One-fourth of India’s monsoon rain evaporates mid-air, says new study, Pg11

Key Highlights:

• A study by the Indian Institute of Tropical Meteorology (IITM), Pune, revealed that approximately 25% of the rain over the North Western Ghats evaporates mid-air during the Southwest Monsoon.
• This marks the first experimental measurement of raindrop evaporation in India, with daily variations ranging from 4% to 61%.
• The findings are crucial for refining existing weather and climate models and enhancing the understanding and prediction of the monsoon.
• The research was published in the peer-reviewed journal Atmospheric Chemistry and Physics.

Detailed Insights:

• Raindrop evaporation cools the sub-cloud layer, which in turn feeds downdrafts and creates cold air pools at the surface.
• This process significantly reshapes atmospheric convection, which is responsible for generating subsequent rainfall.
• Climate and monsoon models have historically faced challenges in accurately representing this crucial evaporation process.
• The measured 25% rain mass loss is at the lower end when compared to global estimates, such as 20% in the tropics and 40% near Zurich.
• Researchers employed isotopic analysis, observing that lighter water molecules evaporate preferentially, leaving heavier isotopes in the remaining raindrops.
• The team collected rainwater and atmospheric vapor in Pune during the 2019 monsoon and analyzed their isotope ratios using a laser spectrometer.
• The collected data was then integrated into a Below Cloud Interaction Model to simulate the fall of individual raindrops.
• The IITM is actively expanding its rainwater-isotope network across India to comprehensively map this evaporation process nationwide.

Scientific/Technical Concepts Involved:

• Evaporation: The physical process where a liquid changes into a gas, absorbing heat from its surroundings.
• Isotopes: Atoms of the same element that have the same number of protons but differ in the number of neutrons.
• Monsoon: A seasonal prevailing wind in South and Southeast Asia, characterized by heavy rainfall.
• Atmospheric Convection: The transfer of heat through the movement of air due due to differences in density.