On The Characteristics Of Aerosol Indirect Effect Based On Dynamic Regimes In Global Climate Models

With the fast developing of our society,the emission of anthropogenic aerosols is getting growing more and more.The emission of anthropogenic aerosols can affect our life a lot.Besides the severe air condition problems,it also brings deep influences to our climate system of the earth.Aerosols can alter the Earth’s radiation balance via interactions with clouds.It is important because it can offset much of the global warming induced by anthropogenic greenhouse gases.However,aerosol-cloud interactions continue to constitute a major source of uncertainty for the estimate of climate radiative forcing.Various scales of involving physical processes and the complexity of microphysical-dynamical-radiative feedbacks make the aerosol climate effect hard to predict.A large number of processes are involved in the chain from emissions of aerosol precursor gases and primary particles to impacts on cloud radiative forcing.Those processes are manifest in a number of relationships that can be expressed as factors dlnX/dlnY driving aerosol effects on cloud radiative forcing.These factors include the relationships between cloud condensation nuclei(CCN)concentration and emissions,droplet number and CCN concentration,cloud fraction and droplet number,cloud optical depth and droplet number,and cloud radiative forcing and cloud optical depth.Uncertainties can arise from uncertainty in the limitations of the formulations of the physical processes represented in the model(structural uncertainty).In this study,we investigate aerosol indirect effect(AIE)uncertainties in global climate models with several different physical parameterizations(so-called structure uncertainty).It is found every process contribute almost equally to the model uncertainties in estimating AIE.This indicates that we need to do something further to study AIE,for example,under some specific cloud regimes,or on different dynamic regimes.Aerosol,clouds,precipitation distributions and dynamical feedbacks are all related to the prevailing meteorological environment.However,few works have paid enough attention to analyze the relationship between aerosol indirect effects and dynamic regimes.The variation of aerosol indirect effects(AIE)in climate models is investigated across different dynamical regimes,determined by monthly mean 500 hPa vertical pressure velocity(ω500),lower-tropospheric stability(LTS)and large-scale surface precipitation rate derived from several global climate models(GCMs),with a focus on liquid water path(LWP)response to cloud condensation nuclei(CCN)concentrations.The LWP sensitivity to aerosol perturbation within dynamic regimes is found to exhibit a large spread among these GCMs.It is in regimes of strong large-scale ascent(ω500<-25 hPa/d)and low clouds(stratocumulus and trade wind cumulus)where the models differ most.Shortwave aerosol indirect forcing is also found to differ significantly among different regimes.Shortwave aerosol indirect forcing in ascending regimes is close to that in subsidence regimes,which indicates that regimes with strong large-scale ascent are as important as stratocumulus regimes in studying AIE.It is further shown that shortwave aerosol indirect forcing over regions with high monthly large-scale surface precipitation rate(>0.1 mm/d)contributes the most to the total aerosol indirect forcing(from 64%to nearly 100%).Results show that the uncertainty in AIE is even larger within specific dynamical regimes compared to the uncertainty in its global mean values,pointing to the need to reduce the uncertainty in AIE in different dynamical regimes.