Chennai Extreme Rainfall Event of 2015 under Future Climate Projections Using the Pseudo Global Warming Dynamic Downscaling Method

Here we report results of a detailed numerical study on the effect of climate change on the characteristics of a very severe rainfall event that occurred in the coastal city of Chennai, Tamil Nadu, India in December 2015. The pseudo global warming (PGW) method was used to obtain the initial and boundary conditions of the future climate and projections were done for the far future, i.e. the year 2075 using the representative concentration pathway scenario of 8.5. The Weather Research and Forecasting (WRF) model was used for simulations with perturbed initial and boundary conditions by the PGW method in a dynamic downscaling framework. The sensitivities of Microphysics and cumulus parameterization schemes in WRF were first studied. The warm rain microphysics (Kessler) scheme and Kain–Fritsch (KF) cumulus scheme showed good agreement with the observed data. Once the best schemes were identified for such an extreme event and for the specific region under consideration, simulations were carried out for future and current climate conditions. Results show that the bulk Richardson number, energy helicity index, K-index, moisture convergence, vertical temperature and mixing ratio all increase significantly in future climate conditions, thereby leading to heavy precipitation. The precipitation in Chennai region increased by 17.37% on the peak rainy day (1 December 2015) in future compared to current. The key takeaway though is that on succeeding days, the amount of precipitation was seen to increase dramatically by 183.5%, 233.9% and 70.8%. This is bound to lead to severe flood events that are likely to continue for more days in the future, thereby posing further risk and potential for damage.