The Coordinated Enhanced Observing Period (CEOP) is a coordinated international activity aimed to establish an integrated global observing system for the water cycle. It contains satellite data, model and assimilation output, and in situ observations, with a focus on hydrometeorological processes in the atmosphere and land surface. In this study, reference site observations during CEOP EOP-1 (covers the time period from 1 July, 2001 to 30 Sept, 2001) and EOP-3 (spans from 1 Oct, 2002 to 30 Sept, 2003) are used to evaluate land surface processes simulated in the NCEP Global Forecast System (GFS). Two versions of GFS are used in this experiment: one is based on the operational version of GFS employing the legacy OSU Land Surface Model (LSM), and the other uses an experimental version of GFS coupled with the latest version of the NCEP Noah LSM.

Comparison between GFS simulations and in situ observations are conducted over the reference sites where both surface and sub-surface fields were measured. For reference sites located in the continental US, the comparisons are extended to include the results from North American Land Data Assimilation System (NLDAS). The periods of study are austral summer (Jul to Aug) for 2001 and 2003, corresponding to EOP-1 and EOP-3, respectively. This study not only evaluates the impact of LSM upgrade on GFS forecasts (by comparing GFS/Noah and GFS/OSU results), but also assesses the impact of land state spin-up on GFS forecasts (by comparing GFS/Noah runs initialized from Noah cycled land states versus GFS/Noah runs initialized from OSU cycled land states). In addition, the land surface processes simulated by Noah LSM in coupled and uncoupled mode are compared (GFS/Noah versus NLDAS).