Next-generation Metop ASCAT Surface Soil Moisture datasets

Abstract. This article presents the next-generation of the Advanced Scatterometer (ASCAT) surface soil moisture (SSM) dataset, bringing the operational near real-time (NRT) product up to date with the historical offline data record. For years, the offline data record has benefited from successive algorithmic improvements while the NRT product has only received minor updates. This release now applies the latest soil moisture retrieval algorithm and consistent fixed-Earth grid to both data streams, creating a unified dataset and representing a major advancement for the ASCAT SSM NRT product. Furthermore, the standard 12.5 km sampling ASCAT SSM dataset is now complemented by a new high-resolution 6.25 km sampling ASCAT SSM product. This is achieved by customising the spatial resampling process of the ASCAT Level 1B full-resolution backscatter data. A new key development in the change detection algorithm for ASCAT SSM concerns the estimation of the dry and wet backscatter references. Specifically, a moving-window approach is now applied instead of the full time series to mitigate artificial trends caused by long-term land cover changes. Additionally, a new monthly subsurface scattering flag has been added to filter out unreliable SSM measurements where backscatter and soil moisture indicate an inverted relationship.

Quality control of the ASCAT SSM datasets is performed by using soil moisture estimates from Noah GLDAS-2.1 and the ESA CCI Passive Soil Moisture (SM) v09.1 product, as well as in-situ observations provided by the International Soil Moisture Network (ISMN). The validation results show that both ASCAT SSM datasets have a comparable performance in terms of the Pearson correlation coefficient (ASCAT SSM 6.25 km vs ESA CCI Passive SM: 17.9 % > 0.75 and 57.8 % > 0.5; ASCAT SSM 12.5 km vs ESA CCI Passiv SM: 19.6 % > 0.75 and 59.2 % > 0.5) and signal-to-noise ratio (SNR) derived using triple collocation analysis (ASCAT SSM 6.25 km SNR: 56.0 % > 0 dB, 35.6 % > 3 dB, ASCAT SSM 12.5 km SNR: 58.1 % > 0 dB, 38.6 % > 3 dB,). The best performance can be found in regions with strong seasonal variability, including monsoonal, savanna, Mediterranean, and tropical wet-and-dry zones. A lower performance can be found in areas characterised by limited soil moisture variability (such as deserts), dense vegetation, pronounced topographic complexity, wetland areas, or higher latitudes (> 60° N) experiencing longer periods of frozen soil and snow cover.

The ASCAT SSM datasets are publicly available online https://doi.org/10.15770/EUM_SAF_H_0011 and https://doi.org/10.15770/EUM_SAF_H_0012 (2025a, c), while ASCAT SSM NRT products are additionally distributed via the broadcasting system EUMETCast.