Observations of Moist Adiabatic Ascent in Northeast Colorado Cumulus Congestus Clouds

Abstract

The characteristics of entrainment in and below 12 developing cumulus congestus clouds in the north-eastern Colorado area were investigated using measurements obtained with the NCAR/NOAA sailplane, supporting aircraft and rawinsondes. A region of moist adiabatic ascent was found in eight of the most vigorous clouds sampled. A gradual increase was noted in the equivalent potential temperature and the ratio of the liquid water content to the adiabatic value from the edge of the updraft region inward to the moist adiabatic core. Previous measurements and conceptual and theoretical models of entrainment are discussed in the context of the present set of measurements. The moist adiabatic core was positioned off-center with respect to the boundaries of the updraft region. The measurements supported previous conceptual cloud models in which the updraft acts as an obstacle to the horizontal wind thereby causing the environmental air to flow around the upshear portion of the cell, protecting that region from entrainment. A turbulent wake would be expected to occur in the down-shear portion of the cell, producing increased turbulence and mixing in that region.