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Relationship between Low-Level Jet Properties and Turbulence Kinetic Energy in the Nocturnal Stable Boundary Layer

Robert M. BantaNOAA/Environmental Technology Laboratory, Boulder, Colorado

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Yelena L. PichuginaCooperative Institute for Research in the Atmosphere, Colorado State University, Fort Collins, Colorado

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Rob K. NewsomCooperative Institute for Research in the Atmosphere, Colorado State University, Fort Collins, Colorado

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Abstract

In the nighttime stable boundary layer (SBL), shear and turbulence are generated in the layer between the maximum of the low-level jet (LLJ) and the earth's surface. Here, it is investigated whether gross properties of the LLJ—its height and speed—could be used to diagnose turbulence intensities in this subjet layer. Data on the height and speed of the LLJ maximum were available at high vertical and temporal resolution using the high-resolution Doppler lidar (HRDL). These data were used to estimate a subjet layer shear, which was computed as the ratio of the speed to the height of the jet maximum, and a jet Richardson number RiJ, averaged at 15-min intervals for 10 nights when HRDL LLJ data were available for this study. The shear and RiJ values were compared with turbulence kinetic energy (TKE) values measured near the top of the 60-m tower at the Cooperative Atmosphere–Surface Exchange Study-1999 (CASES-99) main site. TKE values were small for RiJ greater than 0.4, but as RiJ decreased to less than ∼0.4, TKE values increased, indicating that RiJ does have merit in estimating turbulence magnitudes. Another interesting finding was that shear values tended to cluster around a constant value of 0.1 s−1 for TKE values that were not too small, that is, for TKE greater than ∼0.1 m2 s−2.

Corresponding author address: Robert M. Banta, NOAA (ET2), 325 Broadway, Boulder, CO 80305. Email: robert.banta@noaa.gov

Abstract

In the nighttime stable boundary layer (SBL), shear and turbulence are generated in the layer between the maximum of the low-level jet (LLJ) and the earth's surface. Here, it is investigated whether gross properties of the LLJ—its height and speed—could be used to diagnose turbulence intensities in this subjet layer. Data on the height and speed of the LLJ maximum were available at high vertical and temporal resolution using the high-resolution Doppler lidar (HRDL). These data were used to estimate a subjet layer shear, which was computed as the ratio of the speed to the height of the jet maximum, and a jet Richardson number RiJ, averaged at 15-min intervals for 10 nights when HRDL LLJ data were available for this study. The shear and RiJ values were compared with turbulence kinetic energy (TKE) values measured near the top of the 60-m tower at the Cooperative Atmosphere–Surface Exchange Study-1999 (CASES-99) main site. TKE values were small for RiJ greater than 0.4, but as RiJ decreased to less than ∼0.4, TKE values increased, indicating that RiJ does have merit in estimating turbulence magnitudes. Another interesting finding was that shear values tended to cluster around a constant value of 0.1 s−1 for TKE values that were not too small, that is, for TKE greater than ∼0.1 m2 s−2.

Corresponding author address: Robert M. Banta, NOAA (ET2), 325 Broadway, Boulder, CO 80305. Email: robert.banta@noaa.gov

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