LAUSR

24 Marine stratocumulus clouds contribute significantly to the Earth’s radiation budget due 25 to their extensive coverage and high albedo. Yet, subgrid variability in cloud properties 26 such as aerosol concentration, droplet number and precipitation rates lead to consider27 able errors in global climate models. While these clouds usually have small vertical ex28 tent, turbulent entrainment-mixing and precipitation can generate significant variations 29 in droplet number, size and relative dispersion with altitude. In this paper, we analyze 30 turbulent entrainment-mixing processes and the variability in cloud microphysical prop31 erties as a function of height within a warm marine stratocumulus cloud layer over the 32 Eastern North Atlantic. We use high resolution airborne holographic measurements and 33 compare them with local turbulence measurements. We find that entrainment-mixing 34 is primarily inhomogeneous near cloud top and homogeneous near cloud base. Further 35 analysis of Damköhler number and transition scale number are able to explain the mix36 ing mechanisms at different cloud heights using phase relaxation but not droplet evap37 oration as the microphysical time scale. A modified droplet evaporation time scale which 38 considers local saturation deficit using a simple linear mixing model is developed and it 39 is able to reliably explain the observed mixing mechanisms. This study reinforces the 40 importance of turbulent mixing and use of appropriate microphysical time scales in de41 termining cloud microphysical processes. 42 Plain Language Summary 43 Warm boundary layer clouds over the oceans cover vast extents of the Earth’s sur44 face and influence the Earth’s temperature considerably. We study these clouds over the 45 Eastern North Atlantic using high resolution holographic measurements which can re46 solve microphysical features at small scales. We observe variability in cloud microphys47 ical properties with height within these clouds with the largest droplets occurring near 48 cloud top. We also attempt to explain the vertical variation in turbulent entrainment49 mixing processes using concurrent turbulence measurements and propose a new micro50 physical time scale to explain variation in cloud microphysics with height. 51