Radar Imagery

Forecasters at the SPC relied heavily on radar imagery to track the development, structure and movement of the severe thunderstorms and tornadoes that developed on this date. Radar, a staple of local television weather forecasts, is familiar to everyone. In fact, possession of the latest in Doppler radar technology is frequently used as a marketing tool by television stations attempting to distinguish themselves from their competitors.

Familiarity with radar imagery allows a meteorologist to form opinions about the nature of the storm based upon its appearance on radar. For example, supercell thunderstorms often present a hook echo that marks the location of the storm's mesocyclone. Not all storms with a hook echo produce a tornado, but its appearance alerts the forecaster to the potential. Bow echoes, features that can harbor damaging straight line winds and areas of circulation capable of producing tornadoes, are also easily identified with radar.

Doppler Base Reflectivity

The regional radar mosaic from 245 CST on February 16th (below) clearly shows the line of thunderstorms stretching from northwest Arkansas into central Illinois. Radar mosaics are created by combining images from several radar stations, and are included on the SPC's Mesoscale Analysis Page when severe weather threatens a region. Although the image does not include a scale, the area shaded in yellow and red represents those areas where the precipitation was most intense.

Regional composite reflectivity from the SPC at 2039Z (239 PM CST) on February 16, 2006.

The transmitter associated with a radar station emits a short powerful pulse (450,000 watts) of microwave energy. Objects, such as rain drops, hail, birds, etc., within the path of this energy pulse cause a small fraction of the pulse to be reflected back towards the radar station, where it is detected by the station's receiver. The object's distance from the station is calculated by determining the elapsed time between transmission of the energy pulse and the receipt of the backscattered energy. The strength of the returned energy is measured in decibels of Z (dBZ) with higher values associated with storms producing heavy rain and/or hail.

Base reflectivity radar image at 2034Z (234 PM CST) on February 16, 2006. Image from NCAR.

The base reflectivity image(above) from 234 PM CST on February 16th was produced by the St. Louis station, and shows a distinct line of thunderstorms stretching southwest to northeast that were nearly perfectly aligned with the cold front pushing southeastward. A cropped image from very near the station appears to show a possible "hook echo". A comparison of the radar image and the dBZ scale to the right of the image indicates that the most intense dBZ values associated with the storms were approximately 50 to 60 dBZ. This range of dBZ values suggests heavy thunderstorms with possible hail.

Although it allows a peek inside a storm, radar imagery should be used cautiously. Short-range base reflectivity has a maximum range of only 124 nautical miles. As the title on the above radar image indicates, the scan took place at 1/2 degree above the surface. This elevation, combined with the curvature of the Earth, results in the beam increasing in elevation as the distance from the station increases. Storms further from the station may appear weaker than those that are closer because the beam is sampling the storm at a much higher level.

The trajectory of radar beams can also be altered by differences in atmospheric density, resulting in the beam traveling a significantly different distance than anticipated. The distortion in a radar beam, however, can sometimes be useful in identifying small-scale frontal boundaries. In light of these known irregularities, it is prudent to analyze radar data from more than a single station. By comparing the images, it is possible to identify irregularities that might cause the forecaster to form an incorrect impression about the structure of the storm.

Doppler Base Velocity

The reference to "Doppler" in Doppler radar refers to the change in frequency of waves that occurs when either the source or the receiver of the waves is moving in relation to the other. Since radar stations are fixed, Doppler radar measures the movement of the targets responsible for backscattering the signal. In addition to discerning movement, meteorologists use Doppler velocity radar to identify the nature of the rotation of a storm's mesocyclone, the region of rotation often found in the right rear flank of a supercell thunderstorm.

How velocity radar discerns wind direction. From NWS Jetstream educational site.

Similar to base reflectivity, base velocity scans are executed at an elevation 1/2 degree above the surface. An examination of the base velocity image from 2034Z (234 PM CST) on February 16th (below) indicates the scale is in knots rather than dBZ, and confirms that the graphic is displaying motion. Positive values on a velocity radar image are those winds moving away from the station, while negative numbers represent wind blowing towards the site. Those areas shaded in dark purple indicate Range Folding (RF), a condition where the radar was unable to discern the wind direction.

Interpretation of velocity imagery can be difficult due to the manner in which it is created. The radar can only calculate that portion of the wind's direction that is moving along the path of the radar's beam. The graphic to the left displays the actual flow of the wind and how the radar interprets it. Since a velocity image displays the wind relative to the radar station, it is critical that the forecaster be aware of its location when utilizing this resource.

Base velocity products measure the total speed of the wind, therefore, the speed of the storm's movement is included. Storm relative radar images are particular helpful in identifying small scale circulations because the overall motion of the storm is subtracted from the wind field. In rare circumstances, a velocity image displays small adjoining areas where very strong winds are blowing both towards and away from the station -- the distinctive feature known as a Tornado Vortex Signature (TVS).

Base velocity radar image at 2034Z (234 PM CST) on February 16, 2006. Image from NCAR. Annotated to show the southwesterly winds and region of range folding near Springfield, IL.

The blue shaded areas on the base velocity image from 2034Z (234 PM CST) on February 16th (above) represent inbound wind speeds ranging from 20 to 30 knots, while the dark red areas display outbound wind speeds at 35 knots and above. When taken together, the combination of the inbound and outbound wind values suggest the wind was southwesterly at the time. The meteogram covering the period in question confirms this interpretation. The image also shows a broad area of range folding (areas of dark purple) to the northeast of St. Louis.

Like base reflectivity, velocity radar images are subject to distortion, and should be used cautiously. For example, the velocities calculated near the radar station are very near the surface. Further from the station, reported wind speeds are likely to be well above the surface and could lead to the conclusion that surface winds in the area were higher than actual. Even with these caveats, radar provides information regarding a storm's movement and structure that can not be gleaned in any other manner.

This project has examined only a few of the resources available to SPC forecasters when severe weather threatens. Of course, the point is not the production of snazzy graphics but rather saving lives by alerting the public to dangerous conditions. Technological advances in the collection, analysis and display of meteorological information have dramatically improved in recent years and have significantly lengthened the amount of advance notice associated with NWS warnings -- particularly for tornadoes.

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