Accounting for Wetting and Drying in the NCOM Operational Ocean Model
Accounting for Wetting and Drying in the NCOM Operational Ocean Model
Abstract:
This presentation discusses the implementation and testing of wetting and drying (WAD) in the operational Navy Coastal Ocean Model (NCOM), which is used for regional to coastal scales.
As horizontal scales decrease and the coastline becomes more detailed, the land-sea boundary becomes more fluid. WAD is caused by both tides and meteorological events. Without a WAD capability, operational models can become unstable when tides or offshore winds cause grid cells to dry out. By adding a WAD algorithm to NCOM, the model’s stability and its applicability to the intertidal zone are improved. NCOM’s WAD algorithm uses a set of criteria in the implicit solution of the model's free surface to determine the location of the land-sea boundary. The algorithm is tested using idealized, laboratory, and real-world test cases. Using a laboratory simulation of a dam break flow over a bump, we show that in a controlled environment, the algorithm replicates the flooding and water levels in the laboratory tank. To test the algorithm for a large tidal range, NCOM is applied to Cook Inlet, Alaska, where the tidal range is approximately 10 meters and there are extensive areas of WAD. To evaluate NCOM for the Cook Inlet test case, the predicted tide heights are compared with those at several International Hydrographic Organization (IHO) tide stations. In addition, we focus on the Turnagain Arm area of Cook Inlet, where a tidal bore that travels up the Arm on the flooding tide is a popular attraction. The simulated tidal water levels agree fairly well with the water level data at the IHO stations in Cook Inlet, and the simulated arrival times of the tidal bore at several locations along Turnagain Arm agree with the observed times within about 30 minutes.
As horizontal scales decrease and the coastline becomes more detailed, the land-sea boundary becomes more fluid. WAD is caused by both tides and meteorological events. Without a WAD capability, operational models can become unstable when tides or offshore winds cause grid cells to dry out. By adding a WAD algorithm to NCOM, the model’s stability and its applicability to the intertidal zone are improved. NCOM’s WAD algorithm uses a set of criteria in the implicit solution of the model's free surface to determine the location of the land-sea boundary. The algorithm is tested using idealized, laboratory, and real-world test cases. Using a laboratory simulation of a dam break flow over a bump, we show that in a controlled environment, the algorithm replicates the flooding and water levels in the laboratory tank. To test the algorithm for a large tidal range, NCOM is applied to Cook Inlet, Alaska, where the tidal range is approximately 10 meters and there are extensive areas of WAD. To evaluate NCOM for the Cook Inlet test case, the predicted tide heights are compared with those at several International Hydrographic Organization (IHO) tide stations. In addition, we focus on the Turnagain Arm area of Cook Inlet, where a tidal bore that travels up the Arm on the flooding tide is a popular attraction. The simulated tidal water levels agree fairly well with the water level data at the IHO stations in Cook Inlet, and the simulated arrival times of the tidal bore at several locations along Turnagain Arm agree with the observed times within about 30 minutes.