Climatological Variations in the Upper Atmosphere and Ionosphere I
Presiding: J Lastovicka, Institute of Atmospheric Physics, Czech Academy of Sciences; S C Solomon, National Center for Atmospheric Research
SA51A-01 INVITED 08:30h
On Modeling the Upper Atmosphere and Ionosphere Response to Global Change
Ice core records indicate that the temperature and composition of the atmosphere can change significantly over geologic times. These changes occur naturally, however, recently the releases of trace gases from human activity have been recognized to have a potential for causing a significant change in the climate of the Earth. Most of the effort in investigating the global response to these trace gases has been directed toward the troposphere and stratosphere. Studies have shown that the troposphere will warm and the stratosphere will cool as trace gas concentrations increase in the 21st century. Studies have also been made that suggest that the mesosphere and thermosphere could also cool and affect the compositional structure of the upper atmosphere and ionosphere. We first review previous studies of the upper atmosphere and ionosphere response to trace gas increases. We then use both a global average model and the NCAR Thermosphere - Ionosphere - Mesosphere - Electrodynamics General Circulation Model (TIME-GCM) to investigate the atmospheric response to various scenarios of trace gas increases and compare the modeling results to the present day upper atmosphere and ionosphere structure. We will also discuss the key aeronomic processes that control the structure of the upper atmosphere as well as the extent to which these processes are known.
SA51A-02 08:45h
Multi-Decadal Variations in the Brightness of Polar Mesospheric Clouds
The Solar Backscatter Ultraviolet (SBUV and SBUV/2) instruments on the NOAA polar-orbiting meteorological satellites have measured the properties of Polar Mesospheric Clouds (PMC) continuously since 1978. Over much of this time period there have been two (and occasionally three) SBUV/2 instruments making measurements simultaneously, allowing us to combine data from the multiple instruments to study long-term PMC behavior. The seasonally averaged PMC albedo measured by each of the SBUV instruments at a wavelength of 252 nm provides a measure of the PMCs brightness. These PMC albedos show a clear anti-correlation with the solar activity as measured by the solar Lyman-α flux. After removing this dependence on the solar activity, there is a statistically significant long-term increase in the PMC albedo in both Northern and Southern Hemispheres. The variations over the past 27 years have been examined as a function of latitude, with the results indicating that the largest changes occur at the lowest latitudes.
SA51A-03 INVITED 09:00h
Trends in Mesospheric Dynamics and Chemistry: Simulations With a Model of the Entire Atmosphere
The cooling resulting from infrared CO2 radiative transfer is a major contribution to the energy budget of the middle atmosphere and thermosphere. The rapid increase of the atmospheric CO2 concentration resulting from anthropogenic emissions is therefore expected to lead, in general, to a substantial cooling in this height range. This can potentially be counteracted by heating due to absorption of near infrared radiation by CO2. Changes in ozone as a consequence of increasing methane and water vapor may also have an impact on the energy budget as dynamical changes caused by increased tropospheric temperatures. By means of numerical simulations with a general circulation and chemistry model of the entire atmosphere we will address the following questions: 1.) Can state-of-the-art atmospheric modeling explain the mesospheric temperature trends observed during the last decades? 2.)Which part of the temperature changes resulting from an increase of atmospheric CO2 is caused by local changes in the radiative budget and which part is influenced by remote dynamical effects? The model used is the newly developed Hamburg Model of the Neutral and Ionized Atmosphere (HAMMONIA) that resolves the atmosphere from the Earth's surface up to about 250 km altitude, and is based on the 3-D dynamics from the ECHAM5 general circulation model and the chemistry scheme from MOZART-3. Results from different time slice experiment representative of years 1970 and 2000, and for a doubling of CO2 will be presented.
SA51A-04 09:15h
New Estimates of Greenhouse Cooling in the Upper Atmosphere
Our understanding of the various mechanisms of long-term cooling in the mesosphere and lower thermosphere (MLT) and of their relative contributions is evolving as new data on relevant physical processes, atmospheric composition, and on the atmospheric response become available. For example, such an important parameter as the collisional excitation rate of CO2 molecules by atomic oxygen has recently been reevaluated downward according to laboratory measurements. Estimates of atmospheric cooling over the recent decades will be revisited using, in particular, new CRISTA data on the MLT CO2 densities and compared to forcings from middle-atmospheric composition changes such as the ozone depletion and water vapor increases. Possible implications for the overall energy balance in the MLT will also be briefly discussed.
SA51A-05 09:30h
Detection of Long-Term Climatological Variationsin the Earth's Upper Atmosphere ApparentlyAssociated with Anthropogenic Effects
From a study of long-term orbital decay of Earth satellites compared to standard empirical models (to remove known systematic variations) it has been discovered that thermospheric densities have declined substantially since at least 1976. Detection of this decline was first published by Keating et al (2000) in Geophysical Research Letters. According to theoretical 3D studies of Rishbeth and Roble (1992) maximum thermospheric response to anthropogenic increases of CO2 and CH4 should occur at solar minimum because of the colder thermosphere. We have therefore focused on detecting the trend at years of solar minimum and making small corrections for solar activity variability using the standard empirical models. We have now expanded our study to many more satellites. All the satellites investigated have shown a downward trend in densities over the period 1976-1996. Over this period we find densities near 380km have decreased 10.3 +/- 1.2 percent. It is estimated that CO2 will double before the end of the century. Exponential extrapolation of the observed trend indicates densities near 400km may decrease over this time interval by approximately 50 percent due to strong anthropogenic cooling of the thermosphere. A paper by Emmert et al. (2004) using a very similar approach, but looking at all levels of solar activity empirically show that maximum downward trends occur near solar minimum as we had hypothesized. Our most recent results show little difference between polar and equatorial trends indicating we have detected a global phenomenon. The results appear to be in general accord with the theoretical studies of Roble and Dickinson (1989), Rishbeth and Roble (1992),and Akmaev and Formichev (2000).
SA51A-06 INVITED 09:45h
Building a Climatological Record in the Upper Atmosphere and Ionosphere: Progress of CAWSES Working Group 4.4
A key component of upper atmospheric research is the climatological analysis of data, which requires extensive data sets spanning the range of all parameters (height, solar activity, etc.) that influence the mean behavior of the system. In addition, there is increasing evidence that the upper atmosphere and ionosphere are undergoing secular changes; identification and interpretation of these trends requires continuous monitoring of multiple properties (temperature, density, composition, etc.) of the system. One of the goals of the Climate and Weather of the Sun-Earth System (CAWSES) Working Group 4.4 is to identify gaps in the climatological record. To that end, we present a comprehensive survey of upper atmospheric (50-1000 km) data coverage, and highlight areas where new or continued measurements would have a major impact on our understanding of this region.