Publication

Journal	Atmospheric Chemistry and Physics
Volume (Issue)	17(5)
Page(s)	3573 - 3604
Title of proceedings	Atmospheric Chemistry and Physics
DOI	10.5194/acp-17-3573-2017

URL	www.atmos-chem-phys.net/17/3573/2017/
Type of Open Access	Publisher (Gold Open Access)

We compare simulations from three high-top (with upper lid above 120 km) and five medium-top (with upper lid around 80 km) atmospheric models with observations of odd nitrogen (NOx = NO + NO2), temperature, and carbon monoxide from seven satellite instruments (ACE-FTS on SciSat, GOMOS, MIPAS, and SCIAMACHY on Envisat, MLS on Aura, SABER on TIMED, and SMR on Odin) dur- ing the Northern Hemisphere (NH) polar winter 2008/2009. The models included in the comparison are the 3-D chem- istry transport model 3dCTM, the ECHAM5/MESSy Atmo- spheric Chemistry (EMAC) model, FinROSE, the Hamburg Model of the Neutral and Ionized Atmosphere (HAMMO- NIA), the Karlsruhe Simulation Model of the Middle Atmosphere (KASIMA), the modelling tools for SOlar Cli- mate Ozone Links studies (SOCOL and CAO-SOCOL), and the Whole Atmosphere Community Climate Model (WACCM4). The comparison focuses on the energetic parti- cle precipitation (EPP) indirect effect, that is, the polar winter descent of NOx largely produced by EPP in the mesosphere and lower thermosphere. A particular emphasis is given to the impact of the sudden stratospheric warming (SSW) in January 2009 and the subsequent elevated stratopause (ES) event associated with enhanced descent of mesospheric air. The chemistry climate model simulations have been nudged toward reanalysis data in the troposphere and stratosphere while being unconstrained above. An odd nitrogen upper boundary condition obtained from MIPAS observations has further been applied to medium-top models. Most models provide a good representation of the mesospheric tracer de- scent in general, and the EPP indirect effect in particular, during the unperturbed (pre-SSW) period of the NH win- ter 2008/2009. The observed NOx descent into the lower mesosphere and stratosphere is generally reproduced within 20 %. Larger discrepancies of a few model simulations could be traced back either to the impact of the models’ grav- ity wave drag scheme on the polar wintertime meridional circulation or to a combination of prescribed NOx mixing ratio at the uppermost model layer and low vertical reso- lution. In March–April, after the ES event, however, mod- elled mesospheric and stratospheric NOx distributions de- viate significantly from the observations. The too-fast and early downward propagation of the NOx tongue, encoun- tered in most simulations, coincides with a temperature high bias in the lower mesosphere (0.2–0.05 hPa), likely caused by an overestimation of descent velocities. In contrast, upper- mesospheric temperatures (at 0.05–0.001 hPa) are generally underestimated by the high-top models after the onset of the ES event, being indicative for too-slow descent and hence too-low NOx fluxes. As a consequence, the magnitude of the simulated NOx tongue is generally underestimated by these models. Descending NOx amounts simulated with medium- top models are on average closer to the observations but show a large spread of up to several hundred percent. This is pri- marily attributed to the different vertical model domains in which the NOx upper boundary condition is applied. In gen- eral, the intercomparison demonstrates the ability of state- of-the-art atmospheric models to reproduce the EPP indi- rect effect in dynamically and geomagnetically quiescent NH winter conditions. The encountered differences between ob- served and simulated NOx, CO, and temperature distribu- tions during the perturbed phase of the 2009 NH winter, how- ever, emphasize the need for model improvements in the dy- namical representation of elevated stratopause events in or- der to allow for a better description of the EPP indirect effect under these particular conditions.