Evaluating the WRF-Hydro Modeling System in Alaska

The operational National Water Model (NWM) was implemented in August 2016 as an uncoupled instantiation of the Weather Research and Forecasting hydrological extension package (WRF-Hydro). Currently, the NWM only covers the contiguous United States, but will be expanded to include an Alaska domain in the near future. It is well known that Alaska presents several unique hydrological modeling challenges, including frozen soils and rivers during many months of the year, rapid snowmelt, braided rivers with variable geometries, and a severe lack of in situ observations for model initialization, all of which hinder operational flood forecasting efforts. In anticipation of the NWM’s expansion to Alaska, this project evaluates WRF-Hydro performance in Alaska by focusing on case studies of flood events in central Alaska’s Tanana River and Susitna River basins, namely, the South Central Alaska Flood (September 2012) and the Susitna Valley Flood (November 2015). For this study, WRF-Hydro is configured to mimic NWM v1.0 with a 1-km resolution Noah-Multiparameterization Land Surface Model (Noah-MP LSM) and a 250-m resolution terrain routing grid, generated using the WRF-Hydro GIS Preprocessor Tool. The two-year spin-up period and the two-year calibration period were performed using Global Land Data Assimilation System (GLDAS) meteorological forcing (0.25° resolution). During the validation periods corresponding to each case study, WRF simulations downscale GLDAS meteorological forcing to match the 1-km resolution of the Noah-MP LSM. This presentation will highlight results of validating WRF-Hydro’s streamflow against in situ USGS discharge observations for each of the case studies mentioned above and suggest areas of future research to improve WRF-Hydro model physics for arctic modeling applications. Future work will also use this WRF-Hydro framework for data assimilation experiments with the upcoming Surface Water Ocean Topography (SWOT) mission (to be launched in 2021) in efforts to improve NWM initialization and streamflow prediction over the future Alaskan domain. SWOT will provide global measurements of water surface elevations (WSE) for rivers with widths greater than 50-100 m, thus providing many more observations for model initialization than are currently available from in situ gauges.