referred to as reflectivity and designated by Z; the Doppler (radial) velocity, designated by v or vr; and the width of the Doppler spectrum, designated by σv. These base data variables, derived in the radar data acquisition (RDA) unit, express the zeroth, first, and second moments, respectively, of the Doppler spectrum of the echoes. A value for each quantity is available for every resolution cell of the radar, as defined by its antenna beamwidth and the sampling rate along the beam axis, although the latter is constrained presently to no less than half the pulse duration.
Displays of these quantities, together with other products (Radar Operations Center, 2002) and results of the algorithms discussed below, are developed from the base data in a radar product generator (RPG) unit. The products include estimates of precipitation accumulations for 1-hour and 3-hour periods as well as a storm-total product. In addition, a series of computer algorithms operates on the base data—and some also incorporate auxiliary information such as temperature profiles—examining the echo patterns and their continuity in space and time in order to identify significant weather features such as mesoscylones, tornado vortex signatures, or the presence of hail. Outputs of these algorithms are displayed as icons superimposed on the basic radar displays or in auxiliary tables. The number and the variety of potential algorithms continue to increase as scientific knowledge about the relationship between echo characteristics and storm properties improves and available computational resources increase.
A principal user processor (PUP) associated with each NEXRAD installation, and numerous additional remote PUPs, provided the initial data display capability. The PUP was essentially a mainframe minicomputer, with a monitor, that operated programs to generate displays from a rather limited set of possibilities. As computer technology has advanced, open-systems architecture has been implemented to replace both the RPG and the PUP units. Thus, the current open RPG (ORPG) generates and displays the various products as well as relays the relevant data for display on other systems.
Although the NEXRAD system of radars is of major value as a stand-alone weather-observing network, additional value is obtained through the integration of NEXRAD data with other weather observations (e.g., wind profilers, satellites, the National Lightning Detection Network, surface measurements, other radar systems) and associated analyses. As described earlier, this synthesis is carried out in the National Weather Service (NWS),