Thought I'd add a few things...
The Z/R relationship on the WSR-88D radars is changed depending on the type of precipitation and time of year. This is the way the radar's software calculates precipitation estimates vs actual reflectivity measurements.
The relationships and rain rate can be found on this page:
http://www.roc.noaa.gov/ops/z2r_osf5.asp
The tropical relationship is very important when being involved with landfalling tropical cyclones. I received 5.10" of rain with Isidore last September. Based on radar reflectivities and the duration of the event, I'd normally expect only about half of that amount (differences in raindrop size cause differences in radar reflectivity). I can recall that our old WSR-57 and WSR-74 radars would often only depict D/VIP 3 to 4 at the most with tropical cyclones (up to 45 dBZ), which wouldn't signal high rain rates with most extratropical systems. This is probably attributed to less powerful radar units and beamwidth (2 degrees of the '57 vs .95 degrees of the 88D).
Light precipitation (drizzle and light snow) may or may not be detected by the 88D. My experience shows that drizzle may be detected at ranges up to 50 nm at times, usually at dBZ levels of less than 10. Sometimes this may show up on Clear Air Mode, but that may also be embedded within ground clutter and other non-precipitating echoes. Radar operators often use Clear Air Mode during times of no precipitation, or during snowy situations (at least around here). Some providers that create 'mosaics' of many radars, do not always provide the full 15 color spectrum of the data. TWC doesn't display the 5-15 dBZ data on their images, so light precipitation is often missing from their products.
Notes on severe weather detection with the WSR-88D...
While the radar can detect precipitation out to 248 nm, the algorithms that create the data and severe weather detection do not.
Within 124 nm of the radar (assuming no terrain shadows) at 0.5 degree elevation...the radar can use the velocity products (which include the TVS and MESO algorithms), as well as precipitation estimates and HAIL detection (size and probability). At the edge of the 124 nm circle, the radar's beam will average 14,700 feet ( 2.5 miles) AGL (above ground level), assuming no terrain or temperature influences.
Between 124 and 186 nm, only the reflectivity algorithms work. The radar's beam is approximately 30,300 feet AGL (3.7 miles) at the edge of this circle. Shallow precipitation events will not be detected at this range.
Between 186 and 248 nm, only reflectivity shapes can be seen on the radar display. At this point, the radar beam is approx 50,500 feet AGL (5 miles) at the edge of this circle. Only the tallest thunderstorms will be scanned at this altitude.
Currently, the WSR-88D standard scanning proceedures limit the lowest elevation angle of the antenna to 0.5 degrees. The radar's hardware can scan down to 0 degrees (-1 degree limitation). Our older radars (the '57 and '74s sometimes scanned down to zero degrees).
All of the 88D radars have standard scanning proceedures that they follow. These include two pulse lengths in Clear Air Mode (mode B 31 and 32) and two precipitation mode pulse lengths (mode A 21 for standard and A 11 for severe weather rapid scan). Scanning proceedures can be found here:
http://weather.noaa.gov/radar/radinfo/radinfo.html
I could learn some of that stuff, too... 
