Abstract. Wet deposition processes are highly efficient in the removal of aerosols from the atmosphere, and thus strongly influence global aerosol concentrations, and clouds, and their respective radiative forcings. In this study, physically detailed size-dependent below-cloud scavenging parameterizations for rain and snow are implemented in the ECHAM5-HAM global aerosol-climate model. Previously, below-cloud scavenging by rain in the ECHAM5-HAM was simply a function of the aerosol mode, and then scaled by the rainfall rate. The below-cloud scavenging by snow was a function of the snowfall rate alone. The global mean aerosol optical depth, and sea salt burden are sensitive to the belowcloud scavenging coefficients, with reductions near to 15% when the more vigorous size-dependent below-cloud scavenging by rain and snow is implemented. The inclusion of a prognostic rain scheme significantly reduces the fractional importance of below-cloud scavenging since there is higher evaporation in the lower troposphere, increasing the global mean sea salt burden by almost 15%. Thermophoretic effects are shown to produce increases in the global and annual mean number removal of Aitken size particles of near to 10%, but very small increases (near 1%) in the global mean belowcloud mass scavenging of carbonaceous and sulfate aerosols. Changes in the assumptions about the below-cloud scavenging by rain of particles with radius smaller than 10 nm do not cause any significant changes to the global and annualCorrespondence to: B. Croft (croft@mathstat.dal.ca) mean aerosol mass or number burdens, despite a change in the below-cloud number removal rate for nucleation mode particles by near to five-fold. Annual and zonal mean nucleation mode number concentrations are enhanced by up to 30% in the lower troposphere with the more vigourous sizedependent below-cloud scavenging. Closer agreement with different observations is found when the more physically detailed below-cloud scavenging parameterization is employed in the ECHAM5-HAM model.

Abstract. Dust particles coated with soluble materials, such as sulfate, are frequently observed in the Mediterranean. Thus far, the processes responsible for the sulfate coating of dust particles have still not been identified. One possible explanation is that the formation of the sulfate-coated aerosols is related to cloud processing of dust particles. In this process the scavenging of aerosol particles and gases, such as SO2, Oa, and H202, by the droi•lets and the subsequent impaction scavenging of mineral dust particles followed by evaporation could release into the atmosphere dust particles coated with soluble materials. These modified particles can serve as giant cloud condensation nuclei and thus can have a significant impact on the microphysical development of other clouds. Using an air parcel model with detailed microphysics, it is shown that cloud processing of dust particles is a possible effective pathway to form soluble coatings on these particles. Furthermore, the simulations show that after one or two cycles of particles through convective clouds the contribution of gas uptake by drops and subsequent liquid phase oxidation add considerable mass of soluble material to particles in the size range of 0.05/•m. On the other hand, this process adds about i order of magnitude less mass to the larger particles as compared to the contribution made by coagulation of drops containing soluble aerosols.

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Abstract.A diagnostic cloud nucleation scavenging scheme, which determines stratiform cloud scavenging ratios for both aerosol mass and number distributions, based on cloud droplet, and ice crystal number concentrations, is introduced into the ECHAM5-HAM global climate model. This scheme is coupled with a size-dependent in-cloud impaction scavenging parameterization for both cloud droplet-aerosol, and ice crystal-aerosol collisions. The aerosol mass scavenged in stratiform clouds is found to be primarily (>90%) scavenged by cloud nucleation processes for all aerosol species, except for dust (50%). The aerosol number scavenged is primarily (>90%) attributed to impaction. 99% of this impaction scavenging occurs in clouds with temperatures less than 273 K. Sensitivity studies are presented, which compare aerosol concentrations, burdens, and deposition for a variety of incloud scavenging approaches: prescribed fractions, a more computationally expensive prognostic aerosol cloud processing treatment, and the new diagnostic scheme, also with modified assumptions about in-cloud impaction and nucleation scavenging. Our results show that while uncertainties in the representation of in-cloud scavenging processes can lead to differences in the range of 20-30% for the predicted annual, global mean aerosol mass burdens, and near to 50% for accumulation mode aerosol number burden, the differences in predicted aerosol mass concentrations can be up to Correspondence to: B. Croft (croft@mathstat.dal.ca) one order of magnitude, particularly for regions of the middle troposphere with temperatures below 273 K where mixed and ice phase clouds exist. Different parameterizations for impaction scavenging changed the predicted global, annual mean number removal attributed to ice clouds by seven-fold, and the global, annual dust mass removal attributed to impaction by two orders of magnitude. Closer agreement with observations of black carbon profiles from aircraft (increases near to one order of magnitude for mixed phase clouds), midtroposphere 210 Pb vertical profiles, and the geographic distribution of aerosol optical depth is found for the new diagnostic scavenging scheme compared to the prescribed scavenging fraction scheme of the standard ECHAM5-HAM. The diagnostic and prognostic schemes represent the variability of scavenged fractions particularly for submicron size aerosols, and for mixed and ice phase clouds, and are recommended in preference to the prescribed scavenging fractions method.

