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.
Highlights
Over the last decades, the rate of near-surface warming in the Arctic has been at least double than elsewhere on our planet (Arctic amplification) [1]
The cirrus occurrence frequency was higher in winter (3.6 ± 0.7%) and spring (4.1 ± 4.3%) relative to summer (1.6 ± 0.3%) and autumn (1.2 ± 0.8%). This seasonality is in accordance with Nomokonova et al [17], who reported higher occurrence of ice clouds over Ny-Ålesund in winter and spring using continuous remote sensing observations
On a more detailed analysis, we found that for a well-defined tropopause the relative humidity (RH) declined rapidly to nearly-zero levels due to positive temperature gradients, whereas for a poorly-defined tropopause the RH decrease was smoother in connection with nearly-neutral temperature gradients
Summary
The rate of near-surface warming in the Arctic has been at least double than elsewhere on our planet (Arctic amplification) [1]. Huang et al [3] suggested that clouds are a driving force for sea ice melt from. Ice clouds cause a warming effect within the lower atmosphere, while a cooling effect occurs in higher levels[4]. At the top of the atmosphere (TOA) the global effect of ice clouds was approximated at 5.1 ± 3.8 Wm−2 , while the globally averaged surface effect was negative [4]. Cirrus is the only cloud type capable of inducing daytime cooling or heating at TOA [5]. The rest of the clouds produce solely daytime cooling at TOA. The relative magnitude of short-wave (SW) cooling and infrared warming depends on the cloud properties, solar geometry, thermal contrast to the surface and surface albedo [6].
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