# How Doppler Winds Are Portrayed

## Interpreting The Doppler Wind Images On The Web

Doppler velocity images are now available on the web for a number of Doppler weather radars around Australia. This section gives some guidelines on how to interpret the images.

It should be stated at the outset that, unlike the normal radar images that show reflectivity (a parameter related to rainfall intensity), which are generally easy for the layman to interpret, the Doppler wind data can be quite difficult to understand. The data is available for anybody to view, but many users will find it difficult to interpret and of little value. People should use the data with care, due the complexities of interpretation.

## How Doppler Wind Is Displayed

The Doppler velocity images show radial wind speed (speed towards or away from the radar) at each point, colour-coded according to the following palette.

Figure 1. Doppler velocity palette.

Inbound velocities (towards the radar) are shaded blue, with pale shades for light winds and dark shades for strong winds. Outbound velocities (away from the radar) are shaded orange with pale shades (yellow) for light winds and dark shades (red) for strong winds. The Doppler velocity at any point can be estimated by comparing the colour shown at that point to the velocity palette, which is attached to every image.

It must always be remembered that the Doppler velocity measured by the radar is only a component of the actual wind - the part that is blowing towards or away from the radar. The radar cannot measure the "crosswise" component. The actual wind will be at least as strong as the Doppler velocity, and possibly considerably stronger.

## Doppler Velocity And Actual Wind - Example

To explain the relation between Doppler velocity and the actual wind further we will consider an idealised and very simple example. Suppose that the wind in the field of view of a Doppler radar was everywhere a Westerly wind (i.e. blowing from west to east) at 50 km/h. The Doppler velocity image produced by the radar would look something like the following. Note that the radar is at the centre of the image, true North is at the top of the image and East is to the right. Thus a Westerly wind would blow from left to right across the image, as shown.

Figure 2. Idealised sample Doppler velocity image.

For point A, you can estimate that the Doppler velocity is outbound (away from the radar) at about 50 km/h. At point B the Doppler velocity is inbound (towards the radar) at 50 km/h. At point C, however, the Doppler velocity is zero. Point C is due North of the radar and our Westerly wind has no inbound or outbound component at this point (inbound would be a Northerly wind (blowing from north to south); outbound would be a Southerly wind (blowing south to north).

Readers familiar with trigonometry will notice that the Doppler velocity at a point is proportional to the cosine of the angle between the wind vector and the radial from the radar to the point in question. To see this, consider point D.

## Estimating The Actual Wind Speed And Direction From A Doppler Velocity Image

Our simple example can also be used to demonstrate two techniques for estimating the actual wind speed and direction from a Doppler velocity image.

## Doppler Velocity Images Do Not Show Ground-level Winds!

It is important to be aware that Doppler velocity images (like the normal reflectivity images) actually show what is happening some distance above the ground, rather than portraying conditions on the ground itself. Winds at the height that the radar is "looking" will often be stronger than they are on the ground, though on occasion the opposite may be true.

Weather radars are often compared to a lighthouse - they generate a narrow beam of energy and gradually turn so that this beam sweeps through a full 360 degrees over time (for weather radars a full rotation takes 20 seconds or so). The beam is generally not pointed horizontally, but angled upwards to avoid it bouncing off nearby objects or topography (which would produce a very strong signal that would swamp the generally weak atmospheric returns we are interested in).

The Doppler velocity images on the Web are produced from a sweep at an elevation angle of 0.9 degrees, meaning that the beam is elevated above the horizontal by this amount. Thus the beam gets higher above the ground as it moves away from the radar. This effect is exacerbated by the curvature of the Earth, which makes the ground curve away from the radar beam with increasing distance from the radar. An additional complication is that the radar beam does not really travel in a straight line - varying temperature and humidity conditions tend to refract (bend) the radar beam, usually downwards.

The following graph and table show how the height of the radar beam changes with distance from the radar for the 0.9 degree elevation angle Doppler velocity images. The data refers to typical atmospheric conditions - the true height of the beam will actually vary somewhat from day to day depending on temperature and humidity conditions through the atmosphere at the time.

Figure 3. How the height of the radar beam changes with distance from the radar.

0
0
25
429
50
932
75
1509
100
2158
125
2882
150
3679

Figure 4. How the height of the radar beam changes with distance from the radar.