[1] Numerical simulations were performed to investigate the effect of cloud-processed mineral dust particles on the subsequent development of cloud and precipitation and possible effects on cloud optical properties. A two-dimensional (2-D) nonhydrostatic cloud model with detailed microphysics was used. The initial aerosol spectra used in the 2-D model consisted of both background cloud condensation nuclei and mineral dust particles. These were taken from the results of three successive runs of a parcel model that simulates the interaction of dust and sulfate particles with cloud drops and trace gases and then evaporates the cloud drops. The results show that insoluble mineral dust particles become effective cloud condensation nuclei (CCN) after passing through a convective cloud. Their effectiveness as CCN increases because of a layer of sulfate that is formed on their surface as they are first captured by growing drops or ice crystals and then released as these hydrometeors evaporate. Upon entering subsequent clouds, these particles increase the concentration of the activated drops and widen the drop size distribution. The present work shows that in continental clouds the effect of cloud-processed dust particles is to accelerate the formation of precipitation particles, although the amount of precipitation depends on the concentration of the large and giant CCN. In maritime clouds the addition of cloudprocessed aerosol and mineral dust particles has a minimal effect on precipitation because the cloud starts with many large particles already. The addition of more CCN to either maritime or continental clouds increases their optical depth, even for those cases in which the precipitation amount is increased.

[1] Numerical simulations were performed to investigate the effects of drop freezing in immersion and contact modes for a convective situation. For the description of heterogeneous drop freezing, new approaches were used considering the significantly different ice nucleating efficiencies of various ice nuclei. An air parcel model with a sectional two-dimensional description of the cloud microphysics was employed. Sensitivity studies were undertaken by varying the insoluble particle types as well as the soluble fraction of the aerosol particles showing the effects of these parameters on drop freezing and their possible impact on the vertical cloud dynamics. The soluble fraction e decides whether immersion or contact freezing will be the major process. For high e values, immersion freezing is the dominant process. In such cases the freezing process is strongly temperature-dependent, and the ice nucleation efficiency of the insoluble particle types becomes important for efficient freezing. The freezing point depression can be neglected because of the preferential freezing of large drops. Contact freezing is the major process in cases of lower e values. In these cases the freezing process is less dependent on temperature and aerosol particle type. For conditions of efficient freezing, cold, high-altitude, completely glaciated clouds could form. The presented approaches for immersion and contact freezing can be incorporated further into mesoscale and global models to estimate the effects of specific ice nuclei on ice formation.Citation: Diehl, K., M. Simmel, and S. Wurzler (2006), Numerical sensitivity studies on the impact of aerosol properties and drop freezing modes on the glaciation, microphysics, and dynamics of clouds,

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In situ aerosol and cloud drop microphysical measurements at a high‐alpine site are used to investigate aerosol partitioning between cloud and interstitial phases in natural, mid‐latitude, mixed‐phase clouds. Measurements indicate a decrease in the activated aerosol fraction (FN) for particle diameters dP > 100 nm with cloud temperature from FN ∼ 0.54 in summer liquid‐phase clouds to FN ∼ 0.08 in winter mixed‐phase clouds. The latter may be attributed to the Bergeron‐Findeisen mechanism whereby ice crystals grow at the expense of liquid water drops, releasing formerly activated aerosols back into the interstitial phase. This provides a means to distinguish the indirect effects of aerosols on drops and ice crystals.