Abstract
Abstract. The uncertainties associated with the wet removal of aerosols entrained above convective cloud bases are investigated in a global aerosol-climate model (ECHAM5-HAM) under a set of limiting assumptions for the wet removal of the entrained aerosols. The limiting assumptions for the wet removal of entrained aerosols are negligible scavenging and vigorous scavenging (either through activation, with size-dependent impaction scavenging, or with the prescribed fractions of the standard model). To facilitate this process-based study, an explicit representation of cloud-droplet-borne and ice-crystal-borne aerosol mass and number, for the purpose of wet removal, is introduced into the ECHAM5-HAM model. This replaces and is compared with the prescribed cloud-droplet-borne and ice-crystal-borne aerosol fraction scavenging scheme of the standard model. A 20% to 35% uncertainty in simulated global, annual mean aerosol mass burdens and optical depth (AOD) is attributed to different assumptions for the wet removal of aerosols entrained above convective cloud bases. Assumptions about the removal of aerosols entrained above convective cloud bases control modeled upper tropospheric aerosol concentrations by as much as one order of magnitude. Simulated aerosols entrained above convective cloud bases contribute 20% to 50% of modeled global, annual mean aerosol mass convective wet deposition (about 5% to 10% of the total dry and wet deposition), depending on the aerosol species, when including wet scavenging of those entrained aerosols (either by activation, size-dependent impaction, or with the prescribed fraction scheme). Among the simulations, the prescribed fraction and size-dependent impaction schemes yield the largest global, annual mean aerosol mass convective wet deposition (by about two-fold). However, the prescribed fraction scheme has more vigorous convective mixed-phase wet removal (by two to five-fold relative to the size-dependent impaction scheme) since nearly all entrained accumulation and coarse mode aerosols are assumed to be cloud-droplet borne or ice-crystal borne, and evaporation due to the Bergeron-Findeisen process is neglected. The simulated convective wet scavenging of entrained accumulation and coarse mode aerosols has feedbacks on new particle formation and the number of Aitken mode aerosols, which control stratiform and convective cloud droplet number concentrations and yield precipitation changes in the ECHAM5-HAM model. However, the geographic distribution of aerosol annual mean convective wet deposition change in the model is driven by changes to the assumptions regarding the scavenging of aerosols entrained above cloud bases rather than by precipitation changes, except for sea salt deposition in the tropics. Uncertainty in the seasonal, regional cycles of AOD due to assumptions about entrained aerosol wet scavenging is similar in magnitude to the estimated error in the AOD retrievals. The uncertainty in aerosol concentrations, burdens, and AOD attributed to different assumptions for the wet scavenging of aerosols entrained above convective cloud bases in a global model motivates the ongoing need to better understand and model the activation and impaction processes that aerosols undergo after entrainment into convective updrafts.
Highlights
Aerosols play an important role in the climate system by influencing the Earth’s radiation budget, directly by scattering and absorbing radiation, and indirectly by modifying cloud properties (Twomey, 1991; Charlson et al, 1992)
(1) We quantify the uncertainty in predicted aerosol wet removal, concentrations, burdens and optical depth (AOD), which can be attributed to different assumptions about the wet scavenging of aerosols entrained above convective cloud bases in a global climate model
We consider the contribution of aerosols entrained above convective cloud bases to simulated global, annual wet removal in the aerosol-climate model ECHAM5HAM under a set of assumptions for aerosol uptake into cloud droplets and ice crystals for the purpose of wet removal
Summary
Aerosols play an important role in the climate system by influencing the Earth’s radiation budget, directly by scattering and absorbing radiation, and indirectly by modifying cloud properties (Twomey, 1991; Charlson et al, 1992). Aerosols have important impacts on global air quality (van Donkelaar et al, 2010) and human health (Dockery et al, 1993). The prediction of three-dimensional aerosol distributions is important in both global climate and air quality models. These distributions are strongly influenced by convective transport and wet scavenging in convective clouds. The aerosol-cloud interactions involving convective clouds are complex and in global models must be parameterized since convective clouds occur at scales smaller than the typical model grid box size. The representation of convective processes remains a major uncertainty for aerosol prediction in global models despite ongoing research efforts (Nober et al, 2003; Randall et al, 2003; Menon and Rotstayn, 2006; Lohmann, 2008; Tost et al, 2010; Morales et al, 2011)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
