Prediction of Convective hazards across SpatioTemporal Scale

Severe thunderstorms cause over one billion of Euros worth in damage and dozens of fatalities across Europe each year. While their impact is large, thunderstorms and their hazards, such as large hail and damaging wind gusts, are very difficult to forecast. The fact that thunderstorms are small and relatively short-lived phenomena, poses a challenge to weather prediction models. PreCaST studies the predictability of convective hazards on the time scales of hours (short-range), out to ten days ahead (medium-range) using state-of-the-art weather prediction models. At medium-ranges, models are run with a resolution that is not high enough that severe convective storms themselves are simulated. Instead, the risk of severe weather is assessed using the simulated large-scale conditions that are required for thunderstorm development, such as an unstable atmosphere or a strong vertical wind shear. The European Severe Storms Laboratory has developed a statistical method called AR-CHaMo that can do this with climate model data. In PreCaST, ESSL will adapt AR-CHaMo to be used with the weather prediction system of the European Centre for Medium Range Weather Forecasting (ECMWF). ECMWF itself has developed an Extreme Forecast Index, which tells forecasters how extreme a particular weather situation is compared to what is normal for a given location and time of year. In PreCaST, both approaches will be validated using actual observations of severe weather. Subsequently, AR-CHaMo will be improved by investigating which additional conditions should be incorporated into the model. In order to find this out, PreCaST calls on the help of forecasters of European weather services who participate in the annual ESSL Testbed programme. For short-range forecasts of severe weather, the high-resolution convection-allowing numerical modelling system of the Zentralanstalt für Meteorologie und Geodynamik (ZAMG) will be used in PreCaST, called C-LAEF (Convection Permitting-Limited Area Ensemble Forecasting System). While the resolution of this system is fine enough to simulate some of the processes occurring in thunderstorms, others are approximated. An example of such microphysical processes are those that predict the phase of precipitation particles (i.e., ice or water) and their sizes, which have a profound influence on hail and wind gusts. By performing multiple simulations, called an ensemble, in which slight variations are made to the details of these approximations, the forecasts will reflect a range of possible outcomes. This forms the basis of a probabilistic short-range forecast of hail and severe wind occurrence that will be pioneered in PreCaST. In a last step, the predictions on the medium and short-range that will have been developed will be combined, or smoothly blended, on the basis of a verification of how well they perform for a range of lead times. The result will be a forecast system that does not yield abrupt changes to the forecast as a potential severe weather episode approaches, but instead smoothly transitions from one system to another. The blended system will be evaluated by forecasters from all over Europe who will investigate how well the PreCaST system fares.

No additional funding sources recorded.