Terminal Doppler Weather Radars (TDWR) were developed to detect hazardous wind shear conditions for aviation. There are currently 48 TDWR stations in the United States. They are generally located at or near major airports. TDWRs have a different, more limited product set than NEXRAD radars. Though their available products are similar to their NEXRAD counterparts, it’s important to understand that TDWRs are optimized for aviation applications and have some limitations compared to NEXRAD radars when viewing heavy precipitation over large areas.
Doppler radars work by bouncing radio waves off particles in the air. Those particles could be raindrops, hail, snow, or even dust and insects. The amount of energy that bounces off of those particles and returns to the radar is called “reflectivity” and is represented by the variable “Z”. Reflectivity covers a wide range of signal strength, from very weak to very strong, so it is measured on a decibel (logarithmic) scale in units of dBZ, or decibels of Z. The higher the dBZ value, the larger the number and/or size of the particles the radar beam is seeing.
The dBZ values increase as the strength of the signal returned to the radar increases. The scale of dBZ values is related to the intensity of rainfall. It is important to remember, however, that the radar shows only areas of returned energy and not necessarily precipitation. So the presence of a return, especially a very weak return below 20 dBZ, doesn’t always mean that it’s raining.
The colors along the bottom of the map correspond to precipitation types and intensities. When you move your cursor across the squares, RadarScope will display a value for each color. NEXRAD radars can’t distinguish between different types of precipitation with absolute certainty. However, reflectivity values can be somewhat roughly associated with different precipitation types:
• 10 dBZ (blue) – Very light rain or light snow
• 20 dBZ (green) – Light rain or moderate to heavy snow
• 30 dBZ (yellow) – Moderate rain or sleet showers
• 40 dBZ (orange) – Moderate to heavy rain or sleet showers
• 50 dBZ (red) – Heavy thunderstorms
• 60 dBZ (pink) – Intense to severe thunderstorms with hail
The TDWR long-range reflectivity product is similar to NEXRAD’s base reflectivity.
Reflectivity is the standard reflectivity product distributed in the TDWR Level III product suite. It’s the same data as long-range reflectivity but at a higher resolution and with a reduced coverage area.
Composite reflectivity combines data from all elevation scans, or tilts, to create a single product. The resulting image shows the highest reflectivity value from the vertical cross section at that location. Composite reflectivity can reveal important features in a storm’s structure that might not be seen in the base reflectivity product.
Because it combines data from all the tilts, the composite reflectivity product is one of the last to be produced during a volume scan. As with all NEXRAD products, it’s important to remember that the data displayed in the image depict conditions that have already happened rather than what is happening right now.
Doppler velocity products indicate storm motion toward or away from the radar. The velocity products in RadarScope use the Doppler effect to determine how fast the particles in the air are moving relative to the radar itself. Negative values (green in RadarScope) indicate motion toward the radar, while positive values (red in RadarScope) indicate motion away from the radar. They can be difficult to interpret without training and experience, but Doppler velocity products can be used to detect the overall movement of a storm as well as relative motion within the storm itself, such as rotation. Note that the radar can only detect the component of the velocity vector along the radar beam, so this isn’t a full picture of the wind field. But it gives you a fairly good idea which way a storm is heading.
The classic one-hour rainfall product estimates the amount of rain that has fallen over the past hour. It’s an older, lower-resolution alternative to the Digital Accumulation Array product.
Storm Total Rainfall
The classic storm total rainfall product estimates the amount of rain that has fallen for an arbitrary period determined by the radar operator. It’s an older, lower-resolution alternative to the Digital Storm Total product.
Vertically Integrated Liquid
Vertically integrated liquid (VIL) estimates the amount of water in a column of air. High values for VIL can indicate heavy rainfall or the presence of hail. When VIL values fall rapidly, it may indicate a downburst. VIL is subject to radar limitations and seasonal dependencies, so it’s a tricky product to interpret. The classic VIL product is a older, lower-resolution alternative to Digital Vertically Integrated Liquid.
The echo tops product shows the maximum height of precipitation echoes detected by the radar between 5,000 and 70,000 feet that exceed 18 dBZ. Higher echoes are often associated with stronger areas of a storm. This product is useful for identifying strong updrafts, and a sudden drop can indicate the onset of a downdraft. Some storms are too close to the radar for the beam to see the top, so echo tops is often underestimated for strong storms near the radar. The classic echo tops product is an older, lower-resolution alternative to Enhanced Echo Tops.