In the pilot world there is a ubiquitous debate that continues to thrive over what ground-based radar product is better to use – NEXRAD composite reflectivity or NEXRAD base reflectivity from the lowest elevation angle. Without question, both of these radar mosaics provide a high glance value to the pilot to highlight the location and movement of the truly nasty adverse weather along your proposed route assuming you understand each of their inherent limitations. Now in ForeFlight Mobile 7.7, you’ll have the opportunity to wrangle over which is best since we’ve added a high resolution base reflectivity layer from the lowest elevation angle to complement the current composite reflectivity layer within the app.
But wait…there’s more! In addition to this new layer, we now offer two new low resolution NEXRAD mosaics, namely, a composite reflectivity and lowest elevation angle base reflectivity layer. These two four-color ground-based radar mosaics comply with the dBZ-to-color mapping standards defined by the Radio Technical Commission for Aeronautics (RTCA) documented in Table 3.2 of DO-267A. More on these later.
Base does NOT equal lowest
First, let’s squash a misnomer about base reflectivity. Many pilots (and even weather professionals) may use the term “base” in base reflectivity to imply lowest. That’s not what it means. In fact, every elevation angle generated by the WSR-88D NEXRAD Doppler radars has a base reflectivity product. The amount of energy directed back to the radar is measured and recorded in a logarithmic scale called decibels of Z (abbreviated dBZ), where Z is the reflectivity parameter. Next, these base data returns are processed by a radar product generator (RPG) to produce hundreds of meteorological and hydrological products including a few near and dear to pilots such as reflectivity.
A more accurate description would be to prefix the product with the elevation angle such as “0.5 degree base reflectivity.” Nevertheless, you may see labels like “Composite Reflectivity” and “Base Reflectivity” on various public and subscription-based websites including those from NOAA. It’s likely that the base reflectivity is from the lowest elevation angle (or lowest tilt) of NEXRAD radar. That’s because the lowest elevation sweep is most representative of precipitation that is reaching the surface which is helpful to the average person on the street including hikers, golfers, boaters and anyone else who wants to know if they need to take the umbrella to work. Unfortunately, the elevation angle is usually dropped (likely due to ignorance or brevity) from these labels.
You might be surprised to learn that in many locations across the U.S., the composite reflectivity image you study before or during a flight is largely made up of only three or four of the lowest 14 elevation scans of the radar. So in these areas the composite reflectivity and base reflectivity from the lowest elevation angle are not all that different. These areas include regions where the NEXRAD coverage is sparse. Which surprisingly doesn’t only occur in the western U.S. Places such as my home town of Charlotte, North Carolina have distinct gaps in radar coverage.
Radar to the max
Each NEXRAD radar makes multiple 360° azimuthal sweeps at increasing elevation angles from 0.5° to 19.5° depending on the current mode of operation. The number of elevation angles (or tilts) depends on the scanning strategy or Volume Coverage Pattern (VCP) of the individual radar which is set by the radar operator that is located at the local weather forecast office that monitors and manages that particular radar site. A composite reflectivity image considers the base reflectivity from all of the most recent sweeps at each elevation angle and shows only the maximum reflected energy in the vertical column above each location within the radar’s effective coverage area.
It’s all about range
With respect to ground-based radar, range or distance is the key. Even though the lowest elevation angle is only 0.5°, at 124 nautical miles away the center of the radar beam is already nearly 17,000 feet above the surface due to the curvature of the earth. So it is easy to see how the higher elevation angles may easily overshoot precipitation that is not in the immediate vicinity of a radar site. Moreover, even if the beam is low enough to see the storm, it may still overshoot the precipitation core. Let’s take a look at an example.
Below is a two-image animation from the NEXRAD located at the Greenville-Spartanburg Weather Forecast Office in Greer, South Carolina. This shows the returns received from the lowest elevation angle or lowest tilt of the radar which is 0.5° and the fourth elevation angle which is only 1.7° (remember that 19.5° is the maximum elevation). Notice the radar at the lowest elevation has identified an area of weather over Fayetteville, North Carolina (seen on the far right). This cell is approximately 150 miles away from the radar site in Greer (on the far left). However, given it’s distance from the radar, the 1.7° elevation scan completely overshoots this area of precipitation. That means the composite reflectivity image in the Fayetteville area is likely made up of only the lowest three elevation angles of the radar. The remaining higher 11 elevation angles overshoot the precipitation in this region.
Now it’s true that other adjacent radars such as the one from Raleigh Durham, North Carolina might be able to see this area of weather at higher elevation angles. However, due to the curvature of the earth, the radar beam from the highest elevation angles often overshoots much of the precipitation out there unless it is close to the radar site. This means that locations where there is little overlap between adjacent radars, expect the composite reflectivity image to be very similar to the base reflectivity image for the lowest elevation angle in these gaps.
The four-color radar
If you are flying with airborne radar, you may want to look at the new low resolution four-color NEXRAD mosaic now available in ForeFlight Mobile. The colors depicted in this radar mosaic match the standard color-to-dBZ mapping defined by the RTCA as documented in Section 3.8.2 (Table 3-2) of RTCA DO-267A (shown below). This standard is also used for airborne radar displays.
To see the four-color radar depiction, simply select one of the two radar layers on the Map view. Then tap the gear button next to the Map mode button and scroll down the Settings window until you see the setting switch labeled Four-color Radar just above the Radar Opacity slider. Tapping on the right side of this switch will change the radar depiction from the high resolution radar mosaic to the four-color mosaic. You can also find this four-color switch in the general Map View settings.
If you use the Stratus (FIS-B) to receive weather while in flight, you won’t find the capability to select the lowest tilt, but you will find the four-color radar will also be available for the composite reflectivity mosaic. As you can see below, the four-color radar mosaic (second image) provides a much more ominous depiction of the weather as compared to its higher resolution counterpart (first image).
The reason for this may not be obvious. The data broadcast for FIS-B radar does not specifically include the raw dBZ values. Instead it uses intensity encoded values or “bins” that map to dBZ ranges as shown in the table below. Notice the wide 10 dBZ ranges for intensity encoded values of 2 and 3. Based on the RTCA standard defined in the table above, these are mapped in the ForeFlight four-color radar to green and yellow, respectively. Red is mapped to intensity encoded values of 4 and 5 with magenta mapped to 6 and 7. Because of the wide ranges as they map to the RTCA standards, the four-color radar depiction from FIS-B will use much “warmer” colors than the standard depiction.
Keep in mind that the four-color radar mosaic is a low resolution depiction and will not emphasize storm characteristics like you may see with the Internet radar. This is especially true for the initial evolution of convective cells.