Towards the Use of a Doppler Filter on Radars Operating a 3-PRT Scheme at Météo-France

A major obstacle to the observation of the lower layers of the atmosphere by radars is the presence of clutter. The return from the ground at the lowest elevations tends to dominate (especially near the mountains) the useful signal from the precipitation. To overcome this problem, a simple method is to filter the signal from the clutter (which has zero Doppler and low spectral width) after performing a Fourier transformation of the I and Q time series of the received signal. However, this technique is not directly applicable to Météo France radars that transmit pulses non-regularly spaced in time. This particular transmission mode (called 3-PRT, where PRT stands for Pulse Repetition Time) has the enormous advantage of allowing the measurement of the Doppler velocity unambiguously up to maximum range [1].

Methods to reconstruct the Doppler spectra from non-regularly spaced in time I and Q samples so that a zero notch spectral filter can be applied, have been proposed by Sachidananda and Zrnic (2000 [2] , 2002[3]) but these methods are complex and would be too computationally expensive for 3-PRT scheme used at Météo-France. More recently, J. C. Hubbert (2009)[4] showed that an easier way was to apply adapted filters to each of the uniform time series that compose the staggered PRT scheme (for a 3-PRT scheme, three regular time series with the period PRT1+PRT2+PRT3). This last method is the one that is currently considered at Météo-France to reduce the impact of the clutter. However this approach is not to be considered alone. A 3-PRT scheme is prone to de-aliasing errors that depend on the spectral width, the signal to clutter ratio, the triplet of PRT and the de-aliasing technique used.

Consequently, to minimize the impact of ground clutter, all aspects need to be considered together in order to design an optimal solution. In this paper, the contribution of each of the sources of error will be presented as well as their interactions. Results of the optimal filtering on simulated and real observations will be shown.

[1] Tabary, P., F. Guibert, L. Perier, and J. Parent-du-Chatelet, 2006: An Operational Triple-PRT Doppler Scheme for the French Radar Network. J. Atmos. Oceanic Technol., 23, 1645–1656.

[2] Sachidananda, M. and D. Zrnic, 2000: Clutter Filtering and Spectral Moment Estimation for Doppler Weather Radars Using Staggered Pulse Repetition Time (PRT). Journal of Atmospheric and Oceanic Technology , 17, 323–331.

[3] Sachidananda, M. and D. Zrnic, 2002: An Improved Clutter Filtering and Spectral Moment Estimation Algorithm for Staggered PRT Sequences. Journal of Atmospheric and Oceanic Technology , 19, 2009–2019.

[4] Meymaris, G., J.C. Hubbert and G. Gray, 2009: A Simplified Approach to Staggered PRT Clutter Filtering, 10B.4.