Influences of in-cloud aerosol scavenging parameterizations on aerosol concentrations and wet deposition in ECHAM5-HAM

摘要

A diagnostic cloud nucleation scavenging scheme, which determines stratiformcloud scavenging ratios for both aerosol mass and numberdistributions, based on cloud droplet, and ice crystal numberconcentrations, is introduced into the ECHAM5-HAM global climatemodel. This scheme is coupled with a size-dependent in-cloud impactionscavenging parameterization for both cloud droplet-aerosol, and icecrystal-aerosol collisions. The aerosol mass scavenged in stratiform clouds isfound to be primarily (90%) scavenged by cloud nucleation processes forall aerosol species, except for dust (50%). The aerosol number scavenged isprimarily (90%) attributed to impaction. 99% of this impaction scavengingoccurs in clouds with temperatures less than 273 K. Sensitivity studies arepresented, which compare aerosol concentrations, burdens, and deposition for a variety ofin-cloud scavenging approaches: prescribed fractions, a more computationallyexpensive prognostic aerosol cloud processing treatment, and the new diagnosticscheme, also with modified assumptions about in-cloud impaction and nucleation scavenging.Our results show that while uncertainties in the representation of in-cloudscavenging processes can lead to differences in the range of 20–30% for thepredicted annual, global mean aerosol mass burdens, and near to 50% foraccumulation mode aerosol number burden, the differences in predicted aerosolmass concentrations can be up to one order of magnitude, particularly forregions of the middle troposphere with temperatures below 273 K where mixedand ice phase clouds exist. Different parameterizations for impaction scavengingchanged the predicted global, annual mean number removal attributed to ice cloudsby seven-fold, and the global, annual dust mass removal attributed to impactionby two orders of magnitude. Closer agreement with observations of black carbonprofiles from aircraft (increases near to one order of magnitude for mixedphase clouds), mid-troposphere Pb vertical profiles, and thegeographic distribution of aerosol optical depth is found for the new diagnosticscavenging scheme compared to the prescribed scavenging fraction scheme of the standard ECHAM5-HAM. Thediagnostic and prognostic schemes represent the variability of scavenged fractionsparticularly for submicron size aerosols, and for mixed and ice phase clouds,and are recommended in preference to the prescribed scavenging fractions method.