KUO-NAN LIOU 1944-2021

BY PING YANG, YU GU, AND QIANG FU
SUBMITTED BY THE NAE HOME SECRETARY

KUO-NAN LIOU was a distinguished professor in the Department of Atmospheric and Oceanic Sciences and founding director of the Joint Institute for Regional Earth System Science and Engineering at the University of California, Los Angeles (UCLA). He passed away March 20, 2021, at the age of 76.

He was born November 16, 1944, in Taiwan. He received his BS degree from National Taiwan University in 1965, and then earned his MS (1968) and PhD (1970) in meteorology and oceanography at New York University. After postdoctoral research at the National Aeronautics and Space Administration, in 1975 he became a professor at the University of Utah, where he taught for 22 years before going to work at UCLA. He made seminal contributions to atmospheric science and education/mentoring, with a focus on atmospheric radiation, light scatter ing, remote sensing, and cloud/aerosol radiative forcing effects on the climate system.

Among his most fundamental contributions, he demonstrated that atmospheric radiation should no longer be consigned to the fringes of meteorology but instead should take a central place in climate science. He advanced the field with a quantum leap through his work on the theory of radiative transfer, investigation of radiative effects of clouds and aerosols, and development of methods for inferring atmo- spheric and surface parameters through remote sensing.

Clouds, which cover 60–70 percent of the globe, play an important role in atmospheric radiation. Liou’s monograph, Radiation and Cloud Processes in the Atmosphere: Theory, Observation, and Modeling (Oxford University Press, 1992), coherently integrates radiative transfer and cloud physics and bridges the gap between radiation and climate processes in clouds. The volume contributed to the development of climate models for the investigation of global climate change and remote sensing techniques for the inference of cloud and aerosol properties.

In an earlier paper he demonstrated that cirrus clouds are ubiquitous, particularly in the tropics, and are critical to understanding the global energy budget and water cycle.¹ Their effect on climate was illustrated through a hierarchy of climate models with varying degrees of complexity. Since publication of this paper, numerous field experiments have been undertaken to collect data to quantify the impact of cirrus on the Earth’s radiation budget and climate.

A 1971 paper by Liou and James Hansen (NAS 1996) on light scattering research may have been the first to systematically compare the geometric optics method and Lorenz-Mie theory.² The scattering of light by spheres can be solved by the Lorenz-Mie theory and computation performed accordingly.

Ice clouds in the atmosphere are composed almost exclusively of nonspherical ice crystals—such as solid and hollow columns, plates, bullet rosettes, aggregates, and dendrites— and ice particles have surfaces with varying degrees of roughness. In the early 1970s Liou was the first to study cirrus cloud radiative properties by considering nonspherical ice crystals.³ He developed the theoretical basis for the depolarization of the backscattered signal from nonspherical ice particles with a linearly polarized laser beam.⁴ This work established the basis for cloud phase detection using groundbased, air-borne, or spaceborne lidar.

In the 1980s he pioneered the study of the scattering of polarized light by nonspherical ice crystals by means of the prin- ciple of geometric optics. And in the 1990s he and one of his students developed an innovative physical-geometric optics method, referred to as the improved geometric optics method (IGOM), for light scattering by large particles.⁵ The IGOM substantially overcame the shortcomings of the conventional geometric optics method for light scattering; in particular, it could for the first time depict the variation in extinction efficiency with particle size within the geometric optics framework, and it overcame the inherent singularity, called the delta transmission, associated with the ray-tracing technique for a particle with parallel surface facets.

The IGOM and its subsequent developments in synergistic combination with other methods for small-to-moderate particles provide advanced modeling capabilities for cirrus cloud optical property computations for downstream applications, as summarized in his coauthored text Light Scattering by Ice Crystals (Oxford University Press, 2016). The computations laid the foundation for fundamental datasets for ice cloud radiation parameterization schemes used in many climate models and for radiance simulations under ice-cloudy conditions in radiative transfer models. 

A theorist, Liou also pursued laboratory experiments in light scattering and cloud physics, primarily to test theory. His work in the area of light scattering was recognized in 1996 through a Creativity Award from the Atmospheric Sciences Division of the National Science Foundation for “Light Scattering by Ice Crystals: Theory and Experiment.”

The Nobel Laureate Chandrasekhar had presented radiative transfer in plane-parallel (1D) atmospheres as a branch of mathematical physics and developed numerous solution methods. Liou followed the discrete-ordinates method devel- oped by Chandrasekhar and in 1974 derived the first analytic solution for the four-stream approximation for radiative trans- fer.⁶ On the basis of the delta-four-stream approach, he and a former student constructed the Fu-Liou radiative transfer model, which includes the correlated k-distribution method for the sorting of nongray gaseous absorption in scattering atmospheres and the scattering and absorption properties of hexagonal ice particles.⁷ The Fu-Liou code has been adopted as a standard broadband radiative transfer model to study climate forcing effects of clouds and aerosols, and used by NASA for the retrieval of satellite-observed atmospheric and surface radiative energy fluxes.

