Day-night differences in &delta;¹⁸O and <em>d</em>-excess of convective rainfall in the Rio Claro station, inland tropics of Brazil

Abstract

The tropical central-southern part of Brazil (CSB) is characterized by strong convective systems bringing generous water supply for agro-industrial activities but also pose flood risks for large cities. Here, we present high-frequency (5–10 minutes) rainfall isotopic compositions to better understand those systems, with a total of 90 intra-event samples collected during the period 2019–2021. Convective activity and moisture transport modulate the seasonal rainwater isotopic composition with low δ18O values during summer and high during autumn and spring. In summer, both regional and local factors contribute to the observed depletion in heavy isotope contents of rainfall, with strong, continuous rainout along the trajectories of moisture-laden air masses arriving at the rainfall collection site from the Amazon basin, and diurnal convective activity of the local atmosphere, respectively. This activity generates convective clouds with distinct features (cloud depth and cloud base height) and induces differences in atmospheric conditions below the cloud base level (relative humidity and rainfall rates) modifying isotopic characteristics of rainfall and revealing novel perspective on day-night contrast in δ18O and d-excess values. During daytime, enhanced sub-cloud effects lead to high δ18O and low d-excess while continuous regional rainout during night-time results in low δ18O and high d-excess values of local rainfall. Our results offer a new framework of key drivers controlling the isotopic variability of rainfall in tropical South America that must be considered in future studies of convective systems across the tropics.