Seasonal and Inter-Annual Variabilities of the Marine Boundary Layer Height and Cloud Top Entrainment Rates over the Eastern Pacific

The eastern basin of the subtropical Pacific Ocean adjacent to the continental coasts in both North and South Americas are characterized by shallow well-mixed marine boundary layer (MBL) topped by stratocumulus clouds. Further west and away from the coast, the surface fluxes can no longer maintain the well-mixed MBL and leads to the decoupling and the transition from the stratocumulus to trade cumulus. Entrainment of the clouds through subsiding dry air and advection has been hypothesized as a key process causing the deepening of MBL and the low-level cloud transition from stratocumulus to trade cumulus. In this study, we present a new MODIS cloud top height (CTH) retrieval based on the lapse rate method while accounting for the seasonal and spatial variation of the MBL lapse rate. The entrainment rates are estimated based on the relationship proposed by Wood and Bretherton (2004) using ERA-Interim reanalysis (ERA-I) subsidence rate as well as the wind and pressure tendency at the 2-D MODIS CTH. The seasonal and inter-annual variabilities of the MBL height and the cloud top entrainment rates over the Eastern Pacific region from 2012 to 2013 will be presented. The relationship between entrainment rates and cloud variabilities as well as the role entrainment plays in the cloud transition from stratocumulus to trade cumulus will be discussed.