This project will study lightning and moist convection in giant planet atmospheres and their interaction with clouds and the large-scale circulation. Lightning indicates where moist convection is occurring. Moist convection regulates the cooling of the giant planets over their lifetimes, and, on shorter timescales, helps sustain large-scale features like the global jet streams and giant ovals like the Great Red Spot. The project has two main components: (1) Characterize lightning by analyzing archival data on both Jupiter and Saturn from the Cassini imaging system. The horizontal width of the flashes, their optical energy, and their spectrum are clues to the depth of the lightning and the charging mechanism. (2) Simulate the dynamics of moist convection with a state-of-the-art model. The proposing team will modify the existing Weather Research and Forecasting (WRF) model to simulate the deep sub-cloud layer of giant planet atmospheres. By nesting a high resolution computational domain between eastward and westward jet streams, they will study small scale processes like convection, precipitation, and lightning as they interact with the large-scale flow. The team will train and involve graduate students in the research, will make the modified GiantPlanetWRF model available to the research community and will archive the reduced and analyzed Cassini images in the Planetary Data System.

Project Report

Convection is vertical movement of a fluid driven by buoyancy forces. The most common types of buoyancy forces arise due to temperature differences, for example, hot air rising and cold air sinking. Moist convection occurs when buoyancy forces arise from latent heat released when water vapor or another gas condenses as the air rises. Hurricanes, thunderstorms, and tornadoes are examples of moist convection on Earth, and they may occur on giant planets as well. Many of the most violent weather systems involve moist convection. In addition, much of the vertical transfer of energy in planetary atmospheres is associated with moist convection. Therefore moist convection is a fundamental weather process. Because of its unpredictable and violent nature, moist convection is hard to study and not well understood. The treatment of moist convection is one of the major uncertainties in forecasting weather and climate. We study weather of other planets to see the big picture, to make generalizations, and to understand phenomena in a larger context. Moist convection is an example. We see lightning flashes on the giant planets' atmospheres, which is a sure sign of moist convection. Giant planets do not have oceans to supply moisture, but they do have water vapor mixed in with other gases below the clouds. Also, giant planet have three condensable gases, water, ammonia, and hydrogen sulfide.The main atmospheric constituents are hydrogen and helium, which are lighter than the condensable gases. On Earth the main constituents are nitrogen and oxygen, which are heavier than the condensable gas, which is water. This makes for challenging differences that we have been studying. We have found that thunderstorms exist on the giant planets. We have found weather features that resemble hurricanes, but there is no ocean underneath. We have learned that oceans are not essential for hurricane-like behavior. We have learned that moist convection makes the weather more unpredicatable. The storms like Jupiter's Great Red Spot, which is at least 100 years old, has no lightning and therefore no moist convection. Paradoxically, reduced incidence of moist convection as one moves out from the Sun may allow the large jet streams to blow faster on Neptune than they do on Jupiter. This is despite the fact that Neptune receives only 5% as much sunlight as Jupiter. We can model the giant storms on the giant planets, but we still have trouble modeling the small intense weather systems associated with moist convection. We have shed some light on moist convection, but it still hasen't been entirely solved.

National Science Foundation (NSF)
Division of Astronomical Sciences (AST)
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Maria Womack
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California Institute of Technology
United States
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