Satellite Remote Sensing Research Of Above Cloud Aerosols And Their Impacts On Low Clouds

Above-low-level-cloud aerosols(ACAs)and their effects on low-level-cloud(LC)have gradually gained more interests in recent years.However,their combined aerosol-cloud radiation effects are not well understood,resulting from the lack of comprehensive and accurate retrieval of aerosols and clouds for ACA scenes,thus introducing uncertainties in understanding and evaluating ACA influence on the macrophysics and microphysics of LC.Based on the multi-satellite observations,this paper presents a new retrievals of ACA and their possible impacts on LC.This paper main research contents are as follows:(1)An improved ACA identification and retrieval methodology was developed by combining three-channel CALIOP(The Cloud-Aerosol Lidar with Orthogonal Polarization)observations.By using the 1064 nm observations,the new method can reliably identify and retrieve both thin and dense ACA layers.Moreover,providing consistent results between the day-and night-time retrieval of ACAs by developing new muti-scale smoothing algorithm.Then,a new four-year(2007 to 2010)global ACA dataset was built,and new seasonal mean views of global ACA occurrence and optical depth(OD)were presented and analyzed.Further discussions on the relative position of ACAs to LCs showed that the mean distance between the ACA layer and the LC deck over the Atlantic is less than 0.2 km.This indicates that the ACAs over this region are more likely to be mixed with LCs,thereby possibly directly influencing the cloud microphysics over this region,contrary to greater than 1 km and semi-direct effect in previous studies.(2)Based on the new global ACA dataset from 2007 to 2010,this study discussed the relationship between ACA OD,ACA occurrences and the LC color ratio(CR)over the Atlantic,to analyze the possible impact of the ACA layer on the LC layer.The seasonal mean distribution of integrated attenuated CR(IACR)show that,the IACR of LC is 30%-50%larger over smoke region in smoke outbreak seasons than other regions or seasons.However,the IACR of LC over dust region shows small difference between dust outbreak seasons and other seasons.It indicates that above-LC smoke aerosol can introduce stronger color effect than dust.This has guiding significance for passive retrieve of ACA and LC.Further analysis of the relationship between ACA OD and the integrated corrected CR(ICCR)show that ICCR of LC tends to decrease with increasing above-LC dust OD,while the LC ICCR shows weak relationship with above-LC smoke OD.Therefore,the above-LC dust aerosol could introduce stronger microphysics effect than smoke,that is,the LC droplet size may decrease with increasing burden of dust aerosol above.(3)Based on the global daily-averaged satellite data from 2007 to 2010,this study compared and analyzed the relationship between cloud top temperature,cloud thickness and liquid cloud-water path(LWP)at the same mid-high latitudes in the Northern and Southern Hemispheres,to analyze the aerosol impacts on LC LWP.Results show that under the same cloud top temperature,the average thickness of the non-precipitation water-phase LC in the Northern Hemisphere is significantly smaller than that in the Southern Hemisphere,and the average LWP is larger than that in the Southern Hemisphere.The average thickness and LWP of the non-precipitation mixed-phase LC in the Northern Hemisphere are close to those in the Southern Hemisphere.The seasonal differences in similar non-precipitating LC LWP between Northern and Southern Hemispheres are further compared.Results show that aerosol has little effect on the LWP of the non-precipitation mixed-phase LC and has a greater impact on the water-phase LC.Results also show that the effect of aerosol on water-phase LC LWP is very obvious in the summer and autumn when the aerosol content in the Northern Hemisphere is small,resulting the average LC LWP 8.3 g/m2 larger than that in the Southern Hemisphere.However,in winter and spring when aerosol content is high,the influence of aerosol on LC LWP is small,may resulted by the multiple aerosol-cloud interactions.The study also found that the precipitation rate of the mid-high latitudes LC in the Northern Hemisphere is about 19%higher than in the Southern Hemisphere region.The results not only help us better understand global aerosol transportation and aerosol-cloud interactions but also provide useful information for model evaluation and improvements.