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Volume 11, issue 14
Atmos. Chem. Phys., 11, 7143–7153, 2011
https://doi.org/10.5194/acp-11-7143-2011
© Author(s) 2011. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue: VAMOS Ocean-Cloud-Atmosphere-Land Study (VOCALS) (ACP/OS inter-journal...

Atmos. Chem. Phys., 11, 7143–7153, 2011
https://doi.org/10.5194/acp-11-7143-2011
© Author(s) 2011. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 21 Jul 2011

Research article | 21 Jul 2011

Coupled vs. decoupled boundary layers in VOCALS-REx

C. R. Jones1, C. S. Bretherton2, and D. Leon3 C. R. Jones et al.
  • 1Department of Applied Mathematics, University of Washington, Seattle, Washington, USA
  • 2Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA
  • 3Department of Atmospheric Science, University of Wyoming, Laramie, Wyoming, USA

Abstract. We analyze the extent of subtropical stratocumulus-capped boundary layer decoupling and its relation to other boundary-layer characteristics and forcings using aircraft observations from VOCALS-REx along a swath of the subtropical southeast Pacific Ocean running west 1600 km from the coast of Northern Chile. We develop two complementary and consistent measures of decoupling. The first is based on boundary layer moisture and temperature stratification in flight profiles from near the surface to above the capping inversion, and the second is based the difference between the lifted condensation level (LCL) and a mean lidar-derived cloud base measured on flight legs at 150 m altitude. Most flights took place during early-mid morning, well before the peak in insolation-induced decoupling.

We find that the boundary layer is typically shallower, drier, and well mixed near the shore, and tends to deepen, decouple, and produce more drizzle further offshore to the west. Decoupling is strongly correlated to the "mixed layer cloud thickness", defined as the difference between the capping inversion height and the LCL; other factors such as wind speed, cloud droplet concentration, and inversion thermodynamic jumps have little additional explanatory power. The results are broadly consistent with the deepening-warming theory of decoupling.

In the deeper boundary layers observed well offshore, there was frequently nearly 100 % boundary-layer cloud cover despite pronounced decoupling. The cloud cover was more strongly correlated to a κ parameter related to the inversion jumps of humidity and temperature, though the exact functional relation is slightly different than found in prior large-eddy simulation studies.

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