Abstract
Abstract. By inducing linear contrails and contrail cirrus, air traffic has a main impact on the ice cloud coverage and occurrence. During the COVID-19 pandemic, civil air traffic over Europe was significantly reduced, in March and April 2020, to about 80 % compared to the year before. This unique situation allows us to study the effect of air traffic on cirrus clouds. This work investigates, based on satellite lidar measurements, if and how cirrus cloud properties and occurrence changed over Europe in the course of COVID-19. Cirrus cloud properties are analyzed for different years between 2014 and 2019, which showed similar meteorological conditions for the month of April as in 2020. The meteorological conditions for March, however, were warmer and drier in 2020 than the previous years. The average thickness of cirrus clouds was reduced to 1.18 km in March 2020 compared to a value of 1.40 km under normal conditions, which is stronger than expected from the aviation reduction due to the less favorable meteorology for ice cloud formation. While the April results in 2020 were only slightly reduced, with an average thickness of 70 m thinner than the composite mean of the previous 6 years. Comparing the different years shows that the cirrus cloud occurrence was reduced by about 17 %–30 %, with smaller cloud thicknesses found in 2020 for both months. In addition, the cirrus clouds measured in 2020 possess smaller values of the particle linear depolarization ratio (PLDR) than the previous years at a high significance level for both months, especially at colder temperatures (T<-50 ∘C). The same analyses are extended to the observations over the USA and China. Besides the regional discrimination of cirrus clouds, we reach the final conclusion that cirrus clouds show significant changes in PLDR in both March and April over Europe, no changes in both months over China, and significant changes only in April over the USA.
Highlights
Cirrus clouds have a wide global coverage and, a large effect on the Earth’s radiation budget
Cirrus ice crystals generally form in regions of ascending motions by ice nucleation on aerosol particles in the upper troposphere, or they appear in the cloud outflow of frontal systems or convection as frozen cloud droplets that had formed at lower altitudes and warmer temperatures
Aircraft flying in cold and humid air masses may trigger the formation of contrails by mixing the aircraft exhaust and the surrounding air with water content condensing on the airborne aerosols that might be emitted by aircraft
Summary
Cirrus clouds have a wide global coverage and, a large effect on the Earth’s radiation budget. Like temperature and supersaturation (e.g., Heymsfield, 1977; Khvorostyanov and Sassen, 1998), and the nucleation mode (e.g., Ström and Ohlsson, 1998; Seifert et al, 2004; Urbanek et al, 2018), can influence the microphysical properties of the cirrus clouds. Previous studies reveal that ice crystals in air form and grow as a function of the ambient temperature and relative humidity, and there is a general trend toward larger morphological complexity with increasing supersaturation at all temperatures (e.g., Heymsfield, 2003; Bailey and Hallett, 2004, 2009). Based on laboratory experiments, Bailey and Hallett (2004) reported that different ice crystal habits were observed under conditions with different temperatures. Differences in size and shape have an impact on the particles’ optical properties; it was found that columnar ice crystals generate higher depolarization ratios than plate-like crystals (Noel et al, 2006), Published by Copernicus Publications on behalf of the European Geosciences Union
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