Doppler Velocity - Interpretations and Benefits

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One of the tremendous pluses of the uses of radar is the ability for it to determine the speed (relative to the radar's fixed position) of objects (primarily think of cloud/rain droplets) in the air.

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Perhaps you have heard of the 'doppler effect.' A common demonstration of its application deals with sound. For example, I'll borrow, expound on, and animate this common audible occurrence, which is referenced on the 'PhysicsClassroom' website (linked at the bottom). They diagram how the siren-sound from an emergency vehicle changes as it approaches and then passes a location.

Not only does the volume change (as it gets closer, it gets louder), but the frequency at which sound-waves reach the listener changes too. This is due to the motion of the vehicle. Higher frequency is recognized in the forward direction of the vehicle, while a lower frequency is observed in the opposite direction of vehicle motion. Pitch is directly related to the frequency, which leads to the siren-sound sounding vastly different between approach and passage.

Now that a fundamental foundation has been laid for understanding the doppler effect, some basic differences arise with its implementation to weather radars: (1) The radar is like the fire-truck in that it is the object from which the waves emanate (Microwave radiation vs. Sound Waves), but unlike the truck, the radar is stationary. (2) Like the sound-waves from the truck reaching the persons, the waves from the radar interact with storms and other objects. But in this case, the storms and associated winds/rain are in movement. (3) The radar has to "listen" for echos which are reflected back to it to make its determinations/calculations. (4) Although fairly rapid, the radar rotates and sends/receives signals in a single direction (along the radial, or beam-path), rather than multiple antennas (or speakers, like with the truck).

Velocities can be calculated from the known Transmission Frequency and the received Echo Frequencies. In addition, direction (relative to the radar site; toward or away) is revealed, by the difference between the Transmission Frequencies (f_T) and the Echo Frequencies (f_R). If f_T - f_R < 0, then generally speaking, targets are moving TOWARD the radar site. Conversely, if f_T - f_R > 0, then targets are assumed to be moving AWAY from the radar site.

Basic Applications / Interpretations of Doppler

- First: Know that radars scan the skies at angled heights. So, w/o going into greater detail, the further away an echo appears on radar, the higher-up in the atmosphere it is. And remember, as alluded-to above, velocity imagery doesn't directly give N,S,E,W directions of the wind or storms. It's ONLY interpretting "along-beam" or radial velocities. With sufficient clouds, moisture, etc, enough data is returned in the scans and computers CAN interpret the data and get weather-balloon-like results, but this is not achieved through viewing velocity imagery...stay tuned...

- Second: Circles / "Range-Rings" are displayed/drawn/imagined around the Radar site to assist with interpretation. Intuitively, smaller rings would be much closer to the radar while larger rings would be at a greater horizontal distance. Now recall the last paragraph. The larger rings also infer greater height ...So echos found along these rings are assumed to be at the same heights. A good estimate of the height is simply found via known calculations using the angle of the beam, the typical beam path over the curved earth surface, and the distance from the radar site.


- Third: Be careful with "zero" velocities. They can indicate very weak winds, but often signify a change in how the radar is seeing the winds and NOT a literal decrease/halt in winds over an area. Frequently, an image like below will be produced, where a line of zero-velocities are present.


This line is merely representing winds in the atmosphere that are blowing tangential to range-rings: The point where no portion of the wind is blowing toward or away from the radar; where Echo Frequencies are the same as the Transmitted Frequencies (f_T - f_R = 0).

Okay, now for the heart of the blog post

(1) Vertical Wind Profile of the Atmosphere
...Guess what. You can tell some things about the wind changes / profiles via the imagery!  Assuming sufficient data exists, along a ring (distance) of choice, we can (1) Find "Zero" velocity lines...(2) Along the ring, approximate where the highest magnitude NEGATIVE (green) velocity is occurring...(3) Draw and arrow from the rings toward/across the radar center.


The general direction the winds at the heights of the range rings are now known!
* It can be difficult to determine where the fastest winds are occurring, but often, another method (employed above) is to actually to draw an arrow across the site, which is normal (perpendicular) to an imaginary line joining the zero velocities at that range ring.

Different wind profiles can be inferred by recognizing the shape of the Zero-Velocity Line:


Other wind-profiles and configurations can be inferred from more patterns seen on radar such as confluence, diffluence, and frontal passages. Each kind is important for various purposes.

(2) Storm Convergence / Divergence vs. Rotation
An important use of the Doppler is determining if areas of a storm exhibit rotation (tornadic potential) or if it is just simply convergence or divergence. The key to determining which is occurring is always knowing the pattern's orientation with respect to the radar site.

Both patterns can seem very similar, where areas of local wind maxima going toward or away from the radar are very close together. And although storm type can sometimes be determined from simple reflectivity, velocity imagery confirms what is happening within the storm and highlights where possible tornadic rotations are occurring.

The best way to keep straight what is occurring is to imagine / draw a line (we call it a radial; think radius) from the radar site through the center of the storm and draw arrows parallel to that line, pointing in the implied directions...

* Convergence - If the arrows are converging, convergence is occurring. This can indicate areas of heavy rain or its imminent development.
* Divergence - If the arrows are diverging, divergence is being observed. When this happens near the surface, it can indicate dangerous microbursts within thunderstorms, responsible for strong straight-line winds.
* Rotation - If shearing is implied with the change in direction occurring parallel to the radial, Rotation is occurring. Tornadic possibility can be monitored and gauged (vorticity strength).

Sources:
1). PhysicsClassroom
2). NWS JetStream - Exactly how does radar work?
3). Dr. MD Eastin Lectures