Comparison of SMOS-retrieved and NDVI climatology-derived vegetation optical thickness

The Soil Moisture Active Passive (SMAP) mission may require ancillary vegetation optical thickness (τ) information as part of SMAP´s soil moisture retrieval algorithm. Currently, the ancillary τ data comes from a normalized difference vegetation index (NDVI) climatology that is converted to τ. The Soil Moisture Ocean Salinity (SMOS) satellite measures τ as part of its soil moisture retrieval algorithm. In this paper, we compare SMOS τ to SMAP´s proposed NDVI climatology-derived τ (SMAP τ). During the growing season in heavily cultivated parts of Iowa, SMAP τ is usually larger than SMOS τ. The timing of the peak in τ is similar between the two data sets on the whole, but some SMOS pixels in 2012 peak earlier by 15-20 days.