Holographic measurements of cloud mixing and inhomogeneity

The degree to which an atmospheric cloud entrains and mixes with clear air from the surrounding environment largely determines the maximum liquid water content achieved by the cloud, the droplet size distribution within the cloud, and therefore influences the rate of rain formation and radiative properties of the cloud. We wish to understand how the turbulent mixing progresses and how the cloud particles respond to the fluctuating thermodynamic environment that results from mixing. Simply put, if a cloud is diluted, do all droplets evaporate uniformly or does a subset of droplets evaporate completely, leaving the remaining drops unaffected? We use an airborne digital holographic instrument (Holodec) to show that cloud edges are inhomogeneous down to centimeter scales and that the droplet size distribution fluctuates strongly in number density but with a nearly unchanging mean droplet diameter. Prior measurements with single-drop-counting instruments are unable to unambiguously differentiate between homogeneous and inhomogeneous mixing at these fine scales. The data were collected during the Instrument Development and Education for the Airborne Sciences projects held in 2011 and 2012 over northeastern Colorado and southeastern Wyoming. The Holodec instrument measures droplet spatial and size distributions in individual cloud volumes of approximately 15 cubic centimeters. The resulting three dimensional view of the cloud structure is uniquely suited for studies of mixing and fine scale cloud structure.