HSAF H03B and H05B, Severe Convection RGB, Natural Colour RGB, True Color RGB, IR10.8, HRV, Airmass RGB, Tropical Airmass, scatterometer winds, NIR1.61, NIR2.25, VIS0.49, IR10.4
This case study combines a number of examples of tropical cyclones that formed and travelled over the Indian Ocean from 2020 onwards. We have tracked them using various satellite data and recorded some of the impacts they made in countries such as Myanmar, Madagascar and Mauritius.
Between 6 and 10 May Tropical Cyclone Ansani developed in the Bay of Bengal. In parallel Tropical Cyclone Karim evolved in the south-west part of the Indian Ocean (well off Australia).
Figure 1 shows the situation on 7 May at 21:00UTC when both tropical cyclones were close to their mature stage. The Airmass RGB from Meteosat-8 (with tropical tuning that enhances the towering convection) is overlaid with data from the ERA5 re-analysis, the streamlines at 500hPa on the left image and the divergence at 200hPa on the right side. The streamlines highlight the dominant circulation along the equator and around the tropical cyclones. The divergence field reveals that there was still considerable divergence over the central parts of the tropical cyclones (patches with red to orange contours), sustaining the vertical circulation.
The evolution of the two systems can be followed in the half-hourly sequence of tropically tuned Airmass RGBs between 6 and 10 May.
After reaching the mature stage, Ansani turned westward and made landfall on the Indian coast. Karim travelled southward and dissolved over open waters in the mid-latitudes.
A similar event occurred with the tropical cyclone couple Fani and Lorna at the end of April 2019.
Ana, Batsirai, Dumako, Emnati
4-7 Feb, Mauritius, Réunion, Madagascar
By Vesa Nietosvaara, Ivan Smiljanic, Natasa Strelec Mahovic
In early 2022 four consecutive tropical cyclones developed in the southern Indian Ocean, crossed over Africa and made landfall in Madagascar.
After tropical storm Ana, tearing across Madagascar and southern Africa in the last week of January, the beginning of February saw a category 4 tropical cyclone hitting Madagascar, again with casualties and wide material damage.
Tropical cyclone Batsirai started to form in the last days of January, in the central part of the south Indian Ocean. It passed north of the island of Réunion on 4 February, see the Tropical Airmass (Figure 3), as a category 4 tropical cyclone, and continued towards Madagascar.
The cyclone made landfall on Madagascar's central eastern coast, next to the city of Mananjary on 5 February, with winds up to 200km/h. The winds measured by the Metop-B and -C ASCAT scatterometers before and during the landfall can be seen in Figure 4.
A direct overpass over the cyclone on the day of the landfall was provided by Hai Yang 2B (HY-2B) satellite carrying the SCAT scatterometer, the image of the winds is seen in Figure 5.
The central and southern parts of the Madagascar island received extensive rainfall, as can be seen in the loop in Figure 6.
Locally more than 150mm, and at the eastern coast more than 250mm, of rain fell during three days, according to accumulated precipitation from the HSAF H-05B product (Figure 7).
Flood warnings were issued for the rivers Mananjary and Fiherenana. Properties and infrastructure in the cities of Mananjary, Fianarantsoa and Ambositra were severely damaged. Tens of thousands of people needed to be displaced and at least 10 deaths were reported.
Heavy precipitation was related to convective processes within the cyclone cloud band, as can be seen in the Meteosat-11 IR 10.8 loop in Figure 8.
In several time-steps of the loop, transverse cirrus bands can be seen on top of the cloud spiral, spreading from the centre of the cyclone. This is further confirmed by a respective Metop-B pass, showing in cloud top temperature rays of colder cloud tops reaching from the eye of the storm outwards. Figure 9 shows the comparison of Metop-B with Meteosat-8 satellite. The better spatial resolution of Metop-B brings out more details of the radial features, and even more wavy formations within the cloud tops around the cyclone centre.
The microphysical structure of the storm tops can be viewed in Figure 10, showing an RGB combination of Suomi-NPP VIIRS channels NIR 1.6, NIR 2.25, VIS 0.49. Having both 1.6 and 2.25 micron channel, this combination clearly distinguishes clouds with small particles (light blue) from those with larger ice particles (dark blue). This combination is close to Cloud Phase RGB that will be available with FCI imager on MTG.
The local Red Cross/Red Crescent societies early action protocol in place for such events, so warnings can be communicated to the most at risk communities, and support and supplies can be put in place before a storm arrives. While this does not change the severity of a storm, it can have an a significant effect on the impacts and outcomes for affected. The data shown in this case study are part of the data that are used by the national and regional forecasting centres to provide the initial warnings that trigger such support, for example during the previous Storm Ana.
EUMETSAT, as a member of International Charter Space and Major Disasters community, received and responded to three different charter calls that were related to tropical storm Ana. These calls came from Madagascar, Mozambique and Malawi, in response to hazardous impacts in these countries of floods and landslides, particularly on on crops, livestock, and urban infrastructures.
Figure 11 is an example of data that are sent to the local communities in order to assess the impact of tropical storm, showing the birth and lifecycle of storm Ana over the east Indian ocean, until moment it reached Malawi.
