Effects of surface and top wind shear on the spatial organization of
marine Stratocumulus-topped boundary layers
Abstract
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.