Ice and Mixed-Phase Cloud Statistics on Antarctic Plateau

Abstract

Abstract. Statistics on the occurrence of clear skies, ice and mixed-phase clouds over the Concordia station, in the Antarctic Plateau, are provided for multiple time scales and analysed in relation to simultaneous meteorological parameters measured at the surface. Results are obtained by applying a machine learning cloud identification and classification code (named CIC) to 4 years of measurements between 2012–2105 of down-welling high spectral resolution radiances, measured by the Radiation Explorer in the Far Infrared-Prototype for Applications and Development (REFIR-PAD) spectroradiometer. The CIC algorithm is optimized for Antarctic sky conditions (clear sky, ice clouds, and mixed-phase clouds) and results in a total hit rate of almost 0.98, where 1.0 is a perfect score. Scene truth is provided by LiDAR measurements that are concurrent with REFIR-PAD. The CIC approach demonstrates the key role of far infrared spectral measurements for clear/cloud discrimination and for cloud phase classification. Mean annual occurrences are 72.3 %, 24.9 % and 2.7 % for clear sky, ice and mixed-phase clouds respectively, with an inter-annual variability of a few percent. The seasonal occurrence of clear sky shows a minimum in winter (66.8 %) and maxima (75–76 %) during intermediate seasons. In winter the mean surface temperature is about 9 °C colder in clear conditions than when ice clouds are present. Mixed-phase clouds are observed only in the warm season; in summer they amount to more than one third of total observed clouds. Their occurrence is correlated with warmer surface temperatures. In the austral summer, the mean surface air temperature is about 5 °C warmer when clouds are present than in clear sky conditions. This difference is larger during the night than in daylight hours, likely due to increased solar warming. A comparison of monthly mean results with cloud occurrence/fraction derived from gridded (Level-3) satellite products, from both passive and active sensors, emphasizes the difficulty of adequately inferring cloud/clear-sky properties in the Antarctic region and highlights the ability of the CIC/REFIR-PAD synergy to identify multiple cloud conditions and study their variability at different time scales.

Synergistic Use of Far‐ and Mid‐Infrared Spectral Radiances for Satellite‐Based Detection of Polar Ice Clouds Over Ocean

“…Recent studies have investigated the use of high spectral resolution far‐IR radiances for cloud phase classification (i.e., Cossich et al., 2021; Maestri, Arosio, et al., 2019; Maestri, Cossich, & Sbrolli, 2019; Sgheri et al., 2021) based on observed downwelling radiances and synthetic satellite observations. These studies utilize numerous spectral channels across the far‐IR and mid‐IR for the radiance inputs to a principal component analysis‐based machine learning algorithm for cloud identification.…”

“…The spectral variation of ice cloud scattering also varies across the far‐IR and can be larger in the far‐IR compared to the mid‐IR, which can potentially provide additional information to help discriminate different cloud phases using a bi‐spectral approach. Furthermore, it is possible that a far‐IR BTD cloud phase algorithm can supplement or improve the conventional mid‐IR BTD approach, as multiple studies have indicated that the far‐IR and mid‐IR can be used synergistically for cloud phase determination (e.g., Cossich et al., 2021; Rathke et al., 2002; Rowe et al., 2019; Turner, 2005). It has also been shown that the prominent far‐IR scattering can be used synergistically with the mid‐IR for ice cloud property retrievals (Libois & Blanchet, 2017; Merrelli & Turner, 2013; Saito et al., 2020).…”

Comparison of mid-latitude single- and mixed-phase cloud optical depth from co-located infrared spectrometer and backscatter lidar measurements

“…Within this temperature regime, clouds can occur as liquid only (supercooled water) as well as in the mixed phase, in which ice crystals coexist with water droplets, and also as purely ice. Mixed-phase clouds are very common in polar regions, both in the Arctic and Antarctica (Cossich et al, 2021), but they can also be widely found at mid-latitudes, so much so that Costa et al (2017) show that out of 16 measurement flights of the COALESC (Combined Observation of the Atmospheric boundary Layer to study the Evolution of StratoCumulus) campaign performed with aircraft at mid-latitudes, 14 observations provided the presence of clouds in the mixed-phase regime.…”

Reply on RC1

“…Recently, some authors (such as Maestri et al, 2019a;Saito et al, 2020;Di Natale et al, 2020) have shown the potentialities of exploiting FIR spectrally resolved radiances to refine ice clouds microphysical and optical properties. Maestri et al (2019b) and Cossich et al (2021) also showed the benefit of using FIR channels in combination with MIR ones to largely improve the performances of cloud identification and classification (i.e. between ice and liquid water phase) algorithms.…”