Two Important Features of the 156 Deadliest Tropical Cyclones (1851-1996) in the North Atlantic

Damages and deaths due to a tropical cyclone (TC) can easily occur at three places belong to a TC: the small ring area (SRA) of maximum sustaining wind around TC eye, the large ring area (LRA) of the rotating wind field around SRA and the skin air belt (SKAB) around LRA. However, weather forecasters only use the intensity of the SRA of a TC as the proxy to gauge the whole TC intensity, which led to many “non-major” TCs proved to be “deadly” TCs. This is called as “over-generalization problem” (OGP). Here we introduce a new way to measure the intensity of a TC. After investigating 156 deadly TCs in North Atlantic, we found an important feature: 83.3% of the top 156 deadliest TCs usually made landfalls along “five major tracks”. Furthermore, we found that the new intensity of those “deadliest” TCs kept almost unchanged with the increasing of time intervals at earlier stage from the genesis points, whereas the new intensity of those “safe” TCs increased in a line with much steeper slope. Using these two features, weather forecasters can have two more options to identify those “non-major” and “deadly” TCs by SSHS in future.


Introduction
Pioneer researchers considered that the pathway patterns of tropical cyclones (TCs) are very important information in the study of the TC formation mechanism [1]- [11]. Specifically, the formation of TCs can be better understood by us-DOI: 10.4236/acs.2019.91005 76 Atmospheric and Climate Sciences ing a track classification method to survey historical TCs [12] [13] [14]. Despite many years of effort by scientists [15]- [22], a theory capable of completely explaining the physical mechanism of TC formation has not yet emerged. As such, it is very important to find out the deficiency of present TC intensity evaluating system.
In the present TC intensity evaluating systems in the world, the maximum sustaining winds are the proxy to measure TC intensity, and the Saffir-Simpson Hurricane Scale (SSHS) is the leading TC intensity method. In SSHS, a TC below category 3 is called a "non-major" TC, which means that the TC is a "weak" TC on their intensity, and a TC above category 3 is called a "major" TC, which means that the TC is a "strong" TC. Because only about 10% of TCs were "deadly" TCs, common people often choose to stay home or to leave for a safe place depending on "weak" or "strong" intensity of TCs. Generally, the information of "weak" or "strong" intensity for a TC is according to the information of "non-major" and "major" TCs in TC warnings by weather forecasters. However, this usual policy was not always reliable. For example, Hurricane Gordon, which occurred from November 8 to 21, 1994, was classified as a "non-major" TC according to SSHS, but Hurricane Gordon was the 16th deadliest TC over North Atlantic the period from 1851 to 1996. Some scholars had criticized the SSHS as being too simplistic because of two substantial limitations: its duration problem and its failure to take the physical size of TCs into accounts [23] [24] [25]. In our opinion, the bad performance of TC warnings by weather forecasters was not due to the deficiency of SSHS, but due to that there is an over-generalization problem (OGP) existing in the present TC intensity evaluating systems.
In meteorology, the coverage area of a weather system is also one of the quantities to gauge the intensity of a weather system. In this paper, we collectively call all the three areas as "most direct impact area" (MDIA), the calculating scheme can be seen in the previous literature [ where C is 0.01745329, R is the radius of the earth, and φ is the latitude.
The derivation of (1) is as follows:

The Method to Find the Five Groups of TCs
We use two cross lines X and line Y to divide the North Atlantic Ocean basin.
The equations for X line is in the formula (4) and (5) 0.4172 59.298 Due to the mathematical relation that the product of the slope of the X axis and the slop of the Y axis must equal −1, we can obtain the slope and equation for the Y axis with the following formula: where x is degrees of longitude and y is degrees of latitude.

Results
In Table 1, the first column lists the 0 -5 categories according to SSHS; the second column lists the number of TCs which caused deaths; the third column lists the toll of deaths caused by TCs belong to this category. We can see that the maximum number of TCs causing deaths is category 3, there were 46 deadly TCs during the periodic from 1851 to 1996, which caused 18,340 people to die; then the number order from the second maximum number to minimum number is: category 4, category 2, category 1, category 5 and category 0, respectively, their death numbers are: 45,176, 14,614, 7896, 7110 and 1092. Summing the numbers of deaths belong to category 2, 1 and 0, we obtained 23,602 deaths by "non-major" TCs according to SSHS; Summing the numbers of TCs belong to category 2, 1 and 0, we obtained 55 "non-major" TCs according to SSHS, which is about "35.2%" of 156 deadliest TCs during the period from 1851 to 1996. The reason for this phenomenon is due to that damages and deaths due to a tropical cyclone (TC) can easily occur at three places belong to a TC: the small ring area (SRA) of maximum sustaining wind around TC eye, the large ring area (LRA) of the rotating wind field around SRA and the skin air belt (SKAB) around LRA (see Figure 1). However, in SSHS, the intensity of the SRA of a TC is used to gauge the whole TC intensity, which led to that many "non-major" TCs proved to be "deadly" TCs. This is called as "over-generalization problem" (OGP).
In Figure 2   3) The skin air belt refers to those air belts around LRA. This area is special place due to it receiving kinetic energy when a TC moves. In Figure 3, we illustrate the geographic distribution of the instant MDIA of five groups of TCs and we also give a pathway line of a typical cases' for each of five groups of TCs.
In Figure 4, it shows the average values of instant MDIA with time for both "safer" 1282 TCs and 156 deadliest TCs according to the order numbers of different time from genesis points to dissipation points. We could see that the slope of the instant MDIA curve line for the 1282 "safer" TCs is steeper than that of 156 deadly TCs, while the curve line for the 156 top deadliest TCs keeps almost unchanged if the lifespan of a TC is shorter than 10 days; it increases upward to a peak and decrease downward to a valley for four times during the lifespan from 11 days to 18 days, but the amplitude is much less than that in the curve line for the 1282 "safer" TCs.

Discussion
Due to only the intensity of SRA of a TC is the proxy to measure TC intensity in SSHS used by weather forecasters to submit TC warnings, and due to the intensities in LRA and SKAB remain undefined in the TC evaluating systems in the world; the OGP problem would occur to leads to that many "non-major" TCs proved to be deadly TCs. Table 1 shows the bad performance of the present TC intensity evaluating systems over North Atlantic. 23,602 deaths had been caused by "non-major" deadly TCs in SSHS, which supports our view on OGP in present TC intensity evaluating system in the world. Other scholars had blamed the defects of SSHS as the reason leading to the bad performance, but we consider that the OGP in present TC intensity evaluating is the real reason.  One of the direct ways to overcome OGP is to establish two TC intensity evaluating sub-systems: the sub-system to evaluate the TC intensity of LRA and the

Conclusion
In this paper, we point out that there is an OGP existing in present TC intensity evaluation system; a statistic toll for 156 deadly TCs and new terms of SRA, LRA and SKAB has been used to explain that the view is correct. In order to overcome OGP, we have developed a new track classification method to identify the five kinds of deadly TC tracks, furthermore, an important feature of deadly TCs has been obtained. At last, we suggest two measures to compensate for the OGP.