Research conducted under this grant will study the static stability of that portion of the atmosphere (specifically the troposphere and lower stratosphere) which lies between the warm tropics and the cold polar caps (i.e. the midlatitudes). The static stability of the atmosphere is a key factor which influences many aspects of atmospheric behavior, including the frequency and intensity of convective precipitation, the propagation speed of various kinds of atmospheric wave motions, the horizontal size required for atmospheric circulation patterns to be strongly influenced by the Coriolis force, and even the strength of the atmospheric greenhouse effect. Stability is determined by the vertical variation of temperature with height, and processes which warm the upper atmosphere relative to the lower atmosphere (i.e. lessen the rate at which temperature decreases with height in the troposphere) cause an increase in static stability. The particular focus of this project is the extent to which the condensation of atmospheric water vapor and the attendant warming of the upper atmosphere by latent heating is important for determining the stability of the midlatitude atmosphere. Many previous studies have suggested that the stability of the midlatitude atmosphere is primarily determined by baroclinic instability, which can be thought of roughtly as the action of frontal systems in which warm air masses from the south are displaced by cold air masses from the north. However, some studies have shown that condensational heating could have a potentially large impact on the static stability, and it is generally accepted that condensational heating associated with convection is a primary determinant of static stability in the tropics. Research conducted under this grant will use a hierarchy of atmospheric models of varying degrees of complexity, in combination with the most recent and accurate observational datasets, to understand the fundamental physical and dynamical processes through which moisture influences midlatitude stability.
The determination of midlatitude static stability is a classical problem in fundamental atmospheric science, but it also has practical real-world consequences. Previous work by the principal investigator and others shows that midlatitude stability increases with global warming, with potentially significant consequences for the hydrological cycle. In addition to the scientific broader impacts, the work would contribute to education by funding two graduate students. Scientists conducting the work will also present their research results to the public in a short video, prepared using the resources of their university.
As the atmosphere warms, it can hold substantially more humidity, around 20% more with a 3 degree temperature increase. This project focused on some of the effects that this additional moisture has on the Earth's climate. We particularly studied the midlatitudes, which is the zone between approximately 30 and 60 degrees latitude, and thus includes much of the United States, along with many other populated areas of the world. When moisture condenses to form clouds and rain, heat is released, warming the upper atmosphere. The vertical temperature structure of the atmosphere (also known as the "static stability"), determines how easily updrafts and downdrafts can occur, and is thus very important for influencing the dynamics of weather systems. In our research we have shown that moist processes play an important role in determining the static stability of the atmosphere, even in the midlatitudes. The distribution of continents is also very important in determining the seasonality of static stability in the midlatitudes. This project has contributed to a better understanding of many aspects of the climate in midlatitudes and their interactions with moisture and other processes. For instance, we have studied what determines atmospheric energy transports, which move heat around different parts of the planet. We have examined how the jet stream, the fast moving river of air in the upper atmosphere, changes from year to year, and with climate change. We have developed theories for why the jet stream and Hadley cell are projected to shift poleward in a warming climate. We have studied how precipitation patterns are forecast to change with global warming, and how that rainfall along with evaporative demand will change the aridity of the soil.
