ASCAT -A, -B, and -C ocean surface wind Climate Data Record
The C-band ASCAT scatterometer was first launched on MetOp-A in 2006, and has been processed at RSS since April 2016, from 2007-present, as version V2.1 (Ricciardulli, 2016), using the updated Geophysical Model Function (GMF) C-2015. We recently used the same GMF and wind algorithm to process ocean surface wind retrievals from the newer ASCAT scatterometers on MetOp-B (2012-present), and MetOp-C (2019-present), now released as version V2.1.
Significant efforts were devoted to ensure that the three scatterometers’ wind retrievals were precisely cross-calibrated and meeting the stringent accuracy requirements for a Climate Data Record (CDR): about 0.1 m/s at the global monthly scale (WMO Global Climate System, 2011; Wentz et al., 2017). This was achieved by comparing measurements between these scatterometers and satellite radiometer winds closely collocated in time and space during the overlapping periods. The absolute calibration of each scatterometer has been verified using as ground truth a database of wind measurements from ocean moored buoys from the global network including TAO/TRITON, PIRATA, RAMA, NDBC, and MEDS datasets.
Using this method, we detected two small step-changes (less than 0.1 m/s) in the ASCAT-A V2.1 wind record: one due to an instrument instability (July 2016), and the second to a calibration adjustment that was accidentally disabled in the data processed after October 2019. These step-changes were corrected with very small adjustments (less than 0.1 dB) to the ASCAT-A radar backscatter (sigma0) observations. All ASCAT-A data from July 2016 until present has been reprocessed as V2.1.1, using these small adjustments (see Figure 1).
Figure 1: Monthly globally averaged (55N-55S) timeseries of the wind speed difference between ASCAT-B and ASCAT-A V2.1 (red, dashed line), and ASCAT-C (blue). ASCAT-B and -C have been extremely stable after the first 6 months of their respective missions. Two distinct step-changes emerge for ASCAT-A in RSS V2.1(red dashed line: one in July 2016 and again in October 2019. To correct for these jumps, we reprocessed all ASCAT-A since the beginning of 2016 and applied an adjustment of -0.05dB to the s0 from the L1B files starting on July 22,2016, and restored the +0.08dB adjustment which was accidentally turned off in October 2019. The new adjusted version is referred to as ASCAT-A V2.1.1, and its cross-calibration with ASCAT-B is displayed with the red solid line.
The ASCAT wind retrieval methodology, the GMF, the calibration adjustment and the precise calibration verification versus buoys and additional wind datasets are described in detail in the manuscript Ricciardulli and Manaster (2021), currently under review, which will be shared here as soon as published.
Ricciardulli, L., (2016): “ASCAT on Metop-A Data product update notes: V2.1 data release,” Remote Sensing Systems, Santa Rosa, CA, USA, Tech. Rep. 040416.
Ricciardulli, L., and A. Manaster (2021): Intercalibration of ASCAT Scatterometer winds from MetOp-A, -B, and -C, for a Stable Climate Data Record”, submitted to Remote Sensing.
World Meteorological Organization, (2011): “Systematic observation requirements for satellite-based data products for climate,” WMO, Geneva, Switzerland, Tech. Rep. GCOS-154.
Wentz, F., Ricciardulli, L., Rodriguez, E., Stiles, B., Bourassa, M., Long, D., et al., (2017): Evaluating and extending the ocean wind climate data record. IEEE J. Sel. Top. Appl. Earth Observ. Remote Sens. 10, 2165–2185. doi: 10.1109/JSTARS.2016.2643641