Evidence for Changes in Arctic Cloud Phase Due to Long‐Range Pollution Transport

Abstract

Reduced precipitation rates allow pollution within air parcels from midlatitudes to reach the Arctic without being scavenged. We use satellite and tracer transport model data sets to evaluate the degree of supercooling required for 50% of a chosen ensemble of low-level clouds to be in the ice phase for a given meteorological regime. Our results suggest that smaller cloud droplet effective radii are related to higher required amounts of supercooling but that, overall, pollution plumes from fossil fuel combustion lower the degree of supercooling that is required for freezing by approximately 4 ∘ C. The relationship between anthropogenic plumes and the freezing transition temperature from liquid to ice remains to be explained.Plain Language Summary Anthropogenic pollution plumes from midlatitudes can be transported long distances to the Arctic. In this study, we analyze the impact of these plumes on how easily liquid clouds over the Arctic Ocean freeze by using a novel combination of satellite measurements and a pollution transport model. We find that liquid clouds in polluted air switch phase to become ice clouds at temperatures that are 4 ∘ C higher they would otherwise in pristine air. Because ice clouds in the Arctic precipitate more easily than liquid clouds, the potential is that distant industrial pollution sources are acting to reduce arctic cloud life time.

Space‐Based Analysis of the Cloud Thermodynamic Phase Transition for Varying Microphysical and Meteorological Regimes

“… increases from 26 C for in a bin between 5 and 9 m to 20 C for in a bin between 21 and 25 m. Higher values of are associated with higher in line with previous studies (e.g., Coopman et al, ; Rangno & Hobbs, ; Rosenfeld et al, ). We further classified the data by to retrieve binned according to latitude, , and .…”

“…It is possible that Antarctic clouds glaciate at lower temperature than other clouds because Antarctic clouds are in contact with lower concentrations of aerosols that may serve as potential ice nuclei and facilitate phase transitions. Coopman et al (2018) studied the phase transition of Arctic clouds for different regimes of pollution from fossil fuel combustion and retrieved a glaciation temperature of about −20 • C; the presence of pollution increases the glaciation temperature up to 4 • C. Similarly, Filioglou et al (2019) have shown from CALIPSO and CloudSat measurements that high aerosol loadings increase the glaciation temperature by 10 • C in presence of dust and continental aerosols in the Arctic. We suggest that larger liquid cloud droplets are associated with higher glaciation temperatures because they aid secondary ice nucleation (Rosenfeld et al, 2011).…”

Analysis of the Thermodynamic Phase Transition of Tracked Convective Clouds Based on Geostationary Satellite Observations

“…Between −38°C and 0°C, clouds can be either liquid, ice or a mixture of the two phases and different parameters influence the phase transition: For example, aerosols can act as ice nuclei, while large cloud droplets are more prone to secondary ice production (Rosenfeld et al, 2011); both examples potentially increase the glaciation temperature ( T g ). Parameters, such as the cloud droplet size and aerosol concentration, can be correlated or anti‐correlated with each other, and it is therefore difficult to disentangle their respective effect on T g (Coopman et al, 2018; Gryspeerdt et al, 2016).…”

Detection of Mixed‐Phase Convective Clouds by a Binary Phase Information From the Passive Geostationary Instrument SEVIRI

“…Cloud droplets freeze homogeneously at −37 • C but for temperatures between −37 and 0 • C, supercooled cloud droplets and ice crystals can be observed (Cober et al, 2001;Rauber & Tokay, 1991). The temperature of glaciation of a cloud depends on different parameters, such as cloud altitudes, surface types, cloud droplet sizes, or pollution concentration (Carro-Calvo et al, 2016;Coopman et al, 2018;Rangno & Hobbs, 2001;Rosenfeld et al, 2011;Zamora et al, 2018). Clouds can therefore be composed of either only cloud droplets-referred to as liquid clouds-or only ice crystals-referred to as ice clouds-or a combination of supercooled liquid droplets and ice crystals-referred to as mixed-phase clouds (Korolev et al, 2017).…”