Abstract
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 (δa) with the exception of specific transitional incidence angles. Moreover, U10 and δa 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 δa 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.
Highlights
Spaceborne observations of the ocean surface with active microwave instruments have been performed routinely for more than four decades since the launch of SeaSat in 1978, with most of the sensors working at moderate incidence angles (i.e.,16◦ ≤ θ ≤ 65◦) and near-vertical incidence angles (i.e.,θ 1◦)
The microwave radar backscattered signal from the sea surface at low-incidence angles is dominated by quasi-specular reflection, and the radar backscattering cross section σ0 can be described by the geometrical optics (GO) method [15,16,17]
The probability density function (PDF) is a function of the slope variance, skewness and peakedness which are thought to be directly related to the surface wind and the overall degree of sea state development [5,18]
Summary
Spaceborne observations of the ocean surface with active microwave instruments have been performed routinely for more than four decades since the launch of SeaSat in 1978, with most of the sensors working at moderate incidence angles (i.e.,16◦ ≤ θ ≤ 65◦) and near-vertical incidence angles (i.e.,θ 1◦). In recent years, an increasing number of spaceborne radar systems, including the Ku-band precipitation radar on the Tropical Rainfall Mapping Mission (TRMM PR), the Ku/Ka-band dual-frequency precipitation radar on the Global Precipitation Measurement mission (GPM DPR), the Ku-band surface wave investigation and monitoring radar on the China–France Oceanography SATellite (CFOSAT SWIM) and the Ka-band radar interferometer on the Surface Water Ocean Topography mission (SWOT KaRin), measure the ocean surface at low-incidence angles as such measurements have great potential to obtain the information on sea surface winds and waves very based on the simple quasi-specular reflection theory (e.g., [1,2,3,4]) This triggers the need for well understanding of the characteristics of radar backscatter at Ku- and Ka-band at low-incidence angles. These analyses are still not deep enough as the observation data concerning σ0 at Ka-band are relatively scarce, and the speciality needs further research
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