10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
The Role of Ice Nucleating Particles in Convective Aggregation
HASSAN BEYDOUN, Corinna Hoose, Karlsruhe Institute of Technology
Abstract Number: 1261 Working Group: Unraveling the Many Facets of Ice Nucleating Particles and Their Interactions with Clouds
Abstract Deep convection is the primary mechanism by which energy is vertically transported in the tropical atmosphere. This energy transfer, manifested through latent heating by clouds, counteracts top of the atmosphere radiative cooling. Radiative-convective equilibrium (RCE) is a state in which these two modes of energy transfer balance, and has been shown to be a good approximation of the tropical atmosphere.
For over two decades now, numerical simulations of radiative-convective equilibrium at the cloud resolving scale have unraveled a phenomenon termed convective aggregation, whereby deep convection clusters into a single region of the domain. The implication is a polarized atmosphere, in which one region is dry and clear while the other is cloudy and rainy. Cloud ice is a key element in RCE due to the latent heat release of freezing, the radiative warming induced by anvil and cirrus clouds, and ice’s role in precipitation formation. Despite ice’s contribution to all key modes of the atmosphere’s energy exchange within itself and with its boundaries, no study has yet quantified the sensitivity of the RCE state to ice microphysics and subsequently the sensitivity to aerosol perturbations through ice nucleating particles.
We conducted RCE simulations with the ICOsahedral Non-hydrostatic atmosphere large eddy model (ICON-LEM) on a 700km x 700 km domain with periodic boundary conditions, a two moment microphysics scheme, and a fully interactive radiation scheme. Changes made in each simulation were the ice nucleating particle (INP) concentration, the inclusion (or omission) of the secondary ice formation by the Hallet-Mossop mechanism, and the temperature onset for homogenous freezing. Higher INP concentrations exhibited two fingerprints on the microphysical structure: decreased high altitude ice clouds and increased surface precipitation. Both these effects stem from the lower height at which liquid is converted to ice in the high INP configurations. We found a consistent trend in the simulations’ effect on the RCE state, whereby configurations which enhanced precipitation led to stronger convective aggregation. This is tied to the increased latent heat flux at the surface and decreased high level cirrus clouds which act to reduce the longwave cooling in the dry regions of the domain. However, the magnitude of these effects is very sensitive to the other ice formation processes of homogenous freezing and secondary ice formation. Increasing (or decreasing) the onset temperature of homogenous freezing can dampen (or amplify) the sensitivity to INPs active at low temperatures while inclusion of the secondary ice formation process has a similar impact of INPs active at high temperatures. The implications of these results on mixed phase clouds, deep convection, and climate will be discussed.