Eddy-resolving lidar measurements and numerical simulations of the convective internal boundary layer

This paper will describe our continuing effort to understand the organization of eddies within, and dynamics of, convective thermal internal boundary layers (TIBLs). This region exists from the shore to about 10-km offshore during cold-air outbreaks along the western coast of Lake Michigan. In this region, the shear driven mixed layer (generated over snow-covered Wisconsin) transforms into a mostly convective mixed layer over the water.

Our eddy-resolving tools are 4-D aerosol backscatter lidar observations and large-eddy simulations (LES) using a scalable, quasi-compressible, nonhydrostatic model. Data collected during Lake-ICE with the University of Wisconsin's Volume Imaging Lidar (UW-VIL) will be compared with simulations using the University of Wisconsin's Nonhydrostatic Modeling System (UW-NMS).

To simulate the TIBL, our model domain is typically 12-km in the east-west by 1.6 km in the north-south with 15-m resolution in all directions. The western 6-km of the domain lies over snow-covered land and the eastern 6-km over 279 K water. The model is initialized horizontally homogeneous using an observed radiosonde profile during strong cold-air advection (surface temperatures near -20 C). The observed profile is neutral up to about 400-m AGL above which a strong capping inversion exists. The model generates a turbulent boundary layer on the western side of the domain through shear instabilities. These eddies advect offshore where the surface drag is much smaller and the convective thermal forcing is intense (i.e the right side of the domain). A convective TIBL develops over the right side of the domain.

Our previous results presented at the 13th BL&T Symposium included extracting wind speed and direction every 250-m for the first 10-km offshore from the UW-VIL backscatter data. In this paper, we will present sizes, lifetimes, and orientation of the coherent structures (large-eddies) as a function of offshore distance. This information will be extracted from the 15-m resolution lidar data-set and simulated with the model. A passive-tracer which resembles lidar aerosol backscatter is included in the model to facilitate direct comparison with the observations.