Extended Abstract (2.0M)4.2
Structure of the Atmospheric Boundary Layer in the Central Salt Lake Valley during the Afternoon-to-evening Transition
William J. Shaw, PNNL, Richland, WA; and J. M. Hubbe
As part of a continuing effort to understand the process of turbulence decay in the atmospheric boundary layer, we have measured profiles of winds and turbulence variables in the central Salt Lake Valley. These data were collected in an October 2000 field program that is part of the U.S. Department of Energy's Vertical Mixing (VTMX) initiative. Sampling was done using three collocated instrument systems: a sonic anemometer mounted at 10 m, a single-axis minisodar, and a 915 MHz wind profiler/RASS. These systems operated continuously for nearly three weeks. In treating the wind profiler data, we have made extensive use of the NCAR Improved Moment Algorithm (NIMA) for extracting both winds and turbulence from the profiler. Through reprocessing the profiler spectral data, the NIMA allows credible profiles of winds and, in particular, dissipation rate, to be constructed for periods as short as 5 min.
From the data we have identified two circulation classes that provide a context for the turbulence decay process. One class, which occurred on several days, was dominated by synoptic-scale weather systems and provided a background of moderate, steady winds during the entire surface heating and turbulence decay cycle. The second class, which comprised the large majority of days, was characterized by relatively light winds that were southerly (down-valley) in the pre-dawn and early-morning and switched to northerly (up-valley) during the day. Within this class, there were two distinct divisions. In one, perhaps corresponding to a general mountain-valley circulation, the switch from southerly to northerly flow in the lower boundary layer occurred by mid-morning, that is, early in the diurnal heating period, and the depth of the ensuing northerly flow was 1 to 2 km. In the other division, the direction shift occurred in mid-to-late afternoon. The depth of this latter northerly flow was typically about 500 m, suggesting that was likely a lake breeze generated by Great Salt Lake.
In this paper we will discuss the process of turbulence decay as reflected in measurements of turbulence kinetic energy, dissipation rate, and heat flux from the sonic anemometer in the context of these circulations. We will similarly discuss the time-height structure of dissipation rate and other turbulence variables from the profiler and minisodar.
Session 4, Boundary Layer Turbulence and Diffusion
Monday, 17 June 2002, 4:15 PM-5:30 PM
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