资源环境科技发展态势分析平台

The nature of organized motions within the convective boundary layer (CBL) depends on the surface heating and vertical wind shear. Moderate surface heating and strong shear favor horizontal convective rolls, while strong surface heating and weak shear favor open cellular convection. The transition between rolls and cells over land, particularly complex terrain, is still poorly understood. Due to lack of adequate measurements, variability of structural properties of rolls and cells (such as aspect ratio, wavelength and orientation relative to the mean surface winds) over complex terrain remains unclear. At present resolutions used in operational numerical weather prediction, rolls and cells are now becoming partly resolved, but the reliability of their forecasting in complex terrain remains elusive. The main objective of this research is the investigation of the characteristics of rolls and cells in a mountain valley compared to flat terrain.

To address this objective, we use the Weather Research and Forecasting Model (WRF) in LES mode. The domains cover the Owens Valley, California, the site of the Terrain-Induced Rotor Experiment (T-REX). We show that LES adequately simulated the complex valley-slope flow observed from flux towers and a wind profiler. Additionally, the vertical profiles of higher-order moments agree with those of textbook CBL behaviour. We found the transitional regime between rolls and cells to be broader compared to flat terrain when considering the bulk CBL stability parameter -z(i)/L. The width of the transitional regime depends on the temporal trend of -z(i)/L and the along-valley flow direction. Structural parameters of rolls and cells were different relative to their flat terrain counterparts, and a valley environment tends to introduce additional linearity into convection organization. Our results do not indicate a relationship between different convection modes and upslope flow strength.