Bomb Cyclogenesis: Introduction
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what is the bomb? case study 1 case study 2 case study 3 the page of reference


[What Is The Bomb?]

{The Basics}

_____Simply put, bomb cyclogenesis is the formation of an "extratropical area of low pressure in which the central barometric pressure drops at least 24 millibars in 24 hours." [1] However, it should be noted that this represents the most common case in areas that are north of 60 degrees latitude. Sanders and Gyakum, who coined the term "Bomb Cyclones" set the pressure falls needed to reach bomb status at 19 millibars in 24 hours at 45 degrees and 23 millibars in 24 hours at 55 degrees. [2]

_____The following is a list of four basics facts about bomb cyclones: [3]

__________A) They have cold air and cold air fronts.
__________B) They form under strong uppper level winds.
__________C) They occur during the winter.
__________D) The mainly form in the Northwest Pacific and Northwest Atlantic (which
____________ means that they may have something to do with the formation of Nor'easters).

{Bomb Formation: An Example [4] }

Panel 1: Copyright--USA TODAY

Hour 0

_____The event is triggered by a High Pressure system dropping down along the East Coast, bringing in a large bubble of cold air. Since the ocean temperature remains fairly constant, this creates a pressure gradient along the coast and helps form a weak area of low pressure, along which forms the start of a storm.

_____An upper level disturbance begins to move into the area as it rides along the jetstream.

Hour 12

_____The pressure continues to fall at a rapid rate. The weak storm picks up strength as the upper-level disturbance moves into the area and begins to fuel the surface storm. This results in an onslaught of heavy snow and strong winds in the affected area.

Panel 2: Copyright--USA TODAY

Panel 3: Copyright--USA TODAY

Hour 24

_____Storm has intensified as the pressure has reached a drop of at least 24 millibars. The storm continues to be fueled by the upper-level disturbance. The resulting effect is a combination of heavy snow and extremely strong winds that create a severe blizzard.

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{A More Detailed Look at the Bomb}

[-500 mb circulation and troughs-]

_____A study was conducted of 141 bomb cyclones in conjunction with the attendant 500 millibar circulation changes and the following four conclusions were drawn: [5]

__________A) Most bombs are not associated with locally identifiable 500 mb
____________circulation changes. The bombs connected to these changes tend
____________to be weak in nature.
__________B) Downstream 500 mb ridge amplification (found in one of every
____________four bombs) was more common tahn upstream trough amplification.
__________C) Trough amplifications were associated with southeastward moving
____________500 mb disturbances connected with the surface bombs.
__________D) Most 500 mb cyclone-anticyclone pairs investigated
____________seasons were associated with surface cyclones; the more persistent
____________of these pairs were preceded by bombs.

[-vorticity maxima-]

_____Although the bombs studied were not likely to be associated with circulation changes and those that were tended to be extremely weak in nature, circulation, of course, is a major part of any weather system. Thus, these "explosive cyclogenesis events tend to occur in association with large amiplitude 500 hPa vorticity maxima." [6] Further more it has been found in studies that the rate of development of bombs is proportional to 500 hPa absolute vorticity advection; the advection is due to the vorticity maximum which often is found to exist several days before the surface cyclone developes. [7]


_____Baroclinicty is basically a temperature caused thickness gradient that forms along a constant pressure surface. [8] A study wrote about in a paper by Gaza and Bosart had the following to say in respect to bombs and increasing baroclinicity: [9]

The explosive deepening occured as the large-scale baroclinicity across eastern North America increased in response to the arrival of very cold air behind the deepening northern trough. Observe the strengthening southerly geostrophic flow at 500 mb along the East Coast of the United States as deepening occurs upstream and ridging downstream in the 24 hours... The net effect is to strengthen the jet along the coast and increase the implied advection of cyclonic vorticity by ther thermal wind and resulting lower- and mid-tropospheric ascent over the deepening surface cyclone center.

_____In other words, this increased baroclinicty helped to strengthen both the jetstream and the vorticity advection, which helped to futher deepen and thus strenghten the bomb cyclone.

[-the jet and topography-]

_____The jetstream and topography help to establish the reasoning why bombs occur most frequently on the east coast. These will also be the final items discussed on this site. The following was drawn from several bomb cyclone studies and makes the conclusion concerning how jets and topography work together:

The evolution of the jet/trough systems varies from case to case, but appears to act to increase divergence aloft (and associated positive vorticity advection) immediately prior to and during rapid surface development. The jet/trough systems provide the upper-level divergence/baroclinc instability needed to initiate the development of a cylcone, but topographical features related to the East Coast (in particular, land-sea effects, the Appalachin Mountains and boundary layer processes over the ocean) act to focus or modulate the rapid development phase of the surface cyclone. [10]

_____So while the jetstream helps to provide the instability needed for a bomb cyclone to develope, it's the topography of the East Coast that acts as a focusing and triggering mechanism. This goes to help explain just why bomb cyclones are concentrated on the East Coast and result in the massive storms as discussed in the case studies.

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