Turbulent Mixing Processes and Respective Effects on Microphysical Properties of Shallow Trade Wind Cumuli

The analysis of the entrainment-mixing process of sub-saturated air into shallow trade wind cumuli is presented, based on observations performed during the CARRIBA project over Barbados. Basic meteorological parameters (3D wind vector, temperature, and relative humidity, RH), cloud condensation nuclei concentrations, and cloud microphysical parameters (cloud droplet number size distribution and cloud liquid water content, LWC) are obtained from instrumentation installed on the helicopter payload ACTOS (Airborne Cloud Turbulence Observation System). Taking advantage of the concurrent presence of clouds at different stages in their evolution in the trade wind regime, a total of 361 clouds are classified into actively growing, decelerated, and dissolving clouds, based on LWC, vertical wind velocity, and buoyancy acceleration. For each evolutionary stage the mixing process is analyzed using mixing diagrams, which correlate the cloud droplet number (N) and the cubed droplet mean volume diameter (D_v). A transition from predominantly homogeneous towards inhomogeneous mixing is found during the evolution from actively growing to dissolving clouds. This transition, as well as its effects on N, D_v, and LWC is statistically quantified for all sampled cumuli. It is found that LWC and N in dissolving clouds are decreased by about 50% compared to actively growing clouds. Conversely, D_v remains almost constant, which can be attributed to the existence of a humidity halo around the clouds. It is found that the area in the close vicinity of the clouds is characterized by a high relative humidity (RH > 95%). This area grows during the transition from the actively growing to the dissolving evolutionary stage, which limits the entrainment to more or less purely dilution without droplet evaporation.