Progress in Understanding the Low Marine Cloud-Aerosol Interactions
during CSET using LES
Abstract
Low marine clouds are a major source of uncertainty in cloud simulations
across models from LES to global scale. To address this issue, we
conducted Lagrangian LES experiments that explore the aerosol-cloud
interactions for case studies covering a spectrum of observed ambient
conditions, and evaluated the model against observations. Our LES
benefits from a prognostic aerosol model that simulates aerosol budget
tendencies such as coalescence and interstitial scavenging, surface
sources, and entrainment from free troposphere. To initialize, force,
and evaluate the LES, we used a combination of reanalysis, satellite,
and aircraft data from the Cloud System Evolution in the Trades (CSET)
field campaign in summer 2015 over the Northeast Pacific. The LES
follows two Lagrangian trajectories from subtropical stratocumulus (Sc)
deck region offshore of California to tropical shallow cumulus (Cu)
region near Hawaii. The first trajectory is characterized by a clean,
well-mixed Sc-topped marine boundary layer (MBL) on the first day, and
continuous MBL deepening and precipitation onset after the first day
followed by a clear Sc-to-Cu transition (SCT) and a consistent reduction
of aerosols that ultimately leads to an ultra-clean layer at the top of
MBL. Overall, the LES simulates general MBL features seen in
observations. The runs with enhanced aerosols show distinct changes in
microphysics and macrophysics such as delayed precipitation onset and
SCT. The second trajectory is characterized by an initially polluted and
decoupled MBL, weak or no precipitation, and no clear sign of SCT
throughout the simulations. It is challenging for LES to simulate
observed features, and the LES underestimates (overestimates) low cloud
fraction in the first (last) day. Although enhancing aerosols among
cases leads to distinct changes in microphysics (e.g., enhancement of
cloud optical depth and reduction of effective radius), it does not
affect cloud macrophysical properties significantly. Finally, a
theoretical analysis was conducted to decompose contributions to albedo
of the Twomey effect and cloud adjustments. The cloud radiative forcing
due to the Twomey effect shows an enhancement with an increase in
aerosol, however, the cloud radiative forcing due to cloud adjustments
is strongly dependent on ambient meteorological conditions.