Accumulated cyclone energy

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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[edit]

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[edit]

Categories[edit]

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:

  • Above-normal season: An ACE value above 111 (120% of the 1981–2010 median), provided at least two of the following three parameters are also exceeded: number of tropical storms: 12, hurricanes: 6, and major hurricanes: 2.
  • Near-normal season: neither above-normal nor below normal
  • Below-normal season: An ACE value below 66 (71.4% of the 1981–2010 median), or none of the following three parameters are exceeded: number of tropical storms: 9, hurricanes: 4, and major hurricanes: 1.

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[edit]

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[edit]

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

Season ACE TS HU MH Classification
2005 Atlantic hurricane season 250 28 15 7 Above normal (hyperactive)
1995 Atlantic hurricane season 228 19 11 5 Above normal (hyperactive)
2004 Atlantic hurricane season 225 15 9 6 Above normal (hyperactive)
1950 Atlantic hurricane season 211 16 11 6 Above normal (hyperactive)
1961 Atlantic hurricane season 205 11 8 7 Above normal (hyperactive)
1955 Atlantic hurricane season 199 12 9 6 Above normal (hyperactive)
1998 Atlantic hurricane season 182 14 10 3 Above normal (hyperactive)
1999 Atlantic hurricane season 177 12 8 5 Above normal (hyperactive)
2003 Atlantic hurricane season 175 16 7 3 Above normal (hyperactive)
1964 Atlantic hurricane season 170 12 6 6 Near normal
1996 Atlantic hurricane season 166 13 9 6 Above normal (hyperactive)
2010 Atlantic hurricane season 165 19 12 5 Above normal (hyperactive)
1969 Atlantic hurricane season 158 18 12 5 Above normal (hyperactive)
1980 Atlantic hurricane season 147 11 9 2 Near normal
1966 Atlantic hurricane season 145 11 7 3 Above normal
2008 Atlantic hurricane season 144 16 8 5 Above normal
1951 Atlantic hurricane season 137 10 8 5 Above normal
1989 Atlantic hurricane season 135 11 7 2 Near normal
2012 Atlantic hurricane season 133 19 10 2 Above normal
2011 Atlantic hurricane season 126 19 7 4 Above normal
1967 Atlantic hurricane season 122 8 6 1 Near normal
1958 Atlantic hurricane season 121 10 7 5 Above normal
1963 Atlantic hurricane season 118 9 7 2 Near normal
2000 Atlantic hurricane season 116 15 8 3 Above normal
1954 Atlantic hurricane season 113 11 8 2 Near normal
2001 Atlantic hurricane season 106 15 9 4 Near normal
1953 Atlantic hurricane season 104 14 6 4 Near normal
1988 Atlantic hurricane season 103 12 5 3 Near normal
1971 Atlantic hurricane season 97 13 6 1 Near normal
1981 Atlantic hurricane season 93 12 7 3 Near normal
1979 Atlantic hurricane season 91 9 5 2 Near normal
1990 Atlantic hurricane season 91 14 8 1 Near normal
1960 Atlantic hurricane season 88 7 4 2 Near normal
1985 Atlantic hurricane season 88 11 7 3 Near normal
1952 Atlantic hurricane season 87 7 6 3 Near normal
1965 Atlantic hurricane season 84 6 4 1 Below normal
1957 Atlantic hurricane season 84 8 3 2 Near normal
1976 Atlantic hurricane season 81 10 6 2 Near normal
2006 Atlantic hurricane season 79 10 5 2 Near normal
1959 Atlantic hurricane season 77 11 7 2 Near normal
1992 Atlantic hurricane season 75 7 4 1 Below normal
1975 Atlantic hurricane season 73 9 6 3 Near normal
2007 Atlantic hurricane season 72 15 6 2 Near normal
1984 Atlantic hurricane season 71 12 5 1 Near normal
2002 Atlantic hurricane season 67 12 4 2 Near normal
1978 Atlantic hurricane season 62 12 5 2 Below normal
1974 Atlantic hurricane season 61 11 4 2 Below normal
1956 Atlantic hurricane season 54 8 4 2 Below normal
2009 Atlantic hurricane season 53 9 3 2 Below normal
1973 Atlantic hurricane season 43 8 4 1 Below normal
1997 Atlantic hurricane season 40 7 3 1 Below normal
1993 Atlantic hurricane season 39 8 4 1 Below normal
1962 Atlantic hurricane season 36 5 3 1 Below normal
1986 Atlantic hurricane season 36 6 4 0 Below normal
2013 Atlantic hurricane season 36 14 2 0 Below normal
1968 Atlantic hurricane season 35 8 4 0 Below normal
1970 Atlantic hurricane season 34 10 5 2 Below normal
1987 Atlantic hurricane season 34 7 3 1 Below normal
1991 Atlantic hurricane season 34 8 4 2 Below normal
1994 Atlantic hurricane season 32 7 3 0 Below normal
1982 Atlantic hurricane season 29 6 2 1 Below normal
1972 Atlantic hurricane season 28 7 3 0 Below normal
1977 Atlantic hurricane season 25 6 5 1 Below normal
1983 Atlantic hurricane season 17 4 3 1 Below normal
2014 Atlantic hurricane season 7 1 1 0 In progress

