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|Title:||Visual resolution and optical scintillation over snow, ice, and frozen ground : part 1|
|Authors:||University of Michigan. Meteorological Laboratories.|
Portman, Donald J., 1922-
Elder, Floyd C.
Noble, Vincent E.
|Publisher:||Cold Regions Research and Engineering Laboratory (U.S.)|
Engineer Research and Development Center (U.S.)
Summary: Optical scintillation, visual resolution, and wind and temperature profiles were measured over snow, ice and frozen ground. The data were analyzed to determine relationships between (1) scintillation and visual resolution and (2) scintillation and meteorological and surface conditions. The experimental results included (1) estimates of the limit of visual resolution, (2) telephotometer measurements of the apparent fluctuations in brightness (scintillation) of an artificial light source, and (3) measurements of wind direction and of the vertical distributions of wind speed and temperature. The optical path was 543 m long and 1.5 m above uniform horizontal surfaces. All scintillation and meteorological data are given in an appendix. The principal results of the analysis showed that for turbulent flow in stable stratification over snow (1) visual resolution deteriorated systematically as scintillation increased in intensity and (2) scintilliation intensity increased with increase in vertical temperature gradient. Scintilliation was at a minimum in the absence of thermal stratification and at a maximum (in very stable thermal stratification) during the sudden transition from laminar to turbulent flow. For a given temperature gradient, scintilliation increased with increase in wind speed. When wind and temperature gradients were combined in terms of the Richardson number and related to scintilliation, the data obtained over snow indicated a critical Richardson number of about 0.35. Scintilliation power spectra for eight periods revealed characteristics that could be related to visual resolution, the Richardson number and the mean wind speed component normal to the optical path. NOTE: This file is large. Allow your browser several minutes to download the file.
|Appears in Collections:||CRREL Research Report|
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