Invigoration of Deep Convection in Polluted Environments: Myth or Reality?

Influence of atmospheric pollution on moist convection continues
to be a controversial topic. The prime example is the hypothesized
invigoration of deep convection in high CCN environments. Arguably,
only appropriately designed numerical simulations can clearly
separate impact of aerosols from effects of other factors, such as
the atmospheric sounding, magnitude surface fluxes, or effects of
larger-scale perturbations (e.g., waves) that all affect moist
convection. Moreover, separating the dynamical impact (i.e.,
convective invigoration) from purely microphysical effects (e.g.,
increased upper-tropospheric cloudiness due to higher ice crystal
concentrations and thus smaller particle sizes and lower sedimentation
rates) is difficult using traditional cloud-scale simulations and
virtually impossible in observations. The piggybacking modeling
methodology allows clear separation of dynamical and microphysical
impacts. This presentation will illustrate misconceptions concerning
aerosol effects on deep convection, briefly explain the piggybacking
method, and present results from piggybacking bin microphysics
simulations of daytime convective development over land. This is a
follow-up to previous studies applying double-moment bulk microphysics
(Grabowski and Morrison, J. Atmos. Sci. 2016; Grabowski, J. Atmos.
Sci. 2018). In agreement with those studies, bin simulations show
that microphysical effects dominate aerosol impacts on deep convection
and that dynamical effects play a minor role. In particular, model
results again question the postulated dynamical invigoration of
deep convection in polluted environments.