Sensitivity of Arctic mixed-phase cloud simulations to ice microphysical modifications in the WDM6 scheme of WRF (v4.3.1)

Abstract. Arctic mixed-phase clouds (MPCs) remain challenging to represent in atmospheric models, with bulk microphysics schemes typically biased toward either excessive glaciation or inadequate ice formation. This study evaluates the behavior of the Weather Research and Forecasting (WRF) Double-Moment 6-class (WDM6) scheme and its modified version (WDM6_ICE), which incorporates spherical ice shape, constrained nucleation, and prognostic cloud ice number concentration, under Arctic conditions using the Mixed-Phase Arctic Cloud Experiment (M-PACE) case (9–10 October 2004). WDM6 severely underestimates liquid water through efficient vapor deposition onto bullet-shaped ice. WDM6_ICE exhibits the opposite behavior, maintaining persistent liquid as cloud ice deposition is reduced by about 98 %. The suppressed cloud ice is partially compensated by enhanced snow deposition, resulting in a reduction in total ice water content of about 70 % rather than complete ice elimination. Sensitivity experiments show that ice shape modification is the dominant factor, while nucleation modification alone redistributes ice among hydrometeor categories without reducing total ice content. Comparison with mid-latitude evaluations of the same scheme indicates a regime-dependent response, in which modifications that produce a moderate ice adjustment in mid-latitude cases lead to a more pronounced restructuring of the ice budget under Arctic conditions. Surface energy analysis indicates that balanced phase partitioning is more relevant than liquid water maximization for radiative bias reduction. All configurations underestimate total water path, suggesting that accurate Arctic MPC representation requires coordinated improvements in ice particle properties, ice-nucleating particle recycling, and boundary-layer coupling. These results are based on a single Arctic case and demonstrate a mechanism that remains to be tested across a broader range of conditions.