Here is a method to create a series of site modifiers that can be used to form parabolic forms in a digital terrain model.(DTM). Some steps of this method require the use of the Digital Terrain Modeler(DTM), a feature that comes with the Architect, Landmark or Designer products.
Draw a few vertical lines equidistant apart and group them. To get them spaced properly, snap one end of the lines to one end of the polyline, then use the reshape cursor (diagonal arrow) feature of the 2D selection tool to stretch the lines and snap to the other side of the polyline. To lengthen the lines to meet the edge of the face, reshape each line to the edge of the polyline, looking for the 'object' screen hint of the snap to objects feature. (Fig.2)
Place the polygons over the DTM where desired and choose the Model->Convert to 3D Polys menu command. Then choose the AEC->Send to Surface menu command. (Fig.3)
There are several efficient ways to do this. If you have used an alternative way, please post it.