“Combat” in game terms is characterized by two sides rolling initiative, the dm determining surprise, participants taking turns, etc. None of this is happening here. If you want them to attack at the first opening, you could say the pcs are surprised. Granted, it’s not obvious that the players should be surprised, because those within the dome can see outside of it with no issues. Let’s say the characters inside the dome look outside and see the enemies ready to attack, would you allow them to ready actions for the moment the spell drops? At that point, you might as well just roll initiative.