1. Assessment of hailstorms in WRF weather simulations over Switzerland in summer 2012. Sensitivity, climatology, comparison with observation data Andrey Martynov 1, Luca Nisi 1,2, Olivia Martius 1 1 Institute of Geography and Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland 2 Federal Office of Meteorology and Climatology MeteoSwiss, Locarno-Monti, Switzerland

2. 2 Summertime hailstorms are a major source of property damage in Switzerland. Source: 20Minuten.ch Leser-Reporter Motivation

3. 3 Our goal is to better understand the hailstorm-related risks in current climate and to estimate the climate change-related challenges in the future. The WRF model will be used for simulating hailstorms over Switzerland in current and future climate conditions. It is important to assess the capacity of the model to reproduce hailstorms and to discover its limitations. Questions: - Does the model reproduce the precipitation patterns? - In which conditions can the model reproduce the hailstorms? - In which conditions the model is not able to do it? - Which factors determine the model performance? Model performance assessed for: - 4 microphysics options; - MeteoSwiss weather types. Objectives

4. 4 WRF 3.4.1. Simulation period: JJA 2012 (1.06.2012 – 31.08.2012), 92 days. Single domain, 414 x 375 lanlot grid, 2 km resolution, 35 vertical levels. Simulation settings

5. 5 WRF 3.4.1. Simulation period: JJA 2012 (1.06.2012 – 31.08.2012), 92 days. Single domain: 414 x 375 lan-lot grid, 35 vertical levels. 2 km horizontal resolution, Forcing: ECMWF analysis, 6-hourly, 1/8° No cumulus parameterisation, no nudging Noah land-surface model. Microphysics: 4 options tested. mp = 6 WSM6, Single-Moment 6-class scheme, mp = 8 Thompson et al. scheme, mp = 9 Milbrandt-Yau double-moment 7-class scheme (with explicit hail) mp = 10 Morrison double-moment scheme (graupel mode) Simulation settings

6. 6 Observations: - Combiprecip: - the gridded hourly precipitation rate from MeteoSwiss radar reflectivity- and ground stations; - Radar reflectivity: from 3 dual-polarised C-band radars of MeteoSwiss. In JJA2012: only 63 days out of 92 with hourly data available from all 3 radars (modernisation of Albis radar in June). Simulations: - Total precipitation; - WRF radar reflectivity operator for S-band radars. Compared parameters: - Hourly precipitation accumulation, mm; - Maximum radar reflectivity (MAX ECHO), dBZ Radar reflectivity thresholds: - 25 dBZ rain + hail – noise (~ 1.3 mm/hr) - 45 dBZ : “hail” 15 20 25 30 35 40 45 50 Data: observed and simulated. Maximum radar reflectivity (MAX ECHO), dBZ

7. 7 The hailstorm occurrence and characteristics strongly depend on weather types. Weather types with different prevailing dynamical patterns: a good test for the model. MeteoSwiss weather type classification: GWT10_Z500 (wkwtg1d0). Daily classification with 8 prevailing wind directions and low / high pressure based on daily averaged geopotential height at 500 hPa in ERA-Interim reanalysis. MeteoSwiss weather types GWT10 weather types 1 West 2 SouthWest 3 NorthWest 4 North 5 NorthEast 6 East 7 SouthEast 8 South 9 Low Pressure 10 High Pressure

8. 8 MeteoSwiss weather types Occurrence of GWT10 weather types in JJA2012 GWT10 weather types 1 West 2 SouthWest 3 NorthWest 4 North 5 NorthEast 6 East 7 SouthEast 8 South 9 Low Pressure 10 High Pressure

9. 9 MeteoSwiss weather types Frequency of MAX ECHO-determined precipitation/hail events for different weather types, JJA2012 A non-zero event: threshold exceeded somewhere over Switzerland

10. 10 Quantitative performance measures For a given weather type and microphysics – how can we estimate the model performance? If precipitation or hail over Switzerland is observed (a non-zero event) – does the model also produce precipitation of hail over Switzerland at that instant? → Fraction of reproduced observed non-zero events. If an event has been reproduced – how good? → The SAL method.

