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Doppler radar

See Radar gun for traffic and sports radars.

Table of contents
1 Doppler radar as weather radar
2 Comparison of Reflectivity and Doppler Velocity
3 External link

Doppler radar as weather radar

A simple weather radar can detect precipitation or objects just by the reflection of microwaves. Most weather radar employ pulsed microwave signals. With more precipitation or a bigger object, there is more reflectivity. As in the case with heavy rain or hail, more signal is reflected back to the radar dish.

In meteorology, the Doppler effect becomes especially useful. While Doppler radar can still detect reflectivity, other information is collected from the returning microwave signal.

That information is in the form of Doppler shift of the microwave signal. The information is then collected in real time by computer to derive wind velocity. The velocities that can be detected by a single dish are velocities that are going away from the dish or toward the dish (see vector mechanics).

Even though most weather radar has the ability to collect Doppler wind velocities, it is usually not used for display to the public since it is difficult for the experienced meteorologist to quickly understand. Typically, research meteorologists depend more heavily on the Doppler data for wind vector retrieval. Also, for example, some products from Doppler data are used to indicate (on the reflectivity display) regions of wind shear. Most TV meteorologists refer to their radar products as "Doppler", when in reality their displays are just reflectivity.

Comparison of Reflectivity and Doppler Velocity

The best way to show the vast difference between the Doppler velocity display of a radar and the normally seen reflectivity display is to directly compare the two products from one radar scan. The following image is a vertical radar sample, or RHI, of a rapidly approaching cold front. Essentially, the cold air is forcing the warm-moist air upward so rapidly, that a fast moving thunderstorm is produced at the edge of the cold front.

The above depicts two products of one radar sampling. This image comes from a Doppler capable weather radar. In other words, the radar is capable of retrieving velocity information as well as reflectivity. The main tricky characteristic of doppler velocity is that the values collected are radial velocities. Meaning that all points on the velocity display are velocities that are going away or toward the radar.

On the left:

The Doppler velocities depict a wind pattern consistent with an updraft. But being so colorful, it loses the interpretation of where the rain is actually falling. However, the radial Doppler velocity is also not a true velocity. It is merely a vector component of the real velocity of the particles that have been detected.

On the right:

This display on the right is really a power return product. However, it is directly related to reflectivity. In this display, the red is actually where the rain is.

External link