Long-term ceilometer/wind-lidar observations of aerosol-layer depth in the Arctic

The depth of the aerosol layer at the Villum Research Station at Station Nord in the high Arctic is analyzed based on eight years of observations from a ceilometer. The depth of the aerosol layer is assigned to the inflection point in the attenuated backscatter profile by two methods; one is based on polynomial approximation of the profile and the other is direct numeric differentiation. The analysis is based on two types of hourly profiles; one consists of averaging the attenuated backscatter observations and the other by computing the median. It is noted that the depth of the aerosol layer in the Arctic might be different from the depth of the turbulent layer near the ground.

Near the ground, the observed backscatter exhibits a pronounced seasonal variation, having low values during the summer and high values during the winter. The strength of the seasonal variability decreases with height and is virtually absent at 1 km.

Due to sporadic occurrence of outliers in the range between 50 m and 80 m in the ceilometer observations, this part of the profile is not used in this study. Restricting the observations to heights above100 m, the depths of the aerosol layer are found to be typically 230-250 m. It varies little between winter and summer, but the spread in the depth was larger during the winter as compared to summer.

To extend the profile of the attenuated backscatter below 100 m, the ceilometer measurements were combined with Carrier-To-Noise-Ratio observations from a Doppler wind Lidar. Thus, hourly profiles of attenuated backscatter starting at 40 m, are obtained. The results are available for 2018 only and they show aerosol-layer depths below 100 m as well as depths around 230-250 m and they show few observations of aerosol-layer depths between 100 m and 230 m.

Acknowledgement: This article is based upon work from COST Action CA18235 PROBE, supported by COST (European Cooperation in Science and Technology). www.cost.eu.