Impact of increasing resolution on atmospheric wind forcing over coastal ocean

Abstract:

The impact of resolution on atmospheric predictions that will be used as forcing for oceanic models within regions of complex coastal geometry is evaluated. A quadruple nest of The Coupled Ocean Atmosphere Mesoscale Prediction System, COAMPS™, is used for hourly and 3-hourly surface wind predictions at four resolutions with ranging scales between 27km and 1 km over coastal waters. We studied the Turkish Straits System which is a two-strait region of complex coastline and topography, and the Chesapeake Bay area having a convoluted coastline. The 27-km coarse resolution model product produces a smoother wind field that can successfully simulate the general low-frequency variation but is unable to capture the high frequency peak events. The land-sea interface at 27-km resolution cannot resolve important details over coastal waters, around islands and along straits that will be crucial for coastal ocean modeling. The friction over land and the blocking of winds are affected by the coastline and landmass representation, therefore increase in resolution allows a more realistic coastline resulting in predictions with less error. As the resolution increases especially at those locations close to complex shorelines, the accuracy of atmospheric predictions increases which will allow the generation of better quality forcing files for ocean models. Results appear to converge at 3-km resolution and further increasing the resolution from 3-km to 1-km does not produce a clear improvement. Fast moving storm systems and frontal events with strong wind speeds and sharp gradients in coastal waters can hardly be captured by 3-hrly winds and an increase in temporal frequency of the wind records to hourly for ocean forcing generation is recommended. The analyses suggest the use of hourly atmospheric products at 3-km resolution as oceanic forcing for both the Turkish Straits System and the Chesapeake Bay region. A preliminary analysis on the direct impact of atmospheric forcing products at different resolutions on coastal ocean predictions using the Advanced Circulation Model, ADCIRC, is also presented.