The Subtropical Stratocumulus-Topped Planetary Boundary Layer: A Climatology and the Lagrangian Evolution

Abstract

Abstract Prior work has shown that deeper planetary boundary layers (PBLs) are associated with cloud breakup and reduced droplet concentration in subtropical stratocumulus cloud decks, motivating a need for a thorough understanding of PBL mechanics. Here, 169 000 boundary layer trajectories are calculated in four eastern subtropical ocean basins following reanalysis winds at 925 mb (1 mb = 1 hPa). These trajectories combined with a twice-daily cloud-top-height-inferred PBL depth product allow for a comprehensive Lagrangian analysis of the stratocumulus (Sc)-topped PBL as the cloud deck transitions from Sc to trade cumulus (Cu). Month-to-month variations of this PBL product are strongly positively correlated with an independent PBL product derived from GPS radio occultation. A climatology shows the PBL deepening offshore in every region. The yearly cycle of PBL depth varies in opposition to the yearly cycle of lower-tropospheric stability (LTS), but high-frequency variation between LTS and PBL depth is more complex. Observed geographical patterns of Lagrangian PBL deepening rates appear nonuniform between and within study regions, with smaller regions of maximum deepening rates. A Lagrangian analysis suggests that many variables act to alter the PBL: increased sea surface temperature and droplet concentration act to deepen the PBL, while increases in upper-level humidity, LTS, precipitation, upper-level temperature, and subsidence lead to PBL shallowing.