Kilometer-scale simulations of trade-wind cumulus capture processes of
mesoscale organization
Abstract
The international field campaign for EUREC4A (Elucidating the role of
clouds and circulation coupling in climate) gathered observations to
better understand the links between trade-wind cumulus clouds, their
organization, and larger scales, a large source of uncertainty in
climate projections. A recent large-eddy simulation (LES) study showed a
cloud transition that occurred during EUREC4A (2nd February 2020), where
small shallow clouds developed into larger clouds with detrainment
layers, was caused by an increase in mesoscale organization generated by
a dynamical feedback in mesoscale vertical velocities. We show that
kilometer-scale simulations with the Met Office Unified Model reproduce
this increase in mesoscale organization and the process generating it,
despite being much lower resolution. The simulations develop mesoscale
organization stronger and earlier than the LES, more consistent with
satellite observations. Sensitivity tests with a shorter spin-up time,
to reduce initial organization, still have the same timing of
development and sensitivity tests with cold pools suppressed show only a
small effect on mesoscale organization. These results suggest that
large-scale circulation, associated with an increased vertical velocity
and moisture convergence, is driving the increase in mesoscale
organization, as opposed to a threshold reached in cloud development.
Mesoscale organization and clouds are sensitive to resolution, which
affects changes in net radiation, and clouds still have substantial
differences to observations. Therefore, while kilometer-scale
simulations can be useful for understanding processes of mesoscale
organization and links with large scales, including responses to climate
change, simulations will still suffer from significant errors and
uncertainties in radiative budgets.