A soil moisture and soil temperature analysis for NOAA's GFSv17/GDAS global NWP system.

As a part of a major upgrade to our global NWP system, NOAA is developing a new land data assimilation system. In addition to updating the snow depth analysis, the new land data assimilation system is planned to introduce a soil moisture and soil temperature analysis, which will be the first such analysis in NOAA's global NWP systems. The new soil analysis expands the current GSI Hybrid 4DEnVar assimilation of atmospheric observations to also assimilate screen-level temperature and humidity observations, and to also update the soil moisture and soil temperature states. A series of experiments was carried out to determine the best coupling strategy for the GSI-based land/atmosphere data assimilation system, and the best results were obtained by performing the land/atmosphere data assimilation update as a single (strongly coupled) update. This new land/atmosphere data assimilation system modestly improves the 6-hour low-level atmospheric forecasts, as measured by the data assimilation observation minus forecast statistics for the screen-level variables, while having a minimal, but positive, impact higher in the atmosphere.