Submitted as: model description paper 17 Dec 2020

Submitted as: model description paper | 17 Dec 2020

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

SimCloud version 1.0: a simple diagnostic cloud scheme for idealized climate models

Qun Liu, Matthew Collins, Penelope Maher, Stephen I. Thomson, and Geoffrey K. Vallis Qun Liu et al.
  • College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter, UK

Abstract. SimCloud, a simple diagnostic cloud scheme for general circulation models (GCMs) is proposed in this study. The large-scale clouds, which form the core of the scheme, are diagnosed from relative humidity. In addition, marine low stratus clouds, typically found off the west coast of continents over subtropical oceans, are determined largely as a function of inversion strength. A freeze-dry adjustment based on a simple function of relative humidity may also used to reduce an excessive clouds bias in polar regions. Other cloud properties, such as the effective radius of cloud droplet and cloud liquid water content, are specified as simple functions of temperature. All of these features are user-configurable. The cloud scheme is implemented in Isca, a modeling framework designed to enable the construction of GCMs at varying levels of complexity, but could readily be adapted to other GCMs. Simulations using the scheme with realistic continents generally capture the observed structure of cloud fraction and cloud radiative effect (CRE), as well as its seasonal variation. Specifically, the explicit low cloud scheme improves the simulation of shortwave CREs over the eastern subtropical oceans by increasing the cloud fraction and cloud water path over there. The freeze-dry adjustment alleviates the longwave CRE biases in polar regions especially in winter. However, the longwave CRE in tropical regions and shortwave CRE over extratropics are still too strong compared to observations. Nevertheless, this simple cloud scheme provides a suitable basis for examining the impacts of clouds on climate in idealized modeling frameworks.

Qun Liu et al.

Status: open (until 11 Feb 2021)
Status: open (until 11 Feb 2021)
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
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Qun Liu et al.

Data sets

Outputs from Isca simulations with simple cloud scheme and SOCRATES radiation scheme Qun Liu, Matthew Collins, Penelope Maher, Stephen Thomson, and Geoffrey Vallis

Model code and software

Cloud Scheme Manuscript Figures v1.0 Qun Liu

Qun Liu et al.


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Short summary
Clouds play an vital role in Earth's energy budget and even a small change in cloud fields can have a large impact on climate system. They also bring lots of uncertainties to climate models. Here we implement a simple diagnostic cloud scheme in order to reproduce the general radiative properties of clouds. The scheme can capture some key features of the cloud fraction and cloud radiative properties, thus providing a useful tool to explore the unsolved problems about clouds.