On The Unique Trends between East Asian Cloud Coverage, Urbanization, and Haze

Abstract:

Atmospheric properties may be significantly modified by introduction of urbanization processes, through changes in surface albedo, sensible and latent heat balances, and aerosol emission amounts and distributions. In this study, we choose to focus on East-Asia, as one of the most densely and rapidly economically growing areas, to explore the connection between extensive urbanization processes and cloud properties.

For this purpose, a 14 yearlong surface meteorological observation dataset is co-located with Multi-angle Imaging SpectroRadiometer (MISR) aerosol and cloud product, and with land usage product of the Moderate Resolution Imaging Spectroradiometer (MODIS). We find that in the densely urbanized areas in East Asia, the averaged low-cloud fraction has decreased by ~4% in the recent 10 years. Additional result shows that the low-cloud fraction in East Asia is constantly lower in urbanized areas than it is in rural areas, and that hazy days are characterized by smaller coverage of low clouds. These results, that found to have the opposite trends in any other region of interest, show that in East-Asia, urbanized environment high pollution levels lead to less low-cloud coverage, unlike the expected trend due to the known microphysical (“indirect”) and radiative-dynamical (“semi-direct”) effects of aerosol on clouds.

The observed trend of cloud coverage over urbanized areas in East Asia may influence both radiation budget and precipitation distribution in regional and probably global scales, and therefore should be carefully addressed and assessed. We suggest a combined dynamical and microphysical mechanism to explain this unique trend that is observed in East Asia, and study the optional range of urbanization and climate conditions that may lead to the decrease of cloud coverage due to extensive urbanization processes, using regional and cloud resolving simulations.