NWP ocean calibration of Ku-band scatterometers

Calibration procedures over the ocean using forecast winds have the advantage that they may be applied over a large portion of the globe and consequently may provide accurate results over a relatively short period. Ocean calibration has been applied successfully for the European Remote-Sensing Satellite (ERS) and Advanced SCATterometer (ASCAT) C-band fan-beam scatterometer wind products at KNMI. The OceanSat-2 rotating pencil-beam scatterometer (OSCAT) uses a Ku-band radar wavelength (13.515 GHz) which is strongly affected by the presence of rain within a scatterometer wind vector cell (WVC). In this paper, The Indian Space Research Organisation (ISRO) L2A data and 50km resolution L2B data processed by the OSCAT Wind Data Processor (OWDP) software, which is being developed by the KNMI scatterometer group, and include European Centre for Medium-range Weather Forecasts (ECMWF) Numerical Weather Prediction (NWP) equivalent-neutral winds, are successfully used in OSCAT NWP ocean calibration (NOC). With the NOC results obtained, the backscatter is corrected and wind retrieval statistics between ECWMF NWP winds and OSCAT NOC-corrected winds are computed and shown to be improved w.r.t. the OSCAT winds without NOC corrections applied. Based on the KNMI QC(Quality Control) flag, the rain effects to Ku-band NWP ocean calibration are investigated and NOC corrections based on WVCs that pass the QC are about 0.1dB lower than the NOC corrections without the QC applied. The best wind retrieval statistics are thus effectively obtained after that the QC-ed NOC corrections are applied on Ku-band OSCAT scatterometer backscatter data.