Simulations of Stratospheric Volcanic Aerosol Clouds

Key Investigators: Richard Young', Howard Houbenr, Owen B. Toon, and Robert Bergstrom

Volcanic eruptions throw large amounts of gases and particles into the stratosphere. In particular, the eruption from the Philippines' Mt. Pinatubo on June 15, 1991 is estimated to have injected 20 megatons of sulfur dioxide gas into the stratosphere. The sulfur dioxide gas was then converted by chemical reactions to sulfuric acid particles. These particles remained in the stratosphere for several years and affected the Earth's climate by blocking out the sun's rays. While volcanic eruptions are relatively common, the Mt. Pinatubo eruption was the largest of this century. The effects of the eruption on the stratosphere were significant and were still being felt several years after the eruption.

Scientists at NASA Ames Research Center have developed a combined 3-dimensional circulation and aerosol-transport computer model to simulate the dispersion of volcanic clouds in the stratosphere. Observations show that volcano clouds from eruptions at similar latitudes spread across the Earth in different ways. The model is being used to understand why the spreading differs.

The image shows a depiction of the latitudinal extent of the volcanic cloud predicted by the computer model for the eruption of El Chichon in Mexico in 1982 and Mt. Pinatubo in 1991. The El Chichon eruption was approximately one-third the size of the Mt. Pinatubo eruption. The model shows that El Chichon's cloud spread less rapidly away from the latitude where the volcano is located than did the Mt. Pinatubo cloud.

Ames scientists are also comparing satellite data with the model simulations to learn if the model's predictions are accurate. On the top left side of the image, the optical depth (which is related to the amount of light scattered by a vertical column of aerosol) predicted by the computer model is shown at different latitudes. This representation (called a zonal average) shows that more light is scattered (large optical depth) at the equator than at the poles. The E1 Chichon cloud was much more concentrated in latitude than the Mt. Pinatubo cloud. Data from the Advanced Very High Resolution Radiometer, a satellite instrument that measures the amount of scattered light from the atmosphere, is also shown for the Mt. Pinatubo simulation. As indicated, the model agrees closely with the satellite data.

More simulations and comparisons with available data are planned. The goal is to better understand the influence of these dramatic volcanic eruptions on the Earth's climate.

Collaborators: Patrick Hamill, San Jose State University; Richard Turco, University of California, Los Angeles.

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