| 1. |
- Dimitrelos, Antonios, 1986-
(författare)
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A large-eddy simulation perspective on Arctic airmass transformation and low-level cloud evolution
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Arctic is currently warming faster than other regions of the Earth. Many processes and feedbacks contribute to the enhanced warming. Among these are the radiative effects of clouds. Arctic mixed-phase clouds, which contain both liquid and ice condensate, have high longevity and can exert significant surface warming since the amount of solar radiation in the region is relatively low and the surface reflectivity often is high. In this thesis, we study these clouds utilizing a large-eddy model coupled with one-dimensional thermodynamic sea ice model. The main aim is to understand the interactions between cloud dynamics, microphysics, radiation, and turbulent processes and how these together govern the life cycle and surface warming of the clouds. By comparing a group of models with observations from the summertime high Arctic, we confirm the hypothesis that when aerosol concentrations are low, a small increase in their number concentration can increase the liquid water content of the cloud and in turn, the surface warming. Idealized simulations of moist intrusions into the Arctic show that the surface temperature may increase by more than 15o C if we allow clouds to form during a moist intrusion compared to if the atmosphere is cloud free. The simulations also show that the large-scale divergence rate strongly impacts the maintenance of the liquid layer at the top of these clouds. A main finding of the thesis is that the temperature of the cloud that forms during a moist intrusion is close to the initial dew point temperature. Thus, the surface warming induced by the clouds depends mostly on the initial humidity of the air mass rather than the initial temperature. In addition, the stability of the initial dew point temperature profile largely controls the turbulent state of the cloud. If the profile is unstable, then the cloud can transform from a thin, stable stratus to a deeper stratocumulus cloud, which also enhances the surface warming. Consequently, both the initial amount and the vertical structure of the initial moisture of the intrusion are important for the warming of the sea ice. A change in the number of cloud condensation nuclei does not affect the cloud evolution considerably provided that there is a continuous supply of these nuclei. However, if cloud condensation nuclei sources are absent then the cloud may remain in its stable state. Furthermore, a decrease in the cloud ice condensate, which may be caused by a lack of ice nucleation particles, may delay the transformation of the cloud into a stratocumulus. These results suggest that any future change in aerosol loading and atmospheric moisture transport into the Arctic may alter the surface longwave cloud radiative effect and cause changes in the sea ice evolution.
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| 2. |
- Dimitrelos, Antonios, 1986-, et al.
(författare)
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Controls on surface warming by winter Arctic moist intrusions in idealized large-eddy simulations
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The main energy input to the polar regions in winter is the advection of warm, moist air from lower latitudes. This makes the polar climate sensitive to the temperature and moisture of extra-polar air. Here, we study this sensitivity from an air-mass transformation perspective. We perform simulations of an idealized maritime air mass brought into contact with sea ice employing a three-dimensional large-eddy simulation model coupled to a one-dimensional multilayer sea ice model. We study the response of cloud dynamics and surface warming during the air-mass transformation process to varying initial temperature and humidity conditions of the air mass. We find in all cases that a mixed-phase cloud is formed, initially near the surface but rising continuously with time. Surface warming of the sea ice is driven by downward longwave surface fluxes, which are largerly controlled by the temperature and optical depth of the cloud. Cloud temperature, in turn, is robustly constarined by the initial dewpoint temperature of the air mass. Since dewpoint only depends on moisture, the overall result is that surface warming depends almost exclusively on initial humidity and is largerly independent of initial temperature. We discuss possible climate implications of this result, in particular for polar amplification of surface warming and the role played by atmospheric energy transport.
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| 3. |
- Dimitrelos, Antonios, 1986-, et al.
(författare)
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The turbulent state of Arctic mixed-phase clouds under different conditions of moisture, aerosols, and ice water content
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Previous studies have shown that low-level mixed-phase clouds (MPCs) that form during moist intrusions into the Arctic can exist in either a stable (stratus) or a convective (stratocumulus) state. Here, we examine the process and conditions that promote a transition between the two states through idealized simulations using a three-dimensional large-eddy simulation model coupled with a one-dimensional multilayer sea ice model. We find that the vertical distribution of the initial dew point temperature (Td) profile has a fundamental influence on whether a transition from stable to convective conditions occurs or not. If the initial moisture content of the advected airmass decreased rapidly with height, i.e. if the Td profile is unstable, then a turbulent transition is likely to occur and a stratocumulus cloud can form. However, the availability and properties of aerosols and the cloud ice content can delay or even prevent stratocumulus formation, regardless if the conditions in terms of the initial Td profile are favorable. If no suitable cloud condensation nuclei are available at the base of the cloud at the time of the transition, then no new droplets form, the buoyancy remains low and the cloud remains in its stable state. Furthermore, a decrease in cloud ice water content results in a more stably stratified cloud layer and a delay in the transition. The transition from the stable to the convective state has a substantial effect on the surface warming induced by the cloud in the model; simulations with a transition generally show larger surface warming than simulations without a transition. Our results suggest that the low-level mixed-phase cloud evolution and the thermodynamic transition of an airmass during a moist intrusion are closely linked to the aerosol processing by the cloud, i.e. a chemical transformation, and that the two processes should be considered simultaneously.
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