Liou was a pioneer in the development of 3D radiative transfer theories based on the finite spherical-harmonics expansion of the intensity and scattering phase function. In particular, he developed a successive order-of-scattering approach for 3D radiative transfer that offers an innovative way of constructing a 3D cloud extinction coefficient field from satellite observations. This study corrected the conventional 1D approach to the evaluation of sunlight reflected and absorbed by clouds, which is essential to discussion of the role of clouds/ radiation in climate and climate change. In addition, Liou and his associates worked on 3D radiative transfer over mountains⁸ for high-resolution climate models, seeking to improve regional climate simulations by incorporating the 3D radiation configuration in mountains and snowcovered regions that are especially vulnerable to climate change and global warming.⁹

Liou developed a 1D cloud-precipitation-climate model to investigate the potential link between the perturbed cloud particle size distributions and precipitation produced by greenhouse warming/air pollution.¹⁰ If more small particles are produced, precipitation could decrease, leading to more cloud water in the atmosphere, which implies more reflection of sunlight, leading to cooling and a potential offset of the warming produced by greenhouse gases. A reduction of cloud particle size of about 1 µm in eastern North America has been observed as a result of anthropogenic pollution. Liou’s discovery linking cloud particle size and precipitation in climate change is now referred to as the second indirect climate forcing in aerosol-cloud feedbacks.

Over five decades Liou and his associates conducted numerical simulations involving the effects on precipitation of the increase of anthropogenic aerosols in China, using the UCLA atmospheric general circulation model.¹¹ They showed that increased aerosol optical depths in China led to a noticeable increase in precipitation in the southern part of the country in July due to the cooling in midlatitudes, producing a precipitation pattern referred to as “north drought/south flooding” over the past 50 years. Moreover, black carbon and dust in China would heat the air column in the middle to high latitudes and tend to move the precipitation toward the Tibetan Plateau.

As the first to use long-term satellite data and a comprehensive cloud model to study ice clouds, Liou and colleagues also found compelling evidence that large quantities of ice nucleating particles (an important factor in the formation of ice clouds) are produced by human activities.¹² Because ice clouds play a central role in severe weather and climate change, adequate representation of this process is expected to significantly improve climate projections.

In addition to his accomplishments in radiative transfer, remote sensing, and climate applications, Liou enhanced understanding of microphysics, radiation, and turbulence interactions in clouds. In particular, he and a former student constructed a 2D model to understand the evolution of cirrus clouds.¹³ This study represents the first effort to incorporate in a cirrus model all the pertinent physical processes involving ice crystal formation, radiative transfer in clouds, and secondorder turbulence closure.

Liou was also active in service to the national and international science community throughout his career. To list a few, he chaired the Atmospheric Sciences Section Fellows Committee (2013–14) and Roger Revelle Medal Committee (2017–20) of the American Geophysical Union (AGU), the 1986 International Radiation Symposium, and the Committee on Atmospheric Radiation (1982–84) of the American Meteorological Society (AMS); and he was chair-elect of the AMS Atmospheric Research Awards Committee (2021–22). He was appointed to the National Academies’ Committee on  Evaluating NOAA’s Plan to Mitigate the Loss of Total Solar Irradiance Measurements from Space (2013) and Advisory Panel for the International Satellite Cloud Climatology Project (1984–87). And he was quite involved in the work of the NAE, with extensive service in various roles for his section as well as appointment to the NAE Nominating Committee (2011–13) and Committee on Membership (2008–11).

He was also editor of the Journal of the Atmospheric Sciences (1999–2005) and Journal of Geophysical Research (1987), review editor for the IPCC Report (1998–99), and associate editor of the Journal of Quantitative Spectroscopy and Radiative Transfer (2011–21).

We would be remiss if we did not mention Liou’s contributions to atmospheric education and mentoring. His iconic textbook, An Introduction to Atmospheric Radiation (Elsevier Science, 1980), was translated into Chinese, Russian, Japanese, and Arabic and educated several generations of researchers around the world in the disciplines of atmospheric radiation and remote sensing. The 2002 edition includes about 70 percent new material and is still frequently used by researchers in the areas of radiative transfer, light scattering, and remote sensing.

