Three Modes of Cloud-Boundary Layer Coupling over the Southern Ocean:
Performance of Conventional and Mass-flux PBL Schemes
Abstract
While the continental planetary boundary layer (PBL) structure and model
capability to simulate it are relatively well understood, its structure
and the ability of models to simulate it over the Southern Ocean (SO),
especially in the presence of clouds, are less known. In this study, in
situ soundings and remote sensing data collected from ships during two
field campaigns over the SO, the Measurements of Aerosols, Radiation and
Clouds over the Southern Ocean (MARCUS) and the Clouds Aerosols
Precipitation Radiation and atmospheric Composition over the Southern
Ocean (CAPRICORN) campaigns, and WRF simulations with different PBL
schemes are examined to study the boundary layer structure over the SO,
focusing particularly on the coupling status between the surface-based
boundary layer and the single cloud layer above. Ten single cloud layer
cases, including Dec. 1, 2017, Mar. 21-22, 2018, Mar. 23, 2018, Jan. 10,
2018 detected during MARCUS, and Feb. 17-18, 2018 detected during
CAPRICORN, are examined. The cloud-boundary layer coupling over the SO
for these cases can be classified into three modes: Coupled
cloud-boundary layer in the presence of weak surface positive flux;
Decoupled cloud-boundary layer in the presence of surface negative flux,
with a very shallow surface-based PBL; and Decoupled cloud-boundary
layer in the presence of single-layer high clouds and stronger surface
positive flux, with thicker surface-based PBL. WRF simulations were
conducted for these selected cases using different PBL schemes,
including the Yonsei University (YSU) scheme with and without extra
mixing and entrainment induced by cloud-top cooling (referred to as
YSUtopdown when the cloud-top cooling treatment is included), the
Mellor–Yamada Nakanishi and Niino (MYNN) scheme, and the MYNN scheme
with the eddy-diffusivity (ED) local closure and mass flux (MF) nonlocal
approach (referred to as MYNN-EDMF). For cases with the different
cloud-boundary layer coupling modes, different PBL schemes provided the
best consistency with observations. The MYNN-EDMF scheme is more
consistent with observations than the conventional PBL schemes for the
type 3 coupling mode because of the different vertical extent of local
mixing and nonlocal mass flux in presence of sufficient surface flux.
The YSUtopdown scheme has more consistency with observations than the
YSU scheme for the type 1 coupling mode to simulate higher cloud-topped
boundary layer. For the type 2 coupling mode, the different PBL schemes
perform similarly.