Active Indian Ocean tropical cyclone season
24 September, 24 October 2019–8 November
2019 saw a lot of cyclonic activity in the northern Indian Ocean, with five systems being generated.
10, November 2020
By Hilal Al-Hajri (Muscat, Oman) and Ivan Smiljanic (SCISYS)
Two of the cyclones were pre-monsoon (Fani in April and Vayu in June) and three were post-monsoon.
The first post-monsoon tropical cyclone was Hikka in September, which was a very fast system — from cyclogenesis to landfall and associated decay took only two days. Hikka made landfall in Oman on 24 September, bringing high winds but not causing any major damage.
The next two systems were Kyarr and Maha.
Kyarr's cyclogenesis took place on 24 October and it became the first super cyclonic storm in the North Indian Ocean since Gonu in 2007. It was also the strongest tropical cyclone ever recorded in the Arabian Sea, and the second-most intense tropical cyclone in North Indian Ocean history. It decayed over the sea. In parts of India the high winds and heavy rainfall caused some flooding and damage to trees and powerlines. Due to the high winds coinciding with high tides some part of Pakistan, Oman and the United Arab Emirates were also flooded.
The storm started as a depression off the Indian coast on 24 October. The Meteosat-8 Convection RGB shows that there was a severe thunderstorm developing around the system. The yellow colour indicates the small ice particles associated with the strong updraft.
The system started to move north west and intensified to a tropical storm. It was named Kyarr at 18:00 UTC on 24 October. Metop-A was at the edge of the system at the time and the ASCAT winds product (Figure 2) gave an indication that the wind speeds around the centre were more than 55 km/h.
By 27 October Kyarr had intensified to a Category 4 cyclone, with an eye that continued to grow to almost 50 km diameter. The Metop-A ASCAT wind product (Figure 3) showed the speeds in the middle of the cyclone. It also showed that the outflow cloud band extended across northern India.
The Meteosat-8 sandwich product (Figure 4) showed the severity of the thunderstorm in the cyclone wall and the eye's diameter.
Comparing the infrared image from Meteosat-11 and Meteosat-8, there is a shift between them in the location of the eye of Kyarr, where Meteosat-8 is shifted to the west more than Meteosat-11. This can be clearly seen on Figure 5, which has the ASCAT surface wind field overlaid on both images.
Note: In the imagery Meteosat-11 is positioned further east of the cyclone's eye, than Meteosat-8, due to a higher scanning angle, i.e. higher parallax. However, it better overlaps with the central position of the cyclone suggested by the ASCAT wind pattern. That would mean that the ASCAT scan was also shifted from the true position of the system at 12:00 UTC, towards the east, due to the fact that the scan was performed many hours earlier. The passage of the Metop satellite in this region is around 07:00 UTC.
The IR10.8 image from Meteosat-8 (Figure 6) detected the centre of the cyclone at the same time as Metop passed overhead, thus the ASCAT wind product showed the same position as the Meteosat-8 image.
The system moved northwest and started to decay, due to advection of strong wind shear and dry air intrusion. The Metosat-8 Natural Colour RGB (Figure 7) shows the system started to become smaller and the eye became less visible, although it was still a Category 4 storm.
The red colour in the Airmass RGB (Figure 8) indicates the dry airmass intrusion into the system and the green shades indicate the humid airmass. The dark area on the water vapour channel (Figure 9) also indicates the dry intrusion.
On 30 October, as Kyarr was decaying, another system started to develop as a deep depression, near the Indian subcontinent (Figure 10).
The sea surface temperature (SST) for the Indian Ocean was very high, reaching 28 °C, favourable for cyclogenesis. Figure 11 shows the SST from the Terra satellite, where cooling of the surface temperature caused by Kyarr's passage, can be seen. The Oman south coast had lower temperatures due to the south west monsoon wind and the accompanied upwelling process, when compared to the rest of the west Indian Ocean.
On 1 November satellite images showed that the new system was intensifying to a tropical storm, when it was named Maha. Interestingly, this system followed the same direction as the dissipating cyclone Kyarr (northwest). The Metop-A ASCAT winds at 00:00 UTC (Figure 12) captured them both, showing wind speeds of more than 64 km/h for Maha and 51 km/h for Kyarr.
On 3 November at 00:00 UTC Kyarr dissipated over the sea and Maha was officially classified as a Category 1 tropical cyclone. The Meteosat-8 Airmass RGB (Figure 13) showed the humidity which helped Maha intensify.
Maha reached Category 3 for a short time, before starting to decay. The Natural Colour RGB on 6 October at 06:00 UTC (Figure 14) showed most of the clouds were to the east of the system, where the centre of the system was located at 19.75N 63.8E. At 12:00 UTC on the same day, the system started to drift north east (Figure 15).
The system continued to decay and dissipated over the sea on 8 November (Figure 16). It brought moderate rain to Gujarat (on the Indian coast).
The centre of the system can be better seen using the meteosat-8 High Resolution Visible (HRV) RGB (Figure 17), than using the infrared channels, since the visible (especially HRV) channel is more useful for cloud detection during the day (provided the clouds are not too thin).
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