Symmetric Double-Eye Structure in Hurricane Bertha (2008) Imaged by SAR

Guosheng Zhang,William Perrie

doi:10.3390/rs10081292

Guosheng Zhang, William Perrie

Open Access

https://doi.org/10.3390/rs10081292

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Abstract

Internal dynamical processes play a critical role in hurricane intensity variability. However, our understanding of internal storm processes is less well established, partly because of fewer observations. In this study, we present an analysis of the hurricane double-eye structure imaged by the RADARSAT-2 cross-polarized synthetic aperture radar (SAR) over Hurricane Bertha (2008). SAR has the capability of hurricane monitoring because of the ocean surface roughness induced by surface wind stress. Recently, the C-band cross-polarized SAR measurements appear to be unsaturated for the high wind speeds, which makes SAR suitable for studies of the hurricane internal dynamic processes, including the double-eye structure. We retrieve the wind field of Hurricane Bertha (2008), and then extract the closest axisymmetric double-eye structure from the wind field using an idealized vortex model. Comparisons between the axisymmetric model extracted wind field and SAR observed winds demonstrate that the double-eye structure imaged by SAR is relatively axisymmetric. Associated with airborne measurements using a stepped-frequency microwave radiometer, we investigate the hurricane internal dynamic process related to the double-eye structure, which is known as the eyewall replacement cycle (ERC). The classic ERC theory was proposed by assuming an axisymmetric storm structure. The ERC internal dynamic process of Hurricane Bertha (2008) related to the symmetric double-eye structure here, which is consistent with the classic theory, is observed by SAR and aircraft.

Highlights

Accurate observations of surface winds of a tropical cyclone, the high-resolution structures, play a critical role in improving hurricane dynamic readiness and understanding of its evolution process
The maximum wind speed of the inner eyewall is much smaller than the outer eyewall, the outline of the inner core is very clear in the wind speed field
As the synthetic aperture radar (SAR) imagery of hurricane Bertha (2008) captures a double-eye structure (Figure 5), we address two points consistent with the classic theories for the eyewall replacement cycle (ERC) internal dynamic process: (1) the structure of the sea surface wind field is axisymmetric, and (2) the intensity of the inner eyewall is smaller than the outer eyewall

Summary

Introduction

Accurate observations of surface winds of a tropical cyclone (hurricane or typhoon), the high-resolution structures, play a critical role in improving hurricane dynamic readiness and understanding of its evolution process. Over the past 25 years, prediction skill for hurricane intensity has had comparatively few improvements because of limited knowledge regarding the hurricane internal dynamical processes, whereas its track forecasts errors have steadily declined [1]. Based on this axisymmetric assumption, hurricane internal dynamics have been analyzed using the SFMR measurements, i.e., the vortex Rossby wave dynamics [3], eyewall replacement cycles (ERCs) [4], and hurricane pressure-wind model [5]. The actual hurricane structure, whether axisymmetric or not, is difficult to discern, only based on the aircraft reconnaissance low-level SFMR data. The surface wind fields of hurricanes have high azimuthal asymmetries, and these asymmetries are hard to measure by the aircraft, which typically flies at roughly fixed azimuths with time legs [5,6,7]

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