MODIS Terra, 28 July 2008
MODIS Terra, 28 July 2008

The team has shown that the twilight zone, which used to be considered as a cloud-free region, has unique microphysical and optical properties. By defining the boundaries of cloud fields one can better characterize the importance of this zone and its properties compared to a "cloud-field-free" pixel. In a paper in Environmental Research Letters, the atmosphere is classified into cloud fields and cloud-free – away from a cloud field. Detectable clouds are included in the cloud-field class as a subset. This classification is made using a new algorithm based on how clouds are spatially distributed.

The new findings show that, while the average cloud fraction (the relative area that is covered by detectable clouds only) over the Atlantic Ocean (50° S to 50° N) during July is ~50%, the cloud field coverage (the relative area that is covered by cloud fields) is higher than 90%. This suggests that cloud-field coverage needs to be considered when studying clouds and aerosol properties and interactions. A comparison between aerosol optical properties inside and outside cloud fields reveals differences in how the aerosols reflect light, depending on their location. The observed mean aerosol "optical depth" (how transparent the particles are) inside cloud fields is more than 10% higher than outside, indicating that the location of aerosols inside or outside a cloud field should be taken into account when analysing their properties. This may lead to better estimations of the effect of cloud–aerosol interactions on climate.

The next stages in the group's work will involve a global analysis of cloud fields, such as analysing cloud-field coverage, aerosol optical and physical properties as a function of their location, and the effects of global climate on the properties of cloud fields. These include the differences between latitudinal belts, and differences between cloud fields over sea and land surfaces.