On impacts of overlying solar-absorbing aerosol layers on the transition of stratocumulus to trade cumulus clouds

Abstract:

Early cloud-scale modeling work on effects of solar-absorbing aerosol layers focused on the desiccation of shallow cumulus clouds embedded with such layers, resulting from the reduction in relative humidity induced by solar heating, as well as reduced vertical mixing from stabilization of the boundary layer. Such a cloud response serves as a positive radiative forcing at the top of atmosphere, tending to warm the climate system. Subsequent work has largely targeted the impact of overlying solar-absorbing aerosol layers on stratiform clouds in the marine boundary layer, in which the solar heating increases the strength of the temperature inversion capping the boundary layer, which reduces entrainment of overlying air into the boundary layer. Because entrainment typically (but not always) reduces the average relative humidity of the boundary layer and thereby leads to a thinner cloud layer, a reduction in entrainment induced by an absorbing aerosol layer leads to a thicker cloud layer and a negative radiative forcing at the top of atmosphere, tending to cool the climate system. Here we use large-eddy simulations to assess the effects of overlying solar-absorbing aerosol layers on the transition of stratocumulus to trade cumulus clouds. Beyond the impact on the inversion strength, we also consider the changes induced by microphysical response to entrained aerosol that serve as cloud condensation nuclei, as well as reduction in solar heating of the cloud induced by the overlying aerosol layer. Observationally-based transition cases used in a recent large-eddy simulation intercomparison will be used as a starting point for the model setup, along with idealized aerosol layer properties based on remote sensing and in situ observations. We will also use the same simulation setups to evaluate and compare the response of the single column model version of the GISS climate model (with two-moment microphysics).