Estimation of Precipitable Water Vapor over the Caucasus Region Using the Mesoscale WRF Model

Ground-based millimeter and submillimeter astronomy requires detailed astroweather and astroclimatic studies of potential sites in mountain regions. Using the mesoscale WRF model, validated against data from three upper-air stations, we examined the spatiotemporal distribution of precipitable water vapor (PWV), effective height of water vapor, wind field and cloudiness over the Caucasus region for the period from 26 March to 4 April 2025. Three operating observatories—SAO RAS, GAS GAO and Peak Terskol as well as three mountain summits in Dagestan, namely Dzhufudag, Kurapdag and Shunudag, promising for millimeter and submillimeter observations—were considered. The sites fall into two groups differing in the variability in PWV: a western group with values of 9.3 ± 1.9 mm at SAO RAS and GAS GAO (coefficient of variation 20–21%) and an eastern group (Dagestan) with a drier atmosphere ranging from 2.8 ± 1.2 to 4.3 ± 1.6 mm (36–42%). Given the similar values of effective height, the key factors driving the contrast in moisture content are the heights of sites, synoptic advection, and the specific pattern of airflows around local topographic features. The averaged PWV field is strongly shaped by orography: ranging from 4–8 mm along the high-mountain part of the Caucasus Range to 16–20 mm over the lowlands. On the night of 1–2 April 2025, when the regional minimum was observed, PWV fell by 3.7 and 2.7 mm in the western and the eastern group, the contrast between the sectors rose from a factor of 2.1 to a factor of 3.8, and the effective height dropped to 1497 and 1338 m. The driest site, Kurapdag, proved to be the windiest: the mean night-time wind speed there reaches 6.8 m/s against 1.8 m/s at SAO RAS. The comparison with the ERA5 reanalysis showed that the area-mean PWV over the 1 km domain is 10.6 mm in ERA5 and 9.3–9.6 mm in WRF. ERA5 reproduces the temporal behavior of PWV (r = 0.80–0.88), but owing to its smoothed orography it overestimates PWV over the ridges and underestimates it in the valleys. The differences between the WRF domains are largest on passing from a grid spacing of 9 km to 3 km. A linear regression between the ERA5 and WRF fields removes only 17–29% of the discrepancy at the summits, so that recalculating the reanalysis PWV to the summit level requires methods that explicitly account for the difference in terrain height.