storm detection

(meteorology) Any of the methods and techniques used to ascertain the formation of storms, including procedures for locating, tracking, and forecasting; special tools adapted to this purpose are radar and satellites to supplement meteorological charts and visual observations.

Identifying storm formation, monitoring subsequent storm evolution, and assessing the potential for destruction of life and property through application of various methods and techniques. Doppler radars, satellite-borne instruments, lightning detection networks, and surface observing networks are used to detect the genesis of storms, to diagnose their nature, and to issue warnings when a threat to life and property exists. See also Storm.

Radar surveillance

Radars emit pulses of electromagnetic radiation that are broadcast in a beam, whose angular resolution is about 1° with a range resolution of about 0.5 km (0.3 mi). The radar beam may intercept precipitation particles in a storm that reflect a fraction of the transmitted energy to the transmitter site (generally called reflectivity or the scatter cross section per unit volume). As the transmitter sweeps out a volume by rotating and tilting the transmitting antenna, the reflectivity pattern of the precipitation particles embodied in the storm is defined. Doppler radars also can measure the velocities of precipitation particles along the beam (radial velocity). Reflectivity and velocity patterns of the storm hydrometeors then make it possible to diagnose horizontal and vertical circulations that may arise within the storm, and to estimate the type and severity of weather elements attending the storm, such as rainfall, hail, damaging winds, and tornadoes. See also Doppler radar; Precipitation (meteorology); Radar; Radar meteorology; Wind.

Satellite surveillance

Since the early 1960s, meteorological data from satellites have had an increasing impact on storm detection and monitoring. In December 1966 the first geostationary Applications Technology Satellite (ATS 1) allowed forecasters in the United States to observe storms in animation. A Geostationary Operational Environmental Satellite (GOES) program was initiated within the National Oceanic and Atmospheric Administration (NOAA) with the launch of GOES 1 in October 1975. The visible and infrared spin scan radiometer (VISSR) provided imagery, which significantly advanced the ability of meteorologists to detect and observe storms by providing frequent-interval visible and infrared imagery of the Earth surface, cloud cover, and atmospheric moisture patterns.

The first of NOAA's next generation of geostationary satellites, GOES 8 was launched in the spring of 1994. GOES 8 introduced improved capabilities to detect and observe storms. The GOES 8 system includes no conflict between imaging and sounding operation, multispectral imaging with improved resolution and better signal-to-noise in the infrared bands, and more accurate temperature and moisture soundings of the storm environment. The Earth's atmosphere is observed nearly continuously.

Derived-product images showing fog and stratus areas from GOES 8 are created by combining direct satellite measurements, such as by subtracting brightness temperatures at two different wavelengths. GOES 8 shows the fog and stratus much more clearly because of its improved resolution. This capability enables forecasters to detect boundaries between rain-cooled areas having fog or low clouds, and clear areas. Such boundaries are frequently associated with future thunderstorm development. The sounder on GOES 8 is capable of fully supporting routine forecasting operations. This advanced sounding capability consists of better vertical resolution in both temperature and moisture, and improved coverage of soundings in and around cloudy weather systems. See also Cloud; Fog; Meteorological satellites; Satellite meteorology.

Surface observing systems

Larger convective storm systems such as squall lines and mesoscale convective systems can be detected (but not fully described) by the temperature, moisture, wind, and pressure patterns observed by appropriate surface instrumentation. Automatic observing systems provide frequent data on pressure, temperature, humidity, wind, cloud base, and most precipitation types, intensity, and accumulation. Analyses of these data, combined with improved conceptual models of convective storm systems, enable forecasters to detect and monitor the intense mesoscale fluctuations in pressure and winds that often accompany the passage of convective weather systems such as bow echoes, derechos (strong, straight-line winds), and squall lines. A bow echo is a specific radar reflectivity pattern associated with a line of thunderstorms. The middle portion of the thunderstorm line is observed to move faster than the adjacent portions, causing the line of storms to assume a bowed-out configuration. Other analyses of these mesoscale data fields aid the forecaster in detecting favorable areas for thunderstorm cell regeneration, which may produce slowly moving mesoscale convective storms attended by heavy rains and flash floods. See also Squall line; Weather observations.

Cloud-to-ground lightning detectors

Lightning location stations provide forecasters with the location, polarity, peak current, and number of strokes in a flash to ground within seconds of the flash occurrence. Useful applications have emerged with regard to the detection and tracking of thunderstorms, squall lines, other mesoscale convective systems, and the weather activity that accompany these phenomena, such as tornadoes and hail. See also Lightning; Mesometeorology; Sferics; Weather forecasting and prediction.