Browsing by Subject "boundary layer meteorology"

The planetary boundary layer (PBL) is a layer of the atmosphere directly influenced by the presence of Earth's surface. In addition to its importance to the weather and climate systems, it plays significant role in controlling the air pollution levels and low-level heat conditions, thereby directly influencing the general well-being. While the modification of the boundary layer conditions by varying atmospheric forcings has been widely studied and discussed, it remains unknown what the dominant states of the PBL variation in response to this modification are. In this study, the dominant boundary layer types in both daytime and nighttime layers are examined. To understand the factors contributing to the development of these layers, weather regimes in the northern Atlantic-European region are considered. Machine learning techniques are utilized to study both the boundary layer and the large-scale flow classes, with an emphasis on unsupervised learning methods. It was found that the boundary layers in Helsinki, Finland, can be categorized into four daytime and three nighttime boundary layers, each characterized by the dominant turbulence production mechanism or the absence thereof. During the daytime, layers driven by both mechanical and buoyant turbulence are observed in summer, autumn, and spring, while individually buoyancy-driven layers occur in summer and winter, and individually mechanically-driven layers emerge in autumn, winter, and spring. Additionally, a layer characterized by overall reduced turbulence production is present throughout all seasons. During the nighttime, all three boundary layer types---individually buoyancy-driven, individually mechanically-driven, and stable layer---are observed in all seasons. Each boundary layer type exhibits season-specific variations, whereas daytime and nighttime boundary layers driven by the same mechanisms reflect the diurnal cycle of their relative intensities. The analysis revealed that the weather regimes producing cyclonic and anticyclonic flow anomalies over southern Finland collectively influence the boundary layer conditions, whereas the impact of individual weather regimes remains relatively small. Large-scale flow variation is associated with changes in the boundary layer dynamics through alterations in surface radiation budget (cloudiness) and wind conditions, thereby influencing the relative intensities of mechanical and buoyant turbulence production. However, inconsistencies in the analysis suggest that additional mechanisms, such as mesoscale phenomena, must also contribute to the development of the observed boundary layer types.