Implications of energetic particle precipitation events for the temperature and dynamics of the atmosphere - a model study
Zusammenfassung der Projektergebnisse
In this project, the impact of energetic particle precipitation has been investigated with respect to the change in the chemical composition and a resultant impact on the radiative and chemical heating of the middle and upper atmosphere. Different model calculations have been carried out with the B3dCTM under the conditions of the large solar proton events during October/November 2003. These model calculations have been driven with meteorological data derived from the KMCM general circulation model. Results for the impact on the chemical composition and specially for HOx , NOx and O3 show changes in these species that are comparable to other model studies and observations of the same event. Large increases in HOx and NOx during high particle forcing cause O3 depletions of up to 90%. These O3 depletions cause changes in the radiative heating rate in the middle and upper atmosphere. In high Northern latitudes, the O3 depletion results in less emission of infrared radiation, whereas in sunlit latitudes it causes less absorption of UV radiation. The impact on the radiative heating rate is of maximum during strongest particle forcing of ≈-10 K/day due to less UV absorption and ≈0.5 K/day due to less infrared emission. This results in a additional heating in high northern latitudes during the absence of sunlight and an additional cooling in the sunlit latitudes as a cause of the particle precipitation and its induced chemical change of the middle and upper atmosphere. Large HOx increases and resultant O3 losses have an impact on the heating rates due to different exothermic chemical reactions. The change in the heating rates for several exothermic reactions have been calculated from the results of the chemical model calculations. Results show that the main impact on the chemical heating rate is due to the high O3 loss during large particle forcing. Absolute changes in the chemical heating rates are generally small compared to the change in radiative heating rates, but could strengthen the impact of radiative heating rate changes by about 10%. A model experiment using the changes in heating / cooling rates due to energetic particle precipitation for a period of exceptionally strong particle impacts as an additional forcing factor of a free-running general circulation model has shown a distinct pattern of intrahemispheric coupling in the Southern stratosphere and mesosphere compared to a model run without this additional particle forcing. The emerging pattern of temperature differences clearly follows the prescribed mesospheric cooling rates in December, and decreases again after the forcing has subsided. However, from comparing two model experiments only, it is not possible to distinguish clearly between the impact of the additionally forcing and the internal dynamical variability of the model. To investigate this further, an ensemble of sensitivity runs with and without additional forcing due to energetic particles will be carried out in the near future.
