Dynamic filename generated by two variables

Hi Guys,
if this topic is in the wrong category, please move it to the right.
I´m using Openhab2 with CometVisu.

I´m getting weatherwarnings from DWD (Deutscher Wetterdienst) like the tutorial from Elk and it´s working like a charm.
The original weather icons with level marked colour are based on event_level and named like




I get the variables event and level and the items are DWD_Warnung_event and DWD_Warnung_level.

I´ve read much days and done a lot trial and error, and i will be very happy if someone can help me to generate the icon filename with two catched variables.

Thanks in advance