Idealized large-eddy simulations of stratocumulus advecting over cold
water. Part 1: Boundary layer decoupling
Abstract
We explore the decoupling physics of a stratocumulus-topped boundary
layer (STBL) moving over cooler water, a situation mimicking the warm
air advection (WADV). We simulate an initially well-mixed STBL over a
doubly periodic domain with the sea surface temperature decreasing
linearly over time using the System for Atmospheric Modeling large-eddy
model. Due to the surface cooling, the STBL becomes increasingly stably
stratified, manifested as a near-surface temperature inversion topped by
a well-mixed cloud-containing layer. Unlike the stably stratified STBL
in cold air advection (CADV) that is characterized by cumulus coupling,
the stratocumulus deck in the WADV is unambiguously decoupled from the
sea surface, manifested as weakly negative buoyancy flux throughout the
sub-cloud layer. Without the influxes of buoyancy from the surface, the
convective circulation in the well-mixed cloud-containing layer is
driven by cloud-top radiative cooling. In such a regime, the downdrafts
propel the circulation, in contrast to that in CADV regime for which the
cumulus updrafts play a more determinant role. Such a contrast in
convection regime explains the difference in many aspects of the STBLs
including the entrainment rate, cloud homogeneity, vertical exchanges of
heat and moisture, and lifetime of the stratocumulus deck, with the last
being subject to a more thorough investigation in part 2 of this study.
Finally, we investigate under what conditions a secondary stratus near
the surface (or fog) can form in the WADV. We found that weaker
subsidence favors the formation of fog whereas a more rapid surface
cooling rate doesn’t.