Optically thin midlevel ice clouds derived from Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observations

We use measurements from the Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) to identify optically thin midlevel ice clouds and determine their occurrence and properties. We identify these clouds as having cloud top heights between 2 km above the ground and the tropopause plus 1 km, cloud base temperatures less than −10°C , and cloud top temperatures greater than −38°C . Globally, we find that these clouds occur at least 5% of the time and represent 7% of all tropospheric clouds detected by CALIPSO at a horizontal scale of 10 km. The cloud type fraction decreases with increasing horizontal scale, representing 4% of clouds at 25 km, 2% at 50 km, and less than 0.1% at 100 km. These clouds occur most commonly in the Arctic, most often in winter and least often in summer. During the winter, these clouds occur up to 15% of the time in the Arctic. We found five large ( ∼500  km ) distinct clouds over the Arctic and investigated their meteorological conditions and radiative effects. We find that these thin midlevel ice clouds have a warming effect on the surface from 23 to 48  W/m 2 . Our study highlights the importance of active satellite-based remote sensing in globally detecting and characterizing optically thin clouds.