Urban Heat Island Cluster Effect Over The Yangtze River Delta And Its Impact On Ozone

During the past decades, the Yangtze River Delta (YRD) region is experiencing a fast urbanization process and an increasing urban heat island (UHI) effect. Distances between cities are shortening, and interactions of the UHI effect and air pollutants between cities are increasing. Therefore, population and urban area data for the YRD region from the statistical year book and decades of meteorological data are used to investigate the impact of urbanization on temperature and relative humidity and thermal advection between cities over the YRD region. The coupled WRF-BEP (Building Energy Parameter) and air quality model CMAQ (Community Multiscale Air Quality model) is used to investigate urban land-surface forcing on local and downstream city UHI and ozone air quality.In this study, forty-years (from 1970 to 2009) meteorological observation data are used to investigate, by K-Z filter, the impact of urbanization process on air temperature and relative humidity over the YRD region. Results show that urban temperature/relative humidity increase/decrease significantly by urbanization process. Temperature increases are more significant in spring and summer; relative humidity decrease is strongest in summer, weakest in winter. Interactions between nearby cities are investigated, and warm advection from upstream city to downstream city is increased with wind speed after reaching the maximum then decrease with wind speed.In this study, the meso-scale meteorological model WRF is coupled with a multi-layer urban canopy model BEP to investigate the impact of urbanization of Suzhou-Wuxi-Changzhou (SXC) on the UHI effects and the impact of lake breeze from Taihu lake on the UHI effects over SXC cluster. Significant UHI effects appear over the SXC cluster. The UHI intensity is increased from Suzhou to Changzhou when under the control of southeasterly breeze. UHI can significantly increase downstream temperature. During the daytime, the UHI effects of the three cities merged together under the influence of southeasterly breeze, and result in a stronger UHI circulation. During the nighttime, the UHI circulation is weakened by stable and lower boundary layer, which is favorable for the downstream transport of the UHI temperature. The intensity and vertical extension of the UHI over Wuxi and Changzhou are increased by upstream city. During the daytime, a strong lake breeze is formed by intensive temperature gradient between Taihu lake and land, and hence influence the UHI effects over surrounding cities. Near-surface temperature over Wuxi and Changzhou are decreased by 0.49℃ and 0.52℃, respectively, under the influence of lake breeze. And the interaction of UHI effects between the two cities is weakened.Significant urban dry island (UDI) effects occur over SXC. In this case, the UDI effect is weakest over Suzhou and strongest over Changzhou under the influence of upstream water vapor advection. Upward movements of the UHI circulation transport near-surface water vapor to the upper-air, result in a larger water vapor amount in the urban upper-air than that in rural. The impact of Taihu lake on humidity over SXC is relatively small in the near surface but large in the upper-air. Upward transport of water vapor from urban surface is decreased by weaker UHI circulation and lower boundary layer with the appearance of Taihu lake.Impact of urbanization on downward short-wave radiation is small; however, it decreases urban-surface albedo, increases urban-surface absorptive area and net shot-wave radiation flux, decreases the upward short-wave radiation. Downward long-wave radiation flux is strengthened by the UHI effect, the upward long-wave radiation flux is strengthened around noon weakened in the nighttime and early morning. Increases of surface net radiation flux, by urbanization, is significant in the night and early morning (about 20-30 W m-2) but weak in the afternoon (less than 10 W m"2). Surface sensible/latent heat is increased/decreased significantly by urbanization process. Impact of upstream urbanization on the surface shot-wave radiation flux in Kunshan can be negligible. But upward and downward long-wave radiation fluxes in Kunshan are increased by upstream effect. Under the influence of upstream urbanization, sensible heat flux in Kunshan is slightly decreased; heat storage is increased but has little effect on latent heat.Temperature over Kunshan is increased by Shanghai’s urbanization; the magnitude of temperature increase depends on the UHI intensity and UHI circulation in Shanghai. Although the UHI intensity is strong in the afternoon, the strong UHI circulation inhibits the downstream transport of warm air. However, the warm advection is strengthened by weaker UHI circulation in the nighttime, result in a 2.5℃ temperature increase over Kunshan.During the daytime, near-surface O3 concentration over Shanghai is increased about 10 ppb by urbanization process. While O3 concentration on the top of boundary layer is increased as much as 40 ppb. Urbanization process increases the urban boundary layer and vertical mixing over Shanghai, and hence increases the upward transport of near-surface NOx, result in a decrease of near-surface NOx concentration and increase of upper-air NOx concentration. As a result, O3 titration by high concentration NOx is decreased in the lower boundary layer, and O3 chemical production is increased in the upper-air. In the nighttime, O3 concentration is slightly increased in the lower boundary layer but decreased in the upper-air by urbanization.In the daytime, O3 concentration in the middle and lower boundary layer of Kunshan is slightly decreased by upstream urbanization, but O3 concentration in the upper-air of Kunshan is significantly increased. Horizontal advection of air pollutants from Shanghai to Kunshan are inhibited by strong UHI circulation in the lower boundary layer, thus lower NOx and VOCs concentrations in Kunshan result in lower O3 concentration. In the upper-air, southeasterly breeze is strengthened by strong UHI circulation, and hence increase th& horizontal advection contribution of O3 and its precursors from Shanghai. As a result, O3 chemical production in the upper-air of Kunshan is increased. In the nighttime, upstream urbanization increase near-surface O3 and decrease upper-air O3 over Kunshan.