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 (δ 18 O 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 δ 18 O and δD. The slope of the local meteoric water line (δD-δ 18 O 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 δ 18 O and DSD. The possible explanation is that larger drops are mostly associated with convective rather than stratiform rain, and 18 O (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.

Rainouts over the Arabian Sea and Western Ghats during moisture advection and recycling explain the isotopic composition of Bangalore summer rains

“…The role of factors like wind speed, SST, and raindrop size on over all isotopic ratios of rains has been discussed in the literature [Gat, 1996;Wright et al, 2001;Pang et al, 2004;Rao et al, 2008;Vachon et al, 2010;Srivastava et al, 2012]. But these studies did not evaluate the role of temporal and spatial variability of source parameters on variation of isotopic ratios in continental rainfall.…”

High-frequency Isotope Compositions Reveal Different Cloud-top and Vertical Stratiform Rainfall Structures in the Inland Tropics of Brazil

“…Gedzelman & Lawrence, 1990; T. Han et al, 2020;Muller et al, 2015). Studies using highfrequency sampling schemes have highlighted the relevance of convective and stratiform rainfall types in controlling isotope ratios during extreme events, such as organized mesoscale convective systems (Munksgaard et al, 2020;Srivastava et al, 2012;Sun et al, 2019), squall lines (Landais et al, 2010;Camille Risi et al, 2010;Tremoy et al, 2014), and cyclones (X. Han et al, 2021;Sánchez-Murillo et al, 2019;Xu et al, 2019).…”

Water Isotopologues Long-Term Continuous Rainfall Monitoring Contribution for Modeling Present Climate: Information Obtained from Different Time Scale Observations

“…Locally, when rainfall falls on a collection point, a stage called post-condensation processes, it is also subject to two fractionation processes: (i) rainwater composition balances with the surrounding humidity and becomes enriched. This balance depends on droplet size and relative humidity, which at lower levels is less depleted than rainfall at the cloud base, so the isotopic composition of surface rain closely resembles that of surface moisture [21]; The larger the raindrop, the greater its falling velocity and the less exchange with the surrounding vapor, resulting in δ 18 O depletion [9]; (ii) evaporation of the drops that fall on a layer of low humidity enriches the remaining rain, making the surface rainfall more enriched [22][23][24][25][26]. This process has been observed mainly in desert areas [15,21,27], although it is also seen in continental areas, mainly in light rains in a relatively dry atmosphere [20].…”