On 14 February the next tropical storm, Dumako, reached its peak intensity as a moderate tropical storm, with maximum 10-minute sustained winds of 85km/h (50mph), maximum 1-minute sustained winds of 95km/h (60mph), and a minimum central pressure of 993hPa. It made landfall as a moderate tropical storm near Sainte-Marie Island, Madagascar, with winds of 65km/h (40mph). Although Dumako was a moderate storm, at least 14 people died in Madagascar as a result of flooding.
Figure 12 shows the moment when Dumako moved over Madagascar, passing over the land mass it lost the energy and dissipated on 16 February. At the same time, seen in the same image, Tropical Cyclone Emnati started to deepen further in the east.
The first frame of the Airmass RGB loop in Figure 13 starts with Figure 12 view, and, as well as showing the dissipation of TS Dumako, it continue to show more than eight full days of a lifecycle of Tropical Cyclone Emnati. TC Emnati was a strong, category 3 cyclone that exhibited some typical features related to intense processes within, and around the cyclone, like the eye feature and transverse cirrus bands. It hit Madagascar early on 23 February, with winds of up to 135km/h (84mph).
The eye feature was prominent at the peak of the cyclone's intensity. After that the intensity started to reduce and it made a landfall in south-east Madagascar as category 1 tropical cyclone (see track).
Paths of all four storms, together with impacts on Madagascar up to 21 February (day before TC Emnati made a landfall), are summed up by a graphic in Figure 14.
1-11 April, Australia, Savu Sea
By HansPeter Roesli
Between 1 and 11 April Tropical Cyclone Seroja moved from the Savu Sea (between Flores and Timor) to the Australian west coast. Figure 15 shows the track annotated on Himawari-8 infrared imagery.
On the way, between 7 and 9 April, Seroja had a close encounter with a weaker unnamed tropical cyclone. This tropical cyclone, initially positioned south of Seroja, moved cyclonically around Seroja and approached Seroja's centre, down to a distance of below 1,400km.
During the day on 9 April when the distance fell to around 500km, both tropical cyclones slowed down. But overnight Seroja grew more vigorous and slowly engulfed the unnamed tropical cyclone in the local night — known as the Fujiwhara effect.
Although, tropical cylones passing each other isn't uncommon, see the Lekima-Krosa case study where the two tropical cyclones passed each other practically undisturbed at a distance above 1,400km. However, in this case the unnamed tropical cyclone crossed under the 1,400km limit and, thus, underwent the Fujiwhara effect.
The Fujiwhara effect, sometimes referred to as 'Fujiwhara interaction', says that "binary interaction of smaller circulations can cause the development of a larger cyclone, or cause two cyclones to merge into one".
The effect is named after Sakuhei Fujiwhara, the Japanese meteorologist who initially described the effect. According to Wikipedia "Extratropical cyclones typically engage in binary interaction when within 2,000 kilometres of one another, while tropical cyclones typically interact within 1,400 kilometres of each other".
The couple's life cycle can be seen in Himawari-8 IR10.4 animated imagery at 20-minute intervals (Figure 16).
21-30 Jan, Mozambique, Madagascar, Botswana
By Jochen Kerkmann, Jose Prieto and Ivan Smiljanic
During southern hemisphere summer season 2020/2021, there were two similar tropical low pressure systems in the short time span, both crossed Madagascar and restrengthened over the Mozambique Channel, hitting Mozambique in the region of Beira. The first was Tropical Storm Chalane, which was weaker in intensity but had interesting track, traversing the entire breadth of the southern African subcontinent. The second was Tropical Storm Eloise, which was stronger (Category 2), but moved along a shorter track (Credit: CIMSS). Two consecutive International Charter Calls, and numerous other warnings on severe rainfall, floods and landslides, were raised for affected countries: Madagascar, Mozambique, Zimbabwe, South Africa, Eswatini, even Botswana (Figure 20)
Figures 17-19 show the moments when strong re-intensification of the system took place over very warm waters in the Mozambique Channel, accompanied by clusters of strong convection around a low pressure centre.
The H SAF rain rate product points out intense rainfall region associated with that convection (Figure 19). This product is based on infrared (IR) brightness temperatures (BT), with calibration against microwave data, and becomes saturated when the BTs are very low. Therefore, it is no surprise that when zooming in on the convective system at the south western flank of Eloise (bright yellow system in Figure 17, or saturated feature in Figure 19), BTs of cloud tops reach 180K (ca. -93°C) (Figure 21). Comparing that to local temperature profiles, but also performing dual-view visual assessment, it was estimated these clouds reached heights of around 20km (+/- 3km).
It is interesting to note is that higher resolution IR sensors read even lower temperatures of the coldest parts of convective system (overshooting tops), below 180K (saturated white pixels in Figure 22).
TC Eloise developed a pronounced eye feature, best seen in the Meteosat-11 Night Microphysics animation that shows landfall moment on 23 January, around 03:00 UTC (Figure 23).
