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Effects of surface and top wind shear on the spatial organization of marine Stratocumulus-topped boundary layers
  • Mónica Zamora Zapata,
  • Thijs Heus,
  • Jan Kleissl
Mónica Zamora Zapata
University of California San Diego

Corresponding Author:mzamoraz@eng.ucsd.edu

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Thijs Heus
Cleveland State University
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Jan Kleissl
University of California, San Diego
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The convective nature of Stratocumulus topped boundary layers (STBL) involves the motion of updrafts and downdrafts, driven by surface fluxes and radiative cooling, respectively. The balance between shear and buoyant forcings at the surface can determine the organization of updrafts between cellular and roll structures. We investigate the effect of varying shear at the surface and top of the STBL using LES simulations, taking DYCOMS II RF01 as a base case. We focus on spatial identification of the following features: coherent updrafts, downdrafts, and wet updrafts, and observe how they are affected by varying shear. Stronger surface shear organizes the updrafts in rolls, causes less well-mixed thermodynamic profiles, and decreases cloud fraction and LWP. Stronger top shear also decreases cloud fraction and LWP more than surface shear, by thinning the cloud from the top. Features with stronger top than surface shear are associated with a net downward momentum transport and show early signs of decoupling. Classifying updrafts and downdrafts based on their vertical span and horizontal size confirms the dominance of large objects spanning the whole STBL. Large objects occupy 14% of the volume in the STBL while smaller ones occupy less than 1%. For updraft and downdraft fluxes these large objects explain 33% of the vertical velocity variance and 53% of the buoyancy flux, on average. Stronger top shear also weakens the contribution of downdrafts to the turbulent fluxes and tilts the otherwise vertical development of updrafts.
16 Jun 2021Published in Journal of Geophysical Research: Atmospheres volume 126 issue 11. 10.1029/2020JD034162