Coupled vs. decoupled boundary layers in VOCALS-REx 1Department of Applied Mathematics, University of Washington, Seattle, Washington, USA
21 Jul 2011
2Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA
3Department of Atmospheric Science, University of Wyoming, Laramie, Wyoming, USA
Received: 03 Mar 2011 – Published in Atmos. Chem. Phys. Discuss.: 11 Mar 2011 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.
Revised: 15 Jul 2011 – Accepted: 15 Jul 2011 – Published: 21 Jul 2011
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.
Citation: Jones, C. R., Bretherton, C. S., and Leon, D.: Coupled vs. decoupled boundary layers in VOCALS-REx, Atmos. Chem. Phys., 11, 7143-7153, doi:10.5194/acp-11-7143-2011, 2011.