Using SMOS obervations for science development of the SMAP level 4 surface and root zone soil moisture algorithm

Authors: REICHLE, ROLF - National Aeronautics And Space Administration (NASA); DE LANNOY, GABRIELLE - National Aeronautics And Space Administration (NASA); Crow, Wade; KIMBALL, JOHN - National Aeronautics And Space Administration (NASA); KOSTER, RANDALL - National Aeronautics And Space Administration (NASA); LI, QUI - National Aeronautics And Space Administration (NASA); DRAPER, CLARA - National Aeronautics And Space Administration (NASA)

Submitted to: International Geoscience and Remote Sensing Symposium Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 6/15/2013
Publication Date: 7/21/2013
Citation: Reichle, R., De Lannoy, G., Crow, W.T., Kimball, J., Koster, R., Li, Q., Draper, C. 2013. Using SMOS obervations for science development of the SMAP level 4 surface and root zone soil moisture algorithm. 2013 International Geoscience and Remote Sensing Symposium [abstract]. July 21-26, 2013, Melbourne, Australia. 2013 CDROM.

Interpretive Summary:

Technical Abstract: The NASA Soil Moisture Active and Passive (SMAP) mission is targeted for launch in October 2014. The soil moisture mapping provided by SMAP has practical applications in weather and seasonal climate prediction, agriculture, human health, drought and flood decision support. The Soil Moisture and Ocean Salinity (SMOS) mission was launched by ESA in November 2009 and has since been observing 1.4 GHz (L-band) upwelling passive microwaves. In this presentation we describe our use of SMOS brightness temperature observations for science development of the planned SMAP Level 4 Surface and Root-zone Soil Moisture (L4SM) product. The SMOS interferometric radiometer observes L-band passive microwave emission at a range of incidence angles at a resolution of about 40 km (varying with incidence angle) and with a measurement error standard deviation of approximately 4 K. The SMAP mission will make simultaneous active (radar) and passive (radiometer) measurements in the 1.26-1.43 GHz range (L-band). Similar to SMOS, the SMAP radiometer measurements will be at about 40 km resolution. Unlike SMOS, SMAP will observe brightness temperature at a constant incidence angle of 40-degrees and with a design accuracy of 1.3 K. SMAP and SMOS observations are directly connected to surface soil moisture (in the top 5 cm of the soil column). Several of the key applications targeted by SMAP, however, require knowledge of root zone soil moisture (~top 1 m of the soil column), which is not directly measured by SMAP. The foremost objective of the SMAP L4SM product is to fill this gap and provide estimates of root zone soil moisture that are informed by and consistent with SMAP observations. Such estimates are obtained by merging SMAP brightness temperature observations with estimates from a land surface model in a soil moisture data assimilation system. The land surface model component of the assimilation system, the NASA Catchment land surface model, is driven with observations-based surface meteorological forcing data, including precipitation, which is the most important driver for soil moisture. The model also encapsulates knowledge of key land surface processes, including the vertical transfer of soil moisture between the surface and root zone reservoirs. A radiative transfer model is added to the land surface model to simulate microwave radiances. The horizontally distributed ensemble Kalman filter (“3d EnKF”) update step considers the respective uncertainties of the model estimates and the observations, resulting in a soil moisture and soil temperature analysis at 9 km resolution that is, in theory, superior to satellite or model estimates alone. Moreover, error estimates for the L4SM product are generated as a by-product of the data assimilation system. L-band brightness temperature observations from SMOS and SMAP are only sensitive to soil moisture and soil temperature in a shallow surface layer (~top 5 cm of the soil column). The SMAP L4SM algorithm merges brightness temperature observations with estimates from a land surface model in a soil moisture and soil temperature analysis system and is designed to provide estimates of root zone soil moisture (~top 1 m of the soil column) that are informed by and consistent with SMAP observations. Such estimates are important for several applications targeted by SMAP. SMOS has been providing multi-angular, global passive microwave observations at L-band since its launch in November 2009. These observations are highly valuable for the science development of the SMAP L4SM algorithm. In a first step, the multi-angular SMOS observations were used to calibrate the L4SM microwave radiative transfer model. In a second step, multi-angular SMOS observations were assimilated into the prototype L4SM algorithm. Results show that the assimilation estimates of surface and root zone soil moisture are superi