Role of Radar in the Weather and Climate Observing and Predicting System
Radars today are used to detect and track aircraft, spacecraft, and ships at sea as well as insects and birds in the atmosphere; measure the speed of automobiles; map the surface of the earth from space; and measure properties of the atmosphere and oceans. Principles of radar have led to the development of other similar technologies such as sonar, sodar and lidar (laser radar) that permit detection of phenomena and targets in the oceans and in the optically clear air.
In the past half century, weather radar has advanced greatly and has played increasingly important roles that span a wide spectrum of meteorological and climatological applications. Of particular importance has been its ability to detect and warn of hazards associated with severe local storms that include hail, tornadoes, high winds, and intense precipitation. Weather radar also monitors larger weather systems such as hurricanes that often include similar phenomena but can extend over very large areas. Today, weather radars improve aviation safety and increase the operational efficiency of the entire air transport industry, and they contribute to agriculture alerts and flood warnings through monitoring of rainfall intensity. They are also used regularly for recreational planning and other weather-impacted activities. Radar measurements have also been key to many remarkable advances in our understanding of the atmosphere and to better weather prediction over a variety of temporal and spatial scales. Such advances have been enabled through a combination of progressive improvements in radar hardware, signal processing, automated weather-based algorithms, and displays.
In recent years, added improvements in short-range forecasting and nowcasting have also resulted from the development of integrated observing systems that blend data from weather radar and other instruments to produce a more complete picture of atmospheric conditions. Two examples of such relatively
new systems are the Advanced Weather Interactive Processing System (AWIPS)1 and the Integrated Terminal Weather System (ITWS). AWIPS is a modern data acquisition and distribution system that gives meteorologists singular workstation access to NEXRAD radar products, satellite imagery, gridded weather forecast data, point measurements, and computer- and man-made forecast and warning products. The result is an integrated forecasting process that utilizes a comprehensive set of data for application by National Weather Service (NWS) Offices and others to generate more accurate and timely weather forecasts and warnings (Facundo, 2000). ITWS combines data from a number of weather radars, including NEXRAD, the Terminal Doppler Weather Radar (TDWR), and airport surveillance radars (ASR), with lightning cloud-to-ground flash data and automated weather station measurements to produce a suite of products that display current weather as well as nowcast weather out to around one hour for use by air traffic controllers in the management of airport terminal operations (Evans and Ducot, 1994).
The evolution of weather radar in the United States has been marked by the development and implementation of a series of operational systems, including the CPS-9, the WSR-57, and the WSR-88D (NEXRAD). Each of these systems was a response to the recognition of new needs and opportunities and/or deficiencies in the prior generation radar. The CPS-9 (X-band or 3-cm wavelength) was the first radar specifically designed for meteorological use and was brought into service by the U.S. Air Force USAF Air Weather Service in 1954. The WSR-57 was the radar chosen for the first operational weather radar system of the NWS. It operated at S-band or 10-cm wavelength, chosen to minimize the undesirable effects of signal attenuation by rainfall experienced on the CPS-9 3-cm wavelength radar. The development of the WSR-88D was in response to demand for better weather information and resulted from advances in Doppler signal processing and display techniques, which led to major improvements in capabilities of measuring winds, detecting tornadoes, tracking hurricanes, and estimating rainfall. These remarkable new measurement capabilities were a direct consequence of many engineering and technological advances, primarily advances in integrated circuits, digital signal processing theory, and display systems, and these advances led to advanced research weather radars. Radar meteorology research has also played a critical role in these developments through the generation of new knowledge of the atmosphere, especially regarding cloud and precipitation physics, severe storm evolution, kinematics of hurricanes, and detection of clear air phenomena such as gust fronts and clear air turbulence. Such knowledge has greatly benefited the operational utility of weather radar, particularly through innovations, understanding, and testing of algorithms that process radar data into meaningful physical descriptions of atmospheric phenomena and weather con-
A complete list of acronyms and their definitions is provided in Appendix B.
ditions. It was the combination of technological advances with new scientific knowledge that enabled the deployment of the NEXRAD system and ensured its success as a highly valuable weather observing system.
This history of the national weather radar system and the multiplicity of factors that influenced the development of NEXRAD into its present form is necessarily brief. Most importantly, it does not do justice to the many persons who contributed to the current state of the nation’s NEXRAD system or to the numerous scientific and technological advances that have made the system (current and future) possible. It is not possible to adequately credit all those whose knowledge and skills have led to the current system. However, a number of recent review articles by Rogers and Smith (1996), Serafin (1996), and Whiton et al. (1998) provide a starting point for this analysis. Additionally, a number of books and monographs, including works by Battan (1959, 1973), Doviak and Zrnic (1993), Atlas (1990), Sauvageot (1992), and Bringi and Chandrasekar (2001), provide valuable insight. The American Meteorological Society (AMS) preprints of the Conferences on Radar Meteorology also provide a rich resource on related matters.
As was the case with prior generation radar, the WSR-88D has achieved many more goals than was anticipated at the time of its design. The WSR-88D was motivated largely by the needs for early severe storm detection and warning. In this regard it has proved to be remarkably successful (Serafin and Wilson, 2000) and has become the cornerstone of the modernized weather service in the United States (NRC, 1999). But many other important applications have emerged from experience with NEXRAD and through advances in the research community. Thus, needs and opportunities have expanded and limitations have been found (see Chapter 2). Among the primary new developments in recent years is radar polarimetry. This development allows for data-quality enhancements and improved accuracy in the determination of rainfall. This is consistent with the emphasis on quantitative precipitation estimation (QPE) and quantitative precipitation forecasting (QPF), which have been identified as one of the top priority goals in meteorology by both the U.S. Weather Research Program (USWRP) (Fritsch et al., 1998; USWRP, 2001) and the World Meteorological Organization (WMO) (Keenen et al., 2002). Another advance has been the measurement of air motion in the optically clear air, which provides important wind information fundamental to a variety of applications. A more recent development based upon the long-term behavior of precipitation systems (e.g., Carbone et al., 2002) emphasizes the climatic applications of NEXRAD data.
Moreover, it is no longer appropriate to use the radar network as a standalone system. One cannot overestimate the importance of using the radars as part of an integrated observing system. On regional scales, the combination of the primary radar with subsidiary radars, with satellite data, with automated meteorological measurements from aircraft, and with a network of ground-based meteorological instruments reporting in real time has led to advances in vital
nowcasting applications of severe weather. Such applications include improving the accuracy of severe local storm warnings (including forecasts of storm initiation, evolution, and decay), providing reliable guidance for construction activities, providing better information on current and future road conditions, furthering the needs of the aviation system for improving safety and operational efficiency (both civil and military), and helping individuals plan recreational activities.
The next generation of radars should be designed as part of an integrated observing system aimed at improving forecasts and warnings on relevant time and space scales.