Mountain Waves and Rotors - Revisiting the Concept of the “Lower Turbulent Zone”

In their seminal 1974 paper on “Lower Turbulent Zones Associated with Mountain Lee Waves”, P. F. Lester and W. A. Fingerhut attempted to characterize regions of low-level turbulence in the lee of mountain ranges, commonly associated with large-amplitude mountain waves aloft. Ever since that study, scientists have been trying to refine the conceptual model of the lower turbulent zone (LTZ) and shed more light on the structure and origin of turbulence therein.

The Terrain-Induced Rotor Experiment (T-REX, Sierra Nevada, CA, 2006) is the most recent, major effort organized to investigate the characteristics of LTZs by studying the coupled mountain-wave, rotor, and boundary-layer system. During the T-REX Intensive Observing Periods (IOPs), the University of Wyoming King Air with the Wyoming Cloud Radar on board measured the variation of the complex mountain flow along multiple flight legs across the Sierra Nevada. High-rate in situ and cloud radar measurements from T-REX missions allow us to examine the structure of the LTZ and the turbulence within it at unprecedented spatial resolution.

In this study, we make use of the extensive T-REX datasets from IOPs with strong gravity wave forcing in order to revisit the LTZ concept. The complete analysis of all the relevant IOPs (1, 2, 3, 4, 6, 11, and 13) calls for augmenting the classical picture of the LTZ by several additional elements, including the effective dimensions of the primary wave-generating obstacle, properties of the valley atmosphere (stable vs. convective), and the influence of the secondary ridge on the wave field aloft.