Bomb (meteorology)

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The Braer Storm of January 1993 explosively deepened to a record low of 914 mbar (hPa)

Explosive Cyclogenesis (also referred to as a weather bomb,[1][2][3] meteorological bomb,[4] explosive development,[1] or bombogenesis[5][6][7]) refers in a strict sense to a rapidly deepening extratropical cyclonic low-pressure area. To enter this category, the central pressure of a depression at 60˚ latitude is typically taken to decrease by 24 hPa or more in 24 hours.[8] This is a predominantly maritime, cold-season (winter) event,[8] but also occurs in continental settings.[9][10] They are the extra-tropical equivalent of the tropical rapid deepening.

Definition[edit]

Fred Sanders and John Gyakum in 1980 defined a ‘bomb’ as an extratropical cyclone that deepens by at least (24 sin φ/ sin 60˚)mb in 24 hours, where φ represents latitude in degrees. This is based on the definition for explosive development of a cyclone at 60˚N as deepening by 24mb in 24 hours which was standardised by Tor Bergeron.[11] Sanders and Gyakum noted that an equivalent intensification is dependent on latitude, at the pole it would be a drop in pressure of 28 mb/24 hours, while at 25 degrees latitude it would be only 12 mb/24 hours. All of these rates qualify for what Sanders and Gyakum called “1 bergeron”.[9][8]

Formation[edit]

Baroclinic instability has been cited as one of the principal mechanisms for the development of most explosively deepening cyclones.[12] Though the relative roles of baroclinic and diabatic processes in explosive deepening of extratropical cyclones have been subject to debate (citing case studies) for a long time.[13] Other factors include the relative position of a 500-hPa trough and thickness patterns, deep tropospheric frontogenetic processes which happen both upstream and downstream of the surface low, the influence of air–sea interaction, and latent heat release.[14]

Regions and motion[edit]

The four most active regions where extratropical explosive cyclogenesis occurs in the world are the Northwest Pacific, the North Atlantic, the Southwest Pacific, and the South Atlantic.[15]

In the Northern Hemisphere the maximum frequency of explosively deepening cyclones are found within or to the north of the Atlantic Gulf Stream and Kuroshio Current in the western Pacific,[8] and in the Southern Hemisphere, Australian east coast lows above the East Australian Current, which shows the importance of air-sea interaction in initiating and rapidly developing extratropical cyclones.[16]

It has been noted that many explosively deepening cyclones south of 50˚S show equator-ward movement, in contrast with the pole-ward motion of most Northern Hemisphere bombs.[14] Over the year, 45 cyclones on average in the Northern Hemisphere and 26 develop explosively in the Southern Hemisphere, mostly in the respective hemisphere winter time.[14] Less seasonality has been noticed in bomb cyclogenesis occurrences in the Southern Hemisphere.[14]

The January 2013 Northwest Pacific bomb cyclone east of Japan, which met the conditions of a bomb cyclone

Historical conceptual development[edit]

In the 1940s and 50s meteorologists at the Bergen School of Meteorology began informally calling some storms "bombs" because they developed with a ferocity rarely, if ever, seen over land.[17] By the 1970s the terms "explosive cyclogenesis" and even "meteorological bombs" were being used by MIT professor Fred Sanders (building on work from the 1950s by Tor Bergeron), who brought the term into common usage in a 1980 article in the Monthly Weather Review.[8][17]

Other uses of the term 'bomb'[edit]

The term “weather bomb” is popularly used in New Zealand to describe dramatic and/or destructive weather events. Only very rarely are these events actually instances of explosive cyclogenesis, as the rapid deepening of low pressure areas is a rare event around New Zealand.[18] The "bomb" name may lead to confusion with the more strictly defined meteorological term. The term is often misused in North America,[19] confused with heavy snowfall and nor'easters (which do sometimes meet the rapid pressure drop required to meet the strict meteorological definition). In Japan the term bakudan teikiatsu (爆弾低気圧 literally “low pressure bomb”) is used to refer to a rapid drop in atmospheric pressure that precipitates a sudden and intense storm.[20]

Further reading[edit]

References[edit]

