VAD Analysis with the Inclusion of the Deformation Term

The current Velocity Display Algorithm on the WSR-88D uses a least square fit routine to produce the divergence and mean wind from the azimuthal dependence of the Doppler velocities. A routine product is the wind in a time height display. The least square fit is used so that the procedure accepts data at arbitrary azimuthal increments. The most common cause of unequal azimuthal increments is missing data. The algorithm considers only the zeroth and first harmonic of the data and from these the divergence and mean wind are computed. Nonetheless, the second harmonic is also related to a kinematic property of the wind field, namely deformation. In this abstract presented is the computational method that simplify obtaining the second harmonic. An example of a weather condition in which a front is inside the VAD circle illustrates the need for the second harmonic. In the example the axis of dilatation is compared with a front position on a synoptic chart. Omission of the second harmonic not only deprives users of a valuable information but, if some azimuthal data are missing, introduces errors in deduced winds.