NASA did quite alot of research after the ATR turned over a few years ago as a result of icing. Some of the tests were done on an ATR and some on a Twin Otter: several papers were produced as a result. I don't know where you would get copies, but I saw them at Flight Safety Canada (Toronto): might be a starting point for you. There is also a pink AIC: check your AIP in your ops room, or where ever your company keeps it. In essence it says if in doubt, deice, and be very careful about hold over times, and if you're still in doubt, deice again. It is illegal to take off with contamination on the lift surfaces.
That said, and this was a special situation, in the Sahara in winter there is often a frost at night. The temperatures rise quite quickly as soon as the sun comes up and melts the frost off the wings almost instantaneously. Ground temperatures could be -2 or -3, but temperatures at 50' would be in the order of +10 or so, and there was no deicing on hand anyway. We were allowed by company rules to depart with frost on the wings, but not ice.