Looking at the lifecycle of tropical cyclones Gulab and Shaheen between Bay of Bengal and north Oman in September/October 2021.
21 February 2023
04 October 2021
By HansPeter Roesli (Switzerland), Ivan Smiljanic (CGI), Jochen Kerkmann (Germany) and Hilal Al Hajri, Zamzam Alrawahi and Ibrahim Al Abdusalam (PACA)
Gulab and Shaheen were two related, consecutive tropical cyclones that caused considerable damage between September and October. Gulab developed over the Gulf of Bengal and impacted eastern India. Shaheen re-developed over the Arabian Sea and devastated coastal areas of Pakistan, Iran and north Oman, before making landfall.
The full lifecycle of the two tropical cyclones can be followed in a sequence of hourly Airmass RGBs from Meteosat-8, between 24 September and 5 October (Figure 1).
Between 24 and 25 September, a low-pressure area over the Bay of Bengal quickly organised into a tropical storm and was assigned the name Gulab. On 26 September, Gulab made landfall in northern Andhra Pradesh, India. Weakening overland, it degenerated into a relic low on 28 September, but continued moving north-west to the Gulf of Kutch in India's state of Gujarat.
On 29 September it moved over the Arabian Sea, where it re-generated into a depression early on 30 September, and re-strengthened again into another tropical storm, this time named Shaheen. This transitional phase can be seen in rocking mode in a sequence of half-hourly Airmass RGBs from Meteosat-8, between 26 and 30 September — before it reached the west coast of India (Figure 2).
Looking at these images the relation between Gulab and Shaheen appears to be rather weak. However, as it entered the Sea of Oman the system gradually strengthened again. While initially slowly moving north-westward, the storm accelerated subsequently turning west-south-west and made an extremely rare landfall in the Al Batinah North Governorate of Oman on 3 October, as a category 1-equivalent tropical cyclone. Shaheen then rapidly weakened and dissipated the next day. This intensive phase is shown in a sequence of 15-minute false-colour images of the IR10.8-band from Meteosat-8 between 1 and 4 October (Figure 3), where the red-to-yellow colour wedge covers the cold temperature range between 236K to 180K (-37°C and -93°C).
Landfall of Shaheen is shown in the 15-minute imagery of the HRV-band on 3 October between 02:00 UTC and insufficient illumination after 13:30 UTC (Figure 4).
The descending black pattern is due to the so-called HRV sliding-window of Meteosat-8 operations. An HRV-IR10.8 comparison shows the situation at 08:00 UTC on 3 October, when Shaheen had an eye in its most active moments (Figure 5).
With Shaheen rapidly disintegrating the Arabian Peninsula got swamped with humidity at lower levels (Figure 6). This comparison of the Dust RGB from Meteosat-8 on 28 September and 6 October confirms this humidification by the intensified blue colouring on 6 October over much of the arid landmass, indicative in Dust RGBs for the presence of low-level humidity. Advection of blue colouring, i.e. moisture, with system remnants is nicely shown with this Meteosat-8 Dust RGB hourly loop.
Shaheen brought extreme rainfall to Oman, causing flooding and erosion across a wide area of the country's northeastern coast. The urbanised areas around Muscat saw heavy flooding, with submerged cars and other low-lying objects. Other Omani areas, and neighbouring countries outside the directly impacted areas, experienced some rain after landfall.
Natural Colour RGBs from Sentinel-2’s MSI imaging radiometer at 10m horizontal resolution give a view on the storm's impact in a coastal area just west of the Muscat International Airport. The area is covered by Sentinel-2 overflights around every five days, for this case on 27 September and on 2 & 7 October. These datasets allow for a comparison of the situation prior to the storm's passage (27 September) to the one after landfall (7 October) in an image couple (Figure 7).
Prominent on the image is the Al Khoudh retention/recharge dam just at the right of the image centre, lush green on the inward side of dam prior to the storm and water-filled after the storm.
Also, due to the heavy rain, the stream that drains the AlKhoudh Wadi, one of the major wadis in the Sultanate, can be clearly seen on the after-image is. The dam is around 3km long and has a capacity of around 70 million cubic metres.
The comparison also highlights various other freshly flooded areas — extended ones along the coast and minor ones south-east of the dam. The overflight of 2 October (Figure 8), the day before landfall, although partially cloud-covered, reveals the still empty retention basin and extended electric-blue coloured surf along the coast, hinting at strong winds and high waves over the Sea of Oman.
Before and after imagery comparison, between two Terra overpasses on 18 September and 6 October (Figure 9), indirectly reveal the intensity of the rain that fell over the region NW of Muscat – it is clear how huge amounts of water engraved the water paths (wadies) from the higher elevations towards the sea.
Further Sentinel-2 imagery (Figures 10 & 11) revealed both the flooded and eroded signatures along Oman coast.
Shaheen’s passage in the Sea of Oman and landfall over the Omani coast was reminiscent of another tropical cyclone, Gonu, that hit the same area in June 2007. Gonu was much stronger, but just skirted the coast on its way to make landfall over Iran. Compare the two quite different tracks in Figure 12.