Tuesday, May 31, 2011

The WSR-88D (Weather Surveilance Radar - 88 Doppler) is a doppler weather radar developed by the NEXRAD program, which is a joint effort between the Dept. of Commerce, Dept. of Defense, and Dept. of Transportation. The WSR-88D operates in two basic modes, selectable by the operator – a slow-scanning clear-air mode for analyzing air movements when there is little or no activity in the area, and a precipitation mode, with a faster scan for tracking active weather. The WSR-88D detects precipitation and atmospheric movement or wind. It returns data which when processed can be displayed in a mosaic map which shows patterns of precipitation and its movement. The WSR-88D antenna is not directly controllable by the user. Instead, the radar system continually refreshes its three-dimensional database via one of several predetermined scan patterns. Since the system samples the atmosphere in three dimensions, there are many variables that can be changed, depending on the desired output. There are currently nine Volume Coverage Patterns (VCP) available to NWS meteorologists. Each VCP is a predefined set of instructions given to the antenna that control the rotation speed, transmit/receive mode, and elevation angles.

Under legacy resolution, the WSR-88D provides reflectivity data at 1 km by 1 degree to 460 km range, and velocity data at 0.25 km by 1 degree to a range of 230 km. Super Resolution provides reflectivity data with a sample size of 0.25 km by 0.5 degree, and increase the range of Doppler velocity data to 300 km. Initially the increased resolution is only available in the lower scan elevations. Super resolution of the WSR-88D makes a compromise of slightly decreased noise reduction for a large gain in resolution. The next major upgrade is polarimetric radar, which adds vertical polarization to the current horizontal radar waves, in order to more accurately discern what is reflecting the signal. This so-called dual polarization allows the radar to distinguish between rain, hail and snow, something the horizontally polarized radars cannot accurately do. Early trials have shown that rain, ice pellets, snow, hail, birds, insects, and ground clutter all have different signatures with dual-polarization, which could mark a significant improvement in forecasting winter storms and severe thunderstorms.