Journal cover Journal topic
Geoscientific Model Development An interactive open-access journal of the European Geosciences Union
Geosci. Model Dev., 7, 799-819, 2014
© Author(s) 2014. This work is distributed
under the Creative Commons Attribution 3.0 License.
Development and technical paper
12 May 2014
Development of a new semi-empirical parameterization for below-cloud scavenging of size-resolved aerosol particles by both rain and snow
X. Wang1, L. Zhang2, and M. D. Moran2 1Kellys Environmental Services, Toronto, Ontario, Canada
2Air Quality Research Division, Science and Technology Branch, Environment Canada, 4905 Dufferin St, Toronto, Ontario, M3H 5T4, Canada
Abstract. A parameter called the scavenging coefficient Λ is widely used in aerosol chemical transport models (CTMs) to describe below-cloud scavenging of aerosol particles by rain and snow. However, uncertainties associated with available size-resolved theoretical formulations for Λ span one to two orders of magnitude for rain scavenging and nearly three orders of magnitude for snow scavenging. Two recent reviews of below-cloud scavenging of size-resolved particles recommended that the upper range of the available theoretical formulations for Λ should be used in CTMs based on uncertainty analyses and comparison with limited field experiments. Following this recommended approach, a new semi-empirical parameterization for size-resolved Λ has been developed for below-cloud scavenging of atmospheric aerosol particles by both rain (Λrain) and snow (Λsnow). The new parameterization is based on the 90th percentile of Λ values from an ensemble data set calculated using all possible "realizations" of available theoretical Λ formulas and covering a large range of aerosol particle sizes and precipitation intensities (R). For any aerosol particle size of diameter d, a strong linear relationship between the 90th-percentile log10 (Λ) and log10 (R), which is equivalent to a power-law relationship between Λ and R, is identified. The log-linear relationship, which is characterized by two parameters (slope and y intercept), is then further parameterized by fitting these two parameters as polynomial functions of aerosol size d. A comparison of the new parameterization with limited measurements in the literature in terms of the magnitude of Λ and the relative magnitudes of Λrain and Λsnow suggests that it is a reasonable approximation. Advantages of this new semi-empirical parameterization compared to traditional theoretical formulations for Λ include its applicability to below-cloud scavenging by both rain and snow over a wide range of particle sizes and precipitation intensities, ease of implementation in any CTM with a representation of size-distributed particulate matter, and a known representativeness, based on the consideration in its development, of all available theoretical formulations and field-derived estimates for Λ (d) and their associated uncertainties.

Citation: Wang, X., Zhang, L., and Moran, M. D.: Development of a new semi-empirical parameterization for below-cloud scavenging of size-resolved aerosol particles by both rain and snow, Geosci. Model Dev., 7, 799-819, doi:10.5194/gmd-7-799-2014, 2014.
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