How an Agile-Beam Polarimetric Phased-Array Radar Can Add to the Observing Capabilities of the NWS

A topic that has not been given as much attention as needed is whether dual polarization phased array radar can provide measurements of near-storm atmospheric properties. We present reasons why, in fact, it should be possible to derive environmental winds in the vicinity of storms, profiles in the clear atmosphere; and water vapor content in the planetary boundary layer.

For wind measurements in storm environments the beam agility enables a very high sensitivity mode to be selectively applied in areas of weak reflectivities. The mode consists of a long dwell time (few seconds) and a high compression-ratio waveform followed by spectral analysis and spatial averaging. The high efficiency (power and pulse compression) is achievable with GaN power amplifiers having ~50 W peak power per element and 10% duty cycles. Therefore a WSR-88D sized aperture could radiate roughly two orders of magnitude more power than the average on the WSR-88D. It is expected the mode could retrieve signals with power per pulse of about -30 dB below the noise power. Tradeoff considerations at such low SNRs are explored. Use of polarimetry and polarimetric spectral analysis can separate returns from passive wind tracers (insects, or turbulence-induced fluctuations in refractive index) from returns caused by birds and other fast-flying biota. If winds are locally uniform (over several km) the retrieved Doppler velocities can be combined to produce horizontal winds.

Vertical profiles of winds in clear air and cloudy conditions can also be estimated in this high-sensitivity mode. It is suggested the polarimetric information can be applied to censor Doppler winds caused by strong non-passive scatterers. Scanning techniques developed for UHF/VHF wind profiling radars can then be applied to retrieve vertical profiles of winds. The beam agility makes possible rapid switching between the wind profiling mode and the storm surveillance mode.

The proposed high-sensitivity mode may also detect Bragg scattering which yield the refractive index structure parameter (C_n²) profile with height. This parameter depends on turbulence intensity and gradients of humidity (and to a lesser extent temperature gradients). A bistatic mode for this application offers significant enhancement of detectability due to the increase in scattered Bragg scale.

Technical issues and challenges to make the proposed measurements are addressed.