Boundary Layer Depth, Entrainment, and Decoupling in the Cloud-Capped Subtropical and Tropical Marine Boundary Layer

Abstract

Estimates of marine boundary layer (MBL) depth and degree of decoupling for two regions of the subtropical and tropical east Pacific are presented using satellite observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI). These data are combined in a novel way with NCEP reanalysis data and a mixing line parameterization to estimate the mean entrainment rate we over the two regions. Mean entrainment rates vary geographically and have maxima just downwind of the Californian coast (we ≈ 4–5 mm s−1), and also in the core of the equatorial east Pacific cold tongue where mean we exceeds 6 mm s−1. Entrainment exceeds subsidence by 30% or less in the subtropical stratocumulus regions. North of the equatorial cold tongue entrainment greatly exceeds subsidence, producing a rapid deepening of the MBL as air flows over a marked SST gradient. Shallow MBLs (zi < 500–700 m) are found to be well mixed in general. The decoupling increases markedly for deeper boundary layers and is well parameterized as a function of the thickness of the layer extending from the top of the surface mixed layer to the MBL inversion. This study demonstrates new ways in which large-scale observational and reanalysis datasets may be used to aid understanding of MBL boundary layer and cloud systems.