An Algorithm for Detecting Ice Cloud at Different Altitudes by Combining Dual CrIS Full Spectrum Resolution CO2 Channels

Using infrared sensors to detect ice clouds in different atmospheric layers is still a challenge. The different scattering and absorption properties of longwave and shortwave infrared channels can be utilized to fulfill this purpose. In this study, the release of Suomi-NPP Cross-track Infrared Sounder (CrIS) full spectrum resolution is used to select and pair channels from longwave (~ 15 μm) and shortwave (~4.3 μm) CO₂ absorption bands under stricter conditions, so as to better detect ice clouds. Besides, the differences of the weighting function peaks and cloud insensitive level altitudes of the paired channels are both within 50 hPa so that the variances due to atmospheric conditions can be minimized. The training data of clear sky are determined by Visible Infrared Imaging Radiometer Suite (VIIRS) cloud mask product and used to find the linear relationship between the paired longwave and shortwave CO₂ absorption channels. From the linear relationship, the so-called cloud emission and scattering index (CESI) is derived to detect ice clouds. CESI clearly captures the center and the ice cloud features of the Super Typhoon Hato located above 415 hPa. Moreover, the CESI distributions agree with cloud top pressure from the VIIRS in both daytime and nighttime in different atmospheric layers.