  1. ^ a b "Fierce 'weather bomb' batters Britain". The Telegraph. 9 December 2011. Retrieved 21 March 2013. 
  2. ^ "The worst storm in years?". Met Office Blog. 28 January 2013. Retrieved 21 March 2013. 
  3. ^ Edwards, Tim (9 December 2011). "Scotland storm: what is a weather bomb?". The Week (London, United Kingdom: Dennis Publishing). Retrieved 4 April 2013. 
  4. ^ O'Hanlon, Larry (25 February 2013). "Look out -- 'meteorological bomb' is on the way!". NBC News. Retrieved 21 March 2013. 
  5. ^ "Ryan explains Bomb Cyclogenesis". WBRZ News 2 Louisiana. Retrieved 21 March 2013. 
  6. ^ Freedman, Andrew (1 March 2013). "Meteorological bomb explodes over New England". Washington Post. Retrieved 21 March 2013. 
  7. ^ Rodman, Kristen (23 January 2014). "What is Bombogenesis?". Accuweather. Retrieved 31 January 2014. 
  8. ^ a b c d e Sanders, Frederick; Gyakum, John R. (1980). "Synoptic-Dynamic Climatology of the "Bomb"". Monthly Weather Review 108 (10): 1589–1606. doi:10.1175/1520-0493(1980)108<1589:SDCOT>2.0.CO;2. Retrieved 21 March 2013. 
  9. ^ a b "The Bomb". blog.ametsoc.org. 27 October 2010. Retrieved 21 March 2013. 
  10. ^ Macdonald, Bruce C.; Reiter, Elmar R. (1988). "Explosive Cyclogenesis over the Eastern United States". Monthly Weather Review 116 (8): 1568–1586. doi:10.1175/1520-0493%281988%29116%3C1568%3AECOTEU%3E2.0.CO%3B2. Retrieved 21 March 2013. 
  11. ^ Baker, Laura (2011). Sting Jets in Extratropical Cyclones (Ph.D.). University of Reading. 
  12. ^ WENG, H. Y.; BARCILON, A. (1987). "Favorable environments for explosive cyclogenesis in a modified two-layer Eady model". Tellus A 39A (3): 202–214. doi:10.1111/j.1600-0870.1987.tb00301.x. 
  13. ^ Fink, Andreas H.; Pohle, Susan; Pinto, Joaquim G.; Knippertz, Peter (2012). "Diagnosing the influence of diabatic processes on the explosive deepening of extratropical cyclones". Geophysical Research Letters 39 (7): n/a–n/a. doi:10.1029/2012GL051025. Retrieved 2 June 2013. 
  14. ^ a b c d Lim, Eun-Pa; Simmonds, Ian (2002). "Explosive Cyclone Development in the Southern Hemisphere and a Comparison with Northern Hemisphere Events". Monthly Weather Review 130 (9): 2188–2209. doi:10.1175/1520-0493(2002)130<2188:ECDITS>2.0.CO;2. Retrieved 21 March 2013. 
  15. ^ Black, Mitchell Timothy; Pezza, Alexandre Bernardes (January 2013). "A universal, broad-environment energy conversion signature of explosive cyclones". Geophysical Research Letters 40 (2): 452–457. doi:10.1002/grl.50114. Retrieved 21 March 2013. 
  16. ^ Yoshiike, Satoki; Kawamura, Ryuichi (2009). "Influence of wintertime large-scale circulation on the explosively developing cyclones over the western North Pacific and their downstream effects". Journal of Geophysical Research 114 (D13). doi:10.1029/2009JD011820. Retrieved 9 April 2013. 
  17. ^ a b Williams, Jack (20 May 2005). "Bomb cyclones ravage northwestern Atlantic". USA Today. Retrieved 22 March 2013. 
  18. ^ Kreft, Peter (4 March 2012). "The Bomb". Metservice NZ blog. Retrieved 21 March 2013. 
  19. ^ Day, Cindy. "Weather Bomb Blasts the Maritimes!". CTV News. Retrieved 5 April 2013. 
  20. ^ Milner, Rebecca (3 December 2012). "Japan’s top 10 buzzwords for 2012". Japan Pulse Blog, The Japan Times. Retrieved 25 April 2013. 

External links[edit]