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The relationship between radar backscatter cross section and ocean wave parameters at low incidence angles

Using collocated data set of Tropical Rainfall Mapping Mission (TRMM) precipitation radar (PR) and buoy wind and wave measurements, we study the relationship between the Ku-band radar backscatter cross section (σ°), radar incidence angle (θ), the buoy measured near surface wind speed in 10m (U10), significant wave height (SWH), wave steepness (℘), and wave age (β). Sensitivity analysis of those parameters shows there is a nodal point at incidence angle about 1 1° for radar backscattering, which exhibits not only low sensitivity to wave parameters but also an overall low variability. Tabular empirical functions relating PR σ° to each single wave parameters for incidence angles from nadir to 18° are developed. The dependence of σ° on a single wave parameter in a fixed incidence angle is not unique, so multi-parameter functions of PR σ° are also investigated. Those multi-parameter functions can be used to retrieve wave parameter from radar backscatter with other wave parameters measurements. An example of empirical tabular model function relating wave steepness to PR σ° in terms of θ and U10 is firstly developed from collocated buoys and PR measurements.