High resolution 3d win profiling using an s-band polarimetric fm-cw radar:dealiasing techniques

Abstract

The Transportable Atmospheric RAdar (TARA) is a S-band polarimetric FM-CW profiler that is located at Cabauw, the Netherlands. It is a part of European Union sponsored CESAR Observatory.
At this wavelength (¿=9.1 cm), the wind tracers are raindrops, water cloud droplets, ice crystals and snowflakes. These particles are ranging in sizes from 0.01 mm to 10 mm. Clear air scattering, which consists of reflection echos from spatial differences in refractivity index of the order of ¿/2, gives another possibility of wind tracing in absence of hydrometeors.

The aim of TARA is to provide simultaneous measurements of the dynamics and microphysics of clouds and precipitation by using the Doppler polarimetric measurement methodology. Therefore the TARA measurements can be carried out in single polarization mode, alternating polarization mode and/or wind measurement mode, that employs two offset beams. For the wind mode, which is the purpose of this paper, the radar TARA uses three independent beams to acquire Doppler spectra. The three measured mean Doppler velocities are input for the 3D wind estimates. They are measured with a high temporal resolution (1 s -10 s) and spatial resolution (3 m ¿ 30 m), allowing a detailed study of the troposphere. Because of the importance of a correct estimate of the vertical wind for clouds and boundary layer studies, one offset beam is looking at the zenith while the two other beams are respectively at 75 deg. and 69 deg. of elevation.

A retrieval method of the three dimensional wind velocity is based on processing of Doppler spectra. Because it makes use of three consecutive beam observations, aliasing problem appears as the main drawback of this technique. This limitation is further enhanced by adding dual-polarization measurements for clutter suppression. As the maximum unambiguous measurable velocity decreases dramatically, a very important effort has been dedicated to palliate this important limitation. Therefore, a successful dual-polarization technique has been employed for the main beam dealiasing, while a more classical one was used for the other beams with good results, as well. It is shown that the polarimetric dealiasing technique carries out a significant clutter suppression.
The results show the good behaviour of this wind velocity retrieval method. Different measurements have demonstrated how this technique behaves in different meteorological conditions, with or without precipitation.