Detecting the Phase of Marine Boundary Layer Clouds: Some Implications for Cloud Albedo

This study examines rare examples of marine boundary layer clouds that show a sharp transition from supercooled to glaciated phase without other significant changes in appearance. We used the Cloud‐Aerosol Lidar with Orthogonal Polarization's (CALIOP) cloud phase detection algorithm and cloud top height to identify these cases. Measurements by CALIOP were co‐located with those by the Moderate Resolution Imaging Spectroradiometer (MODIS) to determine spectral changes in reflected and emitted radiation along the glaciated to supercooled transition path. In every case, the MODIS 8.5–11 µm brightness temperature difference abruptly changed from positive to negative, with a relatively constant cloud top temperature. No change was seen in visible reflectivity, and only weak changes were detected in shortwave infrared reflectivity. These glaciated‐to‐supercooled transitions were incorrectly classified with the MODIS cloud phase classification algorithm. We show that, for boundary layer clouds, it is possible to improve the (passive) remote sensing of the cloud phase using only a single spectral difference in brightness temperature. Using this approach, we closely match CALIOP's climatological distributions of the cloud phase. Our work also indicates that a sudden change in the boundary layer cloud thermodynamic phase from supercooled to glaciated is not accompanied by a shift in shortwave albedo.