Soil Moisture Memory in Commonly-used Land Surface Models Differ
Significantly from SMAP Observation
Abstract
Weather and climate forecast predictability relies on Land-Atmosphere
(L-A) interactions occurring at different time scales. However,
evaluation of L-A coupling parameterizations in current land surface
models (LSMs) is challenging since the physical processes are complex,
and large-scale observations are scarce and uncommon. Recent
advancements in satellite observations, in this light, provide a unique
opportunity to evaluate the models’ performances at large spatial
scales. Using 5-year soil moisture memory (SMM) from Soil Moisture
Active and Passive (SMAP) observations, we evaluate L-A coupling
performances in 4 prevailing LSMs with both coupled and offline
simulations. Multi-model mean comparison at the global scale shows that
current LSMs tend to overestimate SMM that is controlled by
water-limited processes and vice versa. Large model spreads in SMM are
also observed between individual models. The SMM biases are highly
dependent on models’ parameterizations, while showing minor relevance to
the models’ soil layer depths or the models’ online/offline simulating
schemes. Further analyses of two important terrestrial water
cycle-related variables indicate current LSMs may underestimate soil
moisture that is directly available for evapotranspiration and global
flood risks. Finally, a comparison of two soil moisture thresholds
indicates that the soil parameters employed in LSMs play an essential
role in producing the model’s biases. The satellite estimation of ET at
the water-limited stage and soil hydraulic parameters provides readily
available information to constrain LSMs, which are essentially important
to improve the models’ L-A coupling simulations, as well as other land
surface processes such as terrestrial hydrological cycles.