Numerical Simulation of the Nocturnal Turbulence Characteristics over Rattlesnake Mountain

A two-dimensional second-order turbulence-closure model based on Mellor-Yamada level 3 is used to examine the nocturnal turbulence characteristics over Rattlesnake Mountain in Washington. Simulations of mean horizontal velocities and potential temperatures agree well with data. The equations for the components of the turbulent kinetic energy (TKE) show that anisotropy contributes in ways that are counter to our intuition developed from mean flow considerations: shear production under stable conditions forces the suppression of the vertical component proportion of total TKE, while potential-temperature variance under stable conditions leads to a positive (countergradient) contribution to the heat flux that increases the vertical component proportion of total TKE. This paper provides a qualitative analysis of simulated turbulence fields, which indicates significant variation over the windward and leeward slopes. From the simulation results, turbulence anisotropy is seen to develop in the katabatic tlow region where vertical wind shears and atmospheric stability are large. An enhancement of the vertical component proportion of the total TKE takes place over the leeward slope as the downslope distance increases. The countergradient portion of the turbulent heat flux plays an important role in producing regions of anisotropy.