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 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
URI	https://reading-clone.eprints-hosting.org/id/eprint/72253
Identification Number/DOI	10.1175/MWR-D-16-0479.1
Refereed	Yes
Divisions	Science > School of Mathematical, Physical and Computational Sciences > Department of Meteorology
Publisher	American Meteorological Society
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