Human Influence on Marine Low-Level Clouds

A study of air mass origin’s effect on marine stratus and stratocumulus clouds has been performed on clouds north of Scandinavia between 2000 and 2004. The aerosol number size distribution of the air masses has been obtained from measurements in northern Finland. A trajectory model has been used to calculate trajectories to and from the measurement stations. The back trajectories were calculated using the measurement site as receptor to make sure the air masses had the right origin, and forward trajectories were calculated from receptor stations to assure adequate flow conditions. Satellite data of microphysical parameters of clouds from the Moderate Resolution Imaging Spectrometer (MODIS) has been downloaded where the trajectories indicated that clouds could be studied, and where the satellite images displayed low-level clouds. The 25 % days with the highest number of aerosol with a diameter over 80 nm (N80) and the 35% with the lowest N80 have been used to represent polluted and clean conditions respectively. After screening trajectories and satellite imagery, 22 cases of clouds with northerly trajectories that had low N80 values (i.e. clean) and 25 southerly cases with high N80 values (i.e. polluted) where identified for further analysis.The average cloud optical thickness (τ) for all polluted pixels was more than twice that of the clean pixels. This can most likely be related to the differences in aerosol concentrations…


1 Introduction
2 Theory
2.1 Aerosols
2.1 The area and air masses
2.2 Cloud optical and geometric thickness
2.3 Effective radius and liquid water path
3 Method
3.1 Aerosol data and station descriptions
3.2 Trajectory model
3.3 Satellite data
3.4 ECMWF data
4 Results
4.1 Aerosol data, trajectories and satellite data
4.2 Cloud Optical thickness
4.3 Effective radius
4.4 Seasonal differences
4.5 Case study
5 Discussion
6 Conclusion

Author: Sporre, Moa

Source: Uppsala University Library

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