why not simply download with an url like this:

https://www.grib.us/download.php?username=me&password=secret&coord1=leftuppercorner&coord2=rightlowercorner&timeinhours=72 etc.

This way users could simply download the requested data totally independent from the device, pc, windows, mac or smartphone.

For grib.us this is the simplest solution i can think of.