Browsing by Subject "evaporation"

Sort by: Order: Results:

Now showing items 1-4 of 4
  • 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.
  • Lindroth, Anders; Holst, Jutta; Linderson, Maj-Lena; Aurela, Mika; Biermann, Tobias; Heliasz, Michal; Chi, Jinshu; Ibrom, Andreas; Kolari, Pasi; Klemedtsson, Leif; Krasnova, Alisa; Laurila, Tuomas; Lehner, Irene; Lohila, Annalea; Mammarella, Ivan; Mölder, Meelis; Lofvenius, Mikaell Ottosson; Peichl, Matthias; Pilegaard, Kim; Soosaar, Kaido; Vesala, Timo; Vestin, Patrik; Weslien, Per; Nilsson, Mats (2020)
    The Nordic region was subjected to severe drought in 2018 with a particularly long-lasting and large soil water deficit in Denmark, Southern Sweden and Estonia. Here, we analyse the impact of the drought on carbon and water fluxes in 11 forest ecosystems of different composition: spruce, pine, mixed and deciduous. We assess the impact of drought on fluxes by estimating the difference (anomaly) between year 2018 and a reference year without drought. Unexpectedly, the evaporation was only slightly reduced during 2018 compared to the reference year at two sites while it increased or was nearly unchanged at all other sites. This occurred under a 40 to 60% reduction in mean surface conductance and the concurrent increase in evaporative demand due to the warm and dry weather. The anomaly in the net ecosystem productivity (NEP) was 93% explained by a multilinear regression with the anomaly in heterotrophic respiration and the relative precipitation deficit as independent variables. Most of the variation (77%) was explained by the heterotrophic component. Six out of 11 forests reduced their annual NEP with more than 50 g C m(-2)yr(-1)during 2018 as compared to the reference year. The NEP anomaly ranged between -389 and +74 g C m(-2)yr(-1)with a median value of -59 g C m(-2)yr(-1). This article is part of the theme issue 'Impacts of the 2018 severe drought and heatwave in Europe: from site to continental scale'.
  • Häkkinen, Ella (Helsingin yliopisto, 2020)
    Atmospheric aerosol particles affect Earth’s radiation balance, human health and visibility. Secondary organic aerosol (SOA) contributes a significant fraction to the total atmospheric organic aerosol, and thus plays an important role in climate change. SOA is formed through oxidation of volatile organic compounds (VOCs) and it consists of many individual organic compounds with varying properties. The oxidation products of VOCs include highly oxygenated organic molecules (HOM) that are estimated to explain a large fraction of SOA formation. To estimate the climate impacts of SOA it is essential to understand its properties in the atmosphere. In this thesis, a method to investigate thermally induced evaporation of organic aerosol was developed. SOA particles were generated in a flow tube from alpha-pinene ozonolysis and then directed into a heated tube to initiate particle evaporation. The size distribution of the particles was measured with parallel identification of the evaporated HOM. This method was capable of providing information of SOA evaporation behaviour and the particle-phase composition at different temperatures. Mass spectra of the evaporated HOM and particle size distribution data were analyzed. The obtained results suggest that SOA contains compounds with a wide range of volatilities, including HOM monomers, dimers and trimers. The volatility behaviour of the particulate HOM and their contribution to SOA particle mass was studied. Furthermore, indications of particle-phase reactions occurring in SOA were found.
  • Apers, S.; De Lannoy, G. J. M.; Baird, A. J.; Cobb, A. R.; Dargie, G. C.; Pasquel, J.; Gruber, A.; Hastie, A.; Hidayat, H.; Hirano, T.; Hoyt, A. M.; Jovani-Sancho, A. J.; Katimon, A.; Kurnain, A.; Koster, R. D.; Lampela, Maija; Mahanama, S. P. P.; Melling, L.; Page, S. E.; Reichle, R. H.; Taufik, M.; Vanderborght, J.; Bechtold, M. (2022)
    Tropical peatlands are among the most carbon-dense ecosystems on Earth, and their water storage dynamics strongly control these carbon stocks. The hydrological functioning of tropical peatlands differs from that of northern peatlands, which has not yet been accounted for in global land surface models (LSMs). Here, we integrated tropical peat-specific hydrology modules into a global LSM for the first time, by utilizing the peatland-specific model structure adaptation (PEATCLSM) of the NASA Catchment Land Surface Model (CLSM). We developed literature-based parameter sets for natural (PEATCLSM(Trop,Nat)) and drained (PEATCLSM(Trop,Drain)) tropical peatlands. Simulations with PEATCLSM(Trop,Nat) were compared against those with the default CLSM version and the northern version of PEATCLSM (PEATCLSM(North,Nat)) with tropical vegetation input. All simulations were forced with global meteorological reanalysis input data for the major tropical peatland regions in Central and South America, the Congo Basin, and Southeast Asia. The evaluation against a unique and extensive data set of in situ water level and eddy covariance-derived evapotranspiration showed an overall improvement in bias and correlation compared to the default CLSM version. Over Southeast Asia, an additional simulation with PEATCLSM(Trop,Drain) was run to address the large fraction of drained tropical peatlands in this region. PEATCLSM(Trop,Drain) outperformed CLSM, PEATCLSM(North,Nat), and PEATCLSM(Trop,Nat) over drained sites. Despite the overall improvements of PEATCLSM(Trop,Nat) over CLSM, there are strong differences in performance between the three study regions. We attribute these performance differences to regional differences in accuracy of meteorological forcing data, and differences in peatland hydrologic response that are not yet captured by our model.