Urban effects on the convergence and convection over Chennai, India

Abstract. The MM5 mesoscale model is used to investigate the urban effects on the sea breeze-induced convergence and convection along India's east coast. Observed and simulated wind directions over Chennai (13.0N, 80.2E) show a well developed sea breeze circulation on 28 June 2003. Ground temperatures were 5° K warmer in the urban area than in the surrounding region. High resolution numerical simulations were performed over the region with and without the urban area. Simulated surface sensible heat fluxes increase by 50 W m-2 (20%) during the day over the city. The increased heating in the urban area of Chennai is shown to enhance simulated onshore flow associated with the sea breeze along the coast by up to 4.0 m s-1 to 8.0 m s-1. Positive vertical velocity along the sea breeze front is also increased by up to 30 cm s-1 to 140 cm s-1. Radar imageries of the sea breeze front indicate the inland progression of the sea breeze front is slowed over the city. Simulated surface wind speeds are in agreement with these observations. A simulated cross section of the sea breeze reveals onshore wind speeds at the surface over Chennai and the offshore return flow aloft are both reduced by up to 3.0 m s-1 by the increased friction over the urban area. Inland precipitation is enhanced by more than 20 mm over a region of 300 km2 by including the urban effect in the model simulation.