Aluminum Door sill and Concrete
There are specialized door sweeps that retract and can provide a good seal to most any type of sill.
I have seen similar units for indoor use and noise control (sound and privacy). The sweep is lowered when the door is closed. Probably no cheap, but it could be a good, smooth solution for limited access.
You may want to look for a low profile sill that is make from a material compatible with the concrete (steel, marble, etc.) and suitable for your climate exposure.