Sea state observation in island-sheltered nearshore zone based on in situ intermediate-water wave measurements and NCEP/CFSR wind data

Abstract

In this study, wind-seas, swells, and the coastal wind pattern are examined to interpret the temporal diversity of the sea state in the island-sheltered nearshore zone off Karwar on the west coast of India. The sea state is analyzed based on the sea swell energy ratio (SSER) criteria and inverse wave age (IWA) criteria. The SSER is estimated following a one dimensional spectral split of in situ intermediate-water waves measured by deploying a directional waverider buoy. The IWA is estimated based on the measured waves and the National Centres for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) wind data followed by validation with the autonomous weather station (AWS) wind data. Additionally, wave transformation in and around offshore islands is examined using the wind wave model SWAN (Simulating WAves Nearshore). The NCEP/CFSR wind data exhibited sea breezes as well as land breezes, and also revealed good correlation to the AWS wind data during sea breeze events. Observation revealed that the SSER criteria is more practical than the IWA criteria for interpreting the sea state in the nearshore zone, where the diversity of the sea states depend significantly on the variation of coexisting wind-seas and swell proportions. The SWAN model revealed that wave propagation and transformation in the island-sheltered nearshore zone is influenced considerably by the direction of the offshore waves to the associated island(s), where the simulated wave characteristics in the SWAN model are found more reliable based on the parametric boundary condition. Further, the study revealed that modeling is a necessary task apart from a single point observation to understand surface wave propagation and transformation in an island-sheltered nearshore zone.