Assessing raindrop evaporation over northern Western Ghats from stable isotope signature of rain and vapour

Abstract. Stable isotopes of hydrogen and oxygen were analysed in rain and vapour samples collected simultaneously from Pune, India, during the 2019 summer monsoon. The δ18O and δD were significantly depleted in four events when the Outgoing Longwave Radiation showed a strong negative anomaly suggesting large-scale convection. The δ18O values of the rain samples are negatively correlated with their d-excess indicating effect of drop evaporation. Isotope exchange between rain and ambient vapour and associated raindrop evaporation in the sub-cloud layer, tracked by Δδ–Δd plot, suggest an equal share of equilibrium exchange and drop evaporation. We used a one-dimensional Below Cloud Interaction Model (BCIM) to quantify sub-cloud processes affecting raindrop evolution. A Rayleigh ascent assumption in the BCIM simulations yields higher rain isotope values. Using radiosonde-based temperature and humidity profiles and constructing vapour isotope profiles from a combination of Tropospheric Emission Spectrometer data and a global circulation model (LMDZ) output, a better agreement is reached between the model and observed values. Sensitivity studies reveal that model values are strongly influenced by vapour isotope profiles, and moderately by drop size, temperature and relative humidity. Raindrop evaporation fraction estimated from the model yields daily-scale values varying from 4 % to 61 % (on average 23 %). This evaporation influences the heat budget and affects the monsoon convection. In addition, our study shows that drop evaporation reduces the rainfall amount considerably, especially in the lower range of precipitation. A precise quantification of raindrop evaporation is required for validation of the models used routinely for monsoon rainfall predictions.