Warm Clouds Biases in CMIP6 Models Linked to Indirect Effects of Falling Ice-Radiation Interactions Over the Tropical and Subtropical Pacific

Abstract

We examine the spatial distributions of CMIP6-simulated cloud liquid water path (CLWP) and content (CLWC) against MODIS and CloudSat synthesized data over the tropical and subtropical Pacific. Three subsets of models are categorized based on their treatments of frozen ice-radiative interactions. CLWP/CLWC are generally well simulated in subset with separately-calculated radiative effects of cloud ice and falling ice (SON2). Too much warm clouds above 750 hPa are produced in either subset with total frozen ice radiative effects (SON1) or subset without radiative effects of falling ice (NOS) and thus CLWP/CLWC are overestimated over the open ocean including the trade-wind regions. Stratocumulus clouds off the coasts of North and South America are severely underestimated in NOS models. We attribute the overestimates of clouds above the trade-wind boundary layers to anomalous ascending motion associated with warmer sea surface temperature and weaker surface wind stress linked to indirect effects of falling ice-radiation interactions.