This is very rough, but I more or less followed the waves with the pencil tool. I applied a gradient across the pencil path with the same color on all three stops, sampled from the hair in the photo, but the outer two have a transparency of zero. I applied a Roughen effect to the strokes. You can play with the stroke width and the Roughen size and detail to get the effect you want. The gradient is the same color in both of the instances below, but the top of the head on the left is full black, while the right is somewhere between the face and darkest hair colors to approximate where the scalp shows through the hair.

Peter