Quantifying entrainment in the planetary boundary layer morning transition

Abstract

The morning transition of the Planetary Boundary Layer (PBL) plays a critical role in controlling the evolution of heat, moisture, and depth of this important layer. Entrainment of air from the free troposphere is an important forcing on this transition, yet its role is not fully understood due to limitations in observing it. A recent advancement in the ability to measure horizontal advection has allowed a pathway to quantify entrainment through the closure of heat and moisture budgets using mixing diagrams constructed from remote sensing observations. This novel budget closure method is tested and applied to the morning transition using ground-based remote sensing instruments at the Depart-ment of Energy Atmospheric Radiation Measurement Southern Great Plains (SGP) site.The result is a dataset of 61 clear air entrainment cases over SGP from 2018-2019 used to understand the strength and variability of entrainment in the morning transition and to analyze its relationship to land and synoptic-scale atmospheric conditions. The results show how the water vapor component energetically dominates the variability in morning entrainment, with weaker and less variable forcing in conditions of high surface pressure and anti-cyclonic winds. The analysis demonstrates how entrainment is governed by a combination of vertical entrainment zone gradients, surface fluxes, surface conditions, and processes unique to the morning transition. The resulting observational dataset provides a foundation for testing PBL entrainment theory and furthering our understanding of land-atmosphere interactions in the PBL morning transition.