SECTION 3. TROPICAL CYCLONE INTENSITY TERMINOLOGY

Although the World Meteorological Organization provides general TC intensity terminology (Huschke, 1959), individual forecast agencies may have their own terminology. This section describes terminology used by the U.S. Department of Defense (DOD) and the U.S. Department of Commerce (DOC).

The DOD classification for TCs is based on the one-minute average mean wind speed. Most other forecast agencies around the world use a ten-minute average mean. This difference in intensity results in confusion between intensity forecasts among the warning agencies. Table A-1 shows a comparison of the one-minute and ten-minute mean winds for a given sea level pressure. The following are the five intensity classes used by DOD.

(1). Tropical Disturbance.
A discrete system of loosely organized convection generally 100 to 300 nm in diameter, originating in the tropics or subtropics, having a non-frontal, migratory character and having maintained its identity for 12 to 24 hours. It may or may not be associated with a detectable wave perturbation of the low-level wind or pressure field.

(2). Tropical Depression.
A TC that may have one or more closed isobars and maximum 1-minute mean sustained surface wind speeds of 33 kt or less.

(3). Tropical Storm.
A TC with maximum 1-minute mean sustained surface wind speeds in the range of 34 to 63 kt inclusive.

(4). Typhoon or Hurricane.
A TC with maximum sustained 1-minute mean surface wind speeds of at least 64 kt. West of 180 degrees longitude they are called typhoons and east of 180 degrees longitude they are called hurricanes.

(5). Super-Typhoon.
A typhoon with maximum sustained 1-minute mean surface wind speeds of 130 kt or greater.

The DOC/NOAA/NHC uses the same intensity classification as DOD does except that there is no super-hurricane class. In addition, the NHC has adopted the Saffir-Simpson Hurricane Scale for Atlantic hurricane intensity (Table 6.4). This scale consists of five categories (weak, moderate, strong, very strong and devastating) indicating damage due to high winds and storm surge. The primary purpose of this scale is to provide hurricane forecasters with the vocabulary to communicate estimates of damage potential to their customers.

The Saffir-Simpson Hurricane Scale is an application of the Fujita Tornado Scale or F-scale (Fujita, 1971, NHOP, 1994) which estimates tornado wind speeds from damage reports. As there are major statistical differences between Atlantic hurricanes, Pacific hurricanes, and Pacific typhoons, (Table 6.4) should be used only for Atlantic hurricanes.

Intensity trend is defined as the change of intensity with time. The terms "intensifying" and "deepening" refer to positive intensity trends while the terms "decaying" and "filling" are used for negative intensity trends.

Tropical cyclones that rapidly deepen (or rapidly intensify) and are not forecast to do so pose a serious threat to operational commanders. Although the concept of rapid deepening is clear, the definition, as shown in (Table 6.5), is rather arbitrary. Globally, about 15% of all TCs experience a period of rapid intensification. When applied to an individual year in the western Pacific, rapid intensification has been as high as 37%. When applied to the South China Sea, this statistic is observed to be as low as 1%.

Tropical cyclones that dissipate over water exhibit a large range of dissipation rates. Storms that weaken slowly usually are affected by moderate vertical wind shear or are moving over slightly lower sea surface temperatures (SSTs). These storms can take three to five days to weaken. Tropical cyclones that decay rapidly over water are either affected by strong vertical wind shear or are moving over much lower SSTs. These storms take one to three days to dissipate. Tropical cyclones that move over land decay rapidly due to the loss of the warm moist maritime heat source. These storms typically dissipate in one to two days, although the strongest winds in the core can weaken significantly in less than one day.

The Dvorak technique (Dvorak, 1973) has been widely adopted for describing satellite-based TC intensity trends. This technique provides the experienced analyst with a procedure in which the interpretation of satellite imagery is combined with a model of TC development. The model describes cyclone development and dissipation as a day-to-day progression through recognizable combinations of cloud characteristics. See Appendix B, for an expanded discussion of this technique.

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