Orographic cirrus and radiative forcing in CESM2

Cirrus clouds play an important role in the Earths energy budget, thereby affecting the climate state and climate change. The dominant ice formation mechanism in cirrus clouds, which is largely driven by cloud updraft vertical velocity (or cooling rate) unresolved by large-scale models, is still unknown. In this study, several parameterizations of subgrid orographic gravity waves (OGWs) are introduced in the Community Earth System Model version 2 (CESM2). OGW generated vertical velocity variance in the upper troposphere is linked to the cirrus ice formation in CESM2. Model simulations with and without the OGW effects on ice nucleation are evaluated against the DOE ARM Small Particles in Cirrus (SPARTICUS) campaign observations of fast-speed vertical velocity variance, in-cloud ice number concentration, ice water content, and ice effective radius in in-situ cirrus. The McFarlane OGW scheme generates more and higher fast-speed vertical velocity variance () than the default model without OGW effects and has a better agreement with the observation. The vertical velocity variance due to OGWs increases the activated aerosol number concentrations for ice nucleation. The dominant ice nucleation mechanism in modeled in situ cirrus switches from heterogeneous to homogeneous nucleation after considering the OGWs effects. Evaluation of the model simulations against the CALIPSO satellite observations indicates that the McFarlane OGW parameterization significantly increases the in-cloud ice number concentrations in the upper troposphere over mountains and high plateaus in the mid-latitudes and polar regions of the winter hemisphere. The increased number concentrations of ice particles with smaller sizes absorb more infrared radiation ( W m-2). At top of the atmosphere (TOA), the net radiative cloud forcing is +0.159 W m-2 due to the OGW effects.