Vertical Variation of Turbulent Entrainment Mixing Processes in Marine Stratocumulus Clouds Using High-Resolution Digital Holography

Marine stratocumulus clouds contribute significantly to the Earth's radiation budget due to their extensive coverage and high albedo. Yet, subgrid variability in cloud properties, such as aerosol concentration, droplet number, and precipitation rates, lead to considerable errors in global climate models. While these clouds usually have small vertical extent, turbulent entrainment-mixing and precipitation can generate significant variations in droplet number, size, and relative dispersion with altitude. Here, we analyze turbulent entrainment-mixing processes and the variability in cloud microphysical properties as a function of height within a warm marine stratocumulus cloud layer over the Eastern North Atlantic. We use high-resolution airborne holographic measurements and compare them with local turbulence measurements. We find that entrainment-mixing is primarily inhomogeneous near cloud top and homogeneous near cloud base. Further analysis of Damkohler number and transition scale number are able to explain the mixing mechanisms at different cloud heights using phase relaxation but not droplet evaporation as the microphysical time scale. A modified droplet evaporation time scale that considers local saturation deficit using a simple linear mixing model is developed, and it is able to reliably explain the observed mixing mechanisms. This study reinforces the importance of turbulent mixing and the use of appropriate microphysical time scales in determining cloud microphysical processes. Plain Language Summary Warm boundary layer clouds over the oceans cover vast extents of the Earth's surface and influence the Earth's temperature considerably. We study these clouds over the Eastern North Atlantic using high-resolution holographic measurements which can resolve microphysical features at small scales. We observed variability in cloud microphysical properties with height within these clouds with the largest droplets occurring near cloud top. We also attempt to explain the vertical variation in turbulent entrainment-mixing processes using concurrent turbulence measurements and propose a new microphysical time scale to explain variation in cloud microphysics with height.

Automated summary

Marine stratocumulus clouds have a significant impact on Earth's radiation budget, but subgrid variability in cloud properties can cause errors in global climate models. This study analyzes turbulent entrainment-mixing processes and cloud microphysical properties at different heights within a warm marine stratocumulus cloud layer over the Eastern North Atlantic. The findings emphasize the importance of turbulent mixing and appropriate microphysical time scales in understanding cloud microphysical processes.