What Is ‘ACE’ That Meteorologists Talk About During Hurricane Season?


As I write this, the hurricane and typhoon seasons are full swing. The sixth named system of the Atlantic season, Tropical Storm Florence, is in the Atlantic Ocean. Forecasters are also keeping an eye on a tropical wave that has an 80% chance of developing once it gets into the Gulf of Mexico by mid-week. If you follow enough meteorologists on social media, you may see them refer to the term “ACE.” What is ACE and why is it important?

The Atlantic Basin with Tropical Storm Florence on the morning of September 2nd, 2018.NOAA

The inspiration for this piece was a tweet by one of the best young tropical meteorologists within our field, Dr. Michael Ventrice. Ventrice, a meteorologist and software engineer with the IBM/Weather Company, tweeted on September 2nd, 2018:

To put things in perspective, the Atlantic’s ACE is at 18.1. Thus the East Pacific’s ACE is nearly almost 10x greater than that of the Atlantic to date.

To the average person, this tweet might sound like a foreign language or my son trying to tell me something while he is chewing a mouth full of fruit snacks. As a meteorologist, I immediately know what he is saying. ACE is an acronym for Accumulated Cyclone Energy and is generally used to measure the combined duration and strength of tropical cyclones. It is also a good metric for scientists to compare storms against each other and across different seasons. According to Dr. Jeff Masters in the outstanding weather blog, Weather Underground,

ACE is calculated as the square of the wind speed every 6 hours, and is then scaled by a factor of 10,000 for usability. The ACE of a season is the sum of the ACE for each storm and takes into account the number, strength, and duration of all the tropical storms in the season. The caveat to using ACE as a measure of the activity of a season is that it does not take the size of the hurricane or tropical storm into account. The damage potential of a hurricane is proportional to the square or cube of the maximum wind speed, and thus ACE is not only a measure of tropical cyclone activity, but a measure of the damage potential of an individual cyclone or a season.

According to Dr. Master’s, the average ACE is 61.2 (per month) and 730.5 for the year. For a comprehensive list of “10 things” that we generally know about ACE, click this link.

As side note, ACE does not really account for the “rainfall or flooding” aspect of tropical systems nor does the Saffir-Simpson scale. As Dennis Mersereau recently wrote in Forbes, 2018 Hurricane Lane and 2017 Hurricane Harvey were record-breaking rainfall producers. Flooding potential from a tropical system is not necessarily correlated with wind speed. Both Lane and Harvey are good examples because much of the flooding associated with these storms happened after they were actually tropical storms. In a 2007 Geophysical Research Letters study, we also noted that weaker tropical systems contribute more rainfall to U.S. coastal regions than the major hurricanes.

That was a slight but important digression but let’s get back to ACE. The original study that most people cite for ACE was published in The Bulletin of the American Meteorological Society. The 2000 paper, authored by Gerald Bell and numerous colleagues, stated on pages S19-S20, 

….the hurricane destruction potential (HDP), which is calculated by summing the squares of the estimated 6-hourly maximum sustained wind speed for all periods in which the system is a hurricane…. A slight modification of the HDP index involves accumulating estimated 6-hourly maximum sustained wind speed for all 6-hourly periods in which the system is either a tropical storm or hurricane, thereby also accounting for the number and duration of storms while at a tropical storm status. This modified HDP index is referred to as accumulated cyclone energy (ACE) index

Dr. Ventrice messaged me to point out that for 2018 Eastern Pacific ACE (see below) is already exceeding that of the seasonal climatological value. He specifically noted that this “eye opening for me in a year that many debate whether El Nino is playing a role. I think this really boils down to our old way of defining El Nino, limiting the definition to ocean temperatures in the far eastern Pacific.” Western Pacific Ace is also running slightly ahead of climatological averages. The lesson here is that just be cause the Atlantic season is “not active” doesn’t mean that the global tropical season is not active. I find that many people in the U.S. often lose sight of this fact.

With the Atlantic hurricane basin starting to heat up literally and figuratively, it will be interesting to see how its ACE changes. In terms of longer term changes, the jury is still somewhat out on the connection between ACE and climate change-hurricane relationships. I will perhaps dig a little deeper on that topic in a future post. For a credible discuss on climate change and hurricanes, I recommend the NOAA GFDL website at this link.

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