Frequently Asked Questions about Reflectivity

Answers

What Is Reflectivity Data And How Is It Gathered?

Reflectivity data shows the radar's signal reflected back to the radar by liquid and frozen droplets in the atmosphere. Base Reflectivity data is gathered at all elevation angles surveyed in each volume scan, but only four of these elevation angles, tilts 1 through 4, are available to users outside the National Weather Service.

Figure 2 shows the elevation angles (tilts) generally used by most NEXRAD sites. Elevation angles at some locations with unique geography may differ somewhat. For example, the Los Angeles, California NEXRAD is stationed at the top of a mountain. Tilt 1 for the Los Angeles radar will be between 0.00° and 0.40°.

What Is Base Reflectivity?

Personal AccuWeather provides access to Base Reflectivity Tilts 1-4 images for the entire network of 143 NEXRAD Doppler Radars in the Continental US and also San Juan, Puerto Rico. The format of the key on the right of the images is as follows:

Doppler Radar
Product (Base Reflectivity)
Date / Time
Site ID / Range
Tilt (Elevation Angle - 0.5 degrees)
Radar Mode
Intensity Key (Increasing Positive Numbers Are Stronger)
Maximum Reflectivity Echo Intensity (in dBZ) The Base Reflectivity, Tilt 1 product, reflectivity data gathered at the 0.50° elevation angle, is the product most like that available from conventional radar. The radar makes one 360° sweep at 0.50° above the local horizon, and the reflectivity data gathered from this sweep is what is displayed on the Base Reflectivity, Tilt 1 product. The elevation angles corresponding to Tilts 2 through 4 are listed in the table above. These are the same elevation angles as those used for the base velocity, Tilts 1 through 4 products.

Base Reflectivity, Tilt 1 shows the location and intensity of precipitation that is reaching the ground. That is the intensity of precipitation, be it rain or snowfall, that is being experienced at ground level. The higher elevation angles display the location and intensity of precipitation aloft which will, in most instances, eventually reach the ground (in situations where the lower levels of the atmosphere, near the ground, are eventually very dry, some of the precipitation aloft may evaporate before reaching the ground; this is known as virga).

Base Reflectivity, Tilt 1 is most useful for identifying where rain and/or snow are falling and how heavy the rain/or snowfall is. The radar does not delineate between rain and snow. The type of precipitation that is falling can be verified by noting the reports of surface observing stations within the coverage area of the radar.

dBZ levels correspond to levels of precipitation intensity in terms of inches of rainfall per hour. The higher the dBZ level, the more intense the precipitation ; they are directly proportional. "dBZ" stands for decibels of Z, Z being the reflectivity factor. The higher the reflectivity factor, the heavier the rainfall. This scale is logarithmic, so there is no factor one can use to multiply the dBZ level and arrive at the corresponding rainfall rate. Figure 3 shows approximate equivalents of dBZ and rainfall rates in inches per hour. Reflectivities in the range between 5 dBZ and 75 dBZ are detected when the radar is in precipitation mode. Reflectivities in the range between -28 dBZ and +28 dBZ are detected when the radar is in clear air mode. dBZ levels correspond to levels of precipitation intensity in terms of inches of rainfall per hour. The higher the dBZ level, the more intense the precipitation; they are directly proportional. "dBZ" stands for decibels of Z, Z being the reflectivity factor. The higher the reflectivity factor, the heavier the rainfall. This scale is logarithmic, so there is no factor one can use to multiply the dBZ level and arrive at the corresponding rainfall rate. Figure 3 shows approximate equivalents of dBZ and rainfall rates in inches per hour. Reflectivities in the range between 5 dBZ and 75 dBZ are detected when the radar is in precipitation mode. Reflectivities in the range between -28 dBZ and +28 dBZ are detected when the radar is in clear air mode.

Individual thunderstorm cells producing locally heavy rainfall and possibly hail can easily be identifies as small areas of very high dBZ level echoes and can be tracked by looping consecutive Base Reflectivity, Tilt 1 images in a time series. The radar does not delineate between thunderstorms and other non-thundering precipitation areas. Thunderstorms on NEXRAD Doppler radar can be verified as such, using lightning data. Areas of precipitation as indicated on radar that correspond with areas of lightning flashes as indicated in the lightning data for the same time period are indeed thunderstorms.

