Properties of Cirrus Clouds over the European Arctic (Ny-Ålesund, Svalbard)

Abstract

Cirrus is the only cloud type capable of inducing daytime cooling or heating at the top of the atmosphere (TOA) and the sign of its radiative effect highly depends on its optical depth. However, the investigation of its geometrical and optical properties over the Arctic is limited. In this work the long-term properties of cirrus clouds are explored for the first time over an Arctic site (Ny-Ålesund, Svalbard) using lidar and radiosonde measurements from 2011 to 2020. The optical properties were quality assured, taking into account the effects of specular reflections and multiple-scattering. Cirrus clouds were generally associated with colder and calmer wind conditions compared to the 2011–2020 climatology. However, the dependence of cirrus properties on temperature and wind speed was not strong. Even though the seasonal cycle was not pronounced, the winter-time cirrus appeared under lower temperatures and stronger wind conditions. Moreover, in winter, geometrically- and optically-thicker cirrus were found and their ice particles tended to be more spherical. The majority of cirrus was associated with westerly flow and westerly cirrus tended to be geometrically-thicker. Overall, optically-thinner layers tended to comprise smaller and less spherical ice crystals, most likely due to reduced water vapor deposition on the particle surface. Compared to lower latitudes, the cirrus layers over Ny-Ålesund were more absorbing in the visible spectral region and they consisted of more spherical ice particles.

Observations of cold cloud properties in the Norwegian Arctic using ground-based and spaceborne lidar

“…the difference can be explained by the restriction to single-layer clouds, while Shupe (2011) takes into account multi-layered clouds as well. On the other hand, our result is still larger than the annual mean cirrus occurrence of 2.7 ± 1.8% reported from Ny-Ålesund by Nakoudi et al (2021a), which is meaningful as their value is according to the authors negatively biased due to the very strict criteria for reliable detection. In terms of geometrical properties, Nakoudi et al (2021a) also find a mean thickness of 2 km and higher cloud bases during summer and autumn than during winter and spring.…”

“…On the other hand, our result is still larger than the annual mean cirrus occurrence of 2.7 ± 1.8% reported from Ny-Ålesund by Nakoudi et al (2021a), which is meaningful as their value is according to the authors negatively biased due to the very strict criteria for reliable detection. In terms of geometrical properties, Nakoudi et al (2021a) also find a mean thickness of 2 km and higher cloud bases during summer and autumn than during winter and spring. Relatively large variations in geometrical thickness with a tendency towards thicker layers in winter seem to be common at Northern high latitudes (Nakoudi et al, 2021a;Devasthale et al, 2011).…”

“…In terms of geometrical properties, Nakoudi et al (2021a) also find a mean thickness of 2 km and higher cloud bases during summer and autumn than during winter and spring. Relatively large variations in geometrical thickness with a tendency towards thicker layers in winter seem to be common at Northern high latitudes (Nakoudi et al, 2021a;Devasthale et al, 2011).…”

“…2b and ranges from 0.2 to 0.3, indicating thin plate-like particles (shape ratio length/diameter < 0.1) and intermediate and irregular shapes with shape ratios up to 0.5 3). This is also the most common category found at Ny-Ålesund, Svalbard, with 73% occurrence (Nakoudi et al, 2021a), and at the subarctic site of Kuopio, Finland, (62.74°N, 27.54°E) with 71% occurrence (Voudouri et al, 2020).…”