High-resolution meteorological CO2 enhancements of German metropolitan areas using WRF

Verifying greenhouse gas (GHG) mitigation efforts of governments using atmospheric observations is a task which is rapidly gaining scientific interest and attention. The United Nations Framework Convention on Climate Change (UNFCCC) requires the compilation of National Inventory Reports and recommends augmenting them with observational data. The joint project ITMS (Integriertes Treibhausgas-Monitoringsystem für Deutschland) is Germany's national contribution to the World Meteorological Organization’s Integrated Global Greenhouse Gas Information System (IG3IS). It will establish the scientific basis and methodology for integrating GHG observations into the national emissions inventories. Our focus within the ITMS joint project is to optimize observation strategies for monitoring fossil CO2 emissions in German urban and metropolitan areas using synthetic studies. Focusing on cities is especially relevant as cities are substantial contributors to total anthropogenic CO2 emissions. 

Our study uses the Weather Research and Forecasting model (WRF v4.3.3) with ECMWF-ERA5 as meteorological input and boundary conditions. As a first step, we have optimized model transport such that our results are representative of real-world conditions as much as possible. Within comprehensive sensitivity studies, we have analyzed the optimal model settings for German urban areas. Our sensitivity studies focus on the Rhine-Neckar region and compare 16 different physics configurations of WRF for 4 months of 2020, representative of the four seasons. Modeled meteorological variables were compared against 19 meteorological observation stations operated by the German Weather Service and 2 radiosonde stations. We found the setup using Mellor-Yamada-Janjic boundary layer, Noah MP land surface, Monin-Obukhov surface layer and BEP urban parametrization scheme has the overall best performance for our use case. 

Using the optimal setup for urban areas in WRF, we have generated a year-long, 1km resolved dataset of German metropolitan areas. This dataset contains meteorological and sector specific CO2 enhancement data for the year 2018. These metropolitan areas include the Rhine-Neckar, Berlin, Rhine-Ruhr, Nuremberg and Munich metropolitan areas. We showcase the usefulness of the dataset by comparison to actual observations in cities.