If you are looking for:
an increase, especially one that is gradual
this is one of the definitions of a build-up from CDO.
In fact, another dictionary (that is LDOCE) specifically mentions the relationship with time:
build-up [countable usually singular]
1: an increase over a period of time
The definitions vary slightly from dictionary to dictionary, one emphasizes something more than the other, so ODO mentions that the word is used typically of something negative, that leads to a problem, but I don't think that this should stop you: typically doesn't mean exclusively; ODO is the only one out of three dictionaries I checked which mentions this.
You mentioned formation of ice, and there are examples of usage with this:
The ice caps of Greenland and Antarctica became more active. The glacial build-up lasted about 100000 years and ended abruptly, over a few hundred years. Within the build-ups and terminations there were short episodes of glacial advance ...
from: Global biogeochemical cycles edited by Butcher
There are a lot more on Google books.
That being said, I agree with the comments which state that you could use accumulation on its own since it means:
1 The acquisition or gradual gathering of something: (ODO)
You should probably base the choice on how much you want to emphasize the time relationship and the terminology that is used in your field.
I must say that I don't quite get your example sentence (probably because I'm not from the field and it seems quite technical), so I had to use your definition and example of your own research as a guide.