On December 1, 2015 Chennai and its surrounding regions experienced an unprecedented, heavy rainfall. In a region where the average rainfall during the season is expected to be 8-10 mm per day, one of the rain gauges in the city recorded an abnormally high, 494 mm, rainfall over 24 hours that day. This led to death of nearly 250 people, and Chennai was declared a ‘disaster zone’. There have been attempts to explain this phenomenon of how clouds remained stationary over this region, continuously giving rain over 24 hours. In a first, Jayesh Phadtare of Centre for Atmospheric and Ocean Sciences, Indian Institute of Science, Bengaluru, links the presence of the Eastern Ghats to this phenomenon, in a paper published in Monthly Weather Review.
Cold pool
When clouds give out water droplets, the droplets evaporate mid-air, as they fall down. This cools the surrounding air, forming a cold pool of air which sinks down and flows horizontally. “The gusty cold wind that heralds an approaching thunderstorm is nothing but a cold pool, which plays a pivotal role in cloud dynamics.” says Jayesh.
Unlike the Western Ghats, which run close to the west coast of India, the Eastern Ghats are nearly 200 km away from the coast. Therefore, the link between the mountains’ orography and the rainfall over the region is not obvious, and this is the first study to link the two. Jayesh, who is studying cloud propagation over the Indian region, could see the connection by observing satellite images: “In Kalpana-I satellite images, I saw that the clouds that gave so much rain over Chennai on 1 December 2015 moved from Bay of Bengal to the coast and became stationary there,” he says. Realising that the Eastern Ghats must be having a role in this, he went on to study a model of the system. “The interaction between mountains, clouds and cold pools became clear after performing the model experiments,” he adds, in an email to The Hindu.
According to the model, the cold pool was obstructed by the Eastern Ghats from flowing downward. Hence it piled up and remained stationary over the Chennai region. “The reason for the clouds remaining stationary was that there was a balance between the piling of cold pool along the mountain and the winds from the bay. This does not happen in all heavy rainfall incidences over Chennai,” says Jayesh.
Sensitivity experiments were done to check this model. In the experimental model in which the orography was absent, the winds just swept downstream and the clouds moved inland. In the model where the evaporative cooling was removed, the cold pool did not form at all and the clouds moved over the Ghats.
Dust storms
Cold pools are known to play an important role in the dust storms (Aandhi) that form in northern India. They form by the evaporation of raindrops. This process is more efficient in the drier and warmer environment as there is lot of scope of evaporation. So, the cold pools that form in these conditions, are deeper and more vigorous. “As pre-monsoon conditions in north India are very dry and warm, cold pools that accompany the pre-monsoon thunderstorms there are far more destructive, causing widespread damages,” says Jayesh. For the first time, this study links cold pools and the mountain structure to explain rainfall over south India.
Though the primary aim of the study is to explain the anomalous rainfall over Chennai on December 1, 2015, "the understanding gained from this analysis can be useful for improving the general weather forecast over this region,” the author writes in the paper.
