Polarimetric radar takes a look at cloud seeding

A cloud seeding experiment has been conducted for the past two Southern Hemisphere summers near Brisbane, Queensland, Australia. Seeding operations were conducted near the CP2 dual wavelength (X- and S-band) polarimetric radar. Previously the CP2 radar was an NCAR facility but was transferred to the Australian Bureau of Meteorology Research now called the Center for Australian Weather and Climate Research (CAWCR). This is the first time a dual-wavelength, dual polarization radar has been used in a cloud seeding experiment. In addition to CP2 there is an Australian Bureau of Meteorology 10 cm Doppler radar 35 km east of CP2 which makes possible Dual Doppler analysis.

A research aircraft from the South African Weather Service was available both summers for hygroscopic and silver iodide seeding. It also carried a suite of instruments for measurement of state, aerosol and cloud physics variables.

It has been theorized that cloud seeding under certain conditions may modify the natural distribution of precipitation particle sizes and concentrations. Since ZDR is at least partially related to drop size this paper will present time-height profiles of the evolution of Z and ZDR for both seeded and unseeded clouds. The non-seeded clouds are made up of clouds that were declared “no seed” from randomized seeding experiments and clouds in close proximity (“sister clouds”) of seeded clouds.