Observational Study On Cloud Identification And Its Radiative Effects At Typical Stations In Northwest China

About 2/3 of the total global surface area is covered by clouds.On the one hand,by directly reflecting and scattering solar shortwave radiation,absorbing and emitting longwave infrared radiation,clouds regulate the radiative budget distribution of the atmosphere;On the other hand,it also cloud affect the transport of water vapor,vertical convection and cooling and condensation processes in the atmosphere,and change the intensity,frequency and distribution of precipitation,which in turn has important effects on the radiative energy balance of the global Earth-atmosphere system,the water cycle process and climate change.Cloud radiative forcing can characterize this effect,and it mainly depends on the macroscopic and microscopic physical parameters of the cloud,such as cloud amount,cloud type,cloud height,cloud water content,cloud phase state,cloud optical depth and so on.The physical parameters of clouds are mainly detected by technical means such as millimeter-wave cloud radar or hyperspectral lidar,but these ground-based observation sites with advanced instruments are unevenly distributed in my country,especially in the vast northwestern region,which is more scarce,making the results of numerical climate models more uncertain.Based on the 1-minute high-precision surface radiation flux and meteorological element data of five typical sites in Northwest China(Lanzhou,SACOL,Minqin,Dunhuang,and Minfeng)from 2007 to 2019,two effective methods for identifying clear sky and cloudy are proposed from the perspectives of shortwave radiation and longwave radiation respectively.The two methods were used to develop the empirical fitting relationship of solar shortwave radiation and downward longwave radiation at the surface under clear sky conditions.Then,the downward shortwave and longwave radiative forcing of cloud in the northwest of our country are calculated.Based on the clear sky shortwave radiation fitting and cloud cover observation data,a cloud cover fitting relationship suitable for different sites was established.Finally,the correlations between cloud cover and shortwave/longwave forcing in this region are analyzed.(1)The parameters in the shortwave radiation(SW)clear sky detecting step are formulated according to the clear sky SW and diffuse radiation flux,diffuse ratio,and solar zenith angle observation data of each site,including the upper and lower limits of the normalized total shortwave radiation(SW?max,SW?min),the maximum diffuse radiation flux threshold(D_max)and the clear sky standard of standard deviation of normalized diffuse ratio(Ratio?std).The sand and dust weather in Minfeng is frequent,so the detecting parameters are obviously different from those of the other four stations.Thresholds for SW?max,SW?min(solar zenith angle less or greater than 80°),D_max and Ratio?std are 1150 Wm^-2,930/850 Wm^-2,300 Wm^-2 and 0.002respectively;The clear sky judgment conditions of Lanzhou and SACOL are relatively similar,and the corresponding detecting parameters are 1250 Wm^-2,1000/900 Wm^-2,250 Wm^-2 and 0.002;Minqin and Dunhuang sites have relatively clean atmosphere,and the detecting parameters are set separately for 1250 Wm^-2,1000/900 Wm^-2,150 Wm^-2 and 0.0015;(2)The LW fluctuation limit and the temperature difference(the difference between the air temperature and the effective sky brightness temperature)limit are two criteria for clear sky detecting of longwave radiation(LW).The LW measurements at the Lanzhou,Minqin and Dunhuang sites under clear sky conditions during the day show significantly higher fluctuations than at night,and the temperature difference also shows a large difference.Taking Lanzhou in July 2014 as an example,the daily average LW fluctuation(1.43)was about three times that of the nighttime(0.47),and the average temperature difference between the daytime(-0.08)and the nighttime(-5.45)was 5.37K.In order to improve the detecting accuracy,we use the daytime clear sky selected by the SW detecting method,the LW variation range of the clear sky,the statistical exclusion of the corresponding time and the percentage standard to formulate the LW detecting curve(detecting conditions);(3)For the SW clear sky fitting coefficient and the LW clear sky fitting coefficient(k)of the months close to summer(from May to September,divided according to the diurnal variation characteristics of k values in each month),the fitting coefficients for cloudy days are obtained by interpolating or averaging the clear sky coefficient.Different from the months close to summer,the k in the months close to winter(from October to the next April)varies greatly in the three periods of the day(daytime,sunset to midnight,and midnight to sunrise),so diurnal variation curve of the k as the clear sky fitting coefficient of the current month needs to be calculated.Finally,based on these clear sky fitting coefficients,the continuous downward SW and LW clear sky radiations at the surface are obtained;(4)From the seasonal average point of view,the maximum value of Lanzhou City's surface downward SW forcing appears in summer(-96 Wm^-2),and the minimum value appears in winter(-26 Wm^-2),and it shows that in spring and summer are obviously higher than in autumn and winter.The downward LW forcing has similar forcing effects in spring,summer and autumn,but smaller in winter,with the largest increase in spring(33 Wm^-2)and the smallest increase in winter(19 Wm^-2).The daily mean value of downward SW/LW forcing at Minqin Station in May 2010(-106.8/27.5 Wm^-2)was significantly higher than that in June(-77.1/21.8Wm^-2);Dunhuang's in April 2012(-50.3/18.6 Wm^-2)was the largest,followed by May(-46.9/16.1 Wm^-2)and the smallest in June(-43.5/13.8 Wm^-2);(5)The process of establishing the cloud cover fitting relationship is mainly the elimination of two types of outliers.The first is the cloudy case with large cloud optical thickness,which is manifested by the non-uniqueness of the normalized diffuse radiative forcing(Dn)where the cloud cover(Scv)equals 1.The other is due to the misiudgment of the inversion of TSI.It is shown that when Scv?1,Dn is small and Scv is super high.They can be eliminated through the constraints of cloudy days with thick cloud optical thickness(mean diffuse ratio(15min,the same below)>0.9;Dn<0.30;standard deviation of diffuse ratio<0.05)and linear coarse elimination and curve fine removal;(6)No matter in terms of monthly seasonal changes,daily average changes,or spatial comparisons,the daytime downward SW and LW forcing effects have a good positive correlation with cloud cover.We fit the quantitative relationship between the daily mean values of the downward SW and LW radiative forcing and cloud cover in four seasons of Lanzhou City.The SW forcing effect of cloud cover in spring and summer is significantly stronger than that in autumn and winter,and the fitting coefficients(-238.8,-291.9;1.31,1.38)are significantly higher than those in autumn and winter(-169.3,-133.7;1.11,0.96);The LW forcing effect of cloud cover is the strongest in summer(75.0;1.34),followed by spring and autumn(66.4,69.0;1.07,1.03),and the weakest in winter(46.6;0.84).