Evaluating the Representation of Tropical Stratocumulus and Shallow Cumulus Clouds As Well As Their Radiative Effects in CMIP6 Models Using Satellite Observations

Low clouds over tropical oceans reflect a great proportion of solar radiation back to space and thereby cool the Earth, yet this phenomenon has been poorly simulated in several previous generations of climate models. The principal aim of the present study is to employ satellite observations to evaluate the representation of marine tropical low clouds and their radiative effect at the top of the atmosphere in a subset of latest climate models participating in CMIP6. We strive for regime-oriented model validation and hence introduce a qualitative approach to discriminate stratocumulus (Sc) from shallow cumulus (Cu). The novel Sc-Cu categorization has a conceptual advantage of being based on cloud properties, rather than relying on a model response to a cloud-controlling factor. We find that CMIP6 models underestimate low-cloud cover in both Sc-regions and Cu-regions of tropical oceans. A more detailed investigation of cloud biases reveals that most CMIP6 models underestimate the relative frequency of occurrence (RFO) of Sc and overestimate RFO of Cu. We further demonstrate that tropical low cloudiness in CMIP6 models remains too bright. The regime-oriented validation represents the basis for improving parameterizations of physical processes that determine the cloud cover and radiative impact of Sc and Cu, which are still misrepresented in current climate models. Similar as white snow and ice caps, bright low clouds have a high shortwave albedo, reflecting a huge amount of sunlight back to space and thereby helping us counteract global warming. The shadowing effect of bright low clouds is especially pronounced over tropical oceans, since equatorial regions of our planet receive most sunshine, which is in clear skies otherwise practically entirely absorbed within the contrastingly dark ocean. Climate models had traditionally struggled simulating these clouds by underestimating their areal extent and simultaneously overestimating their reflectivity. In other words, simulated clouds were commonly found to be "too few" and "too bright" compared to observations, which introduced a substantial uncertainty to climate projections. Herein, we proposed a novel approach to proficiently decompose tropical low cloudiness into stratocumulus and shallow cumulus regime, which is essential to provide a proper guidance for climate model development. We subsequently showed that the newest generation of climate models still suffers from the "too few, too bright" tropical low-cloud problem within both stratocumulus and shallow cumulus regimes, which thus needs to be further tackled with the greatest possible endeavor. We introduce a new approach to distinguish stratocumulus and shallow cumulus regimes over tropical oceans based on cloud coverThe "too few, too bright" tropical low-cloud problem persists in 12 CMIP6 models within stratocumulus and shallow cumulus regimesMost CMIP6 models underestimate (overestimate) the relative frequency of occurrence of stratocumulus (shallow cumulus)