Development and numerical simulation of a coupled atmosphere-wave modelling system (GRIST-WW3) based on ESMF/NUOPC

Atmosphere-wave interactions are crucial dynamics at the air-sea interface, with the sea surface momentum roughness length being a key variable in coupled atmosphere-wave modelling system. The Global-Regional Integrated Forecast System (GRIST), a next-generation unstructured-grid unified weather and climate modelling system, has been independently designed and developed by Chinese team in recent years. By employing the ESMF/NUOPC framework, GRIST has been integrated with the WW3 model to create the coupled atmosphere-wave modelling system (GRIST-WW3). In this system, the atmospheric model provides the 10 m wind fields over the sea surface to drive the wave model, while the sea surface roughness, derived from a wave parameterization scheme, is fed back into the atmospheric model. Preliminary results show that the GRIST-WW3 system accurately captures the spatial distribution of the sea surface wind field and significant wave height, aligning well with observations. However, in regions such as the Southern Hemisphere"s westerlies and areas near typhoons, where the wind speeds are notably high, the model tends to overestimate the 10 m wind speed and significant wave height. The two-way coupling process increases the average and dispersion of sea surface roughness, further reducing the wind speed bias in areas of high wind speed. In terms of typhoon simulation, improvements in typhoon trajectory and 10 m maximum wind speed accuracy are evident with the atmosphere-wave coupled modelling system, though the minimum sea level pressure remains unaffected. In the coupled atmosphere-wave modelling system, wave parameterization scheme of sea surface roughness is essential for accurately simulating high wind speed areas. The optimization of scheme should be guided by the atmospheric model"s bias characteristics, with the primary goal of reducing bias in the high wind speed regions.