1. Case study of a gust front passing the Danish island Funen on 9 November 2008 Majken Demant Meteorologist, aviation forecaster, DMI msd@dmi.dk Eumetcal NWP Applications course, Helsinki, 30 November - 4 December 2009

2. What happened? 800 tons of steel (”small” crane) knocked over by big crane Wind gust/squall at 110 m height reported to be ~ 32 m/s ~ 5 minutes NWP predicted wind speed at 110 m: 15 m/s !

3. Synoptic situation Occluding frontal system has just passed the shipyard… …followed by a SW-ly flow of unstable cold air Surface analysis at 1200 UTC on 9 November 2008

4. Radar images 1136 UTC1146 UTC1156 UTC 1206 UTC1216 UTC1226 UTC

5. Wind observations from SYNOPs on island Funen

6. Assens/Torø Local time (UTC+1h) Mean wind speed v10m (m/s) T2m (°C) Max gust: 25.2 m/s

7. Wind observations from SYNOPs on island Funen Odense/Beldringe Mean wind speed v10m (m/s) T2m (°C) Max gust: 22.1 m/s Local time (UTC+1h)

8. Wind observations from SYNOPs on island Funen Årslev Mean wind speed v10m (m/s) T2m (°C) Max gust: 20.6 m/s Local time (UTC+1h)

9. Wind observations from SYNOPs on island Funen Sydfyn/Tåsinge Mean wind speed v10m (m/s) T2m (°C) Max gust: 16.5 m/s Local time (UTC+1h)

10. Upper level analyses and soundings MSLP and 500 hPa geopot. height & temp. 12 UTC06 UTC De Kooy/Den HelderSchleswig Soundings

11. IR satellite image

12. Evidence: Elongated radar echo with narrow line of high reflectivity Sudden peak winds and temperature drops coinciding with passage of radar echo at four individual SYNOP stations Unstable air mass Strong east-moving trough (positive vorticity advection, increasing with height) Strong veering and increase in wind speed with height

13. =>Conceptual model: Gust front

14. HIRLAM-S05 6H prognosis valid 12 UTC 10 m wind For comparison: Observed 10 m gusts on island Funen: 25.2 m/s 22.1 m/s 20.6 m/s 16.5 m/s m/s Barbs: mean wind Colours: gusts

15. HIRLAM-S05 6H prognosis valid 12 UTC 850 hPa mean wind For comparison: Observed 10 m gusts on island Funen: 25.2 m/s 22.1 m/s 20.6 m/s 16.5 m/s m/s

16. HIRLAM-S05 6H prognosis valid 12 UTC Instability index

17. Conclusions Peak 10 m winds were not predicted by HIRLAM-S05, i.e. gust front not resolved by the model Indications from HIRLAM-S05 850 hPa wind field that the squall line phenomenon was to some extent predicted by the model No distinct conclusion to draw from HIRLAM-S05 instability index prediction in this case

18. Recommendations Direct model output should not be blindly trusted – especially when phenomena of smaller scale than the model resolution can occur In case of organized convection and gust fronts, (NWP-predicted) winds aloft (e.g. at 850 hPa level) could be a fair indicator of low level gust potential, perhaps in combination with Instability index For nowcasting purposes: Always monitor nearby (wind) observations and radar images – look out for signs of downdrafts or gust fronts Human forecaster can add value to direct model output

19. Questions? Thanks to my colleagues Thomas Mørk Madsen Niels Woetmann Nielsen Claus Petersen for their help with retrieving data