Impacts of different urban canopy schemes in WRF/Chem on regional climate and air quality in Yangtze River Delta, China

Yangtze River Delta (YRD) region has experienced a remarkable urbanization during the past 30 years, and regional climate change and air pollution are becoming more and more evident due to urbanization. Impacts of urban canopy on regional climate and air quality in dry- and wet-season are investigated in this paper, utilizing the Weather Research and Forecasting/Chemistry (WRF/Chem) model. Four regimes of urban canopy schemes with updated USGS land-use data in actual state of 2004 base on MODIS observations are examined: (1) SLAB scheme that does not consider urban canopy parameters (the control experiment in this paper); (2) a single-layer urban model with a fixed diurnal profile for anthropogenic heat (UCM); (3) multilayer urban canopy model (BEP-Building effect parameterization); (4) multilayer urban models with a building energy model including anthropogenic heat due to air conditioning (BEP + BEM). Results show that, compared with observations, the best 2-m temperature estimates with minimum bias are obtained with SLAB and BEP + BEM schemes, while the best 10-m wind speed predictions are obtained with BEP and BEP + BEM scheme. For PM10 and ozone predictions, BEP + BEM scheme predicted PM10 well during January, while the best estimate of PM10 is obtained with UCM scheme during July, BEP + BEM and SLAB schemes best estimated ozone concentrations for both the two months. Spatial differences of meteorological factors between canopy schemes and control scheme show that compared with SLAB scheme, BEP and BEP + BEM schemes cause an increase of temperature with differences of 0.5 °C and 0.3 °C, respectively, UCM scheme simulates lower temperature with decrease of 0.7 °C during January. In July, all the canopy experiments calculates lower air temperature with reduction of 0.5 °C-1.6 °C. All the canopy experiments compute lower 10-m wind speed for both January and July. Decreases were 0.7 m/s (0.8 m/s) with UCM, 1.7 m/s (2.6 m/s) with BEP, and 1.8 m/s (2.3 m/s) with BEP + BEM schemes in January (July), respectively. For chemical field distributions, results show that, compared with SLAB scheme, UCM scheme calculates higher PM10 concentration in both January and July, with the differences of 22.3% (or 24.4 μg/m3) in January, and 31.4% (or 17.4 μg/m3) in July, respectively. As large as 32.7% (or 18.3 μg/m3) of PM10 increase is found over Hangzhou city during July. While 18.6% (or 22.1 μg/m3) and 16.7% (or 24.6 μg/m3) of PM10 decreases are fund in BEP and BEP + BEM schemes during January. Compared with control experiment during January, 6.5% (or 2.6 ppb) to 10.4% (4.2 ppb) increases of ozone are computed over mage-cities by canopy experiments. All the three canopy schemes predict lower ozone concentrations and as large as 30.2% (or 11.2 ppb) decrease is obtained with UCM scheme, and 16.5% (6.2 ppb) decrease with BEP scheme during July. The SLAB scheme is suitable for real-time weather forecast while multiple urban canopy scheme is necessary when quantify the urbanization impacts on regional climate.