Assessing the SMAP Level-4 Carbon Product Over the Arctic and Subarctic Zones

The soil moisture active passive (SMAP) satellite mission distributes a product of CO$_{\text{2}}$ flux estimates (SPL4CMDL) derived from a terrestrial carbon flux model, in which SMAP brightness temperatures are assimilated to update soil moisture (SM) and constrain the carbon cyclemodeling. While the SPL4CMDL product has demonstrated promising performance across the continental USA and Australia, a detailed assessment over the arctic and subarctic zones (ASZ) is still missing. In this study, SPL4CMDL net ecosystem exchange (NEE), gross primary production (GPP), and ecosystem respiration (R$_{\text{E}}$) are evaluated against measurements from 37 eddy covariance towers deployed over the ASZ, spanning from 2015 to 2022. The assessment indicates that the NEE unbiased root-mean-square error falls within the targeted accuracy of 1.6 gC.m$^{\text{-2}}$.d$^{\text{-1}}$, as defined for the SPL4CMDL product. However, modeled GPP and R$_{\text{E}}$ are overestimated at the beginning of the growing season over evergreen needleleaf forests and shrublands, while being underestimated over grasslands. Discrepancies are also found in the annual net CO$_{2}$ budgets. SM appears to have a minimal influence on the GPP and R$_{\text{E}}$ modeling, suggesting that ASZ vegetation is rarely subjected to hydric stress, which contradicts some recent studies. These results highlight the need for further carbon cycle process understanding and model refinements to improve the SPL4CMDL CO$_{\text{2}}$ flux estimatesover the ASZ.