Optical Turbulence Simulations from a Numerical Weather Prediction Model in Support of Air to Air Laser Communications

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Monday, 18 January 2010
Randall Alliss, Northrop Grumann Corporation, Chantilly, VA; and B. Felton

Handout (480.1 kB)

Optical turbulence (OT) acts to distort light in the atmosphere, degrading imagery from large astronomical telescopes and possibly reducing data quality of air to air laser communication links. 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 the turbulence at the location of interest. Therefore, it is vital to understand the climatology of optical turbulence at such locations. In many cases, it is impractical and expensive to setup instrumentation to characterize the climatology of OT, so simulations become a less expensive and convenient alternative.

The strength of OT 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, Cn2 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). In this work we use the Weather Research and Forecast (WRF) NWP model to generate Cn2 climatologies in the planetary boundary layer and free atmosphere, allowing for both point-to-point and ground-to-space seeing estimates of the Fried Coherence length (ro) and other seeing parameters. Simulations are performed using the Maui High Performance Computing Centers Jaw's cluster.

The WRF model is configured to run at 1km horizontal resolution over a domain covering the islands of Maui and the Big Island. The vertical resolution varies from 25 meters in the boundary layer to 500 meters in the stratosphere. The model top is 20 km. We are interested in the variations in Cn2 and the Fried Coherence Length (ro) between the summits of Haleakala and Mauna Loa. Over six months of simulations have been performed over this area. Simulations indicate that the vast lava fields which characterize the Big Island to the shoreline have a large impact on turbulence generation. The same turbulence characteristics are also present in the simulations on the Southeastern face of Haleakala. Turbulence is greatest during the daytime when the lava fields produce tremendous heat fluxes. Good agreement is found when the WRF simulations are compared to in situ data taken from the Advanced Technology Solar Telescope (ATST) Site Survey Working Group at the Mees Solar Observatory on Haleakala. The ATST used a solar DIMM instrument; therefore comparisons were limited to daytime. Both the WRF simulations and ATST showed ro values bottoming out in the 3-4 cm range during daytime. Analysis of the horizontal path between Haleakala and Mauna Loa show minimum ro dropping below 1 cm during the peak heating of the day. We are awaiting horizontal observations of Cn2 to become available to continue the validation exercises. Results of these analyses are assisting communication engineers in developing state of the art adaptive optic designs for aircraft to aircraft laser communications. Detailed results of this study will be presented at the conference.