Urbanization Signals In Observational Data Series Of Precipitation And Humidity In China

Urbanization leads to a systematic bias in the observational record of the urban stations,resulting in the main uncertainty in the research of the regional even global climate change monitoring,detection and attribution.In order to deepen understanding of regional and even global climate changes and their possible causes,it is necessary to evaluate the bias of urbanization effect on the data sets,since the data had been extensively used by researchers.At the regional scale,the urbanization effect on the surface air temperature changes in China has been fully understood.However,it is not clear whether there is an obvious local human influence on the observed precipitation and humidity records,which are also the basic surface climate factors.In-depth discussion of the issue is of great importance and immense value for understanding the potential problems of surface climate observational data,the uncertainty of long-term changes of precipitation and atmospheric humidity,and also the characteristics of urban climate change.Most of the related researches have estimated the urbanization effect on observed series based on a comparison of urban and rural/reference stations.However,only urban land use or urban population around the stations have been considered as urbanization indicator to select rural stations.Based on a new method,which is developed to determine the urbanization level around observational sites on varied spatial scales and to classify the stations,China reference surface observational network is formed.Then the reference observational data set of crucial factors,such as surface air temperature,precipitation,and relative humidity are established.The urbanization effect on and its contribution to the trends of total precipitation,extreme precipitation,light precipitation,and relative humidity are evaluated based on a frequently applied data set of observational station network in China for the past six decades.Based on hourly rain gauge observations,the effect of urban heat island on the diurnal variation of summer total precipitation is explored on Wuhan city,China.The results are summarized as follows:1.A new classification method of stations with different urbanization levels is proposed,which can be used to evaluate the urbanization effect on observational surface record.Based on the data set of satellite visible spectral remote sensing,a new method is developed to determine the urbanization level around observational sites on varied spatial scales and to classify the sites into different categories of stations(U1,U2,…,U6),with U1 the least and U6 the largest affected by urbanization.China reference surface observation networks are formed,with the spatial resolution of1.0°*1.0° and 2.0°*2.0°,through multiple specific criteria.2.Quantifying the urbanization effect on the trend of extreme precipitation in the national surface observation network for the period of 1960-2018.A significant positive urbanization effect is found on the long-term changes of the annual and summer total days and amount of extreme precipitation.The urbanization effect on and its contribution to the estimated trends of the standardized anomalies of the annual and summer total extreme precipitation days and amount are about-0.010 /10 yr and 30 %,respectively.The trend of extreme precipitation series has experienced a significant urbanization effect in south China,where the urbanization contributions to the trends of standardized anomalies of the annual total extreme precipitation days and amount reach 22-45 %.3.Quantifying the urbanization effect on the trend of light precipitation in the national surface observation network for the period of 1960-2018.The significant negative urbanization effects are found on the long-term changes of the annual and seasonal total days and amount of light precipitation.The most obvious negative effect appears in the change of trace precipitation,and the urbanization effect on and its contribution to the estimated trends of the standardized anomalies of the annual total trace precipitation days and amount reach-0.079 /10 yr and 27.2 %,respectively.The trend of trace precipitation has experienced a significant urbanization effect throughout southeastern China,especially in central and east China,where the urbanization contributions reach 29.8 % and 23.2 %,respectively.4.Quantifying the urbanization effect on the trend of total precipitation in the national surface observation network for the period of 1960-2018.The significant urbanization effects are found on the long-term changes of the annual days,amount and intensity of total precipitation.The most obvious effect appears in the change of summer total precipitation.The urbanization effect on and its contribution to the estimated trends of the standardized anomalies of the summer total precipitation days,amount,intensity reach-0.007 /10 yr and 7.7 %,-0.004 /10 yr and 12.4 %,-0.017 /10 yr and 16.0 %,respectively.5.It reveals that an obvious impact of urban heat island on the diurnal variation of summer total precipitation in Wuhan city.The peaks of the diurnal variation of the rural and urban summer precipitation events occur at 0900 and 0400(Beijing Time)around Wuhan,China,respectively,and the difference is more obvious under a strong summer urban heat island intensity.The nocturnal water vapor flux convergence in the Jianghan Plain is concentrated around Wuhan urban,after a positive nocturnal urban heat island intensity anomaly is appeared in Wuhan urban.The nocturnal urban heat island intensity enhances atmospheric instability,cooperates with water vapor fluxes convergence caused by mesoscale circulation,it is the reason for the trigger,development,and maintenance of the second half of the night urban events.6.Quantifying the urbanization effect on the trend of relative humidity in the national surface observation network for the period of 1960-2015.The significant negative urbanization effects are found on the long-term changes of the annual and seasonal relative humidity in the national stations,and it reaches-0.074 %/10 yr to the trend of the annual mean relative humidity anomaly for the past 56 years.The urbanization contribution to the estimated trend of the annual mean relative humidity anomaly reaches 93.1 %.Urbanization caused a significant increase in the trend of vapor pressure deficit,and also in the trends of the annual,winter and spring actual vapor pressure.The trend of relative humidity/vapor pressure deficit has experienced an obvious negative/positive urbanization effect in east China,respectively,and the trend of actual vapor pressure has experienced an obvious positive urbanization effect in west China.