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Publications which I have co-authored. Doug's research links
Stratospheric Dynamics and Tracer Transport My research has generally focused on the dynamics and tracer transport in the troposphere, stratosphere, and mesosphere. The first half of my Ph.D. research involved the study of wave dynamics in the stratosphere via analyses of satellite measurements of geopotential height, temperature, and ozone (see numbers 2 and 3 in list of publications in curriculum vitae). We identified the "4-day wave" in Upper Atmosphere Research Satellite (UARS) Microwave Limb Sounder (MLS) data in the mid- to upper-stratosphere in the Antarctic winter. After identifying the wave in geopotential height and temperature data, we examined the effect on the ozone distribution and showed good agreement between observed and expected ozone anomalies. We also found the wave signal in derived potential vorticity fields and postulated the wave dynamics to be similar to the "PV charge" concept. The second half of my Ph.D. research involved the analysis of UARS Improved Stratospheric and Mesospheric Sounder (ISAMS) CO data in the stratosphere and mesosphere (see numbers 5 and 7 in publication list). In the winter stratosphere, the photochemical lifetime of CO is very long, and it therefore acts as a passive tracer. We examined the seasonal cycle of CO, analyzed the sources and sinks of CO in the tropics, and studied in detail the dynamical evolution of the breakdown of the winter vortex. Comparison with CO simulations from the Goddard Space Flight Center 3D Chemistry and Transport Model provided further insight into the dynamics and chemistry of CO in the middle atmosphere. My current research involves the examination of mixing processes in the stratosphere and upper troposphere using a novel diagnostic developed by Prof. Noboru Nakamura, my postdoctoral supervisor at The University of Chicago. The diagnostic, which has been coined "effective diffusivity," provides a Lagrangian determination of the horizontal mixing processes on isentropic levels that is more robust than the conventional mixing parameterization methods (e.g., Kyy determined from observations, potential vorticity, or trajectory dispersal). The effective diffusivity, Keff, is both easier to calculate (can be done with one global "snapshot" of a tracer field) and easier to interpret than its counterparts. Minima in Keff indicate barriers to horizontal mixing of tracers, while maxima indicate regions where significant mixing occurs. We calculated Keff using satellite observations of tracers and found that it clearly revealed the mixing barriers at the edge of the winter and summer polar vortices and at the edge of the so-called "tropical pipe" region. Obtaining a global picture of Keff throughout the stratosphere and for all seasons proved to be a very difficult task using satellite observations. As a surrogate, we developed our own synthetic data set by numerically advecting a passive tracer on isentropic surfaces from 350 to 1900 K using United Kingdom Meteorological Office (UKMO) assimilated wind fields. A seamless picture is thereby obtained of the state of mixing throughout the stratosphere, and comparison can easily be made between different geographical regions and between different seasons and years. The seasonal cycle of formation and demise of the winter and summer stratospheric vortices is easily seen from the analysis as well as the seasonal evolution of the tropical barrier. The mid-latitude tropopause barrier is identified as a minimum in Keff at 350 K. Significant interannual variability associated with the equatorial quasi-biennial oscillation (QBO) is seen in the tropics as well as in the extratropics. Our recent paper, which includes the validation of the Keff calculation and a detailed description of the Keff climatology (1992-1999), will soon be in print (see number 8 in publication list). Impact of Future Arctic Ozone Hole So far, ozone depletion on the scale of the Antarctic ozone "hole" has not been seen in the Arctic region. The Arctic stratosphere contains sufficient levels of inorganic chlorine to cause significant destruction of ozone, but the temperatures generally are not cold enough to form polar stratospheric clouds (PSCs), which significantly accelerate the ozone photochemistry. Also, the Arctic polar vortex doesn't generally last long enough into the spring for sufficient levels of sunlight to be present. The stability and temperature of the stratospheric polar vortices depend on the degree to which large amplitude planetary waves are forced in the troposphere and propagate upward to the stratosphere, where they break, causing significant transport of heat, momentum, and material. The Arctic vortex is warmer and less stable than its Antarctic counterpart