Relationship between stable isotope ratios and drop size distribution in tropical rainfall

Abstract

We carried out simultaneous measurements of drop size distribution (DSD) and stable oxygen and hydrogen isotopic compositions (δ18O and δD) of rain at the National Atmospheric Research Laboratory (NARL), Gadanki (13.5°N, 79.2°E), southern India, during September–October 2006, with the aim of understanding microphysical processes leading to rain formation. The MST radar at NARL was operated continuously during rain events, while rain samples were collected at very short time intervals (<1 h), to capture small changes (>0.2‰ and >2‰) in their δ18O and δD. The slope of the local meteoric water line (δD–δ18O line), was 8.07 ± 0.47, similar to that of global meteoric water line, confirming that the precipitation occurred under isotopic equilibrium, and was unaffected by some anomalous process; further, the evaporation of rain drops at the cloud base was insignificant. Whenever the isotopic variations were larger during a rain event (>2‰) there was a significant negative correlation between the δ18O and DSD. The possible explanation is that larger drops are mostly associated with convective rather than stratiform rain, and 18O (and D) depletion in convective rain is relatively more. Bin-resolved microphysical models incorporating water isotopologues could benefit by considering drop size spectra, which could improve the match with stable isotope observations of precipitation.