Validation Dataset for ERA5-Derived Atmospheric Refraction Coefficient (κ): Sağrak–Shkhara Line-of-Sight Path

Overview This deposit contains the independent-validation data supporting the revised Section 4.7 of "Real-Time Atmospheric Refraction Coefficient Determination Using ERA5 Reanalysis Data for Line-of-Sight Visibility and Geodetic Applications" (Alkalali, K.Z.A., under revision, Geodesy and Geodynamics). It validates the ERA5 L137-derived refraction coefficient (κ) for the Sağrak–Shkhara 493 km line-of-sight path (15 December 2024, 04:00 UTC) against four WMO radiosonde soundings, and documents the ERA5 land-sea-mask diagnosis of a coastal κ discrepancy. Path Observer: Sağrak Summit, Turkey (40.515°N, 38.185°E, 3035 m). Target: Shkhara, Georgia (43.001°N, 43.112°E, 5203 m). Great-circle distance: 493.26 km. ERA5 result: κ = 0.1467, required κ = 0.1527, deficit = −0.0060 (blocked). Contents Ten files: a README describing method and provenance; three JSON files (radiosonde path-κ validation, near-surface drift-free comparison, and ERA5 land-sea-mask diagnosis); two CSV files (along-path sampling-density convergence and 30-point detail); and the four full raw radiosonde soundings (Samsun 17030, Erzurum 17095, Tuapse 37011, Mineral'nye Vody 37055; all 00 UTC 15 December 2024) from the University of Wyoming Upper Air Archive, included in full for independent reproducibility. Method κ is computed identically for ERA5 and radiosonde data, differing only in data source: refractivity N = 77.6·P/T + 3.73×10⁵·e/T² (Smith–Weintraub / ITU-R P.453-14); vapour pressure from dewpoint via the Buck (1981) formula for soundings; the path altitude band (3035–5203 m) with linearly interpolated endpoints; ordinary least-squares gradient of N versus geometric altitude; κ = −R·(dN/dz)·10⁻⁶ with R = 6,371,000 m. Key findings The four-station radiosonde mean κ (0.1369) agrees with the ERA5 L137 value (0.1467) to within 6.7%, independently confirming that December 2024 conditions were insufficient for visibility along this path. Near-surface refractivity is reproduced by ERA5 to within a fraction of a percent at both coastal stations, and both data sources independently show a near-surface κ of 0.19–0.25 — roughly double the conventional design value of 0.13. The quality of κ-gradient agreement at coastal stations is governed by the ERA5 grid cell's land-sea fraction (land-dominated Samsun cell: 25% gap; sea-dominated Tuapse cell: 3.3% agreement) — a spatial-resolution effect, not a drift or measurement artefact. Reproducibility and verification The radiosonde-derived κ values in this deposit were independently recomputed directly from the raw soundings included here, and the land-sea-mask fields were retrieved via an authenticated Copernicus CDS API query; both are reproducible by any reader from the files in this deposit and the method described above, without access to the authors' proprietary computation backend.