Observed and model simulated thermodynamic processes associated with urban heavy rainfall events over the Bangalore city, India

by P Ajilesh, V Rakesh, Sanjeeb K Sahoo and S Himesh

In this study, 32 rainfall events spanning from 2012 to 2014 over the urban Indian city, Bangalore were simulated using the Weather Research and Forecast (WRF) model. Model simulations were carried out with a four‐nested domain initialized with Global Forecast System (GFS) data and the forecast was generated on an hourly basis. The forecasted rainfall at hobli‐level (Bangalore has 34 hobli divisions with an area of each hobli of the order of ~10 km2) was evaluated in terms of their intensity and pattern of spatial distribution by comparing with corresponding rain‐gauge observations. Also, the rainfall forecast skill of the model was evaluated statistically by computing Root Mean Square Error (RMSE), Bias, and Mean Absolute Error (MAE). Thermodynamic variables like Equivalent Potential Temperature, Convective Available Potential Energy (CAPE), Convective Inhibition (CIN), K Index (KI), Lifted Index (LI), and Total Totals Index (TTI) were also derived from simulated model parameters for all the events and verified against corresponding observations. Results showed that the WRF model could simulate the rainfall events and associated thermodynamic features qualitatively; however, there were few hoblis where the relative errors in the forecast were more than 100%. The forecast errors were relatively lower for cases during the south‐west monsoon season compared to other seasons. It was found that the model underestimated thermodynamic indices like CAPE, dew point depression and the simulated LI were positive; these were indicative of model's limitation in simulating intense convection and a possible reason for underpredicted rainfall simulations.


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