The Impact of Atmosphere–Ocean–Wave Coupling on the Near-Surface Wind Speed in Forecasts of Extratropical Cyclones

Abstract

Accurate modelling of air–sea surface exchanges is crucial for reliable extreme surface wind-speed forecasts. While atmosphere-only weather forecast models represent ocean and wave effects through sea-state independent parametrizations, coupled multi-model systems capture sea-state dynamics by integrating feedbacks between the atmosphere, ocean and wave model components. Here, we investigate the sensitivity of extreme surface wind speeds to air–sea exchanges at the kilometre scale using coupled and uncoupled configurations of the Met Office’s UK Regional Coupled Environmental Prediction system. The case period includes the passage of extra-tropical cyclones Helen, Ali, and Bronagh, which brought maximum gusts of 36 m s$$^{-1}$$ - 1 over the UK. Compared with the atmosphere-only results, coupling to the ocean decreases the domain-average sea-surface temperature by up to 0.5 K. Inclusion of coupling to waves reduce the 98th percentile 10-m wind speed by up to 2 m s$$^{-1}$$ - 1 as young, growing wind waves reduce the wind speed by increasing the sea-surface aerodynamic roughness. Impacts on gusts are more modest, with local reductions of up to 1 m s$$^{-1}$$ - 1 , due to enhanced boundary-layer turbulence which partially offsets air–sea momentum transfer. Using a new drag parametrization based on the Coupled Ocean–Atmosphere Response Experiment 4.0 parametrization, with a cap on the neutral drag coefficient and reduction for wind speeds exceeding 27 m s$$^{-1}$$ - 1 , the atmosphere-only model achieves equivalent impacts on 10-m wind speeds and gusts as from coupling to waves. Overall, the new drag parametrization achieves the same 20% improvement in forecast 10-m wind-speed skill as coupling to waves, with the advantage of saving the computational cost of the ocean and wave models.

The Regional Coupled Suite (RCS-IND1): application of a flexible regional coupled modelling framework to the Indian region at km-scale

“…This contrasts with the sensitivity found for extra-tropical cyclones (Gentile et al 2021a), potentially as the wave feedback on drag saturates for the higher wind speeds found in TCs.…”

“…A key consideration is the representation of surface drag at high wind speeds (more than 30 ms -1 ). Different approaches to change the UM drag parameterisation under investigation were discussed recently by Gentile et al (2021a) in the context of km-scale coupled UM simulations of extratropical cyclones. This includes testing the impact of moving to the COARE 4.0 parameterisation at lower wind speeds, with a cap and reduction in drag coefficient at higher wind speeds.…”

A Spatial Downscaling Approach for WindSat Satellite Sea Surface Wind Based on Generative Adversarial Networks and Dual Learning Scheme

“…In this study, we use the bulk algorithm COARE 4.0 algorithm (packages: https:// cerform.readthedocs.io/en/latest/api/cerform.flux.html#module-cerform.flux.coare4 (accessed on 3 February 2022)) to calibrate the NDBC, and RAMA winds to neutral-equivalent winds at a standard 10 m reference height, which are comparable with the satellite observations wind products [26,27]. The COARE bulk algorithm iteratively solves equations involving the air-sea exchanges of momentum, heat, and water vapor to arrive at the wind speed profile in the lower atmospheric boundary layer [28].…”

The impact of time‐varying sea surface temperature on UK regional atmosphere forecasts

“…Previous work considering the sensitivity of UK weather to SST variability around the North-West Shelf region has included case studies focused on sea breeze evolution (Tang, 2012;Sweeney et al, 2014), heavy precipitation (Warren et al, 2014), boundary layer clouds (Fallmann et al, 2017), sea fog (Fallmann et al, 2019), near-surface wind gusts (Gentile et al, 2020) and heat waves (McCarthy et al, 2019;Petch et al, 2020).…”