Design concepts and test results are summarized for a Doppler weather radar system suitable for precipitation measurements over a wide span of radial velocities and slant ranges, even in the presence of ground clutter. The radar transmits two uniform pulse trains at 2.710 and 2.760 GHz. Uniformly spaced pulses permit ground clutter cancellation of up to 50 dB to be achieved with a three-pole elliptic filter. Pulse spacing at one frequency is consistent with long-range coverage in reflectivity, while spacing of the second is consistent with a wide unambiguous velocity measurement span.

A functional description of an automatic radar system performance monitoring network is presented. A network of microprocessor controlled monitoring units were interfaced to various sensors at key areas in a Doppler weather radar in order to collect system status information and display it at a central point near the operator's console. This status information is displayed on a color terminal in a way that allows the systems operator to verify the proper operation of the radar with just a cursory glance at the terminal. Fault location and fault tolerant techniques were employed to provide the system user with quality assured data.

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The AFGL 10-cm Doppler weather radar was modified to enable the measurement of the differential reflectivity between horizontal and vertical polarizations in addition to the absolute reflectivity and the Doppler mean velocity and spectrum variance. Polarization switching is achieved by means of a diplexer, which separates at the antenna the transmitted signals of two frequencies, 2710 and 2760 MHz, and permits nearly simultaneous transmission of signals of orthogonal polarizations. Signals of these frequencies can be transmitted either with horizontal and vertical polarization, respectively, or with left and right circular, polarization, respectively. We describe the design and performance of the diplexer and the performance of the real-time data processor, present examples of meteorological measurements, and discuss future developments of the radar system. Keywords: Polarization diversity weather radar, Dual polarization radar, and Polarization switching.

Observations of severe convective storms in June 1987 by an 11-cm polarization diversity Doppler radar are described. Specific features include heavy rain, large hail, melting ice particles, lightning, and signal propagation effects. Of particular interest are the first measurements of the polarization differential reflectivity of lightning channels, which reveal the predominant orientations of the channels. Successive radar pulses were transmitted with alternating horizontal and vertical polarization by means of a high-power microwave switch to permit derivation of the differential reflectivity with a small standard error of estimate.

Weather radar is a vital instrument for observing the atmosphere to help provide weather forecasts and issue weather warnings to the public. The current Next Generation Weather Radar (NEXRAD) system provides Doppler radar coverage to most regions of the United States (NRC, 1995). This network was designed in the mid 1980s and deployed in the 1990s as part of the National Weather Service (NWS) modernization (NRC, 1999). Since the initial design phase of the NEXRAD program, considerable advances have been made in radar technologies and in the use of weather radar for monitoring and prediction. The development of new technologies provides the motivation for appraising the status of the current weather radar system and identifying the most promising approaches for the development of its eventual replacement. The charge to the committee was to determine the state of knowledge regarding ground-based weather surveillance radar technology and identify the most promising approaches for the design of the replacement for the present Doppler Weather Radar. This report presents a first look at potential approaches for future upgrades to or replacements of the current weather radar system. The need, and schedule, for replacing the current system has not been established, but the committee used the briefings and deliberations to assess how the current system satisfies the current and emerging needs of the operational and research communities and identified potential system upgrades for providing improved weather forecasts and warnings. The time scale for any total replacement of the system (20- to 30-year time horizon) precluded detailed investigation of the designs and cost structures associated with any new weather radar system. The committee instead noted technologies that could provide improvements over the capabilities of the evolving NEXRAD system and recommends more detailed investigation and evaluation of several of these technologies. In the course of its deliberations, the committee developed a sense that the processes by which the eventual replacement radar system is developed and deployed could be as significant as the specific technologies adopted. Consequently, some of the committee's recommendations deal with such procedural issues.