P 7 . 10 The production of high quality Doppler velocity fields for dual PRT weather radar

Doppler weather radar suffer from well known limitations associated with the trade off between range and velocity aliasing. This is particularly severe as the radar wavelength decreases. There are a number of operational 5 cm wavelength Doppler radars around the world and for reasonable maximum ranges of about 150 km, the unambiguous velocity is only about 13 ms. Automated processing techniques to de-alias the resulting radar data are fairly limited in there usability for such a small Nyquist. An alternative strategy that has been employed is the use of multiple pulse repetition times. A widely used approach to mitigate this problem is using a dual PRT (Pulse Repetition Time). This may be on a pulse pair to pulse pair basis, where the time between pulses is varied on every pulse pair (referred to as a staggered sampling), or where alternate records of, for example 32 pulses, are sampled at a single, but different PRT (dual PRT). The former has intrinsic advantages for unambiguous unfolding, but the staggered sampling limits the performance of simple ground clutter filters (e.g. Banjanin and Zrnic, 1991). A common approach to avoid this is the latter strategy which is an approximation of the first. This approximation is employed on operational Doppler radars in Canada and Australia, the research radar at ETH and is available on some commercial signal processors (SIGMET, 1992; Joe et al, 1998). However, there are problems unfolding the data if the azimuthal gradient of the radial velocity is large or if the variance in the velocity estimate becomes comparable to the Nyquist velocity. The goal of this paper is to evaluate and demonstrate the advantages and limitations associated with the dual PRT method in the presence of wind fields with large horizontal shears and to assess the magnitude of problems inherent in the dual PRT approach.