Dominant Patterns of Variability of the Planetary Boundary Layer Height in Romania During the Warm Season from ERA5 Reanalysis Data (1989–2022)

The planetary boundary layer height (PBLH) represents an essential parameter of the climate system, which controls the exchanges of momentum, heat, moisture, and pollutants between the terrestrial surface and the free atmosphere. In this study, the spatiotemporal variability of the PBLH in Romania during the warm season (May–September) was investigated utilizing the ERA5 reanalysis data for the 1989-2022 period. To identify the dominant modes of variability, the Empirical Orthogonal Function (EOF) analysis was applied, conducted separately for the daily, daytime, and nighttime fields of the PBLH. The results highlight the fact that the first EOF mode explains more than half of the total PBLH variability and is associated with the variability of the large-scale atmospheric circulation, represented by the North Atlantic Oscillation (NAO). The subsequent EOF modes reflect the influence of regional thermodynamic processes, heatwaves, atmospheric moisture, and the meridional circulation modulated by the Carpathian topography. The findings indicate that the evolution of the planetary boundary layer in Romania is determined by the interaction between the hemispheric-scale atmospheric circulation, regional thermodynamic processes, and the topographic effects associated with the Carpathian arc. These results contribute to the understanding of the mechanisms that control PBLH variability in Southeastern Europe and provide useful information for climatological and air quality studies.