A typical search would be to retrieve all the values of a <variable> for or for a particular year+yday combination.
If those are the typical search patterns, is a relational database the right hammer to reach for? Why not flat files? Appending to a flat file is a quick operation. If you keep one flat file per day, scanning for the subset of records for that day is trivial. If you have to do a full scan through all the data, one file per day is also easy, and allows the scan to be parallelized if you ever need to throw more machines at the problem.
10Tb over 20 years is 500Mb a year, or very roughly 1.5Mb a day. That's not an unreasonable size for a flat file.