At NCEP, the operational mesoscale Eta model (including its companion, continuously cycled Eta Data Assimilation System known as EDAS) is coupled to the NOAH land-surface model (LSM). The physics, initialization, calibration, and validation of the NOAH LSM has been periodically upgraded by NCEP throughout the 1990's, in large part as a result of the numerous NCEP collaborations with principals of the NOAA- and NASA-sponsored GEWEX American Prediction Project or GAPP (formerly GCIP). The focus of the most recent upgrades of the NOAH LSM has been a) cold season processes (snowpack, soil freeze/thaw) and b) early spring wet soils (over the central and eastern U.S.). Impacts of these upgrades in uncoupled, land-only simulations with the NOAH LSM have been reported earlier, for example in papers from PILPS-2d or the AMS 15th Conference on Hydrology.

This paper emphasizes very recent COUPLED testing of the above NOAH LSM upgrades. Specifically, we report on the notable positive impact from (and mechanisms of) the above upgrades in the land-atmosphere interactions of the coupled NCEP Eta model. From extensive parallel execution of Eta 3-day forecasts over the CONUS for month-long testing periods, we present a wide range of validation results that encompass near-surface and PBL profiles of temperature and dew point, in terms of both individual case studies of hourly time series at validating surface stations, as well as aggregate monthly verification statistics by region. The case studies focus on the impacts of 1) our introducing fractional ("patchy") snow cover attributes to the snowpack physics, 2) reformulating the ground heat flux under snowpack, 3) adopting a new function for the thermal diffusivity in the soil column, and 4) modifying the dependence of bare soil evaporation on near-surface soil moisture.

Lastly, we briefly outline 1) other improvements (precipitation assimilation in EDAS, snow albedo) and 2) formal NCEP initiatives to unify the NOAH LSM as the land-surface model throughout all NCEP global and regional weather and climate prediction models, including their data assimilation systems, such as the NCEP 25-year EDAS-based Regional Reanalysis.