Single-site NEXRAD base reflectivity data are updated every 5, 6, or 10 minutes, depending on whether the radar
is in normal precipitation
precipitation mode, or clear air mode. The same NEXRAD base reflectivity information (excluding clear air mode data) is incorporated into the WSI NOWrad national and regional mosaics, except that the NOWrad images go through rigorous quality control procedures that remove most false echoes caused by ground clutter and anomalous propagation (AP). Caution must be exercised in attempting to interpret the single site NEXRAD images because they often contain 'artifacts' that are not actual precipitation echoes.
Base Reflectivity is a measure of the intensity of precipitation occurring, and is reported in units of DBZ. The WSR-88D emits pulses of energy into the atmosphere at regular intervals. When this energy impacts something (i.e. a raindrop
, a snowflake
, a mountain, etc.), some of the energy is scattered back to the radar dish. The amount of energy which is received back at the radar dish is measured in units of DBZ (decibels). The higher the DBZ value the larger the object. Large raindrops
and hail, for example, produce high DBZ values. In general, DBZ values greater than 15 indicate areas where precipitation
is reaching the ground; DBZ values less than 15 usually are an indication of very light precipitation
which in most cases is evaporating in the atmosphere before it reaches the ground.
NEXRAD or Nexrad (Next-Generation Radar) is a network of 158 high-resolution Doppler weather radars
operated by the National Weather Service, an agency of the National Oceanic and Atmospheric Administration (NOAA) within the United States Department of Commerce. Its technical name is WSR-88D, which stands for Weather Surveillance Radar, 1988, Doppler. NEXRAD 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 radar system 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 time for tracking active weather. NEXRAD has an increased emphasis on automation, including the use of algorithms and automated volume scans.