Multi-beam raindrop size distribution retrievals on the Doppler spectra
Acquiring the raindrop size distribution from radar data is still a challenge. Generally this distribution is retrieved using the reflectivity, Z, and the differential reflectivity, Zdr, at S-band. The specific differential phase, Kdp, provides a third radar observable in the case of heavy precipitation to strengthen the retrieval technique. For S-band profiling radars, Kdp cannot be used, mainly because of the limited number of range bins containing rain. The Zdr measurement cannot be employed when the elevation is near the vertical, or in the case of drizzle and light rain. Therefore retrievals have to be performed on the measured Doppler spectra. For this purpose, the Doppler spectra of rain are modeled using Rayleigh scattering, the gamma distribution for the raindrop size, a size-shape relationship and a Gaussian kernel for the turbulence. Comparing the measured Doppler spectrum with the modeled one, a non-linear least-square fit technique is employed to obtain the parameters of the raindrop size distribution (D0, ?) and the turbulence broadening factor of the raindrop size spectrum (?0). The intercept parameter (Nw) and the radial wind component (v0) are estimated by scaling the broadened drop size distribution (DSD) along the spectral reflectivity axis and the Doppler velocity axis respectively. This approach has been proposed in (Moisseev et al, 2006). So far, this technique has not been further developed, exploited and validated. It is our intention to investigate this methodology using the Doppler-polarimetric TARA radar to retrieve raindrop size distribution from drizzle to heavy precipitation with high spatial and time resolution. Since TARA can profile in three directions, three raindrop size distribution profiles are estimated, which can give insight in the microphysical homogeneity of the precipitation
The new real-time measurement capabilities of the profiling TARA radar
In the past 10 years, the S-band FM-CW TARA (Transportable Atmospheric RAdar), placed at the Cabauw Experimental Site for Atmospheric Research (CESAR), provided in real-time vertical profiles of the Doppler moments. Classical spectral processing was carried out. The polarimetric and multi-beam measurement capabilities of the radar were not exploited in real-time. It was only possible to acquire raw data for case studies. Based on them, new algorithms were developed using spectral polarimetry and the multi-beam capability of TARA. They have been tested during the COPS (2007) and EUCAARI-IMPACT campaigns (2008). To measure in real-time the Doppler moments of three beams, the differential reflectivity, the linear depolarization ratio, the horizontal wind and the vertical mean Doppler velocity, it became necessary to upgrade TARA. This resulted in a new design of the radar control unit and a new processing based on spectral polarimetry. This major upgrade took place in 2011. TARA can now deliver multi-parameters profiles with high spatial and time resolution and raw data in real-time. They are stored with the NetCDF format. Furthermore, detailed quick-looks of all the observables are available in real-time at http://ftp.tudelft.nl/TUDelft/irctr-rse/tara/index.html. For the design of the radar control unit and processing, a flexible solution that can process the data in a high level programming language, was chosen. This was done to be able to easily implement future developments in radar signal processing and algorithms.
A new method to separate ground clutter and atmospheric reflections in the case of similar Doppler velocities
This paper introduces a new ground clutter suppression technique which preserves weather echoes. This clutter suppression method uses both statistical and polarimetric properties of the target and clutter. This technique is intended for use in atmospheric studies for weather echoes the spectral properties of which do not differ much from those of ground clutter. This technique can be applied both to the total signal or to its separate Doppler frequency components.