A prerequisite for a successful development of the multi-mission wind (MMW) product is to ensure good inter-calibration of the different extreme wind datasets to be integrated in the product. Since the operational hurricane community is working with the in-situ dropsondes as wind speed reference, which are in turn used to calibrate the NOAA Hurricane Hunter Stepped Frequency Microwave Radiometer (SFMR) wind data (Uhlhorn et al., 2007), MAXSS uses the latter to ensure inter-calibration among the Tier-1 medium-resolution swath-based scatterometer and radiometer systems described in Section 2.2 and listed in Tables 1 and 2 of [MAXSS Team, 2021]. In short, these are: the Advanced Scatterometers onboard the Metop series (i.e., ASCAT-A, -B, and -C), the scatterometers onboard Oceansat-2 (OSCAT) and ScatSat- 1 (OSCAT-2), and onboard the HY-2 series (HSCAT-A, -B); the Advanced Microwave Scanning Radiometer 2 onboard GCOM-W1(AMSR-2), the multi-frequency polarimetric radiometer (Windsat), and the L-band radiometers onboard the Soil Moisture and Ocean Salinity (SMOS) and the Soil Moisture Active Passive (SMAP) missions.
Within the SMOS+STORM [Reul et al., 2017] and CHEFS [Stoffelen et al., 2020] projects, a preliminary calibration of, respectively, SMOS and ASCAT-A extreme winds has been carried out, using a long time series of collocated satellite-SFMR wind data in storm-centric coordinates, which are based on Best Track data. In particular, in the framework of CHEFS, SFMR winds are upscaled to account for ASCAT-A resolved spatial scales (or representativeness), and then used as reference for ASCAT wind recalibration purposes. The so-called ASCAT dropsonde-scale winds show sensitivity to high winds, and an overall correlation with SFMR upscaled winds of about 0.9 [Polverari et al., 2022]. The saturation of the GMF at extreme winds is somehow compensated by the high calibration stability of the ASCAT-A instrument. This SFMR-based re-calibration approach is revisited in MAXSS and applied to all the mentioned medium-resolution scatterometer and radiometer systems to ensure extreme wind inter-calibration of such systems.
Note that several sources of collocation errors have been reported, namely the Best Track temporal sampling and geolocation inaccuracies and the temporal differences between the SFMR and scatterometer acquisitions [Stoffelen et al., 2020]. However, mission-long datarecords are collocated with SFMR in order to mitigate the effect of collocation errors in the recalibration process. Moreover, the assumption of proper inter-calibration among satellite winds from the same data provider (i.e., the Ocean and Sea Ice Satellite Application Facility or OSI SAF for scatteromters and the Remote Sensing Systems or REMSS for all radiometers but SMOS) is used to improve the recalibration fit.
The following document describes the re-calibration in detail;