Ku-Band Sea Surface Radar Backscatter at Low Incidence Angles under Extreme Wind Conditions

Xiuzhong Li,Yijun He,Biao Zhang,Alexis Mouche,William Perrie

doi:10.3390/rs9050474

Abstract

This paper reports Ku-band normalized radar cross section (NRCS) at low incidence angles ranging from 0° to 18° and in the wind speed range from 6 to 70 m/s. The precipitation radar onboard the tropical rainfall measuring mission and Jason-1 and 2 have provided 152 hurricanes observations between 2008 and 2013 that were collocated with stepped-frequency microwave radiometer measurements. It is found that the NRCS decreases with increasing incidence angle. The decrease is more dramatic in the 40–70 m/s range of wind speeds than in the 6–20 m/s range, indicating that the NRCS is very sensitive to low incidence angles under extreme wind conditions and insensitive to the extreme wind speed. Consequently, the sea surface appears relatively “smooth” to Ku-band electromagnetic microwaves. This phenomenon validates the observed drag coefficient reduction under extreme wind conditions, from a remote sensing viewpoint. Using the NRCS dependence on incidence angle under extreme wind conditions, we also present an empirical linear relationship between NRCS and incidence angles, which may assist future-satellites missions operating at small incidence angles to measure sea surface wind and wave field.

Highlights

Tropical cyclones are always associated with high wind speeds
Active microwave remote sensors have advantages for estimating wind speeds because the normalized radar cross section (NRCS, σ0 ) of the sea surface varies with the roughness and whitecaps or the foam effects that are driven by high wind speeds
This study newly presents the NRCS characteristics at extremely high wind speeds

Summary

Introduction

Tropical cyclones ( called hurricanes or typhoons) are always associated with high wind speeds. The wind speed plays a crucial role in air–sea interactions, numerical forecasting models, and tropical cyclone intensity. Active microwave remote sensors have advantages for estimating wind speeds because the normalized radar cross section (NRCS, σ0 ) of the sea surface varies with the roughness and whitecaps or the foam effects that are driven by high wind speeds. Besides wind speeds at the sea surface, the NRCS can be used to detect ship targets at the sea surface [1], ocean surface currents [2], ocean wave spectra [3], and other ocean surface information. Several researchers have analyzed the C-band NRCS saturation at wind speeds up to 35 m/s [4,5,6]

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