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Evaluation of the performance of the wave boundary layer model with the OpenIFS

Makrygianni, Nefeli 2022. Evaluation of the performance of the wave boundary layer model with the OpenIFS. PhD Thesis, Cardiff University.
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Abstract

The impact of the air–sea interactions on the atmospheric and the ocean processes has been extensively studied for over three decades, showing the importance of the input term and the transfer of momentum in the air-sea interface for wind and wave predictions. Despite the significant improvement in modelling of the atmosphere, accurate predictions of the sea state under tropical cyclone conditions still remain highly challenging. Evidence shows that the air-sea-waves interaction over the ocean surface can significantly impact on the coupled atmosphere-ocean systems, through momentum, mass, and energy exchanges. In particular, the momentum exchanges have been found to affect both the structure of the wave boundary layer and the sea state, through the wave evolution. For many decades, studies suggested different parameterisations of the momentum fluxes, through the drag coefficient (Cd) and the roughness length (z0). The analysis of the Wave Boundary Layer (WBL) has been used in several studies to improve the wind and wave predictions. In recent years, research has been focused on the theoretical approaches of the momentum parameterisation within the WBL in order to obtain the best computation of surface stresses. However, the WBL was used only to resolve the z0 and Cd, but with no change in the computation of the source functions in wave models. Only recently, Du et al. (2017, 2019) proposed the use of the WBL model (WBLM) in the calculation of the input source function. However, the work was based on a standalone model, and not in a coupled system. Nevertheless, as the atmosphere and ocean need to be consider as one system the importance of fully shifting towards coupled systems, in order to improve the wind and wave predictions, has been proved even in early studies (Janssen et al., 1989; Janssen, 1991) Based on the above, in this study the WBLM is implemented and evaluated in a coupled system (OpenIFS). The main aim is to test if and how this resent approach can improve the wind and wave predictions, and became an available source input function choice for operational forecasting. The new wind input term is then tested using numerical model simulations for four tropical cyclone cases, and its validation is based on in-situ (buoy) and altimeter (satellite) data. In addition, the WBLM, as well as the observations are compared also to the default source input function of OpenIFS (Janssen, 1991), in order to understand the possible improvements of the new approach. Results of this study hint that the use of the WBLM, reduced the commonly overestimated Cd and in times shifts the wind and wave predictions, coming in agreement with previous studies of the literature. However, comparisons with the observations showed that in cases Janssen (1991) still gives better predictions of wind and wave. Most importantly, it is also found here that the WBLM is more computationally costly (for about 50%) than the default parameterisation, which is a key issue for operational forecasting.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Engineering
Uncontrolled Keywords: Wind – wave interaction , Air-sea interaction , Wave Boundary Layer Model , OpenIFS , WAM , Source Input Term
Date of First Compliant Deposit: 20 July 2022
Last Modified: 20 Jul 2022 15:08
URI: https://orca.cardiff.ac.uk/id/eprint/151375

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