Uncertainty associated with convective wet removal of entrained aerosols in a global climate model

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 sizedependent 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 ...

Weak global sensitivity of cloud condensation nuclei and the aerosol indirect effect to Criegee + SO<sub>2</sub> chemistry

“…As these estimates are only for the cloud albedo AIE, the total AIE may be higher if aerosol lifetime effects were included. Similar to the results with CCN, this cloud albedo AIE change is significantly smaller than the uncertainties due to nucleation mechanisms (Pierce and Adams, 2009c;Wang and Penner, 2009;Kazil et al, 2010), SOA amount (Pierce and Adams, 2009c), primary emissions amount (Adams and Seinfeld, 2003;Pierce and Adams, 2009c;Spracklen et al, 2011a) and size (Bauer et al, 2010;Spracklen et al, 2011a), SOA condensational behavior and wet deposition (Croft et al, 2012).…”

Sensitivity of remote aerosol distributions to representation of cloud–aerosol interactions in a global climate model

“…This new scheme effectively reduces the BC aloft and better simulates the observed BC profiles with decreasing mixing ratios in the mid-to upper troposphere, especially in the tropics and mid-latitudes. Croft et al (2012) also found that treatments of scavenging of aerosols entrained above cloud base strongly affect aerosol concentrations in the upper troposphere.…”

The importance of interstitial particle scavenging by cloud droplets in shaping the remote aerosol size distribution and global aerosol-climate effects

“…Interstitial scavenging has aerosol-climate effects of similar magnitude as uncertainties in nucleation (Merikanto et al, 2009;Pierce and Adams, 2009;Reddington et al, 2011;Spracklen et al, 2008;Wang and Penner, 2009), primary emissions (Adams and Seinfeld, 2003;Adams, 2006, 2009;Reddington et al, 2011;Spracklen et al, 2011), wet/dry deposition (Croft et al, 2012) and other factors ; hence, since significant effort is put into improving these other processes in models, we recommend attention be paid to the coagulation of interstitial particles by cloud droplets. Our simple methods here may be further refined by including online schemes that calculate aerosol activation from updraft velocities and the aerosol size distribution (e.g., Nenes and Seinfeld, 2003;Abdul-Razzak and Ghan, 2000).…”