The research project entails a modeling effort with the goal of better understanding the role of energetic particle precipitation at mesospheric altitudes on temperature and chemical composition at stratospheric altitudes. The nitric oxide produced by the precipitation of energetic electrons has a long lifetime to be transported downwards into the stratosphere where it can affect ozone chemistry. This process is especially effective during the polar winter. The different components of this stratosphere-mesosphere coupling path are not understood at present. The research work will be directed toward the evaluation and improvement of modeling of this coupling in the framework of National Center for Atmospheric Research (NCAR) Whole Atmosphere Community Climate Model (WACCM). In particular, improved gravity wave parameterizations will be developed for WACCM to more accurately describe the downward transport of nitric oxide and nitrogen dioxide induced by the precipitation. Furthermore, the sensitivity of WACCM simulations to changes in gravity wave parameterization will be quantified as well as nitric oxide transport variability.
Energetic particle precipitation (EPP) refers to the process by which energetic electrons and protons from the sun and magnetosphere impinge on the Earth’s atmosphere. EPP is responsible for the aurora – the northern and southern lights – that are commonly seen in the night sky at high latitudes. EPP also causes changes in the atmosphere that are not visible to the human eye. Of particular interest to this project is that EPP leads to the production of molecules called nitric oxides, which are abbreviated as "NOx", in the polar upper atmosphere. During the winter this NOx can descend into the stratosphere, where it participates in chemistry that affects the stratospheric ozone layer. The primary goal of this project was to better understand the degree to which EPP affects the stratosphere, and specifically how the prevailing weather in the middle and upper atmosphere influences the EPP effects. Our focus was on the rate of downward transport in the winter polar region and confinement of air in the polar vortex. The polar vortex is a cyclonic wind system that exists between about 5 and 40 miles above the Earth's surface; it is very large, and can cover the entire polar region in the winter hemisphere. The Arctic polar vortex is sometimes perturbed by phenomena called stratospheric sudden warmings (abbreviated as SSWs). As the name suggests, when SSWs occur, they cause the stratosphere to warm. They also disrupt the structure of the vortex, leading to an irregularly shaped vortex and large changes in upper atmospheric air circulation. These circulation changes can cause more NOx produced by EPP (referred to as EPP-NOx) to descend into the stratosphere, where the NOx can affect ozone. In 2004, 2006, and 2009, satellite measurements showed that much more EPP-NOx descended into the Arctic stratosphere than ever before observed. One of our objectives was to explore whether this was because there truly was more descent in these years, or whether we simply did not have sufficient observations to measure EPP-NOx that might have descended in previous years. Our conclusion, which was based on satellite data and thus pertained only to years in the satellite era since 1978, was that the lack of reported EPP-NOx descending to the NH stratosphere in the 1980s and 1990s was most likely due to a lack of appropriate measurements. It is thus possible that at least as much EPP-NOx descended to the stratosphere in prior years, but was undetected. One of the symptoms of enhanced descent that could bring more EPP-NOx down to the stratosphere is changes in the stratopause, the top boundary of the stratosphere. In particular, when the stratopause is elevated in height, more EPP-NOx descends. Because 3-dimensional studies are time- and data-intensive, studies of the stratopause often use "zonal averages", where the stratopause characteristics are defined as a function of latitude, but not longitude. Because the Arctic vortex is often not circular, our work focused on defining stratopause characteristics as functions of both latitude and longitude. We found that the stratopause inside the vortex is warmer and higher inside the vortex than outside, highlighting the need to consider zonal asymmetries when investigating stratopause characteristics. Using the National Center for Atmospheric Research Whole Atmosphere Community Climate Model (WACCM), we also showed that elevated stratopause (ES) events, which lead to more EPP-NOx descent, occur preferentially over the Canadian Arctic and Norwegian Sea. Our work resulted in confirmation that WACCM simulations of the processes relevant to descending EPP-NOx are robust. A new satellite, the Aeronomy of Ice in the Mesosphere, was launched in 2007. The overall goal of this mission is to study polar mesospheric clouds, which are the highest clouds in the Earth's atmosphere. Onboard is an instrument called the Solar Occultation For Ice Experiment (SOFIE). One of the recent results of the work on this CEDAR project is showing that SOFIE makes measurements that are useful not only for PMCs, but also for studies of EPP-NOx. Specifically, we showed that the measurements can be used to trace the transport of molecules from the upper atmosphere down into the stratosphere. This suggests that future satellite missions will benefit from including instruments that can make the same types of measurements as SOFIE.
