why does the subsurface modifier create such a harsh edge here I have tried various fixes like removing doubles, altering the mesh, the problem persists.
Here's the .blend:
https://drive.google.com/open?id=1OwhsqAuXHCTySSfbciQxV9f1igz4xLQ1
It looks like it's a problem of normals, select all and ShiftN to recalculate and it will fix it.