Abstract

This Algorithm Theoretical Basis Document (ATBD) presents a prototype soil moisture (SM) retrieval algorithm for the Copernicus Imaging Microwave Radiometer (CIMR) mission. In doing so, the forthcoming CIMR mission carries on the legacy of successful L-band missions such as SMOS, SMAP, and Aquarius.

The proposed algorithm is based on the simultaneous retrieval of soil moisture and vegetation properties. It builds on the tau-omega radiative transfer model and heritage of the SMOS-IC algorithm and SMAP dual channel algorithms, while evolving to reduce dependencies on ancillary information by exploiting CIMR multifrequency capabilities. It utilizes both H-polarized and V-polarized brightness temperature (TB) observations at the L-band frequency (~1.4 GHz) to estimate soil moisture. Across CIMR bands, L-band has the highest sensitivity to soil moisture but also the coarsest spatial resolution.

CIMR aims to provide two soil moisture products: the first is based on the inversion of L-band TBs at their native resolution (<60 km, hydroclimatological), and the second involves the inversion of L-band TBs at an enhanced spatial resolution by means of sharpening with C/X-band TBs (~15 km, hydrometeorological). In addition to soil moisture, the algorithm will also provide a secondary output: the vegetation optical depth (VOD or τ), which represents the degree of attenuation of the microwaves through the canopy. This microwave vegetation parameter is an ecological indicator that correlates well with diverse vegetation attributes such as water content and biomass.