The Impact of Warm and Moist Airmass Perturbations on Arctic Mixed-Phase Stratocumulus

Abstract

The Arctic is known to be particularly sensitive to climate change. This Arctic amplification has partially been attributed to poleward atmospheric heat transport in the form of air mass intrusions. Locally, such air mass intrusions can introduce moisture and temperature perturbations. The effect of air mass perturbations on boundary layer and cloud changes and their impact on the surface radiative balance has received increased attention, especially over sea ice with regard to sea ice melt. Utilizing cloud-resolving model simulations, this study addresses the impact of air mass perturbations occurring at different altitudes on stratocumulus clouds for open ocean conditions. It is shown that warm and moist air mass perturbations substantially affect the boundary layer and cloud properties, even for the relatively moist environmental conditions over the open ocean. The cloud response is driven by temperature inversion adjustments and strongly depends on the perturbation height. Boundary layer perturbations weaken and raise the inversion, which destabilizes the lower troposphere and involves a transition from stratocumulus to cumulus clouds. In contrast, perturbations occurring in the lower free troposphere lead to a lowering, but strengthening of the temperature inversion, with no impact on cloud fraction. In simulations where free tropospheric specific humidity is further increased, multi-layer mixed-phase clouds form. Regarding energy balance changes, substantial surface longwave cooling arises out of the stratocumulus break-up simulated for boundary layer perturbations. Meanwhile, the net surface longwave warming increases resulting from thicker clouds for air mass perturbations occurring in the lower free troposphere.

A central arctic extreme aerosol event triggered by a warm air-mass intrusion

“…The intrusion caused the highest CCN number concentrations observed throughout the entire MOSAiC expedition year, together with the advected moisture. While the net impact of increased CCN concentrations in this environment involves a complex web of cloud microphysical processes 70 , such an increase could mean that optically thicker clouds can form over the Arctic pack ice leading to stronger downwelling longwave radiation 5 , which was observed to be abnormally high in this case 10 , thereby positively re-enforcing the warming effect. The increase in CCN could be impacting the time scales associated with the air-mass transformation leading to a potential extension of the central Arctic clouds' lifetime leading to longer time and larger spatial extent of cloud radiative effects during such intrusion events.…”

“…Altogether, changes in aerosol size distribution, mass composition and hygroscopicity associated with the warm intrusion event affect the CCN number and properties significantly. During the intrusion period, we observed a substantial increase in CCN number and a decrease in hygroscopicity, which can result in an optically thicker cloud cover and an increase in the net surface longwave warming 5 . It is worth mentioning here that the instrumentation deployed during MOSAiC included measurements of ice nucleating particles (INPs) which can also have a significant impact on cloud properties and radiative balance.…”

“…During the Polarstern drift, the haze condition was terminated by an extreme, record-breaking increase of near-surface temperature 10 accompanied by the arrival of an airmass originating from Eurasia on April 15th 2020. The air-mass arriving from North-western Russia and North-eastern Europe brought higher moisture, with strong effects on Arctic cloud properties 5 . The extreme event caused an increase in aerosol number concentration and aerosol mass transforming the Arctic from a remote low concentration environment into an area comparable to an urban setting in central Europe.…”

“…2A, B right, Supplementary Figs. [5][6]. Air-masses transported via such "intrusion pathways" from lower latitudes to the Arctic extend over thousands of kilometers, and occur over only several hours to a few days 2 , but are nevertheless subject to substantial atmospheric chemical and microphysical transformation.…”

“…Thus, unless prompt actions to significantly reduce emissions in the source regions are taken, such intrusion events are expected to continue to affect the Arctic climate.The Arctic is warming at a rate roughly thrice as fast as the rest of the globe due to Arctic amplification 1,2 . While its detailed causes remain to be quantified 3,4 , a large number of studies attributed the amplification to anomalous poleward atmospheric transport in the form of warm airmass intrusions [5][6][7][8] . Air-mass intrusions arriving from mid-latitudes can introduce moisture and perturb Arctic temperatures.…”