19 Apr 2022
19 Apr 2022
Status: this preprint is currently under review for the journal ACP.

Dynamics of aerosol, humidity, and clouds in air masses travelling over Fennoscandian boreal forests

Meri Räty1, Larisa Sogacheva2, Helmi-Marja Keskinen1,a, Veli-Matti Kerminen1, Tuomo Nieminen1,3, Tuukka Petäjä1,4, Ekaterina Ezhova1, and Markku Kulmala1,4,5 Meri Räty et al.
  • 1Institute for Atmospheric and Earth System Research (INAR) / Physics, Faculty of Science, University of Helsinki, Helsinki, Finland
  • 2Climate Change Programme, Finnish Meteorological Institute, Helsinki, Finland
  • 3Institute for Atmospheric and Earth System Research (INAR) / Forest Sciences, Faculty of Agriculture and Forestry, University of Helsinki, Helsinki, Finland
  • 4Joint International Research Laboratory of Atmospheric and Earth System Sciences, School of Atmospheric Sciences, Nanjing University, Nanjing, China
  • 5Aerosol and Haze Laboratory, Beijing Advanced Innovation Center for Soft Matter Sciences and Engineering, Beijing University of Chemical Technology (BUCT), Beijing, China
  • acurrent: independent researcher, Finland

Abstract. Boreal forests cover vast areas of land in the high latitudes of the northern hemisphere, which are under amplified climate warming. The interaction between the forests and the atmosphere are known to generate a complex set of feedback processes. One feedback process, potentially producing a cooling effect, is associated with an increased reflectance of clouds due to aerosol-cloud interactions. Here, we investigate the effect that the boreal forest environment can have on cloud-related properties during the growing season. The site investigated was in Hyytiälä, Finland, in a region near the edge of the biome. Air mass back trajectories were the basis of the analysis and were used to determine the time each air mass had spent over land prior to its arrival at the station. This enabled tracking the changes occurring in originally marine air masses as they travelled across the forested land. Only air masses arriving from the north-western sector were investigated, as these areas have a relatively uniform forest cover and relatively little anthropogenic interference. We connected the air mass analysis with comprehensive in-situ and remote-sensing data sets covering up to eleven growing seasons. We found that the properties of air masses with short land transport times, thereby less influenced by the forest, differed from those exposed to the forest environment for a longer period. The latter were associated with higher number concentrations of cloud condensation nuclei and increased water vapour content. Indications of corresponding transformations in the cloud layer were observed from satellite measurements. Lastly, longer transport times over forest seem also to slightly enhance the observed precipitation frequency. Most of the variables showed an increase with an increasing land transport time until approximately 60 hours, after which a balanced state with little variation seemed to have been reached. This appears to be the approximate time scale in which the forest-cloud interactions take effect, and the air masses adjust to the local forest environment.

Meri Räty et al.

Status: open (until 23 Jun 2022)

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Meri Räty et al.

Meri Räty et al.


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Short summary
We utilised back-trajectories to identify the source region of air masses arriving in Hyytiälä, Finland, and their travel time over forests. Combined with atmospheric observations, they revealed how air mass transport over the Fennoscandian boreal forest during the growing season produced an accumulation of cloud condensation nuclei and humidity, promoting cloudiness and precipitation. By 60 hours of transport, air masses appeared to reach a balanced state with the forest environment.