Characterizing the Turbulent Structure of the Convective Boundary Layer Using ARM/ASR Observations and LES Model Output

Principal Investigator(s):
David Turner, University of Oklahoma

Thijs Heus, Cleveland State University
Richard Ferrare, NASA Langley Research Center

The convective boundary layer (CBL) is the lowest part of the atmosphere where the dominant turbulent mechanism is surface-driven buoyancy, although strong winds or wind shear can also induce mechanically driven turbulence. The turbulent motions in the CBL are important for redistributing trace gases, particles, aerosols, heat, and momentum between the surface and the free troposphere, and parameterizing these motions in large scale models is still a challenge. High temporal and vertical resolution observations of turbulence as a function of height throughout the CBL, together with high-resolution output from large eddy simulation (LES) models that can model turbulent motions for the larger eddies, are needed to evaluate and improve these parameterizations.

Our project will use a combination high-resolution ground-based remote sensing observations and LES model simulations to better understand the evolution, turbulent structure and characteristics of the boundary layer with particular attention on the entrainment zone, which lies on the top of the CBL. The entrainment zone is a critical portion of the CBL, as this zone is where shallow land-forced cumulus exist and turbulence in this region controls the exchange of energy and moisture between the CBL and the free troposphere.

Our research has five foci:

  1. Characterizing the diurnal evolution of the boundary layer thermodynamic structure
  2. Characterizing the turbulent structure of the CBL and the entrainment zone
  3. Evaluating the ability of LES models to simulate the thermodynamic and turbulent structure of the CBL
  4. Understanding how perturbations in the surface properties and CBL properties influence boundary layer clouds
  5. Comparing and contrasting the boundary layer and cloud interaction in the marine vs. continental CBL

To accomplish these tasks, we will initially focus on the continental CBL and utilize observations from the Southern Great Plains (SGP) site. In particular, the high temporal and vertical resolution observations from the Raman and Doppler lidars, together with thermodynamic retrievals from the AERI, at the SGP central facility will be used to address the first two points. Observations from the AERI and Doppler lidars at the new boundary facilities (as part of the reconfigured SGP site) and surface turbulent flux observations from the eddy correlation systems will be used to investigate how the surface properties and CBL varies spatially, and to increase the number of observations used to derive distributions of cumulus cloud properties for different environmental conditions. A large number of LES model runs will be performed to evaluate the accuracy of the model in reproducing the observed boundary layer evolution and turbulent structure; the range of observations should allow any weaknesses in the model to be identified and improved. One of our main foci will be to understand how different environmental factors affect the entrainment zone; for example, we will study both how the skewness of the water vapor mixing ratio distribution in the entrainment zone impacts cumulus cloud formation and the factors that modulate water vapor skewness at that level, and the exchange of water vapor and aerosols from the CBL into the free troposphere through the entrainment zone. Finally, we will extend our analysis to include observations from marine environments, so that we can evaluate how the interaction between cumulus and the CBL may be different for marine vs. continental locations. We expect our results to culminate in improved parameterizations, which will be tested with help of single column model simulations.