Contrasting ice formation in Arctic clouds: surface coupled vs decoupled clouds

• Main Authors: Griesche, Hannes J., Ohneiser, Kevin, Seifert, Patric, Ansmann, Albert, Engelmann, Ronny
• Format: Text
• Language: English
• Published: 2020
• Subjects: Arctic; Svalbard
• Online Access: https://doi.org/10.5194/acp-2020-1096; https://acp.copernicus.org/preprints/acp-2020-1096/

In the Arctic summer of 2017 (June, 1st to July, 16th) measurements with the multiwavelength polarization lidar Polly XT -OCEANET, 35-GHz cloud radar of the OCEANET platform, and radiosonde measurements were conducted during cruise PS106 of the research vessel Polarstern around Svalbard. In the scope of the presented study, the influence of cloud height and surface coupling on the probability of clouds to contain and form ice is investigated. The analyzed data set shows a significant impact of the surface-coupling state on the probability of ice formation. Surface-coupled clouds, identified by a quasi-constant potential temperature profile from the surface up to liquid layer base, in the same cloud-top temperature range contain ice more frequent than decoupled clouds by a factor of up to 5 for cloud-top intervals between −7.5 and −5 °C (169 vs. 31 profiles). These findings provide evidence that heterogeneous ice formation in Arctic mixed-phase clouds occurs by a factor of 2–5 more likely when the cloud layer is coupled to the surface. In turn, for cloud-top temperatures below −15 °C, the frequency of ice-containing cloud profiles for coupled and decoupled conditions approached the respective curve for the Central-European site of Leipzig, Germany (51° N, 12° E). This provides further evidence that the free-tropospheric ice nucleating particles (INP) reservoir over the Arctic is controlled by continental aerosol. One possible explanation for the observation is that turbulent mixing of the air below surface-coupled clouds allows ice particles, acting as seeds for ice multiplication, or marine aerosols, acting as INP, to be transported into the cloud layer more efficiently than in the case of decoupled conditions. This hypothesis is corroborated by recent in-situ measurements of INP in the Arctic, of which much higher concentrations were found in the surface-coupled atmosphere in close vicinity to the ice shore. Using lidar measurements we also found evidence for enhanced INP number concentrations (INPC) within surface-coupled cloud-free air masses. The INPC have been estimated based on particle backscatter profiles, published freezing spectra of biogenic INP and existing parameterizations.