Researchers have found the mechanics behind a devastating extreme rainfall event that dumped more than 250 millimetres of rain on the Western Ghats mountain range in just one day. Using weather radar, researchers discovered that persistent, mid-sized clouds, rather than massive thunderstorms, were the true culprits behind the catastrophic July 2018 floods in Mahabaleshwar.
The research team, hailing from Savitribai Phule Pune University and the Indian Institute of Tropical Meteorology, set out to understand the dynamics of extreme monsoon rains over complex, mountainous terrain. Extreme rainfall events are becoming more frequent and intense globally as a warming climate allows the atmosphere to hold more moisture. To investigate how the 2018 deluge occurred, the scientists used a high-altitude X-band weather radar alongside an impact-based device called a disdrometer, which measures the size and the speed of individual raindrops. This method allowed the researchers to probe the three-dimensional structure of the clouds and track their evolution over time.
They discovered that the 2018 event was triggered by the rare, simultaneous occurrence of three major weather systems: a low-pressure zone over the Bay of Bengal, a mid-level cyclone over Gujarat, and a trough off the western coast. Together, these systems supercharged the monsoon winds, creating a powerful, low-level jet stream loaded with moisture from the Arabian Sea.
The extreme rainfall happened from the way this jet stream interacted with the local geography. When the fierce, wet winds slammed into the steep slopes of the Western Ghats, the mountains acted as a massive physical barrier. This forced the moist air to rise rapidly, a process known as orographic lifting. Surprisingly, the radar data revealed that this lifting did not create the towering, deep thunderclouds usually associated with extreme storms. Instead, it generated cumulus congestus clouds, mid-sized clouds that reached up to nine kilometres into the sky. Because of the continuous wind forcing, these clouds were sustained over the mountain peaks for hours on end. Inside the clouds, the disdrometer detected a unique microphysical signature. The raindrops grew through a process of collision and coalescence, resulting in fewer but significantly larger raindrops. This steady, heavy rain fell relentlessly through the night and early morning.
By feeding these precise radar observations and cloud behaviours into computerized weather models, meteorologists can greatly improve the accuracy of extreme rainfall predictions. Better forecasts mean earlier, more reliable warnings, giving vulnerable mountain and coastal communities the precious time they need to evacuate and protect themselves from catastrophic landslides and flash floods.
