10 Jun 2021

10 Jun 2021

Review status: this preprint is currently under review for the journal ACP.

Optically thin clouds in the trades

Theresa Mieslinger1, Bjorn Stevens2, Tobias Kölling2, Manfred Brath1, Martin Wirth3, and Stefan A. Buehler1 Theresa Mieslinger et al.
  • 1Universität Hamburg, Faculty of Mathematics, Informatics and Natural Sciences, Department of Earth Sciences, Meteorological Institute, Hamburg, Germany
  • 2Max Planck Institute for Meteorology, Hamburg, Germany
  • 3Institut für Physik der Atmosphäre, Deutsches Zentrum für Luft- und Raumfahrt e.V., Wessling, Germany

Abstract. We develop a new method to describe the total cloud cover including optically thin clouds in trade wind cumulus cloud fields. Climate models as well as Large Eddy Simulations commonly underestimate the cloud cover, while estimates from observations largely disagree on the cloud cover in the trades. Currently, trade wind clouds contribute significantly to the uncertainty in climate sensitivity estimates derived from model perturbation studies. To simulate clouds well and especially how they change in a future climate we have to know how cloudy it is.

In this study we develop a method to quantify the cloud cover from a clear-sky perspective. Using well-known radiative transfer relations we retrieve the clear-sky contribution in high-resolution satellite observations of trade cumulus cloud fields during EUREC4A. Knowing the clear-sky part, we can investigate the remaining cloud-related contributions consisting of areas detected by common cloud masking algorithms and those undetected areas related to optically thin clouds. We find that the cloud-mask cloud cover underestimates the total cloud cover by a factor of 2. Lidar measurements on board the HALO aircraft support our findings by showing a high abundance of optically thin clouds during EUREC4A. Mixing the undetected optically thin clouds into the clear-sky signal can cause an underestimation of the cloud radiative effect of up to −32 %. We further discuss possible artificial correlations in aersol-cloud cover interaction studies that might arise from undetected optically thin clouds. Our analysis suggests that the known underestimation of trade wind cloud cover and simultaneous overestiamtion of cloud brightness in models is even higher than assumed so far.

Theresa Mieslinger et al.

Status: open (until 22 Jul 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Theresa Mieslinger et al.

Data sets

Retrieval results for optically thin clouds in the trades Theresa Mieslinger

Model code and software

Simplified Clear-Sky radiative transfer Model (SCSM) Theresa Mieslinger, Tobias Kölling

Theresa Mieslinger et al.


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Short summary
The trades are home to a plethora of small cumulus clouds, often barely visible to the human eye and difficult to detect with active and passive remote sensing methods. With the help of a new method and by means of high-resolution data we can detect small and particularly thin clouds. We find that optically thin clouds are a common phenomenon in the trades covering a large area and influencing the radiative effect of clouds if they are undetected and contaminate the clear-sky signal.