Detecting Supercooled Water Clouds Using Passive Radiometer Measurements

• Published in: Geophysical research letters Vol. 49; no. 4
• Main Authors: Zhou, Ganning, Wang, Jianjie, Yin, Yan, Hu, Xiuqing, Letu, Husi, Sohn, Byung‐Ju, Yung, Yuk L., Liu, Chao
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 28.02.2022
• Subjects: Accretion; Air safety; Aircraft; Aircraft accidents & safety; Aircraft icing; Algorithms; Artificial precipitation; Aviation; Brightness temperature; Channels; Cloud properties; Clouds; Detection; Droplets; Earth; Freezing; Freezing point; Ice accretion; Ice accumulation; Infrared radiometers; Latitude; Lidar; Lidar measurements; Melting points; Meteorological satellites; Microwave imagery; Oceans; passive radiometers; Precipitation; Properties; Radar; Radiance; Radiation; Radiation balance; Radiation budget; Radiometers; Rainmaking; Reflectance; Safety; Satellites; Supercooled water; supercooled water clouds; Surface radiation temperature; Synchronous satellites; Temperature differences; Temperature gradients; Water; Water droplets; Water drops; Weather; Weather modification
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2021GL096111

Supercooled water clouds (SWCs) have significant impacts on the Earth's radiation balance, aircraft ice accretion and precipitation augmentation. This study introduces an efficient algorithm to detect SWCs from passive radiometers, which combines information from the reflectance difference between 1.61 and 2.25 μm channels, the brightness temperature difference between the 8.5 and 11 μm channels, and the cloud top temperature. Validated by space radar and lidar measurements, our algorithm can correctly detect 91% of SWC pixels, better than current Visible Infrared Imager Radiometer Suite operational product. SWCs are found mostly over the mid‐ to high‐latitude oceans and have a global occurrence frequency of ∼8% in cloudy skies. Since the channels used for the detection are available in most current operational polar and geostationary satellite radiometers, this SWC detection algorithm can be easily implemented for operations such as cloud monitoring, aviation safety, and SWC‐related weather modification. Plain Language Summary Supercooled water clouds (SWCs) are clouds with temperatures under the freezing point but still containing liquid water droplets. The detection of SWCs is crucial for aviation safety as aviation accidents are frequently caused by the accumulation of supercooled water droplets on airplanes. Detecting SWCs is also critical for artificial precipitation because the implementation is highly dependent on SWC contents. Moreover, SWCs are of great importance in the Earth's radiation budget. However, since SWCs from passive radiometer measurements are normally determined with empirical relationships of the channel radiance and cloud properties, the results are often subject to certain biases or required pre‐retrieved cloud properties. This study introduces a simple but efficient detection algorithm to identify SWCs on a global scale from the Visible Infrared Imager Radiometer Suite (VIIRS) onboard the Suomi‐National Polar‐orbiting Partnership (NPP) satellite. Our algorithm is able to detect over 90% occurrence of SWCs, which accounts for ∼8% among all cloudy sky, with most located over the mid‐ to high‐latitude oceans. Our algorithm can be used for geoscience researches, such as estimating the cloud radiative effect, and for applications such as aviation safety and weather modification operations. Key Points An efficient algorithm to detect supercooled water clouds (SWC) from passive radiometer measurements is developed The algorithm correctly identifies 91% of SWCs determined by active sensors, significantly better than that in the Visible Infrared Imager Radiometer Suite (VIIRS) operational product The passive radiometer VIIRS suggests a global SWC occurrence frequency of ∼8% in cloudy skies