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September 13, 2019
The University of Tokyo
National Institute for Environmental Studies

Smaller Cumulonimbus Cloud Ensembles in the Tropics Due to Global Warming: Do Clouds Exacerbate Warming?

1. Key Points

According to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (AR5–IPCC), the main factor contributing to uncertainty in the rise of global temperatures caused by increased greenhouse gas concentrations is the uncertainty of predictions of clouds.
We analyzed high-resolution climate simulation data derived from global nonhydrostatic icosahedral atmospheric model (NICAM) predictions for approximately 100 years in the future. We found that under an atmosphere with advanced global warming, cloud organization in the tropics became impaired, making the development of large cumulonimbus cloud ensembles less likely.
It has been suggested that increases in the number of small cumulonimbus cloud ensembles could cover the upper atmosphere, which may exacerbate the greenhouse effect of infrared radiation and thereby accelerate climatic warming.

2. Overview

A collaborative study was conducted by groups at the Research Center for Environmental Modeling and Application (CEMA) of the Research Institute for Global Change at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), the Atmosphere and Ocean Research Institute at the University of Tokyo, and the National Institute for Environmental Studies. Data were obtained from simulated predictions of the Earth approximately 100 years in the future using a global nonhydrostatic icosahedral atmospheric model (NICAM*1). Analysis of this data indicated the possibility that populations of cumulonimbus clouds in the tropics may become smaller (i.e., cloud disorganization) (Figure 1).

The tropics are the primary determinant of Earth’s thermal balance, and clouds in the tropics are a driving energy source for atmospheric flow on a global scale. In the study reported here, detailed predictions of future cloud formation and elimination suggested that clouds in the tropics would become disorganized, and that the weakening of global atmospheric flow in the tropics (hereafter “general atmospheric circulation”) was a contributing factor (Figure 2). Additionally, the size distribution of cold pools of air created by clouds was also consistent with cloud disorganization (Figure 3). This suggests that tropical clouds enhance warming in an atmosphere with advanced warming due to the increased emissions of anthropogenic greenhouse gases (Figure 4). The results also show that there may be an effect on the frequency of mild winters in Japan and the number of typhoons generated each year. We hope that this research will contribute to future reports, such as the Sixth Assessment Report (AR6) of the Intergovernmental Panel on Climate Change (IPCC).

This study was implemented with support from the Integrated Research Program for Advancing Climate Models of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) and Priority Issue 4, “Advancement of Meteorological and Global Environmental Predictions Utilizing Observational ‘Big Data,’” of the application development and research into high-priority social and scientific issues to be tackled using the “Post-K” supercomputer (“Fugaku”) (Project Nos.: hp160230, hp170234, hp180182, and hp190152). The results of this study was published in the online edition of the Journal of Advances in Modeling Earth Systems on September 13, 2019 (JST).

Title: Responses of clouds and large-scale circulation to global warming evaluated from multi-decadal simulations using a global nonhydrostatic model
Authors: Akira T. Noda1, Chihiro Kodama1, Yohei Yamada1, Masaki Satoh2,1, Tomoo Ogura3 and Tomoki Ohno1
1. Research Institute for Global Change, JAMSTEC
2. Atmosphere and Ocean Research Institute, The University of Tokyo
3. National Center for Environmental Studies

*1 NICAM: The first global atmospheric model capable of directly calculating cloud motions for all global regions. Prior worldwide atmospheric models calculated cloud formation and elimination processes based on artificial assumptions, causing uncertainties in the predicted changes of clouds in the future; eliminating/minimizing these uncertainties enables highly accurate predictions of such changes. In simulations at the multi-day scale, NICAM uses a horizontal resolution (mesh spacing) of 870 m, while simulations at the multi-decadal scale in the study reported here used a 14-km resolution.


Figure 1. Predicted changes in clouds in the tropics over approximately 100 years. The color indicates the organization of clouds, with current levels set to 0, an increased organization of clouds is represented by positive values (warm colors) and greater cloud disorganization by negative values (cold colors). Overall, disorganization has increased.


Figure 2. The organization of clouds relative to the strength of atmospheric circulation for the results shown in Figure 1. After approximately 100 years, warming causes weakened general atmospheric circulation and the reduced organization of clouds.


Figure 3. Predicted changes in the size distribution of cold pools of air associated with tropical clouds.


Figure 4. Conceptual schematic of clouds formed in organized and disorganized manners, and their interactions with infrared radiation. The deep blue near the clouds represents areas of cool air. Greater thickness and coloration of the arrows indicate a larger amount of infrared radiation.


(For this study)
Akira T. Noda, Scientist, Research Institute for Global Change, JAMSTEC
Masaki Satoh, Professor, Atmosphere and Ocean Research Institute, The University of Tokyo
Tomoo Ogura, Senior Researcher, Climate Modeling and Analysis Section, Center for Global Environmental Research
(For press release)
Public Relations Section, Marine Science and Technology Strategy Department, JAMSTEC
Public Relations Office, Atmosphere and Ocean Research Institute, The University of Tokyo
Public Relations, National Institute for Environmental Studies
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