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

Abstract

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.

Large-scale urbanization effects on eastern Asian summer monsoon circulation and climate

“…Our results, especially the early summer response of monsoon circulation to the urbanization partly agree with Feng et al (2015). However, the simulated later summer responses of the monsoon circulation are similar to results of Shao et al (2013) and Ma et al (2015). Such a discrepancy can be attributed to different urban scenarios, urban parameterization schemes, numerical models, parameters and so on, which implies that there are still large uncertainties in largescale urban impacts on the monsoon system.…”

“…It is worthy noting that Shao et al (2013) suggested that the large-scale changes from natural-surface to high-density urban land cover tend to weaken the summer monsoon, while Feng et al (2015) found that the summer monsoon is slightly strengthened by the large-scale urbanization. Our results, especially the early summer response of monsoon circulation to the urbanization partly agree with Feng et al (2015).…”

“…Ose (1998) also pointed out that the monsoon circulation, diabatic heat source together with the response of the monsoon circulation to diabatic heating exhibit distinct seasonal changes. Shao et al (2013) also showed that the response of the EASM to large-scale urbanization exhibits differences between strong and weak monsoon years. Since the basic states of the atmosphere in summer over this region are distinct from month to month, does the month-based monsoon circulation exhibit different responses to the urbanization-induced thermal forcing?…”

“…However, to what extent can the large-scale urbanization in China alter the land surface thermal forcing, and thus influence the summer monsoon system? Shao et al (2013) recently evaluated possible impacts of urbanization in East Asia on the summer monsoon and precipitation in China using a regional climate model (RegCM3). Their study suggested that the large-scale changes from natural-surface to high-density urban land cover tend to weaken the summer monsoon.…”

“…It is worth noting that the anthropogenic heat and an explicit description of the urban canopy, which were included in the study of Feng et al (2015), were not considered by Shao et al (2013). Besides, there are some evident deficiencies in simulating the planetary scale monsoon circulation system and evaluating the large-scale circulation adjustment induced by urbanization using regional models due to impacts of the fixed lateral boundary conditions.…”