Analysis of future tropical cyclone characteristics in the Bay of Bengal
3.5 Characteristics of cyclone parameter .1 Central pressure
One of the most important characteristics of a cyclone is central pressure. To find the intensification, wind speed, and the category of a cyclone central pressure makes an important role. Thus, I have investigated the mean, standard deviation, maximum and minimum central pressure of a cyclone in each region of Bangladesh coast. The central pressure of a cyclone in different climate scenarios is also investigated. The strike cyclone of each coastal zone in Bangladesh has different characteristics of central pressure. The detailed central pressure behavior
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of low-intense and intense cyclone in different scenarios for every part of the coastal zone is given in Fig. 3.8.
Fig. 3.8 Central pressure behavior of different coastal zone of Bangladesh
From this figure, it is evident that the central pressure for low-intense cyclone is stable but for the intense cyclone, it is unstable. West part cyclone is more unstable than east part. The mean central pressure of each scenario for east is lower than the middle and west part. I have found that the western parts of the Bangladesh coast will more venerable due to the intense cyclone in future. But, most of the dangerous and catastrophic cyclone will strike on the eastern coast of Bangladesh.
Because, the cyclone of lowest central pressures are found in this region more than the western part. So, it is important to concentrate the development of the coastal
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structure and disaster prevention policy along this area. Though this part is very important for future disaster-prevention plan due to the development of the Bangladesh economy and economic zone, the present development did not meet the expectation. Moreover, there is no proper investigation of future disaster risk along this region.
The central pressure of the strike cyclone in each part of the coastal area of Bangladesh shows different characteristics under present and future climate condition. In the scenario of CC, GF, HA, MI, MP and MR, the central pressure of a cyclone is unstable for intense cyclone but the low-intense cyclone represents the stable changes due to climate change. The mean and standard deviation uncertainty of cyclone central pressure for present and future scenario is shown in the figure (see Fig.3.8). From this study, it is evident that the eastern part of Bangladesh coast faces dangerous cyclone but most of the cyclone strike western part of Bangladesh.
3.5.2 Landfall angle
Landfall angle is an important factor for disaster risk analysis. The landfall angle may have an influence in the surge height along this region.
Fig.3.9 Schematic figure of before and after landfall point
Due to the climate change impact, the cyclone landfall behavior also changed. For the different climate scenario, I have observed the characteristics of cyclone landfall angle. The overall characteristics of landfall angle depend on the coastal structure. Landfall angle was calculated from the study of azimuth distance. To find the landfall angle, the consecutive two points of a cyclone eye are needed,
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which are in sea and land. Then the following relation is used to calculate the landfall angle.
2 1 2 1
Land fallangle ( ) a tan(x x , y y ) (3.3) where, landfall angle, (x , y )1 1 latitude and longitude before landfall,
2 2
(x , y ) latitude and longitude after landfall.
The polar histograms of landfall angle are calculated for both the low-intense and intense cyclone. For the different climate scenario, I have also observed the characteristics of cyclone landfall angle.
Fig.3.10 Polar histogram of landfall angle for low-intense cyclone
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Fig.3.11 Polar histogram of landfall angle for intense cyclone
Fig.3.10 and Fig.3.11 show the polar histograms of low-intense and intense cyclone landfall angle. In this study, it is found that the landfall angle would be shifted northeast to north-west direction in the future climate scenario along the coast of Bangladesh. The landfall angle of low-intense cyclone has some uncertainty in the west part of Bangladesh than intense cyclone. It is found that the landfall angle is same or stable for the intense cyclone. The polar histogram gives the detail information about the landfall angle in different parts of Bangladesh coast in different climate scenario for present climate and the future CC scenario.
3.5.3 Maximum sustained wind radius
Maximum sustained wind radius is an important parameter of a tropical cyclone.
This parameter depends upon the central pressure and wind velocity. To explain the tropical cyclone, the tropical cyclone is considered a function of some parameters like wind speed, central pressure, maximum sustained wind radius etc.
The maximum sustained wind radius has great impact on surge height. Due to the global warming, the maximum sustained wind radius of a cyclone may change. To understand its nature in the Bangladesh coast, I have investigated the cyclone information for the six different climate scenarios. From the study of Hsu and Yan,
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(1998) the mean Rmax was 47 km for the central pressures of 909–993 hPa in 1893– 1979. Fujii (1998) found that the typhoon with central pressure ≤ 980 hPa that hit the Japan main islands has the Rmax of 84–98 km. The maximum sustained wind radius Rmax is depending on the cyclone track and the characteristics of each cyclone. Therefore, some scientist proposed several estimation models for Rmax (see, Kossin et al. (2007); Quiring et al. (2011)). However, the maximum wind velocities are differently defined depending on different ocean basin where the tropical cyclone transits. Different countries classified the sustained wind in different consideration. For instance, 10 min maximum sustained wind speed was considered for Japan Meteorological Agency (JMA) and 1 min for United States National Weather Service (NWS). Some well-defined empirical relationship developed by some scholars (see, e.g., Kawai et al., 2005, Takagi et al., 2012;
Nakajo et al., 2014;). In this study, I have calibrated the cyclone track data under present and future climate condition to find the appropriate empirical relation. The modified empirical relation from Kawai et al., 2005 is
1 0.644 max c
P 1013 V 13
(3.4)
Pc 967 61.5
Rmax 94.89*exp
(3.5) where, Pc=Central pressure (hPa); Vmax= maximum velocity (m/s); Rmax= maximum sustained wind radius (km).
Fig.3.12 shows the mean and standard deviation of maximum sustained wind radius for the different part of Bangladesh coast. In the figure (a) and (b) represent the mean value for intense and low-intense cyclone and the figure (c) and (d) represent standard deviation. From this analysis, it found that the mean radius range is 62-64 km for the cyclone of Bangladesh.
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Fig.3.12 Mean and standard deviation of maximum sustained wind radius
This study found that the maximum sustained wind radius decreases in the east coast and increases in the middle and west coast in future. From the wind-pressure relation, it is evident that the intense cyclone activity will increase in the east coast.
It also found that the fluctuation of maximum sustained wind radius is very high along the east coast of Bangladesh. I have used this information to find the dangerous cyclone for surge simulation in the chapter 6.
3.5.4 Translation speed
Translation speed of a tropical cyclone represents the movement speed of a cyclone. This study investigated the movement speed of a cyclone, which strike in the coast of Bangladesh. Due to the climate change, there is an impact on cyclone translation speed. Cyclone translation speed has a great impact on storm surge.
Therefore, for the future dangerous cyclone risk analysis, it is important to find the behavior of the cyclone movement speed.
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Fig.3.13 Mean and standard deviation of translation speed
To find the cyclone translation speed, I have used the general definition of speed.
At first, find the distance of consecutive two points (before landfall and after landfall). After that, the distance is dividing by the passing time. Basically, this movement speed is the speed of crossing the coast. For each part of Bangladesh coast, I have found the movement speed of individual cyclone. I have found that the climate change and the coastal location have some impact on movement speed.
Different climate scenario shows different translation speed. Fig. 3.13 shows the mean and standard deviation of translation speed. Figure (a) and (b) show the mean translation speed for the low-intense and intense cyclone. Figure (c) and (d) show the standard deviation of translation speed for the low-intense and intense cyclone.
From this analysis, it was found that the fluctuation (standard deviation) of translation speed is zero for the HA scenario at the middle part. The mean translation speed is 15-20 km/h for the low-intense cyclone but 20-25km/h for intense cyclone.
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