Weather radar is a sophisticated instrument that allows meteorologists to peer inside a storm, detecting precipitation and the wind’s invisible motion. The technology used today is primarily Doppler radar, which transmits pulses of microwave energy into the atmosphere. When this energy encounters precipitation particles, a small fraction scatters back to the radar antenna. Analyzing the returning signal allows meteorologists to determine the storm’s structure and identify specific signatures indicating a forming or existing tornado.
Understanding Reflectivity and Velocity Data
The foundation of modern radar interpretation rests on two fundamental data types: reflectivity and velocity. Reflectivity measures the intensity of the signal returned to the radar, showing where precipitation exists and how heavy it is. This is typically displayed using a color scale where blues and greens represent lighter rain, while reds and purples indicate heavier precipitation, often suggesting large hail. Higher reflectivity values, measured in decibels of Z (dBZ), mean a greater concentration of targets like raindrops or hailstones.
Velocity data utilizes the Doppler effect to measure the movement of precipitation particles relative to the radar site. This product allows meteorologists to measure the wind within a storm. The color scheme typically uses shades of red to represent wind moving away from the radar and shades of green or blue to show wind moving toward the radar. The intensity of these colors corresponds to the speed of the wind, providing a direct measurement of atmospheric motion.
The Classic Reflectivity Signature
The first visual sign of a potentially tornadic storm on the reflectivity product is often the “Hook Echo.” This signature appears as a curved appendage extending from the main body of a supercell thunderstorm. It forms because the strong, rotating updraft, called a mesocyclone, wraps precipitation around itself. The resulting circulation draws rain and hail, creating this distinct, comma-shaped pattern on the radar screen.
The location of the tornado, if one is present, is usually found near the tip of this hook feature. Another related, vertically oriented signature is the Bounded Weak Echo Region (BWER), sometimes called a vault. This appears as a small area of low reflectivity surrounded by much higher reflectivity aloft and to the sides. The BWER marks the location of the storm’s powerful updraft, where vertical wind is so strong that precipitation cannot fall through it.
Unmasking Rotation Using Velocity Data
While the hook echo suggests a strong rotating storm, definitive proof of a tornado threat comes from examining the velocity data. Meteorologists look for a “Velocity Couplet,” which is the visual manifestation of intense rotation. This couplet is characterized by a small, adjacent pairing of bright red and bright green pixels. The bright red indicates air moving rapidly away from the radar, and the bright green indicates air moving rapidly toward the radar.
When these opposing wind vectors are tightly packed together, it reveals strong wind shear, the hallmark of a spinning column of air. A larger, broader couplet represents a mesocyclone, the storm-scale rotation that precedes a tornado. When the red and green pixels tighten into a very small, intense core, it is classified as a Tornado Vortex Signature (TVS). The TVS pinpoints a tight circulation that is highly likely to be a tornado or one about to form.
The tighter the couplet, the more intense the rotation is, increasing the likelihood of a tornado reaching the ground. Forecasters estimate the wind speed within the circulation by calculating the difference between the maximum inbound and outbound velocities. Detecting a persistent, low-level TVS over several scan updates is strong evidence for issuing a tornado warning.
The Final Confirmation of a Tornado
Dual-Polarization (Dual-Pol) radar provides objective confirmation that a tornado is causing surface damage. Dual-Pol systems send out both horizontal and vertical radio waves, allowing the radar to analyze the size and shape of the targets it detects. One specific product is the Correlation Coefficient (CC), which measures the uniformity of the targets within the radar beam.
A high CC value, typically near 1.0, means the radar is seeing uniform targets, such as spherical raindrops. Conversely, a sudden drop in the CC value to below 0.8, co-located with the tight velocity couplet, signifies the presence of non-meteorological debris, known as a Tornado Debris Signature (TDS). The low CC value occurs because the tornado has lofted a chaotic mix of irregularly shaped objects—like trees, shingles, and soil—into the air.
The detection of a TDS provides definitive proof that a tornado is on the ground and causing damage. While a strong velocity couplet suggests a tornado is imminent, the TDS confirms the circulation is interacting with the ground and lifting debris. This signature is valuable in low-visibility conditions, such as at night or when the tornado is obscured by heavy rain.
Caveats and Look-Alikes
Interpreting radar data is challenging, as some non-tornadic phenomena can mimic severe weather signatures. One issue is “ground clutter,” which appears near the radar site as non-moving, low-level returns caused by the radar beam hitting stationary objects. These returns can contaminate the lowest-level scans, making true low-level rotation difficult to discern.
Another limitation is the increasing height of the radar beam as it travels farther away due to the curvature of the Earth. At long distances, the radar cannot sample the atmosphere close to the ground, potentially missing a weak or shallow tornado. Furthermore, not every Hook Echo or velocity couplet produces a tornado; strong circulations aloft can exist without tightening down to the surface.