Role of Urban Land Use on Mesoscale Circulations and Precipitation

Abstract

A high resolution mesoscale meteorological model (MM5) was employed to study urban effects on rainfall over Oklahoma City, U.S.A. and Chennai, India. Numerical modeling results for Oklahoma City show that urban land use increases the daytime sensible heat flux while it decreases the latent heat flux over the city. Height of the level of free convection (LFC) is reduced by 100 hPa downwind of Oklahoma City because of increased vertical mixing. Increased friction over the city reduced the near surface wind speed by up to 30% as compared to that over the surrounding rural regions during the simulation period. Surface stress over an urban area is shown to increase convergence on the wind ward side of the city and decrease convergence on the lee side of the city. Simulated maximum vertical velocity associated with a line of thunderstorms is enhanced by 1.4 m s-1 by the urban effect of Oklahoma City. Increased rainfall amounts of 25 mm were simulated around 55 km downwind of Oklahoma City.

Rainfall observations and numerical modeling results were used to investigate the impact of Chennai urban land use on the sea breeze circulation and rainfall amounts during the southwest monsoon. Simulated wind speeds show that the urban region of Chennai increases onshore flow associated with the sea breeze by 4.0 m s-1. Inland propagation of the sea breeze front is reduced over and immediately downwind of the city due to higher friction. During the research period, positive vertical velocity is enhanced along the leading edge of the sea breeze front by more than 1.0 m s-1 because of increased low-level convergence over the city. Rainfall amounts were increased up to 25 mm well inland due to urban effects. Observations indicate occurrence of rainfall over the city during late evening and nocturnal hours, possibly due to the interaction between receding sea breeze circulation and the urban heat island. This process could not be simulated due to possible deficiencies in the model physics.