Mesoscale modelling of optical turbulence in the atmosphere: the need for ultrahigh vertical grid resolution

Abstract

The high-fidelity modeling of optical turbulence is critical to the design and operation of a new class of emerging highly-sophisticated astronomical telescopes and adaptive optics instrumentation. In this study, we perform retrospective simulations of optical turbulence over the Hawaiian islands using a mesoscale model. The simulated results are validated against thermosonde data. We focus on turbulence in the free atmosphere, above the atmospheric boundary layer. The free-atmosphere is particularly important for adaptive optics performance and for sky coverage calculations and hence has significant impact on performance optimisation and scheduling of observations. We demonstrate that a vertical grid spacing of 100 m or finer is needed to faithfully capture the intrinsic variabilities of observed clear air turbulence. This is a particularly timely study because the next generation of Extremely Large Telescopes are currently under construction and their associated suite of instruments are in the design phase. Knowledge of the expected accuracy of optical turbulence simulations and real-time forecasts will enable the design teams to i) test and develop instrument designs and ii) formulate operational procedure.

Global atmospheric turbulence forecasting for free-space optical communications

“…Figure 5 shows the comparison between a state-of-the-art meso-scale model 16 and this model for the day of December 11th to 12th October at Bradshaw Army Airfield, Hawaii. The Pearson correlation of the logarithm of the turbulence strength is 0.75.…”

“…An alternative is to use measurement campaigns or small-spatial scale (meso-scale) simulations (see for example [13][14][15][16] ) to estimate the turbulence above the site. These both have the advantage of capturing the dynamics of the turbulence.…”

Computation of Optical Refractive Index Structure Parameter From Its Statistical Definition Using Radiosonde Data

“…From these quantities, empirical (e.g., Dewan model (Dewan, 1993), Hufnagel-Valley model (Andrews & Phillips, 2005;Hufnagel, 1985)) or theoretical (e.g., Tatarskii (1971)) models can be used to determine 𝐴𝐴 𝐴𝐴 2 𝑛𝑛 . Alternatively, NWP software can directly solve for 𝐴𝐴 𝐴𝐴 2 𝑛𝑛 using the turbulent kinetic energy, see Basu et al (2020) and Masciadri et al (2017). There also exist other approaches to characterize the scintillation index 𝐴𝐴 𝐴𝐴 2 𝐼𝐼 or the variance Regarding the determination of 𝐴𝐴 𝐴𝐴 2 𝑛𝑛 profiles based on meteorological quantities, focus will be given here to radiosonde measurements as they have an improved spatial resolution compared to NWP simulation data, especially in the vertical direction (Martini et al, 2017).…”

“…Alternatively, NWP software can directly solve for $${C}_{n}^{2}$$ using the turbulent kinetic energy, see Basu et al. (2020) and Masciadri et al. (2017).…”

Sensitivity study on the number of height levels used in a numerical weather model to determine the vertical profile of Cn2

“…This makes our study more comprehensive, since we now have two pathways to evaluate 𝐶 𝑛 2 . With the increasing recognition that high-resolution vertical modelling is essential to accurately evaluate 𝐶 𝑛 2 profiles, 6 our investigation contributes to the ongoing dialogue by addressing the critical role of vertical levels in forecast accuracy.…”