Sub‐Cloud Turbulence Explains Cloud‐Base Updrafts for Shallow Cumulus Ensembles: First Observational Evidence
Youtong Zheng, Daniel Rosenfeld, Zhanqing Li
Abstract
Abstract Sub‐cloud turbulent kinetic energy has been used to parameterize the cloud‐base updraft velocity ( w b ) in cumulus parameterizations. The validity of this idea has never been proved in observations. Instead, it was challenged by recent Doppler Lidar observations showing a poor correlation between the two. We argue that the low correlation is likely caused by the difficulty of a fixed‐point Lidar to measure ensemble properties of cumulus fields. Taking advantage of the stationarity and ergodicity of early afternoon convection, we developed a Lidar sampling methodology to measure w b of a shallow cumulus (ShCu) ensemble (not a single ShCu). By analyzing 128 ShCu ensembles over the Southern Great Plains, we show that the ensemble properties of sub‐cloud turbulence explain nearly half of the variability in ensemble‐mean w b , demonstrating the ability of sub‐cloud turbulence to dictate w b . The derived empirical formulas will be useful for developing cumulus parameterizations and satellite inference of w b .