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Compensating Feedback Mechanisms in Aerosol Perturbed Stratiform Mixed-Phase Clouds

Figure: Process schematic of aerosol cloud effects in boundary layer mixed-phase clouds (See text for explanation).

In the Arctic, climate change effects are proceeding more rapidly than the rest of the world, which is commonly referred to "Arctic amplification". Due to the rapid melt of the Arctic sea ice sheet, previously inaccessible shipping routes such as the Northern Sea Route, may soon be ice-free for up to 6 months of the year opening new opportunities for long-distance trans-Arctic shipping trade.

In the current Arctic climate, the oceans are cloud covered at least 60% of the time and

most of these clouds (over 70%) are low-lying boundary layer clouds containing a mixture of liquid droplets and ice crystals and are hence referred to as "mixed-phase" clouds. These clouds contribute considerably to the radiative budget in the Arctic, but feedback processes and their net effect on the cloud's radiative forcing to aerosol perturbations remain largely unexplored. The radiative forcing of these clouds in the longwave and in the shortwave (during summer) is governed by their liquid and ice water contents and the droplet or crystal sizes within the cloud. These cloud properties in turn are constrained by the large-scale meteorological conditions and aerosol properties (aerosol composition and number concentration).

In this study we investigate the sensitivity of mixed-phase boundary layer clouds to aerosol perturbations affecting either the liquid (cloud condensation nuclei, CCN, emissions) or the ice phase (ice nucleating particle, INP, emissions). Our results show that feedback processes in the ice phase act to compensate known feedback processes in the liquid phase. As the cloud is perturbed by CCN, the liquid precipitation formation is suppressed as the liquid water content is distributed onto more numerous cloud droplets. In this manner the liquid water content near cloud top is increased, which increases the cloud top radiative cooling. In a pure liquid cloud, or a cloud containing only few ice crystals due to low background INP concentrations (see Figure: bottom row), this may lead to enhanced vertical mixing, cloud deepening and an increased liquid water path. However, in mixed-phase clouds forming in environments of high background INP concentrations which contain more numerous ice crystals, the strengthened radiative cooling is compensated by enhanced levels of freezing within the cloud, which keeps the turbulent fluxes and the liquid water path of the cloud unchanged (Figure: top row). The additional emission of INPs alongside CCN acts to strengthen this process loop for mixed phase clouds exposed to high potential INP concentrations (Figure: top-right panel) and triggers this process loop in clouds surrounded by fewer potential background INP (Figure: bottom-right panel).

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