Doppler algorithms

The algorithms Doppler indicate automatic processes of recognition of certain circulations in the data of the weather Radars. In and under the storms, one can have rotations which can lead to Tornade S. One also has zones of convergence along the faces of gusts and the divergence S at the time of downward gusts.

Rotation

During rotation, the winds turn around a center as in the part the top on the left of the image. The radial component with the radar, located in bottom of the image, thus varies according to the part of the rotation which one looks at. It passes from a maximum approaching of the radar to another moving away on the right while passing by a line where the radial wind is null in the center because perpendicular to the beam radar. This gives gives an owner Doppler speeds on the right with a doublet of colors green-red separated by an grey area null radial speeds.

Convergence and divergence

If the winds convergent or diverge, one obtains zones where the winds also change direction but according to an owner different from rotations.

the case of convergence corresponds to air coming from various places and forced to go up in altitude at the time of their meeting. There are thus vectors of wind of different directions meeting along a line or of a point meets where speed becomes null. The radar thus sees a doublet green-red with a line of zero perpendicular to the direction with the radar.

the case of divergence corresponds contrary, that is to say downward winds which are forced to be spread out in range by the meeting of the ground. One thus obtains an owner similar to convergence but with the color reversed.

Mésocyclone

As a mésocyclone is a rotation in the cloud, one expects the identitfer by an owner of rotation. However, in general one also observes convergence in the low levels of detection of a storm and the owner will be often a mixture of rotation and convergence as to the extreme right-hand side of the image. The axis of the zero radial speed will be then between the parallel and the perpendicular with the radar.

Algorithms

A computer program thus passes the Doppler data point by point according to the distance to the radar and the angle of sight. According to the type of ciculation which one wants to find, one seeks in the data the passage speeds moving away to approaching as in the examples above. One thus obtains zones which correspond to the definition but it should be remembered that these zones are weak rays and one uses a criterion of form to eliminate from bad detections. For example, in a uniform circulation with large scales the radial component follows a value equivalent to a cosine what can resemble a zone of divergence (max of approach-zero-max of distance).

Once the algorithm found zones corresponding on an angle of elevation with the criteria, it can compare equivalent structures so exist on several angles at the same place and determine the vertical continuity of circulation.

See too

Storm
weather Tornado
Radar