The rigid calibration factors were not calibrated using the MERRA climatic data set. The application will automatically require NARR climatic inputs when rigid designs are selected.
You can bypass this by creating "custom" climate files that use the MERRA data and then use those in your design.
To create custom climate files you need to navigate to C:\ProgramData\AASHTOWare\ME Design\HCD\CustomHcd directory and modify the CustomStations.dat file. You'll enter one row in the .dat file for each MERRA climate station you'd like to use. The row must be correctly formatted (there is an example row in the file already for you).
Note that the first entry in each row is a unique identifier for your custom climate file. You can then take a MERRA file from the InfoPave site at https://infopave.fhwa.dot.gov/Tools/MEPDGInputsFromMERRA. The ID in the CustomStation.dat file field must match the name of the custom climate file (.hcd file) you download from the InfoPave site.
Once you have your "custom" hcd files (as MERRA .hcd files) in the custom HCD directory and you have the links to those files created in CustomStation.dat, then you can open your design in PMED (Pavement ME Design) and select the "Use Custom Stations" checkbox. The application will then check the CustomStation.dat file and pull in the data to the google map. You can then select stations and include them in the design normally.