Abstract/Summary

The spatial and temporal scale of large aperture scintillometry (LAS) observations are closer to current operational numerical weather prediction (NWP) models cf. other observation types, with source area (SA) extents comparable to O(1 km) NWP grid-boxes. Despite this, there is a lack of previous studies using LAS-derived sensible heat fluxes (QH) for NWP evaluation. Here, we introduce a methodology for using LAS to perform the first NWP evaluation over an urban area focusing on the densely built-up city centre of London. NWP at both O(1 km) and O(100 m) horizontal resolutions are considered. In the former turbulence is parameterised, whereas in the latter turbulence is partially resolved. To ensure variables are spatially and temporally comparable, horizontal and vertical resolution characteristics of both datasets need to be considered. A new methodology for determining the probable LAS-SA is defined. As the LAS-SA location and extent changes with wind direction and stability, the NWP grid-boxes involved in the comparison also change. Consideration is given to the comparable vertical levels from the model and the observation’s effective measurement height (zf). Larger differences are found between modelled QH at the surface and closest to zf (despite both being within the inertial sub layer), compared to differences between adjacent grid-boxes which overlap the LAS-SA. The LAS fluxes’ time and space averaging allows the QH to be obtained for periods similar to the model’s timestep (e.g., 1-min), but O(1 km) models do not explicitly represent higher frequency turbulent motion, and comparisons using longer observation averages are more appropriate. At higher resolutions, LAS-SAs can inform the choice in area over which the explicit component of the modelled flux is calculated. LAS observations can provide novel and underutilized insights to model evaluation studies, with increasing utility at higher model resolution.