1. How to make a (weather) “bomb” Tim Baker Thanks to: Doug Parker, Peter Knippertz, Alan Blyth

2. Contents Introduction o What is a “weather bomb”? o Why should we care? o How common are they? Case Study - Cyclone Friedhelm o Background o Development + Jet Stream + Upper-Level PV + Diabatic PV o Role of latent heating Summary

3. What is a “weather bomb”? 0600 02/01/2012 0600 03/01/2012

4. Why do we care about them?

5. From Martinez-Alvarado, 2014, Monthly Weather Review Pressure gradient is key to determining wind strength.

6. How common are they? From Roebber 1984 Most Cyclones do not exhibit explosive cyclogenesis, however a sizeable minority do. Different detection methods give different numbers ranging from yearly averages of 30-80 events in the Northern Hemisphere (Allen, 2010, Journal of Climate).

7. Cyclone Friedhelm Cyclone Friedhelm passed Scotland on the 8 th Dec. 2011. Also known as #HurricaneBawbag Had deepened by 44 hPa in 24 hours! Caused widespread disruption and damage. Maximum wind gust of 165 mph at Cairn Gorm Summit. Investigated as part of the DIAMET project.

8. Cyclone Friedhelm

9. Ingredients for a “bomb”: Jet Stream Cyclone Friedhelm occurred during a period of strong Jet Stream. Caused by a very high NAO (North Atlantic Oscillation) of 2.52 during Dec. 2011. This was the highest December NAO, and third highest for any month, since 1950. From Vaughan et al. 2014

10. Ingredients for a “bomb”: Jet Stream Strong Jet Stream drives storms across the Atlantic. Enhances upper-level divergence, increasing the deepening. Vorticity changes on around the Jet Stream can enhance the circulation.

11. Ingredients for a “bomb”: PV What is PV? Combination of the vorticity and stability. It is conserved in adiabatic conditions, making it a useful tracer. Diabatic heating can create regions of strong PV. Generally high PV in stratosphere and lower in the troposphere. Can be “inverted” to ascertain thermodynamic variables (wind speed, temperature etc.).

12. Ingredients for a “bomb”: PV Intrusion

14. Ingredients for a “bomb”: Low-level PV Strong band of low- level PV. Created by latent heat release through cloud processes. Increases low-level circulation. PV @ 850 hPa

15. Ingredients for a “bomb”: PV towers Upper- and lower-level PV anomalies can align. These create “PV towers” This coupling can enhance the circulation and deepening Once the upper-level anomaly moves past the lower-level anomaly the system often begins to weaken.

16. How important is Latent Heating? Latent heating removed from the model simulation. Removal of latent heating makes little difference to the storm track. However, latent heating makes a significant difference to the deepening and wind maxima.

17. How does removing LH change the PV? Low-level PV anomaly is not generated without latent heating. Upper-level anomaly is still present although somewhat altered CTRLNO LH

18. How important is Latent Heating? CTRL run wind speeds and SLP. No latent heating run wind speeds and SLP. Moisture and latent heat release key in deepening cyclones What effect might this have in a warmer, moister future climate? 0600 UTC 1200 UTC

19. Summary A “weather bomb” is an extratropical cyclone which deepens by roughly >24 hPa in 24 hours. A number of factors contribute and influence the deepening. These include: o Jet stream and related background conditions (NAO). o Upper-level PV anomalies. o Low-level PV production. Latent heating and PV generation is a key component to the deepening.

20. Thanks. Any Questions?

21. Where do the high winds occur?

23. What is a “weather bomb”?