HIRDLS and CALIPSO observations of tropical cirrus

Monthly geospatial averages of cloud frequency of occurrence between 90 and 177 hPa are similar and correlate well with Microwave Limb Sounder (MLS) relative humidity with respect to ice (RHI) data, though clouds are present at individual RHI values less than 100%, due to the 5, 1, and sub-1 km vertical resolutions of the MLS, HIRDLS, and CALIPSO experiments.

Seasonal variations in cloud frequency of occurrence are similar, with largest frequencies during winter (December–February). Though the CALIPSO and HIRDLS experiments employ nadir- and limb-viewing observational geometries, respectively, CALIPSO horizontal scales of cirrus are frequently larger than 100 km, and therefore similarities are present in the geospatial distributions of cloud occurrence. Isolated laminar cirrus is most prevalent away from the equator.