Comsol (4.3): convert parametric surface into block?
 

I am new at Comsol (vn4.3.x) CFD and have a problem in creating a geomery:

I am not able to convert a parametric surface into a block for further manipulation and meshing. Please help!

Problem is the following:
I have to create a cubic volume, its lower boundary shall represent a rough surface. This surface comes out from a measurement and was successfully read in Comsol as a parametric surface. Viewed from above, this surface is a square.

Then, I draw a solid block around the parametric surface, so that the surface is located approximately in the middle of the block (cube).

Now I have to cut out the lower section of the block because it is not needed in solving the problem, I just need the upper part of the block in which the read-in surface would be the lower boundary.

Unfortunately, I do not find a way to do this. "Split" does not work. "Intersection" works only on solids and the surface is no solid, of course.

Converting the parametric surface into a solid in principal works - but the resulting surface/solid then vanishes from the "Graphics" window, so that I cannot select it with the mouse when I am attempting to intersect the two solids.

Also strange is, that I cannot extrude the parametric surface in order to get a block.

Does someone has a suggestion how to achieve what I want? I googled a lot and also looked into the Comsol forum,cbut couldn't find anything which helped.

Thanks