Cloud microphysical and radiative properties for parameterization and satellite monitoring of the indirect effect of aerosol on climate

Abstract

[1] The spatial variability of the microphysical fields in stratocumulus clouds is documented in this paper with statistics of droplet number concentration, droplet mean volume radius, and liquid water content for eight cases of the second Aerosol Characterization Experiment. Statistics are calculated in five sublayers, from cloud base to cloud top, and they are utilized for deriving estimates of cloud optical thickness and liquid water path, by assuming either random or maximum overlap. The resulting in situ frequency distributions of optical thickness and liquid water path are validated against distributions of these two parameters retrieved from independent remote sensing measurements of cloud radiances. They are also used for testing parameterizations of optical thickness based on liquid water path and either the droplet effective radius or the cloud droplet number concentration. This unique data set of extensive, concomitant, and independent measurements of cloud microphysical and radiative properties is finally used for assessing the detectability of the aerosol indirect effect through examination of the correlation between cloud optical thickness and droplet effective radius. If only cases of comparable values of geometrical thickness are considered, the correlation between optical thickness and effective radius is negative, as anticipated by Twomey [1977]. However, if the most polluted cases are also accounted for, the trend suggests a positive correlation. In fact, the most polluted cloud systems sampled during ACE-2 were slightly drier, hence thinner, than the marine and intermediate cases, hence producing a positive correlation between optical thickness and droplet effective radius. This study demonstrates that the monitoring of the aerosol indirect effect with satellite observations requires an independent retrieval of the liquid water path together with the cloud optical thickness and droplet effective radius.

Vertical profiles of droplet effective radius in shallow convective clouds

“…As shown in many field observations, droplet size not only exhibits horizontal heterogeneity, but also vertical stratification (e.g., Warner, 1955;Brenguier et al, 2000;Miles et al, 2000;Hudson and Yum, 2001;Twohy et al, 2005;Jiang et al, 2008;Lu et al, 2008;Arabas et al, 2009). The vertical stratification of droplet size must be resolved because it is central to both the cloud albedo and the precipitation process (Brenguier et al, 2003;Rosenfeld and Lensky, 1998). Although in situ measurements can resolve vertical profiles of droplet size, they cannot provide regional or global scale data sets for understanding and parameterization of aerosol effects on climate.…”

“…They confirmed that the composite properties from tracking the cells reproduce the properties in the snapshot. Because entrainment-mixing is significant for convective clouds, and because the top and the sides of convective clouds may experience different degrees of mixing (e.g., Warner, 1955;Blyth et al, 1988;Blyth, 1993;Burnet and Brenguier, 2007;Small and Chuang, 2008;Jiang et al, 2008;Lu et al, 2008), cloud droplet size at cloud top and inside of clouds and clouds at different stages of the lifecycle may deviate from the adiabatic value to different degrees. However, if entrainment mixing is inhomogeneous, then cloud droplet size is likely independent on the degree of mixing (Freud et al, 2008).…”

“…More details of the simulations can be found in Xue and Feingold (2006). The spatial variability of r e is investigated to shed light on implications for both satellite retrieval and model parameterization of the aerosol effects (Brenguier et al, 2003). We investigate whether the in-cloud r e profiles can be constructed using cloud-top r e and the corresponding cloud-top height from satellite measurements for convective clouds.…”

“…Brenguier et al (2003) pointed out that cloud vertical stratification must be taken into account in climate model parameterizations of cloud radiative properties and can be approximated with an adiabatic model for stratus clouds. It should also be noted that the stratified cloud model has been used to develop procedures for the retrieval of cloud geometrical thickness, liquid water content, and drop number concentration from the measurement of cloud radiances for stratiform clouds (Schüller et al, 2005).…”

Simultaneous observations of aerosol–cloud–albedo interactions with three stacked unmanned aerial vehicles

“…Numerous studies have demonstrated that an increase in aerosols increases n D (8)(9)(10)(11)(12); however, fewer have actually measured the impact of aerosols on ␣ c (13,14). Several studies have not detected the expected effect of aerosols on clouds, possibly because of variations in liquid water path (15)(16)(17), and employ aerosol and cloud properties in radiative transfer models to estimate radiative forcing.…”