The spatial variability and structure of turbulent kinetic energy in the convective boundary layer over an isolated mountain

Abstract

Convective boundary layer (CBL) turbulence is not well understood over mountainous terrain. In the presences of topography, orographic forcings can create a multitude of sub-mesoscale phenomena that affect turbulence variability in the CBL. Improvement of numerical weather prediction relies on observations and an understanding of the effects of such phenomena on turbulence. The objectives of this study are to investigate the spatial variability of turbulent kinetic energy (TKE) and to distinguish the underlying mechanisms generating turbulence in a CBL over and around the isolated mountain, Granite Peak, Utah. To address these objectives, the analysis utilizes in-situ 10 Hz meteorological and Doppler Wind Lidar observations collected by a Navy Twin Otter research aircraft in the Fall 2012 Mountain Terrain Atmospheric Observations and Modeling Experiment (MATERHORN). The spatial variability of TKE is investigated by calculating TKE from in-situ airborne observations over and around Granite Peak. To examine the mechanisms important for TKE generation, the TKE budget terms of shear production, buoyancy production, and dissipation are estimated from in-situ aircraft and Twin Otter Doppler Wind Lidar observations. TKE estimated from aircraft show TKE maxima directly over ridge tops, the crest of Granite Peak, and in and at the exit of a terrain gap. Results from the TKE budget analysis indicate buoyancy production is a dominant contributor to TKE in the CBL at lower levels over ridge tops and the crest of Granite Peak. However, shear production is also a dominant source of turbulence within and above the CBL due to entrainment of higher momentum from aloft. During periods with winds > 5 m s-1, dynamically forced flows are an important mechanism for shear production of TKE above the CBL. Based on our findings, we show that TKE variability is significantly influenced by topography and that shear production can be a dominant production mechanism in and above CBL over an isolated mountain. The variability and generation of TKE are due to a complex array of phenomena including buoyant plumes, entrainment processes, and dynamically driven terrain flows. This study provides information for improving the representation of turbulence variability and the mechanisms important for turbulence generation over complex terrain.