Echo Tops on Doppler Radar Imagery
Radar is an excellent short-term forecasting tool, particularly during outbreaks of severe weather. The wide variety of products available from the National Weather Service's (NWS) Doppler Radar network provide forecasters with the critical resources they need to issue timely warnings in life-threatening situations. Dramatic improvements in warning lead-times for tornadoes over the past twenty years can largely be attributed to advances in Doppler radar technology and the development of new radar-derived products. In addition to determining the location and movement of thunderstorms, some of these new products, such as echo tops, allow forecasters to "peer" inside storms to assess the potential for transition of the convective mode and intensity.
Radar Imagery Types
The images from the squall line that ravaged northern Illinois on June 23, 2010 (below) are examples of the most familiar and readily-available Doppler radar products -- base reflectivity and base velocity. Both images were produced by KLOT, the NWS radar site located southwest of Chicago. Other Doppler products are more obscure and harder to locate on the Internet. Echo top imagery, a representation of estimated relative storm height, falls into this category.
Echo Top Imagery
Echo top imagery is derived from reflectivity data and displays an estimate of the highest altitude where the value of dBZ exceeds a specified threshold. The threshold adopted by the NWS and most providers of radar imagery on the Internet is 18.5 dBZ. Please note that care should be taken to check the dBZ threshold when viewing this type of imagery. A comparison of the 2302Z image (below), and the color scale at the left-hand side, indicates that the highest echo tops were approximately 50,000 feet and were located along the leading edge of the squall line to the southwest of the radar station (KLOT).
Echo tops should not be confused with the actual height of the storm. Storm height is higher, as illustrated on the cross-section of composite reflectivity below. Based upon a dBZ value of zero, the top of the storm measured approximately 55,000 feet. The threshold value of 18.5 dBZ was located in the dark-to-light-blue region at an approximate height of 52,300 feet -- 3,000 feet below the maximum height of the storm. As shown on the storm cross-section, dBZ values within a storm typically decline with increasing height. There are exceptions, such as the bright purple area at approximately 16,000 to 21,000 feet, that most likely signaled the presence of hail in the storm's updraft. However, the high reflectivity values associated with the hail don't alter the fact that dBZ values decreased as heights increased as represented on the image by the transition from warm to cool colors.
In addition to measuring the intensity of the backscattered energy (dBZ), the station also calculates the distance and height of the object responsible for the backscattering. This data, in combination with the tendency for dBZ values to decrease with height, allows for the creation of echo top imagery. As the radar station scans the mid- and upper-levels of the storm, it encounters generally decreasing values of dBZ. Eventually, the entire depth of the storm is scanned and the height where the dBZ value fell below the threshold (NWS=18.5 dBZ) is recorded as the echo top of the storm.
Currently (August 2010), two versions of echo top products were in use at NWS radar stations. The legacy product (ET 41) is produced by a coarser algorithm that results in echo top imagery where heights are reported in 5,000 foot increments. Digital echo tops (EET 135), a higher resolution product capable of calculating heights in 1,000 foot intervals, results in imagery that has smoother and more realistic height transitions. The sample imagery that appears in this article is the digital echo top product.
A correlation exists between the height of a storm and its intensity, and echo tops imagery allows forecasters to focus their attention on the portion of an approaching system where the risk of property damage and personal injury is the greatest. A rapidly decreasing echo top may also signal the formation of a destructive downdraft. Unfortunately, the usefulness of echo tops imagery is regularly diminished by the character and behavior of radar beams.
The close-up version of the Echo Top imagery (above) shows four concentric rings surrounding the radar site. The inner-most ring shows an absence of color suggesting echo tops of zero feet. The remaining three rings suggest storms possessing echo tops of a consistent height, and show a stair-step transition in echo top height occurring where the color of the ring abruptly changes. The echo top data within the four rings varies dramatically from the data further from the site and is inconsistent with basic thunderstorm structure. A check of the Echo Top imagery produced by the Davenport, Iowa site (KDVN) proves that storm heights were much higher near KLOT. The explanation for the unrealistic echo tops near KLOT lies with how radar beams sample the atmosphere.
