Examining Boundary Layer Evolution during the Morning Transition using Observations and a Single-Column Model

The morning transition is the period when the planetary boundary layer (PBL) transitions from stable to unstable conditions. Understanding the contribution of the various forcings, and how the relative importance of each change, to the PBL heat and moisture budgets during the morning transition is important for forecasting air quality. This study investigates the processes that contribute to PBL evolution during the morning transition at the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site by applying a mixing diagram framework to observations and output from a single-column model (SCM). Although the evolution of mean PBL potential temperature and specific humidity in the SCM is comparable to observations, PBL growth is substantially delayed in time and growth occurs over a shorter time period in the SCM. This delay is due to both a larger gradient between skin and near-surface air temperatures at sunrise, as well an initially slower rate of surface heating in response to incoming solar radiation. Replacing the land surface model with prescribed surface fluxes and skin temperatures from observations can reduce the delay in PBL growth; however, mixing diagrams show that the atmospheric response within the SCM is not proportional to that shown by observations. The results of this study highlight the utility of mixing diagrams for assessing the representation of transitions within numerical weather prediction models.