Abstract
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/m2. Our study highlights the importance of active satellite-based remote sensing in globally detecting and characterizing optically thin clouds.
Highlights
Fundamental questions on the climate system remain unanswered because of our limited understanding of how clouds, atmospheric circulation, and climate interact
Midlevel clouds have been documented by several field campaigns in the tropics (e.g., Refs. 9–11), midlatitudes (e.g., Refs. 12–14), and polar regions (e.g., Refs. 15–17)
We present the occurrence of these clouds in the context of the low, mid, and high-level clouds that are detected by CALIPSO
Summary
Fundamental questions on the climate system remain unanswered because of our limited understanding of how clouds, atmospheric circulation, and climate interact. The radiative effects of clouds depend on their macrophysical (altitude, temperature, and geometrical thickness) and microphysical (particle phase and size distribution) properties.[1,2,3] Despite their importance, the accurate representation of cloud types in global/regional climate models remains a challenge. Altocumulus clouds form as cellular structures and are relatively thin often with ice virga precipitating from the cloud base These traditional midlevel clouds are optically thick, several studies indicate they can exist as optically thin clouds.[18,20,21] Such optically thin altostratus and altocumulus clouds are least studied because of the detection limitations of both ground-based measurements and satellite passive sensors. We determine their radiative impact in the Arctic, where these clouds are observed most commonly
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