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A Statistical Analysis of Tropical Cyclone Intensity

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  • 1 Program in Atmospheres, Oceans and Climate, Massachusetts Institute of Technology, Cambridge, Massachusetts
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Abstract

Cumulative distribution functions (CDFs) of tropical cyclone wind speeds are calculated using best track data from the North Atlantic and western North Pacific basins. Wind speeds are normalized by theoretical potential wind speeds derived from reanalysis datasets, and the individual storms are classified according to whether their maximum intensities were limited by landfall, passage over cold water, or other factors. For each classification, CDFs were calculated and the evolution of the storm wind speed was composited relative to the time at which each storm achieved its lifetime maximum wind speed.

For storms of hurricane strength whose maximum intensity is not limited by declining potential intensity (landfall or passage over cold water), the normalized CDFs of storm lifetime maximum wind speed are nearly linear, in contrast to the lognormal distributions found with many other geophysical phenomena, such as earthquakes. Thus there is a roughly equal likelihood that any given tropical cyclone of hurricane strength will achieve any given intensity, up to but not beyond its potential intensity. Tropical cyclones of tropical storm strength also have linear CDFs, but their slope is distinctly greater, indicating a greater likelihood of finding storms with wind speeds below hurricane strength. There is a nearly equal probability of finding any individual storm at a normalized intensity of any given fraction of its maximum normalized intensity. Combining this with the CDFs of the storm lifetime maximum wind speed shows that, up to the time a storm reaches its lifetime maximum intensity, the probability of encountering hurricane-strength maximum normalized winds in excess of υ is given by
PP0υυυ
where P0 varies with location and season.

For storms whose maximum intensity is not limited by declining potential intensity, the evolution of storm intensity is remarkably similar in the Atlantic and western North Pacific basins, with average intensification and decay rates of around 12 m s−1 day−1 and 8 m s−1 day−1, respectively. The average hurricane-strength storm in both basins reaches a sharp peak in intensity followed by a decline at a rate roughly two-thirds that of its prior intensification, a behavior distinctly different from that of axisymmetric numerical models. Moreover, this class of storms achieves almost the same intensity in the Atlantic and western North Pacific regions, while storms whose maximum intensity is limited by declining potential intensity are significantly more intense in the Pacific region, showing that the main reason for the greater intensity of western North Pacific tropical cyclones is the greater length of the average storm track over warm water. Other results from this study include the finding that average rates of decline of tropical cyclone intensity over warm and cold water are very similar and are about half the average rate of decline of landfalling storm intensity.

Corresponding author address: Dr. Kerry Emanuel, Room 54-1620, Massachusetts Institute of Technology, Cambridge, MA 02139.

Email: emanuel@texmex.mit.edu

Abstract

Cumulative distribution functions (CDFs) of tropical cyclone wind speeds are calculated using best track data from the North Atlantic and western North Pacific basins. Wind speeds are normalized by theoretical potential wind speeds derived from reanalysis datasets, and the individual storms are classified according to whether their maximum intensities were limited by landfall, passage over cold water, or other factors. For each classification, CDFs were calculated and the evolution of the storm wind speed was composited relative to the time at which each storm achieved its lifetime maximum wind speed.

For storms of hurricane strength whose maximum intensity is not limited by declining potential intensity (landfall or passage over cold water), the normalized CDFs of storm lifetime maximum wind speed are nearly linear, in contrast to the lognormal distributions found with many other geophysical phenomena, such as earthquakes. Thus there is a roughly equal likelihood that any given tropical cyclone of hurricane strength will achieve any given intensity, up to but not beyond its potential intensity. Tropical cyclones of tropical storm strength also have linear CDFs, but their slope is distinctly greater, indicating a greater likelihood of finding storms with wind speeds below hurricane strength. There is a nearly equal probability of finding any individual storm at a normalized intensity of any given fraction of its maximum normalized intensity. Combining this with the CDFs of the storm lifetime maximum wind speed shows that, up to the time a storm reaches its lifetime maximum intensity, the probability of encountering hurricane-strength maximum normalized winds in excess of υ is given by
PP0υυυ
where P0 varies with location and season.

For storms whose maximum intensity is not limited by declining potential intensity, the evolution of storm intensity is remarkably similar in the Atlantic and western North Pacific basins, with average intensification and decay rates of around 12 m s−1 day−1 and 8 m s−1 day−1, respectively. The average hurricane-strength storm in both basins reaches a sharp peak in intensity followed by a decline at a rate roughly two-thirds that of its prior intensification, a behavior distinctly different from that of axisymmetric numerical models. Moreover, this class of storms achieves almost the same intensity in the Atlantic and western North Pacific regions, while storms whose maximum intensity is limited by declining potential intensity are significantly more intense in the Pacific region, showing that the main reason for the greater intensity of western North Pacific tropical cyclones is the greater length of the average storm track over warm water. Other results from this study include the finding that average rates of decline of tropical cyclone intensity over warm and cold water are very similar and are about half the average rate of decline of landfalling storm intensity.

Corresponding author address: Dr. Kerry Emanuel, Room 54-1620, Massachusetts Institute of Technology, Cambridge, MA 02139.

Email: emanuel@texmex.mit.edu

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