Cirrus Clouds and Climate

Key Investigator: Stefan Kinne

The climate of the Earth results from an energy balance between absorbed sunlight and radiative losses of heat from Earth and its atmosphere to space. Clouds are an important modulator in this balance. Clouds reflect sunlight back toward space, which reduces the solar energy available to the Earth (albedo effect). Clouds also reduce radiative heat losses to space (greenhouse effect). Which of these two opposing processes dominates depends on many parameters including cloud particle composition, cloud structure, cloud cover, and cloud location. Changes to only one of these parameters can have significant implications for climate.

The greenhouse effect is weak for low altitude clouds, so their albedo effect dominates and they cool the Earth's climate. In contrast, cold high altitude cirrus clouds may either cool or warm the climate. They have a strong greenhouse effect, which may outweigh their albedo effect losses. As the importance of both opposing effects depends critically on little understood cirrus properties, theoretical calculations of the climatic effects of cirrus are controversial. The main uncertainty stems from a current inability to calculate the scattering of sunlight in cirrus clouds, since they contain a multitude of ice crystal shapes and sizes, and are irregular in structure (see Figure, top insert).

The climatic importance of cirrus has lead to many cirrus cloud field experiments over the last decade. At NASA's Ames Research Center we analyzed and compared remote sensing data from the ground, from satellites, and from NASA aircraft with measurements of cirrus taken simultaneously by balloon and other aircraft. Cloud properties and climate effects are linked via a model. Our calculated transmission of sunlight through cirrus is much larger than measurements indicate (see Figure bottom insert). Since the transmission is too large, the reflection must be too small. Hence, the cirrus albedo effect is severely underestimated by calculations. Yet these are the type of calculations that are repetitively used in climate prediction models.

To better understand the impact of cirrus clouds on the Earth's climate, we are improving the cirrus cloud representation with the aid of more complete process models. Some models simulate scattering processes on observed ice crystals shapes rather than on approximate shapes. Other models mimic a non-uniform cloud structure, which is typical of cirrus, yet commonly over-looked. Validated by field experiment data, new cirrus cloud model representations should enhance the ability to predict cirrus cloud effects on climate.

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