Iridescent Clouds High Above the Polar Regions

Atmospheric physics is a discipline that uses the methods of physics to study the structure and motion of the atmosphere and phenomena that occur in the atmosphere. One of the benefits of research into atmospheric physics and meteorology is our daily weather forecast.

Clouds may be one of the most visible and easily understandable meteorological phenomena. Assistant Professor Masashi Kohma studies polar stratospheric clouds, which form in the stratosphere above the polar regions of the Arctic and Antarctic.

“What we usually see are clouds in the troposphere, the lowest layer of the atmosphere. Because the troposphere is immediately adjacent to the ocean, it holds large quantities of water vapor, so clouds are more likely to form there. Most weather phenomena, such as clouds and rain, occur in the troposphere. In the stratosphere above the troposphere, on the other hand, the amount of water vapor is less than 1/100th of that in the troposphere, and hence clouds rarely form. In winter, however, the polar regions experience continuous nights, and the stratosphere cools to below minus 80 degrees Celsius. Even the slightest amount of water vapor in the stratosphere will change phase and form cloud particles, creating polar stratospheric clouds.”

Beautiful polar stratospheric clouds, also called mother-of-pearl clouds because of their pale iridescent colors, are closely associated with depletion of atmospheric ozone. When chlorine compounds of human origin such as chlorofluorocarbons (CFCs) are released in the troposphere, they enter the stratosphere from the sky above the tropics and are carried to the poles, where they are transformed into stable compounds (reservoirs). Unchanged, there would be no problem, but in the presence of polar stratospheric clouds, these reservoirs elicit a chemical reaction on the surface of the cloud particles, producing chlorine molecules. When sunlight strikes in the spring, chlorine molecules are converted through a photochemical reaction to chlorine monoxide, which catalyzes the breakdown of stratospheric ozone and the formation of a hole in the ozone layer.

The ozone hole over Antarctica is shrinking, thanks due to measures such as controls on CFC emissions. In order to make long-term and seasonal forecasts of the ozone hole as we work to finally solve the problem, it is necessary to understand the exact mechanism by which polar stratospheric clouds form. In my Ph.D. program I used satellite data to study the dynamics of the Antarctic atmosphere. I found that a weather phenomenon called a blocking high, which occurs in the troposphere over Antarctica, is a factor in the appearance of polar stratospheric clouds.

Assistant Professor Kohma is also involved in research on polar mesospheric clouds, which appear in the mesosphere, the layer of the atmosphere directly above the stratosphere. These clouds, which appear during the summer months in the polar regions, have attracted attention because they are thought to be indicators of climate change and the amount of carbon dioxide in the troposphere.

Observational data and computer simulations are the key tools used by meteorologists, and Assistant Professor Kohma has served as a wintering team member of the Japanese Antarctic Research Expedition for eighteen months. In addition to providing operational support for the Antarctic Syowa MST/IS Radar installed at Syowa Station, the team used meteorological balloons to measure variables including the temperature and humidity of the upper atmosphere.

“So many people have shared their wisdom and worked hard to acquire the atmospheric data that researchers use. I am constantly aware of my debt to them.”

The motivation for Assistant Professor Kohma’s research on the atmosphere and weather lies in the “spirit of science.”

“I’d like to understand the true nature of atmospheric phenomena and atmospheric dynamics. It’s the task of science to try to understand the true nature of atmospheric and meteorological phenomena through simplification and modelling. The fact that the findings from our research can be used to help predict daily weather patterns and understand climate change is also a driving force behind our research,” he added.

Interview and text: Masatsugu Kayahara
Photography: Junichi Kaizuka

Originally published in The School of Science Brochure 2020

KOHMA Masashi

Assistant Professor, Department of Earth and Planetary Science

Graduated from the Faculty of Science, Kyoto University in 2009. Received his Ph.D. in 2014 from the Department of Earth and Planetary Science, Graduate School of Science, the University of Tokyo, and has been in his current position since then. He was a member of the 57th Japanese Antarctic Research Expedition wintering party (December 2003 to March 2005). In 2004, he received the Yamamoto Award from the Meteorological Society of Japan.