Observations of the Variability of Shallow Trade-Wind Cumulus Cloudiness and Mass Flux in Relation to Boundary Layer Structure

English Abstract:

Oceanic cumuli are ubiquitous in the subtropics and help regulate the thermodynamic structure of the lower troposphere and the large-scale tropical circulation. Yet, observations of the vertical structure of marine cumulus cloudiness and mass flux profiles remain sparse. Fortunately, modern island-based remote sensors more specifically, the Barbados zenith cloud radar posses the sensitivity to study hydrometers which, when small enough, can also be used as tracers of air motion.

Over the 2-years analyzed, non-precipitating cumuli (peak cloud fraction 4.5% at 710m) exhibit an elevated reflectivity core and ascendant center surrounded by a subsiding shell. Bulk (3h) statistics reveal that these cloud are active and organized. They contain 79% updrafts with 86% of them being organized in large coherent structures contributing to a maximum updraft mass flux of 0.008-0.036kgm^-2s^-1 just above cloud base. Alternatively, downdrafts contribute insignificant mass flux and show little vertical and temporal variability (0.000-0.007kgm^-2s^-1). Together with complementary Raman lidar information, normalized profiles suggest that updraft mass flux profile slope is inversely related to environmental relative humidity. Altogether, the analysis presented here is useful for evaluating the representation of cloudiness and mass flux schemes in numerical models.