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 North Atlantic hurricane season. It uses an approximation of the wind 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. In recorded history, the highest ACE worldwide is 82, for Hurricane Ioke, in 2006.[1]

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 (65 km/h) or higher), at six-hour intervals. Since the calculation is sensitive to the starting point of the six-hour intervals, the convention is to use 0000, 0600, 1200, and 1800 UTC. If any storms of a season happen to cross years, the storm's ACE counts for the previous year.[2] The numbers are usually divided by 10,000 to make them more manageable. The unit of ACE is 104 kn2, and for use as an index the unit is assumed. Thus:

\text{ACE} = 10^{-4} \sum v_\max^2

where vmax 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.[3]

Atlantic basin ACE

Categories

Atlantic basin cyclone intensity by Accumulated cyclone energy, timeseries 1895-2007

A season's ACE is used by NOAA and others to categorize the hurricane season into 3 groups by its activity.[4] Measured over the period 1951–2000 for the Atlantic basin, the median annual index was 87.5 and the mean annual index was 93.2. The NOAA categorisation system divides seasons into:

The term hyperactive is used by Goldenberg et al. (2001)[5] based on a different weighting algorithm[6] which places more weight on major hurricanes, but typically equating to an ACE of about 153 (175% of the 1951–2000 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, Hurricane Donna in 1960, with an ACE of 64.6, Hurricane Isabel in 2003 with an ACE of 63.28, and the Great Charleston Hurricane of 1893 with an ACE of 63.5.


Atlantic hurricane seasons, 1950–2014

For the in-progress season, the ACE is preliminary based on National Hurricane Center bulletins, which may later be revised.

SeasonACETSHUMHClassification
2005 Atlantic hurricane season25028157Above normal (hyperactive)
1995 Atlantic hurricane season22819115Above normal (hyperactive)
2004 Atlantic hurricane season2251596Above normal (hyperactive)
1950 Atlantic hurricane season21116116Above normal (hyperactive)
1961 Atlantic hurricane season2051187Above normal (hyperactive)
1955 Atlantic hurricane season1991296Above normal (hyperactive)
1998 Atlantic hurricane season18214103Above normal (hyperactive)
1999 Atlantic hurricane season1771285Above normal (hyperactive)
2003 Atlantic hurricane season1751673Above normal (hyperactive)
1964 Atlantic hurricane season1701266Near normal
1996 Atlantic hurricane season1661396Above normal (hyperactive)
2010 Atlantic hurricane season16519125Above normal (hyperactive)
1969 Atlantic hurricane season15818125Above normal (hyperactive)
1980 Atlantic hurricane season1471192Near normal
1966 Atlantic hurricane season1451173Above normal
2008 Atlantic hurricane season1441685Above normal
1951 Atlantic hurricane season1371085Above normal
1989 Atlantic hurricane season1351172Near normal
2012 Atlantic hurricane season13319102Above normal
2011 Atlantic hurricane season1261974Above normal
1967 Atlantic hurricane season122861Near normal
1958 Atlantic hurricane season1211075Above normal
1963 Atlantic hurricane season118972Near normal
2000 Atlantic hurricane season1161583Above normal
1954 Atlantic hurricane season1131182Near normal
2001 Atlantic hurricane season1061594Near normal
1953 Atlantic hurricane season1041464Near normal
1988 Atlantic hurricane season1031253Near normal
1971 Atlantic hurricane season971361Near normal
1981 Atlantic hurricane season931273Near normal
1979 Atlantic hurricane season91952Near normal
1990 Atlantic hurricane season911481Near normal
1960 Atlantic hurricane season88742Near normal
1985 Atlantic hurricane season881173Near normal
1952 Atlantic hurricane season87763Near normal
1965 Atlantic hurricane season84641Below normal
1957 Atlantic hurricane season84832Near normal
1976 Atlantic hurricane season811062Near normal
2006 Atlantic hurricane season791052Near normal
1959 Atlantic hurricane season771172Near normal
1992 Atlantic hurricane season75741Below normal
1975 Atlantic hurricane season73963Near normal
2007 Atlantic hurricane season721562Near normal
1984 Atlantic hurricane season711251Near normal
2002 Atlantic hurricane season671242Near normal
2014 Atlantic hurricane season67862Near normal
1978 Atlantic hurricane season621252Below normal
1974 Atlantic hurricane season611142Below normal
1956 Atlantic hurricane season54842Below normal
2009 Atlantic hurricane season53932Below normal
1973 Atlantic hurricane season43841Below normal
1997 Atlantic hurricane season40731Below normal
1993 Atlantic hurricane season39841Below normal
1962 Atlantic hurricane season36531Below normal
1986 Atlantic hurricane season36640Below normal
2013 Atlantic hurricane season361420Below normal
1968 Atlantic hurricane season35840Below normal
1970 Atlantic hurricane season341052Below normal
1987 Atlantic hurricane season34731Below normal
1991 Atlantic hurricane season34842Below normal
1994 Atlantic hurricane season32730Below normal
1982 Atlantic hurricane season29621Below normal
1972 Atlantic hurricane season28730Below normal
1977 Atlantic hurricane season25651Below normal
1983 Atlantic hurricane season17431Below normal