NWS radar sites sample the atmosphere at several distinct angles over a period of several minutes, a routine known as a Volume Coverage Pattern (VCP). The NWS uses a variety of VCPs, depending upon the type of weather that is being sampled. KLOT was using VCP 212 as the June 23, 2010 squall line passed through northern Illinois. Containing fourteen individual scan angles (from .5° to 19.5°), VCP 212 completes an entire scan of the atmosphere every 4.5 minutes. Owing to the concentration of scans at the lowest levels, VCP 212 is typically used when severe weather is both widespread and rapidly evolving. Squall lines fit this description perfectly.
Regardless of the angle at which the radar beam is transmitted, the height of the beam above the ground increases as distance from the site increases. Because they are aimed at a higher angle from the beginning, higher elevations scans reach loftier altitudes at significantly shorter distances than beams transmitted at lower angles. For example, the typical .5° beam reaches a height of approximately 18,000 feet when it is 120 nautical miles from the station, while the 19.5° scan reaches the same height in less than 10 miles.
In addition to increased height with distance, radar beams also spread as they travel, thereby reducing resolution as distance increases. In the case of echo tops, this beam widening may result in height errors as large as 5,000 feet. The combination of these beam dynamics should foster skepticism among forecasters as storms may be dramatically different than they appear on radar.
Cone of Silence
When VCP 212 is in use, the region of the atmosphere above 19.5° is not scanned. A reflectivity cross-section through the June 23, 2010 squall line (below) distinctly shows a large funnel-shaped data void, known as the cone of silence. The radar station is located in the middle of the image. It is this cone of silence, combined with the geometry of the 19.5° scan, and the methodology used to render the imagery that produced the odd rings surrounding KLOT on the close-up echo top imagery above.
GR Level 2, a powerful software program for processing radar imagery, allows a user to determine the approximate distance from the radar site and height above the surface of a radar beam. The two images below have been annotated with this information. On the left, a scale has been added to show the distance of the concentric rings around the radar site from 5 to 20 nautical miles. The image on the right indicates the echo height values beginning at 10,000 feet and ending at 40,000 feet. It should be noted that the echo heights steadily increase along the scale at intervals of 1,000 feet.
The reflectivity cross-sections below display a slice of the atmosphere beginning at KLOT and extending southwesterly through the leading edge of the squall line. The image on the left was produced from data at KLOT, while the image on the right was produced from data collected at Davenport, Iowa (KDVN). The distance scale on both images are in nautical miles and begin at KLOT.
The southwestern half of the unsampled cone of silence above the 19.5° scan is readily apparent on the imagery from KLOT (above left). In contrast, the image from KDVN (above right) shows that the mid- to upper-atmosphere above KLOT was well sampled, with echo tops reaching approximately 42,000 feet. (The lowest levels were not sampled by KDVN due to the distance from the radar site.) A side-by-side comparison of these images confirms that the geometry of the 19.5° scan is predominantly responsible for the uniform echo top donuts surrounding KLOT.
A review of the reflectivity cross-section from KLOT (above left) indicates that at approximately 5 nautical miles from KLOT, the 19.5° scan is unable to sample the atmosphere above approximately 10,000 feet. This trend continues at roughly 10,000 foot intervals for each additional 5 nautical miles traveled from the radar site. At approximately 20 nautical miles, the maximum height of the scan reaches nearly 40,000 feet.
In contrast to the steady decrease in dBZ with height associated with a fully sampled region, the dBZ abruptly reaches zero in the unsampled area above the 19.5° scan. For example, at 5 nautical miles from KLOT and at approximately 10,000 feet, the value of dBZ quickly changes from a range of 40-50 to zero. Since the value dropped below the echo top threshold of 18.5 dBZ, the echo top algorithm assumes the radar beam has sampled the upper-level of the storm and records an echo top of approximately 10,000 feet -- the echo top value associated with the inner limit of the first concentric circle around KLOT. This value is well below the 42,000 feet suggested by the KDVN imagery at the same time. The erroneous echo top values increase as distance from the radar site increases, however they still fall well below the true height of the 18.5 dBZ threshold.
The concentric rings on the echo top imagery are simply an artifact of the radar station's inability to sample the upper portions of a storm near the site. Generally, echo top imagery displays unreasonably low heights when the storm is within 25 nautical miles of the radar site. The solution to this problem is to simply refer to a neighboring station.