Examining Surface Momentum Balance and Boundary Layer Conditions in Extreme Tropical Cyclones

On a seasonal basis for both the Atlantic and Pacific basins, strong tropical cyclones (TCs) can develop but rarely reach extreme intensities (i.e. surface wind speeds > 155 kts, 80 m/s). When intense TCs develop, one way to measure their surface winds is by using the Stepped Frequency Microwave Radiometer (SFMR) on the NOAA WP-3D and the Air Force Reserve Command (AFRC) WC-130J aircraft. Oftentimes, proximity to these extreme storms prevents the capture of this high wind data. During the 2005 Atlantic hurricane season, hurricanes Katrina, Rita, and Wilma all attained category five status and were all reachable by aircraft, but the SFMR never measured wind speeds greater than 75 m/s in these storms. However, during a 2007 NOAA flight into hurricane Felix, surface wind speeds were measured near 84 m/s, and during the 2010 ITOP experiment, an AFRC flight into supertyphoon Megi consistently observed surface wind speeds in excess of 85 m/s near the eyewall of the cyclone. For this work, the goal is to understand the momentum budget and boundary layer conditions in extreme cases where the surface wind speed exceeds 80 m/s, which is an extension of the Uhlhorn et al. (2011) presentation.

In order to accomplish this goal, the TC inflow equation taken from Malkus and Reihl (1960) and recent boundary layer work by Zhang (2010) are used to estimate the cross-isobaric inflow angle. Because the surface wind speed, drag coefficients (Powell et al. (2003); Donelan et al. (2003)), and the inflow angle are known, it is possible to solve for the inflow height that is necessary for balancing the momentum equation. The frictional component of the momentum budget equation is then rearranged and the inflow height is determined for the extreme cases. Due to the inflow height's dependency on storm intensity, the momentum balance is expected to be altered for the extreme cases compared to weaker TCs. To quantify this momentum balance change, radial profiles of radial and tangential wind speed are calculated, providing radial profiles of expected wind speed. The budget for extreme cases is then compared to that of weaker TCs.