The main attraction is food whether there are thermals or no thermals. If raptors can get food easily, and peace to it eat, then they'll start arriving. Of course, soaring on thermals is not a very useful method of searching for food in a high-rise city, so expect variations. I expect that putting carrion out on your rooftop garden would attract birds, but you may get more corvids than raptors, depending on where you live.