In situ cloud ground-based measurements in the Finnish sub-Arctic: intercomparison of three cloud spectrometer setups

Abstract

Continuous, semi-long-term, ground-based in situ cloud measurements were conducted during the Pallas Cloud Experiment (PaCE) in 2013. The measurements were carried out in Finnish sub-Arctic region at Sammaltunturi station (67∘58′ N, 24∘07′ E; 560 m a.s.l.), part of Pallas Atmosphere – Ecosystem Supersite and Global Atmosphere Watch (GAW) program. The main motivation of the campaign was to conduct in situ cloud measurements with three different cloud spectrometer probes and perform an evaluation of their ground-based setups. Therefore, we mutually compared the performance of the cloud and aerosol spectrometer (CAS), the cloud droplet probe (CDP) and the forward-scattering spectrometer probe (FSSP-100) (DMT; Boulder, CO, USA). We investigated how different meteorological parameters affect each instrument's ground-based setup operation and quantified possible biases and discrepancies of different microphysical cloud properties. Based on the obtained results we suggested limitations for further use of the instrument setups in campaigns where the focus is on investigating aerosol–cloud interactions. Measurements in this study were made by instruments owned by the Finnish Meteorological Institute and results concern their operation in sub-Arctic conditions with frequently occurring supercooled clouds. The measured parameter from each instrument was the size distribution, and additionally we derived the number concentration, the effective diameter, the median volume diameter and the liquid water content. A complete intercomparison between the CAS probe and the FSSP-100 ground setups and additionally between the FSSP-100 and the CDP probe ground setups was made and presented. Unfortunately, there was not a sufficient amount of common data to compare all three probes together due to operational problems of the CDP ground setup in sub-zero conditions. The CAS probe that was fixed to one direction lost a significant number of cloud droplets when the wind direction was out of wind iso-axial conditions in comparison with the FSSP-100 and the CDP, which were both placed on a rotating platform. We revealed that CAS and FSSP-100 had good agreement in deriving sizing parameters (effective diameter and median volume diameter from 5 to 35 µm) even though CAS was losing a significant amount of cloud droplets. The most sensitive derived parameter was liquid water content, which was strongly connected to the wind direction and temperature.