Abstract
The effects of coupling between the atmospheric model of the Consortium for Small-Scale Modelling-Climate Limited-area Modelling (CCLM) and the wind wave model (WAM) on the lower atmosphere within the North Sea area are studied. Due to the two-way coupling between the models, the influences of wind waves and the atmosphere on each other can be determined. This two-way coupling between these models is enabled through the introduction of wave-induced drag into CCLM and updated winds into WAM. As a result of wave-induced drag, different atmospheric parameters are either directly or indirectly influenced by the wave conditions. The largest differences between the coupled and reference model simulation are found during storm events as well as in areas of steep gradients in the mean sea level pressure, wind speed or temperature. In the two-way coupled simulation, the position and strength of these gradients vary, compared to the reference simulation, leading to differences that spread throughout the entire planetary boundary layer and outside the coupled model area, thereby influencing the atmosphere over land and ocean, although not coupled to the wave model. Ultimately, the results of both model simulations are assessed against in situ and satellite measurements, with a better general performance of the two-way coupled simulation with respect to the observations.
Highlights
Wind induced waves at the surface of the ocean are well known to affect the overlying atmosphere.In 1989, Janssen [1] investigated the wave-induced stress and airflow drag over sea waves
In the coupled model simulation, the roughness length calculated by wave model (WAM) is passed to Consortium for Small-Scale Modelling-Climate Limited-area Modelling (CCLM) to ensure a roughness length over the ocean that is more precise than the parametrised roughness length used within the reference model simulation
This reduction is due to the wind speed being reduced (Figure 12a), as the wind speed is passed from the atmospheric model to the wind wave model and directly influences the significant wave height
Summary
Wind induced waves at the surface of the ocean are well known to affect the overlying atmosphere. The largest changes in the surface roughness and friction velocity occur in areas of young sea states [10] This increased friction leads to more direct airflow into the centre of the low-pressure system, and the system fills up more quickly. The temperature difference between the ocean and the atmosphere can become quite large In this case, the heat flux can play a major role in the development of the low. The two-way coupling of wave and atmospheric models was introduced into the operational forecasts of the European Centre for Medium-Range Weather Forecasts (ECMWF) in 1998 This led to substantial improvements in various surface parameters, such as the 10 m wind speed and the significant wave height, and had modest impacts on the 1000 hPa and 500 hPa geopotential heights [7,8].
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