Sensitivity of the East Asian Summer Monsoon Circulation and Precipitation to an Idealized Large-Scale Urban Expansion

A numerical simulation was performed here using a regional climate model (RegCM3) at 60-km horizontal resolution to estimate the maximum impact of urbanization in East Asia on the summer monsoon precipitation in China. A new bulk urban parameterization was added into the land-surface scheme to capture the thermal, dynamic, and hydraulic effects of urban land cover. The 9-year simulation results showed that the large-scale change from natural-surface to high-density urban land cover induced an enhancement in the southwesterly wind speed in the early monsoon season across eastern China. In July and August, the enhanced southwesterlies moved to northern China, while a weakened southwesterly wind speed appeared south of the Yangtze River valley (YRV). Meanwhile, a large reduction in rainfall occurred in southern China with most reduction accounted for by convective precipitation reduction. This corresponded to the reduced water-vapor supply from the weakened southwesterlies in southern China, decreased convergence in the YRV, reduced convective instability in the lower troposphere, and lessened evapotranspiration from the impervious urban surface. Accompanied by the increased southwesterlies in the west and northwest of the urban region, a moderate increase in rainfall occurred in northern China. The changes in southwesterly wind speed and rainfall in southern China were more evident in the weak East Asian summer monsoon (EASM) years than in the strong ones. This underlines the impact of change in large-scale intensive urban land use on regional climate in the weak phase of the EASM. As fractional urban land cover was reduced, the changes in wind and precipitation were still shown but the spatial coverage and magnitude of these changes were reduced.