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. / Graduation date: 1980
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/27891 |
| Date | 07 September 1979 |
| Creators | Brubaker, John Merrill |
| Contributors | Dillon, Thomas M. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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