Inter-calibration of Metop-A and Metop-B scatterometers using ocean measurements

We recently developed a method for inter-calibrating spaceborne scatterometers. This method was successfully applied to ERS-1/ERS-2 and Metop-A/ERS-2 C-band scatterometers. The method is based on combining different natural targets (ocean, sea ice and rainforest) and associated geophysical models. In this paper, the inter-calibration method is applied to Metop-A and Metop-B scatterometers data with a focus on the ocean measurements. Additionally, the correction coefficients obtained from the ocean are compared to and validated on other independent targets i.e., rainforest and sea ice. Calibration of the scatterometer over ocean is widely used for monitoring and correction of the backscattering coefficients. The method is based on the assessment of the difference between the measured and the simulated backscatter using NWP winds and Geophysical Model Functions (GMF's) such as CMOD5. The method provides the instrument bias against the GMF. It was found that this bias varies spatially and temporally. This temporal and spatial variation of the bias could lead to discrepancies of up to 0.1 dB, which is significant compared to the calibration accuracy (0.2 dB). This adds to the actual bias (instrument drift) an artificial error which is due to the misfit of the input wind distribution. It is shown that this discrepancy is due to the sensitivity of the GMF to the wind speed distribution and this consequently yields the calibration over ocean to be sensitive to the wind speed distribution. The wind speed distribution variation in time and space is analyzed. The sensitivity of the calibration over the ocean to the wind speed distribution variation is assessed. Finally, a method is proposed to mitigate this variation and thus reduces the misfit error.