When wind flowing horizontally through the atmosphere hits high ground, it creates waves of air on the lee—or wind sheltered—side of the mountain. Airflow over mountainous regions can be tricky to reproduce. Although earlier versions of the vector vorticity equation model (VVM) have successfully replicated atmospheric flow over steep slopes and rugged terrain, modeling the dynamics of airflow over smaller mountains, for which the spacing of the vertical grid exceeds the mountain’s height, remains a challenge.
To improve these simulations, Chien and Wu have updated how surface topography is represented in the VVM. Using the immersed boundary method, the authors implemented a new partial step approach, which divides one previously large vertical step into a series of incremental ones. Because the height of the mountain is no longer restricted by the model’s grid spacing, this approach better represents gentler topography while preserving the same grid structure.
To validate this approach, the team conducted a standard performance test that simulated mountain waves. Although both methods produced similar shapes, the partial step approach better captured the wave’s smoothness on the lee side compared with the more jagged results of the full step approach.
The results of additional experiments, including simulations of orographic precipitation—rain or snow produced as moist air rises and traverses a mountain range—and conditions comparable to a valley’s slope winds, consistently demonstrated that the partial step approach creates smoother results without increasing the vertical resolution, which would increase the model’s computational requirements. The authors thus conclude that the model is ready to be used in simulations involving lower topography. (Journal of Advances in Modeling Earth Systems (JAMES), doi:10.1002/2015MS000514, 2016)
—Terri Cook, Freelance Writer
Citation: Cook, T. (2016), Improved models of wind flow over mountains, Eos, 97, doi:10.1029/2016EO045937. Published on 15 February 2016.