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

Abstract

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.

Vertical variation of tropical cyclone size in the western North Pacific

“…The TC size calculation is not very sensitive to the TC centre definition, as long as the TC centre is located within the area of relative vorticity >1 × 10 −5 s −1 . Most of the time, the Rvor size can be correctly calculated, except that at the upper levels under the influence of the anticyclonic flow and tilting, the TC core is displaced from the surface centre (e.g., Cohen et al ., 2019), and hence the calculation of the TC size may be contaminated. Nevertheless, at the upper levels, the area of relative vorticity >1 × 10 −5 s −1 is not compact but scattered, and the “exact” centre does not guarantee its location in the area of relative vorticity larger 1 × 10 −5 s −1 , and thus a “perfect” method to obtain the TC size probably does not exist.…”

Lagrangian trajectories at the outflow of tropical cyclones

“…(2019) provided observational evidence for such geopotential surfaces; additional evidence regarding high‐pressure development in TC outflow is cited in Cohen et al . (2019).…”

“…Surfaces of 150 hPa geopotential produced by the two‐Gaussian model are compared with observed geopotential fields of the two storms taken from Cohen et al . (2019). Figure 3a,b, respectively, show observed 150 hPa in Edouard (2014) and Patricia (2015) while Figure 3c,d show the corresponding fields as reproduced by the two‐Gaussian model.…”

“…Thus, the upper-level geopotential map of tropical cyclones may exhibit a low-pressure centre surrounded by annulus, or horseshoe-shaped, high pressure. Cohen et al (2019) provided observational evidence for such geopotential surfaces; additional evidence regarding high-pressure development in TC outflow is cited in Cohen et al (2019).…”

“…Furthermore, both models (Cohen et al, 2017) and observations (Cohen et al, 2019) show that in such geopotential maps, the gradient wind balance is often violated around the high-pressure regions. This is due to strong outward-pointing centrifugal and pressure gradient forces that cannot be counterbalanced by the inward-pointing Coriolis force (see Cohen et al (2015) for a concise mapping of the solution of the gradient balance).…”