Understanding the Effects of Land Hydrology, Water Volatility, and Rotation Rate on Clouds, Climate, and Circulation in a Hierarchy of Models

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Any first-principles theory of the climate dynamics of the Earth should apply across a broader range of conditions than those found on Earth. Such a theory should predict, for example, how the jet streams would change if the planetary rotation rate were faster or slower, or how clouds and precipitation would change if water were more or less abundant. But such Earth analogs are not readily available to test our theories and help us gain confidence in our understanding of the Earth. Work performed under this award explores the idea that Titan, despite being a moon of Saturn with a surface temperature of -290F, can serve as a useful Earth analog. The analogy is based on observations from the Cassini mission showing that Titan has all the features of an Earth-like hydrological cycle: clouds, rain (often torrential rain from large storms), and surface evaporation from lakes and seas, only with methane instead of water. The Principal Investigator (PI) of this award has developed a "water volatility" parameter that allows methane at Titan-like temperatures to be treated as a kind of water equivalent in simulations of Earth's atmosphere. With this treatment Earth and Titan can be treated as climate systems that fall along a continuum, so that a Titan-like hydrological cycle can be produced from Earth's hydrological cycle by increasing the value of the volatility parameter from one to about three. Two other parameters that must also be changed to produce a Titan-like climate state are the planetary rotation rate, as a day on Titan lasts 16 Earth days, and the amount and distribution of land area, as the tropical latitudes of Titan are entirely land covered. The project tests the extent to which these three parameters suffice to account for the essential differences between Earth-like and Titan-like climates.One issue addressed here is the behavior of low and high clouds under changing water volatility, as previous work by the PI (see AGS-1912673) shows that low clouds tend to descend while high clouds ascend under increasing volatility. The two layers merged at low volatility and are abruptly replaced by a single very thick layer at high volatility. This behavior, which was produced in a single-column atmospheric model, is further explored using a general circulation model and a global storm-resolving model (the ICON model from the Max Planck Institute for Meteorology). Additional work considers changes in atmospheric circulation as parameters are varied from Earth-like to Titan-like. One consideration is that the tropical land cover on Titan causes the bulk of surface evaporation to occur at higher latitudes than on Earth and another is that the slower rotation rate causes different types and sizes of atmospheric wave motions to occur on Titan.Broader impacts of the project include a number of educational activities, one of which is a research cruise on Santa Monica Bay during which students collect data related to the temperature and circulation of the bay. Another is the development and construction of a laboratory device to demonstrate density-driven overturning circulations relevant to oceans and planetary atmospheres. The award also provides support and training for a graduate student, thereby promoting workforce development in this research area.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.