Evaluation of CryoSat-2 ocean products through routine monitoring and a 13-year long consistent time series dataset

CryoSat-2 (C2) was launched in 2010 and has been retrieving values of sea surface height anomaly (SSHA), significant wave height (SWH) and wind speed (WSP) ever since. Although these oceanographic variables were secondary aims, an operational CryoSat Ocean Processor was implemented in 2014. This study provides details on the routine monitoring process for the various products alongside additional analyses using a 13-year dataset of quality-controlled Baseline C data. Most of this study focussed on Low Resolution Mode (LRM) data with some specific studies of Synthetic Aperture Radar (SAR) mode. No substantial issues with the data were observed but additional studies of SWH and WSP are recommended. Global mean sea level (GMSL) from C2 has a trend of 4.2±0.3 mm/year which agrees, within uncertainties, from other data sources. Regionally the patterns in SSHA trend agree with other sources. SWH from similar analyses found no significant trend but WSP trend was 3.3±0.2 (cm/s)/year. For GMSL and WSP data from descending passes (C2 moving south) have non-significantly higher trends compared to data from ascending passes. Regionally, apart from a few, small areas (primarily around Southeast Asia), trends in C2 WSP are positive and agree with a previous study using multiple altimeters. Biases have been identified including between LRM and SAR using successive differences in SSHA, SWH and WSP. Similarly, seasonally varying biases between ascending passes and descending passes are found, which in turn vary by hemisphere. Comparisons were made between tide gauges based on data from the Permanent Service for Mean Sea Level (PSMSL) and gridded (1˚) SSHA from C2. For both LRM and SAR, the significant correlations were positive and similar, albeit slightly lower, to previous studies. Routine reports compare C2 SWH to that from the WaveWatch3 model (WW3) with the latter on average (mean) ∼15 cm higher with a few exceptions. This was confirmed using WW3 and C2 SWH on the same monthly grid where the difference was found to be 10 to 20 cm, and all correlations were positive (∼0.8–0.9). There are strong differences between C2 SWH and WW3 by hemisphere (bias is greater in Southern Hemisphere) and the variability of correlations is greater in the Northern Hemisphere. The relationship is more complex as there is some evidence that at low SWH WW3 tends to be higher than C2 whereas at higher SWH C2 SWH is higher than WW3. There is a bias between C2 WSP and that from ERA5 that is negative (ERA5 WSP higher than C2 WSP) at higher WSP and a less pronounced positive bias at low WSP. Comparisons of trends in WSP from C2 compared to ERA5 are more complex and patterns in both do not fully match. These results are possibly as a result of altimeters being unable to resolve low WSP (<∼2 m/s).