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Dynamics of the coastal Stratocumulus cloud dissipation
  • Mónica Zamora Zapata,
  • Thijs Heus,
  • Jan Kleissl
Mónica Zamora Zapata
University of California San Diego

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Thijs Heus
Cleveland State University
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Jan Kleissl
Univ of California, San Diego
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Coastal Stratocumulus clouds (Sc) can greatly impact the available solar radiation in areas like southern California. These clouds usually dissipate during the morning, as surface fluxes increase in the day. The dissipation process begins by the thinning of the cloud layer, that then becomes a broken cloud field until the clouds completely dissipate. A better understanding of the process could improve predictions of dissipation time and solar variability for this type of clouds. We investigate the evolution of a broken cloud field using LES simulations of coastal Sc clouds over Southern California (Ghonima et al, JAS 2016). We identify and characterize coherent updrafts, downdrafts, and cloud elements using flow field variables (vertical velocity and liquid water mixing ratio). The cloud layer is observed to become a broken cloud field around hour 8 of the simulation, after which the thinning tendency strongly diminishes and the variability of cloud fraction, cloud thickness, and liquid water path suddenly increases. The few cloud elements remaining at noon are found to be located close to cloud base, unlike at night, when they are distributed throughout the whole cloud layer. The largest updrafts and downdrafts contribute to a large portion of the turbulent fluxes along the day (on average, 40% of vertical velocity variance and 48% of the buoyancy flux), and their relative contribution changes during the day as updrafts become stronger and downdrafts weaken around noon.