Azimuthal Dependence of GNSS‐R Scattering Cross‐Section in Hurricanes

Rajeswari Balasubramaniam,Christopher Ruf

doi:10.1029/2020jc016167

Abstract

AbstractGlobal Navigation Satellite System‐Reflectometry (GNSS‐R) measurements of the ocean surface are sensitive to roughness scales ranging from a few cms to several kms. Inside a hurricane the surface roughness changes drastically due to varying sea age and fetch length conditions and complex wave‐wave interactions caused by its cyclonic rotation and translational motion. As a result, the relationship between the surface roughness at different scale sizes becomes azimuthally dependent, as does the relationship between scattering cross‐section and wind speed as represented by a Geophysical Model Function (GMF). In this work, the impact of this azimuthal variation on the scattering cross‐section is assessed. An empirical GMF is constructed using measurements by the NASA CYclone GNSS (CYGNSS) matched to HWRF reanalysis surface winds for 19 hurricanes in 2017 and 2018. The analysis reveals a 2–8% variation in scattering cross‐section due to azimuthal location, and the magnitude of the azimuthal dependence is found to grow with wind speed.

Highlights

The measurement of hurricane wind fields has a long history, ranging from airborne measurements (Jones et al, 1981; Uhlhorn et al, 2007; Wright et al, 2001) to spaceborne observations made by microwave radiometers and radars (Ebuchi et al, 2002; Figa‐Saldaña et al, 2002; Gaiser et al, 2004)
The analysis reveals a 2–8% variation in scattering cross‐section due to azimuthal location, and the magnitude of the azimuthal dependence is found to grow with wind speed
For a fully developed sea (FDS), which constitutes the majority of the measurements, the observables are matched to the ground truth reference which is the combination of European Centre for Medium‐Range Weather Forecasts (ECMWF) and Global Data Assimilation System (GDAS) reanalysis wind speed products

Summary

Introduction

The measurement of hurricane wind fields has a long history, ranging from airborne measurements (Jones et al, 1981; Uhlhorn et al, 2007; Wright et al, 2001) to spaceborne observations made by microwave radiometers and radars (Ebuchi et al, 2002; Figa‐Saldaña et al, 2002; Gaiser et al, 2004). The sea age and fetch length conditions can vary significantly with azimuthal location due to its rotational and translational motion This can perturb the balance between the roughness at different scales and alter the measured scattering cross‐section. The directional wave spectra acquired from several hurricane reconnaissance missions suggest that local wind and wave directions vary sinusoidally with the azimuth angle referenced to the hurricane heading and have a weak radial dependence (Hwang et al, 2017) These results suggest that remote sensing techniques such as GNSS‐R, which depend on surface wave scattering, should consider azimuthal wind‐wave response functions for accurate modeling and subsequent wind retrieval. The impact of azimuthal variation on the measured scattering cross‐section is assessed using CYGNSS data over 19 major hurricanes across different basins during 2017 and 2018 For this analysis, HWRF reanalysis hurricane winds are used as a reference.

Theory and Observations

Harmonic Model Function

Performance Assessment

Discussion and Conclusion

Findings

Data Availability Statement