Observations of the variability of shallow trade wind cumulus cloudiness and mass flux

Abstract

AbstractTwo years of ground‐based remote sensing observations are used to study the vertical structure of marine cumulus near the island of Barbados, including their cloud fraction and mass flux profile. Daily radar derived cloud fraction profiles peak at different height levels depending on the depth of the cumuli and thus the extent to which they precipitate. Nonprecipitating cumuli have a peak cloud fraction of about 5% near mean cloud base (700 m), whereas precipitating cumuli tend to have a peak of only 2% near cloud base. Nineteen percent of the precipitating cumuli are accompanied by large cloud fractions near the detrainment level of cumulus tops (~1700 m). Day‐to‐day variations in cloud fraction near cloud base are modest (~3%). Nonprecipitating cumuli have their largest reflectivities near cloud top and an ascending core surrounded by a subsiding shell. Precipitating cumuli with enhanced elevated cloudiness (stratiform outflow) are deeper and contain larger vertical gradients in reflectivity and Doppler velocity than precipitating cumuli without such outflow. Bulk (3 h) statistics reveal that nonprecipitating shallow cumuli are active and organized. They contain on average 79% in‐cloud updrafts with 86% of them being organized in large coherent structures contributing to a maximum updraft mass flux of 8–36 gm−2 s−1 just above cloud base. Alternatively, downdrafts contribute insignificantly to the mass flux and show little vertical and temporal variability (0–7 gm−2 s−1). Complementary Raman lidar information suggests that updraft mass flux profile slope is inversely related to environmental relative humidity.