Abstract
Abstract. Three-dimensional large-eddy simulations (LES) with detailed bin-resolved microphysics are performed to explore the diurnal variation of marine stratocumulus (MSc) clouds under clean and polluted conditions. The sensitivity of the aerosol-cloud-precipitation interactions to variation of sea surface temperature, free tropospheric humidity, large-scale divergence rate, and wind speed is assessed. The comprehensive set of simulations corroborates previous studies that (1) with moderate/heavy drizzle, an increase in aerosol leads to an increase in cloud thickness; and (2) with non/light drizzle, an increase in aerosol results in a thinner cloud, due to the pronounced effect on entrainment. It is shown that for higher SST, stronger large-scale divergence, drier free troposphere, or lower wind speed, the cloud thins and precipitation decreases. The sign and magnitude of the Twomey effect, droplet dispersion effect, cloud thickness effect, and cloud optical depth susceptibility to aerosol perturbations (i.e., change in cloud optical depth to change in aerosol number concentration) are evaluated by LES experiments and compared with analytical formulations. The Twomey effect emerges as dominant in total cloud optical depth susceptibility to aerosol perturbations. The dispersion effect, that of aerosol perturbations on the cloud droplet size spectrum, is positive (i.e., increase in aerosol leads to spectral narrowing) and accounts for 3% to 10% of the total cloud optical depth susceptibility at nighttime, with greater influence in heavier drizzling clouds. The cloud thickness effect is negative (i.e., increase in aerosol leads to thinner cloud) for non/light drizzling cloud and positive for a moderate/heavy drizzling clouds; the cloud thickness effect contributes 5% to 22% of the nighttime total cloud susceptibility. Overall, the total cloud optical depth susceptibility ranges from ~0.28 to 0.53 at night; an increase in aerosol concentration enhances cloud optical depth, especially with heavier precipitation and in a more pristine environment. During the daytime, the range of magnitude for each effect is more variable owing to cloud thinning and decoupling. The good agreement between LES experiments and analytical formulations suggests that the latter may be useful in evaluations of the total cloud susceptibility. The ratio of the magnitude of the cloud thickness effect to that of the Twomey effect depends on cloud base height and cloud thickness in unperturbed (clean) clouds.
Highlights
Aerosols influence the microphysical properties of clouds and affect their radiative properties, amount, and lifetime (IPCC, 2007)
A second goal of the present study is to evaluate analytical formulations of total cloud optical depth susceptibility to aerosol perturbations, including the Twomey, droplet dispersion, cloud thickness effects
Dispersion in the droplet distribution is related to aerosol composition (e.g., Feingold and Chuang, 2002), microphysics, and dynamics (Wood et al, 2002; Lu and Seinfeld, 2006). It is noted from observational data (Martin et al, 1994; Ackerman et al, 2000; Liu and Daum, 2002) that the dispersion forcing would offset the cooling from the Twomey effect as the competition for water vapor in the relatively polluted, condensation-dominated regime leads to spectral broadening and negative dispersion effect (Feingold and Siebert, 2009)
Summary
Aerosols influence the microphysical properties of clouds and affect their radiative properties, amount, and lifetime (IPCC, 2007). Evaporation-entrainment effect (non-drizzling cloud): Na increase → smaller, more numerous droplets → more efficient evaporation → higher TKE → stronger entrainment → LWP decrease → lower albedo (Wang et al, 2003; Xue and Feingold, 2006; Hill et al, 2008) Meteorological conditions, such as free tropospheric humidity, large-scale divergence (D), and sea surface temperature (SST), have strong impacts on cloud responses to aerosol perturbations. A second goal of the present study is to evaluate analytical formulations of total cloud optical depth susceptibility to aerosol perturbations, including the Twomey, droplet dispersion, cloud thickness effects While such analytical formulations are generally based on simplistic assumptions, they offer the advantage of concisely encapsulating complex responses.
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