Using Mixing Diagrams to Evaluate the Evolution of theConvective Boundary Layer in NWP Models

Land-atmosphere feedbacks can play a critical role in daytime convective boundary layer (CBL) evolution. Mixing diagrams provide a framework for disentangling the relative contributions of entrainment, advection, and surface fluxes in the evolution of the CBL moisture and energy budgets. Entrainment is particularly difficult to observe with operational ground-based sensors, and mixing diagrams provide a way to quantify this contribution to the CBL.

We use this framework to evaluate the evolution of the CBL as represented by the Rapid Refresh (which has 13-km grid) and the High-Resolution Rapid Refresh (which has a 3-km grid) numerical weather prediction models.  We have identified 30 cases in a three-month period between May and July 2019 at the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) site in north-central Oklahoma; over this period of time the land surface transitioned from a moist soil with green vegetation to a relatively dry soil with harvested crops.  A key instrument in this analysis is an infrared spectrometer (IRS), from which profiles of temperature and humidity can be retrieved at 5-min resolution.  At this ARM site, a network of IRS and Doppler lidars surround the central facility, from which we are able to derive the advective fluxes of temperature and water vapor.  Our analysis focused on how well the two modeling systems represent the relative contributions from entrainment, surface fluxes, and advection over this three-month period.