Observed impact of meso-scale vertical motion on cloudiness

• Published in: Journal of the Atmospheric Sciences
• Main Authors: George, Geet, Stevens, Bjorn, Bony, Sandrine, Klingebiel, Marcus, Vogel, Raphaela
• Other Authors: Laboratoire de Météorologie Dynamique (UMR 8539) (LMD), Institut national des sciences de l'Univers (INSU - CNRS)-École polytechnique (X), Institut Polytechnique de Paris (IP Paris)-Institut Polytechnique de Paris (IP Paris)-École des Ponts ParisTech (ENPC)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Département des Géosciences - ENS Paris, École normale supérieure - Paris (ENS-PSL), Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL)-École normale supérieure - Paris (ENS-PSL), Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2021
• Subjects: Atmosphere; Clouds; Convergence/divergence; Vertical motion; Boundary layer; Cloud cover; Cumulus clouds; Subsidence; Dropsondes; Kinematics; Lidars/Lidar observations; Measurements; Microwave observations; Radars/Radar observations; Remote sensing; Satellite observations; [SDU.OCEAN]Sciences of the Universe [physics]/Ocean; North Atlantic
• Online Access: https://hal.sorbonne-universite.fr/hal-03448536; https://hal.sorbonne-universite.fr/hal-03448536/document; https://hal.sorbonne-universite.fr/hal-03448536/file/George_etal_jas21_clouds_mesoscalemotion.pdf; https://doi.org/10.1175/jas-d-20-0335.1

International audience We use estimates of mesoscale vertical velocity and collocated cloud measurements from the second Next-Generation Aircraft Remote Sensing for Validation campaign (NARVAL2) in the tropical North Atlantic to show the observed impact of mesoscale vertical motion on tropical clouds. Our results not only confirm previously untested hypotheses about the role of dynamics being nonnegligible in determining cloudiness, but go further to show that at the mesoscale, the dynamics has a more dominant control on cloudiness variability than thermodynamics. A simple massflux estimate reveals that mesoscale vertical velocity at the subcloud-layer top explains much of the variations in peak shallow cumulus cloud fraction. In contrast, we find that thermodynamic cloud-controlling factors, such as humidity and stability, are unable to explain the variations in cloudiness at the mesoscale. Thus, capturing the observed variability of cloudiness may require not only a consideration of thermodynamic factors, but also dynamic ones such as the mesoscale vertical velocity. SIGNIFICANCE STATEMENT: Knowing how low clouds link to atmospheric circulation over a few hundred kilometers will reduce current uncertainties in the sensitivity of Earth's climate to warming. Such investigations have previously been limited by lack of circulation measurements at the mesoscale. However, using measurements now available from a recent field campaign over the tropical North Atlantic along with cloud measurements, we demonstrate how atmospheric vertical motion especially in the lower layers can influence the extent and structure of clouds. We find that the kinematics have a more dominant control on low-level cloudiness than conventionally studied thermodynamics. Our results show why it is important to focus attention to the circulation to improve our understanding of the variability in cloudiness.