Low-level Turbulent Zones and Associated Rotors in the Lee of Mountains

Northwest Research Associates, Incorporated, Seattle WA

Investigators

Abstract

This research utilizes data from numerical simulations and field observations to evaluate and extend existing dynamical understanding of low-level turbulent zones (LLTZ) and rotors associated with mountain waves. In recent decades, LLTZ have been left largely unstudied, despite their potential feedbacks on mountain wave characteristics and structure, their influence on destructive down-slope windstorms, dispersion, and the danger they present to aircraft operations throughout the world. Advances in mesoscale models and computing power have made high-resolution simulations of this atmospheric phenomenon feasible, thereby creating the opportunity for potentially large advances in understanding of these important counterparts to mountain waves. The authors hypothesize the following: 1) The most intense and turbulent LLTZ occur in association with high-amplitude mountain waves; less intense LLTZ form in association with trapped lee waves. 2) The severity of LLTZ depends on the horizontal vorticity budget, which in turn depends on details of the upstream environment; hence the LLTZ intensity can be quantified and forecast. 3) The diurnally evolving boundary layer can significantly modify the spatial structure of LLTZ, which then modifies the overlying mountain wavelength and amplitude. The research will employ numerical simulations of increasing complexity. Simulations will use fine resolution, with grid spacing of O[100m], since most previous numerical mountain wave studies have used grid spacing too coarse to adequately capture details within LLTZ. The work begins with two-dimensional simulations initialized using a single upstream sounding with no moisture and using no surface heat fluxes. Realistic surface heating will be added over a diurnal cycle to more completely study the boundary layer effects on LLTZ. Recognizing the likely importance of three-dimensional effects, a series of idealized three-dimensional experiments similar to those in two dimensions will be run. Finally, the validity of the stated hypotheses will be explored by investigating the role of changing synoptic conditions superimposed on the diurnal cycle via a select number of fully three-dimensional case studies. Wherever possible observations from past and upcoming field campaigns will be utilized to validate conclusions obtained from simulations. It is expected that this study will yield a far more complete understanding of LLTZ and their relation to the overall mountain-wave system, and result in benefits within the atmospheric science, aviation, and air pollution disciplines.