since the wave forcing (from land-sea temperature contrasts and orography) is larger in the Northern Hemisphere. However, recent studies have shown that increased greenhouse gases may actually play a role in reducing the stratospheric wave activity, thereby stabilizing and cooling the Arctic vortex. Indeed, the years 1997 and 2000 have seen significant springtime ozone depletion over large geographical regions in the high northern latitudes (see figure at top of previous page) combined with very low heat flux from planetary waves. In the extreme case, the Arctic stratosphere could potentially experience ozone depletion of the same magnitude as in the Antarctic. It is important to understand how this low ozone would affect the highly populated areas of the Northern Hemisphere. The most vulnerable period is during the springtime breakup of the Arctic vortex, as air with low ozone levels is transported away from the pole, during a time of decreasing solar zenith angle. It is important to quantify the biological risk people in these regions face, in the light of a future Arctic ozone hole. To elucidate the dynamics and biological impact of low ozone anomalies resulting from Arctic ozone depletion, I am currently using a combination of satellite data, a 2D isentropic advection code, and a detailed radiative transfer model. The satellite data (from NASA's TOMS instrument) are used to quantify the anomalous values of total ozone that occur following the evolution and breakup of the Arctic vortex. I use the 2D code to advect a passive tracer at multiple isentropic levels in order to examine the dynamics of the anomalies. A novel Lagrangian approach is taken in the dynamical analysis that effectively accounts for perturbations due to wave activity. This is necessary due to the significant amount of daily variability in the ozone distribution caused by both planetary and synoptic scale waves (see number 4 in publication list). To quantify the biological risk I calculate the percent change in biologically relevant UV radiation for a given change in the column ozone, using the radiative transfer model developed by Prof. John Frederick at The University of Chicago. Preliminary analyses of transport processes and UV changes during a low ozone anomaly that occurred on 10 April 2000 over western Europe show very interesting results. Transport of Tropospheric Aerosols When I was a National Research Council Research Associate at Goddard Space Flight Center I was involved with a project that examined the transport of tropospheric aerosols using the Total Ozone Mapping Spectrometer (TOMS) aerosol index data and trajectory mapping methods. (see number 6 in publication list). The TOMS instrument essentially measures total column UV-absorbing aerosol, providing no information on the vertical distribution of aerosol within the column. In order to convert the aerosol index to optical depth, the essential quantity for determining the radiative effects of the aerosol, the vertical distribution must be assumed unless independent observations are available. My supervisor, Dr. Mark Schoeberl, had the idea to run trajectories at multiple levels in an attempt to track the cloud. By eliminating trajectories that did not faithfully track the aerosol cloud measured by TOMS, the method obtains the estimated 3D shape of the aerosol cloud. I applied this method to the volcanic ash cloud from the September 1992 eruption of Mt. Spurr, Alaska. Although good agreement with radar and pilot reports was obtained between the estimated cloud base and height from my analysis using United Kingdom Meteorological Office (UKMO) winds, poor agreement was obtained with National Centers for Environmental Prediction (NCEP) winds. This showed that the method may be useful, but it is very sensitive to errors in the wind field. Subsequent analysis of Saharan dust clouds showed that the method might have some usefulness in characterizing the seasonal cycle of the height to which the dust can attain. I am interested in pursuing this research further if funding becomes available. History of Meteorology Finally, I am also interested in the history of meteorology. I was program co-organizer for the "Hudnall Symposium on the 60th Anniversary of the Founding of the Institute of Meteorology at the University of Chicago." While organizing the symposium I researched the history of meteorology at Chicago, interviewing people who were present in the early days such as George Platzman, Dave Fultz, Norman Phillips, Joanne Simpson, and Roscoe Braham. This led to the writing of two articles that I recently submitted to the AMS Bulletin (see numbers 9 and 10 in publication list). I am interested in doing further work in this area, particularly regarding the development of meteorological education in the United States from 1940-2000. |