The Hadley circulation is a dominant feature of the climate system. Its strength and extent are connected to the distribution of clouds, water vapor, precipitation, surface winds, and ocean currents throughout the tropics and subtropics. Despite the primary role of latent heating in driving the Hadley circulation, most previous theoretical work has used dry models. The research goals of this project are to test theories for the Hadley circulation using a hierarchy of general circulation models, with particular focus on an idealized moist model that uses simplified parameterizations and allows for large parameter variations and ease of interpretation and reproducibility of results. The importance of physical effects, such as eddy fluxes of moisture, heat, and momentum in modifying the zonally symmetric theories in the past will be investigated. Simulations with a range of prescribed ocean heat fluxes (both symmetric about the equator and centered off-equator), with zonally asymmetric boundary conditions, with changes to the height of the tropopause, and with changes to the convective parameterization will be performed in order to investigate how these different factors and processes affect the Hadley circulation. Simulations with an idealized coupled ocean-atmosphere model will also be performed. The hierarchy of models is chosen to isolate individual physical effects in models of maximal simplicity first, then add in different effects systematically, and then compare with more comprehensive models.
The educational goals of this project are to inform high school students, undergraduates, and the general public about the importance of condensation as a source of heat in the atmosphere. At the high-school level, this will be accomplished with a series of laboratory experiments with students from two different climate zones. The students will measure the effect of condensation on the outside of cooled drink cans on raising the temperature of the liquid therein, and will compare data between the two sites. The Principal Investigator (PI) will visit the site in central Washington State, which has a high percentage of minority and low-income students, to give presentations about the effects of condensation on the atmosphere. In addition to the outreach experiments, educational and research goals will be integrated by using undergraduate and graduate research assistants in the project and by presenting research results and concepts to the general public in the form of essays.
The Hadley circulation is a massive overturning of the tropical atmosphere, with hot air rising near the equator and sinking in the desert regions around 30 degrees latitude. The rising branch of the Hadley circulation is closely associated with a rainfall feature called the intertropical convergence zone, or ITCZ. In the ITCZ, water vapor condenses in massive amounts, producing deluges that continually inundate some of the rainiest places on Earth. Condensation also releases latent heat, which provides fuel for tropical weather systems. In our project, we studied aspects of the Hadley circulation and the ITCZ in nature and in climate models. One of our most important results involved finding a surprising source of one of the most persistent problems with climate models known as the "double ITCZ" problem. Climate models tend to produce two rain bands when in reality one is much more prominent. We found that the second rain band appears due to a lack of cloudiness over the Southern Ocean. We hope that this finding will eventually lead to improvements in climate models, and better rainfall forecasts for future decades. We also proposed a new theory for why the ITCZ is located north of the equator. By analyzing observations and performing simulations with climate models, we found that the ITCZ is in the Northern Hemisphere because of the global ocean circulation, which transports a large amount of heat northward across the equator. In another study, we found that tropical rainfall shifted southward during the 1970-80s, causing devastating droughts in places like the Sahel region of Africa. We compared precipitation observations with climate models, which also showed a southward shift in that time period, although to a lesser extent. We found that the shift in models was primarily due to air pollution from dirty coal burning, which reflected back sunlight in the Northern Hemisphere and caused the southward movement of rainfall. Our project also contributed to public education about the basic physics of the atmosphere. We developed a simple experiment to demonstrate in an everyday context the power of heat release from condensation, which provides energy for many types of weather systems. Our experiment involves cold drink cans brought outside in different climates. When a cold drink is brought outside in a hot, humid location, droplets of condensation form on the outside, and this condensation helps to warm the can. We performed controlled experiments with an environmental chamber that allowed us to test a wide range of atmospheric conditions. We found that condensation can cause your drinks to warm twice as much on a hot day in New Orleans-like conditions than in a dry heat of a location like Phoenix. We wrote an article for a popular science journal about this, and also filmed a YouTube video about a version of the experiment that you can try at home.
