The Science Behind “Atmospheric Bomb” Cyclones

The first full week of January featured a powerful winter storm – known as a nor’easter – intensify off the east coast of the United States causing snowfall from the North Florida to Maine into Atlantic Canada, along with widespread power outages from strong winds as well as storm surge flooding and battering waves.

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Powerful nor’easter offshore Long Island Thursday afternoon.
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Vehicles navigate Monument Square on Sunday in North Adams, Massachusetts, which was under a snow emergency. Gillian Jones / AP

The storm underwent rapid intensification known in meteorological slang as “bombogenesis”. An “atmospheric bomb” occurs when a developing cyclone’s low pressure center intensifies explosively…defined as at least 1 millibar or 1 hectopascal drop per hour on average during a 24 hr period. This system had a pressure drop of 54 millibars in 24 hrs (1004 to 950 millibars). This bombogenesis phase can occur in both frontal cyclones seen in the mid-latitudes such as with this week’s storm or with tropical cyclones. A famous example would be Hurricane Patricia in the Eastern Pacific in 2015 which experienced a minimum central pressure drop of 95 millibars during a 24 hr period (967 to 872 millibars).

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Bombogenesis in mid-latitude cyclones occurs when there are favorable jet stream dynamics which allow for strong vertical motion, to force air up and away from a developing surface low. These include very strong upper-level winds and diverging flow. This allows for a high rate of decrease in surface pressure, intensifies the pressure gradients, reinforces the “conveyor belts” of warm, moist air flowing into the cyclone for clouds, releases latent heat and producing precipitation, which further strengthens the storm.

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For frontal cyclones, the most intense atmospheric “bombs” occur when you have a merging or “phasing” of the northern and southern jet streams (basically the polar jet with much colder air to its north and the subtropical jet with far richer moisture sources to its south). This “phasing” of jet streams occurred with the most recent nor’easter.

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Upper-atmospheric air flow maps from Wed-Thurs morning showing the phasing of the polar and sub-tropical branches of the jet stream, which intensified the massive East Coast nor’easter.

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“Bomb” cyclones are nothing new. Unfortunately for us who have to live and deal with their impacts, human-induced climate change has forced our world to retain a significant amount of heat energy. These major changes on climate in just the past 20-30 years have caused statistical changes in observed weather. And one of those changes is in rapid intensification of cyclones. With tropical cyclones, there is evidence that a warming ocean and lower atmosphere (with greater moisture/latent heat release) is playing a role in increasing the frequency of rapidly intensifying tropical cyclones (here’s a paper by Kishtawal et al. on the topic). With mid-latitude cyclones, there is ongoing debate on the issue. However, there ongoing research suggests that in addition to thermodynamic roles, the increasing “waviness” of the polar jet stream theorized to occur in a warming world may have impacts on mid-latitude weather and long-term climate patterns. High amplitude jet streams produce greater mixing of air masses at lower levels of the atmosphere between the polar regions and sub-tropics (a process known as temperature advection). The increase in jet stream amplitude acts as a feedback to further amplify Arctic warming rapidly relative to the mid-latitudes as much warmer air advects into the far north (jet slows slightly with less temperature gradient, but becomes much more amplified, enhancing warming further). While the effect of the mid-latitudes circulation patterns on the Arctic seems more well-established because of the rapid changes in the far north, climate scientists are in much higher disagreement on the effects of feedbacks back on the mid-latitudes. Dr. Jennifer Francis (Rutgers University; see short webinar on possible connection between Arctic warming and mid-latitude extreme weather), among other scientists continue to do research actively on jet stream dynamics in the mid-latitudes with regards to climate change. But such a combination of warming energy sources and amplified jet stream patterns could further the development “bomb” cyclones in the future as the world continues to warm, at least while there remains strong temperature gradients between air masses to fuel mid-latitude storms (mid-latitude cyclones may be weaker and/or found much farther north in a much warmer planet). And there has already been a statistically detectable shift northward in winter storm tracks in the Northern Hemisphere and an increase in the severity (intensity of cyclones and precipitation rates) and frequency of “atmospheric river” events in the Eastern Pacific toward North America since the 1950s (see Key Finding #4-5/Chapter 9 of US Climate Report).

What “bomb” means as far as hazardous impacts will depend on the specific storm, but when it comes to ocean storms, like what was witnessed this week, obviously, damaging winds, heavy surf, storm surge flooding and heavy precipitation which can cause dangerous disruptions are what are all possible. In this case, much of it was all snow and ice. In the warm season, it can be flooding rainfall. But human-induced forcing (retaining of heat in Earth’s system) is now known to play a role in the attribution of the intensification of these large-scale weather systems within the changing climate regime.

–Meteorologist Nick Humphrey

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Author: Meteorologist Nick Humphrey

Meteorologist and geoscientist in Lincoln, NE. Seattle, WA native. Love weather, storm chasing/photography and planetary science.

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