11. 11 SAL: a quantitative method of assessing gridded simulated fields in comparison with observations. Wernli, Paulat, Hagen, C. Frei, 2008. Mon. Wea. Rev., 136, 4470 Structure, Amplitude, and Location: 3 quantitative parameters. 0: perfect reproduction. Deviation from 0 = quantitative imperfection measure. Data assessment method: SAL Source: Wernli et al. 2008

12. 12 median Q1 Q3 Q1 Q3 Data assessment method: SAL For each weather type and microphysics SAL diagrams were build (dots = hourly data). - S,A,L parameters: - median, - interquartile spread Q3 – Q1 are measures of performance in simulating reproduced events.

13. 13 Threshold: 5% of the maximum grid value More than 70% of all non-zero events are reproduced WT 2, SouthWest, reproduced better than other frequent WTs. Microphysics: no strong distinction between 4 options. Hourly precipitation

14. 14 Hourly precipitation: S, A, L parameters Threshold: 5% of the maximum grid value No systematic difference between mp options in frequent weather types (West, SouthWest, NorthWest). Systematic underestimation of precipitation size factor S and total amplitude A by the model. 5 th percentile threshold – absolute values different In two compared fields. S A L

15. 15 MAX ECHO > 25 dBZ More than 60% of all non-zero events are reproduced WT 2, SouthWest, reproduced better than other frequent WTs. Microphysics: no strong distinction between 4 options. Weather types: fraction of reproduced events

16. 16 Weather types: fraction of reproduced events MAX ECHO > 45 dBZ Much less events are reproduced, than at 25 dBZ. WT 2, SouthWest: reproduced better than other frequent WTs. Microphysics: no strong distinction between 4 options.

17. 17 Weather types: S, A, L parameters MAX ECHO > 25 dBZ mp = 10 (Morrison): remarkably biased. Other options: no evident winner. S: negative or zero bias A: small or zero bias L: similar to hourly precipitation. Large spread of S, A, L values within weather types. S A L

18. 18 Weather types: S, A, L parameters MAX ECHO > 45 dBZ Large spread of S, A, L values within weather types. mp = 10 (Morrison): remarkably biased. Other options: no evident winner. S A L

19. 19 Radar observations Number of days, when the 25 dBZ threshold value was exceeded in hourly MAX ECHO snapshots. MAX ECHO climatology overview: 25 dBZ

20. 20 Number of days, when the 25 dBZ threshold value was exceeded in hourly MAX ECHO snapshots. Radar observations mp = 06 mp = 08 mp = 09mp = 10 The observed patterns are in general reproduced. MAX ECHO climatology overview: 25 dBZ

21. 21 Number of days, when the 45 dBZ threshold value was exceeded in hourly MAX ECHO snapshots. Radar observations mp = 06 mp = 08 mp = 09mp = 10 Simulated MAX ECHO stronger than observed, especially at mp =10 (Morrison) MAX ECHO climatology overview: 45 dBZ

22. 22 MAX ECHO climatology overview: diurnal cycle MAX ECHO diurnal cycle, 25 dBZ

23. 23 MAX ECHO climatology overview: diurnal cycle MAX ECHO diurnal cycle, 45 dBZ

24. 24 The simulated hourly precipitation and MAX ECHO have been compared with ground stations- and weather radar-based MeteoSwiss data. Main features of precipitation- and severe weather-related radar reflectivity over Switzerland are reproduced by WRF simulations: -the climatological patterns over Switzerland, -the shape of the diurnal cycle. The WRF model reproduces precipitation over Switzerland in more than 60% of all studied cases; the precipitation/hail reproduction rate strongly depends on the prevailing weather type. The dependence on microphysics is however weak. There is no major difference between microphysics schemes WSM6, Thomson et Milbrandt- Yau in precipitation and hail characteristics. The Morrison scheme produces much stronger radar echo, than other schemes, but the hourly precipitation rate is comparable with other microphysics. The Milbrandt-Yau microphysics, containing the explicit hail, will be used in further studies. Current work: a new series of short runs with WRF 3.5.1 is underway. Sensitivity to physics, dynamics settings as well as to the domain parameters is being studied. JJA2013 and JJA2014 will be simulated (better radar data availability than in 2012). Summary and future plans