Observed and Simulated Characteristics of 2015 Chennai Heavy Rain Event: Impact of Land-Use Change, SST, and High-Resolution Global Analyses

The southern Indian city of Chennai received more than 1200 mm of rainfall between 5 November and 10 December 2015. The record-breaking rain exceeded the previous 24-h precipitation record (290 mm) on 1 and 2 December and resulted in severe urban flooding. Meteorological analysis from observational and reanalysis data sets shows that this rainfall event was the culmination of three synoptic-scale systems with oceanic moisture trajectories. The city has also witnessed dramatic urban sprawl in recent decades, and the feedback of urban heating on regional convection is postulated to be another critical factor in modulating the mesoscale environment and heavy rains. Accordingly, the study seeks to assess whether the Weather Research Forecasting (WRF) modeling system typically used for heavy rain prediction can better simulate this event by considering enhanced meteorological setup and urban feedback. Simulations were conducted using the WRF modeling system with different model configurations for 40 days (00 UTC 1 November until 00 UTC 11 December 2015), with a triple-nested domain and the finest grid spacing of 1.2 km centered over Chennai. The results indicate that the WRF modeling system broadly agrees with the observations, such as from the GPM-IMERG rainfall products. Model experiments also reveal the impact of increased urban built-up area on the rainfall distribution and the improvement in the model results using high-resolution atmospheric and SST initialization. The findings indicate that even for this synoptic-driven event, representing realistic urban land cover can aid the simulation of the heavy urban rainfall.

Automated summary

The city of Chennai in southern India experienced record-breaking rainfall in 2015, leading to severe urban flooding. The study suggests that this rainfall event was the result of three large-scale systems and the feedback of urban heating. Using the WRF modeling system, simulations were conducted which showed agreement with observations. The study also found that an increase in urban built-up area affected rainfall distribution and that high-resolution atmospheric and sea surface temperature initialization improved model results.