Observation of low-depolarization contrails at Florence (Italy) using a 532-1064 nm polarization LIDAR

Abstract

Contrail backscatter and polarization measurements made using a 532-1064 nm LIDAR in Florence (Italy) indicated that, in most of the cases, the contrail depolarisation and color ratio resemble the usual cirrus. In 20% of the cases, a depolarisation below 15% was observed, both in isolated contrails and in cirrus-embedded contrails. In a few cases, a very low depolarisation, down to 3%, was observed at temperatures in the -40° to -65° C range. In two measurements, a low depolarization was coupled with a high color ratio in the 2-4 range. The low-depolarisation, high color ratio contrail observations are discussed in the light of the results of recent T-matrix scattering simulations. The two events lasted at least 10-20 minutes in the LIDAR data, suggesting a very slow growth or no growth at all of the initial micron-sized contrail particles in the large ice crystals typical of cirrus.