The Land-Ocean Freshwater Flux in the Regional Arctic System Model (RASM): Assessing Model Performance Using Streamflow

The Regional Arctic System Model (RASM) is a high-resolution Earth System model that covers the Arctic drainage basin and is developed to better understand polar climate variability, system interconnectivity and improve and understand constraints on decadal predictions in high northern latitudes. RASM uses the flux coupler (CPL7) within the Community Earth System Model framework to couple regional configurations of the Weather Research and Forecasting model (WRF), Parallel Ocean Program (POP), Los Alamos sea ice model (CICE), and Variable Infiltration Capacity land hydrology model (VIC). Recent work has focused on the development of a routing model to complete the hydrological cycle and represent the freshwater flux from the land surface to the Arctic Ocean. The routing model is a source-to-sink model that solves a linearized version of the Saint-Venant equations. Whereas previous hydrology models coupled to CPL7 have routed runoff within the land model, the routing scheme being applied here is independently coupled to RASM as a separate entity. This provides more flexibility for delivery of fresh water runoff and biogeochemical constituents to the Arctic Ocean, the former of which has been shown to be an important driver of recent changes in Arctic Ocean circulation.

In addition to providing the ocean model with the terrestrial freshwater flux, streamflow serves as an integrator of coupled land-atmosphere processes and is arguably the most accurately measured, large-scale component of the hydrological cycle. Measured and simulated streamflow can therefore be used to evaluate RASM's abilities to capture key features of the hydrological cycle. We compare daily, seasonal, and annual RASM simulated streamflow with in-situ observations from gauging stations. Results from initial simulations indicate that RASM is performing well on annual and season timescales when compared to observed records, with less agreement for timescales less than a week. Streamflow patterns are further analyzed by a comparison of other hydrological variables; such as snow cover extent, with observations and reanalysis products and by a comparison of fully-coupled and uncoupled simulations. In the latter, the land surface is forced with meteorological observations or model output, without a feedback to the atmosphere.