Climatology of the subvisual cirrus clouds as seen by OSIRIS on Odin

The InfraRed Imager (IRI) subsection of the OSIRIS instrument onboard the Odin spacecraft collects limb images of the 1.53 μm sunlight that is scattered from the upper troposphere and lower stratosphere. Due to the enhancement provided by limb geometry, the IRI is capable of detecting cloud layers with vertical equivalent optical depths as low as 10−5. These measurements can be used to map subvisual cirrus clouds present in the upper troposphere. The dusk-dawn orbit of the Odin spacecraft provides sunlit coverage of the equatorial region throughout the year. This work presents the subvisual cirrus cloud climatology that has been measured by the InfraRed Imager. The global seasonal distribution of sub-visible clouds detected by OSIRIS is similar to that measured previously by other instruments, but is obtained on a shorter timescale due to the sampling characteristics of the limb scatter data.