Cloud microphysical relationships and their implication on the mixing processes in the stratocumulus clouds measured during the VOCALS-REx

Abstract:

Understanding the nature of marine stratocumulus clouds (MSCs) have been recognize as crucially important in climate change prediction due to their important roles in modulating radiation budget on a very wide spatial scale. However, despite the seemingly simple shape of MSCs, microphysical and dynamical characteristics of these clouds are very complex. MSCs are usually topped by much warmer and drier air and entrainment and mixing at the top of these clouds modulate the cloud microphysics and dynamics. The important point is that depending on how entrained air mixes with the cloudy air, further development of the cloud can turn out to be different. Here we examine this problem for the MSCs observed over the Southeastern Pacific. The airborne data we used were obtained from the G-1 aircraft during Vamos Ocean Cloud Atmosphere Land Study-Regional Experiment (VOCALS-REx). The data were recorded at various rates but the ones presented in this study are the 1Hz and 40Hz data. Cloud microphysics data obtained from horizontal penetrations through the clouds are analyzed to find the evidence for the dominant mixing mechanism of the entrained dry air from above the cloud top. We find that the dominant feature is the positive relationship between cloud droplet mean volume (V) and liquid water content (L). Despite this apparent trait of homogeneous mixing, relevant scale parameters (i.e., transition length scale and transition scale number) consistently suggest that inhomogeneous mixing would be dominant for the VOCALS MSCs. Here we propose that this clear discrepancy can be explained by vertical circulation mixing that can modulate the V-L relationship to suggest homogeneous mixing at the measurement altitudes, which was on average 10.2 mb down from the cloud top where actual mixing may have occurred inhomogeneously.