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Does the representation of flow structure and turbulence at a cold front converge on multi-scale observations with model resolution?

Harvey, B. ORCID: https://orcid.org/0000-0002-6510-8181, Methven, J. ORCID: https://orcid.org/0000-0002-7636-6872, Eagle, C. and Lean, H. (2017) Does the representation of flow structure and turbulence at a cold front converge on multi-scale observations with model resolution? Monthly Weather Review, 145 (11). pp. 4345-4363. ISSN 0027-0644

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To link to this item DOI: 10.1175/MWR-D-16-0479.1

Abstract/Summary

In situ aircraft observations are used to interrogate the ability of a numerical weather prediction model to represent flow structure and turbulence at a narrow cold front. Simulations are performed at a range of nested resolutions with grid spacings of 12 km down to 100 m and the convergence with resolution is investigated. The observations include the novel feature of a low-altitude circuit around the front that is closed in the frame of reference of the front, thus allowing the direct evaluation of area-average vorticity and divergence values from circuit integrals. As such, the observational strategy enables a comparison of flow structures over a broad range of spatial scales, from the size of the circuit itself ($\approx$100 km) to small-scale turbulent fluctuations ($\approx$10 m). It is found that many aspects of the resolved flow converge successfully towards the observations with resolution if sampling uncertainty is accounted for, including the area-average vorticity and divergence measures and the narrowest observed cross-frontal width. In addition, there is a gradual handover from parametrized to resolved turbulent fluxes of moisture and momentum as motions in the convective boundary layer behind the front become partially-resolved in the highest resolution simulations. In contrast, the parametrized turbulent fluxes associated with subgrid-scale shear-driven turbulence ahead of the front do not converge on the observations. The structure of frontal rainbands associated with a shear instability along the front also does not converge with resolution, indicating that the mechanism of the frontal instability may not be well represented in the simulations.

Item Type:Article
Refereed:Yes
Divisions:Science > School of Mathematical, Physical and Computational Sciences > Department of Meteorology
ID Code:72253
Publisher:American Meteorological Society

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