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Creation of the mosaic
Removal of nonprecipitation echoes
Limitations on accuracy
This radar mosaic is created twice per hour, from observations taken at WSR-88D sites around 00:15 and 00:45. The mosaic is composed of the highest observed reflectivity category within map grid boxes approximately 10 km (~5 nautical miles) on a side. It is designed to give you an overall picture of the position, movement, and evolution of precipitation on a synoptic scale. The National Weather Service also distributes this mosaic in digital GRIdded Binary (GRIB) format under WMO message header HAXA00 KWBC.

The reflectivity categories indicate the density of precipitation backscatterers (raindrops, snowflakes, hailstones, or ice pellets) within the lowest 10,000 feet of the atmosphere. Reflectivity and approximate surface rainfall rates for each category are...

Level dBZ Rain Rate (in/hr)
0 <15 0 or Trace
1 15-29 0.01 to 0.09
2 30-39 0.1 to 0.4
3 40-44 0.5 to 0.9
4 45-49 1.0 to 1.9
5 50-54 2.0 to 3.9
6 55> >4

These rainrates are approximations that hold only as long-term averages. Also, there is presently no reliable way to interpret reflectivity in terms of snowfall rate in cold-weather conditions. However, in any region in any given situation, higher reflectivity generally indicates a higher rate of precipitation at the surface.

Gray areas on the graphic are those that are more than 230 km (approximately 120 nautical miles) from the nearest reporting radar, or which are shadowed by terrain features from any reporting radar.

Though the WSR-88D detects reflectivity at much higher spatial and temporal resolution and with greater precision than that appearing in the Radar Coded Message, the reduced data format was chosen due to communications limitations present during the early part of the radar network's life cycle, in the late 1980's and early 1990's.

Creation of the mosaic
Individual radars transmit Radar Coded Messages (RCMs) twice per hour. The RCM contains a coded text summary of reflectivity features in the local area, along with information on convective storms and the vertical wind profile near the radar. The reflectivity data have already been navigated to a fixed national map grid.

The individual RCMs are composited into one map grid at NWS headquarters. For grid boxes in which reflectivity is detected by more than one radar, the highest reported value is put into the final product.

Removal of nonprecipitation echoes
Weather radars generally detect many nonprecipitation features, including echoes from terrestrial objects, aircraft, birds, and insects. In the WSR-88D, the reflectivity data put into RCM's has received automated quality control that generally removes ground clutter echoes near the radar, and anomalous propagation (AP) echoes that appear when atmospheric temperature inversions cause return from the earth's surface itself. However, aerial targets generally remain.

Most of these nonprecipitation echoes are detected and removed from the mosaic by checking for the presence of precipitating clouds in the vicinity of the echoes, utilizing infrared satellite data and relative humidity information from rawinsondes. Echoes from migrating birds and insects can be recognized as circular patterns of level 1 reflectivity centered near radar sites. Echo features failing the infrared satellite check or resembling features commonly due to birds or insects are removed before the mosaic is disseminated.

Limitations on accuracy
Because each grid box in the mosaic contains the highest reflectivity observed within that box, precipitation areas which appear continuous in this graphic may actually contain many "holes" in the precipitation field. Light snow often returns too little radio energy to appear in this mosaic, which has a minimum threshold of 15 dBZ. Moderate and heavy snow generally do appear.

Despite the automated quality control described above, nonprecipitation features sometimes appear in the disseminated mosaic. These are most often due to migrating birds, and appear during nighttime in the spring and late summer through autumn. More rarely, real precipitation features are removed.

 National Weather Service, NOAA
 1325 East-West Highway
 Silver Spring, MD 20910
Page Authors: Dennis R. Cain and Paul Kirkwood 
Last Modified: January 13, 2003