Urban turbulent kinetic energy budgets as influenced by nocturnal low-level jets

Nocturnal low-level jets (LLJs) are responsible for mesoscale transport and dispersion, for example advecting large quantities of moisture into the U.S. Great Plains during summer. LLJs are also assumed to affect smaller-scale transport and dispersion processes. The present study quantifies the mechanisms by which mesoscale phenomena, like nocturnal LLJs, affect turbulence and transport within urban areas.

To document atmospheric transport and dispersion in urban environments, the Joint URBAN 2003 field experiment took place in Oklahoma City, Oklahoma, during July 2003. Over one hundred three-dimensional sonic anemometers were deployed in and around the urban area to monitor wind speed, direction, and turbulence during dispersion experiments in the city center. Sonic anemometers were sited in various urban microenvironments, such as within a street canyon, at an intersection of two boulevards, atop a building in the built-up city center, in a less-built-up area just downwind of the city center, etc. The inertial dissipation method has been applied to data from these sonic anemometers to calculate turbulence kinetic energy (TKE) dissipation rates. In the case of a profile of sonic anemometers, all the terms of the TKE budget have also been calculated. Although daytime values of dissipation rates are typically at least a factor of two larger than nighttime values of disspation, some significant nocturnal events include short “bursts” with dissipation rates on the order of daytime values.

Concurrently, an hourly climatology of nocturnal low-level jet (LLJ) behavior in the OKC region has been prepared using boundary-layer wind profilers and sodars. Analysis of these datasets indicates that episodic turbulence bursting events induced by low-level jets descend to the levels observed with sonic anemometers, and enhanced TKE and dissipation rates are observed coincident with these bursting events, suggesting that urban studies must incorporate consideration of mesoscale events such as LLJs.