Cloud Lifetime Effects Of Aerosols Based On Long-term Satellite Observations Over The East Coast Of The United States

Climate forcing associated with aerosol-cloud interactions has been singled out as contributing the largest uncertainty to total radiative forcing estimates,and much of such uncertainty comes from the controversy about the lifetime effect of aerosols,especially for the marine low-level clouds.The main reasons for the large uncertainty above are mainly in two aspects: First,cloud condensation nuclei(CCN,aerosol particles on which water vapor condenses)is difficult to observe directly,and different CCN proxy will have a significant impact on the strength of cloud lifetime effects;secondly,changes in both aerosols and meteorology will affect clouds,so it is difficult to separate the effects of aerosols from meteorology.Long-term satellite observations provide a good opportunity to explore the cloud lifetime effects of aerosols.Based on the long-term(2003-2018)MODIS satellite observations and reanalysis data,we explored the strength of the cloud lifetime effect of aerosols over the East Coast of the United States(ECUS)for the first time with more reasonable CCN proxy and a statistical model to separate the aerosol effects from meteorology.With the regression model,we identified four crucial cloud-controlling parameters over ECUS,including cloud droplet number concentrations(N_d,the CCN proxy),cloud geometrical thickness(CGT),lower tropospheric stability(LTS),and relative humidity at 950 h Pa(RH₉₅₀).These four factors explained most of the cloud cover variability in ECUS.The cloud cover increases with the increase of N_d,CGT,LTS,and RH₉₅₀.N_d plays an important role in regulating the dependence of cloud fraction on RH₉₅₀.In a relatively clean environment,the cloud cover almost does not change with the increase of RH₉₅₀.This is because the probability of precipitation increases with the increase of RH₉₅₀ in a clean environment,and the generation of enough heavy precipitation will make the cloud broken and reduce the amount of clouds.However,in the relatively polluted environment,where the N_d is large,the effective radius of cloud droplet is small,then,precipitation is suppressed.Finally,the cloud cover increases significantly with the increase of RH₉₅₀.The decrease of cloud cover caused by the decreased N_d(a proxy for cloud condensation nuclei,CCN)is offset by the increase of cloud cover caused by the increase of RH₉₅₀,finally leading to no obvious change of cloud cover over ECUS.Anthropogenic emissions of the east United States are significantly reduced,resulting in a significant reduction in N_d over ECUS.The enhancement of the low-level warm air advection in this area increases the inversion strength in the lower part of the marine boundary layer,and the inversion layer traps more water vapor in the lower part of the boundary layer,leading to the increase of RH₉₅₀.The CGT and LTS show no obvious changes.With the regression model,we found that the observed annual-mean CF shows no significant trend due to the cancellation from the opposite trends in N_d and RH₉₅₀.Based on the long-term satellite observations,reanalysis data,and regression model,we found that the effect of aerosol on cloud cover is significant.When other meteorological factors are fixed,the decrease of 40% in N_d should lead to a decrease of 22% in cloud cover.The susceptibility of cloud cover to N_d [defined as dln(CF)/dln(N_d)] is 0.55.Based on the long-term satellite observation data,we found that the cloud lifetime effect of aerosol is significant.Our results show that it is necessary to consider the influence of N_d when studying the low-level cloud feedback based on observations.