Discovery Magazine

Quantifying the Stable Boundary Layer Structure and Evolution during T-REX

Quantifying uncertainty in predictive models of the planetary boundary layer is the main theme of this research. The PI of this project is Dr. Sen Chiao at San Jose State University; Dr. George Maul of Florida Tech is the grant administrator.

This research focuses on quiescent Enhanced Observation Periods (EOPs) that occurred during the Terrain-Induced Rotor Experiment (T-REX) field project in spring 2006. These cases are associated with nocturnal low-level jet and valley wind/drainage flows in the Owens Valley, CA. The inhomogeneous land surface types in association with orographic effects over mountain-valley areas in the planetary boundary layer are not easy to simulate. The multi-scale interactions occur between the atmosphere and mountain-valley thermal contrast, which pose the difficulty in depicting downslope flows.

The Advanced Weather Research and Forecasting (WRF-ARW) model will be employed to learn how and the stable planetary boundary layer structure changed in terms of the onset of drainage flow and the occurrence of a nocturnal low-level jet in Owens Valley. In a typical WRF model simulation, subgrid mixing is parameterized within the planetary boundary layer physics. It is assumed that there is a clear scale separation between the horizontal and vertical mixing, where the vertical mixing is dominant. However, this assumption may not be valid when the horizontal grid spacing approaches 1 km or less, and a fully three-dimensional subgrid turbulence closure should replace the planetary boundary layer parameterized mixing.

In this project, a comprehensive study integrates surface observations, data from in-situ measurements and a nested numerical model with two related topics: (1) Turbulent Closure: To evaluate the two primary planetary boundary layer (PBL) parameterizations (Mellor-Yamada-Janjic scheme (MYJ) and Yonsei University scheme (YSU)) as well as quantify differences at a fine scale model output using the different turbulent mixing/diffusion options in the WRF-ARW model; and (2) Terrain Influences: To investigate the terrain slope effects upon the horizontal and vertical eddy viscosity coefficients calculation. The different terrain averaging and filtering/smoothing schemes will be examined in order to quantify their impact on stable planetary boundary layer forecast results.