Polar mesospheric clouds

Stern s research has taken him to the South Pole, to a number of major astronomical observatories, and to the upper atmosphere aboard high-performance military aircraft. His areas of interest include spacecraft rendezvous theory, terrestrial polar mesospheric clouds, galactic astrophysics, and tenuous satellite atmospheres. He has been principal investigator for a number of space projects, including the European Space Agency s Rosetta/ ALICE Extreme Ultraviolet Spectrometer Experiment (a mission to study Comet 46P/Wirtanen), two Space Shuttle projects, three airborne research projects, and two research rocket projects. In 1995, Stern was selected to be a Space Shuttle Mission Specialist finalist for a forthcoming flight. 

The transition between the mesosphere and the thermosphere, called the mesopause, is characterized by a temperature minimum whose location varies significantly with latitude and season (see Figure 3.32). In the tropics during all seasons, and in the winter mid- and high-latitudes, this transition takes place at approximately 100 km altitude. At the winter pole, the mesopause temperature is typically 180-200 K. At mid- and high latitudes during summer, the mesopause is located near 85 km, and the temperature at the pole reaches values as low as 120-140 K (coldest place in the Earth s atmosphere where polar mesospheric clouds are occasionally observed). 

Summers, M.E., R.R. Conway, C.R. Englert, D.E. Siskind, M.H. Stevens, J.M. Russell III, L.L. Gordley, and M.J. McHugh, Discovery of a water vapor layer in the Arctic summer mesosphere Implications for polar mesospheric clouds. Geophys Res Lett 28, 3601, 2001. 

Plate 13. Noctilucent clouds observed over Ktthlungsbom, Germany (54°N) on 10 July 1997 at 01 00 GMT. These polar mesospheric clouds are found at approximately 90 km altitude and are illuminated by the Sun, which at that time is located approximately 11 degrees below the horizon. Courtesy ofF.-J. LUbken, Leibniz Institute for Atmospheric Physics, Kiihlungsbom, Germany. 

In many cases, the NOx family is formed as the sum of NO and N02, and accounts for the most reactive nitrogen species. The NOx/ NOy concentration ratio, which is often reported from field observations, is an indicator of the reactivity of odd nitrogen and its ability to destroy stratospheric ozone (or to affect other chemical families including chlorine and bromine compounds). The value of this ratio increases with altitude above 30 km to reach a value of nearly one in the upper stratosphere and mesosphere. It decreases substantially when the stratospheric aerosol load is enhanced, for example, after large volcanic eruptions (Fahey et al, 1993), and substantial amounts of nitrogen oxides are converted to nitric acid by heterogeneous reaction (5.152). It is also low in the polar regions, especially in air masses processed by polar stratospheric clouds. 

While PSCs form in the winter lower polar stratosphere, there is a second class of high altitude clouds that are formed during summertime at mesopause levels, when the temperature observed at these heights drops below approximately 150 K (see Plate 13). The chemical role of these mesospheric clouds is not yet well understood, but it has been suggested that the frequency of appearance of such clouds could increase in the future in response to human-induced cooling of the middle atmosphere (associated with enhanced levels of CO2 and other greenhouse gases in the atmosphere). 

Measurements of the gaseous sulfur dioxide released were obtained with the Total Ozone Mapping Spectrometer (TOMS Krueger, 1983) and with the Solar Backscatter Ultraviolet Spectrometer (SBUV Heath et d., 1983), both carried on the Nimbus 7 satellite. Three instruments on board the Solar Mesosphere Explorer (SME) also revealed features of the cloud the Infrared Radiometer measured the thermal emission from the aerosols, while the Visible and Near Infrared Spectrometers measured the backscat-tered solar radiation. The three instruments are limbscanning and view the atmosphere along the track of the sunsynchronous polar orbit (Barth et d., 1983 Thomas et d., 1983). Ground based and airborne spectro-photometric measurements of sulfur dioxide have also been carried out (Evans and Kerr, 1983). 

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