A Satellite-based Study Of The Effect Of Aerosols On Warm Marine Clouds And Precipitation

Aerosol-cloud interactions affect the global energy budget.The effective radiative forcing from aerosol-cloud interactions remains the largest uncertainty in our estimate of anthropogenic radiative forcing.Long-term aerosol and cloud satellite observations help us to understand the cloud response to changes in aerosol loading from the perspective of long-term synergistic change,while instantaneous satellite observations can be used to quantitatively analyze key processes in aerosol-cloud interactions.In the context of a widely spatial variation of aerosol concentrations,previous studies lack comprehensive analysis of changes in aerosol and cloud properties in long-term scale,especially for synergistic changes in cloud droplet number concentration(CDNC)and aerosol.In the analysis of quantitative indicators based on instantaneous observations,there is currently lack of uncertainty study on the quantitative indicators themselves.For instance,previous satellite-based studies show large discrepancy in estimates of precipitation susceptibility(referring to the sensitivity of precipitation to aerosol).Based on A-Train satellite products,this study comprehensively analyzes spatiotemporal distributions of aerosol and cloud properties over the past 15 years,focusing on the synergistic change in CDNC and aerosol index(AI)at both global and regional scales.Finally,this study systematically compares precipitation susceptibilities among different metric and different satellite products,and further explores how precipitation susceptibility might be different under different atmospheric stability conditions and different rain types.This study shows that aerosol properties(including aerosol optical depth(AOD),angstrom exponent(AE)and aerosol index(AI))significantly decrease over the northern Atlantic and the east coast of China during the period 2003-2017.Over the same region,CDNC reduces with time,and the change in liquid water path(LWP)is not evident,which likely mean that the role of cloud lifetime effect(the response of LWP to aerosol change)may not be significant on the period we studied.We also find that,for a given AOD value,due to the significant increase in proportion of large particle aerosols,CDNC for global ocean warm clouds decreases and cloud effective radius(CER)increases.In addition,over three key regions(eastern coast of US(EUS),western coast of Europe(WEU)and eastern coast of China(EC)),PDFs of CDNC,CER and COT all shift from the first five years(2003-2007)to the last five years(2013-2017).CDNC at low values occurs more frequently while the occurrence frequency of large CDNC decreases.The opposite pattern is seen for CER.Occuraance frequency of high COT(>30)strikingly decreases.With pollution redction over the three regions,clouds with low COT and high CER occur more often,while clouds with high COT and low CER occur less often.By conducting a comprehensive analysis of comparison of precipitation susceptibility among the six different metrics and multiple satellite products,we find that SPOP(precipitation frequency susceptibility)demonstrates relatively robust features throughout independent LWP products and diverse rain products.In contrast,the behaviors of S:(precipitation intensity susceptibility)and SR(precipitation rate susceptibility)are subject to LWP or rain products used to derive them.Aerosol products with different spatial resolutions(i.e.,10km vs.1°)have less impact on SX_AI(precipitation susceptibility with respect to AI),which suggests that aerosol layers over oceans are relatively homogeneous.In addition,the results show that the discrepancy in magnitude between Sx_AI and SX_CDNC(precipitation susceptibility with respect to CDNC)can be mainly attributed to the dependency of CDNC on AI.On the global scale,our results show that Sx_AI is about one-third of Sx_CDNC.This relationship is more applicable to SPOP and is less applicable to SI.In addition,SR?SI +SPOP is generally true for different LWP products and over different stable conditions.The results of this study indicate that SPOP strongly depends on atmospheric stability,with larger values under more stable environments.This dependence is true throughout different satellite products and different rain types,but is less robust for SI.Our results show that precipitation susceptibility for drizzle(with a-15dBZ rainfall threshold)is significantly different than that for rain(with a OdBZ rainfall threshold).Onset of drizzle is not as readily suppressed in warm clouds as rainfall,whereas drizzle intensity is more susceptible to aerosol perturbations than rain intensity.