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Case study of a gust front passing the Danish island Funen on 9 November 2008

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. What happened ?.

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Case study of a gust front passing the Danish island Funen on 9 November 2008

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  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. Whathappened? • 800 tons of steel (”small” crane) knocked over by bigcrane • Wind gust/squall at 110 m heightreported to be • ~ 32 m/s • ~ 5 minutes • NWP predictedwind 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 unstablecold air Surfaceanalysis at 1200 UTC on 9 November 2008

  4. Radar images 1136 UTC 1146 UTC 1156 UTC 1206 UTC 1216 UTC 1226 UTC

  5. Wind observations from SYNOPsonislandFunen

  6. Wind observations from SYNOPsonislandFunen Assens/Torø Max gust: 25.2 m/s T2m (°C) Meanwind speed v10m (m/s) Local time (UTC+1h)

  7. Wind observations from SYNOPsonislandFunen Odense/Beldringe Max gust: 22.1 m/s T2m (°C) Meanwind speed v10m (m/s) Local time (UTC+1h)

  8. Wind observations from SYNOPsonislandFunen Årslev Max gust: 20.6 m/s T2m (°C) Meanwind speed v10m (m/s) Local time (UTC+1h)

  9. Wind observations from SYNOPsonislandFunen Sydfyn/Tåsinge Max gust: 16.5 m/s T2m (°C) Meanwind speed v10m (m/s) Local time (UTC+1h)

  10. Upper level analyses and soundings MSLP and 500 hPageopot. height & temp. 06 UTC 12 UTC De Kooy/DenHelder Schleswig Soundings

  11. 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

  12. => Conceptual model: Gust front

  13. HIRLAM-S05 6H prognosis valid 12 UTC 10 m wind • Barbs: meanwind • Colours: gusts m/s • For comparison: • Observed 10 m gustsonislandFunen: • 25.2 m/s • 22.1 m/s • 20.6 m/s • 16.5 m/s

  14. HIRLAM-S05 6H prognosis valid 12 UTC 850 hPameanwind m/s • For comparison: • Observed 10 m gustsonislandFunen: • 25.2 m/s • 22.1 m/s • 20.6 m/s • 16.5 m/s

  15. HIRLAM-S05 6H prognosis valid 12 UTC Instabilityindex

  16. 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

  17. 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

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

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