Supercooled liquid fogs over the central Greenland Ice Sheet

• Published in: Atmospheric chemistry and physics Vol. 19; no. 11; pp. 7467 - 7485
• Main Authors: Cox, Christopher J, Noone, David C, Berkelhammer, Max, Shupe, Matthew D, Neff, William D, Miller, Nathaniel B, Walden, Von P, Steffen, Konrad
• Format: Journal Article
• Language: English
• Published: Katlenburg-Lindau Copernicus GmbH 05-06-2019; Copernicus Publications
• Subjects: Aerodynamics; Air; Climatology; Cloudbows; Cold; Condensation; Damping; Diurnal cycle; Diurnal variations; Environmental aspects; Fluxes; Fog; Glaciation; Greenland ice sheet; Ice; Ice sheets; Low temperature; Melting; Meteorology; Observations; Preconditioning; Radiation; Radiation (Physics); Radiative forcing; Remote sensing; Spectrometers; Summer; Supercooled liquids; Surface temperature; Water; Weather; Winter; Greenland
• DOI: 10.5194/acp-19-7467-2019
• EISSN: 1680-7324
• ISSN: 1680-7324; 1680-7316

Radiation fogs at Summit Station, Greenland (72.58∘ N,
38.48∘ W; 3210 m a.s.l.), are frequently reported by observers. The
fogs are often accompanied by fogbows, indicating the particles are composed
of liquid; and because of the low temperatures at Summit, this liquid is
supercooled. Here we analyze the formation of these fogs as well as their
physical and radiative properties. In situ observations of particle size and
droplet number concentration were made using scattering spectrometers near 2 and 10 m height from 2012 to 2014. These data are complemented by
colocated observations of meteorology, turbulent and radiative fluxes, and
remote sensing. We find that liquid fogs occur in all seasons with the
highest frequency in September and a minimum in April. Due to the
characteristics of the boundary-layer meteorology, the fogs are elevated,
forming between 2 and 10 m, and the particles then fall toward the surface.
The diameter of mature particles is typically 20–25 µm in summer.
Number concentrations are higher at warmer temperatures and, thus, higher in
summer compared to winter. The fogs form at temperatures as warm as −5 ∘C, while the coldest form at temperatures approaching −40 ∘C. Facilitated by the elevated condensation, in winter two-thirds of
fogs occurred within a relatively warm layer above the surface when the
near-surface air was below −40 ∘C, as cold as −57 ∘C,
which is too cold to support liquid water. This implies that fog particles
settling through this layer of cold air freeze in the air column before
contacting the surface, thereby accumulating at the surface as ice without
riming. Liquid fogs observed under otherwise clear skies annually imparted
1.5 W m−2 of cloud radiative forcing (CRF). While this is a small
contribution to the surface radiation climatology, individual events are
influential. The mean CRF during liquid fog events was 26 W m−2, and
was sometimes much higher. An extreme case study was observed to
radiatively force 5 ∘C of surface warming during the coldest part
of the day, effectively damping the diurnal cycle. At lower elevations of
the ice sheet where melting is more common, such damping could signal a role
for fogs in preconditioning the surface for melting later in the day.