Characterization of the mass-flux in fair-weather cumuli using long-term cloud radar measurements at the SGP site

 

Authors

Bruce A. Albrecht — University of Miami
Stephen Klein — Lawrence Livermore National Laboratory
Pavlos Kollias — Stony Brook University
Yunyan Zhang — Lawrence Livermore National Laboratory
Arunchandra Susheela Chandra — McGill University

Category

Dynamics/Vertical Motion

Description

The diurnal cycle of fair-weather cumuli over land is strongly coupled to the surface forcing. Fair-weather cumuli are one of the main mechanisms for moistening the lower troposphere and preconditioning the deep convection. Despite their importance and relevance to both weather and climate, fair-weather cumuli are sparsely observed and hence poorly understood. The first step towards understanding fair-weather cumuli is to document their properties using a statistically large sample. The diurnal cycle and the mass-flux profile associated with these clouds are the two most important areas that can lead to increasing our understanding of the coupling of these clouds to the surface and the role of their life cycles in regulating cloudiness and mass-fluxes. Using the entire cloud radar data set from SGP, statistics parameters of the fair-weather cumuli fields have been derived—in particular, the cloud fraction, the cloud aspect ratio, cloud spacing, and the profile of the in-cloud mass flux. The derived cloud base mass flux is used to evaluate aspects of proposed mass flux parameterizations (e.g., mass flux, critical velocity, updraft/downdraft fraction, vertical velocity distributions at the cloud base, etc). In addition, the in-cloud mass flux profiles of individual clouds are used to classify the clouds into active and passive and study the diurnal variation of these cloud categories. The goal is to identify the active cloud fraction and investigate its relationship to surface and subcloud conditions.