Bomb Cyclone

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Recently, the readers might have come across viral photos and videos of homes, cars, roads, lawns etc. covered in four feet of snow near the Great Lakes Region in North America. This snowfall occurred, unexpectedly, in a very short period. The meteorologist attributed this atmospheric event to the Bomb Cyclone. However, most people don’t know the meaning of a bomb cyclone. So, what is a bomb cyclone?

What is a Bomb Cyclone?

The question arises: what is a bomb cyclone? Why don’t we just call it a temperate cyclone. Let us first define the term bomb cyclone and then disaggregate the concept.

  • Definition: Bomb cyclone is a cyclonic activity in which two air masses of different properties collide resulting in very high intensity rainfall, snowfall or other forms of precipitation.
  • It is clearly discernible from this definition that a bomb cyclone is like a temperate cyclone but the intensity of rainfall or snowfall in a bomb cyclone is so high that it does not come across to people as a normal cyclonic weather event. For example in Buffalo, New York, U.S.A., many people died due to extreme snowfall in the last week of December 2022.
  • This term is primarily used to describe the sudden change in weather conditions like a bomb explosion changes the surroundings.
  • This term points to a disastrous weather event.
  • Hurricane is a form of tropical cyclone whereas the bomb cyclone is a form of temperate cyclone.

Bombogenesis versus Cyclogenesis

Cyclones are rotating masses of air. In other words, when the winds blow inwards towards a low pressure zone then we term such atmospheric phenomenon as cyclones. Cyclogenesis is the process of creation of cyclones. Similarly, Bombogenesis is the process of creation of a bomb cyclone. Let us understand the phenomenon step-by-step as follows.

Normal Temperate Cyclone

  1. The cyclones are of two types i.e. Tropical Cyclone and Temperate Cyclone.
  2. The bomb cyclone is a mid-latitude occurrence, hence it is related to temperate cyclones.
  3. Normally, the dry and cold polar winds become stronger in winter and blow southwestward. The warm and moist tropical winds flow northeastward. These winds collide in the mid-latitudes and form frontal or temperate cyclones.
  4. In temperate cyclones, the cold wind pushes the warm wind upwards and precipitation starts. Precipitation may be in the form of rain, snowfall, hail, sleet etc. These cyclones are much stronger in winter and cause snowfall in most of Canada and the U.S.A. In common parlance, such storms are called blizzards. For details on formation of temperate cyclones and fronts, CLICK HERE.

Bomb Cyclone

Fig. 1: Collision of Cold and Warm Air Masses over Great Lakes (Source: USA Today)
  1. In Bombogenesis, some additional climatic conditions increase the intensity of a frontal cyclone. For instance, the Great Lakes are inter-connected large water bodies in central Northeast America around the 49th Parallel and create certain conditions which helps bombogenesis.
  2. A huge amount of water evaporates from the surface of these lakes. The water vapor rises upwards creating a low pressure zone on the surface of the Great Lakes. As a result, the winds from the surrounding areas rush into a low pressure zone. These winds are polar winds from the north and tropical winds from Caribbean Sea (see Fig. 1).
  3. The convection of water vapor becomes stronger when there is presence of Jet Streams in the upper atmosphere. The Jet Stream acts as a vacuum machine and quickly sucks the water vapor upwards from the lakes’ surface.
  4. Consequently, the atmospheric pressure drops suddenly in the center of the low pressure zone. If the drop in the atmospheric pressure is greater than 25 millibars then the speed of winds from surrounding areas increases to more than 80 Kilometers/hour.
  5. The winds increase the rate of evaporation The huge amount of upward rising water vapor cools at Dry Adiabatic Lapse Rate which causes very intensive rainfall in summer and snowfall in winter. Meteorologists measure the intensity of rainfall in terms of rainfall or snowfall in an hour. This intense cyclone is termed a bomb cyclone.

In the case of the U.S.A. and Canada, the bomb cyclone led to 566 mm of snowfall within a day breaking all previous records of diurnal snowfall.

Impact of Climate Change

  • There is an increase in the global air temperature in the post-industrial revolution period due to use of fossil fuels. Therefore, more evaporation of water takes place from the water surface.
  • The impact of the water evaporation on cyclogenesis is much more prominent in winter. For instance, the temperature of Great Lakes’ water is more than the temperature of surrounding air mass but due to global warming, this difference has increased. Hence, the Pressure Gradient from surrounding areas towards the lakes is much greater than before, especially during the winter season. This leads to greater wind speed and frequent occurrence of Bomb Cyclones.
  • The warming of lake water means stronger convective currents and stronger storms. With increase in global warming, the frequency of such disasters is estimated to increase.

Conclusion

We can conclude that bomb cyclones are extreme weather events which are hazardous to cultural and natural landscapes. It is urgent to remedy the causes of global warming to reduce such catastrophes. Additionally, the governments must incorporate disaster risk reduction (DRR) strategies as per Sendai Framework to minimize loss.