The radar that is used by most National Weather Service offices is called the NEXRAD WSR-88D. It was built in 1988 and operates in over 160 locations now.

What makes WSR-88D unique is that it transmits at 750,000 watts and can operate at 14 different elevations (”tilts”). It can go from 0° to 60° but usually maxes out around 19.5° for operational use. Things that can be seen on radar can be moisture (rain/ice), birds/insects, debris, wind turbines, and airplanes.

You can read more about WSR-88D here: https://www.weather.gov/iwx/wsr_88d

Important radar principles to understand

1. The radar sends out a pulse of a radio wave. If that radio wave encounters something in its path, it will reflect a radio wave back to the radar. This response is called an Echo.
2. A Radial is the angle or degree of the beam relative to center. As the radar spins around, it makes a complete 360° rotation. In a horizontal plane, Radial is equivalent to the heading on a compass.
3. A radar beam acts similar to a beam of light from a flashlight. The farther away the beam gets from the radar, the wider the area of coverage will be. However, this also results in a decrease of resolution.
4. Each angle that a radar sends out a signal at is called a Tilt.

IMPORTANT: Factors like radar tilt and distance from the radar will impact what objects the radar can detect.

Let’s say the radar is looking at a storm off in the distance. Below are examples that show the impact of tilt and distance.

• After increasing the tilt, the beam will now see a different (higher elevation) part of that storm.
• After increasing the tilt enough, eventually the beam will overshoot the storm.
• If the tilt was kept the same, but the storm moves farther away from the radar, the beam will start seeing higher (elevation) parts of the storm
• If the tilt was kept the same, but the storm moves closer to the radar, the beam will start seeing lower (elevation) parts of the storm

These principles and factors are important for understanding what the radar is able to see and what the information being provided can tell you.

IMPORTANT: As the beam gets farther from the radar, it does not just measure straight out horizontally, it also increases height.

Base Radar Products

Reflectivity

Measures the diameter and density of objects in the scanned area areas. Values are measured in Decibels Relative to Z (dBZ). This is helpful for determining things like intensity and structure. There are two types of Reflectivity: Base and Composite.

Base Reflectivity (Z) – reflectivity at a single tilt elevation. This is is helpful for understanding where precipitation may exist in the atmosphere. Is rain primarily low to the ground? Is it elevated? Which part of the storm is the strongest?

Composite Reflectivity – (technically, not a “base” product) a blend of all tilts, showing the max reflectivity for that area from any angle. This is helpful for seeing overall intensity of a storm and how it is trending. However, this product has 1/4 the resolution of Base Reflectivity…and due to Principle #2 above…it isn’t as accurate at detecting precipitation at longer distances (over 286 miles). It is also limited, if using alone, because you can miss important storm structure if there is greater reflectivity above that structure (in elevation).

Velocity

Measures radial motion of wind. There are two main types of velocity products: Base Velocity and Storm Relative Velocity.

IMPORTANT: The radar only measures the radial velocity of wind. This means, wind moving parallel to the radar beam radial. If an object is moving perfectly perpendicular (sideways) to the radar beam, the radar will return a velocity of 0. In practice, it’s rare for a storm to move perfectly in these ways, but just know that velocity values are usually more than what is reported because of these angles.

Base Velocity (V) – velocity/direction at a single tilt. This is helpful for telling overall motion of winds, speed of a cold front, strong winds from a downburst (aka “downburst” / “straight line winds”).

Storm Relative Velocity (SRV) or Storm Relative Motion (SRM) – subtracts the overall motion of a storm from the velocity value, so you can tell what the winds are doing within the storm as it moves. For example, smaller scale rotation (mesocyclone)…which is often the areas where tornadoes can form.

It’s common for velocity products to use colors to describe velocity values. Red often indicates motion moving away from the radar (”red is bad, go away”) and green is often used for motion towards the radar (”green is good, come here”).

Detecting “rotation” and translating that into a prediction of what is occurring, requires are a number of advanced concepts and techniques. It’s something I might get into another section.

IMPORTANT:

1. At a basic level, keep in mind that “rotation” (aka mesocyclone or tornado) will have a VERY TIGHT couplet of BRIGHT opposing colors close to each other. For the at home weather enthusiast…If you are in doubt, it is likely nothing. If it’s something, it should be obvious.

