Abstract
Abstract. In situ aircraft measurements obtained during the VAMOS (Variability of the American Monsoons) Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) field campaign are analyzed to study the aerosol–cloud interactions in the stratocumulus clouds over the southeastern Pacific Ocean (SEP), with a focus on three understudied topics (separation of aerosol effects from dynamic effects, dispersion effects, and turbulent entrainment-mixing processes). Our analysis suggests that an increase in aerosol concentration tends to simultaneously increase both cloud droplet number concentration (Nd) and relative dispersion (ε), while an increase in vertical velocity (w) often increases Nd but decreases ε. After constraining the differences of cloud dynamics, the positive correlation between ε and Nd becomes stronger, implying that perturbations of w could weaken the aerosol influence on ε and hence result in an underestimation of dispersion effect. A comparative analysis of the difference of cloud microphysical properties between the entrainment and non-entrainment zones suggests that the entrainment-mixing mechanism is predominantly extremely inhomogeneous in the stratocumulus that capped by a sharp inversion, whereby the variation in liquid water content (25 %) is similar to that of Nd (29 %) and the droplet size remains approximately constant. In entrainment zone, drier air entrained from the top induces fewer cloud droplets with respect to total in-cloud particles (0.56±0.22) than the case in the non-entrainment zone (0.73±0.13) by promoting cloud droplet evaporation. This study is helpful in reducing uncertainties in dispersion effects and entrainment mixing for stratocumulus, and the results of this study may benefit cloud parameterizations in global climate models to more accurately assess aerosol indirect effects.
Highlights
Stratocumulus clouds play a key role in the radiative energy budget of the Earth by reflecting incoming shortwave radiation and cooling the planet surface and offsetting the warming by greenhouse gases (Hartmann et al, 1992)
Based on the useful information on the microphysical properties of aerosols and clouds provided by previous studies, in this study, we conduct additional explorations regarding aerosol–cloud interactions over the southeastern Pacific Ocean (SEP) by employing in situ aircraft data collected by the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft during VOCALS-REx, which include the following: (a) investigating the controlling factors of cloud droplet formation, (b) evaluating the dispersion effect under relatively constant cloud dynamical conditions, and (c) re-examining the entrainment-mixing mechanism by using a different approach to that of Yum et al (2015)
It is interesting to note that the effective radius of aerosol particles (Ra) below cloud is larger than that above cloud, which is probably attributable to the differences in aerosol sources and aerosol properties
Summary
Stratocumulus clouds play a key role in the radiative energy budget of the Earth by reflecting incoming shortwave radiation and cooling the planet surface and offsetting the warming by greenhouse gases (Hartmann et al, 1992). Based on the useful information on the microphysical properties of aerosols and clouds provided by previous studies, in this study, we conduct additional explorations regarding aerosol–cloud interactions over the SEP by employing in situ aircraft data collected by the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft during VOCALS-REx, which include the following: (a) investigating the controlling factors of cloud droplet formation (e.g., cloud dynamics and aerosols), (b) evaluating the dispersion effect under relatively constant cloud dynamical conditions, and (c) re-examining the entrainment-mixing mechanism by using a different approach to that of Yum et al (2015)
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