<p>An experimental setup within ICON (<strong>ICO</strong>sahedral <strong>N</strong>on-hydrostatic)-ART (<strong>A</strong>erosols and <strong>R</strong>eactive <strong>T</strong>race gases) has been &#160;carried out to simulate the atmospheric CH<sub>4</sub> concentration over Europe during the investigation period from 01 May 2018 to 30 June 2018.</p> <p>Modelling CH<sub>4</sub> in the Limited Area Mode (Europe, 6.5 x 6.5 km), the model requires as accurate as possible initial and boundary atmospheric conditions as well as spatially highly resolved emissions. Temporal resolved emissions are to be included in the next step. While the intial data denote here the state of the atmosphere (meteorological and CH<sub>4</sub> concentration fields) at the start of the model run, the boundary conditions shall denote the data in the lateral boundary zone where the model is forced by the meteorological and CH<sub>4</sub> concentration data outside the domain. We have used DWD's operational numerical weather prediction output as meteorological boundary conditions. The Copernicus Atmosphere Monitoring Service (CAMS) provides the necessary initial and boundary CH<sub>4</sub> data, which are made applicable &#160;for the ICON-ART before the model run in Limited Area Mode. The regional CH<sub>4</sub> emissions for Europe have been &#160;provided by TNO and are processed with the ART module. &#160;</p> <p>Since CAMS uses a vertical coordinate of a hybrid sigma-pressure system, the data had been horizontally and vertically interpolated to the height based SLEVE coordinate system of ICON. The sectorial CH<sub>4</sub> emissions for Europe and for Germany were mapped separately to the target ICON grid by preprocessing the corresponding reported methane emissions of various sectors (resulting in 36 distinct methane variables in the model). The 50 largest point emissions from each sector are treated separately, smaller point emissions are treated together with the area emissions.</p> <p>To run a hourly experimental setup for two month the Basic Cycling environment (BACY) tool was used. The fields for meteorological parameters were initialized daily by using the DWD's operational data, while the atmospheric CH<sub>4</sub> concentrations are taken from the previous ICON-ART CH<sub>4 </sub>simulation results (e.g., the 24 h CH<sub>4</sub> forecast from the previous day). Then, the merged concentration fields for meterological conditions and atmospheric methane are used as &#8220;DWD first guess&#8221;, which served for a daily start of the simulation process in the ICON-ART Limited Area Mode. In order to compare the model results and measurements from the Integrated Carbon Observation System (ICOS) stations, the model equivalents have been extracted at the locations of the &#160;ICOS montoring stations&#160; using the &#8220;Model Equivalent Calculator&#8221;.</p> <p>In this work, the ICON-ART CH<sub>4</sub> simulation setup for Limited Area Mode (Europe) was forced by ICON meteorology and CAMS CH<sub>4</sub> boundary data and had been started daily by the merged &#8220;DWD first guess&#8221;. These are shown to be a useful method to simulate the CH<sub>4</sub> atmospheric concentrations at the regional scale.</p>
<p>Methane is emitted during the production and transportation of fossil fuels. Methane emissions result also from intensive livestock farming and agricultural practices as well as by the decay of organic waste. The leakage throughout the extraction, processing and transportation of natural gas releases methane straight into the atmosphere. Due to the damage to the Nord Stream gas pipelines on 26 September 2022 leaks have appeared close to the Danish island of Bornholm in the Baltic Sea, which releases large amounts of methane in the pipeline into the atmosphere within just a few days. In our study, we simulated the transport of methane plume in Nord Stream case by using DWD&#8217;s regional Icosahedral Nonhydrostatic (ICON) model with its transport scheme ART (Aerosols and Reactive Trace gases) extension.</p> <p>The model is run for Limited Area Mode (LAM) with a horizontal spatial resolution of 6.5 km and 60 model levels. As source strength of methane emissions were used the estimates which were calculated by the German Federal Environmental Agency (Umwelt Bundesamt). An assumption, that a constant 700 kg/s of gas had been leaking since September 26 was used for the hourly model run to simulate the methane plume between September 26 and October 1 2022.</p> <p>The model results had been compared to the potential methane signals of Nord Stream leaks detected at Integrated Carbon Observation System (ICOS) stations. According to our simulations, we found a good fit with respect to ICOS observations for the stations Hyltemossa, Birkenes and Norunda. Further analysis has been conducted to look at vertical profiles at different heights and also into correlation coefficients between the model and observations.</p> <p>In this Nord Stream case, our simulation demonstrates modelling capabilities of the ICON-ART model and its associated quantitative assessment of methane emissions.</p> <p>This work has been funded by the German Federal Ministry for Digital and Transport programme for Development and Implementation of Copernicus services for public needs within the HoTC project.</p>
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