Abstract
The planetary-boundary layer (PBL) plays an important role in air-pollution studies over urban/industrial areas. Therefore, numerous experimental/modelling efforts have been conducted to determine the PBL height and provide statistics. Nowadays, remote-sensing techniques such as ceilometers are valuable tools in PBL-height estimation. The National Observatory of Athens operates a Vaisala CL31 ceilometer. This study analyses its records over a 2-year period and provides statistics about the PBL height over Athens. A specifically developed algorithm reads the CL31 records and estimates the PBL height. The algorithm detects an upper and a lower PBL curve. The results show maximum values of about 2500 m above sea level (asl)/3000 m asl in early afternoon hours in all months for upper PBL, and particularly the summer ones, under all-/clear-sky conditions, respectively. On the contrary, the lower PBL does not possess a clear daily pattern. Nevertheless, one morning and another afternoon peak can be identified. The intra-annual variation of the upper PBL height shows a peak in August in all-weather conditions and in September under clear-sky ones. Season-wise, the upper PBL height varies showing an autumn peak for all-weather cases, while the lower PBL height shows a winter maximum due to persistent surface-temperature inversions in this season.
Highlights
The planetary-boundary layer (PBL) is the lower part of the troposphere where theEarth’s surface interacts with large-scale atmospheric flows
This methodology was later applied to solar radiation, illumination as well as atmospheric turbidity studies over Athens [79,80,81] and is applied to the present study too
It is seen that the uPBLH (MLH) forms a ridge with higher values across almost the entire day in the warm season of the year (June–September); this ridge becomes wider from midday to early afternoon hours
Summary
The planetary-boundary layer (PBL) is the lower part of the troposphere where the. Earth’s surface interacts with large-scale atmospheric flows. Substances emitted into the PBL disperse through atmospheric turbulence horizontally and vertically in a gradual way. Compton et al [2] report that the daytime PBL consists of a near-ground unstable surface layer (SL) with a depth of a few tens of metres, the ML on top of the SL with a thickness of few kilometres, and the ML is capped by the entrainment zone (EZ). Of a depth of a few hundred metres. They report that at night, the ML collapses and forms a stable nocturnal-boundary layer (NBL), a few hundred metres thick over the ground. The height of the ML is called the mixing-layer height (MLH)
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