An important part of Liou’s legacy is reflected by the number and quality of the graduate students he trained— he guided the completion of 33 doctoral dissertations and many master’s theses. Many of his former students are now prominent researchers. He also mentored many early-career researchers who worked with him as visiting scholars.

Liou’s significant contributions were well recognized. In addition to his election to the US National Academy of Engineering (1999), he was a fellow of the Academia Sinica (2004) and foreign member of the Chinese Academy of Sciences (2017). He received the AMS Jule G. Charney Medal (1998), Biennial William Nordberg Medal (2010) from the Committee on Space Research, International Radiation Commission Quadrennial Gold Medal (2012), AGU Roger Revelle Medal (2013), and AMS Carl-Gustaf Rossby Research Medal (2018), and was included in the Nobel Peace Prize bestowed on the Intergovernmental Panel on Climate Change (IPCC) in 2007. He was a fellow of AMS, AGU, the American Association for the Advancement of Science, and the Optical Society of America.

The passing of Kuo-Nan Liou is a great loss to atmospheric science. He was an expert in many research areas, most significantly in atmospheric radiation. His remarkable scientific insight led to paradigm-shifting contributions to atmospheric science, and his teaching and mentoring exemplified his self-lessness and generosity.

With his passion for science and pursuit of excellence in the three key areas of university faculty—teaching, research, and service—Liou was a role model for all. He was generous and supportive of others, and wrote numerous letters in support of colleagues’ promotions and nominations for awards or recognitions. He liked to encourage early-career researchers by reciting an ancient Chinese poem: “The setting sun leans on the furthest mountains to disappear, the Yellow River flows into the sea. To see a thousand miles further, ascend another story.”

In tribute to Liou for his extraordinary accomplishments in scientific research, his dedication to service for the science community, his integrity, and his generosity and kindness, we borrow from Margit Dirac’s speech at the dedication of the Paul A.M. Dirac Science Library: “It is customary to praise those who are not with us anymore—often quite undeservedly, but not in this case. No praise will exaggerate or be too glowing.”

Liou is survived by his wife Agnes, daughter Julia, son Cliff, and a granddaughter.

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Adapted from the Bulletin of the American Meteorological Society (August 2021, pp. 778–82), which includes more extensive technical discussion of his contributions. 
¹ Liou K-N. 1986. Influence of cirrus clouds on weather and climate processes: A global perspective. Monthly Weather Review 114(6):1167–99.
² Liou K-N, Hansen JE. 1971. Intensity and polarization for single scattering by polydisperse spheres: A comparison of ray optics and Mie theory. Journal of the Atmospheric Sciences 28(6):995–1004. 
³ Liou K-N. 1972. Light scattering by ice clouds in the visible and infrared: A theoretical study. Journal of the Atmospheric Sciences 29:524–36.
⁴ Liou K-N, Lahore H. 1974. Laser sensing of cloud composition: A backscattered depolarization technique. Journal of Applied Meteorology 13:257–63.
⁵ Yang P, Liou K-N. 1996. Geometric-optics–integral-equation method for light scattering by nonspherical ice crystals. Applied Optics 35(33):6568–84.
⁶ Liou K-N. 1974. Analytic two-stream and four-stream solutions for radiative transfer. Journal of the Atmospheric Sciences 31:1473–75.
⁷ Fu Q, Liou K-N. 1992. On the correlated k-distribution method for radiative transfer in nonhomogeneous atmospheres. Journal of the Atmospheric Sciences 49(22):2139–56. 
⁸ Liou K-N, Gu Y, Leung LR, Lee WL, Fovell RG. 2013. A WRF simulation of the impact of 3-D radiative transfer on surface hydrology over the Rocky Mountains and Sierra Nevada. Atmospheric Chemistry and Physics 13:11709–21.
⁹ For a less technical perspective on Liou’s work, see “Mapping the Frozen Sky: Study Looks at Clouds from Both Sides Now,” published in ScienceNews in June 2002.
¹⁰ Liou K-N, Ou S-C. 1989. The role of cloud microphysical processes in climate: An assessment from a one-dimensional perspective. Journal of Geophysical Research 94(D6):8599–607.
¹¹ Gu Y, Liou K-N, Chen W, Liao H. 2010. Direct climate effect of black carbon in China and its impact on dust storms. Journal of Geophysical Research 115(D7). 
¹² Zhao B, Wang Y, Gu Y, Liou K-N, Jiang JH, Fan J, Liu X, Huang L, Yung YL. 2019. Ice nucleation by aerosols from anthropogenic pollution. Nature Geoscience 12:602–07.
¹³ Gu Y, Liou K-N. 2000. Interactions of radiation, microphysics, and turbulence in the evolution of cirrus clouds. Journal of the Atmospheric Sciences 57:2463–79.