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September 22, 2021

Alleviation of global warming due to reduction of tropical upper cloud cover through altitude changes in upper clouds:
the impact of microscale physics on warming predictions

1. Key Points

The Sixth Assessment Report of the United Nations Intergovernmental Panel on Climate Change has indicated that uncertainty in global warming predictions has a particularly large impact on uncertainty associated with upper cloud cover at tropical latitudes.
Using tools such as the Earth Simulator, the changes accompanying the warming of the internal processes taking place inside upper clouds in the tropics were examined using the world's first model that directly calculates microscale phenomena within clouds. The results indicated that a warmed environment accelerates cloud droplet development processes, shortens the lifespan of tropical upper clouds, and reduces tropical upper clouds.
Reduction in upper clouds at tropical latitudes weakens the infrared-fueled greenhouse effect and weakens the global warming. This study clarified one of the causes of uncertainty in warming predictions.

2. Overview

Research Center for Environmental Modeling and Application (CEMA) operated by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) collaborated with a research group at the Atmosphere and Ocean Research Institute, University of Tokyo, to conduct and analyze a high-resolution numerical simulation using the NICAM* global cloud-resolving model via the Earth Simulator and other tools. The results indicated that changes of microscale proesses inside the clouds that spread over the upper troposphere at tropical latitudes (hereinafter “upper clouds”) associated with global warming alleviate global warming (Fig. 1).

In the 50 years from 1970 to 2019, global warming accelerated at a pace more rapid than in any 50-year period in the past 2000 years. While the primary cause is the increase in greenhouse gases due to human activity, uncertainties associated with future global warming predictions linger. In particular, uncertainties are indicated regarding tropical upper clouds affected by the greenhouse effect. The numerical models used in previous climate prediction research calculate internal processes within clouds based on artificial hypotheses due to computer resource issues, creating a major source of uncertainty.

This study conducted and analyzed a simulation that directly calculated the microscale cloud droplet development processes (hereinafter the “cloud microphysical processes”). It indicated that three cloud physical microprocesses are vital to the life cycle of the upper clouds: the gravitational settling process, the collision & merger process between cloud droplets, and the cloud particle-water vapor deposition/sublimation process. It was also clarified that the three aforementioned processes progress as the atmospheric pressure at the altitude of the upper clouds decreases with global warming, resulting in a decrease in the upper clouds (Fig. 2). Upper clouds are known to have a warming effect that inhibits infrared radiation from escaping to space. These results, therefore, indicate that cloud microphysical processes reduce upper cloud cover during periods of warming, leading to a reduced global warming due to weakening of the greenhouse effect.

The results strongly suggest cloud microphysical processes and their properties which were paid little attention that have the potential to impact warming predictions.

This study was conducted with support from the TOUGOU program by the Ministry of Education, Culture, Sports, Science and Technology (the Integrated Research Program for Advancing Climate Models) and “post Kei” grant (the Fugaku super computer) under Priority 4 group, “Upgrades to climate and global environment predictions using big data in earth observation” (topic nos.: hp120279, hp130010, hp140219, hp150213, hp160230, and hp170234) in application of research & development related to pressing social and scientific issues.

The results are scheduled to be published on September 22 (Japan time) in Geophysical Research Letters, the journal of the American Geophysical Union.

Importance of Pressure Changes in High Cloud Area Feedback Due to Global Warming
Tomoki Ohno1, Akira T. Noda1, Tatsuya Seiki1, and Masaki Sato1,2
1. Japan Agency for Marine-Earth Science and Technology
2. Atmosphere and Ocean Research Institute, University of Tokyo

【Supplemental information】

NICAM (Nonhydrostatic Icosahedral Atmospheric Model): the first atmospheric model in the world capable of direct calculation of the microscale processes that occur within clouds (cloud microphysical processes) according to the laws of physics, using the entire globe as the calculation area.

Figure 1. Overview of responses to warming in the upper clouds. The thickness of the arrows represents the speed of the process. In response to warming, upper clouds migrate to an altitude with less atmospheric pressure. Through this process, the speed at which cloud droplets fall and the frequency of inter-droplet collisions increase, promoting development via collisions and mergers. Additionally, the pressure reduction of the upper cloud layer facilitates an easier movement of the airborne water molecules and promotes the deposition/sublimation process. In this manner, the processes associated with the life cycle of upper clouds are promoted, and upper cloud cover is reduced.


Figure 2.(a)Upper cloud cover amounts in a standard experiment and in an experiment where the pressure dependence of the sublimation process and cloud droplet fall velocity in the upper troposphere were controlled. Controlling pressure dependence suppresses differences in the speed of the processes that accompany differences in the atmospheric pressure at the altitude of the upper clouds in the current and the future climate.(b)Upper clouds increase with warming in all cases, but the amount of increase is greater in the experiment where the pressure dependency of the cloud droplet fall velocity and the speed of the deposition/sublimation process are controlled compared to the standard experiment.(c)As estimated from differences in the response to warming of the upper cloud cover, the contribution of pressure dependence in fall velocity and sublimation process speed to cloud cover response is indicated to be negative in all cases, and an action that reduces cloud cover during warming is indicated.


(For this study)
Tomoki Ohno, Researcher, Research Institute for Global Change (RIGC), Research Center for Environmental Modeling and Application (CEMA), Cloud-Resolving Model Development and Application Group (CRM-DAG), JAMSTEC
(For press release)
Public Relations Section, Marine Science and Technology Strategy Department, JAMSTEC
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