Potentials of frequency agile cloud radars

Radars at Ka and W frequency bands are widely used in cloud remote sensing. Results of frequency agile technique for these bands are presented. Frequency agile Ka and W band radars allow implementing multi-wavelength techniques with a single radar. Due to scattering resonance effects on droplets it is possible to infer information on scattering particles by making measurements at different wavelengths within the bands. It is shown that power measurements with a 10% frequency chirp can be used to estimate the maximal size of scattering particles. For a parameterized drop size distributions of a form No exp(-D/Do), power measurements at two frequencies within a band allow determining No and Do . This technique does not use particles' fall speed parameters so it can be utilized for any antenna elevation angles.

The Doppler spectra at vertical incidence are analyzed as well. Power spectral differences in the Doppler spectra measured at two wavelengths within a band can reach several dB. These differences do not depend on the mean wind velocity in the radar volume and allow obtaining non parametric particles' size distributions.

A frequency agile Ka band radar with the bandwidth from 30 to 36 GHz is described. Several initial experiments have been conducted outdoors. The portable radar transmits a continuous waveform composed of 30 to 36 GHz frequencies in steps of 10 MHz. The in and quadrature (IQ) phase received signals are digitized by the data acquisition unit (DAQ). The DAQ connects the front end to a laptop computer via a serial communication channel, which allows the digitized IQ data to be uploaded. The compact, portable radar is currently known as the “shoebox” radar, because of its small size and low-power consumption.