Diagnostic Analysis of extremely severe cyclonic storm ‘Fani’ using Microwave satellite images

Habibur Rahaman Biswas India Meteorological Department Meteorological Centre, Bhubaneswar, India Email: [email protected] Classification of Low pressure System in India Maximum Sustained wind speed (3 minute Low Pressure System average) in Knots LOW <17 Depression 17-27 Deep Depression 28-33 Cyclonic Storm 34-47 Severe Cyclonic Storm 48 - 63 Very Severe Cyclonic Storm 64 - 89 Extremely Severe Cyclonic Storm 90-119 Super Cyclonic Storm >119 Background: Extremely severe cyclonic storm ‘Fani’ is one of the rare and strongest originated over in the month of April and crossed , India coast on 3rd May, 2019. Salient features of Fani : 1. Genesis of Cyclone “Fani” was observed near equator near Latitude 2.70N and it has one of the longest track about 3030 km in the Bay of Bengal Basin. Genesis of the cyclonic disturbance in such a lower latitude is very rare, last such activity was observed over the north Indian Ocean in January, 2005. 2. Fani has phase and maximum sustained wind speed increased 45 knots to 95 knot within 30 hours period only. 3. It recurves towards north northeastwards and then attained Maximum intensity with Maximum sustained surface wind Speed 115 Knot gusting to 230 Knots. 4. Very heavy rainfall activity associated with Fani was comparatively less though it maintain cyclonic storm intensity even upto 21 hours after landfall. Objective of the present study is to investigate these features through Microwave Channel images as it has advantage to see cloud properties beneath the top of the cloud.

Genesis Properties

IMD analysis : 26.04.2019 1200 UTC Depression : Centre: 3.2°N/89.2°E Intensity: T1.5 Microwave 37.9GHz Channel : Low cloud ring around centre and deep convention in forward sector. IMD analysis : 27.04.2019 0000 UTC Deep Depression : Centre: 4.5°N/88.8°E Intensity: T2.0 Microwave 37.9GHz Channel : Deep convection patch near centre, outer band convection developing , especially in northward sector. IMD analysis : 27.04.2019 0900 UTC Cyclone : Centre: 5.4°N/88.5°E Intensity: T2.5(MSW 40KT) Microwave 37.9GHz Channel : Convective bands towards NNW Sector. Thus development of strong convective cloud pattern in the forward sector of low level vortex observed unlike intense convection behind the trough line of easterly wave. Rapid intensification Phase

Change of Wind Speed (in Knot)with respect the Time of Cyclone Fani Important factor for intensity change of tropical cyclone

❖ wall diameter ❖ Eye Wall Brightness Temperature ❖ Eye Temperature ❖ Ring Percentage with deep Convection (Percentage of ring (eye wall inner boundary) colder than hot spot(warmest eye pixel) in the eye by 20k or ‘the percentage that is colder than 232k and colder than the hot spot by 10k’) ❖ Pattern of Convective cloud in Eye wall and interchange between primary and secondary wall cloud ring.

Temporary weakening of deep convection in primary wall cloud ring before recurvature of storm ‘Fani’ Deep convection in primary wall cloud ring regaining after recurvature process of storm ‘Fani’. Also shift of convection pattern observed. Maximum Intensity Just before Landfall

Very Intense convection in the wall cloud region has been significantly decrease In consequence, it was observed that amount of very heavy rainfall associated with the cyclone was less. Conclusion: 1. Significant changes in convection pattern in association with Low level vortex may be an indicator factor for genesis of Cyclone Fani in lower Latitude.

2. Rapid intensification of ‘Fani’ could be recognized through change in curvature and intensity of convection in eye wall both primary and secondary around the storm centre in microwave satellite images.

3. Intensity of the convection in primary wall cloud region may decrease temporarily in case of recurving system.

4. Significant changes in convection pattern in wall cloud region just before or at the time of recurvature of the system were observed which can provide important indication to the forecaster for assessment of the system track and intensity.