Hole-punch clouds over Helsinki, Finland

 

Author

Ewan James O'Connor — University of Reading

Category

Cloud Properties

Description

Several hole-punch clouds were observed simultaneously over Helsinki, Finland, on November 5, 2011, in a supercooled liquid layer at an altitude of 7.5 km. The temperature of this layer was about -32°C. Formation of these circular holes is a result of glaciation; under certain conditions, the presence of ice nuclei in a supercooled liquid layer can produce large numbers of ice crystals that grow rapidly by deposition of water vapour in the supersaturated (with respect to ice) conditions. This rapid growth of ice crystals is maintained at the expense of evaporation of liquid droplets, causing a gap to form in the cloud, and the larger and heavier ice crystals falling in a visible streak below the liquid layer.

However, ice nuclei are not normally present in high enough concentrations in the atmosphere to cause such glaciation. The mechanism responsible for glaciation is usually thought to be that ice nuclei originate via rapid freezing of water when an aircraft passes through the liquid layer.

Here, we posit another mechanism that can also provide ice nuclei in sufficient quantities under certain conditions: intermittent fallstreaks from a higher cirrus layer, analogous to the seeder-feeder mechanism for enhanced orographic precipitation. This is based on observations by a collocated vertically pointing Doppler cloud radar (Metek 36) and Doppler lidar (Halo Photonics), situated on the roof of the Finnish Meteorological Institute in Helsinki, together with scans from the prototype C-band weather radar (Vaisala) on the roof of the University of Helsinki and photos from human observers. Interaction between a higher cirrus layer (8–11 km) and a lower mixed-phase layer 1-km deep (supercooled layer at 7.5 km with ice falling up to 1 km below) was noted in the cloud radar Doppler velocity spectra, where the velocity distribution of the falling ice from the cirrus layer broadened, then shifted to higher falling velocities after encountering the supercooled liquid layer. No aircraft were flying in this region at the altitude of the hole-punch cloud events.