Concentric Rings on Radar ImageryThe composite radar image below was produced at 2156Z on November 6, 2007 by the National Weather Service (NWS) site located at Caribou, Maine. The image has been annotated to highlight the series of rings of increasing diameter and enhanced reflectivity extending out from the radar site. The presence of these rings do not represent a rare or unusual form of precipitation. They are instead a result of stratiform precipitation and the methodology used by the radar station to scan the atmosphere and create the composite image.
Before delving into an explanation of the concentric rings, a quick summary of the weather conditions at Caribou, Maine on November 6, 2007 is in order. As shown on the surface analysis map at 18Z, Caribou was northwest of a warm front associated with an occluded low pressure system located over Ontario. The cold front extending from this system stretched from Maine all the way to Arizona's southeast corner.
The meteogram for November 6 indicates that precipitation began at Caribou at approximately 20Z and persisted beyond 0Z on November 7. In response to the steady southeasterly winds, the surface temperature increased from an early morning low of approximately 25° F to near 39°F by 18Z. The onset of precipitation initiated a slight lowering of the temperature to 37° F, where it remained until 23Z. It was a cold, dreary and uncomfortable day.
Volume Coverage Patterns
A discussion of composite imagery should begin with an overview of the scanning methodology used by radar stations. A volume coverage pattern (VCP) is the name given to a complete scan of the atmosphere by a radar station. Each VCP is comprised of a series of complete 360° rotations of the station's antenna at a variety of elevations. There are several VCP options available to NWS radar stations, and VCP 21 was used to create the annotated composite image above. The graphic below provides a cross-section of the elevation angles used in a VCP 21 scan.
As the VCP 21 cross-section graphic indicates, the elevation angles in a VCP 21 scan are approximately 0.5°, 1.5°, 2.4°, 3.4°, 4.3°, 6.2°, 10.0°, 14.0° and 19.5°. In order to understand the appearance of the concentric rings, two facets of the VCP should be discussed. First, the lowest five scan angles (0.5° to 4.3°) touch one another, providing a continuous scan through the lowest level of the atmosphere (from the surface to approximately 4.3 degrees). In contrast, the radar beams associated with the higher scans (6.2° to 19.5°) do not touch and therefore result in four discrete scans. In other words, when a VCP 21 is used the narrow slices of the atmosphere between these discrete scans are intentionally not sampled by the station.
The second feature of the VCP is the tendency of the radar beam to increase in altitude as the distance from the station increases. For example, under normal circumstances, at a distance of approximately sixty miles, the 10° scan would be sampling the atmosphere at a height of nearly seventy-thousand feet. The next highest scan, 14°, reaches seventy-thousand feet less than fifty miles from the station. At the extreme, the 19.5° scan reaches this level at a distance of little more than thirty miles from the station.
Composite Reflectivity Imagery
Although a VCP may contain as many as nine scan elevations, most Internet sources of imagery provide only base scans (0.5°) and composite imagery to visitors. Composite radar imagery is the result of combining data from multiple scan elevations into a single image. The advantage of composite imagery is its ability to present a more comprehensive assessment of the precipitation, but as demonstrated by our sample image from Caribou, there can be drawbacks.
|Mouseover the elevation angle to the right to display the radar image created from that particular scan. Radar images from NCDC.||0.5||1.5||2.4||3.4||4.3||6.0||9.9||14.6||19.5||Composite|
For a better understanding of the process, please refer to the interactive graphic above to examine the nine individual scans used to create the composite image at 2156Z on November 6, 2007. As the elevation of the scan increases, the diameter and overall width of the area of enhanced reflectivity (in shades of yellow) gets noticeably smaller. This tendency reaches an extreme in the 19.5° scan, which shows a ring with a radius of only a few miles and a very narrow band of enhanced reflectivity.
|Bright band graphic from Lyndon State College.|
Broad areas of stratiform precipitation, such as the one affecting Caribou on November 6, are recognized for their association with the radar feature known as bright banding. Bright banding is defined as an area of enhanced radar reflectivity corresponding with the atmospheric layer where snow melts to rain.
As shown on the graphic to the right, relatively slow-falling snowflakes begin to melt as they cross into a layer where the temperature is warmer than 0° C. The melting process starts from the outer edges of the snowflakes, so initially the melting snowflakes are surrounded by a thin layer of water. In this transitional phase, melting snowflakes appear as large raindrops to the radar, resulting in enhanced radar reflectivity. When the melting process is complete, the resulting raindrops are much smaller and fall faster than the melting snowflakes. The transition from the high reflectivity associated with gently-falling melting snowflakes and the relatively lower reflectivity attributed to faster falling raindrops results in the melting layer appearing "brighter" on radar imagery, hence the name bright band.
The atmospheric sounding from 0Z on November 7, 2007 (below), indicates the temperature was above 0° C from the surface to a height of approximately five thousand feet. Melting was the fate of snowflakes descending into this layer from colder regions above. The above freezing temperature ensured that the transition to raindrops was complete and that the precipitation remained in liquid form for the remainder of the journey to the surface.
Based upon the sounding and an understanding of bright banding, it appears the layer of enhanced reflectivity was located at an elevation of near five thousand feet.
Summary The weather pattern on November 6, 2007 was a classic scenario for the appearance of bright banding radar imagery. When combined with the discrete high elevation scans in a VCP 21, the result was a composite radar image that contained concentric bands of enhanced reflectivity.
A bright band appeared in response to each radar beam scanning the melting level, observed on the sounding to be at approximately 5,000 feet. As the elevation of the scan increased, this bright band moved progressively closer to the radar station. At the four highest elevations (shown below), not only did the diameter of the bright band move closer to the station, the discrete nature of the scans produced very narrow rings of enhanced reflectivity.
Ultimately, when the composite image was compiled, these bright bands from the highest scans were overlaid on the much lower "background" reflectivity present on the lowest level scans. The result was the dark yellow donuts surrounding Caribou, Maine.