Tropical Cyclone Center Positions from Sequences of HDSS Sondes Deployed along High-Altitude Overpasses

A method is developed to calculate the zero-wind center (ZWC) position from a sequence of Yankee High Density Sounding System (HDSS) dropwindsondes deployed during a high-altitude overpass of a tropical cyclone. The approach is similar to the Willoughby and Chelmow technique in that it utilizes the intersections of bearings normal to the wind directions across the center to locate the ZWC position. Average wind directions over 1-km layers are calculated from the accurate global positioning system (GPS) latitude–longitude positions as the HDSS sonde falls from the 60 000-ft flight level of the NASA WB-57 to the ocean surface. An iterative procedure is used to also account for the storm translation, which is necessary to put these high-frequency HDSS observations into a storm-relative coordinate system. The Tropical Cyclone Intensity (TCI-15) mission into Hurricane Joaquin on 4 October 2015 is examined here. The ZWC positions from two center overpasses indicate the vortex tilts from 1- to 10-km elevation and rotates cyclonically with height.