An evaluation of several turbulence closure schemes for modelling thermally induced slope flows

Abstract

A one-dimensional high-resolution atmospheric boundary layer (ABL) model for studying slope flows has been developed and used to examine several turbulence closure schemes to understand the effects of turbulence closure on model results. The different schemes are compared in terms of their influences on mean variables, turbulent fluxes, turbulent kinetic energy (TKE), TKE budget and eddy diffusivity. The results show that simulations from standard E- ε closure and its modification differ significantly from the other four closure schemes, which all produce similar slope flows with minor differences in mean profiles and major differences in eddy diffusivities. Model simulations using E- ℓ and q²ℓ Model I are also compared with observations of katabatic flows taken over the sloping ice surface of the Vatnajökull in Iceland. Generally, it is found that model simulations are in good agreement with observations for mean wind and temperature profiles as well as turbulent quantities with a slightly under-predicted TKE. E- ℓ and q²ℓ Model I are also used to simulate a diurnal cycle of idealized slope winds under calm background winds and stably stratified ambient conditions. Differences of thermodynamic and dynamic structures between downslope and upslope winds are investigated.

Key words: ABL model, Slope winds (Katabatic, Anabatic), Turbulence closure schemes