Abstract
In this study, a climatology of two key boundary layer features, the Planetary Boundary Layer Height (PBLH) and the wind field over Greece is derived. The climatology is based on daily soundings collected in Athens, Thessaloniki and Heraklion and spanning a 32-year period. The PBLH is estimated using a method based on the gradient of potential temperature and a method based on the bulk Richardson number. The wind field is analyzed by calculating the wind shear and the turning angle of the wind vector between the surface and the top of the boundary layer. The PBLH of the daytime boundary layer over Athens and Thessaloniki is found to exhibit seasonal variability with summer maxima and winter minima and has annual median values in the range of 1.4–1.7 km estimated using the gradient method. The PBLH over Heraklion is found to exhibit weak seasonal variability with a lower median value of 1.2 km. The nighttime boundary layer over all three sites is found to be much shallower with PBLH values in the range of 150–200 m with no seasonal variations. In addition, the bulk Richardson number method is found to systematically underestimate the PBLH compared to the gradient method. The wind field in the daytime boundary layer at all three sites is found to have small shear of the order of 1 ms−1 and wind turning angles that are lower than 15 degrees, while in the nocturnal boundary layer it has larger shear of the order of 5–10 ms−1 with turning angles lower than 20 degrees. In addition, for both the daytime and the nighttime boundary layer there is no general preference for veering or backing.
Highlights
The Planetary Boundary layer (PBL) is the lowest layer of the atmosphere where the Earth’s surface interacts with the free troposphere through energy, momentum, moisture, and chemical compounds exchanges
The uniform vertical distributions of these dynamic variables extend until the stable inversion layer that forms a transition zone to the free troposphere as shown in Figure 3a illustrating the vertical profile of potential temperature for a typical Convective Boundary Layer (CBL) over Athens
The Planetary Boundary Layer Height (PBLH) was derived by first categorizing the boundary layer in three types based on the gradient of potential temperature near the surface: the Convective Boundary Layer (CBL), the Neutral Boundary Layer (NBL) and the Stable Boundary
Summary
The Planetary Boundary layer (PBL) is the lowest layer of the atmosphere where the Earth’s surface interacts with the free troposphere through energy, momentum, moisture, and chemical compounds exchanges. The key role of the PBL in many aspects of weather, climate and air quality has long been recognized since it is involved in many processes such as convection, turbulent mixing, low-level cloud and fog formation, pollutants dispersion and the surface energy budget. The realistic parametrization of PBL characteristics and their temporal evolution is critical to weather forecast and to climate and air pollution models. The PBL structure is determined by the complex interactions between the surface forcing, the local circulation and the synoptic flow and exhibits variability in a large range of spatial and temporal scales [1,2]. To characterize the complex PBL structure, two key features have been widely used: the Planetary Boundary Layer Height (PBLH) and the mean wind field as described by the wind shear and the turning of the wind vector. The PBLH determines the vertical extent of Atmosphere 2020, 11, 910; doi:10.3390/atmos11090910 www.mdpi.com/journal/atmosphere
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