On open boundary condition for tidally and sub-tidally forced circulation in a limited-area coastal model

Abstract:

Inconsistent usage of open boundary condition (OBC) in the limited-area ocean modeling studies, in which the circulation is concurrently forced by both tidal and sub-tidal flows in the open boundary, has been frequently witnessed. Orlanski-type radiation OBCs have been proved to be reasonably efficient in handling the sub-tidally forced flow, while Flather-type OBC has been commonly adapted for the tidally forced flow. However, neither of them performed well when the flows were forced simultaneously by both tidal and sub-tidal flow in the boundary. In this study, we developed a novel OBC that accommodates consistently for the respective tidal and sub-tidal forcing in the open boundary of the limited-area model. Our OBC applied the active OBCs of Gan and Allen (2005) in the sub-tidal forcing component and Flather-type OBC in the tidal forcing component concurrently. Using the Regional Ocean Modeling System (ROMS), we applied this OBC to the shallow East China Sea (ECS) shelf, where the circulation is jointly governed by strong tidal and sub-tidal forcing over the complex shelf topography. The OBC well reproduced the observed tidal and sub-tidal circulation in the ECS, and the spurious disturbances induced by the OBC were constrained within 2% as compared with the result obtained from a vastly expanded model domain. The disturbances generated inside the domain were able to travel outward across the open boundaries with nominal spurious reflection and the mass conservation was also fulfilled by the OBC. Our numerical experiments and analyses suggested that the newly developed OBC considerably suppressed the reflective spurious disturbances near the open boundary and performed better than either Orlanski-type or Flather-type in reproducing realistic tidally and sub-tidally forced shelf circulation in the ECS.