CAREER: Understanding the Transport Circulation of the Troposphere

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

This is a CAREER award in which the research component examines the transport circulation of the atmosphere. Here transport circulation refers to the movement of air masses and atmospheric constituents, including water vapor and chemical tracers which move over long distances before being rained out or removed by chemical reactions. The transport circulation of the atmosphere is of interest for two practical reasons: first, it plays a key role in determining air quality, as local air quality can be affected by pollutants transported from remote sources. Second, the transport of water vapor, energy, and momentum largely determines the evolution of weather patterns. This is true not only for daily weather but also for month-to-month and year-to-year change in weather patterns such as those associated with El Nino events, fluctuations of the jet streams, and global climate change. In the stratosphere the transport circulation determines the distribution of ozone and the size and severity of the ozone hole. In this project the primary focus is on the longitudinally averaged transport circulation in the north-south direction, for example the movement of constituents into and out of the tropics and polar caps, and the extent to which this movement is enhanced or impeded by various dynamical processes. For example, to what extent do the midlatitude jet streams constitute a mixing barrier which reduces exchange between the polar caps and lower latitudes? More specifically, the research seeks to 1) characterize and quantify the transport circulation using both observed winds (from reanalysis products) and mean tracer motion in numerical simulations; 2) investigate and isolate the mechanisms of tropospheric transport in idealized models; 3) examine the causes of interannual variability in the transport circulation; and 4) investigate the changes in transport circulation associated with changes in time-mean (Eulerian) circulation induced by climate change (for example, the widening of the tropics and the poleward shift of the jet streams). The numerical simulations are performed using a hierarchy of models in which the most realistic is the Whole Atmosphere Community Climate Model (WACCM), which has a well-resolved stratosphere and sophisticated representations of atmospheric chemistry. Other models include an aquaplanet configuration of a global atmospheric model, used to study the transport circulation in an idealized setting which includes the effects of atmospheric moisture (including parameterized convection). Variations in the width of the Hadley cell, positions of the storm tracks and jet streams, and other circulation features would be created in the model by varying the imposed sea surface temperature, and the transport response to these variations would be examined. Further experiments would be performed using an idealized dry atmospheric model driven by highly simplified physics (primarily Newtonian damping), in which transport can be studied as a function of basic factors including atmospheric stability, equator-to-pole temperature gradient, and planetary rotation rate. Much of the diagnostic analysis of observations and model output focuses on the effective diffusivity of the longitudinally-averaged mean circulation. A new formalism based on equivalent latitude coordinates is used in which the longitudinally averaged meridional transport of tracers is represented as a resolved eddy flux and an effective downgradient diffusive flux. The education component of this CAREER proposal is based on demonstrations and experiments performed with a portable rotating tank. The tank is rotated to induce a Coriolis force and circulations can be generated in the tank which are analogous to the motions of Earth's atmosphere. The tank is used both on campus, in undergraduate and graduate courses, and in museum-based outreach to middle and high school student groups and to the general public. The on-campus activity consists of a week-long lab module incorporated into six different classes, in which students identify a phenomenon of interest (for example, a midlatitude cyclone) and design an experiment in the tank to simulate and study it. The rotating tank lab is motivated by a desire to present students with a hands-on learning experience which will allow students to develop an intuitive, hands-on appreciation for atmospheric dynamics. In the absence of such hands-on work students in atmospheric dynamics can become so focused on mastering the mathematical formalisms that they fail to appreciate the physical significance of the dynamical equations. The museum-based outreach would take place at teh Paleontological Research Institute's Museum of the Earth. Two forms of outreach are planned, one consisting of 50-minute demonstrations targeted to groups of middle and high school students, and another consisting of video presentations of atmospheric circulation features and their analogs in the rotating tank. The video exhibits would be complemented by 30-minute demonstrations of the rotating tank, to be performed on a monthly basis. The museum activities will be documented, evaluated by an external evaluator, and shared with other museums. In addition to the broader impacts of the education component, work under this award has societal relevance due to the effect of the transport circulation on atmospheric weather and pollution, as noted above. In addition, the work will have scientific broader impacts by building connections between the research communities engaged in atmospheric chemistry and atmospheric dynamics. The work will also support and train a graduate student, thereby providing for the future workforce in this research area.