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May 6, 2019

How does a meteorological bomb develop?

Meteorological bom off the US East Coast
Summary
A meteorological bomb is a rapidly developing cyclone.

It refers to the unofficial term for when a low pressure deepens with at least 24 hPa within 24 hours.
Such a rapid deepening of a low can be classified as a meteorological bomb – also called a weather bomb, bombogenesis, explosive cyclogenesis, bomb cyclone etc. - which ever you prefer.
In a cyclone, the air is rotating inwards counterclockwise (on the Northern Hemisphere) and are commonly associated with destructive winds, high waves and severe weather.

The recipe for a meteorological bomb

The main ingredient for forming a meteorological bomb is a large air temperature difference.
Gravity is sorting the air so that the heaviest air will be underneath lighter air. As cold air is relative heavier than the warmer air, the cold air rushes below the warmer air. This rush is what we call wind as wind is moving air. When the cold air displaces warmer air, we call it a cold front.
In that way, potential energy is being transformed into kinetic energy. The potential energy is due to the two different airmasses with different densities laying side by side but as the cold air rushes beneath the warmer and lighter air, the potential energy becomes transformed into kinetic energy (the increasing winds).
The larger the temperature difference is, the more intense may the front be and the larger is the potential energy and thus resulting wind speed.

Meteorological bombs occurs most frequently over the ocean during the winter season downstream of upper-level troughs and near the strongest sea surface temperature difference  – like the northern edge of the Gulf Stream south of Nova Scotia and Newfoundland in the North Atlantic and the northern edge of the Kuroshio Current east of Japan in the North Pacific.
Below image is an example of meteorological bomb over the Gulf Stream.

Meteorological bomb over the Gulf Stream
Met. bomb over the Gulf Stream on January 4th, 2018.

Meteorological bombs are especially dangerous to shipping as they form very rapidly and often culminates as a hurricane force low with very high waves as a result.

Other reinforcing factors are the evaporation over the relative warm waters that releases heat energy during the phase shift from water vapor to liquid water (water droplets in the condensing clouds). This will be heating the air over the sea surface further and as the winds and turbulence increases the evaporation becomes even stronger. 

Large values of warm advection at higher levels over the area will also reinforce the deepening of a low as well as air flow divergence over the area - but we should not go further along that nerdy path!

If the jet stream is very strong and a jet streak develops (an area where the wind speed in the jet stream culminates), it can further enhance the low development if the jet streak is positioned in the right place regarding to the initial surface low pressure. The right spot is either if it’s the left exit area or the right entrance area of the jet streak that will slowly move over the surface low.

Jet streak
A jet streak over Japan. The mentioned “left exit area” is above and east of Hokkaido and the “right entrance area” is above and west of the Shanghai area.

 

To recap we have mentioned the following ingredients:

  • An initial minor surface low at a weather front,
  • A large air and sea surface temperature difference,
  • Heating of the surface air by evaporation,
  • Increasing winds over sea generates turbulence and further evaporation,
  • Warm advection at higher levels in the troposphere,
  • Air flow divergence at higher levels,
  • Left exit area or right entrance area of a jet streak above the initial surface low pressure.

 

The perfect storm
If you add all above ingredients in the right way, all processes mentioned may act to reinforce each other and therefore generate a more rapid surface pressure fall than would be otherwise with each alone. When all these factors go hand in hand you may have “the perfect storm”.

Below two analysis charts shows the development of meteorological bomb that was forecast to generate waves of 16-19m Hs (significant wave height) when culminating as a deep hurricane force low.

Two weather charts
The development of a met. bomb over and east of Newfoundland as seen in analysis charts. From November 14th to 15th, 2018. The centre pressure dropped from 975 hPa to 948 hPa. The swell waves from this low slammed the Canary Islands several days after and made severe destruction to buildings at the northwest-facing coasts.

The strongest winds and highest waves are mainly south and west of a low (when moving eastwards on the Northern Hemisphere). Large low-pressure systems typically have a wide wind field, and the strongest winds normally have some distance to the low-pressure center.
It can be rather difficult to forecast the track and the intensity of a meteorological bomb as rather small differences in the position of a jet streak or the timing of a deepening surface pressure can have a huge impact on the forecast just a few days after.
The strategy for making the optimum route design when passing an ocean is to begin to circumnavigate a coming potential meteorological bomb several days before passing it as they move rather fast. However, this is of course depending on if the vessel is east going or west going.
But be careful – you have to have a good indication and understanding of the track of the low or else you may sail directly towards the area which has the highest waves a few days after!

If you want to read more about different kinds of lows you can read this article.

With this short article, we hope that you in some degree have learned how a meteorological bomb form and why some lows only become minor lows and others become hurricanes when culminating.
Please write to us if you want to have anything of the above clarified or if you have any feedback - we will appreciate that.