New Treatments of Aerosol Formation, Resuspension, Ice Nucleation, and Light-Absorption in Snow/Ice

Wednesday, May 6, 2015 - 07:00
Add to Calendar

Several improvements to the representations of clouds, aerosols, and cloud-aerosol interactions planned for the ACME V1 model have been implemented or are underway. In order to improve the simulated concentrations of smaller aerosol particles (< 100 nm), the treatments of H2SO4 vapor production and loss have been modified to use a parallel time-split approach. H2SO4 vapor strongly affects new particle formation, and because of its short lifetime it is sensitive to the time splitting. Simulated aerosol number concentrations are now in much better agreement with observations. A significant fraction of the aerosol that is wet-scavenged within clouds is resuspended when raindrops evaporate below clouds, particularly for CAM5 stratiform clouds. The existing treatment in CAM5 releases the aerosols back to their originating mode, rather than to the coarse mode as relatively large particles, and this noticeably affects simulated CCN concentrations and submicron mass loadings. The aerosol wet-scavenging modules are being revised to use a more physically-based treatment of this resuspension process. In order to improve the simulation of ice clouds and their impacts on radiation and the hydrological cycle, several new parameterizations are implemented in ACME, including a new treatment of the sub-grid updraft velocity that determines the ice nucleation rate, an updated ice nucleation parameterization that considers the impact of pre-existing ice crystals in cirrus clouds, and a classical-nucleation-theory based parameterization for heterogeneous ice nucleation in mixed-phase clouds. These new treatments remove some artificial thresholds applied in the existing ice nucleation schemes and improve the consistency between the representations of various ice nucleation modes. The ACME model is also being modified to improve the compatibility between MAM aerosols and light-absorbing particles deposited on snow/ice and simulation of their impacts on radiation and snow/ice melting. Preliminary results show significant changes to aerosol-in-snow forcing induced by the new treatments. The code is being structured to function with future treatments of light-absorption by brown carbon particles in addition to black carbon and dust.