21 February 2003 Low-altitude atmospheric turbulence characteristics at Mt. Wilson Observatory
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Abstract
Previous measurements of atmospheric density fluctuations have shown that a substantial fraction of seeing fluctuations occur within 100 feet of the ground, and that the power spectrum of path length fluctuations through the atmosphere has a somewhat smaller slope than that predicted by the Kolmogorov-Taylor approximation. To provide some possibility of appreciable path-length corrections, the ISI has assembled a system capable of measuring temperature changes at fifteen foot intervals of heights up to 70 feet from the ground. Analysis of temperature measurements made under a variety of conditions confirms previous results concerning the decrease in the magnitude of the fluctuations with altitude near the ground: the rms magnitude of the temperature fluctuations at an elevation of 70 feet is, on average, 52% of the mean rms value at 9 feet. However, these new measurements made at point locations show a power spectrum close to the Kolmogorov-Taylor prediction at frequenices up to 1.0 Hz, for average wind speeds above 2 m/s. In addition, correlation analysis between sensors located at the same elevation but separated by a given distance shows up to 50% correlation out to separations as large as 24 meters with wind speeds of a few meters per second, and indicate that Taylor's approximation applies over spatial distances in the range of 24 - 85 meters, or on timescales as large as ten seconds, and perhaps as large as 14 or 15 seconds. This makes path length corrections possible by temperature measurements at nearby locations.
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Nicholas Short, Walter Fitelson, David D. Snyder Hale, Charles H. Townes, "Low-altitude atmospheric turbulence characteristics at Mt. Wilson Observatory", Proc. SPIE 4838, Interferometry for Optical Astronomy II, (21 February 2003); doi: 10.1117/12.456998; https://doi.org/10.1117/12.456998
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