Intra-event isotope and raindrop size data of tropical rain reveal effects concealed by event averaged data

• Published in: Climate dynamics 2016-08, Vol.47 (3-4), p.981-987
• Main Authors: Managave, S. R., Jani, R. A., Narayana Rao, T., Sunilkumar, K., Satheeshkumar, S., Ramesh, R.
• Format: Article
• Language: English
• Subjects: Atmospheric models; Climate; Climatology; Condensation; Deuterium; Earth and Environmental Science; Earth Sciences; Evaporation; Geophysics/Geodesy; Greenhouse gases; Isotopes; Meteoric water; Oceanography; Rain; Water vapor
• ISSN: 0930-7575; 1432-0894
• DOI: 10.1007/s00382-015-2884-7

Evaporation of rain is known to contribute water vapor, a potent greenhouse gas, to the atmosphere. Stable oxygen and hydrogen isotopic compositions (δ 18 O and, δD, respectively) of precipitation, usually measured/presented as values integrated over rain events or monthly mean values, are important tools for detecting evaporation effects. The slope ~8 of the linear relationship between such time-averaged values of δD and δ 18 O (called the meteoric water line) is widely accepted as a proof of condensation under isotopic equilibrium and absence of evaporation of rain during atmospheric fall. Here, through a simultaneous investigation of the isotopic and drop size distributions of seventeen rain events sampled on an intra-event scale at Gadanki (13.5°N, 79.2°E), southern India, we demonstrate that the evaporation effects, not evident in the time-averaged data, are significantly manifested in the sub-samples of individual rain events. We detect this through (1) slopes significantly less than 8 for the δD–δ 18 O relation on intra-event scale and (2) significant positive correlations between deuterium excess ( d -excess = δD – 8*δ 18 O; lower values in rain indicate evaporation) and the mass-weighted mean diameter of the raindrops ( D m ). An estimated ~44 % of rain is influenced by evaporation. This study also reveals a signature of isotopic equilibration of rain with the cloud base vapor, the processes important for modeling isotopic composition of precipitation. d -excess values of rain are modified by the post-condensation processes and the present approach offers a way to identify the d -excess values least affected by such processes. Isotope-enabled global circulation models could be improved by incorporating intra-event isotopic data and raindrop size dependent isotopic effects.