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| Thursday, 4 December 2003 |
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What led to the May floods? by Dr. Lareef Zubair, Coordinator, Sri Lanka Meteorology, Oceanography and Hydrology Network and International Research Institute for Climate Prediction, New York Heavy rainfall deluged South-Western Sri Lanka between the 11th and 19th of May 2003. Floods and landslides claimed 260 lives. Schools, homes and other infrastructure were destroyed. The historical average rainfall in the South-Western corner between Ratnapura, Galle and Colombo for May is 275 mm. The heaviest rainfall on record is 600 mm during 1936; the lowest is 18 mm in 1953. So the regional average rainfall for 2003 of 450 mm is high but not extreme. But last May's rainfall was concentrated in a few places and in one week and particular on the 17th. The heaviest rainfall for May 2003 of 899 mm was recorded at Gonapenigala Iranganie Estate and the third heaviest rainfall of 755 mm was recorded at Panilkande Estate. The monthly rainfall in Ratnapura was 718 mm with half of that falling on the 17th of May. What were the weather conditions that led to this rainfall? Strangely, it turns out that it was due to a cyclonic storm. Strange because, there has been no record of such a storm making landfall in Sri Lanka during May. This is one reason why people were caught unawares. The 2003 cyclonic storm did not make landfall in Sri Lanka but was far away in the Bay of Bengal! The storm started 700 km to the East of Sri Lanka on the 11th of May and made its way to the North and North-East until it reached Myanmar on the 20th. there was heavy rainfall along the path of the cyclone and in a spot far away in South-Western Sri Lanka. Why that corner of Sri Lanka got soaked when a cyclone travelled so far away is a freak combination of geography and wind patterns. In that fateful week unusual North-Westerly regional wind patterns stalled the cyclone in the middle of the Bay of Bengal for a few days and one of the octopus like spiral arms of the cyclone (which draws in expanses of air to feed its core) gusted over Sri Lanka. During May, the cloud bands that usually deluge Kerala at the end of month hovers over the latitudes of Sri Lanka and the cyclonic gusts entrained these clouds towards Sri Lanka. These moist gusts were interrupted by the Sri Lankan mountains precipitating the deluge on their southern-western slopes on the windward side. The rainfall patterns resemble a shadow of Sri Pada or Adams Peak towards the South-West. Later, we found out that Kerala and Tamil Nadu had rainfall deficits in June. This mechanism of mountain-induced rainfall is the reason why the windward westward slopes of the central mountain ridge running from Kirigalpotta to Hantana mountains and Knuckles get such heavy rainfall during the middle of the year as strong wind blows from the West. This time however the wind was from the South-West and the rainfall pattern for May 2003 is on largely on the South-Westward slopes. This is why Deniyaya received heavy rainfall while Watawala and Matale received relatively low rainfall last May. The major disasters affecting Sri Lanka, namely floods, landslides, droughts and cyclones have hydro-meteorological antecedents. Last May's flooding and landslides and previous hydro-meteorological disasters such as the cyclones in 1978 and 2000 that affected the North-East and the recurrent drought that affects the South-East and North-West underscores the urgent need for local computational weather prediction. Of course, it is not only the matter of issuing hazard warnings that shall reduce damages but also its translation to hazard warning and the implementation of appropriate communication of warnings and disaster preparedness and mitigation and response systems. Even with all its shortcomings, computational weather prediction has the potential to forewarn of weather and climate spawned hazards. Indeed, the National Centre for Medium Range Weather Forecasting of India was able to predict high rainfall in Sri Lanka with useful accuracy three days in advance last May 2003. The development of indigenous computational weather and climate prediction capability should be a national priority. The author has presented a proposal for developing climate prediction technology to the Ministry of Science and Technology after consultations with senior officials of the Department of Meteorology and other scientists. Given the local availability of computer and Internet resources and skilled scientific and computational expertise, such prediction technology can be developed indigenously in three years as long as there is a sustained will to do it and the right people are engaged and a suitable environment is provided for them to work in. |
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