Study Of Precipitable Water Vapor Over China Based On Ground Observations And Satellite Remote Sensing

Based on measurements of the 936 nm water-vapor channel and the 870 nm,and 1020 nm atmospheric window band for the CE-318 sunphotometer of CARSNET,clear-sky direct solar radiation observational data and relative optical air masses of between 2 and 5,and considering sensitivity parameters such as pressure,altitude,wavelength,and AOD936 nm,which may affect the calibration result,a method to calibrate the 936 nm water-vapor channel is presented.The daily average precipitable water vapor(PWV)over urban Beijing was retrieved using three methods(Methods A,B and C),and compared with the AERONET water-vapor product.The calibration results showed the calibration value to differ from the original value by about 4%.The PWV retrieval result showed that AERONET and the three methods of retrieval for PWV were highly correlated.The monthly average variation trends of AOD500 nm and PWV were similar,with the maximum occurring in July,and the minimum in December.The seasonal average variation trends of AOD500 nm and PWV were also similar,but could be placed in the following order: summer > autumn > spring > winter.The seasonal variation of ?440–870 versus AOD500 nm,?440–870 versus PWV,and AOD500 nm versus PWV,showed increasing AOD500 nm with increasing PWV,which reflected the hygroscopicity of aerosol fine particle growth characteristics.Summer was mainly characterized by fine particles;while spring,autumn and winter were a mix of coarse and fine particles,showing typical urban aerosol properties.This study assessed the four PWV products of quality and accuracy at six typical sites in China from 2011 to 2013,including Global Positioning System(GPS)PWV,radiosonde(RS)PWV,MODerate resolution Imaging Spectroradiometer near-infrared(MODIS-NIR)clear PWV and Aerosol Robotic Network(AERONET)sunphotometer PWV.Intercomparison results showed that GPS-PWV and RS-PWV had a slightly higher correlation(R2 = 0.975)at 0000 UTC than that at 1200 UTC(R2 = 0.967).The mean values of Bias,SD,and RMSE between GPS-PWV and RS-PWV(GPS-RS)were-0.03 mm,2.36 mm,and 2.60 mm at 0000 UTC,and-0.23 mm,2.76 mm,and 2.95 mm at 1200 UTC,respectively.This showed that GPS-PWV was slightly lower than RS-PWV,and this difference was more obvious during the nighttime.The MODIS-NIR-Clear PWV product had a similar correlation coefficient(R2 = 0.88)with GPS-PWV compared with RS-PWV.In addition,MODIS-NIR-Clear PWV was greater than GPS-PWV and RS-PWV.The MODIS-NIR-Clear PWV showed a larger deviation from GPS-PWV(MODIS-GPS Bias = 1.50 mm,RMSE = 5.76 mm)compared with RS PWV(MODIS-RS Bias = 0.75 mm,RMSE = 5.31 mm).The correlation coefficients between AERONET-PWV and the PWV from GPS,RS,and MODIS-NIR-Clear were 0.970,0.963,and 0.923(with RMSE of 2.53 mm,3.67 mm,and 4.39 mm),respectively.In the Beijing area,the overall mean bias of the AERONET-PWV product with GPS-PWV,RS-PWV and MODIS-NIR-Clear PWV was-0.09 mm,-1.82 mm,and-1.54 mm,respectively,which shows that the AERONET-PWV product was lower than the other three PWV products.This study analyzed the temporal and spatial distribution of precipitable water vapor(PWV)in China.The analysis of the temporal and spatial distribution showed that the PWV distribution in China has clear geographical differences,and its basic distribution characteristics gradually change from the coast in the southeast to inland in the northwest.Affected by the East Asian monsoon,the PWV over China showed clear seasonal distribution features,with highest values in the summer,followed by autumn and spring,and the lowest values in winter.Overall,the annual mean PWV between 2001 and 2015 are lower than it between 1979 and 1999,the maximum difference areas are located in central China,east China and south China and other regions,with the maximum difference can reach to-3mm.This study reveals the variations in PWV trends over China based on PWV datasets from homogenized radiosonde records during 1979 to 2015 and satellite MODIS observations during 2001 to 2015.Based on radiosonde data,PWV increased significantly(confidence level 95%)for the majority of sites in China during 1979 to 1999,with a maximum China-averaged increasing trend of 0.110 mm yr-1 occurring in summer,and decreased significantly(confidence level 95%)throughout China during 2000 to 2015,with a maximum China-averaged decreasing trend of 0.170 mm yr-1 occurring in winter.Fifteen years of MODIS PWV datasets also showed a similar decreasing trend across the majority of regions in China during 2001 to 2015,with a China-averaged decreasing trend of 0.061 mm yr-1.In addition,it was shown that the magnitude of the annual mean PWV trends for three site types is in the order main urban > provincial capital > suburb over the two periods(1979–1999 and 2000–2015),which may be related to greater anthropogenic activities at the main urban sites.This study analyzes the annual and seasonal trends in surface temperature(Ts)over China from 1979 to 2014,and the relationships between PWV and Ts,using the monthly Ts data at 160 weather stations in China and the monthly Ts gridded data from University of Delaware(UDel).The results revealed the increasing trend in surface temperature was much more prevalent over 1979 to 1999,with the china-averaged increasing of 0.047 ? yr-1(confidence level 95%).While sudden decreases occurred in the most regions of China after 2000,which is consistent with the slowdown in the global warming rate around 2000.The annual mean PWV is well correlated with Ts on the interannual and decadal timescales in China,and regions with positive(negative)PWV trends generally show a warming(cooling)Ts trend.The correlation between MODIS PWV and UDel gridded Ts data during 2001 to 2015 shows large seasonal differences,with the largest correlation coefficient of 0.732(confidence level 95%)occurring in winter and the lowest correlation coefficient of 0.546(confidence level 95%)occurring in spring during 2001 to 2015.