East Pacific ACE

Accumulated Cyclone Energy is also used in the eastern and central Pacific Ocean. Data on ACE is considered reliable starting with the 1971 season. The season with the highest ACE since 1971 is the 1992 season. The 1977 season has the lowest ACE. The most recent above-normal season is the 2014 season, the most recent near-normal season is the 2012 season, and the most recent below normal season is the 2013 season.[7] The 35 year median 1971–2005 is 115 x 104kn2 (100 in the EPAC zone east of 140°W, 13 in the CPAC zone); the mean is 130 (112 + 18).

The (unofficial) categorisation of seasons for this table is based mutatis mutandis on that used in the Atlantic basin:

Observed monthly values for the PDO index, 1900–present.
Historical East Pacific Seasonal Activity, 1971–2007.
SeasonACETSHUMHClassification
1992 Pacific hurricane season290281610Above normal
1990 Pacific hurricane season24921166Above normal
1978 Pacific hurricane season20719147Above normal
1983 Pacific hurricane season20621128Above normal
1993 Pacific hurricane season20115119Above normal
2014 Pacific hurricane season19822169Above normal
1984 Pacific hurricane season19321137Above normal
1985 Pacific hurricane season19224138Above normal
1994 Pacific hurricane season18520105Above normal
1991 Pacific hurricane season17814105Above normal
1997 Pacific hurricane season1671997Above normal
1982 Pacific hurricane season16123125Above normal
2006 Pacific hurricane season15519116Above normal
1971 Pacific hurricane season13918126Above normal
1972 Pacific hurricane season1361484Near normal
1998 Pacific hurricane season1341396Near normal
1998 Pacific hurricane season1341396Near normal
1987 Pacific hurricane season13220104Near normal
1988 Pacific hurricane season1271573Near normal
2009 Pacific hurricane season1252085Near normal
2002 Pacific hurricane season1241586Near normal
1976 Pacific hurricane season1211595Near normal
2011 Pacific hurricane season11811106Near normal
1973 Pacific hurricane season1141273Near normal
1975 Pacific hurricane season1121794Near normal
1989 Pacific hurricane season1101794Near normal
1986 Pacific hurricane season1071793Near normal
1995 Pacific hurricane season1001073Near normal
2012 Pacific hurricane season9817105Near normal
2005 Pacific hurricane season961572Near normal
2000 Pacific hurricane season951962Near normal
1974 Pacific hurricane season9018113Near normal
2001 Pacific hurricane season901582Near normal
1999 Pacific hurricane season90962Near normal
2008 Pacific hurricane season831572Below normal
1980 Pacific hurricane season771473Below normal
1981 Pacific hurricane season721581Below normal
2013 Pacific hurricane season722091Below normal
2004 Pacific hurricane season711263Below normal
1979 Pacific hurricane season571064Below normal
2003 Pacific hurricane season561670Below normal
1996 Pacific hurricane season53952Below normal
2007 Pacific hurricane season521141Below normal
2010 Pacific hurricane season49832Below normal
1977 Pacific hurricane season22840Below normal

See also

References

  1. ↑ Mark Saunders, Adam Lea (2007). "Summary of 2006 NW Pacific Typhoon Season and Verification of Authors’ Seasonal Forecasts" (PDF). Tropical Storm Risk. Retrieved 2013-09-01.
  2. ↑ Last advisory for T.S. Zeta 2005
  3. ↑ Bell GD, Halpert MS, Schnell RC et al. (2000). "Climate Assessment for 1999" (PDF). Bulletin of the American Meteorological Society 81 (6): 1328. Bibcode:2000BAMS...81.1328B. doi:10.1175/1520-0477(2000)081<1328:CAF>2.3.CO;2.
  4. ↑ Climate Prediction Center (May 22, 2014). "Background information: the North Atlantic Hurricane Season". United States National Oceanic and Atmospheric Administration. Retrieved June 5, 2014.
  5. ↑ Goldberg SB, Landsea CW, Mestas-Nuñez AM, Gray WM (July 2001). "The Recent Increase in Atlantic Hurricane Activity: Causes and Implications" (PDF). Science 293 (5529): 474–9. Bibcode:2001Sci...293..474G. doi:10.1126/science.1060040. PMID 11463911. Supplementary material
  6. ↑ Summary of 2000 Atlantic tropical cyclone activity and verification of authors’ seasonal activity prediction.
  7. ↑ East North Pacific ACE (through 30 Nov. 2005)

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