A standard radar will assume normal refraction takes place. Radar determines an echo height by calculating how much the beam changes in elevation with distance from the radar and how the earth's surface curves under the radar beam.
Errors in the echo height can occur from the beam not refracting normally and land surface elevation changes at the earth's surface. The land surface elevation change errors can be removed if the radar is given topographic data of the earth's surface. The refraction errors can be reduced from soundings inputted into the radar so that the radar determines whether refraction will be more than normal, normal or less than normal.
If the radar assumes normal refraction, significant echo height errors can occur when superrefraction and subrefraction take place. Suppose there is a storm that is 100 kilometers from the radar site and the echo top of the storm in the actual troposphere is 40,000 feet. Suppose superrefraction is taking place and the radar assumes normal refraction. The radar will not indicate the actual echo top of 40,000 feet since the beam is not refracting as the radar assumes it is. The radar under superrefraction conditions will indicate an echo top greater than 40,000 feet in this example. Thus superrefraction overestimates the echo top height. Using this same line of logic, subrefraction underestimates the echo top height thus it will indicate a echo top of less than 40,000 feet in this example.