Seamless Stratocumulus Simulation across the Turbulent Gray Zone

I. A. Boutle Met Office, Exeter, United Kingdom

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J. E. J. Eyre Met Office, Exeter, United Kingdom

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A. P. Lock Met Office, Exeter, United Kingdom

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Abstract

A pragmatic approach for representing partially resolved turbulence in numerical weather prediction models is introduced and tested. The method blends a conventional boundary layer parameterization, suitable for large grid lengths, with a subgrid turbulence scheme suitable for large-eddy simulation. The key parameter for blending the schemes is the ratio of grid length to boundary layer depth. The new parameterization is combined with a scale-aware microphysical parameterization and tested on a case study forecast of stratocumulus evolution. Simulations at a range of model grid lengths between 1 km and 100 m are compared to aircraft observations. The improved microphysical representation removes the correlation between precipitation rate and model grid length, while the new turbulence parameterization improves the transition from unresolved to resolved turbulence as grid length is reduced.

Corresponding author address: Ian Boutle, Met Office, FitzRoy Road, Exeter, EX1 3PB, United Kingdom. E-mail: ian.boutle@metoffice.gov.uk

Abstract

A pragmatic approach for representing partially resolved turbulence in numerical weather prediction models is introduced and tested. The method blends a conventional boundary layer parameterization, suitable for large grid lengths, with a subgrid turbulence scheme suitable for large-eddy simulation. The key parameter for blending the schemes is the ratio of grid length to boundary layer depth. The new parameterization is combined with a scale-aware microphysical parameterization and tested on a case study forecast of stratocumulus evolution. Simulations at a range of model grid lengths between 1 km and 100 m are compared to aircraft observations. The improved microphysical representation removes the correlation between precipitation rate and model grid length, while the new turbulence parameterization improves the transition from unresolved to resolved turbulence as grid length is reduced.

Corresponding author address: Ian Boutle, Met Office, FitzRoy Road, Exeter, EX1 3PB, United Kingdom. E-mail: ian.boutle@metoffice.gov.uk
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