Influence of frozen hydrometeor–radiation interactions on moisture biases in CMIP6 models over the tropical and subtropical pacific

Abstract

Using satellite observations from Atmospheric Infrared Sounder (AIRS) and Global Navigation Satellite System (GNSS) Radio Occultation (RO), this study evaluates air-specific humidity (QA) biases in CMIP6 models and Community Earth System Model version 2 (CESM2) sensitivity experiments with and without falling-ice radiative effects (FIREs). CMIP6 models are classified by their treatment of radiative effects of frozen hydrometeors: SON2 separates cloud and falling ice, SON1 combines them, and NOS includes only cloud ice. SON1 and NOS exhibit larger upper-tropospheric humidity biases than SON2, indicating that neglecting or simplifying FIREs enhances vertical moisture transport and moistening aloft. The explicit treatment in SON2 produces a more realistic humidity structure, closer to observations. CESM2 sensitivity experiments confirm these results: disabling FIREs reduces surface wind stress, warms sea surface temperatures, and intensifies convection over trade-wind regions, generating upper-tropospheric moistening patterns resembling SON1 and NOS (subgroups in CMIP6). These findings highlight the critical role of FIREs in cloud–radiation–circulation coupling and support broader adoption of separate frozen-hydrometeor radiative treatments in future CMIP frameworks.