Entrainment, Mixing, and Microphysics in RICO Cumulus

This study addresses small-cumulus physical processes of entrainment, mixing, and the resulting changes in microphysics, all of which require better understanding. In this regard RICO Flight RF12 on January 11, 2005 is analyzed in detail utilizing the probes on the NCAR C-130 research aircraft. This flight is chosen over others, given a sufficient amount of available data at 5 different levels in the Cu, a maximum Cu thickness of about 1,500 m, and a drizzle and precipitation content that is not excessive. Key data for this analysis is the 1000-hz data (~10-cm incloud resolution) from the PVM-100 which provided liquid water content (LWC) and an indication of droplet size changes from measurements of the droplets' effective radius (Re). Evidence is presented to support the following conclusions: We find that a comparison between inhomogeneous and homogeneous mixing following entrainment yields an average inhomogeneous mixing fraction that dominates at all levels of the Cu. However, this fraction is not constant with height in the Cu, since the homogeneous mixing fraction increases downward towards the LCL. We attribute this behavior due to increased entrainment of moistened air near cloud base, rather than the TKE dissipation time constant being smaller than the droplet response time constant for a saturation deficit. We also conclude that the scales of the entrained air volumes are smaller than hitherto thought, because the high-resolution PVM data finds a lack of cloud-free voids within the Cu. This suggests that entrainment-related mixing occurs initially and primarily near the surface of the Cu. And finally, droplet spectra broadening is affected most by both the homogeneous mixing near cloud base and by the activation of new CCN introduced into the Cu by entrainment.