Effect of High Resolution Winds on Wind-Wave Simulations in Arabian Gulf
Authors: Nayak Sashikant , Sandeepan B.S , Panchang Vijay 
Published: July 2019

Reliable knowledge of wave climate in a large water body such as in the Arabian Gulf for navigation, recreational, maintenance and installation of offshore-infrastructure (e.g. oil platforms) etc. is an important pre-requisite.

Predicting waves in a region like Arabian Gulf, which is approximately 1000 km in length (north to south) and 550 km in width and shallower (average water depth is ∼35 m) requires a well-developed wave model that can account for the shallow water wave mechanisms like their generation, propagation and dissipation. Wave models like SWAN (Simulating WAves Nearshore), WAM (WAve Model) and WAVEWATCH are now routinely run in several parts of the world for predicting and forecasting the ocean waves. Among these, SWAN wave model has been found to be a better model for the shallower and coastal environments.

The quality of wave hind-casts/forecasts made, largely depends on the quality of wind fields (speed and direction) that are feed into these models. For hind-cast, these wind fields can be obtained from observational measurements or reanalysis data from centers such as ECMWF (European Center for Medium Range Weather Forecast), NCEP (National Center for Environmental Prediction). For forecast, the wind fields are obtained from global weather forecast models that are routinely run in centers like ECMWF, NCEP or running weather forecasting models such as WRF (Weather Research Forecasting models) regionally. Further the resolution of wind fields both temporally and spatially affects the quality of wave prediction. For Arabian Gulf, the spatial and temporal resolution of wind fields requires finer resolution than the available reanalysis/forecasts data from NCEP (∼0.5°) or ECMWF ERA-interim (∼ 0.72°).

This study reports on the setting up of SWAN wave model and the improvement in wave modeling by using two sources of surface winds viz. from ECMWF ERA-Interim daily and from a meso-scale high resolution atmospheric model, WRF. Two types of bathymetry are also taken into consideration for running the SWAN wave model. One using a regular bathymetry grid (∼1’, ETOPO1 from NGDC) and the other one using an unstructured mesh bathymetry (for the same ETOPO 1). The advantage of unstructured grid over the regular grid is that it accounts much better for the complex coastal boundaries and islands in the region of interest. Initially, the simulation is carried out for the month of October, 2015. The presence of mesoscale locally convective phenomena's such as land-sea breeze are dominant in a water body like Arabian Gulf. Era Interim reanalysis datasets considered to be the best available wind sources misses these features (because of low resolution) and in turn when used as forcing to the wave models, may result in predicting less accurate wave fields, mostly the directions which in turn effect the non-linear interaction of waves and distribution of energy. The high resolution, mesoscale atmospheric model WRF modeled winds clearly reflects these mesoscale phenomena's and upon forcing to wave model reflects a more accurate wave fields. It is also found that the waves coming from Arabian Sea doesn't impact much in the evolution of waves in Arabian Gulf. The reason may be, their dissipation of energy traveling through the narrowness of Strait of Hormuz. The results are compared with the available buoy data from Qatar Meteorological Department (web portal). Simulated significant wave height (SWH) are also compared with the Satellite SWH's for the same period.