Observed Boundary Layer Wind Structure and Balance in the Hurricane Core. Part I: Hurricane Georges

Abstract

The GPS dropsonde allows observations at unprecedentedly high horizontal and vertical resolution, and of very high accuracy, within the tropical cyclone boundary layer. These data are used to document the boundary layer wind field of the core of Hurricane Georges (1998) when it was close to its maximum intensity. The spatial variability of the boundary layer wind structure is found to agree very well with the theoretical predictions in the works of Kepert and Wang. In particular, the ratio of the near-surface wind speed to that above the boundary layer is found to increase inward toward the radius of maximum winds and to be larger to the left of the track than to the right, while the low-level wind maximum is both more marked and at lower altitude on the left of the storm track than on the right. However, the expected supergradient flow in the upper boundary layer is not found, with the winds being diagnosed as close to gradient balance. The tropical cyclone boundary layer model of Kepert and Wang is used to simulate the boundary layer flow in Hurricane Georges. The simulated wind profiles are in good agreement with the observations, and the asymmetries are well captured. In addition, it is found that the modeled flow in the upper boundary layer at the eyewall is barely supergradient, in contrast to previously studied cases. It is argued that this lack of supergradient flow is a consequence of the particular radial structure in Georges, which had a comparatively slow decrease of wind speed with radius outside the eyewall. This radial profile leads to a relatively weak gradient of inertial stability near the eyewall and a strong gradient at larger radii, and hence the tropical cyclone boundary layer dynamics described by Kepert and Wang can produce only marginally supergradient flow near the radius of maximum winds. The lack of supergradient flow, diagnosed from the observational analysis, is thus attributed to the large-scale structure of this particular storm. A companion paper presents a similar analysis for Hurricane Mitch (1998), with contrasting results.

Slab- and height-resolving models of the tropical cyclone boundary layer. Part I: Comparing the simulations

“…Their arguments were strongly influenced by the slab-model results of , so the fact that slab models overestimate the depth-mean inflow and the strength of the supergradient flow may be cause to doubt their reasoning. Indeed, while emphasize the ubiquity of supergradient flow in the slab model, analysis of observations shows that not all storms contain strongly supergradient flow in the boundary layer, consistent with simulation of these storms by the height-resolving model (Kepert, 2006a;Schwendike and Kepert, 2008). Further, do not give the mechanism by which the boundary-layer dynamics and supergradient flow lead to an adjustment in the cyclone's mass field, necessary for intensification.…”

Paradigms for tropical cyclone intensification

“…The boundary layer spin-up mechanism explains why the maximum azimuthally-averaged tangential wind speeds are located near the top of the boundary layer in the model calculations of Smith et al (2009) and others (e.g. Braun and Tao 2000, Zhang et al 2001, and in observations (Kepert 2006a, b, Montgomery et al 2006, Bell and Montgomery 2008, Schwendike and Kepert 2008, Sanger et al 2014, Zhang et al 2011a). …”

An Estimation of Turbulent Characteristics in the Low-Level Region of Intense Hurricanes Allen (1980) and Hugo (1989)

“…most theoretical models of hurricanes (e.g., Kepert 2001;Smith et al 2008;Bryan and Rotunno 2009b;Smith and Thomsen 2010), we believe it worthwhile to estimate the vertical diffusivity here using the data in Hurricanes Hugo and Allen. Typically, the vertical eddy diffusivity is defined as…”