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Bomb Cyclones: The Forgotten Latitude Weighting
Tomer Burg • 27 March 2022 • Analysis
Post Highlights
Over the last few years, the term "bomb cyclone" or "bombogenesis" has become increasingly popularized within and beyond the weather community. A bomb cyclone is commonly defined as a decrease in central minimum pressure of 24 hPa or more within 24 hours. This definition, however, is incomplete.

The original definition of a "bomb cyclone" from Sanders and Gyakum (1980) included a latitude weighting factor, such that the threshold for a bomb cyclone increases the further north you go. This post reviews how this latitude weighting affects bomb cyclone criteria, as well as how this definition can be optimized in future work.
What is a "Bomb Cyclone"?
2018 Bomb Cyclone satellite image, courtesy NOAA.
Many storms may come to mind when hearing the term "Bomb Cyclone". One such storm may be the infamous January 2018 "Bomb Cyclone", a historically intense but fast-moving cyclone reaching an extraordinarily low minimum mean sea level pressure (MSLP) of 949 hPa off the New England coast.

As the term "bomb cyclone" became popularized, some suspected it was merely a made-up term, a product of clickbait headlines for media articles. This may seem reminiscient of how "Polar Vortex" became popularized following the 2014 Cold Air Outbreak, and has generally become synonymous with extremely cold air.

Similarly to how the Polar Vortex is an actual meteorological term that became misrepresented, the same can be said for Bomb Cyclones. Originally defined in Sanders and Gyakum (1980), a "bomb cyclone" was defined as the minimum central pressure of a cyclone deepening by 24 hPa or more within 24 hours... but with a twist!
Latitude Weighting for Bomb Cyclones
Common usage of the term "Bomb Cyclone" in recent years refers to a decrease in central minimum pressure of a cyclone of 24 hPa or more in 24 hours. It should be noted this is specifically in reference to the cyclone's minimum pressure, not the pressure at a single station/location. Sanders and Gyakum (1980) defined a bomb cyclone as exceeding 1 "bergeron" units, defined as the decrease in central minimum MSLP over a 24-hour period, but with the addition of normalizing this rate by a latitude weighting factor of sin(latitude) / sin(60°).

Where did the 60°N latitude normalization came from, you may be wondering? It is reputed that Tor Bergeron characterized a rapidly deepening cyclone as one whose central minimum pressure decreases by 24 hPa over a 24 hour period. This likely was in reference to Bergen, Norway, which is at a latitude of 60°N.

This means that as one goes farther north, the criteria for how much a cyclone has to deepen over a 24-hour period to meet bomb cyclone criteria increases. Conceptually this makes sense, as one ingredient for rapid cyclogenesis is strong baroclinicity (e.g., a strong temperature gradient), which is scarce in lower latitudes where deep cold air masses are highly uncommon.

Let's consider New England, where bomb cyclones are somewhat common in the winter. At Boston's latitude of 42.3°N, the bomb cyclone criteria is about 18.6 hPa over 24 hours. This decreases farther south towards Washington D.C., where at a latitude of 38.9°N the criteria further decreases to about 17.4 hPa over 24 hours. If you want to calculate this for your location, the Bomb Cyclone Calculator tool on my website can easily do the task. Therefore bomb cyclones are actually more common in the United States than one might get using the rule of thumb definition of 24 hPa over 24 hours!
Optimizing the Bomb Cyclone Definition
Bomb Cyclone climatology from Attard and Lang (2019).
Looking at the equation and plot in the previous section, one might assume that bomb cyclones increase in frequency the further north one goes. This is actually not the case; a bomb cyclone climatology from Attard and Lang (2019) shows two maxima in bomb cyclone tracks; one off the eastern North American coast, and the other off of eastern Asia.

There's numerous reasons why bomb cyclones are most common in these two regions, which will be the subject of a future blog post. But the takeaway point is that bomb cyclone frequency actually decreases in higher latitudes the further north one goes! This suggests that there is room for opportunity for improving the definition of anomalous cyclone deepening.

At the time of Sanders and Gyakum (1980), the recent addition of global satellite data and the relative infancy of numerical analyses and modeling made for challenges in creating high-quality, long-term cyclone climatologies. Nowadays with much higher resolution reanalyses such as the ERA-5 and much more advanced cyclone tracking techniques and computational abilities, we are able to analyze much more data than in the past.

How can we use these improvements to optimize the definition of a bomb cyclone? I'm not sure there is a single right answer to this question, and there could easily be multiple approaches. One such potential would be to create a gridpoint based climatology of cyclone deepening rates, and derive the percentile rank of a cyclone's deepening rate relative to all historical cyclones within a neighboring radius and within a surrounding time frame to account for seasonality differences in cyclone intensity climatology.

It may not be as simple to calculate and remember as 24 hPa / 24 hours, but it may ultimately yield a more accurate answer for just how unusual a cyclone is relative to climatology.
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