A comprehensive numerical study of aerosol-cloud-precipitation interactions in marine stratocumulus

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, largescale 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 % Correspondence to: J. H. Seinfeld (seinfeld@caltech.edu) 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.

Does precipitation susceptibility vary with increasing cloud thickness in marine stratocumulus?

“…Aerosol indirect effects (AIEs), broadly defined here as the effect of aerosols on the cloud albedo, are strongly sensitive to the amount of drizzle falling from warm clouds (Rotstayn and Liu, 2005) and to the amount evaporating below cloud (Wood, 2007;Chen et al, 2011). The AIEs have been difficult to quantify, in part because different cloud types respond differently to perturbations in aerosol concentration (Ackerman, 2004;Stevens and Feingold, 2009).…”

Impact of natural and anthropogenic aerosols on stratocumulus and precipitation in the Southeast Pacific: a regional modelling study using WRF-Chem

“…The main advantage of LES is its ability to resolve turbulent eddies. LES are often idealized and conducted with small domains (on the order of 10 2 km 2 or smaller) to study the equilibrium state or diurnal cycle of clouds under horizontally uniform forcing conditions (e.g., Berner et al, 2011;Chen et al, 2011;Lu and Seinfeld, 2005;Sandu et al, 2008;Wang and McFarquhar, 2008a, b). With a relatively large domain (on the order of 10 4 km 2 ), the LES can simulate the response of mesoscale cloud structures to aerosol perturbations, but only in very ideal meteorological scenarios with prescribed large-scale forcings (Kazil et al, 2011;Mechem et al, 2012;Wang and Feingold, 2009a, b).…”

A statistical analysis of North East Atlantic (submicron) aerosol size distributions

“…Particularly uncertain are the ones influencing the radiative balance and properties of clouds, such as the number size distribution, chemical composition and particle mixing state. The lack of a proper representation concerning size distributions of the aerosol in global and regional models is a major reason why direct and the indirect climate effects (Twomey 1974;Albrecht 1989;Chen et al, 2011) of the atmospheric aerosols constitute the largest uncertainty in our present understanding of the anthropogenic climate forcing (IPCC, 2007). Atmospheric aerosol particles span over several orders of magnitude in diameter (D p ), from a few nanometer to hundreds of micrometer.…”