Space-time scales of temperature variability in the seasonal thermocline of Lake Tahoe Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/xs55mg117

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  • Thermal structure in the seasonal thermocline at Lake Tahoe was investigated through the analysis of vertical temperature profiles taken in the upper 70 m during late summer stratification. Different ranges of horizontal and temporal scales were sampled in each of three subsets of finestructure profiles: (1) a three-week sequence at a midlake station, (2) a 2 1/2 hour, 12 km transect across the lake, and (3) a 2 1/2 hour sequence of intensive sampling over a 200 m transect line at midlake. The ensemble mean profile was much the same for each set, but the ensemble temperature variance, similar for (1) and (2) , was smaller at all depths and distributed differently in the vertical for (3). As a working hypothesis, it is assumed that a substantial amount of the observed temperature variability is due to internal wave displacements. Within this framework, the observations of set (3) are found to be consistent with generation by a field of small-scale internal waves obeying WKB displacement scaling. In contrast, over the longer time scales of (1) , or the larger area of (2), the dominant contribution to the variance appears to have come from a few, low order, vertical modes. Vertical wavenumber spectra of temperature fluctuations for all three sets were similar for wavenumbers greater than about 0.1 cpm, falling as wavenumber to the -5/2 power. When interpreted as spectra of vertical displacements, the level in the 0.1 to 1.0 cpm decade was fairly constant even though the local buoyancy frequency for individual records varied from 3 to 13 cph; this spectral level was somewhat lower (by a factor of 0.3 to 0.7) than that reported for various oceanic results. WKB internal wave scaling did not improve the spectral collapse effected by the "displacement scaling" of temperature spectra. Horizontal-temporal coherence fell to 0.5, for vertical wavelengths of 13 m, at a separation of 70 in and 12 minutes. For 6.5 m wavelengths the separation was 44 in and 7 minutes. A set of microstructure profiles was also obtained during the same project. These revealed the characteristic patchiness of microstructure activity, and the occurrence of the most intense signals in a shore-bound mixing layer over the steeply sloped bottom. With two horizontally separated thermistors on one instrument, it was determined that the shape of microscale features varied with scale, larger features being more flattened. The shape also depended on the local stratification.
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