Investigating the “Too Bright” Issue Pertaining to Non-PBL Clouds Over the South Pacific Trade-Wind Region in CMIP6 Global Climate Models

Abstract

This paper examines the “too bright” issue pertaining to non-planetary boundary layer (PBL) clouds over the South Pacific trade-wind region and its potential link to the falling ice radiative effects (FIREs). We run sensitivity experiments with CESM2-CAM6 (CESM2) global climate model with FIREs on (SON) and off (NOS). The model exhibits more in-cloud liquid water content (CLWC) and droplet above the PBL in NOS, leading to larger shortwave (SW) reflectivity at the top of the atmosphere than in SON over the trade wind regions. CMIP6 models are divided into three subsets: separately calculates the radiative effects of cloud ice and falling ice (SON2), combined (SON1) and without falling ice (NOS). SON2 models exhibit improved CLWC and SW reflectivity similar to CESM2-SON, while NOS and SON1 models are akin to CESM2-NOS owing to weaker surface wind stress and warmer ocean surface, caused by the lack of FIREs over the convective zones.