Case Studies


HEPPA-I: Composition changes after the “Halloween” solar proton event

The HEPPA-I exercise has focused on the intercomparison of MIPAS/Envisat data obtained in the aftermath of the “Halloween” SPE (26 October – 30 November 2003) with state of the art GCMs and CTMs. The large number of models participating in this exercise allowed for an evaluation of the overall ability of atmospheric models to reproduce observed atmospheric perturbations generated by SPEs, particularly with respect to NOy and ozone changes. This model validation represents a mandatory first step towards an accurate implementation of particle precipitation effects in long-term climate simulations. Further, the quasi-instantaneous perturbation of the atmosphere due to the SPE acts as a natural laboratory for studying stratospheric and mesospheric chemistry. This has allowed to test and to identify deficiencies in the chemical schemes, particularly with respect to nitrogen and chlorine chemistry, being relevant for stratospheric ozone.  (read more....)


HEPPA-II: EEP indirect effects during the unusual 2009 NH polar winter

HEPPA-II exercise focuses on EPP indirect effects (i.e., polar winter NOx descent). This is motivated, on one hand, by the higher potential of EPP indirect effects to influence middle atmospheric composition on longer time scales compared to direct effects (i.e., SPEs) and, on the other hand, by its large variability related to dynamical modulations, making its representation in current atmospheric models challenging. In particular, the 2008/2009 NH polar winter resulted to be a very interesting period because of the peculiar dynamic conditions which were characterized by an unusually strong and persistent stratospheric sudden warming (SSW) that occurred in January, followed by the reformation of a strong upper stratospheric vortex and leading to very efficient descent during the following weeks. Similar situations were also found in the NH during January 2004 and 2006, however, the period November 2008 – May 2009 was better covered by satellite data than the previous winters. (read more....)