Abstract
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 ([Formula: see text]100 km) to small-scale turbulent fluctuations ([Formula: see text]10 m). It is found that many aspects of the resolved flow converge successfully toward 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 parameterized 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 parameterized 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.
Highlights
Atmospheric frontal systems are associated with numerous high-impact weather phenomena
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
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
Summary
Atmospheric frontal systems are associated with numerous high-impact weather phenomena. Development is underway at several forecasting centres on experimental local-area NWP models with sub-kilometer resolutions, down to O(100 m) grid spacings In this case, shallow boundary-layer convective motions will be partially-resolved, resulting in a reduced need for the parametrization of non-local boundary-layer mixing. In terms of surface impacts, the most intense precipitation along the front falls between the vortices, on narrow filaments of strong shear and temperature gradient, and tornadic structures, when they occur in the UK, typically occur in the braids joining such vortices (Clark and Parker, 2014; Mulder and Schultz, 2015) The presence of such rainband segments in the case studied here provides both opportunities and complications.
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