Abstract/Summary

Short lead-time forecasts using the operational United Kingdom Variable-Resolution ('UKV) configuration of the Met Office’s Numerical Weather Prediction model, with horizontal grid-length 1.5 km over the UK, with and without a representation of the 20 March 2015 eclipse, has been used to simulate the impact of the eclipse on UK weather. The major impact was surface-driven through changes to surface heat and moisture fluxes which changed the boundary-layer development. In cloud-free areas, the nocturnal stable boundary-layer persisted or quickly re-established during the eclipse. Surface temperatures were reduced by 7-8 °C, near-surface air temperature by 1-3 °C, near-surface winds were backed, typically by 20 °. Impacts on wind speed were small and variable and would have been very difficult to detect. Smaller impacts occurred beneath cloud. However, the impact was enhanced because most of incoming radiation which reached the surface was driving surface sensible heat flux rather than moisture flux, and the near surface air temperature impact (0.5-1 °C) agrees reasonably well with observations. The modelled impact of the eclipse was substantially reduced in urban areas due to their large thermal inertia. Experience from other assessments of the model suggests that this lack of response may be exaggerated. Surface impacts propagated upwards and down stream with time, resulting in a complex pattern of response, though generally near-surface temperature differences persisted for many hours after the eclipse. The impact on atmospheric pressure fields was insufficient to account for any significant perturbations to the wind field when compared with the direct impacts of surface stress and boundary-layer mixing.