Abstract:
Errors in soil moisture adversely impact the modeling of land-atmosphere water and energy fluxes and, consequently, near-surface atmospheric conditions in atmospheric data assimilation systems (ADAS). To mitigate such errors, a land surface analysis is included in many such systems, although not yet in the currently operational NASA Goddard Earth Observing System (GEOS) ADAS. This study investigates the assimilation of L-band brightness temperature (Tb) observations from the Soil Moisture Active Passive (SMAP) mission in the recently developed GEOS weakly-coupled land-atmosphere data assimilation system (LADAS) during summer 2017. The SMAP Tb analysis improves the correlation vs. in situ measurements of LADAS soil moisture by ~0.1-0.26 over that of the ADAS; the unbiased root-mean-square error (ubRMSE) of LADAS soil moisture is reduced by 0.002-0.008 m3 m-3 from that of ADAS. Furthermore, the global average RMSE vs. in situ measurements of LADAS screen-level air specific humidity (q2m) and daily maximum temperature (T2m_max) is reduced by 0.05 g kg-1 and 0.04 K, respectively, from that of ADAS. Regionally, the RMSE of LADAS q2m and T2m_max is improved by up to 0.4 g kg-1 and 0.3 K, respectively. Improvements in LADAS specific humidity extend into the lower atmosphere (below ~700 mb), with relative improvements in q2m bias and ubRMSE of 15-25% and 1-3%, respectively. LADAS air temperature bias slightly increases but ubRMSE is reduced relative to that of ADAS. Finally, the root-mean-square of the LADAS Tb observation-minus-forecast residuals is smaller by ~0.1 K than in a land-only assimilation system, which corroborates the positive impact of the Tb analysis on the modeled land-atmosphere coupling.