Modelling of hurricane Dorian via the implementation of Wave Boundary Layer Model (WBLM) within the OpenIFS

Abstract

For over three decades numerous studies have tried to understand the processes and impacts of air–sea interactions on the atmosphere and oceans, particularly in predicting winds and waves under tropical cyclone conditions. Literature has highlighted the critical role of momentum transfer, with various parameterisations proposed for the momentum fluxes, through the drag coefficient ($C_d$) and the roughness ($z_0$). However, accurate predictions still remain a significant challenge. Recently, Du et al. (2017,2019) proposed a comprehensive calculation of the source input function using a Wave Boundary Layer Model (WBLM). However, their study used a standalone model rather than a coupled system. Given the established significance of two-way wind-wave systems (Janssen, 1991), this study implements the WBLM within a coupled model (OpenIFS), to evaluate its impact and discuss the potential and limitations of the method. Numerical simulations were conducted using the WBLM scheme for a selected tropical cyclone, with results compared against in-situ buoy measurements and satellite altimeter data. Furthermore, the new approach’s results were compared with outputs using the default, well-established source input function of OpenIFS (Janssen et al., 1989; Janssen, 1991) to further assess its effectiveness. The findings suggest that the WBLM tends to reduce the commonly overestimated drag and Charnock coefficients. However, comparisons with in-situ observations indicate that the new approach requires substantial refinements to improve wind and wave predictions, since there are cases that the WBLM scheme under-performs the default scheme. This discrepancy may be attributed to the calculation of high-frequency impacts on momentum exchanges.