The 60 dBZ level would correspond approximately to 8 inches of rain per hour if all the precipitation were to fall as liquid rain. Sometimes there is hail falling in a storm with a reflectivity of 60 dBZ, so the precipitation that is falling is not all liquid. Above 60 dBZ, it is very difficult to arrive at a corresponding estimate for the rate of rainfall as there is likely hail mixed with the raindrops. Base Reflectivity, Tilt 1 shows the location and intensity of precipitation that is generally reaching the ground. That is the intensity of precipitation, be it rain or snowfall, that is being experienced at ground level. The higher elevation angles display the location and intensity of precipitation aloft, which will, in most instances, eventually reach the ground (in situations where the lower levels of the atmosphere, near the ground, have low relative humidities, the precipitation aloft may evaporate before reaching the ground). Base Reflectivity, Tilt 1 is most useful for identifying the location and intensity of rain and/or snow. The radar does not delineate between rain and snow. The type of precipitation that is falling can be verified by noting the reports of surface observing stations within the coverage area of the radar. Individual thunderstorm cells producing locally heavy rainfall, ice pellets, and possibly hail can easily be identified as small areas of very high dBZ level echoes and can be tracked by looping consecutive Base Reflectivity, Tilt 1 images in a time series. Since radar does not delineate between thunderstorms and other non-thundering precipitation areas, you can verify thunderstorms by using lightning data.

When looking at Base Reflectivity data, it is important to remember that as you look farther away from the radar site in any direction, the radar beam is looking higher in the sky. Close in to the radar site, the radar beam is looking very near the Earth's surface. However, due to the curved surface of the Earth, the Earth literally runs out from underneath the radar beam as you get farther away from the radar site, the result being that the farther from the radar site you go, the higher up in the atmosphere you are looking. Figure 4 below gives corresponding range and height coordinates for each Base Reflectivity tilt angle.

Figure 4: Base Reflectivity Tilt Numbers and Range Distance
Base Reflectivity Tilt # Height at 30 miles Height at 60 miles Height at 90 miles Height at 120 miles Height at 143 miles
Tilt 1 2,000 ft. 5,000 ft. 8,000 ft. 13,000 ft. 16,000 ft.
Tilt 2 5,000 ft. 10,000 ft. 16,000 ft. 24,000 ft. 28,000 ft.
Tilt 3 8,000 ft. 15,000 ft. 24,000 ft. 35,000 ft. 40,000 ft.
Tilt 4 11,000 ft. 20,000 ft. 32,000 ft. 46,000 ft. 52,000 ft.

These range distances can be drawn as concentric circles on the display, each with a radius corresponding to the ranges listed above. Everything appearing on the Base Reflectivity product in the vicinity of a range ring of this type is at the same height above the ground. For example, on the Tilt 1 product, everything appearing on the display at a range of 30 miles from the center of the image in all directions (at a 30 mile range ring) is being detected at 2,000 ft.

What Is Composite Reflectivity?

Composite Reflectivity is useful for viewing the broad pattern of precipita- tion surrounding the radar site. This product (along with Extended Reflectivity) has the largest range of any NEXRAD products, showing an area surrounding the radar site out to 286 miles. Composite Reflectivity is also useful in detecting thunderstorms in their development stage, before they produce precipitation. This can be seen when relatively high level precipitation echos appear on a Composite Reflectivity product before they appear on a Base Reflectivity, Tilt 1, product. In these cases, Composite Reflectivity shows the rain drops that are being held within the clouds by the thunderstorm's updraft before they become too heavy and start falling to the Earth as rain. The Composite Reflectivity product displays the highest reflectivity (the greatest precipitation intensity) recorded at any altitude above each location on the display. It is a composite of all the reflectivity scans at each elevation angle that a NEXRAD Doppler Radar makes. Therefore, the intensity of precipitation shown on this product is not necessarily the intensity with which the precipitation is reaching the ground, but is rather the potential intensity of precipitation that may reach the ground at some time in the very near future.

What are the Layer Composite Reflectivity Products?

The Layer Composite Reflectivity product displays the highest reflectivity recorded at any altitude within the layer of the atmosphere being displayed. It is a composite of all the reflectivity scans at each elevation angle that a NEXRAD Doppler Radar makes, divided up into three layers. The lowest layer displays reflectivities detected in the atmospheric tier between the Earth's surface and 24,000 ft. The highest reflectivity detected at any altitude within this layer is shown on the display. This product typically shows the location and highest intensity of precipitation reaching the ground. The middle layer shows reflectivities detected in the atmospheric tier between 24,000 ft and 33,000 ft and the highest layer shows reflectivities detected in the atmospheric zone between 33,000 feet and 60,000 ft. Again with both of these products, the highest reflectivity detected within the layer is what is shown on the display. These products show the location and highest intensity of precipitation within their respective atmospheric layers. They show reflectivities on what can be considered a constant altitude slice of the atmosphere. It is extremely useful to aviators as it shows flight level weather hazards. The reflectivities shown on the display reveal that precipitation is occurring within the atmospheric layer being shown. dBZ levels correspond to levels of precipitation intensity in terms of inches of rainfall per hour. They have the same meaning as they do for Base Reflectivity data.