IISc scientists decode Bengaluru's raindrops

BENGALURU: A team of scientists at the Indian Institute of Science (IISc) have meticulously tracked how seas and oceans influence the composition of raindrops in Bengaluru.

In their research called 'the journey of moisture from ocean to the land', scientists at the Centre for Earth Sciences and the Divecha Centre for Climate Change at IISc explored the relationship between rain and the ocean. It studied isotopes (variants of elements with different number of neutrons) of oxygen and hydrogen in raindrops over Bengaluru. They examined the difference in isotopes compared to the distance from the source (middle of the Arabian Sea).

The team found that different physical parameters, along with four processes, determined the final vapour composition. They observed that partial rain over the sea and moisture recycling from nearby land majorly modified the original isotopic composition of vapour generated from sea water.

"The thought was new and not well accepted," said team member Prof Prosenjit Ghosh, adding, "Our study showed the importance of the ocean and its physical state driving the isotopic signature of rain water in Bengaluru."

The team collected several rainwater samples and compared the isotopic data with other meteorological parameters viz amount of rainfall, temperature and relative humidity. Using a global database, they were able to pinpoint the exact source of moisture. The isotopic signature of water vapour reaching Bengaluru was predicted through four processes: evaporation at source region, transport of moisture and rain over sea and nearby land, recycling of moisture from land where rain evaporates again, and rain at the Bengaluru collecting station.

While previous such studies mainly considered intensity of rainfall and temperature at the place of condensation, this study mainly identified the role of moisture source parameters. Since Bengaluru's rain is caused by both moisture drawn from the Arabian Sea and the Bay of Bengal with varying contributions during summer and winter, it was the perfect setting to test how source and distance make a difference in isotopic signature of rain water.

Complicated process

The ocean is a vast reservoir of water and provides moisture for rain in the sub-continent. The process involves distillation, which gets extremely complicated due to the influence of atmospheric and oceanic circulation. The presence of salt also dampens the rate of evaporation. The process of bringing fresh water to the surface and wind-driven phenomena like ocean upwelling modify the salinity of surface water and alter cloud formation.

Prosenjit Ghosh | associate professor, Centre for Earth Sciences, IISc