Title:                     Soil moisture data assimilation and the SMAP L4_SM Data Product

Speaker:              Rolf Reichle, NASA/GSFC  

Abstract:

L-band (1.4 GHz) brightness temperature (Tb) observations collected by the NASA Soil Moisture Active Passive (SMAP) 
mission are directly related to surface (0-5 cm) soil moisture.  The primary objective of the SMAP Surface and Root 
Zone Soil Moisture (L4_SM) data product is to provide model-based estimates of “root zone” (0-100 cm)  soil moisture 
that are spatially and temporally complete and that are informed by and consistent with SMAP observations.  Such 
estimates are obtained by merging SMAP Tb observations with estimates from a land surface model in the NASA Goddard 
Earth Observing System, version 5 (GEOS-5) land data assimilation system.  The L4_SM product is global, at 9 km and 
3-hourly resolution, and publicly available with a mean latency of about 2.5 days.  

This presentation will provide an overview of the L4_SM algorithm and product, including calibration and 
validation.  The system was calibrated using L-band observations from the Soil Moisture Ocean Salinity (SMOS) 
mission.  A comparison with in situ measurements indicates that the SMAP L4_SM product meets the root-mean-square 
error target of 0.04 m3/m3 (after removal of the long-term mean differences) and is better than a model-only 
simulation that does not benefit from SMAP observations.  An investigation of the observation-minus-forecast 
residuals and the analysis increments of soil moisture suggests that the assimilation system is reasonably unbiased, 
but that the assimilation system tends to overestimate or underestimate the actual errors.