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Abstract. Water vapour and ozone are important for the thermal andradiative balance of the upper troposphere (UT) and lowermost stratosphere(LMS). Both species are modulated by transport processes. Chemical andmicrophysical processes affect them differently. Thus, representing thedifferent processes and their interactions is a challenging task fordynamical cores, chemical modules and microphysical parameterisations ofstate-of-the-art atmospheric model components. To test and improve themodels, high-resolution measurements of the UT–LMS are required. Here, weuse measurements taken in a flight of the GLORIA (Gimballed Limb Observerfor Radiance Imaging of the Atmosphere) instrument on HALO (High Altitude and LOng Range Research Aircraft). The Germanresearch aircraft HALO performed a research flight on 26 February 2016 that covered deeply subsidedair masses of the aged 2015/16 Arctic vortex, high-latitude LMS air masses,a highly textured region affected by troposphere-to-stratosphere exchangeand high-altitude cirrus clouds. Therefore, it provides a challengingmultifaceted case study for comparing GLORIA observations withstate-of-the-art atmospheric model simulations in a complex UT–LMS region ata late stage of the Arctic winter 2015/16. Using GLORIA observations in this manifold scenario, we test the ability ofthe numerical weather prediction (NWP) model ICON (ICOsahedralNonhydrostatic) with the extension ART (Aerosols and Reactive Trace gases)and the chemistry–climate model (CCM) EMAC (ECHAM5/MESSy Atmospheric Chemistry – fifth-generation European Centre Hamburg general circulation model/Modular Earth Submodel System) to model the UT–LMS composition of water vapour (H2O), ozone(O3), nitric acid (HNO3) and clouds. Within the scales resolved bythe respective model, we find good overall agreement of both models withGLORIA. The applied high-resolution ICON-ART set-up involving an R2B7 nest(local grid refinement with a horizontal resolution of about 20 km),covering the HALO flight region, reproduces mesoscale dynamical structureswell. Narrow moist filaments in the LMS observed by GLORIA at tropopausegradients in the context of a Rossby wave breaking event and in the vicinity ofan occluded Icelandic low are clearly reproduced by the model. UsingICON-ART, we show that a larger filament in the west was transportedhorizontally into the Arctic LMS in connection with a jet stream splitassociated with poleward breaking of a cyclonically sheared Rossby wave.Further weaker filaments are associated with an older tropopause fold in theeast. Given the lower resolution (T106) of the nudged simulation of the EMACmodel, we find that this model also reproduces these features well. Overall,trace gas mixing ratios simulated by both models are in a realistic range,and major cloud systems observed by GLORIA are mostly reproduced. However,we find both models to be affected by a well-known systematic moist bias inthe LMS. Further biases are diagnosed in the ICON-ART O3, EMAC H2Oand EMAC HNO3 distributions. Finally, we use sensitivity simulations toinvestigate (i) short-term cirrus cloud impacts on the H2O distribution(ICON-ART), (ii) the overall impact of polar winter chemistry andmicrophysical processing on O3 and HNO3 (ICON-ART and EMAC), (iii) the impact of the model resolution on simulated parameters (EMAC), and (iv) consequences of scavenging processes by cloud particles (EMAC). We find thatchanging the horizontal model resolution results in notable systematicchanges for all species in the LMS, while scavenging processes play a roleonly in the case of HNO3. We discuss the model biases and deficits found inthis case study that potentially affect forecasts and projections(adversely) and provide suggestions for further model improvements.

atmospheric chemistry and physics

Challenge of modelling GLORIA observations of upper troposphere–lowermost stratosphere trace gas and cloud distributions at high latitudes: a case study with state-of-the-art models