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

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.