Mixing length controls on high resolution simulations of convective storms

Hanley, K. E., Plant, R. S. ORCID: https://orcid.org/0000-0001-8808-0022, Stein, T. H. M. ORCID: https://orcid.org/0000-0002-9215-5397, Hogan, R. J. ORCID: https://orcid.org/0000-0002-3180-5157, Nicol, J. C., Lean, H. W., Halliwell, C. and Clark, P. A. ORCID: https://orcid.org/0000-0003-1001-9226 (2015) Mixing length controls on high resolution simulations of convective storms. Quarterly Journal of the Royal Meteorological Society, 141 (686). pp. 272-284. ISSN 1477-870X doi: 10.1002/qj.2356

Abstract/Summary

We perform simulations of several convective events over the southern UK with the Met Office Unified Model (UM) at horizontal grid lengths ranging from 1.5 km to 200 m. Comparing the simulated storms on these days with the Met Office rainfall radar network allows us to apply a statistical approach to evaluate the properties and evolution of the simulated storms over a range of conditions. Here we present results comparing the storm morphology in the model and reality which show that the simulated storms become smaller as grid length decreases and that the grid length that fits the observations best changes with the size of the observed cells. We investigate the sensitivity of storm morphology in the model to the mixing length used in the subgrid turbulence scheme. As the subgrid mixing length is decreased, the number of small storms with high area-averaged rain rates increases. We show that by changing the mixing length we can produce a lower resolution simulation that produces similar morphologies to a higher resolution simulation.