NEXRAD Doppler radar - What's a negative dBZ?


Of all of the weather products available to pilots prior to a flight, the images produced by the NWS WSR-88D NEXRAD Doppler radars are likely the most widely used. These images have an extremely high glance value and are packed with important life-saving guidance assuming that a pilot knows how to interpret all the pretty colors.


The colors you see on the NEXRAD image displayed by your favorite website depends on many factors. Unfortunately, accepted standards do not exist per se. The private organization, vendor or government agency is free to map the data (reflectivity) to colors of their choosing. Depending on their operational requirements, they may use three colors representing light, moderate or heavy precipitation or they are free to use 30 different colors. So it is important to always reference the scale (such as the one shown below on the left) that is normally located somewhere on the image or around the image border.


More importantly, there are many kinds of images and products that you may encounter. Therefore, knowing the kind of radar image you are viewing is also paramount. The image may display base reflectivity from a single NWS NEXRAD radar that is in clear air mode. Or it may be one of the volume products such as composite reflectivity or echo tops. Another likely possibility is that the image may represent a radar mosaic that has combined the base reflectivity or composite reflectivity data from multiple NEXRAD sites into a regional or national image.


While forecasters at the various NWS local Weather Forecast Offices (WFOs) do have real-time access to all of the data, the "raw" data that is generated by the WSR-88D NEXRAD Doppler radars is not distributed directly to other government and private organizations in real time. Therefore, a NEXRAD Information Dissemination Service (NIDS) has been established that includes only a subset of the entire WSR-88D base and derived products for use by external users. Below is the RPG console that you'll find at every WFO that manages a WSR-88D.

One of the most ubiquitous base products is called base reflectivity. Note that the term base does not mean lowest as most pilots assume. It comes from the term "base data" since every elevation scan has a base reflectivity product (note in the image above that says "Base Data Display"). How you interpret the base reflectivity image will depend on the mode of the radar.


The WSR-88D radar operates in one of two modes: precipitation and clear air. The main difference between the two is that clear air mode offers the advantage of greater sensitivity due to a slower antenna rotation rate, which allows more energy to be returned back to the radar. However, clear air mode takes twice as long to generate a product so it suffers from temporal resolution, but is able to detect smaller objects in the atmosphere such as light snow or drizzle.


The radar sends out a known pulse of energy in the microwave band (10 to 11 centimeter wavelength). Some of this energy strikes airborne objects referred to as hydrometeors. This includes rain, snow, hail, dust, birds, insects, etc. and the power returned is referred to as the reflectivity parameter or Z. In basic terms, Z is the density of water drops (measured in millimeters raised to the sixth power) per cubic meter of air. As you might expect, there is a very wide range of possible Z values. Converting Z to dBZ (decibels of Z) makes that range smaller and easier to use.

When the radar is in precipitation mode, the range of dBZ values displayed can be as low as 5 to a maximum of 75, whereas clear air mode offers a range from -28 to +28. The reason negative dBZ values can occur in clear air mode is because the dBZ is a logarithmic function. So an increase of only 3 dBZ actually represents a doubling of power returned!  Anytime Z is less than 1 mm6/m3, dBZ becomes negative. Negative dBZs are only found when the radar is in clear air mode such as shown above. This radar image shows light snow falling around the Bismarck, North Dakota area.  Given that light snow is falling, the radar operator needs the radar in its most sensitive mode, namely, clear air mode. Notice the negative dBZ values in the dark taupe color.  If the radar were in precipitation mode, the amount of coverage would be limited to the blue areas.  


A negative dBZ means that the radar is detecting very small hydrometeors.  As mentioned above, this is great way for forecasters to detect very dry light snow or drizzle which have lower reflectivities.  It may also be useful to detect outflow boundaries and drylines.


One of the disadvantages of clear air mode is that any dBZ value under 10 gets filtered by the datalink weather.  The NEXRAD clear air mode image above from Bismarck shows a rather wide area of precipitation.  However, the XM weather image shown below only includes the returns that are greater than 10 dBZ.  The areas shown in the taupe color have been effectively filtered out due to their lower dBZ values. 


Most pilots are weatherwise, but some are otherwise™


Scott Dennstaedt

Weather Systems Engineer

CFI & former NWS meteorologist

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