Understanding Processes Controlling the Temporal and Spatial Variations of PBL Structures Over the ARM SGP Site

Abstract

The surface heat, moisture, and momentum fluxes are transferred to the atmosphere above through the planetary boundary layer (PBL), where vertical mixing due to turbulent eddies of different sizes play critical roles. Therefore, reliably representing PBL processes in numerical models is critical for weather, climate, and air quality prediction. Currently, there are over ten PBL schemes that are selectable within the advanced research version of the Weather Research and Forecasting (WRF) model, indicative of the challenges in capturing the impacts of turbulence within the PBL in models. Further improvements in PBL parameterizations are needed for both weather and climate models as emphasized in many recent national reports. Our ability to represent PBL processes depends on understanding the underlying boundary layer processes from observations.  However, there are few routine measurements of the surface fluxes and PBL structure to support the PBL process study. Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) megasite is one of few sites, if not the only site in the world, offering multi-year routine surface flux and PBL structure observations, especially with enhancements since 2016. The project takes advantages of ARM’s recent investments in the atmospheric boundary layer observations and modeling to understand key physical processes controlling the mixed layer development and water vapor transport in the PBL because of their importance for cloud/precipitation development and improve their parameterizations in numerical models. The project has the following three main objectives:

1. Characterize PBL structure and variations: Long-term DOE ARM observations of fine-scale temperature, water vapor, and wind profiles from Raman lidar and Doppler lidar together with other measurements will be used to characterize the PBL structure, including PBL height, mixed layer height, vertical turbulent mixing and water vapor flux profile at the SGP site. The spatial and temporal variations of the PBL structure will be documented under different meteorological and thermodynamic conditions.
2. Understand processes controlling PBL variations: PBL evolutions are controlled by multi-scale processes, including surface fluxes, radiation, dynamics, and turbulence, as well as clouds and precipitation systems. The ARM facility at the SGP site offers the necessary complement of measurements for the process-oriented study. We will focus on processes important for mixed layer development and vertical water vapor transport because of their importance for cloud/precipitation development. The impact of land-atmosphere interactions on PBL developments will be constrained by observed surface latent and sensible heat fluxes. Analyses will be performed to understand the roles of multi-scale dynamics interactions in controlling the mixed layer development and vertical water transport, the impacts of meteorological and thermodynamic conditions on PBL evolution, and the surface and cloud/precipitation control of PBL spatial heterogeneity at the SGP site.
3. Improve PBL modeling in WRF: The third objective tries to evaluate and improve the PBL parameterizations in representing turbulent mixing of moisture in the convective boundary layer (CBL) regime using the following approaches. First, we will combine the validated Large-Eddy Simulation (LES) ARM Symbiotic Simulation and Observation (LASSO) and Land-Atmosphere Feedback Experiment (LAFE) LES simulations and observational results to explore ways to improve the PBL parameterizations. Second, we will perform Single Column Model (SCM) simulations under the observational constraints to discover the consistent deficiencies of selected WRF PBL schemes using both observations and validated LASSO simulations and test potential improvements for the selected schemes. The SCM framework provides an observation-constrained setup to better isolate the PBL parameterized physics from dynamics-physics coupled system. Third, we will conduct three-dimensional WRF simulations using the original and updated PBL schemes for two to three LASSO and/or LAFE cases to assess the performance or improvement of modified PBL schemes in the CBL regime.