Numerical Simulation of the Impact of Urban Non-uniformity on Precipitation

To evaluate the influence of urban non-uniformity on precipitation, the area of a city was divided into three categories （commercial, high-density residential, and low-density residential） according to the building density data from Landsat satel- lites. Numerical simulations of three corresponding scenarios （urban non-uniformity, urban uniformity, and non-urban） were performed in Nanjing using the WRF model. The results demonstrate that the existence of the city results in more precip- itation, and that urban heterogeneity enhances this phenomenon. For the urban non-uniformity, uniformity, and non-urban experiments, the mean cumulative summer precipitation was 423.09 mm, 407.40 mm, and 389.67 mm, respectively. Urban non-uniformity has a significant effect on the amount of heavy rainfall in summer. The cumulative precipitation from heavy rain in the summer for the three numerical experiments was 278.2 mm, 250.6 mm, and 236.5 mm, respectively. In the non- uniformity experiments, the amount of precipitation between 1500 and 2200 （LST） increased significantly. Furthermore, the adoption of urban non-uniformity into the WRF model could improve the numerical simulation of summer rain and its daily variation.

Enhancement and Removal of PM_(2.5)by Cold and Warm Air Masses Accompanying Certain Synoptic Weather Systems across Hangzhou,China during 2014-2020

Local air pollution is strongly affected by synoptic weather systems,such as fronts,troughs,low-altitude vortices,or high-altitude ridges.Nevertheless,few studies have analyzed the meteorological properties of cold or warm air masses associated to these systems and their impact on local air quality.In this study,hourly observations of fine particulate matter(diameter of up to 2.5µm,i.e.,PM_(2.5)),wind(V),temperature(T),pressure(P),and precipitation(R),acquired in Hangzhou in 2014-2020,were analyzed.From this analysis,weather patterns were categorized into 27 types;89 and 94 cases illustrating the passage of warm and cold air masses over Hangzhou were identified,respectively;the influence of air mass temperature,wind speed,and wind direction on PM_(2.5) concentrations and local accumulation or removal was quantified.The main results are as follows.(1)Pollution events occurred more frequently for cold than for warm air masses,but average pollutant concentration was lower for cold air masses;(2)48%of the cold air mass cases corresponded to PM_(2.5) decreases and 52%to PM_(2.5) increases,with strong cold air masses(ΔT_(24h)>4°C;∣V∣_(average)>4 m s^(−1))markedly reducing local pollution,but weak cold air masses(ΔT24h<2°C;∣V∣_(average<)2 m s^(−1))primarily inducing pollutant transport and accumulation;(3)for warm air masses,PM_(2.5)accumulation or removal occurred in 60%and 40%of the cases,respectively:warm air masses(ΔT24h>4°C)reduced the PM_(2.5) concentration whereas weaker winds(∣V∣_(average)<2 m s^(−1))increased it;and(4)PM_(2.5) concentration decreased sharply within 4 h after the passage of strong cold air masses,but more gradually within 14 h after the passage of strong warm air masses.These results considerably improve the current understanding of the influence of cold and warm air masses on local pollution patterns.

Advances in Urban Meteorological Research in China

Over the past decades,a large number of studies have been carried out in the field of urban meteorology in China.This paper summarizes the main progress in urban meteorology research from four aspects:urban meteorological observation network and field campaign,multi-scale model of urban meteorology,interaction between urban meteorology and atmospheric environment,and the impacts of urbanization on weather and climate.Major advances are as follows.China’s major cities have established or are improving comprehensive urban meteorological observation networks characterized by multi-platform,multi-variable,multi-scale,multi-link,and multi-function.Beijing,Nanjing,Shanghai,and other cities carried out urban meteorological field campaigns,which were included in the WMO research demonstration project.Wind tunnel experiments and scale-model outdoor experiments were successfully conducted.Multi-scale urban meteorological and air quality prediction numerical model systems have been developed and put into operational use.The urban heat island effect;urban impacts on precipitation,regional climate,and air quality;urban planning;and interaction between urban meteorology and atmospheric environment are extensively investigated.Finally,efforts to improve observational technology,data assimilation,and urban system modeling,to explore the impacts of urbanization on environment and human health,and to provide integrated urban hydro-meteorological climate and environmental services are planned ahead.

