Explosive cyclogenesis (also referred to as a weather bomb, meteorological bomb, explosive development, or bombogenesis) 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 required to decrease by 24 mb (hPa) or more in 24 hours.
In the 1940s and 50s meteorologists at the Bergen School of Meteorology began informally calling some storms that grew over the sea "bombs" because they developed with a ferocity rarely, if ever, seen over land.
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. In 1980, Sanders and his colleague John Gyakum 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, standardised by Bergeron, for explosive development of a cyclone at 60˚N as deepening by 24mb in 24 hours. Sanders and Gyakum noted that an equivalent intensification is dependent on latitude: at the poles this 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 these rates qualify for what Sanders and Gyakum called "1 bergeron".
Baroclinic instability has been cited as one of the principal mechanisms for the development of most explosively deepening cyclones. However, 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. 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.
Regions and motion
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, 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.
Explosively deepening cyclones south of 50˚S often show equator-ward movement, in contrast with the poleward motion of most Northern Hemisphere bombs. Over the year, 45 cyclones on average in the Northern Hemisphere and 26 in the Southern Hemisphere develop explosively, mostly in the respective hemisphere's winter time. Less seasonality has been noticed in bomb cyclogenesis occurrences in the Southern Hemisphere.
Other uses of "weather bomb"
The term "weather bomb" is popularly used in New Zealand to describe dramatic and/or destructive weather events. Only very rarely are these actually instances of explosive cyclogenesis, as the rapid deepening of low pressure areas is rare around New Zealand. This use of "bomb" may lead to confusion with the more strictly defined meteorological term.
The term is often misused in North America, confused with heavy snowfall and nor'easters (which do sometimes undergo the rapid pressure drop required to meet the strict meteorological definition). In Japan, the term bomb cyclone (爆弾低気圧 bakudan teikiatsu?) is used both academically and commonly to refer to an extratropical cyclone which meets the meteorological "bomb" conditions.
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