I grow my dwarf pomegranate in full sun and it does beautifully. Try it and if it doesn't do well you can move it back to the windowsill. Might the heat on the rooftop be an issue? Is the roof dark or a light reflective color? If it's dark it might get too hot.

They are adapted to heat, full sun, and drought but as the above poster suggests, roofs get mighty hot and NYC and environs have been extremely hot this year. The bigger the pot the better in this department. I have my ornamental dwarf in the ground--it's deciduous but cold hardy in zone 7. In a pot, you'll have to protect--I would imagine.

shouldnt be a problem, i would suggest using a bigger pot so the soil doesn;t dry up too quickly. and might need to water daily.

i bought 2 small angel reds last fall, i've had them out in full texas sun all summer, i water every 2-3 days and they have grown about 10 times in size.

Should not be a problem at all. I grow two punica granatum nana's in small pots with minimal watering and care. They are both out in full sun with temps ranging in the 80s most of the time.