Tropical Cyclone Ashobaa
06 June 2015 06:30 UTC—11 June 06:00 UTC
In early June Tropical Cyclone Ashobaa formed over the Indian Ocean, then headed towards Oman.
14 November 2020
06 June 2015
By Hilal Al-Hajri and Shima Al-Yazidi (PACA), Jochen Kerkmann (EUMETSAT), HansPeter Roesli (Switzerland)
After forming as tropical storm off the Indian subcontinent, Ashobaa moved over the Arabian Sea, becoming a Category 1 cyclone. According to the Oman Department of Meteorology, the island of Masirah (off the east coast of Oman) received 239.4 mm of rain during the three days that Ashobaa affected the country.
The progress of the tropical cyclone can be seen in the Meteosat-7 infrared animation, 5 June 12:00 UTC–15 June 06:00 UTC, from sudden birth on 5 June, explosive on 7 June, to slow dissipation from 10 to 15 June, after landfall there was recurrent severe convection near Masirah Island.
Ashobaa started to develop in the Indian Ocean, near Indian subcontinent, and was classified as a low pressure on 5 June. It was 1400 km away from the Oman coast.
The low pressure started to intensify and move north to north west. On 7 June the surface wind around the centre reached between 46–56 km/h ( 25–30 kts) and the system was classified as a deep depression.
At that time the system was still travelling north to north west and was located 1200 km away from the Oman coast.
On 8 June the system started to intensify very quickly (Figure 2) and became a named tropical storm (Ashobaa), see Met-10 Dust RGB image with ECMWF 10 m model winds overlaid (Credit: EUMeTrain). The wind speed around the centre was between 64–74 km/h (35–40 kts) and the movement became more west to north west, see also Day Microphysics RGB (8 June 11:00 UTC) from the Indian Meteorological Agency's Insat 3D satellite.
The Metop AVHRR image (Figure 3) shows the extensive, high-level cirrus outflow from the relatively small tropical storm.
On day-night band imagery from NOAA's Suomi-NPP satellite, from 8 June at 20:45 UTC the cyclone's structure could be clearly seen.
On 9 June the tropical storm moved closer to Oman — approximately 500 km off the coast. At that time the advocated high and medium cloud touched the Oman coast and sea condition were rough, with waves reaching 3.5 m.
For tropical storms like Ashobaa it is often difficult to locate the centre (defined as low level circulation centre) of the system in simple VIS or IR images. In these cases, it is recommendable to use microwave imagery like the 89 GHz Channel from MHS on Metop, or comparable channels on other satellites like the 37 or 85 GHz channels on TMI, or the 85 GHz channel on SSMI, or, even better, to use the wind vectors from scatterometer instruments like ASCAT to locate the centre of the cyclone. For more info, have a look at the lecture from Sheldon Kusselson (NOAA) , presented in 2012 during a satellite training workshop in Pretoria, South Africa
Figure 4 shows the AVHRR IR image from 9 June 05:29 UTC and compares it to the 89 GHz image from MHS. The centre of the cyclone is not visible on the IR image but clearly appears on the MHS image.
Likewise, the scatterometer winds shown in Figure 5 help to locate the centre of Ashobaa that is not visible in the AVHRR VIS/IR images.
Ashobaa's closer proximity to the Oman coast could be seen in both Meteosat-10 and Metop-B imagery. This Meteosat-10 HRV loop from 02:00–09:30 UTC shows the cyclone approaching the coast.
As the cyclone moved closer to Oman, the need to make a rainfall estimation forecast became more important, as it was likely that the country could receive substantial amounts of rain.
For countries which don't have a rainfall radar network, NOAA has developed a method called eTrap (ensemble Tropical.Rainfal Potential). This method allows for the generation of probabilistic forecasts of rainfall in addition to deterministic rainfall totals.
Each eTRaP is made up of forecasts using LEO satellite observations from NOAA/Metop (MHS), NASA (TRMM) and DMSP (SSMIS).
The three basic assumptions for eTrap are:
- satellite rain rate estimates are accurate;
- forecast tracks are accurate, and
- rain rates over 24 h period are in steady state.
By 10 June, as the cyclone started to move over the Omani coast, it started to weaken slightly. Looking at satellite imagery it appeared that this was due, in part, to dry-dusty air being ingested into the system. An area of low-level dust north-east of the system can be clearly seen on the Meteosat-10 Dust and Natural Colour RGB images (Figure 8). The dust, which originated over Iran and Afghanistan, is visible over Pakistan, the Sea of Oman and northern Oman. There are also large areas of sun glint (over the sea and some low-level clouds, see Natural Colour RGB), which should not be confused with dust.
On 11 and 12 June, Ashobaa very slowly moved westward approaching Masirah Island on the south coast of Oman, but not making landfall. As can be seen in Figures 9 and 10, the structure of Ashobaa is better seen in Meteosat-7 visible imagery than in Meteosat-10 HRV images. Both have comparable horizontal resolution for this area, but Meteosat-7 visible images have a higher contrast for thin and thick clouds due to the position of Meteosat-7 over the Indian Ocean (57 deg E, nearly NADIR viewing for Oman area).
Cyclone Ashobaa over the Arabian Sea (NASA Earth Observatory)
Ashobaa effect in Oman: Heavy rain floods Sur, Masirah areas; rescue work on (Oman Times)
No Ashobaa casualties (Oman Observer)
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