A Comparison of Potential Temperature Variance Budgets over Vegetated and Non-Vegetated Surface

Abstract:

Over the past decades, researchers have achieved a fundamental understanding of the budgets of turbulent variables over simplified and (more recently) complex terrain. However, potential temperature variance budgets, parameterized in most meteorological models, are still poorly understood even under relatively idealized conditions. Although each term of the potential temperature variance budget has been studied over different stabilities and surfaces, a detailed understanding of turbulent heat transport over different types of surfaces is still missing. The objectives of this study are thus: 1) to quantify the significant terms in the potential temperature variance budget equation; 2) to show the variability of the budget terms as a function of height and stability; 3) to model the potential temperature variance decay in the late-afternoon and early-evening periods. To do this, we rely on near-surface turbulence observations collected within the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) program, which was designed to better understand the physics governing processes in mountainous terrain. As part of MATERHORN, large field campaigns were conducted in October 2012 and May 2013 in western Utah. Here, we contrast two field sites: a desert playa (dry lakebed), characterized by a flat surface devoid of vegetation, and a vegetated site, characterized by a low-elevation valley floor covered with greasewood vegetation. As expected, preliminary data analysis reveals that the production and molecular dissipation terms play important roles in the variance budget, however the turbulent transport term is also significant during certain time periods at lower levels (i.e., below 5 m). Our results also show that all three terms decrease with increasing height below 10 m and remain almost constant between 10 m to 25 m, which indicates an extremely shallow surface layer (i.e. 10 m). Further, at all heights and times an imbalance between production and dissipation terms is observed.