Abstract:
The NASA Soil Moisture Active Passive (SMAP) mission Level-4 Soil Moisture (L4_SM) product provides global, 9-km resolution, 3-hourly surface (0-5 cm) and root-zone (0-100 cm) soil moisture from April 2015 to present with a mean latency of 2.5 days from the time of observation. The product is based on the assimilation of SMAP L-band (1.4 GHz) brightness temperature observations into the NASA Catchment land surface model as the model is driven with observations-based precipitation forcing.

In the current Version 6 of L4_SM, satellite- and gauge-based precipitation from the NASA Integrated Multi-satellitE Retrievals for the Global Precipitation Measurement mission (IMERG) are used in two ways. First, the climatology to which all L4_SM precipitation forcing inputs are rescaled is based on IMERG-Final (Version 06B) data, replacing the Global Precipitation Climatology Project v2.2 data used in L4_SM Version 5. Second, in L4_SM Version 6 the precipitation forcing outside of North America and the high latitudes is corrected to match the daily totals from IMERG, replacing the gauge-only, daily product or uncorrected weather analysis precipitation used there in L4_SM Version 5. Specifically, L4_SM Version 6 re-processing used the satellite- and gauge-based IMERG-Final product and forward-processing uses the satellite-only IMERG-Late product.

Compared to L4_SM Version 5, the Version 6 surface and root-zone soil moisture is wetter in much of South America and Australia and drier in most of Africa, owing primarily to the revised precipitation climatology. Evaluation based on independent radar soil moisture retrievals reveals that the anomaly time series correlation skill of L4_SM Version 6 surface soil moisture is considerably improved (relative to Version 5) in South America, Africa, Australia, and parts of East Asia. Particularly large improvements are seen in central Australia and Myanmar, where the quality of the gauge-only precipitation product used in L4_SM Version 5 was particularly poor. Smaller but nevertheless important skill improvements are also seen in supplemental L4_SM soil moisture estimates that were derived from just the satellite-only IMERG-Late precipitation for the entire period of record (thus mimicking the quality of the forward-processing product).