Abstract. Water vapor is an important component in the water and energy cycle of the Arctic. Especially in light of Arctic amplification, changes in water vapor are of high interest but are difficult to observe due to the data sparsity of the region. The ACLOUD/PASCAL campaigns performed in May/June 2017 in the Arctic North Atlantic sector offers the opportunity to investigate the quality of various satellite and reanalysis products. Compared to reference measurements at R/V Polarstern frozen into the ice (around 82∘ N, 10∘ E) and at Ny-Ålesund, the integrated water vapor (IWV) from Infrared Atmospheric Sounding Interferometer (IASI) L2PPFv6 shows the best performance among all satellite products. Using all radiosonde stations within the region indicates some differences that might relate to different radiosonde types used. Atmospheric river events can cause rapid IWV changes by more than a factor of 2 in the Arctic. Despite the relatively dense sampling by polar-orbiting satellites, daily means can deviate by up to 50 % due to strong spatio-temporal IWV variability. For monthly mean values, this weather-induced variability cancels out, but systematic differences dominate, which particularly appear over different surface types, e.g., ocean and sea ice. In the data-sparse central Arctic north of 84∘ N, strong differences of 30 % in IWV monthly means between satellite products occur in the month of June, which likely result from the difficulties in considering the complex and changing surface characteristics of the melting ice within the retrieval algorithms. There is hope that the detailed surface characterization performed as part of the recently finished Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) will foster the improvement of future retrieval algorithms.

Abstract. Water vapor is an important component in the water and energy cycle of the Arctic. Especially in the light of Arctic amplification, changes of water vapor are of high interest but are difficult to observe due to the data sparsity of the region. The ACLOUD/PASCAL campaign performed in May/June 2017 in the Arctic North Atlantic sector offers the opportunity to investigate the quality of various satellite and reanalysis products. Compared to reference measurements at R/V Polarstern frozen into the ice (around 82° N, 10° E) and at Ny-Ålesund, the Integrated Water Vapor (IWV) from IASI shows the best performance among all satellite products. Using all radiosonde stations within the region indicates some differences that might relate to different radiosonde types used. Though the region is well sampled by polar orbiting satellites daily means can deviate by up to 50 % due to strong spatio-temporal IWV variability associated with atmospheric river events. For monthly mean values, this weather induced variability cancels out but systematic differences dominate which particularly appear over different surface types, e.g. ocean, sea ice. In the data sparse central Arctic above 84° N, strong differences of 30 % in IWV monthly means between satellite products occur in the month of June which likely results from the difficulties to consider the complex and changing surface characteristics of the melting ice within the retrieval algorithms. There is hope that the detailed surface characterization performed as part of the recently finished Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) will foster the improvement of future retrieval algorithms.

Abstract. Evaluating the global chemistry models in the upper troposphere – lower stratosphere (UTLS) is an important step toward a further understanding of its chemical composition. The latter is regularly sampled through in situ measurements based on passenger aircraft, in the framework of the In-service Aircraft for a Global Observing System (IAGOS) research infrastructure. This study focuses on the comparison of the IAGOS measurements in ozone, carbon monoxide (CO), nitrogen reactive species (NOy) and water vapour, with a 25-year simulation output from the LMDZ-OR-INCA chemistry-climate model. For this purpose, we present and apply an extension of the Interpol-IAGOS software that projects the IAGOS data onto any model grid, in order to derive a gridded IAGOS product and a masked model product that are directly comparable to one another. Climatologies are calculated in the upper troposphere (UT) and in the lower stratosphere (LS) separately, but also in the UTLS as a whole, as a demonstration for the models that do not sort out the physical variables necessary to distinguish between the UT and the LS. In the northern extratropics, the comparison in the UTLS layer suggests that the geographical distribution in the tropopause height is well reproduced by the model. In the separated layers, the model simulates well the water vapour climatologies in the UT, and the ozone climatologies in the LS. The opposite biases in CO in both UT and LS suggest that the cross-tropopause transport is overestimated. The NOy observations highlight the difficulty of the model in parameterizing the lightning emissions. In the tropics, the upper-tropospheric climatologies are remarkably well simulated for water vapour, as the observed CO peaks due to biomass burning in the most convective systems, and the ozone latitudinal variations. Ozone is more sensitive to lightning emissions than to biomass burning emissions, whereas the CO sensitivity to biomass burning emissions strongly depends on the location and on the season. Through this evaluation, the present study demonstrates that the Interpol-IAGOS software is a tool facilitating the assessment of the global model simulations in the UTLS, potentially useful for any modelling experiment involving chemistry-climate and chemistry-transport models.

<p class="western" align="justify">Water vapor is an essential component for regulating the Earth's radiation budget. To realistically determine the global radiation budget, an accurate description of the water vapor distribution in the upper troposphere and lower stratosphere (UTLS) is therefore indispensable. For example, small changes in water vapor concentration can lead to significant changes in local radiative forcing, especially in the dry lower stratosphere. The change in this region can be even stronger if condensed water in the form of ice clouds is present instead of solely water vapor.</p> <p class="western" align="justify">The formation and evolution of ice clouds is crucially determined by the saturation ratio over ice (Si). Ice crystals can only form (and grow) at supersaturated conditions (i.e. Si>1), i.e. in so-called ice supersaturated regions (ISSRs), which also constitute potential regions for the formation and existence of persistent aircraft contrails. Knowing and precisely forecasting the occurrence of ISSRs can help reducing the contribution of aviation to man-made climate change, as contrails usually have a warming effect on the climate.</p> <p class="western" align="justify">Ice supersaturation is often observed in the UTLS. However, despite their importance, the large-scale three-dimensional structure of ISSRs is widely unknown. Therefore, we present a three-dimensional climatology of ice supersaturation in the UTLS over the North Atlantic for the years 2010 to 2019. This climatology is based on the recent ERA5 reanalysis data set of the European Center for Medium Weather Forecast (ECMWF), which explicitly allows ice supersaturation in cloud-free conditions. To quantify the quality of the ERA5 data set with respect to ice supersaturation, we use the long-term in-situ measurements of the European Research Infrastructure ’In-service Aircraft for a Global Observing System’ (IAGOS; www.iagos.org) (Petzold et al., 2015).</p>

<p>Ice clouds are challenging because of the high complexity and diversity of their composition  (microphysics) as well as formation and growth processes. As a result, there has been little constraint from observations until recently, resulting in significant limitations in our understanding and representation of ice clouds. A major problem with satellite measurements is the lack of information on the environmental context, which is necessary to identify and understand the formation mechanism and evolution of clouds; these renditions indeed only represent a snapshot of the state of a cloud and its microphysical properties at a given time. This work tackles this issue by providing additional metrics on ice cloud history and origin along with operational satellite products.</p><p>Here, we present a novel framework that combines geostationary satellite observations with Lagrangian transport and ice microphysics models, in order to obtain information on the history and origin of air parcels that contributed to their formation. The trajectory of air parcels encountered along the DARDAR-Nice track has been traced using the air mass transport models CLAMS (Chemical LAgrangian Model of the Stratosphere). CLaMS - Ice model is jointly used to simulate cirrus clouds along trajectories derived by CLaMS. This approach provides information on the cloud regime as well as the ice formation (in-situ vs liquid origin) pathway. For tropical cirrus of convective origin, a Time Since Convection dataset from geostationary observations can also be incorporated into this approach. Preliminary results of this approach obtained on case studies representative of multiple cloud types will be shown here.</p>