Randall Alliss, Northrop Grumman Corporation
Keywords: Optical turbulence seeing parameters, AO design, high performance computing
Abstract:
Quantifying atmospheric seeing for applications such as LADAR, spaced based imaging, and optical communications is not new. Much effort and expense has been expended to develop ground-based instrumentation to estimate optical turbulence so that mitigation techniques can be developed. Instruments, such as a Differential Imaging Motion Monitors (DIMM), which track stars or solar DIMMs which track the sun are often deployed to collect measurements. These field collection campaigns require many years of observations which are costly but essential to develop mitigation techniques. Adaptive optics (AO) systems are a common form of ground-based turbulence mitigation, particularly for space domain awareness and coherent space to ground optical communications, but they are often custom and very expensive. Because of the complexity and high costs, it is critical to avoid over engineering the AO system based on observations that can take many years to collect. As ground-based optical ground stations begin to proliferate the ability to quickly model and estimate atmospheric seeing is critical and is now possible given the availability and low cost of high-performance computing.
The severity of optical turbulence can be characterized by the refractive index structure function Cn2, which in turn is used to calculate atmospheric seeing parameters through various integration equations. While attempts have been made to characterize Cn2 using empirical models (e.g., Huffnagel-Valley), a novel method for computing Cn2 has been developed. Using an atmospheric Numerical Weather Model (NWM), the index of refraction is more directly computed using three dimensional modeled pressure, temperature, thermal stability, vertical wind shear, turbulent Prandtl number, and turbulence kinetic energy (TKE). NWMs are three dimensional models of the atmosphere that include full physics, dynamics, and thermodynamics, by solving the Navier-Stokes equations numerically. These models have been greatly improved over the years and are enhanced by advances in numerical data assimilation particularly from meteorological satellites. In this work, we use an advanced NWM customized to generate high resolution vertical profiles of Cn2 from the surface layer to the top of the atmosphere allowing for both horizontal, slantwise and vertical seeing estimates of the Fried Coherence length (ro), Greenwood frequency (Fg) and Isoplanatic angle (Qo). Simulations, which take just a few weeks to run on a modest high-performance computing system, are capable of producing multiple years of realistic seeing parameters.
The custom NWM model is configured to run at 1 kilometer (km) horizontal resolution over a 60 km by 60 km region. The resolution of the vertical levels is approximately 60-m below 3 km above ground level (AGL), 70-200 m between 312 km AGL, and 200-500 m up to the model top (10 millibars). The Mellor-Yamada-Janjic (MYJ) TKE scheme is 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 these kinds of models. The modified MYJ scheme increases the probability and strength of TKE in thermally stable conditions thereby increasing the probability of optical turbulence. Over two years of simulations have been generated for several desert locations, one in each hemisphere. Results indicate realistic values of the seeing parameters and model output matches the expected diurnal variations for one of the desert sites. As expected, seeing is worse during the daytime summer (2 cm ro) than at night with large ros observed just after sunset and just before sunrise during the atmospheric neutral event.
This paper will review the NWM and the necessary modifications required for accurate seeing parameter estimations. Results from several desert locations, thought to have the most challenging optical turbulence for an AO system, will be shown including the seasonal and diurnal variations evident in the data.
Date of Conference: September 19-22, 2023
Track: Atmospherics/Space Weather