate that the phenomena responsible for the unusual variations in the eyewall boundary layer structure may be associated with micro-vortices embedded within the eyewall.