Thanks for reaching out. I wouldn’t necessarily credit Tara for this method. In a blog post that came out about the same time as her book, she states:
About twelve years ago, a number of Buddhist teachers began to share a new mindfulness tool that offers in-the-trenches support for working with intense and difficult emotions. Called RAIN
When I was digging around to find the origins of the term, I also found a claim that it was coined by someone else in the Buddhist community long before Tara Branch:
The acronym RAIN, first coined about 20 years ago by Michele McDonald, is an easy-to-remember tool for practicing mindfulness.
As if that weren’t enough, I found roots of the method in the non-spiritual realm — specifically Cognitive Behavioral Therapy.
I absolutely want to credit someone who invented some idea that I write about, but the problem for so many of these kinds of things is that no one is quite sure who coined them. But boy are they helpful!