Downloader & cmorizer for NOAA GML surface flask trace gases

[jlenh]

Description

• Closes Add CMORized NOAA Global Monitoring Lab surface flask concentration measurements of trace gases (CH4, CO2, N2O) #3839
• Link to documentation:

This work adds a downloader and a formatter to ESMValTool to download and CMORize surface flask measurements of trace gases (CH4, CO2, and N2O) from the Global Monitoring Lab (GML) of NOAA. This work contributes to the ongoing development of a recipe and diagnostic to evaluate model output of surface concentrations of trace gases against the NOAA GML station measurements #3838.

The downloader retrieves event-based and monthly station averages of surface flask measurements of CH4, CO2 and N2O (see NOAA-GML) as an archive. Only the monthly averages of each station are CMORized here and are already computed following the methodology described on NOAA's GML website from the event-based files also present in the downloaded archive. The dataset archives are downloaded separately for each trace gas.

The formatter CMORizes the station monthly averages for a given trace gas by creating a NetCDF file containing all stations for which a location was available in the corresponding event files. The NetCDF file holds a time series for each station with the coordinates station_id, latitude and longitude. The general dataset architecture is taken from #3227 which implemented a CMORizer for time series of surface station observations. This allows the access to all stations data in a single file, speeding up loading times and processing operations. The general formatting functions are implemented in a separate file from which a single function is then imported and called in the respective trace gases formatter files.

Note: the CMORized variables produced here (ch4s, co2s, n2os) are not available in the CMOR table CMIP6 - Amon. However, in the case of model outputs, they need to be derived from their 3D counterparts (ch4, co2, n2o, see #2611) which are present in the corresponding CMOR table.

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