Severe weather events: sensitivity to horizontal resolution and microphysics for HARMONIE

On 10 September 2011, a supercell storm travelled from Southwest to Northeast across The Nether- lands leading to excessive precipitation, lightning and severe wind gusts. This severe weather event has been investigated with the non-hydrostatic numerical weather prediction model HARMONIE. The case has been run with two versions of the HARMONIE model with increasing horizontal resolution up to 100 m and altered microphysics and turbulence settings. A large factor 10 difference in cloud ice is observed between the two model versions. The new model version shows cloud ice concentrations that are comparable to observations. The overprediction of spots with high amounts of precipitation is hereby reduced. A “sanity check” experiment resulted in a correct inner domain model environment and the removal of a bug in the HARMONIE nesting procedure. The nesting of two HARMONIE envi- ronments results in counterintuitive behaviour of the model. At 100 m resolution, almost nothing is left of the original precipitation pattern. Rain intensity increases with increasing resolution and is con- sistent with existing literature. A higher rain evaporation rate leads to less precipitation reaching the surface at 100 m resolution. A new turbulence scheme (HaRatu) does not show a large impact. At larger scale (2.5 km), a higher evaporation rate also leads to new triggered convection, resulting in more precipitation. The new turbulence scheme results in a decreased accumulated precipitation and is not entirely consistent with earlier tests. Runs with HARMONIE at 100 m resolution are possible, but a lot more work has to be done to be able to use it in operational meteorology.