I haven't operated modern radar but certainly we used maximum manual gain at high altitude cruise in the 737 with Collins WXR-700 radar over the Pacific and that would pick up CB tops which had little moisture. It would show as a tiny echo at 40 miles or thereabouts but enough to alert an alert pilot of something big ahead. All you did then was use the tilt control to scan below and pick up the body of the CB and take it from there - ie divert around it. The auto gain control was useless at high altitude which is why MAX gain was used to scout for CB tops.