There is definitely a "roof down" mode. I noticed on mine, that if it is in "auto" and a low temp setting, ie max cooling, then putting the roof down results in it going from auto to manual after a short while.
I think it's so that the system isn't trying to achieve an "unobtainable" temp one the roof has been dropped. Pusing the auto button after it has done this will allow it back to auto, BUT it will then change back to maual. It doesn't do this with the roof up.
HTH
Mike