A consortium of GAPP-sponsored groups is undertaking the development of an uncoupled North American Land Data Assimilation System (LDAS). NCEP, NASA/GSFC, NWS Hydrology Lab, NESDIS, Princeton University, University of Washington, University of Maryland, and Rutgers University have undertaken the development and prototype realtime demonstration of national, realtime, hourly, 15-km, uncoupled land-surface models forced by observed precipitation and observed GOES-derived solar insolation. This system is referred to as the LDAS, as future enhancements will include the assimilation of satellite-derived land-surface fields such as skin temperature, soil moisture, and snowpack.

The hallmarks of this LDAS project are 4 separate realtime land-surface models (LSMs) running in parallel in the NCEP computer environment on a common 0.125 degree grid using common a) NCEP-derived hourly atmospheric forcing, b) common land-surface characteristics (to the extent possible), and c) a common streamflow routing model. The land-surface state variables (e.g. soil moisture and temperature, snowpack) of the 4 LSMs will all be continuously cycled forward in realtime. The participating LSMs so far include the NOAH LSM, the MOSAIC LSM, the VIC-3L LSM, and the Sacramento Model.

In this talk we intercompare LDAS results of snowpack simulations over the North American CONUS and streamflow simulations (about 25 small to medium sized catchments) from the land-surface models participationg in LDAS. Results will be shown from the realtime LDAS (April 99 to present), as well as from the retrospective runs (see Cosgrove et al., this conference) from September 1996 to September 1999. Comparisons will highlight differences in model snow physics and runoff production, and show the correlation of these events. Model results will be compared to observed streamflow and estimated snowcover and snow water equivalent products from NESDIS, US Air Force, and NOHRSC.