November 16th, 2012 by Joshua S Hill
The small Indian town of Leh was struck by flash flooding on the night of August 5, 2010, which left 193 dead, hundreds missing, thousands homeless, and caused severe damage throughout the town.
During the 24 hour period of the flash flooding the rainfall in the area measured 8 inches, when the average for the whole of August is normally just half an inch.
“Flash flooding events don’t happen often but when they do they are some of the scariest, most dangerous and quickest natural disasters that can happen,” saidKristen Rasmussen, a University of Washington graduate student in atmospheric sciences. “But now that we know what types of conditions to look out for, flash flood warnings in remote regions of India might be possible.”
Rasmussen and colleague Robert Houze studied satellite images and re-analysis data to attempt to understand the cause for such horrific flash flooding, and they’ve published their results in the latest edition of the Bulletin of the American Meteorological Society.
Their conclusions showed that the flash flooding was the result of a series of unusual weather events.
On three consecutive days clouds had formed high in the mountains to the east over the Tibetan Plateau, an occurrence that was, in and of itself, not that unusual. “What’s different in this case is that there was the unusual wind coming from the east and blowing west,” said Rasmussen. As a result, the clouds clumped together and built into a larger storm system capable of creating heavy rain over Leh.
To make matters worse low-level winds carried moisture from the Bay of Bengal and the Arabian Sea. “The storm, forming just up the slope, was able to tap into that additional moisture,” she said.
Now under normal circumstances the setting of the sun would see the warm air creating the clouds lift and become cooler, allowing the clouds to die out throughout the evening. However, during those three days the winds kept blowing through the night, continuing the cloud formation.
Finally, with so little rainfall in the region, the soil does not absorb water well.
It all added up to a fatal mix of events.
“The key is that this happened for three successive days. If the third day hadn’t happened or if the first two days hadn’t set the process in motion, there probably wouldn’t have been such a devastating flash flood,” Rasmussen said.
Now that researchers have identified these common elements, including organized clouds high in the mountains on the edge of an arid plain with unusual access to moisture, weather forecasters can potentially warn people who could be in danger if a flash flood happens, she said.
Image Source: Jennifer Spatz, Global Family Travels via University of Washington
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