Azimuthal Dependence of Sea-State Development inside Tropical Cyclones as Measured by CYGNSS

Earth remote sensing using reflected GNSS signals is an emerging trend, especially for ocean surface wind measurements, primarily due to its all day/all weather capability. GNSS-R measurements of ocean surface scattering cross section are directly related to the surface roughness at scale sizes ranging from small capillary waves to long gravity waves. In a fully developed sea state, the wind field has a constant speed and direction over a sufficiently long time and distance and can be assumed to be spatially homogeneous, thus the surface roughness at all scale sizes can be assumed to be wind driven. In this case, a single Fully Developed Seas (FDS) Geophysical Model Function (GMF) can be constructed which relates the scattering cross section to the wind speed. However, the sea age and fetch length conditions inside a tropical cyclone are in general not consistent with a fully developed sea state. Said conditions can also vary significantly with azimuthal location within the storm due to changes in the relative alignment of the storm’s forward motion and its cyclonic rotation. As a result, the relationship between the surface roughness at different scale sizes will become azimuthally dependent, as will the relationship between scattering cross section and wind speed represented by the GMF. A modified Young Sea Limited Fetch (YSLF) GMF is proposed that is a function of the azimuthal location inside a tropical cyclone. An empirical YSLF GMF is constructed using measurements by the NASA Cyclone Global Navigation Satellite System (CYGNSS) which are nearly coincident with HWRF reanalysis winds for 19 hurricanes in 2017 and 2018.The results demonstrate an azimuthal dependence of scattering cross section on wind speed that is consistent with expected azimuthal variations in sea age and fetch length within an organized storm.