Mesoscale optical turbulence simulations at Dome C: refinements

In a recent paper the authors presented an extended study aiming at simulating the classical meteorological parameters and the optical turbulence at Dome C during the winter with the atmospherical mesoscale model Meso-NH. A statistical analysis has been presented and the conclusions of that paper have been very promising. Wind speed and temperature fields revealed to be very well reconstructed by the Meso-NH model with better performances than what has been achieved with the European Centre for Medium-Range Weather Forecast (ECMWF) global model, especially near the surface. All results revealed to be resolution-dependent and it has been proved that a grid-nesting configuration (3 domains) with a high horizontal resolution (1km) for the innermost domain is necessary to reconstruct all the optical turbulence features with a good correlation to measurements. High resolution simulations provided an averaged surface layer thickness just ~14 m higher than what is estimated by measurements, the seeing in the free atmosphere showed a dispersion from the observed one of just a few hundredths of an arcsecond (~0.05"). The unique limitation of the previous study was that the optical turbulence in the surface layer appeared overestimated by the model in both low and high resolution modes. In this study we present the results obtained with an improved numerical configuration. The same 15 nights have been simulated, and we show that the model results now match almost perfectly the observations in all their features: the surface thickness, the seeing in the free atmosphere as well as in the surface layer. This result permits us to investigate now other antarctic sites using a robust numerical model well adapted to the extreme polar conditions (Meso-NH).

💡 Research Summary

This paper presents a comprehensive refinement of the mesoscale atmospheric model Meso‑NH for the simulation of optical turbulence over the Antarctic plateau, specifically at Dome C during the winter season. The authors build upon their earlier work (Lascaux et al., 2009), which demonstrated that a three‑level grid‑nesting configuration (horizontal resolutions of 25 km, 10 km, and 1 km) could reproduce the surface‑layer thickness and free‑atmosphere seeing with reasonable accuracy, but suffered from a systematic over‑estimation of turbulence within the surface layer. To overcome these limitations, two major upgrades were introduced.

First, the digital elevation model (DEM) was replaced. The previous GTOPO30 DEM, which provides coarse topographic detail, was substituted with the high‑resolution RAMPDEMv2 (Radarsat Antarctic Mapping Project DEM version 2). RAMPDEMv2 captures fine‑scale orographic features at resolutions finer than 5 km, correctly locating the Concordia station on the local summit and representing subtle slope variations around Dome C. This improvement yields more realistic near‑surface wind and temperature gradients, which are critical drivers of turbulence generation.

Second, the surface scheme of Meso‑NH, the ISBA (Interaction Soil‑Biosphere‑Atmosphere) module, was re‑parameterized for polar conditions. The thermal conductivity of the soil was adjusted for extreme cold, and two free parameters—a climatological deep‑soil temperature (T_c) and a relaxation term (γ)—were calibrated using a full year of surface measurements (solar radiation, air temperature, wind speed, pressure, humidity). By minimizing the discrepancy between simulated and observed temperatures at −5 cm and −30 cm depths, the authors achieved a highly accurate representation of the surface temperature (T_s) and the subsurface temperature (T_2). This, in turn, improves the calculation of the sensible heat flux (H), which governs buoyancy‑driven turbulence in the surface layer.

The refined model was applied to the same fifteen winter nights (July–September 2005) for which high‑resolution C_N² profiles were available from Trinquet et al. (2008). Surface‑layer thickness (h_sl) was defined as the height containing 90 % of the integrated C_N² within the first kilometre. In the high‑resolution (1 km) configuration, the mean h_sl was 44.2 ± 6.6 m, compared with the observed 35.3 ± 5.1 m; the discrepancy is only ~3 m when statistical error (σ/√N) is considered. The low‑resolution (100 km) configuration yielded a mean h_sl of 68.4 ± 6.8 m, confirming that fine horizontal resolution is essential for accurate surface‑layer representation.

Seeing statistics were also evaluated. The median total seeing (ε_TOT) in the high‑resolution run was 1.7 ± 0.21 arcsec, virtually identical to the observed 1.6 ± 0.2 arcsec. The free‑atmosphere seeing (ε_FA) matched perfectly, with a median of 0.30 ± 0.17 arcsec versus the observed 0.30 ± 0.20 arcsec. In contrast, the low‑resolution run over‑estimated total seeing (2.05 ± 0.21 arcsec). Vertical C_N² profiles further illustrate the improvement: the high‑resolution model now reproduces the steep decline of turbulence within the first 60 m above ground, aligning closely with measurements, whereas the previous configuration displayed a more gradual decrease. Some residual discrepancies remain above 60 m, indicating room for further refinement of turbulence parameterizations.

Overall, the study demonstrates that the combination of a high‑resolution, three‑domain nesting strategy, an accurate DEM, and a polar‑optimized surface scheme enables Meso‑NH to predict optical turbulence over Dome C with both qualitative and quantitative fidelity. The authors argue that this robust configuration can be transferred to other sites on the internal Antarctic plateau (e.g., Dome A, Dome F) without site‑specific recalibration, facilitating comparative site testing for astronomical facilities.

Future work suggested includes extending simulations to multi‑month or multi‑year periods to assess seasonal variability, integrating the model into real‑time forecasting pipelines for adaptive optics operations, and acquiring additional in‑situ turbulence measurements (e.g., SODAR, balloon‑borne microthermal sensors) to further constrain model parameters. Such developments will be crucial for the design and operation of next‑generation telescopes in Antarctica, where accurate knowledge of the vertical distribution of optical turbulence directly impacts the performance of high‑resolution imaging and interferometry.