Abstract
Abstract. A recent parcel model study (Reutter et al., 2009) showed three deterministic regimes of initial cloud droplet formation, characterized by different ratios of aerosol concentrations (NCN) to updraft velocities. This analysis, however, did not reveal how these regimes evolve during the subsequent cloud development. To address this issue, we employed the Active Tracer High Resolution Atmospheric Model (ATHAM) with full microphysics and extended the model simulation from the cloud base to the entire column of a single pyro-convective mixed-phase cloud. A series of 2-D simulations (over 1000) were performed over a wide range of NCN and dynamic conditions. The integrated concentration of hydrometeors over the full spatial and temporal scales was used to evaluate the aerosol and dynamic effects. The results show the following. (1) The three regimes for cloud condensation nuclei (CCN) activation in the parcel model (namely aerosol-limited, updraft-limited, and transitional regimes) still exist within our simulations, but net production of raindrops and frozen particles occurs mostly within the updraft-limited regime. (2) Generally, elevated aerosols enhance the formation of cloud droplets and frozen particles. The response of raindrops and precipitation to aerosols is more complex and can be either positive or negative as a function of aerosol concentrations. The most negative effect was found for values of NCN of ~ 1000 to 3000 cm−3. (3) The nonlinear properties of aerosol–cloud interactions challenge the conclusions drawn from limited case studies in terms of their representativeness, and ensemble studies over a wide range of aerosol concentrations and other influencing factors are strongly recommended for a more robust assessment of the aerosol effects.
Highlights
Clouds have a considerable impact on the radiation budget and water cycle of the Earth (IPCC, 2007)
To cover a wide range of conditions, the updraft velocities range from ca. 0.25 to 20 m s−1 in previous cloud parcel model simulations (Reutter et al, 2009), which www.atmos-chem-phys.net/15/10325/2015/
As variations in aerosol number concentrations have very little effect on the temperature profile, we show this relationship for only one aerosol concentration (NCN = 5000 cm−3) as an example
Summary
Clouds have a considerable impact on the radiation budget and water cycle of the Earth (IPCC, 2007). Aerosol effects on clouds and precipitation have been suggested to influence the formation, persistence, and ultimate dissipation of clouds and its climate effects (Stevens and Feingold, 2009; Tao et al, 2012) and have been studied intensively through cloud-resolving model simulations, analysis of satellite data, and long-term observational data (Tao et al, 2012). D. Chang et al.: Regime dependence of aerosol effects on pyro-convective clouds et al, 2008; Lee et al, 2008; Teller and Levin, 2008; Fan et al, 2013; Camponogara et al, 2014). Changing aerosol concentrations have been found to exert nonmonotonic influences (either positive or negative) on a wide range of cloud properties, such as homogeneous freezing (Kay and Wood, 2008), frozen water particles (Saleeby et al, 2009; Seifert et al, 2012), and convection strength (Fan et al, 2009)
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