Using high-resolution, visible spectral channels, the flooded areas in the Beira region, close to the mouth of Pungwe river, could be clearly seen. The MODIS True Color RGB reveals the muddy colours of water in flooded basin (Figure 24), while the SEVIRI HRV channel shows reduced reflection in the same region (Figure 25).
Tropical Storm Chalane was a tropical cyclone that made landfall in Madagascar and Mozambique in December 2020 and crossed the entire South African subcontinent to emerge into the South Atlantic on 3 January 2021.
Chalane affected Madagascar, Mozambique, Zimbabwe, Botswana and Namibia with strong rainfall and heavy winds: up to 351mm/24 hours fell in Madagascar, seven casualties were reported in Mozambique and 150mm of rain was reported in central/northern Namibia.
Originally a tropical depression, Chalane strengthened into a Tropical Storm on 24 December. Two days later, on 26 December, Chalane made landfall in Madagascar and weakened, before emerging into the Mozambique Channel on 27 December (see Figure 26).
Subsequently, Chalane restrengthened, before making landfall in Mozambique on 30 December (in Mozambique’s Sofala province — which had been struck by Tropical Cyclone Idai in March 2019). The system weakened as it moved inland, degenerating into a remnant tropical low later that day. However, Chalane's remnants continued moving westward for another several days (see unusual track of Chalane, Credit: Wikipedia), as can be seen in the animation of the Tropical Airmass RGB (Figure 27). This RGB is identical to the Airmass RGB, but tuned to tropical conditions (much colder, higher clouds). High clouds appear as ochre, with the highest clouds (overshooting tops) as white.
The westward journey of ex-Chalane across the African subcontinent can also be monitored using daily Natural Colour RGB images (Figure 28). To help identifying the tropical low, the 850hPa geopotential is plotted on the satellite images. The system weakened considerably on its way westward, however, it remained strong enough to generate considerable rainfall in Botswana and Namibia, by advecting even more moisture an instability in the region.
Only a few tropical lows have been observed to travel such a long distance inland bringing substantial rainfall over arid to semi-arid areas like Namibia and Botswana (eg tropical cyclone Eline in February 2000 ). In exceptional cases they can traverse the entire breadth of the southern African subcontinent, from the Indian Ocean to the Atlantic coast: like ex-tropical cyclone Bonita in January 1996, and this case, ex-tropical storm Chalane in January 2021.
21-23 Nov, Africa, Western Asia, Somalia, Yemen, Ethiopia
By Jochen Kerkmann
Tropical Cyclone Gati was the strongest tropical cyclone on record to make landfall in Somalia, when it hit the country in late November 2020.
Storm Gati was the strongest tropical cyclone on record to make landfall in Somalia (source Wikipedia), and one of few tropical cyclones to do so in the country.
The third cyclonic storm of the 2020 North Indian Ocean cyclone season, Gati formed from an area of low pressure in the Arabian Sea, on 21 November. The storm then explosively intensified (in 12 hours, Gati’s winds intensified from 65 km to 185 km per hour), becoming a severe tropical cyclone. It reached its peak intensity on 22 November, as a category 2 on the Saffir-Simpson scale, see track of Gati (Credit: CIMSS).
Gati weakened slightly before making landfall in the Bari region of northeastern Somalia on 22 November at around 13:00 UTC. Gati was the first hurricane-force cyclone to make landfall in Somalia since satellite records began 50 years ago. Gati then weakened and became disorganised as it moved inland.
At least nine people died as a result of the storm; almost 4,000 properties, belonging to nomadic communities, were destroyed; an estimated 10,000 animals were killed in Ufeyn, and 42,000 people were displaced. Minor impacts were also observed on the Yemeni island of Socotra and in the Ethiopian Highlands.
Cyclone Gati was well observed by both Meteosat satellites, Meteosat-8 and Meteosat-11. However, Meteosat-8 was better positioned for the observation of Eastern Africa (Somalia is at about 50 deg East), as seen in the animations in Figures 29 and 30.
Figure 29 shows the Meteosat-8 Tropical Airmass RGB from 21 November to 23 November. This RGB is identical to the Airmass RGB, but tuned to tropical conditions (much colder, higher clouds). High clouds appear as ochre, with the highest clouds (overshooting tops) as white. Both, the rapid intensification of Gati on 22 November, with the formation of a distinct eye, as well as the landfall and the decay over the Gulf of Aden, can be seen.
Figure 30 which displays the High Resolution Visible channel (HRV, 1km sampling), gives a more detailed view of the hours before landfall of Gati. The structure of the eye (diameter of about 30km), the spiral bands and related convection and the weakening of the cyclone shortly before landfall are well observed.
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.
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 three-minute sustained wind speeds of 240km/h (150mph), one-minute sustained wind speeds of 260km/h (160mph), and a minimum central barometric pressure of 925mbar. 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 31) 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 32. Though when at peak intensity the cyclone was Category 5, by the time it made landfall it was only a Category 2 storm. Figure 32 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 33) 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 34) shows the eyewall structure for the same period, at a resolution of 500m.
The Meteosat-8 infrared animation (Figure 34) 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 36–41 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 37 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 38 and 41 clearly show the shape and the size of individual pixels in visible and infrared spectral regions.