Intra-event Isotope and Drop Size Data Reveal Post-condensation Effects in Tropical Rain
Wednesday, 17 December 2014
Stable oxygen and hydrogen isotopic compositions (δ18O and δD, respectively) of precipitation are usually measured/ presented as values averaged over rain events or monthly mean values. Such data, especially from tropical stations, seldom throw light on post-condensation processes, viz., isotopic equilibration of rain with water vapour at the cloud base, and evaporation during atmospheric transit, known to affect the isotopic compositions of precipitation and moisture recycling in the atmosphere. Further, the slope ~8 of the linear relationship between such time-averaged values of δD and δ18O (called the Meteoric Water Line, MWL) is widely accepted as proof of equilibrium condensation with little post-condensational alteration. Here, through a simultaneous investigation of isotopic and drop size distributions of seventeen rain events on an intra-event scale at Gadanki (13.5ºN, 79.2ºE), southern India, we show that post-condensation effects, not evident in the time-averaged data, are significantly manifested in sub-samples of individual rain events. We reveal this through (i) slopes significantly less than 8 for the δD - δ18O relation; (ii) significant correlations between deuterium excess (d-excess = dD – 8*δ18O; lower values in rain indicate evaporation) and the mass-weighted average diameter of raindrops (Dm): positive when raindrops evaporate and negative when they isotopically equilibrate with vapor at cloud base. We demonstrate that post-condensation processes, in addition to environmental conditions at source, could affect the d-excess of rain. An estimated ~65% of rain is influenced by the post-condensation processes at the cloud base, of which ~44% is affected by evaporation. Isotope-enabled global circulation models could be improved by incorporating intra-event isotopic data and raindrop size dependent post-condensation processes.