Boundary-layer transitions are challenging for atmospheric models, including Large Eddy Simulation (LES), numerical weather prediction models, and Earth System Models, like the Department of Energy’s (DOE) Energy Exascale Earth System Model (E3SM). New remote sensing data sets, such as those collected at the DOE Atmospheric Radiation Measurement (ARM) User Facility Southern Great Plains site, allow us to examine conditions during the evening transition. These data sets can be used to better understand key physical processes and model performance.
In this study we combine observations from ARM Doppler and Raman Lidars, Atmospheric emitted radiance interferometer (AERI), eddy-flux systems, and radiosondes, to examine the how quickly turbulence decays as a function of the wind shear and static stability for three nights selected from Holistic Interactions of Shallow Clouds, Aerosols and Land Ecosystems (HI-SCALE) study. This period was selected as it was relatively cloud free over two of the three nights, although some cloud cover was observed on the third day that impacted the surface heat flux at the site. We also utilize LES ARM Symbiotic Simulation and Observation Activity (LASSO)-like LES that extend into the evening transition using two different horizonal grid spacings (100 and 25 m) as well as results from E3SM v1. We scale the results by the boundary-layer depth and the surface vertical velocity variance as the sensible heat flux approaches zero to help reduce the day-to-day variability associated with different depths and turbulence intensities.
The time constant of the observed turbulence decay (as inferred from the vertical velocity variance) measured near the middle of the boundary layer increases from 0.5 to 2 hours over the three nights, while closer to the surface increases from 0.9 to 1.5 hours over the same period. There is generally good agreement between the observations and the high-resolution LES. The coarse-resolution LES generally overestimates the rate of decay aloft, but underestimates it near the surface. The results highlight the need for high-resolution simulations to accurately simulate the evening transition using LES. The rate of decay of turbulence is a function of destruction of turbulence associated with stability and the generation of turbulence by wind shear. We show that the increasing time constants through the period are related to increases in wind shear and a systematic decrease in the bulk Richardson number over the three nights.