Radiative vs. Entraining Effects of Overlying Humidity on the Lagrangian Evolution of Subtropical Stratocumulus

This study makes use of nearly 170,000 24-hour trajectories following the flow at 925mb within the boundary layer. Trajectories are computed for four eastern subtropical ocean basins in the NE Pacific, SE Pacific, SE Atlantic, and E Indian. An analysis has been developed to quantify and compare the effects of different cloud controlling variables on cloud cover and on PBL depth. This new measure of PBL depth comes from co-located MODIS cloud top temperatures, CALIPSO lidar data, and ERA-Interim sea surface temperatures. After accounting for several biases, results suggest that increases in cloud drop concentration (from MODIS) lead to anomalous deepening of the stratocumulus-topped PBL and to anomalous increases in cloud cover after 24 hours. The effects of several other variables are compared, which suggest that the most important driver of Lagrangian PBL depth variability is the humidity above the PBL and that the dominant driver of Lagrangian cloud variability is the Lagrangian change in subsidence.