Superrefraction: Bending of the radar beam in the vertical which is greater than sub-standard refractive conditions. This causes the beam to be lower than indicated, and often results in extensive ground clutter as well as an overestimation of cloud top heights.

From:

http://weather.about.com/newsissues/weather/library/glossary/blglossary_s.htm

A summary picture of Subrefraction and Superrefraction from the AG Study Guide:

Figure 1-11. Anomalous Propagation.

Superrefraction. When warm, dry air overlies relatively cool, moist air, as in an inversion, superrefraction may occur. In this case, the radar beam is refracted below its normal path, as illustrated in figure 1-11.

The greater the increase of temperature and decrease of moisture, or both, with height, the greater degree of superrefraction. In an extreme case, a radar beam becomes trapped in a "duct" beneath the inversion and may travel for long distances without appreciable attention. Figure 1-11 illustrates the ducting phenomenon. Under such conditions, low-level targets a few hundred miles distant, and not normally detected, may be displayed by the radar.

Probably one of the greatest limitations caused by superrefraction is the overestimating of cloud/precipitation heights. As the beam bends below its normal path it strikes a target and returns energy to the radar. This returned energy is interpreted by radar based on the standard refraction of a radar beam. Thus, the actual height of the cloud/precipitation is lower than the displayed height values.

Subrefraction. Straightening of the beam upward, or subrefraction, may occur during atmospheric conditions where the water vapors content increases with height and the temperature decreases with height. This subrefraction, shown in figure 1-11 is the opposite of superrefraction.

Subrefraction reduces the maximum range of detection of low-level targets because the beam quickly overshoots these low targets. Such a condition may occur in certain types of fog, but it does not occur in precipitation. This abnormal condition is relatively unimportant for storm detection.

As with superrefraction, subrefraction has a limitation on displayed cloud/precipitation heights. As the radar beam bends above its normal path it strikes a target and returns energy to the radar. This returned energy is interpreted by radar based on the standard refraction of the radar beam. Thus, the actual height of the cloud/precipitation is higher than the displayed height values.

Still another view:

http://www.crh.noaa.gov/mkx/radar/part1/slide10.html