Abstract
Clouds in the tropical western Pacific are dominated by reflective deep
convective cores with gradually thinning trailing anvil clouds, which
play a crucial role in determining tropical cloud radiative effects. The
microphysical controls of the thinning process and its changes when
subjected to a future warmer climate are still very uncertain.We use the
high resolution version of the Exascale Earth System Model (E3SM) to
study the thinning process in present day and future climate. We apply
Lagrangian forward trajectories starting at peaks of convective activity
of the detected mesoscale convective systems (MCS) and track detrained
ice crystals to better understand the processes controlling the
transition of a thick detrained anvil cloud into a thin cirrus cloud.
The trajectories are computed offline from the 1-hourly model-calculated
velocity fields and ice crystal sedimentation velocities. The modeled
MCS in present day climate have a comparable time evolution to those
observed by Himawari geostationary satellite data, with an average
lifetime of about 10-15 hours, which accounts only for the optically
thick part of the cold cloud shield. However, E3SM fails to simulate the
strongest storms, which is reflected by an underestimation of albedo and
overestimation of outgoing longwave radiation. The analysis of ice
sources (detrainment, vapor deposition, new nucleation events) and sinks
(sublimation, aggregation, sedimentation) along trajectories highlights
the crucial role of the balance between depositional growth and
precipitation formation for the maintenance of aging anvil clouds.
Interestingly, deposition of ice on detrained ice crystals contributes
to the majority of the upper tropospheric ice mass. On the other hand,
about 80% of the initial cloud mass is removed in the form of
precipitation within the first 10 hours of the detrainment event,
changing its radiative effect from net negative to net positive. The
future climate simulation shows an increase in storm frequency and
intensity, an increase in ice water content and albedo in convective
cores and thick anvils. The trajectory calculations reveal a 15%
decrease in cloud lifetime due to climate change, suggesting a decrease
in thin high clouds due to a higher precipitation efficiency and a shift
to more negative net cloud radiative effects.