Evaluating the representation of tropical stratocumulus and shallow cumulus clouds as well as their radiative effects in CMIP6 models using satellite observations

Abstract

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- 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.

Climate Models Underestimate Dynamic Cloud Feedbacks in the Tropics

“…Moreover, shallow cumulus cloud typically has a smaller magnitude SW CRE than stratocumulus due to having lower cloud cover (e.g., Tselioudis et al., 2021). In this context, the climate model SW CRE biases for the subsidence regimes are consistent with those previously identified for stratocumulus and shallow cumulus clouds (e.g., Crnivec et al., 2023; Konsta et al., 2022), with the climate models overestimating the brightness of shallow cumulus cloud (corresponding to 10–30 hPa day −1 ) and underestimating the brightness of stratocumulus (30–50 hPa day −1 ). Consequently, improving the representation of these cloud regimes in climate models may be a path to reducing the climate model dynamic cloud feedback biases.…”

An observation-based method to assess tropical stratocumulus and shallow cumulus clouds and feedbacks in CMIP6 and CMIP5 models

“…Similarly, we find that the overestimate of the shortwave radiative effect -clouds being too bright-has improved in the CMIP6 ensemble but remain large (figure S1). The persistence of the 'too few too bright' problem in CMIP6 models is consistent with a recent study (Konsta et al 2022) and is explored in more depth for Sc and Cu regimes by Crnivec et al (2023).…”

“…However, although collectively, CMIP6 models have substantially increased Sc cloud fraction and slightly increased Cu cloud fraction, both cloud fractions are largely underestimated compared to observations, especially over Sc decks. This issue is investigated into more detail by Crnivec et al (2023) with a slightly larger number of models.…”

An Observation-Based Method to Assess Tropical Stratocumulus and Shallow Cumulus Clouds and Feedbacks in Two CMIP Generations

“…traditionally used to separate Sc and Cu cloud regimes (e.g.,Myers et al 2021, Medeiros and Stevens 2011, Nam et al 2012 despite shortcomings(Crnivec et al 2023, Cesana anddel Genio 2021). It is based on the greater area coverage of Sc compared to Cu clouds, that is, their cloud fraction is typically larger than that of Cu over the typical size of an ESM gridbox (~ 100 km; Cesana et al 2019a).…”