Clouds in the trade cumulus regime - the "too few, too bright" problem

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Description

Many global models have long-standing biases in top-of-the-atmosphere shortwave radiation over the subtropical oceans. Although these biases have been reduced over the years, they are still present, with an albedo that is too low over the maritime stratocumulus decks and too high over the regions of trade cumulus, with consequences for the model radiation budget and response of the ocean in coupled models. The cause of radiation biases in the stratocumulus areas is generally consistent with a lack of cloud cover and liquid water path and is reasonably straightforward to interpret (though by no means easy to remedy). However, for the transition and trade cumulus regions, the clouds in many models are still too bright. For the ECMWF global forecast model, satellite observations show that cloud albedo in the trade cumulus regions is overestimated, even if cloud cover and water path appear to be underestimated. We look at this in more detail and explore the use of ground-based observations from ARM sites to shed light on the apparent inconsistencies of the model comparison with satellite observation products for radiation, cloud cover, and water path. We investigate the role of cloud heterogeneity, 3D radiation, and cloud vertical structure on the resulting cloud albedo. We also assess changes to the ECMWF model microphysics warm-rain formation and evaporation processes that lead to an increase in cloud water and fraction in the stratocumulus regime and a decrease in the shallow cumulus regime. These changes are tested and evaluated in the single-column model framework for cases from the MAGIC campaign and the ARM ENA Azores site with the aim of improving global radiation biases over the subtropical oceans.