Assessing raindrop evolution over northern Western Ghat from stable isotope signature of rain and vapour

Isotope exchange between vapor and rain critically influences rain isotope values, which are useful in modeling raindrop evolution. A one-dimensional Below Cloud Interaction Model (BCIM) has been used to quantify sub-cloud processes affecting raindrop evolution in extratropical regions. However, its applicability has not been tested in a tropical monsoon region, where both advection of moisture and raindrop evaporation are significant. Here, we evaluate the applicability of BCIM using simultaneous surface measurements of rain and vapor isotopes over Pune, a tropical rain-shadow region, during the 2019 Indian Summer Monsoon. Analysis of these data indicates strong isotope exchange and significant raindrop evaporation in the sub-cloud layer. A Rayleigh ascent in BCIM overestimates rain isotope values (by about 6 ‰ for δD), although model and observed values are well correlated. Using radiosonde-based temperature and humidity profiles and constructing vapour isotope profiles from a combination of satellite (Tropospheric Emission Spectrometer) data and the LMDZ model outputs, simulations improve. Further tuning of vapour isotope inputs while preserving the shape of the profiles yields still better agreement. Sensitivity studies reveal that model outputs are strongly influenced by vapour isotope profiles, and moderately by drop size and relative humidity. We used BCIM to estimate raindrop evaporation, which shows that, on average, 23 % of rain mass evaporated over Pune. Our results emphasize the importance of rain evaporation over the Indian continent during the Monsoon season, in particular, over complex orography, and illustrate the use of water isotopes to constrain this key process.