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- Baró Pérez, Alejandro, 1991-
(författare)
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Aerosol impacts on subtropical low-level clouds: a satellite and modelling perspective
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Complex interactions between aerosols, clouds, and radiation impact Earth's climate. However, several aspects of these interactions remain uncertain, which has led to extensive research over the last decades. This thesis explores some unresolved aspects by focusing on subtropical low-level stratocumulus (Sc) clouds, which have a significant cooling effect on climate. The clouds are also sensitive to varying aerosol conditions, which can influence their formation, properties, and lifetime. Clouds over the South East Atlantic have been studied in detail, using both numerical modeling and satellite observations, to shed light on the interactions between aerosols, clouds, and radiation. This geographical region displays a large and semi-permanent Sc cloud deck and is also subjected to meteorological conditions that bring large amounts of light-absorbing aerosols from biomass fires over the African continent. The biomass-burning plumes also bring enhanced levels of moisture, and the individual influence of the aerosols and the moisture on the low-level cloud properties have been investigated.The analysis of satellite retrievals showed a radiative impact (sensitive to aerosol composition and aerosol optical depth) of moist aerosol layers in the free troposphere over the South East Atlantic; however, it was not possible to observe a clear influence of these humid aerosol layers on the underlying low-level clouds. Aerosol-radiation interactions were implemented in a large eddy simulation (LES) code that was used to model stratocumulus to cumulus transitions (SCT) in weather situations where moist absorbing aerosol layers were in contact with low-level clouds and mixed into the marine boundary layer (MBL). In these simulations, the heating by the absorbing aerosol within the MBL affected the persistence of the Sc clouds by accelerating the SCT, especially during daylight and broken cloud conditions. However, the humidity accompanying the absorbing aerosol was also found to be important -- it reduced the deepening of the MBL when located above the Sc deck and delayed the SCT when in contact with clouds. Furthermore, the additional moisture resulted in a radiative cooling effect that was comparable to the radiative cooling effect caused by the aerosol itself. The simulated SCTs were found to be mostly driven by increased sea surface temperatures, regardless of aerosol conditions. This result was different compared to two other LES models where the SCT was driven by drizzle under the same low aerosol conditions. On a larger scale, it was found that an explicit description of aerosol-cloud interactions in a climate model led to smaller differences between the simulated and mean observed values of the shortwave cloud radiative effect compared to when a non-interactive parameterization was used.
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| 2. |
- You, Cheng, 1990-
(författare)
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Arctic Atmospheric Rivers : Eulerian and Lagrangian features, and trends over the last 40 years
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Arctic Atmospheric rivers, termed ‘warm-and-moist intrusion’ (WaMAI) in this thesis, transporting heat and moisture into the Arctic from lower latitudes, is a key contributor to the amplified warming in the Arctic under global change (Arctic Amplification). However, the warming effect of WaMAIs and its transformation along the trajectories into high Arctic still remain unclear, as well as their relation with the large-scale atmospheric circulation.A positive trend of poleward moisture and heat transport during 1979-2018 has been identified over the Barents Sea in winter and East Siberian Sea in summer. These positive trends are attributed to an increased blocking occurrence, as quantified by a blocking track algorithm. Given the increase in poleward energy transport and its impacts on the atmospheric energy budgets, it is necessary to focus on the Arctic atmospheric rivers in more detail. Therefore, a method is developed to detect WaMAIs during December~Febrary, June~August from 1979 to 2018, and to identify the Lagrangian transformation of warm-and-moist air mass in temperature, humidity, cloud water path, surface and boundary-layer energy-budget, along the trajectories of WaMAIs. The analysis shows that WaMAIs, driven by blocking high-pressure systems over the respective ocean sectors, induce an air mass transformation in the atmospheric boundary layer, resulting in surface warming and presumably additional sea ice melt, from positive anomalies of surface net longwave irradiance and turbulent flux. In summer, from a Lagrangian perspective, the surface energy-budget anomaly decreases linearly with the downstream distance from the sea-ice edge, while total column cloud liquid water (TCLW) increases linearly. An initially stably stratified boundary layer at the ice edge transforms into a deepening well-mixed boundary layer along the trajectories, from the continuous turbulent mixing. The boundary-layer energy-budget structures are categorized into two categories: one dominated by surface turbulent mixing (TBL) and one dominated by cloud-top radiative cooling (RAD). The magnitude of the large-scale atmospheric vertical motion, the subsidence, is critical in determining if the boundary layer develops into TBL or RAD.In winter, over the completely ice-covered ocean sectors where the sea ice reaches all the way to the coast, the Lagrangian transformation of the boundary-layer energy-budget is similar to summer WaMAIs, while the surface energy-budget is dominated by longwave irradiance. On the other hand, over the Barents Sea, with an open ocean to the south, the net surface energy budget is dominated by the surface turbulent fluxes. The boundary-layer energy-budget over the Barents Sea can also be categorized into RAD and TBL, but are different from their summer counterparts.
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