Analysis of Ku- and Ka-Band Sea Surface Backscattering Characteristics at Low-Incidence Angles Based on the GPM Dual-Frequency Precipitation Radar Measurements.

The co-located normalized radar backscatter cross section measurements from the Global Precipitation Measurement (GPM) Ku/Ka-band dual-frequency precipitation radar (DPR) and sea surface wind; wave and temperature observations from the National Data Buoy Center (NDBC) moored buoys are used to analyze the dependence and sensitivity of Ku- and Ka-band backscatter on surface conditions at low-incidence angles. Then the potential for inverting wind and wave parameters directly from low-incidence σ⁰ measurements is discussed. The results show that the KaPR σ⁰ is more sensitive to surface conditions than the KuPR σ⁰ overall. Nevertheless; both the KuPR σ⁰ and KaPR σ⁰ are strongly correlated with wind speed (U₁₀) and average wave steepness (δ_a) with the exception of specific transitional incidence angles. Moreover, U₁₀ and δ_a could be retrieved from pointwise σ⁰ near nadir and near 18°. Near 18°; wind direction information is needed as the effect of wind direction on σ⁰ becomes increasingly significant with incidence angle. To improve the performance of U₁₀ retrieval; especially for low U₁₀; auxiliary δ_a information would be most helpful; and sea surface temperature is better taken into account. Other wave parameters; such as significant wave height; wave period and wave age; are partly correlated with σ⁰. It is generally more difficult to retrieve those parameters directly from pointwise σ⁰. For the retrieval of those wave parameters; various auxiliary information is needed. Wind direction and wave direction cannot be retrieved from pointwise σ⁰. [ABSTRACT FROM AUTHOR]