Looks like you have multiple such curves in your data. And they don't have an exact profile as the one that you show.
So questions:
- What is the definition of "Top" given the real data?
- Do you want the all Top's of individual curves or, a "Top" of all curves? Definition.
Here are a couple of possible approaches.
First one. If your definition can define one and only one point on individual curves, or all curves, express your definition as a formula, which returns 1 for match the definition, 0 otherwise. I have tried a couple of my imaginations, and seems such a rigorous definition is not trivial.
Second one. If the desire is just a rough quantity of some sort of "bottom" at the "top", seems even a histogram can be an interesting approach.