The Aerosol Radiative Effect on a Severe Haze Episode in the Yangtze River Delta

Due to increased aerosol emissions and unfavorable weather conditions, severe haze events have occurred fre- quently in China in the last 10 years. In addition, the interaction between the boundary layer and the aerosol radiative effect may be another important factor in haze formation. To better understand the effect of this interaction, the aero- sol radiative effect on a severe haze episode that took place in December 2013 was investigated by using two WRF- Chem model simulations with different aerosol configurations. The results showed that the maximal reduction of re- gional average surface shortwave radiation, latent heat, and sensible heat during this event were 88, 12, and 37 W m2, respectively. The planetary boundary layer height, daytime temperature, and wind speed dropped by 276 m, I^C, and 0.33 m s-l, respectively. The ventilation coefficient dropped by 8%-24% for in the central and northwestern Yangtze River Delta （YRD）. The upper level of the atmosphere was warmed and the lower level was cooled, which stabilized the stratification. In a word, the dispersion ability of the atmosphere was weakened due to the aerosol radi- ative feedback. Additional results showed that the PM2.5 concentration in the central and northwestern YRD in- creased by 6-18 p.g m3, which is less than 15% of the average PM2.5 concentration during the severely polluted peri- od in this area. The vertical profile showed that the PM2.5 and PM10 concentrations increased below 950 hPa, with a maximum increase of 7 and 8 gg m-3, respectively. Concentrations reduced between 950 and 800 hPa, however, with a maximum reduction of 3.5 and 4.5 p.g rn-3, respectively. Generally, the aerosol radiative effect aggravated the level of pollution, but the effect was limited, and this haze event was mainly caused by the stagnant meteorological condi- tions. The interaction between the boundary layer and the aerosol radiative effect may have been less important than the large-scale static weather conditions for the formation of this haze episode.

Development of CMAQ for East Asia CO_2 data assimilation under an EnKF framework: a first result

Under an Ensemble Kalman Filter(EnKF)framework,Regional Atmospheric Modeling System and Models-3 Community Multi-scale Air Quality(RAMS–CMAQ)modeling system is developed to be a CO2data assimilation system EnKF–CMAQ,and the EnKF–CMAQ system is then applied to East Asia for validation with real continuous surface CO2concentration observations available in the study domain instead of using an observation simulation system experiment.Experiments with an experimental period of January 23 to February 7,2007 are conducted,and the experimental results of the EnKF–CMAQ system and the RAMS–CMAQ model are compared against continuous surface CO2observations from assimilation sites and independent reference sites.Distributions of daily mean CO2concentration increments show that the EnKF–CMAQ system confines the update of daily mean CO2within areas nearby and downwind of the assimilation sites.Both the CO2concentration ensemble spreads and background error covariances show flow-dependent patterns.The results indicate the crucial role of wind transport in the CO2data assimilation,which agrees with the previous studies.The average bias and the average root-mean-square error(RMSE)of daily mean CO2concentration at the assimilation sites are reduced by 1.00 and1.83 ppm,respectively,and those at the reference sites are reduced by 0.24 and 0.22 ppm,respectively.The results demonstrate the EnKF–CMAQ system is capable of assimilating the continuous surface CO2concentration observations to improve the simulation accuracy of the atmospheric CO2synoptic variation.Since growing CO2observations over East Asia are being available nowadays,this work is our first step to generate consistent spatial and temporal atmospheric CO2concentration fields over East Asia,particularly over China,using both in situ and satellite observations.