2. Velocity should always be paired with other products (e.g. Reflectivity) and you should never look at a single image…it needs to be evaluated over time to tell whether those velocities were a blip or sustained (e.g. seen in 2 or more volume scans).

3. Rotation is typically counter-clockwise in northern hemisphere (but not always!). It is critical to know where the radar is in relation to what you are analyzing.

Dual-Polarization Products (help determine shapes and types)

Differential Reflectivity (ZDR)

this is helpful in determining shape of the object, for example: hail, where hail is melting to water, updraft strength/height, rain vs. snow. Values are measure on a scale from -8 (perfectly tall) to +8 (perfectly flat), where 0 is perfectly circular. Most commonly values are looked at from -4 to

0 = perfectly circular
+ (positive) = wide
– (negative) = tall

Here is a table of typical ZDR values on the range and what they can mean.

ZDR Uses

Hail Identification

We often look for a ‘hail core’ in very extreme reflectivity values on Reflectivity (in the example below, 60dBZ and higher show up as pink to white). One way to confirm hail is to look at the shape of the objects detected by the radar. Within the suspected hail core, we can see values < 1 dB and close to 0 dB (perfectly circular)…or even possibly negative but still close to 0 dB.

Melting Layer

What layer of the atmosphere is ice crystals melting to rain. Scanning through tilts, you can see in the image below from the 3.4° tilt a ring of noisier ZDR. As more uniform snow/ice melts it takes on a more horizontal shape.

Updraft

An updraft can be see within the more dense part of Reflectivity (left). On the ZDR (right), small areas of enhanced (> 2 dB) values can indicate rain drops shape. Scanning through multiple tilts will help you understand how high isolated column of rain is getting lifted via the updraft. Eventually you will hit an area of surrounding lower dB values (in the example image below right), with an isolated column of HIGHER dB values representing the counter-clockwise inflow/updraft.

This would be different/opposite than tornado debris, because tornado debris will show the rain around it (high dB surrounding area) and then an isolated area of LOWER dB values representing the debris.

Correlation Coefficient (CC)

Measure of how similar the objects are detected one another. This is excellent and commonly used for detecting debris (from a tornado) in the air alongside water droplets. If things are perfectly similar, the value will be 1. How much lower the value is from 1, represents how dissimilar those objects are from one another.

Here is a table of typical CC values on a range and what they can mean

CC Uses

Meteorological vs. Non-Meteorological Things (water vs. insects)

Birds, insects, and other undeseriable artifacts will show up on Reflectivity, but this will show massive differences in CC values. (Water and bugs are not similarly shaped). Values are less important here. You can can see weird Reflectivity on the left with a clear matching “difference” (gap) in CC on the right.

Melting Layer (hail melts to rain)

What layer of the atmosphere is ice crystals melting to rain. Low CC values (0.85) surrounding higher values (0.98) can have a signature ‘donut’ shape. Keep in mind Principle #2 above. The ring away from the radar represents both distance AND elevation…so while you think of this as the distance from the radar (true) the more important takeaway is the elevation that beam is at.

Rain vs. Snow

The white line represents the Rain/Snow line. East of the line, values are 0.9 to 0.95 (think ”difference = downpour”). West of the white line values are 0.99 (”same = snow”)

Giant Hail (> 2”)

We often look for a ‘hail core’ in very extreme reflectivity values on Reflectivity (in the example below, 60dBZ and higher show up as pink to white). A way to determine how BIG the hail is, could be to use CC. Using CC, hail within that high dBZ reflectivity area will typically show up lower than 0.93 and be as low as 0.8 for +3.5” hail.

Tornado Debris

Location of a tornado should be correlated with Reflectivity (Z) and Velocity (V) products. CC can help confirm touchdown and intensity by seeing debris in the air. This is extremely helpful for nighttime or rain-wrapped tornadoes.

Other Products

Echo Tops

measure of the max height of reflectivity (> 18dBZ). This is a great way to get a quick read on convection. Assumption: greater convection will result in higher storm heights. Also, are the tops growing in height or collapsing?

