Effects of lower troposphere vertical mixing on simulated clouds and
precipitation over the Amazon during the wet season
Abstract
Planetary boundary layer (PBL) schemes parameterize unresolved turbulent
mixing within the PBL and free troposphere (FT). Previous studies
reported that precipitation simulation over the Amazon in South America
is quite sensitive to PBL schemes and the exact relationship between the
turbulent mixing and precipitation processes is, however, not
disentangled. In this study, regional climate simulations over the
Amazon in January-February 2019 are examined at process level to
understand the precipitation sensitivity to PBL scheme. The focus is on
two PBL schemes, the Yonsei University (YSU) scheme, and the asymmetric
convective model v2 (ACM2) scheme, which show the largest difference in
the simulated precipitation. During daytime, while the FT clouds
simulated by YSU dissipate, clouds simulated by ACM2 maintain because of
enhanced moisture supply due to the enhanced vertical moisture relay
transport process: 1) vertical mixing within PBL transports surface
moisture to the PBL top, and 2) FT mixing feeds the moisture into the FT
cloud deck. Due to the thick cloud deck over Amazon simulated by ACM2,
surface radiative heating is reduced and consequently the convective
available potential energy (CAPE) is reduced. As a result, precipitation
is weaker from ACM2. Two key parameters dictating the vertical mixing
are identified, p, an exponent determining boundary layer mixing and λ,
a scale dictating FT mixing. Sensitivity simulations with altered p, λ,
and other treatments within YSU and ACM2 confirm the precipitation
sensitivity. The FT mixing in the presence of clouds appears most
critical to explain the sensitivity between YSU and ACM2.