Influence of computational grid resolution on the quality of forecasts of dangerous convection phenomena: a case study of August 11, 2017

On August 11, 2017, a violent convection phenomenon took place in northwestern Poland, i.e., a storm combined with intense rainfall and hurricane winds. This paper presents an attempt to analyze this case by using the results of a numerical weather model, at grid spacings of 7 km, 2.8 km, and 0.7 km. Various convective indicators were analyzed to assess the nature of the event. The key question the authors try to answer is: „To what extent, if any, did a tenfold increase in resolution improve the quality of the numerical forecasts?” This question, however, has not been conclusively resolved. The most likely cause of this event was a supercell rapidly moving from south to northeast. This supercell's path has been mapped (qualitatively at least) by the Supercell Detection Index at all resolutions used. As the resolution increased, the forecasted maximum gusts also increased from 25 m/s in the domain with a resolution of 7 km to 35 m/s at a resolution of 2.8 km and up to about 50 m/s at the highest resolution of 0.7 km. A key conclusion is that the results of the model at a resolution of 2.8 km are much closer to reality than at 7 km. This effect did not pertain to differences between the 2.8 km and 0.7 km models. The latter increase in resolution did not significantly improve the quality of the forecast.