Examining the sensitivity of ocean response to oceanic grid resolution in COAMPS-TC during Hurricane Irma (2017)

Abstract

Using the Navy's Coupled Ocean-Atmospheric Mesoscale Prediction System for Tropical Cyclones (COAMPS-TC), we investigate the sensitivity of upper ocean momentum processes to varying ocean horizontal and vertical resolutions during Hurricane Irma (2017). This study aims to examine the impact of ocean resolution sensitivity on the representation of the ocean mixed layer (OML) and thermocline heat and salt budget when the tropical cyclone (TC) is at maximum intensity east of the Lesser Antilles. Results indicate that changes in ocean resolution over the forecast period do not greatly affect TC track but have variable influence on TC intensity. OML budget analyses reveal that at coarser horizontal resolutions, the overall OML response is sensitive to vertical ocean resolution. Additionally, the dominant OML processes change as horizontal resolution increases with the dependence shifting from horizontal advection to vertical mixing. Examining budget tendencies with respect to TC quadrant reveals that the right-rear quadrant ocean response is least sensitive to grid spacing. However, the OML response within the front quadrants is most sensitive to resolution changes. Moreover, thermocline downwelling is apparent in both front quadrants and the left-rear quadrant. This downwelling response is caused by a mass convergence within the OML, pumping surface water into the ocean interior. Ocean grid resolution sensitivities in coupled TC forecast models are vital to understand since OML response is indicative of air-sea response and, thus, may contribute to forecasted TC intensity. • Modeled TC intensity varies up to 12 kt at 24 h by only changing ocean grid resolution. • Dominant oceanic processes are sensitive to ocean grid resolution and change with respect to TC quadrant. • Consider increasing upper ocean vertical resolution for horizontal resolutions ≥ 10 km.