Impacts of Ice-Particle Size Distribution Shape Parameter on Climate Simulations with the Community Atmosphere Model Version 6 (CAM6)

Abstract. The impacts of the ice crystal size distribution shape parameter (μ_i) were considered in the two-moment bulk cloud microphysics scheme of the Community Atmosphere Model Version 6 (CAM6). The calculating formulas of statistical mean radii indicate that, under the same mass (q_i) and number (N_i) of ice crystals, the ratios of the mass-weighted radius (R_qi, not related to μ_i) to other statistical mean radii (e.g., effective radiative radius) are completely determined by μ_i. Off-line tests show that μ_i has a significant impact on the cloud microphysical processes owing to the μ_i-induced changes in ice crystal size distribution and statistical mean radii (excluding R_qi). Climate simulations show that increasing μ_i would lead to higher q_i and lower N_i in most regions, and these impacts can be explained by the changes in cloud microphysical processes. After increasing μ_i from 0 to 5, the longwave cloud radiative effect increases (stronger warming effect) by 5.58 W m⁻² (25.11 %), and the convective precipitation rate decreases by −0.12 mm day⁻¹ (7.64 %). In short, the impacts of μ_i on climate simulations are significant and the main influence mechanisms are also clear. This suggests that the μ_i-related processes deserve to be parameterized in a more realistic manner.