East Pacific ACE[edit]

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 2006 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:

  • Above-normal season: An ACE value above 135 (117% of the median), provided at least two of the following three parameters exceed the long-term average: number of tropical storms (16), hurricanes (9), and major hurricanes (4).
  • Near-normal season: neither above-normal nor below normal
  • Below-normal season: An ACE value below 86 (75% of the median)
Observed monthly values for the PDO index, 1900–present.
Historical East Pacific Seasonal Activity, 1971–2007.
Season ACE TS HU MH Classification
1992 Pacific hurricane season 290 28 16 10 Above normal
1990 Pacific hurricane season 249 21 16 6 Above normal
1978 Pacific hurricane season 207 19 14 7 Above normal
1983 Pacific hurricane season 206 21 12 8 Above normal
1993 Pacific hurricane season 201 15 11 9 Above normal
1984 Pacific hurricane season 193 21 13 7 Above normal
1985 Pacific hurricane season 192 24 13 8 Above normal
1994 Pacific hurricane season 185 20 10 5 Above normal
1991 Pacific hurricane season 178 14 10 5 Above normal
1997 Pacific hurricane season 167 19 9 7 Above normal
1982 Pacific hurricane season 161 23 12 5 Above normal
2006 Pacific hurricane season 155 19 11 6 Above normal
1971 Pacific hurricane season 139 18 12 6 Above normal
1972 Pacific hurricane season 136 14 8 4 Near normal
1998 Pacific hurricane season 134 13 9 6 Near normal
1987 Pacific hurricane season 132 20 10 4 Near normal
1988 Pacific hurricane season 127 15 7 3 Near normal
2009 Pacific hurricane season 125 20 8 5 Near normal
2002 Pacific hurricane season 124 15 8 6 Near normal
1976 Pacific hurricane season 121 15 9 5 Near normal
2011 Pacific hurricane season 118 11 10 6 Near normal
1973 Pacific hurricane season 114 12 7 3 Near normal
1975 Pacific hurricane season 112 17 9 4 Near normal
1989 Pacific hurricane season 110 17 9 4 Near normal
1986 Pacific hurricane season 107 17 9 3 Near normal
1995 Pacific hurricane season 100 10 7 3 Near normal
2012 Pacific hurricane season 98 17 10 5 Near normal
2005 Pacific hurricane season 96 15 7 2 Near normal
2000 Pacific hurricane season 95 19 6 2 Near normal
1974 Pacific hurricane season 90 18 11 3 Near normal
2001 Pacific hurricane season 90 15 8 2 Near normal
1999 Pacific hurricane season 90 9 6 2 Near normal
2008 Pacific hurricane season 83 15 7 2 Below normal
1980 Pacific hurricane season 77 14 7 3 Below normal
1981 Pacific hurricane season 72 15 8 1 Below normal
2013 Pacific hurricane season 72 20 9 1 Below normal
2004 Pacific hurricane season 71 12 6 3 Below normal
1979 Pacific hurricane season 57 10 6 4 Below normal
2003 Pacific hurricane season 56 16 7 0 Below normal
1996 Pacific hurricane season 53 9 5 2 Below normal
2007 Pacific hurricane season 52 11 4 1 Below normal
2010 Pacific hurricane season 49 8 3 2 Below normal
2014 Pacific hurricane season 40 9 3 2 Ongoing
1977 Pacific hurricane season 22 8 4 0 Below normal

See also[edit]

References[edit]