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 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. [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. 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 kt2, and for use as an index the unit is assumed. Thus:
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. [1]
Climatology
Measured over the period 1951–2005 for the Atlantic basin:
A season's ACE is used to categorize the hurricane season by its activity. NOAA categorisation system[3] 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, 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.
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 1950–2009 by ACE index
The term hyperactive is used by Goldenberg et al. (2001) [4][5] based on a different weighting algorithm[6] which places more weight on major hurricanes, but typically equating to an ACE of about 153 (171% of the current median).
Key • ACE Accumulated cyclone energy • TS Number of tropical storms • HR Number of hurricanes (S-S Category 1 – 5) • MH Number of major hurricanes (Category 3 – 5) (Fields with record values are in bold)
For definitions of the terms "above", "near", and "below" normal, see the Climatology section above.
Mean 1950 – 2005: 102.3
Median 1950 – 2005: 89.5
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 2006 season, the most recent near-normal season is the 2002 season, and the most recent below normal season is the 2007 season. [7] The 35 year median 1971–2005 is 115 x 104kt2 (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)
Median = 115
Calculations from Eastern North Pacific Tracks File at NHC[1]
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.
See also
References
- ^ a b 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
- ^ Last advisory for T.S. Zeta 2005
- ^ Climate Prediction Center – Background Information: The North Atlantic Hurricane Season
- ^ The Recent Increase in Atlantic Hurricane Activity: Causes and Implications
- ^ Supplementary material. The Recent Increase in Atlantic Hurricane Activity: Causes and Implications
- ^ Summary of 2000 Atlantic tropical cyclone activity and verification of authors’ seasonal activity prediction.
- ^ East North Pacific ACE (through 30 Nov. 2005)
External links
- NOAA ACE by year from 1851
- National Climatic Data Center - Atlantic Basin 2004 Accumulated Cyclone Energy (ACE) Index
- National Climatic Data Center — Atlantic Basin 2005 Accumulated Cyclone Energy (ACE) Index
- 2004 Pacific NW Typhoon Season ACE pdf
- Global Tropical Cyclone Best Track Database
- Hurricane Metrics