Super Tropical Cyclone Amphan in Bay of Bengal

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Low pressure area that formed on 13 May intensified into a Super Tropical Cyclone Amphan on 18 May, due to exceptionally warm waters in the Bay of Bengal.

Super Tropical Cyclone Amphan in Bay of Bengal
Date & Time
18 May 2020 05:00 UTC–20 May 10:00 UTC
Meteosat-8, Himawari-8
Infrared Channel, High Resolution Visible (HRV), Natural Colour

By Ivan Smiljanic (CGI)

Amphan was a powerful and deadly tropical cyclone that caused widespread damage in East India and Bangladesh. It was first super cyclonic storm to occur in the Bay of Bengal, since Odisha in 1999.

The first tropical cyclone of the 2020 North Indian Ocean cyclone season, Amphan originated on 13 May from a low pressure area 300 km east of Colombo, Sri Lanka. It became organised as it tracked north-eastward, due to exceptionally warm sea surface temperatures and was upgraded to tropical depression on 15 May.

On 17 May, Amphan underwent rapid intensification and within 12 hours became an extremely severe cyclonic storm. The following day, at around 12:00 UTC, Amphan reached peak intensity, with 3-minute sustained wind speeds of 240 km/h (150 mph), 1-minute sustained wind speeds of 260 km/h (160 mph), and a minimum central barometric pressure of 925 mbar. Eyewall replacement began shortly after, but dry air and wind shear disrupted the process and caused Amphan to gradually weaken near the Indian coastline.

On 20 May, between 10:00 and 11:00 UTC, the cyclone made landfall in West Bengal. At the time, the Joint Typhoon Warning Center (JTWC) estimated Amphan's 1-minute sustained winds to be 155 km/h (100 mph). Amphan rapidly weakened once inland and soon dissipated.

The Meteosat-8 blended Natural Colour RGB and HRVIS image (Figure 1, top right, click to expand) shows Tropical Cyclone Amphan just before the landfall, close to the India-Bangladesh border.

The track and the intensity prior to landfall can be seen on Figure 2. Though when at peak intensity the cyclone was Category 5, by the time it made landfall it was only a Category 2 storm. Figure 2 also shows the high sea surface temperatures (SST), peaking at more than 31 °C in the Bay of Bengal. Reduction of SST can be seen right behind the system, where the heat has been used by both the system and water.

Figure 2
Figure 2: Sea surface temperatures and track on 20 May at 06:00 UTC

The Himawari-8 infrared animation (Figure 3) provides a view over the cyclone when it was at the full strength, showing the warm core and the extent of the convection related to this system (from central India to Myanmar). The darkest red shades indicate the cloud-top temperatures lower than -90 °C.

Figure 3: Himawari-8 infrared (IR104) animation, 18 May 05:00–19:00 UTC

The Himawari-8 visible animation (Figure 4) shows the eyewall structure for the same period, at a resolution of 500 m.

Figure 4: Himawari-8 visible animation, 18 May 05:00–10:30 UTC

The Meteosat-8 infrared animation (Figure 5) captured the moment of landfall. During this stage the tropical cyclone weakened, mostly due to a high vertical shear, having dense overcast in the centre of the system. However, the extent of the system was still very broad, showing associated cloud formations from southern India to Tibet, and even further north.

Figure 5: Meteosat-8 infrared animation, 20 May 00:00–10:00 UTC

The Bay of Bengal is an interesting region for comparison between Meteosat-8 and Himawari-8 satellite data, being central to position of both satellites over equator (roughly 91 °E). Figures 6–11 show comparison of visible ( 1km v 500 m resolution) and infrared (3 km v 2 km resolution) channels from SEVIRI and AHI instruments, respectively.

From the visible imagery comparison the parallax shift between two is apparent (Figure 7 provides the zoomed-in view at the centre of the system). This shift is also visible through infrared imagery comparison, but isn't as easily traced due to reduced resolution. Meteosat-8 visible imagery is darker at 10:00 UTC (sunset in the region) due to sun-glint affecting Himawari-8.

It is worth noting through the comparison of the infrared images that the both instruments show similar brightness temperatures, but the AHI instrument reveals areas of colder cloud tops, due to advanced resolution. Figures 8 and 11 clearly show the shape and the size of individual pixels in visible and infrared spectral regions.

Image comparison
Met-8 HRV Himawari-8 VIS064
Figure 6: Comparison of Meteosat-8 and Himawari-8 visible images on 20 May at 10:00 UTC.
Image comparison
Met-8 HRV Himawari-8 VIS064
Figure 7: Comparison of zoomed in Meteosat-8 and Himawari-8 visible images on 20 May at 10:00 UTC.

Figure 8a: Met-8 1 km HRV, pixel size 1350x1350 m at 21.2N and 90.9 E
Figure 8b: Himawari-8 500m VIS0.64, pixel size 700x700 m at 21.2N and 90.9 E
Image comparison
Met-8 IR108 Himawari-8 IR1084
Figure 9: Comparison of enhanced Meteosat-8 and Himawari-8 infrared images on 20 May at 10:00 UTC.
Image comparison
Met-8 IR108 Himawari-8 IR108
Figure 10: Comparison of enhanced, zoomed in Meteosat-8 and Himawari-8 infrared images on 20 May at 10:00 UTC.

Figure 11a: Met-8 3 km IR108, pixel size 4050x4050 m at 21.2N and 90.9 E
Figure 11b: Himawari-8 2 km IR104, pixel size 2800x2800 m at 21.2N and 90.9 E

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