Revisiting the iris effect of tropical cirrus clouds with TRMM and A-Train satellite data

Just as the iris of human eye controls the light influx (iris effect), tropical anvil cirrus clouds may regulate the Earth's surface warming by controlling outgoing longwave radiation. This study examines this possible effect with monthly satellite observations such as Tropical Rainfall Measuring Mission (TRMM) precipitation, Moderate Resolution Imaging Spectroradiometer cirrus fraction, and Clouds and the Earth's Radiant Energy System top-of-the-atmosphere radiative fluxes averaged over different tropical domains from March 2000 to October 2014. To confirm that high-level cirrus is relevant to this study, Cloud-Aerosol Lidar with Orthogonal Polarization high cloud observations were also analyzed from June 2006 to December 2015. Our analysis revealed that the increase in sea surface temperature in the tropical western Pacific tends to concentrate convective cloud systems. This concentration effect very likely induces the significant reduction of both stratiform rain rate and cirrus fraction, without appreciable change in the convective rain rate. This reduction of stratiform rain rate and cirrus fraction cannot be found over its subregion or the tropical eastern Pacific, where the concentration effect of anvil cirrus is weak. Consistently, over the tropical western Pacific, the higher ratio of convective rain rate to total rain rate (i.e., precipitation efficiency) significantly correlates with warmer sea surface temperature and lower cirrus fraction. The reduced cirrus eventually increased outgoing longwave radiation to a greater degree than absorbed solar radiation. Finally, the negative relationship between precipitation efficiency and cirrus fraction tends to correspond to a low global equilibrium climate sensitivity in the models in the Coupled Model Intercomparison Project Phase 5. This suggests that tropical anvil cirrus clouds exert a negative climate feedback in strong association with precipitation efficiency. Plain Language Summary The Earth may have a cooling mechanism, so called the iris effect, under green house gas forcing, as if human eyes control the light influx. This feedback process of the tropical cloud to increased sea surface temperature is confirmed using various satellite observation. It is found that the precipitation efficiency increases as the temperature increases and the cirrus decreases as the precipitation efficiency increases in the tropical Western Pacific. This negative relationship suggests the presence of the cooling mechanism (negative feedback). Our climate models also tend to show lower warming rate if the observed negative relationship is properly mimicked, vice versa.This study is supported by "Development of Climate and Atmospheric Environmental Applications" project, funded by ETRI, which is a subproject of "Development of Geostationary Meteorological Satellite Ground Segment (NMSC-2017-01)" program funded by NMSC of KMA. Y.-S. Choi and L. Huang are supported by the Jet Propulsion Laboratory, California Institute of Technology. W. Kim is supported by APEC Climate Center. S.-W. Yeh and H.-S. Jo are supported by the Korea Ministry of Environment (MOE) as "Climate Change Correspondence Program". We thank Richard S. Lindzen for valuable comments, Roberto Rondanelli and two anonymous reviewers for reviewing the paper, and Jung Ok for data display. All the data used in this study are publicly open and can be accessed as described in section 2