Estimating the Decoupling Degree of Subtropical Marine Stratocumulus Decks From Satellite. Issue 22 (20th November 2018)

• Record Type: Journal Article
• Authors: Zheng, Youtong; Rosenfeld, Daniel; Li, Zhanqing
• Is Part Of: Geophysical research letters. Volume 45:Issue 22(2018)
• Page Start: 12, 560
• Page End: 12, 568
• Publication Date: 2018-11-20
• Subjects: stratocumulus -- decoupling degree -- satellite retrieval; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2018GL078382
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstracts: The decoupling degree of stratocumulus (Sc) decks is an important quantity dictating evolutions of Sc. In subtropical oceans, the Sc decoupling is a key intermediate process of the Sc‐to‐cumulus transitions, a persistent phenomenon that is not fully understood. This study introduces a new approach for estimating the degree of decoupling of subtropical Sc decks using passive satellite sensors. This method is limited to regions where Sc decks are advected over progressively warmer water. This is most common in the subtropics. The estimation concept is that decoupled Sc clouds under cold‐advection conditions are fed by spreading of the tops of cumulus clouds that are coupled. The cumulus clouds constitute a much larger liquid water path over small areas, which is identified by a positive skewness of the liquid water path, a quantity measurable from high‐resolution satellite data. The decoupling degree here is defined as the difference between the Sc cloud‐base height and lifting condensation level that is the cumulus cloud‐base height under cold‐advection conditions. This concept and the satellite‐based estimations are supported by ship measurements over the Northeast Pacific. One‐year climatology of the satellite‐inferred decoupling degree was generated over the same region, revealing a coherent pattern of offshore decoupling, consistent with previous theory and field‐campaign observations. Plain Language Summary: Our climate system is considerably sensitive to marine low clouds that reflect a great amount of incoming solar radiation and cool the earth. These marine clouds are intimately coupled with the underlying sea surfaces that feed moisture and energy to the clouds for sustaining them. The degree of the surface‐cloud coupling (SCC) considerably influences the cloud properties, which in turn modulates the temperature of our climate system. Despite its significance to climate, the SCC degree has never been measured from satellite, the only observational tool that offers global coverage. This study addresses this long‐lasting issue by developing an innovative method to estimate the SCC degree using satellite data. The validity of this method is demonstrated by validating the SCC degree estimated by this new method against ground truth measurements over the Northeast Pacific. This method offers unprecedentedly new geophysical information, namely, the SCC degree that is crucial for the understanding of behavior of marine low clouds and its interactions with our climate system. This will eventually improve the accuracy with which future climate change is projected. Key Points: A new satellite‐based method for inferring the decoupling degree of Sc decks over subtropical oceans is proposed This method is based on an empirically found relationship between the decoupling degree and the satellite‐derived skewness of liquid water path This method is not expected to be valid in regions where clouds are decoupled due to low‐level warm advection to colder waters