What is Accumulated Cyclone Energy?

Depending on how deep you follow tropical cyclones you may have heard of a term called ACE index or Accumulated Cyclone Energy.  But, what exactly is ACE?

Simply put, it is a measurement of the estimated energy of a tropical cyclone over its life span.  ACE takes into account strength and duration of each tropical cyclone which can also be compiled into seasonal statistics to reveal unique characteristics of tropical cyclones and hurricanes.

For you math whizzes out there, ACE is calculated with every advisory – every six hours as follows:

ACE relies indirectly on satellite imagery; it’s value is, of course, entirely dependent upon the known strength of the system (reconnaissance) or by Dvorak estimates the Tropical Analysis and Forecast Branch (TAFB) of the National Hurricane Center assigns.  Therefore, to an extent, ACE can be flawed; particularly for deep oceanic cyclones (Tropical Storm Grace) and storms that existed prior to the satellite age.

Ryan Maue, a PhD student at Florida State University, Department of Meteorology, writes regular updates on ACE values during the season and has tons of great resources to read.

Using recent memory, Hurricane Bill, the strongest of the 2009 Atlantic Hurricane Season so far with 115kts (132mph or 213kph) winds, earned an ACE value of 26.47.  By comparison, Hurricane Jimena, which peaked at 135kts (155mph or 250kph) only had an ACE value of 24.0175.

This, however, doesn’t necessarily mean Hurricane Bill contained more energy than Hurricane Jimena.  Instead, the overall survival period of the cyclone as well as how long the cyclone maintained its severity play important factors.

Hurricane John is the longest active tropical cyclone on Earth in recorded history.  Hurricane John had an ACE value of 70.4 – nearly three times as much as Bill.  John peaked with winds of 150kts (173mph or 278kph) and was a hurricane for only 8 1/2 days out of its 31 days of existence.

The highest ACE value of known tropical cyclones is Hurricane San Ciriaco at 73.57.

Hurricane seasons can also have ACE values.  The Atlantic hurricane season with the highest ACE value is the 2005 hurricane season with an accumulated ACE of 248.

Close on the heels of the 2005 hurricane season is that of the 1950 hurricane season which actually had less than half as many storms as 2005, yet 1/4 fewer hurricanes and more major hurricanes (by 1).  The ACE value for 1950:  243.

For the East Pacific, two hurricane seasons had a higher ACE than the 2005 Atlantic season:  1992 and 1990 which recorded 290 and 249, respectively.  Only the 1992 East Pacific season had more named storms than the 2005 Atlantic season.  Both East Pacific seasons had more hurricanes than the 2005 Atlantic season while the 1992 East Pacific hurricane season recorded an additional three major hurricanes while the 1990 season recorded one fewer major hurricane than the 2005 Atlantic season.

The following chart obtained from Wikipedia shows total ACE values for the Atlantic hurricane season since 1950:

The red line shows the average ACE per storm while the black line shows the total ACE for the season.  While 2005 was a very active season, by looking at ACE values we can see the season fits in with seasons past.  The 1933 hurricane season, considered the second busiest in the Atlantic on record, only has an ACE of 213 (not visible on chart).

ACE can help provide deeper insight, of which at least one example can be seen in the image above, on the unique characteristics of each season.  Using tools such as ACE should help meteorologists and forecasters have a better understanding of cyclic activity in the Atlantic and East Pacific basins.

By thoroughly exploring each season in different ways, it also helps us to have a better understanding of whether a particular hurricane season is truly unique.

This insight simply cannot be obtained by counting from one every June.