Cloud-system resolving model simulations of aerosol indirect effects on tropical deep convection and its thermodynamic environment

摘要

This paper presents results from 240-member ensemblesimulations of aerosol indirect effects on tropical deep convection and itsthermodynamic environment. Simulations using a two-dimensional cloud-systemresolving model are run with pristine, polluted, or highly polluted aerosolconditions and large-scale forcing from a 6-day period of active monsoonconditions during the 2006 Tropical Warm Pool – International CloudExperiment (TWP-ICE). Domain-mean surface precipitation is insensitive toaerosols primarily because the large-scale forcing is prescribed anddominates the water and static energy budgets. The spread of thetop-of-atmosphere (TOA) shortwave and longwave radiative fluxes amongdifferent ensemble members for the same aerosol loading is surprisinglylarge, exceeding 25 W m even when averaged over the 6-day period.This variability is caused by random fluctuations in the strength and timingof individual deep convective events. The ensemble approach demonstrates asmall weakening of convection averaged over the 6-day period in the pollutedsimulations compared to pristine. Despite this weakening, the cloud topheights and anvil ice mixing ratios are higher in polluted conditions. Thisoccurs because of the larger concentrations of cloud droplets that freeze,leading directly to higher ice particle concentrations, smaller ice particlesizes, and smaller fall velocities compared to simulations with pristineaerosols. Weaker convection in polluted conditions is a direct result of thechanges in anvil ice characteristics and subsequent upper-troposphericradiative heating and weaker tropospheric destabilization. Such a conclusionoffers a different interpretation of recent satellite observations oftropical deep convection in pristine and polluted environments compared tothe hypothesis of aerosol-induced convective invigoration. Sensitivity testsusing the ensemble approach with modified microphysical parameters or domainconfiguration (horizontal gridlength, domain size) produce results that aresimilar to baseline, although there are quantitative differences inestimates of aerosol impacts on TOA radiative fluxes.