Aerosol-Cloud-Precipitation Interactions - Studied using combinations of remote sensing and in-situ data

Author

Summary, in English

Cloud droplets never form in the atmosphere without a seed in the form of an aerosol particle. Changes in number concentrations of aerosol particles in the atmosphere can therefore affect the number of droplets in a cloud. Higher concentrations of aerosol particles in the atmosphere lead to clouds with more droplets and if the amount of liquid water in the clouds stay the same, the droplets become smaller. Clouds with more, smaller droplets reflect more sunlight and may take longer to produce precipitation. In the research presented in this thesis, satellite data of clouds are combined with a range of other datasets to investigate how sensitive the cloud properties are to changes in the concentration of aerosol particles.

Cloud droplets were found to be smaller in low-level clouds formed in air with higher aerosol number concentrations over the ocean north of Scandinavia. This was also true for low-level and convective clouds over land in Sweden and Finland. The results regarding cloud optical thickness (COT), which is a measure of how much light a cloud reflects, was not as conclusive. For the low-level clouds over the ocean, the COT was higher in air masses with higher aerosol number concentrations. Differences in meteorological conditions in the clean and polluted air masses may however explain some of the differences in COT. The low-level and convective clouds over land did not show any significant changes in COT with varying aerosol number concentrations. This may be caused by changes in cloud dynamics due to the smaller droplets in the clouds. Hence, the indirect aerosol effect could not be observed for clouds studied over land.

The precipitation intensity from the clouds over land and how this varied with changing aerosol loading was also investigated. For both low-level and convective clouds, the precipitation was found to decrease somewhat with increasing aerosol number concentrations. However, for the convective clouds, this relationship only appeared when the clouds were sorted according to vertical extent, as higher convective clouds tend to produce heavier precipitation.

How cirrus clouds at midlatitudes in the northern hemisphere are affected by the mass concentration of particulate sulphate present in the lowermost stratosphere (LMS) was investigated using satellite data. Changes in the LMS particle levels were caused by explosive volcanos that emit gases and particles into the stratosphere. Due to subsidence in the stratosphere at midlatitudes, the volcanic sulphate eventually enters the upper troposphere, increasing its sulphate concentration. The reflectance of the cirrus clouds decreased when there were more sulphate particles present in the LMS. Cirrus clouds warm the climate and a decrease in their reflectance hence cools the climate.

Topic

  • Meteorology and Atmospheric Sciences

Keywords

  • clouds
  • aerosol particles
  • climate
  • precipitation
  • satellite data
  • Fysicumarkivet A:2016:Sporre

Status

Published

Research group

  • Aerosol, Nuclear Physics-lup-obsolete

Supervisor

ISBN/ISSN/Other

  • ISBN: 978-91-7623-589-8
  • ISBN: 978-91-7623-590-4

Defence date

19 February 2016

Defence time

09:15

Defence place

Lecture hall Rydbergsalen, Physics Department, Professorsgatan 1, Lund University, Faculty of Engineering