Estimating Wind Speed And Direction From a Doppler Wind Image
The Max/Min Method For Estimating Wind Speed and Direction
It should be emphasized before starting that the Max/Min Method for estimating Wind Speed and Direction relies on a major simplifying assumption - that the wind is uniform in both speed and direction at all points around the radar. This is never exactly true, but is often approximately true, at least in close proximity to the radar.
The method is as follows:
- Locate the point where you want to estimate the wind speed - we've chosen point P in our example below
- Draw a circle through the point, centred on the radar
- Find the point on the circle with the strongest inbound Doppler velocity - we've labelled ours Q
- Find the point on the circle with the strongest outbound Doppler velocity - we've labelled ours R
- R should be lie exactly opposite to Q on the circle. If Q and R are not opposite (or at least approximately so), it indicates that our assumptions are not valid (ie the wind is not uniform at all points around the radar), and consequently the method will not give a reliable result. It may not be possible to estimate the wind speed and direction in this case.
- If R is opposite Q, then draw an arrow from strongest inbound (Q) to strongest outbound (R). This arrow is the wind direction at point P (and in fact all points on the circle - remember the method assumes that the wind is the same at all these points). Note that all Bureau radar images use the convention that true North is at the top of the image and East is to the right of the image.
- Estimate the strength of the Doppler velocity at point Q by comparing the colour shown there to the velocity palette. Do the same for point R. These should be the same (or at least nearly the same). If not then our assumptions are not valid, and the method will not give a reliable result.
- The strength of the Doppler velocity at Q (or at R) is the wind strength at point P (and in fact all points on the circle - remember the method assumes that the wind is the same at all these points)
The wind direction in this simple example is a Westerly (blowing from west to east). Click here for a larger image
Figure 1. Estimating wind direction using the Max/Min Method.
The wind speed in this simple example is 50 km/h. Click here for a larger image
Figure 2. Estimating wind speed using the Max/Min Method.
The Zero Isodop Method for Estimating Wind Direction
One feature that is often easy to identify on a Doppler velocity image is the zero isodop. An isodop is a line connecting points with equal Doppler velocity. The zero isodop is a line joining places with zero Doppler velocity. It shows up as a line a white separating a region of inbound (blue) velocities from a region of outbound (orange) ones.
The zero isodop for our simple uniform Westerly wind example is shown below. Click here for a larger image
Figure 3. The zero isodop for a simple uniform Westerly wind example.
In the real world, the zero isodop is usually a curved line instead of a straight line, but the technique described below is still valid.
The Zero Isodop Method for Estimating Wind Direction at any point on the Zero Isodop is as follows:
- Locate a point on the zero isodop - we've chosen point X in our example
- Draw a radial from the radar to the point of interest - this is the arrow from R (the radar) to X in our example
- Draw another arrow perpendicular (at right angles) to the first arrow and orient it so that it points from the inbound side of the zero isodop to the outbound side. In our example the arrow must run from Y to Z, not Z to Y.
- This second arrow (Y to Z) represents the wind direction at point X. Again, the convention used for all Bureau radar images is that true North is at the top of the image and East is to the right of the image.
Figure 4. The zero isodop method for estimating wind direction.
Note that the wind direction arrow is to be drawn perpendicular to the radial from the radar to the point of interest, not perpendicular to the zero isodop. In our simple example the wind direction is also perpendicular to the zero isodop, but this is not always the case!
Real-Life Example
To learn more about using these two techniques to estimate the wind speed and direction from a Doppler wind image, see this example from the Buckland Park radar in Adelaide.
Back to How Doppler Winds Are
Portrayed