E1.5 Ice nucleation in E3SM and climatic effects of dust on mixed-phase clouds

Abstract

Atmospheric dust is the dominant ice nucleating particles (INP) at temperatures colder than -10°C, and thus may exert a critical role in the ice formation in mixed-phase clouds, frequently observed in the Arctic and mid-latitude storm tracks. In this study, we first implement two deterministic ice nucleation parameterizations for dust INPs (Niemand et al. 2012 and DeMott et al. 2015) in the DOE Energy Exascale Earth System Model version 1 and version 0 (E3SMv1 and E3SMv0). Model simulated INP concentrations based on Niemand et al. and DeMott et al. are compared with the default ice nucleation scheme, namely, the classical nucleation theory (CNT), and with observation data collected in different geographic regions of the globe. For both model versions (v1 and v0), all of the three parameterizations show relatively good agreements with INP observations at temperatures lower than -20°C. But CNT significantly underestimates the INP concentrations at warmer temperatures. Moreover, modeled INP concentrations are too low (by 1-2 order of magnitudes) in the Arctic primarily due to the underestimation of dust transport when compared with CALIPSO dust extinction and the surface dust observation at Canada, Alert station. With the improvement in aerosol wet removal and cloud microphysics in E3SMv1, the aerosol (dust) transport to the polar regions is much more efficient and thus it presents around 100 times more INP than E3SM v0 in the Arctic regions for all of the three parameterizations. The impacts of dust INPs on mixed-phase cloud water and cloud radiative forcing are investigated by comparing with the model simulations turning off dust INPs with the three parameterizations in E3SMv1. Sensitivity tests with enhanced dust INP number concentrations are also conducted to mimic the underestimation of dust INP in high latitudes.