EGU21-1273
https://doi.org/10.5194/egusphere-egu21-1273
EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

The impact of atmosphere-ocean-wave coupling on extreme surface wind forecasts

Emanuele Silvio Gentile1, Suzanne L. Gray1, Janet F. Barlow1, Huw W. Lewis2, and John M. Edwards2
Emanuele Silvio Gentile et al.
  • 1University of Reading, Department of Meteorology, Reading, U.K. (e.gentile@pgr.reading.ac.uk)
  • 2Met Office, FitzRoy Road, Exeter EX1 3PB, U.K.

Accurate modelling of air-sea surface exchanges is crucial for reliable extreme surface wind 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 atmosphere, ocean and wave model components.

Here, we present the results of studying the sensitivity of extreme surface wind speeds to air-sea exchanges at kilometre scale using coupled and uncoupled configurations of the Met Office's UK Regional Coupled Environmental Prediction (UKC4) system. The case period includes the passage of extra-tropical cyclones Helen, Ali, and Bronagh, which brought maximum gusts of 36 ms-1 over the UK.

Compared to the atmosphere-only results, coupling to ocean decreases the domain-average sea surface temperature by up to 0.5 K. Inclusion of coupling to waves decreases the 98th percentile 10-m wind speed by up to 2 ms-1 as young, growing wind waves decrease wind speed by increasing the sea aerodynamic roughness. Impacts on gusts are more modest, with local reductions of up to 1ms -1, due to enhanced boundary-layer turbulence which partially offsets air-sea momentum transfer.

Using a new drag parametrization based on the COARE~4.0 scheme, with a cap on the neutral drag coefficient and decrease for wind speeds exceeding 27 ms-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 skill as coupling to waves, with  the  advantage  of saving the computational cost of the ocean and wave models. 

How to cite: Gentile, E. S., Gray, S. L., Barlow, J. F., Lewis, H. W., and Edwards, J. M.: The impact of atmosphere-ocean-wave coupling on extreme surface wind forecasts, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1273, https://doi.org/10.5194/egusphere-egu21-1273, 2021.

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