Impacts of Urbanization, Aerodynamic roughness and Land Surface Processes on the Extreme Heavy Rainfall over Chennai, India

In this study, the impacts of urban land surface processes on the extreme heavy rainfall event on 01 December 2015 over Chennai, located in north coastal Tamil Nadu, India are analyzed using convection permitting WRF simulations. A series of numerical experiments are conducted using different land cover data (USGS-1992, NRSC-2004, NRSC-2015), aerodynamic roughness, and land surface models (LSM) to assess their sensitivity on the predicted rainfall. Results suggest that experiments with NRSC-2015 with increased urban extent improved the rainfall prediction in terms of rainfall intensity and its distribution. Though temperatures, sensible heat, and Planetary Boundary Layer height (PBLH) increased due to urbanization in both dry and wet phases, the humidity and Convective Available Potential Energy (CAPE) reduced during the dry phase suggesting thermal convection played a secondary role in rainfall. Considerable increase of surface drag, momentum transport, wind shear and Turbulent Kinetic Energy are found in simulations with updated land use and roughness, which determined the location of the cyclonic circulation, convergence and maximum precipitation. LSM sensitivity experiments indicated that while the five-layer model substantially increased the sensible heat, temperature and PBLH, it reduced the moisture convergence and CAPE relative to Noah and Noah-MP thus resulting in low rainfall. The simulation with Noah-MP enhanced the low-level shear and convergence over other LSMs thus produced a wide spread rainfall along the coast. Our results demonstrated that the momentum transport due to urban drag played a vital role by strengthening the low-level convergence and moist convection, which caused heavy precipitation over Chennai.