What explains the population of daytime, optically-thin clouds below one km in the marine trade wind region?

The cloud fraction of shallow non-precipitating cumulus residing at the lifting condensation level (LCL) increases in the afternoon, most evident in airborne lidar observations from EUREC4A. Observations from the HALO platform and from the R/V Ronald H. Brown are used to articulate the responsible process. Three hypotheses are investigated: 1) afternoon increases of the ocean sea surface temperature help support buoyancy fluxes that lift air parcels to saturation, as seen in tropical regions under low wind speeds; 2) shortwave absorption of the sub-cloud layer helps deepen the sub-cloud layer, so that its mixed-layer height can reach the LCL; 3) clouds form where the cloud layer is already moist. We invite the reader to take a moment here to choose which hypothesis you think is correct.

Analysis to date suggests #3 is the correct explanation. If so, then the next question is to identify why the daytime cloud layer is more or less moist in some places and times. Ideas for this can either be moisture redistribution from shallow circulations occurring at scales of approximately 200 km, or, moisture transport occurring at larger scales. These will be explored prior to the meeting, as well as ramifications for the diurnal cycle.