B112 requires with Area Effect.
It says "incompatible with Melee Attack and ranged attack modifiers"
- 102 suggests GMs might allow Explosion to substitute for AE requirement.
- 165 defines what is included
- Ordinarily ranged attacks modified with Emanation,
- advantages such as Obscure and Mana Damper (Switchable),
- and similar zero-range area-effect abilities.
- page 20 gives an example of combining it with Malediction and Selective Area
- page 26 gives example of combining with Bombardment
- Emanation never requires an attack roll. You only bother rolling if your attack is dependent upon your margin of success (e.g., it's a Malediction) -- and even then, if you fail the roll, you're still considered to have succeeded with a margin of 0. (This last rule is a general-case one for any situation where you're guaranteed success but must still roll to determine your margin of success.)
- Emanation obviates the need for an attack roll. It's one of its built-in benefits, implied heavily in the Basic Set but then made more explicit elsewhere. Aura is similar in this way, just longer-lasting.
- Otherwise, whether attacking the ground you're standing on with an area effect requires an attack roll is up to the GM. There are about a million different variations on how this could go down, so use common sense. If the attack could feasibly miss, then ask for an attack roll -- though speaking as GM I'd give both the usual +4 for attacking an area and a +2 circumstance bonus for "shooting straight down" (a.k.a. the Inverted Takedown bonus for those of you who play Arkham: Batman games).
- No roll to hit is required – that's the official answer, and PK has already given it.
- As to why Emanation is still a limitation, that has to do with the lack of range and the requirement to buy Area Effect. The true cost of Emanation is in fact +30% because it always comes with Area Effect, 2 yards, +50% at a minimum . . . and if you want to not fry friends, you'll need Selective Area, +20% as well, for a net +50%. For the same +50%, you could affect any two-yard-radius area you like at Range 10/100. Yes, there would be risk of scatter, but you wouldn't have to stand next to people who could just punch you in the face.