Friday, May 16th, 2025
 
 
announcement
 

Dear users,

The Metop ASCAT Soil Moisture products currently generated at the EUMETSAT Central Facility, and distributed on EUMETCast, will be replaced by new, upgraded soil moisture products generated by the H SAF, as follows:

  • H16 (Large-scale surface soil moisture by radar scatterometer) and H104 (Metop-C ASCAT NRT SSM orbit geometry 12.5km sampling) will be replaced by H29 (Metop ASCAT Surface Soil Moisture Near Real-Time 12.5km sampling V2).
  • H103 (Metop-B ASCAT NRT soil moisture 25km sampling) and H105 (Metop-C ASCAT NRT SSM orbit geometry 25km sampling) will be replaced by H122 (Metop ASCAT Surface Soil Moisture Near Real Time 6.25km sampling).

Main changes introduced with the new products include:

  • Projection on to a fixed Earth grid (6.25km for H122, 12.5km for H29) using the ASCAT Level 1b full resolution backscatter product as input. As well as dropping the coarse resolution ASCAT SSM product (25km sampling) and introducing a better spatial resolution (6.25km sampling), one of the key differences is that there is no sampling of data using swath orbit nodes. The new fixed Earth grid makes it easy to build time series without applying any resampling procedure beforehand. In addition, individual backscatter echoes provided in the ASCAT Level 1b full resolution backscatter product allow for more accurate filtering of signals coming from areas not sensitive to soil moisture changes (eg, open water, cities).
  • The underlying core soil moisture retrieval algorithm — a semi-empirical change detection method — will not change. However, the estimation of the model parameters (slope/curvature, dry/wet backscatter reference) has evolved (Hahn et al., 2017, Hahn et al. 2020), and is based on a much longer backscatter time series (2007–2023) than before. The retrieval method itself is not changing, but the updated model parameters have a significant impact on the dynamics of soil moisture and its extremes, rendering the old ASCAT SSM NRT product inconsistent with the upcoming SSM NRT product.
  • A more adaptive vegetation correction has been introduced by using spatially varying cross-over angles instead of static angles. In addition, the computation of the slope and curvature climatology is now based on a kernel smoother function. Moreover, long-term land cover changes are introducing trends in backscatter, eg, in areas affected by urban growth. In order to mitigate those effects, the dry/wet backscatter reference is now estimated on a yearly basis, taking data from neighbouring years into account.

Main characteristics of ASCAT SSM 6.25km (H122) and ASCAT SSM 12.5km (H29):

  • Variable: Soil moisture content in the surface layer (< 5cm)
  • Unit: Degree of saturation (0–100%)
  • Spatial sampling/resolution: 6.25km/15x15km2 or 12.5km/25x25km2
  • Spatial coverage: Global (65°S, 180°W, 80°N, 180°E)
  • Timeliness: 115 min (for the NRT product) Format: netCDF4 and BUFR
It is planned that the old products will be discontinued in Q2 2025. To ensure a smooth transition for users, the new products, H29 and H122, will be distributed on EUMETCast, in parallel to the old products for a few months. More information will be provided in due course. In the meantime, sample files of the new products are available on the H SAF FTP server: ftphsaf.meteoam.it (registration on the H SAF website is required).