Turbulence Effects on the Formation of Cold Fog Over Complex Terrain With Large-Eddy Simulation

Abstract

A large eddy simulation (LES) embedded in the mesoscale community Weather Research and Forecasting model is used to examine the effects of turbulence on cold fog formation over Heber Valley in northern Utah in the U.S. The LES results indicate that large-scale turbulent eddies prevail and dominate the mixing effects in the planetary boundary layer (PBL). The combination of turbulence mixing effects, mountain-valley flow, and ultra-cold valley temperature lead to fog formation in the LES simulation. The omission of the turbulent eddies in the PBL parameterization results in a weak mixing effect in the PBL and weakens the near-surface air cooling, thus failing to reproduce the fog in the simulation. This study indicates the essential role of turbulence in fog formation over complex terrain. Turbulent eddies exist over complex terrain and can enhance mixing effects, and thus the conditions for fog formation.