Chaff on Radar Imagery
The series of images below are Doppler radar base reflectivity scans from 1511Z to 1550Z on March 5, 2007 from the National Weather Service (NWS) Key West site. At first glance, it appears that a collection of unusually linear bands of precipitation were present over southern Florida and the Keys.
 |
 |
|||||||||
Doppler radar base reflectivity imagery from NWS Key West on March 5, 2007. Mouseover times to change images. |
||||||||||
|
1511Z |
1515Z | 1520Z | 1524Z | 1528Z | 1532Z | 1537Z | 1541Z | 1545Z | 1550Z | |
As you pass your mouse over the times of the various radar images, you will note that the overall shape of the blobs of reflectivity remains remarkably consistent as each drifts in an easterly direction over the Keys and southern Florida.
A review of the HPC surface analysis valid at 12Z on March 5, 2007 shows an absence of frontal boundaries or other precipitation triggers near south Florida. The meteogram (below) confirms that only scattered clouds and gusty, northerly winds were observed at Key West during the time of these radar images. No precipitation was present. The meteogram for Miami indicates more cloud cover, but confirms the absence of precipitation.
|
Meteogram from March 4, 2007 from the University of Wyoming. No precipitation was observed during the entire twenty-four period. |
Although the meteograms above summarize the conditions at only two specific observation points, when combined with the surface analysis and visible satellite imagery it seems very likely that no precipitation fell over south Florida between 15Z and 16Z on March 5, 2007. If not precipitation, then what explains the unusual features on the radar images?
Chaff
The answer lies not with meteorology, but with
aircraft based at the U.S. Naval
Air Facility located near Key West, strategically known as the Gibraltar
of the West. Military aircraft are vulnerable to radar and heat-seeking missiles launched from ground-based sites and
from other aircraft.
Some aircraft, such as the B-2, attempt to
avoid detection by radar- and infrared-based early-warning systems
using unusual designs, composite materials, and special
coatings, collectively referred to as "stealth" technology. In its role
as a heavy
bomber, the B-2 also avoids anti-aircraft technology by operating at very high altitudes.
|
More traditional aircraft lack such stealth technology and must rely upon defensive countermeasures. Primary among these is chaff, a fibrous substance composed of glass fibers and aluminum packed into small bundles. The pilot launches the bundle into the aircraft's slipstream, where it opens and releases nearly five million small fibers (1 to 2cm in length) into the atmosphere forming a highly-reflective masking "cloud."
The dispersal of the chaff is intended to confuse a missile's radar guidance system by creating a more attractive target than the plane that released it. Multiple aircraft acting in unison can create a chaff curtain to protect a much larger air space.
Since the rapid dispersal of the chaff cloud would be counter-productive, the material is designed to linger in the atmosphere for an extended period. Falling at a rate of approximately ten meters per minute, chaff dispersed at an altitude of ten thousand feet doesn't reach the ground for nearly ten hours. The fibers are also highly resistant to breaking apart during descent helping to prolong their protective nature.
Summary
Designed to disrupt radar guidance systems by flooding the atmosphere
with an abundance of long-lived targets, it is no surprise that chaff is highly
reflective to Doppler weather radar systems. The proximity of the Naval Air
Station both to Key West and Miami results in the regular appearance of
chaff on radar imagery from these sites. In this particular instance, the abundance of chaff was
colorfully mentioned
in the Area Forecast Discussion (click here for full version) issued by the NWS Key West Office
at 9:55pm on March 5, 2007.
An interesting aspect of the chaff clouds is their easterly motion in an environment characterized by northerly surface winds at both Miami and Key West. For the explanation, we will examine the winds above the surface by referring to the 12Z sounding (below) at Key West. For those unfamiliar with sounding data displayed on a SkewT-LogP diagram, a primer can be found here.
 |
Sounding from 12Z on March 5, 2007 from the Plymouth State Weather Center. |
An examination of the wind barbs on the 12Z sounding (click here for close-up) indicates that the wind between 825mb and 750mb -- approximately 1750 to 2500 meters above the surface -- was from the west-southwest in the range of ten to twenty knots. More precise observations of the winds aloft are available by viewing the sounding data in tabular format. If the chaff fibers were floating at this level, they would naturally drift in a leisurely manner to the east northeast.
It is relatively easy to determine the height of the objects backscattering energy towards the radar's antenna by determining the distance of the objects from the radar site and the elevation of the beam being examined, and then reviewing this information against the natural tendency for the height of the beam above the ground to increase as the distance from the radar site increases. (Additional information on the nature of radar beams may be found here.) Nexrad software packages, such as GRLevel3, and Internet radar sources readily provide users with the beam height.
In this instance, GRLevel3 indicated that the chaff cloud over Key Largo was sixty miles from the radar site and approximately six thousand feet (1828 meters) above the surface. A quick check of the sounding data confirms that this height was very close to 825mb, where the wind was observed to be from a west southwesterly (250°) direction at eight knots. The previously mentioned difference in the direction of surface winds and the movement of the chaff cloud is not at all unusual. Even at distances very close to the radar station, the height of the beam is well above the level (10 meters) at which surface winds are measured. While it is possible to infer wind direction by watching the movement of radar echoes, this ability does not extend to surface winds.
It is clear that precipitation is not the only thing that appears on Doppler weather radar. But a quick review of meteorological data and familiarity with the local environment (the presence of the Naval Air Station) allows the puzzle to be solved.