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TROPICAL CYCLONE INTENSITY ANALYSIS.— For over 25 years meteorologists have studied satellite imagery of tropical cyclones. An outgrowth of studies correlating cloud patterns with tropical cyclone intensity levels was the development of an intensity analysis technique. Today, fairly accurate estimates are possible based on cloud patterns seen in imagery. The technique currently in use was developed by Mr. Vernon Dvorak. The Dvorak technique is based on cloud feature measurements, models, and cloud pattern changes. The technique is applicable to all types of imagery (visual, IR, and enhanced IR [EIR]). The procedures and rules for each type of imagery are included in their entirety in appendix 2. The following terminology is associated with satellite cloud pattern analysis and the Dvorak technique. Banding features (BF). The amount of coiling (banding) in the cloud system.

Central cold cover (CCC). A circular, cold overcast near the storm center that develops as the curved band cloud pattern dissipates. ‘This feature is only distinguished using enhanced IR imagery. It can occur at any stage of storm development and may last for several hours or several days. When it persists, it signifies the end of tropical cyclone development.

Central dense overcast (CDO). The dense overcast mass of clouds covering the most tightly curved cloud bands. It may cover the lower, curved cloud features or surround the eye of a well-developed tropical cyclone. This pattern pertains to visual imagery only.

Central features (CF). The character of the cloud mass associated with the cloud system center and the overcast around the center. A central dense overcast and eye are examples of central features.

Cloud features. Those aspects of the cloud system used in intensity analysis; for example, central features, central dense overcast, banding, etc.

Cloud system center (CSC). The focal point of all curved cloud lines or bands of the tropical cloud system, or put another way, the point toward which all cloud lines or bands merge. T-number (tropical numbers). The classifi-cation system used to denote the intensity of tropical cyclones. A T1 is representative of minimal tropical cyclone intensity, while a T8 represents maximum possible intensity.

Figure 10-3-12.—Procedures for T-number determination.

PROCEDURES AND RULES.— The analysis procedure consists of two major parts. In part 1, intensity estimates are obtained by measuring cloud features seen in imagery. In part 2 the cloud patterns seen in imagery are compared to model cloud patterns. Part 2 is only necessary when cloud measurements aren’t possible and/or to limit the intensity level (T-number) obtained from the measurements made in Part 1. Analysis pro-cedures are summarized in figure 10-3-12.

The procedure consists of 10 steps in which you determine the intensity of a tropical cyclone by first locating the CSC (step 1) and then analyz-ing the center’s appearance and its relationship to the dense (cold) clouds of the pattern. The CSC is located at the center of curvature of a partial wall cloud in developing cyclones, and at the center of the eye in mature cyclones. If the CSC is not obvious, a model cloud pattern is selected that compares to the one seen in imagery, and the model’s expected CSC is used.

The first intensity estimate is made in step 2 by measuring the cloud features related to intensity. This is done when the cloud pattern contains cloud features similar to those in the cloud pattern descriptions listed in steps 2A to 2E. Since these patterns are seen in IR imagery, as well as visual imagery, you should know the importance of IR pictures in tropical cyclone analysis and the major differences with regard to intensity analysis. Infrared pictures permit tropical cyclone monitoring day and night. In fact, the day-by-day changes in the cloud patterns are often followed in IR pictures with much the same quality as they are in visual pictures. Visual and IR images of tropical cyclones are very similar; however, there are two major differences. First, thin cirrus clouds, which are normally transparent in visual imagery, are often opaque in IR, thereby obscuring the view of lower cloud features. When the cirrus obscures the boundaries of lower cloud features, no intensity measurements are possible or erroneous ones are taken. This means the CDO size and embedded eye distance measurements cannot be interpreted in unenhanced IR pictures.

Second, the low cloud lines used to pinpoint the storm center may not be visible because of the cirrus. With these differences in mind, let us now look at the cloud band pattern types seen in imagery and the type of imagery used to obtain the intensity estimate. Refer to figure 10-3-13.

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