Physics – Optics
Scientific paper
Sep 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011amos.confe..43a&link_type=abstract
Proceedings of the Advanced Maui Optical and Space Surveillance Technologies Conference, held in Wailea, Maui, Hawaii, September
Physics
Optics
Scientific paper
Clouds and optical turbulence are key drivers in the performance of optical imaging and communication systems. Clouds are composed of liquid water and/or ice crystals and depending on the physical thickness can produce atmospheric fades easily exceeding 10 dB. In these more common cases, impacts on optical imaging and communication systems may be severe. On the other hand, there are times when cloud fades may be as low as 1 or 2 dB as a result of thin, ice crystal based cirrus clouds. In these cases, the impacts on imaging and communication collectors may be limited. Atmospheric optical turbulence acts to distort light in the atmosphere, degrading imagery from telescopes. The quality of service of a free space optical communications link may also be impacted. Some of the degradation due to turbulence can be corrected by adaptive optics. However, the severity of optical turbulence, and thus the amount of correction required, is largely dependent upon distributions of turbulence at the location of interest. Large variations in the Fried Coherence Length (ro) are common as a function of time of day and by location and can range from just a few centimeters to tens of centimeters.
The ability to characterize the distribution and frequency of clouds and optical turbulence are critical in order to understand and predict atmospheric impacts. A state-of-the-art cloud detection system has been developed, validated and applied to produce high resolution climatologies in order to investigate these impacts. The cloud detection system uses global in coverage, geostationary, multi-spectral satellite imagery at horizontal resolutions up to one kilometer and temporal resolutions up to fifteen minutes. Multi-spectral imagery from the visible wavelengths (0.6 μm) through the longwave infrared (15 μm) are used to produce individual cloud tests which are combined to produce a composite cloud analysis. The basis for the detection algorithm relies on accurate modeling of the clear sky background (CSB). The CSB represents a recent depiction (one month weighted average) of what the scene looks like, radiometrically, in the absence of clouds so that it can be compared with imagery at the requested analysis time. If the actual imagery compared to the CSB differs by more than a specified threshold then clouds are indicated. Cloud properties such as cloud top heights and bases and optical depths are subsequently derived. The result represents a high spatial and temporal resolution climatology that can be used to derive accurate Cloud Free Line of Sight (CFLOS) statistics in order to quantify atmospheric effects on optical imaging and communication systems. For example, clouds over the State of Hawaii are quite variable in frequency ranging from less than 15% in some of the sheltered coastal waters and local summits to greater than 70% on the mauka (windward) sides of the islands. Vertical optical depths from the summit can range from 0.5dB to greater than 50dB.
Optical turbulence is characterized by the refractive index structure function Cn2, which in turn is used to calculate atmospheric seeing parameters. While attempts have been made to characterize Cn2 using empirical models, it can be calculated more directly from Numerical Weather Prediction (NWP) simulations using pressure, temperature, thermal stability, vertical wind shear, turbulent Prandtl number, and turbulence kinetic energy (TKE). A modified version of the Weather Research and Forecast (WRF) model is used to generate Cn2 throughout the atmospheric column, allowing for ground-to-space seeing estimates of ro. Simulations are performed using the Maui High Performance Computing Centers (MHPCC) Mana cluster.
Detailed results from both the clouds and turbulence simulations will be shown at the conference with specific applications to space imaging and communication systems.
Alliss R.
Felton B.
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