Relating Observations of Gradient Nonbalance at the Top of Hurricanes With Their Warm Core Structures

Recent analysis of Weather Research and Forecasting simulations shows that the flow around high-pressure regions that often develop at the top of Hurricanes (15-km altitude) violates the gradient wind balance, hence termed as “gradient nonbalance.” While observations at such high altitudes are rare, recently, NASA-HS3 and the Office of Naval Research-Tropical Cyclone Intensity campaigns deployed dropsondes from unprecedented levels around 18 km. In this work we use a wavenumber decomposition method to reproduce 2-D slices and 3-D structures from three storms from these campaigns: Edouard (2014), Joaquin (2015), and Patricia (2015). The analyzed data show that the development of high pressure at the hurricane's top leads to various degrees of gradient nonbalance. These highs are a hydrostatic reflection of the storm tilt, which offsets the warm core with respect to the near surface low and the slanted eyewall. The implications of these findings and the relations with recent reports of upper-level wind field are discussed.