Detection of Upper and Lower Planetary-Boundary Layer Curves and Estimation of Their Heights from Ceilometer Observations under All-Weather Conditions: Case of Athens, Greece

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.