The multi‐institution North American Land Data Assimilation System (NLDAS): Utilizing multiple GCIP products and partners in a continental distributed hydrological modeling system

TLDR

The North American Land Data Assimilation System (NLDAS) initiative is largely sponsored by the GEWEX Continental‑Scale International Project (GCIP). This paper presents and evaluates a three‑year NLDAS execution (1996‑1999) from a multi‑institution partnership developing real‑time and retrospective land data assimilation. NLDAS runs four uncoupled land models on a 1/8° continental US grid with common hourly forcing and streamflow routing, and its validation uses mesoscale observations, regional snowpack, streamflow, and satellite retrievals of snow cover, land surface temperature, and insolation. The evaluation revealed substantial intermodel differences in evaporation, runoff, soil moisture, snowpack, and land surface temperature, with unexpected midday summer LST variations linked to aerodynamic conductance, and confirmed that geostationary satellite‑derived LST is of good quality for model validation.

Abstract

Results are presented from the multi‐institution partnership to develop a real‐time and retrospective North American Land Data Assimilation System (NLDAS). NLDAS consists of (1) four land models executing in parallel in uncoupled mode, (2) common hourly surface forcing, and (3) common streamflow routing: all using a 1/8° grid over the continental United States. The initiative is largely sponsored by the Global Energy and Water Cycle Experiment (GEWEX) Continental‐Scale International Project (GCIP). As the overview for nine NLDAS papers, this paper describes and evaluates the 3‐year NLDAS execution of 1 October 1996 to 30 September 1999, a period rich in observations for validation. The validation emphasizes (1) the land states, fluxes, and input forcing of four land models, (2) the application of new GCIP‐sponsored products, and (3) a multiscale approach. The validation includes (1) mesoscale observing networks of land surface forcing, fluxes, and states, (2) regional snowpack measurements, (3) daily streamflow measurements, and (4) satellite‐based retrievals of snow cover, land surface skin temperature (LST), and surface insolation. The results show substantial intermodel differences in surface evaporation and runoff (especially over nonsparse vegetation), soil moisture storage, snowpack, and LST. Owing to surprisingly large intermodel differences in aerodynamic conductance, intermodel differences in midday summer LST were unlike those expected from the intermodel differences in Bowen ratio. Last, anticipating future assimilation of LST, an NLDAS effort unique to this overview paper assesses geostationary‐satellite‐derived LST, determines the latter to be of good quality, and applies the latter to validate modeled LST.