Planetary boundary layer influence on optical turbulence at AlUla Manara

We present the first in-situ characterisation of the AlUla Manara astronomical site, a mid-elevation desert plateau at 1.1 km AMSL in northwestern Saudi Arabia, using a one-year campaign combining a 30 m meteorological tower, a HATPRO microwave radiometer and a Shack–Hartmann Image Motion Monitor (SHIMM). We characterise the nocturnal boundary-layer regime and the vertical partition of optical turbulence, focusing on November–February. The cold-season median seeing is 1.12″, and the median fraction of integrated turbulence in the lowest SHIMM bin (0–4 km AGL) is 0.72, with a 95 per cent confidence interval of [0.71, 0.74]. Inversion strength, quantified by the squared Brunt–Väisälä frequency at the inversion top, N²ᵢₙᵥ, is modestly but significantly associated with this fraction (Spearman rs = +0.25), whereas inversion height shows no detectable association (rs = −0.05). After removal of the mean nocturnal cycle, the absolute 0–4 km integral shows no residual association with inversion strength, while the fractional association persists (rs = +0.18), reflecting anomalously weak free-atmospheric turbulence under stronger inversions. Bad-seeing observations are preferentially associated with westerly and southerly winds. During the 2025–2026 campaign, the sampled cold-season window exhibited better seeing than the sampled warm half-year, contrary to the seasonal ordering of the published ERA5-based benchmark. These results establish an in-situ observational baseline for AlUla and provide constraints for future higher-resolution, multi-season turbulence monitoring.