Diurnal cloud cycle biases in climate models

Jun Yin,Amilcare Porporato

doi:10.1038/s41467-017-02369-4

Abstract

Clouds’ efficiency at reflecting solar radiation and trapping the terrestrial radiation is strongly modulated by the diurnal cycle of clouds (DCC). Much attention has been paid to mean cloud properties due to their critical role in climate projections; however, less research has been devoted to the DCC. Here we quantify the mean, amplitude, and phase of the DCC in climate models and compare them with satellite observations and reanalysis data. While the mean appears to be reliable, the amplitude and phase of the DCC show marked inconsistencies, inducing overestimation of radiation in most climate models. In some models, DCC appears slightly shifted over the ocean, likely as a result of tuning and fortuitously compensating the large DCC errors over the land. While this model tuning does not seem to invalidate climate projections because of the limited DCC response to global warming, it may potentially increase the uncertainty of climate predictions.

Highlights

Clouds’ efficiency at reflecting solar radiation and trapping the terrestrial radiation is strongly modulated by the diurnal cycle of clouds (DCC)
We focus on the total cloud coverage, whose diurnal cycle is closely related to that of total cloud water path[13, 14] and plays a critical role in the energy budget
That the centroid and standard deviation are usually very similar to the amplitude and phase of the first harmonic. These three indexes of cloud climatology are computed for the outputs of the general circulation models (GCMs) participating in the Fifth Phase of the Coupled Model Intercomparison Project, and are compared with those from the International Satellite Cloud Climatology Project (ISCCP)[16] and from the European Centre for Medium-Range Weather Forecasts (ECMWF) twentieth century reanalysis (ERA-20C)[17], all of which have high-frequency (3-h) global coverage for the period 1986–2005

Summary

Introduction

Clouds’ efficiency at reflecting solar radiation and trapping the terrestrial radiation is strongly modulated by the diurnal cycle of clouds (DCC). As efficient modulators of the Earth’s radiative budget, clouds play a crucial role in making our planet habitable[1] Their response to the increase in anthropogenic emissions of greenhouse gases will have a substantial effect on future climates, it is highly uncertain whether this will contribute to intensify or alleviate the global warming threat[2]. Such uncertainties are well recognized in the state-of-the-art general circulation models (GCMs)[3] and are typically associated with their performance in simulating some critical cloud features, such as cloud structure and coverage[4]. We show how the resulting DCC model discrepancies influence the global radiation balance, contributing to increased uncertainties in climate projections

Methods

Results

Conclusion