Browsing by Subject "atmospheric moisture"

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  • Naakka, Tuomas (Ilmatieteen laitos - Finnish Meteorological Institute, 2022)
    Finnish Meteorological Institute Contributions 179
    Water vapour is an effective greenhouse gas, but clouds, which are formed when water vapour condenses into water droplets or ice crystals, may have an even greater effect on radiative energy transfer through the atmosphere. In addition, absorption or release of the latent heat of vaporization and transport of water vapour are part of the heat transport from the Tropics towards the Poles. Thus, atmospheric water vapour greatly affects the energy balance of the atmosphere and is also an important component of the water cycle. This thesis addresses the subject of atmospheric moisture and the processes affecting it in the Arctic and Antarctic. The studies comprising the thesis are mostly based on atmospheric reanalyses. In the polar regions, meteorological observation networks are sparse, due to their remoteness and the harsh environment, and therefore traditional observations have not provided a comprehensive picture of atmospheric conditions in the polar regions. In recent years, atmospheric reanalyses have also become more accurate in remote areas, which has enabled detailed studies of atmospheric moisture in the polar regions. In the polar regions, the mostly negative radiation budget of Earth’s atmosphere-surface system shapes the distribution of water vapour in the atmosphere, especially the vertical structure of specific humidity. The polar regions are sinks for atmospheric water vapour, due to their typically small local evaporation, and even condensation of moisture on the surface. Therefore, moisture transport from the lower latitudes balances the moisture budget in the polar regions. This type of moisture budget favours the formation of specific humidity inversions. Our results show that specific humidity inversions are common in the polar regions, and their occurrence near Earth’s surface is linked with surface conditions: radiative surface cooling, occurrence of temperature inversions in winter and cold sea surfaces or melting of sea ice in summer. Advection of warm, moist air masses over a cold surface in summer is vital for formation of specific humidity inversions. Below the approximately 800-hPa level, interactions between the atmosphere and Earth’s surface clearly affect both the atmospheric moisture content and moisture transport. Our results show that the northward moisture transport near the surface is mostly balanced by southward transport. Moisture transport clearly shapes the spatial distribution of the atmospheric moisture content. Regional trends in atmospheric moisture content in the Arctic are also mostly the results of long-term variations in atmospheric circulation. The negative net radiation budget, weak evaporation and extensive contribution of moisture transport to atmospheric moisture content also characterize moisture conditions in the Antarctic. The results show that, due to geographical conditions, specific humidity inversions in Antarctica are even more persistent than those in the Arctic. This is associated with stronger isolation of air masses in inner Antarctica from advection of warm, moist air masses than in the Arctic. The results also show that when a cold, dry air mass flows from the continent towards the ocean, it undergoes adiabatic warming, which together with downward sensible heat fluxes enables evaporation on Antarctic slopes. Overall, this thesis contributes to our understanding of how the spatial distribution of atmospheric moisture content interacts with moisture transport and with physical processes such as evaporation and condensation in polar regions.