Randall Alliss (Northrop Grumman Information Technology, TASC), Billy Felton (Northrop Grumman Information Technology, TASC), and Eric Kemp (Northrop Grumman Information Technology, TASC)
Keywords: Atmospherics, Space Weather
Abstract:
Optical turbulence (OT) acts to distort light in the atmosphere, degrading imagery from large astronomical telescopes and possibly reducing data quality of laser communication links. Some of the degradation due to turbulence can be corrected by adaptive optics. However, the severity of OT, and thus the amount of correction required, is largely dependent on the turbulence at the location of interest. Therefore, it is vital to understand the climatology of OT at such locations. In many cases, it is impractical and expensive to set up 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. Although 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 Forecasting (WRF) NWP model to generate Cn2 climatologies in the planetary boundary layer and free atmosphere, allowing 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 Center’s Jaws cluster.
The WRF model is configured to run at 1-km horizontal resolution over a ~60-km by 60-km domain. The vertical resolution varies from 25 m in the boundary layer to 500 m in the stratosphere. The model top is 20 km. The Mellor-Yamada-Janjic (MYJ) TKE scheme has been modified to diagnose the turbulent Prandtl number as a function of the Richardson number, following observations by Kondo and others. This modification deweights the contribution of the buoyancy term in the equation for TKE by reducing the ratio of the eddy diffusivity of heat to momentum. This is necessary particularly in the stably stratified free atmosphere where turbulence occurs in thin layers not typically resolvable by the model. The modified MYJ scheme increases the probability and strength of TKE in thermally stable conditions, thereby increasing the probability of OT. Over 12 months of simulations have been generated. Results indicate realistic values of the ro are obtained when compared with observations from a Differential Image Motion Monitor instrument. Seeing is worse during day than at night with large ro’s observed just after sunset and just before sunrise. Three-dimensional maps indicate how seeing varies as a function of location and elevation. This study has shown that urban heat islands and nighttime low-level jets can greatly influence the production of OT. Detailed results of this study will be presented at the conference.
Date of Conference: September 16-19, 2008
Track: Atmospherics/Space Weather