I found this entry.
こまか 【細か】の意味 国語辞典 - goo辞書
To me it seems the same as 細かい but this one is a な adjective.
The first 2 examples being:
細かな砂
細かな網の目
I don't know if there is a real difference in usage in that kind of sentence though since the first 2 examples of 細かい are:
細かい砂
細かい雨