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# Accumulated cyclone energy

Accumulated cyclone energy (ACE) is a measure used by the National Oceanic and Atmospheric Administration (NOAA) to express the activity of individual tropical cyclones and entire tropical cyclone seasons, particularly the Atlantic hurricane seasons. It uses an approximation of the energy used by a tropical system over its lifetime and is calculated every six-hour period. The ACE of a season is the sum of the ACEs for each storm and takes into account the number, strength, and duration of all the tropical storms in the season. [http://ams.allenpress.com/perlserv/?request=get-pdf&doi=10.1175%2F1520-0477%282000%29081%3C1328%3ACAF%3E2.3.CO%3B2 Bell, Gerald D., Halpert, Michael S., Schnell, Russell C., Higgins, R. Wayne, Lawrimore, Jay, Kousky, Vernon E., Tinker, Richard, Thiaw, Wasila, Chelliah, Muthuvel, Artusa, Anthony. "Climate Assessment for 1999." Bulletin of the American Meteorological Society 2000 81: 1328-1328] ]

Calculation

The ACE of a season is calculated by summing the squares of the estimated maximum sustained velocity of every active tropical storm (wind speed 35 knots or higher), at six-hour intervals. If any storms of a season happen to cross years, the storm's ACE counts for the previous year. [ [http://www.nhc.noaa.gov/archive/2005/dis/al302005.discus.030.shtml? Last advisory for T.S. Zeta 2005] ] The numbers are usually divided by 10,000 to make them more manageable. The unit of ACE is 104 kt2, and for use as an index the unit is assumed.

Thus: $ACE = \left\{10^\left\{-!4 sum_\left\{\right\}^\left\{\right\} \left\{\left(\left\{v_mathrm\left\{max^\left\{!2\right\}\right)\right\}$

...where "v"max is estimated sustained wind speed in knots.

Kinetic energy is proportional to the square of velocity, and by adding together the energy per some interval of time, the accumulated energy is found. As the duration of a storm increases, more values are summed and the ACE also increases such that longer-duration storms may accumulate a larger ACE than more-powerful storms of lesser duration. Although ACE is a value proportional to the energy of the system, it is not a direct calculation of energy (the mass of the moved air and therefore the size of the storm would show up in a real energy calculation).

A related quantity is hurricane destruction potential (HDP), which is ACE but only calculated for the time where the system is a hurricane.

Climatology

Measured over the period 1951-2005 for the Atlantic basin:
* Median annual index: 89.5
* Mean annual index: 102.3

A season's ACE is used to categorize the hurricane season by its activity. NOAA categorisation system [ [http://www.cpc.noaa.gov/products/outlooks/background_information.shtml Climate Prediction Center - Background Information: The North Atlantic Hurricane Season] ] divides them into:
* "Above-normal season": An ACE value above 103 (115% of the current median), provided at least two of the following three parameters exceed the long-term average: number of tropical storms (10), hurricanes (6), and major hurricanes (2).
* "Near-normal season": neither above-normal nor below normal
* "Below-normal season": An ACE value below 66 (74% of the current median)

Individual storms

The highest ever ACE estimated for a single storm in the Atlantic is 73.6, for Hurricane San Ciriaco in 1899. This single storm had an ACE higher than many whole Atlantic storm seasons. Other Atlantic storms with high ACEs include Hurricane Ivan in 2004, with an ACE of 70.4, and Hurricane Donna in 1960, with an ACE of 64.6.

ACE index for hurricane seasons 1950-2006

The scale on the left represents the total Accumulated Cyclone Energy Index for the Atlantic Season, and the scale on the right the ACE/Storm average for that season.

Atlantic hurricane seasons 1950-2008 by ACE index

The term "hyperactive" is used by Goldenberg "et al." (2001) [ [http://www.aoml.noaa.gov/hrd/Goldenberg/science01.pdf The Recent Increase in Atlantic Hurricane Activity: Causes and Implications] ] [ [http://www.aoml.noaa.gov/hrd/Goldenberg/science-supp01.pdf Supplementary material] . "The Recent Increase in Atlantic Hurricane Activity: Causes and Implications"] based on a different weighting algorithm which places more weight on major hurricanes, but typically equating to an ACE of about 153 (171% of the current median).

:

Median = 115

Calculations from Eastern North Pacific Tracks File at NHC [http://www.nhc.noaa.gov/tracks1949to2006_epa.txt]

West Pacific ACE

ACE is also used in the western Pacific. 2004 had an ACE of 464 104 kt2 for the NW Pacific. This was the third highest since 1965, after 1992 and 1997. The 40 year norm (1965-2004) is 305 +/-99 104 kt2.

* List of Atlantic hurricane seasons

References

* [http://www.aoml.noaa.gov/hrd/tcfaq/E11.html NOAA ACE by year from 1851]
* [http://www.ncdc.noaa.gov/oa/climate/research/2004/ann/atlantic-2004-ace.html National Climatic Data Center - Atlantic Basin 2004 Accumulated Cyclone Energy (ACE) Index]
* [http://www.ncdc.noaa.gov/oa/climate/research/2005/2005-atlantic-trop-cyclones.html National Climatic Data Center - Atlantic Basin 2005 Accumulated Cyclone Energy (ACE) Index]
* [http://tsr.mssl.ucl.ac.uk/docs/TSRNWP2004Verification.pdf 2004 Pacific NW Typhoon Season ACE pdf]
* [http://www.atmos.albany.edu/student/carl/index.html Global Tropical Cyclone Best Track Database]

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