Development and validation of an Alaskan coupled storm surge, tide, wind wave, and sea ice forecasting system

Western Alaska regularly experiences storm surge events induced by extra-tropical storms, most active during fall, winter, and spring. Among others, the presence of sea ice in Western Alaska seawater poses a challenge in modeling storm surge in this area. Existing storm surge models rarely consider sea ice effects together with wind-induced wave effects. In this paper, we present an ALaska Coastal Ocean Forecast System (ALCOFS) which considers sea ice and wave effects for a real time storm tide forecasting. The system is based on a tightly coupled ADCIRC (a hydrodynamics model used widely for tide and storm surge modeling based on shallow water equations) and SWAN (wind wave model governed by spectrum action balance equation). The sea ice effect is included by incorporating a parameterization of air-sea-ice drag in the ADCIRC storm surge model, and of the wave energy dissipation caused by sea ice is considered in SWAN. The model utilizes an unstructured mesh with variable resolution (ranging from 20km to 70m) to achieve accurate predictions and fast run times. The model was exercised carefully with tidal tests to obtain good quality of tidal results and the optimized parameter setups. The impact of sea ice and waves was examined with several storm surge events. In addition, a three year long storm surge hindcast has been conducted to test the model robustness and to examine the sea level variation trends. Furthermore, an efficient real time continuous storm tide, wave and surge forecasting scheme which performs a cycle with a one day nowcast and then a five day forecast is proposed. The performance of the forecasting system is demonstrated and evaluated through a year long forecast. To examine the effectiveness of the forecasting scheme, it is demonstrated through the SWAN+ADICRC and stand alone WAVEWATCH III (WWIII) models. The recorded forecast results for the past show good performance by comparing with observations. This paper underscores the importance of incorporating sea ice and wave effects into simulations of storm surges for the area with sea ice conditions, and presents the skill of the forecasting system.