Volume Coverage Pattern (VCP)

Is the algorithm a radar uses to scan different tilt/elevations during it’s full cycle. At the foundation, there are two main categories of VCP modes: clear air vs. storms. These are, as you can imagine, for different situations related to whether or not storms are predicted. There used to be many variations of these patterns and it got to be difficult for forecasters to remember each one and the strengths and weaknesses with each. They have been simplified over time, so I’ll overview just the major ones you will likely see.

Why is this important: Knowing which VCP mode is running will help you understand which tilts will be available, how long the full scan will take to complete, and how frequently (or delayed) the ‘Base’ products are being updated.

Clear Air Mode – VCP 31 (winter) or 35 (default/summer)

VCP 35 – scans 9 elevation tilts in 7 minutes

VCP 31 – scans 5 elevation titls in 10 minutes.
Uses a longer pulse length which is helpful in the winter, detect snow or freezing moisture (despite clear air)

Once radar detects moisture in the atmosphere, it will automatically switch to Precip Mode.

Precip Mode – VCP 212 (storms) or VCP 215 (non-storms)

VCP 12 (parent category) – all scan 14 elevation tilts. This is best for thunderstorms. VCP 12 is used when range folding concerns are low.

VCP 212 (variation of VCP 12) – scans 14 elevation tilts in 4.5 – 5 minutes
Uses an algorithm to help mitigate range folding problems.

VCP 112 – (variation of VCP 12) – scans 14 elevation tilts in 5.5
Uses a unique algorithm for velocity, helpful near tropical cyclones

VCP 215 – scans 15 elevation tilt in 6 minutes.
Better vertical profile, but spins slower. Better quality data but too slow for rapidly developing thunderstorms.

Problems with Radar

Range Folding

a phenomenon where some data doesn’t make it back to the radar until after the next pulse is sent. This means the radar doesn’t know which value it’s reading and makes it very difficult to interpret reflectivity and velocity data.

Solving for Range Folding:

Velocity – There are unfolding algorithms that help with this. VCP 212 has a better algorithm (albeit not perfect) for the range folding problem at a price of ~30s longer run.

Reflectivity – There is also a similar problem with reflectivity…however no acceptable loss would be ok for reflectivity. So the radar needs to impose a different strategy.

For this, the radar can scan the lowest 3 elevations (0.5° 0.9° and 1.3°) each twice.
1. The first uses a faster pulse (to get responses quicker) and yield good quality reflectivity data.
However this (by itself) would have a counter-productive result because faster pulses won’t return good velocity readings…
2. So the radar does a second scan at those three elevations, this time with a slower pulse, to measure for good velocity readings.

Time For a Full Volume Scan To Complete

Storms are developing rapidly; 0.5° tilt offers valuable info about reflectivity, velocity, etc. near the surface; it’s the first scan in the volume, with many other tilts after it. Waiting 5 minutes for a full VCP cycle to complete to get another update of 0.5° is costly.

Solution? SAILS.

Supplemental Adaptive Intra-Volume Low Level Scan (SAILS) – allows the lowest elevation to be scanned more frequently, during a severe weather event. This can mean low-levels get scanned every 1.8-2.5 minutes, almost twice as fast as the ~5 min run the full cycle takes. Operators at the NWS can choose to run 1, 2, or 3 additional SAILS scans (versus the ‘single’ scan).

Here’s the impact of SAILs

	Total Full Cycle Scans (per hr)	Number of scans at 0.5 elevation (per hr)
VCP 212	13	13
VCP 212 w/ SAILS	11**	22*

Example VCP 212 w/ SAILSx2 (bold are supplemental SAILS scans):

0.5 (Reflectivity), 0.5 (Velocity),
0.9 (Reflectivity), 0.9 (Velocity),
1.3 (Reflectivity), 1.3 (Velocity),
0.5 (Reflectivity), 0.5 (Velocity),
1.8, 2.4, 3.0, 3.1,
0.5 (Reflectivity), 0.5 (Velocity),
3.8, 4.0…[up to]..19.5 (repeat)

New technology exists now to interpret data coming back from each scan and adjust the number of supplemental SAILS scans and placement within the full range scan. So, depending on the situation, your radar may be operating slightly differently.

If you subscribe to Radar Omega’s “RapidSweep” add-on, you can see this all in action. Not only can you see the progress of each scan (and receive faster/real-time scan data), but you and see how frequently the base levels get scanned. It will also tell you when a SAILS run is executing.