Super Tropical Cyclone Amphan in Bay of Bengal

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 300km 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 240km/h (150mph), 1-minute sustained wind speeds of 260km/h (160mph), 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 155km/h (100mph). Amphan rapidly weakened once inland and soon dissipated.

The Meteosat-8 blended Natural Colour RGB and HRVIS image (Figure 1) 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.

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.

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

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.

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 500m resolution) and infrared (3km v 2km 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 sunglint 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.