Influence of stratiform clouds on δD and δ18O of monsoon water vapour and rain at two tropical coastal stations

Studies of moisture dynamics associated with major tropical rain activities with the aid of stable oxygen and hydrogen isotopic (δ18O and δD) measurements in precipitation and water vapour are of current research interest. Here we present such measurements of rain and water vapour samples collected at two tropical coastal hill stations of southern peninsular India during April-October, 2012. An apparent seasonal cycle is observed both in rain and vapour isotopes. Despite of the isotopic equilibrium observed between rain and vapour during the whole sampling period, it often deviates in sub seasonal time scales. Rain-vapour isotopic separation factors are lower compared to the equilibrium separation values during stratiform rain events of Indian summer monsoon, while the separation factors are close to the equilibrium values during convective events. This is possibly due to the decreased vertical mixing of moisture during stratiform rain events.