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Diurnal and annual variations in mean profiles of Cn²
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1991
[PDF-73.55 MB]
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Description:Individual profiles of turbulent refractive index structure function parameter, Cn2 , exhibit considerable vertical structure and temporal variability. The planetary boundary layer (PBL) is characterized by significant diurnal variations over land because of the daily solar heating cycle. Above the PBL, the turbulence is primarily due to instabilities associated with breaking gravity waves and local fluctuations due to a broad spectrum of gravity waves. A typical average C profile for optical wavelengths that takes these physical differences into account by considering the PBL and free atmosphere separately was constructed. Physical models of turbulence in these two regimes are used as a basis for constructing the climatology. The PBL profiles are obtained from a boundary layer turbulence model driven by climatological values of surface forcing (winds and net radiation), boundary layer depth, and internal energy partitioning (Bowen ratio and soil flux). The free atmospheric profile is obtained using a gravity-wave-induced turbulence model, which implies that the temperature structure function parameter, CT2 , is constant with altitude within an atmospheric height regime (either the troposphere or the stratosphere). The values chosen are a compromise between the model predictions (based on an estimate^f the global gravity wave energy) and 5 years of measurements of radar Cn obtained by NOAA jjadar wind profilers in Colorado. In the free atmosphere variations in C are lognormally distributed (although it may be bimodal) at a selected height. Roughly 68% of high-resolution measurements fall within a factor of 10 of the median (or log average)• Linear averages are a factor of 6 greater than log averages in the troposphere and a factor of 40 greater in the stratosphere.
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