Aerosol Indirect Effects on Cirrus Clouds Based on Global Aircraft Observations

Abstract

Cirrus clouds have large climate impacts, yet aerosol indirect effects on cirrus microphysical properties remain highly uncertain. There is a lack of observational analysis on thermodynamic, dynamical, and aerosol indirect effects simultaneously, which limits the quantification of each effect. Using seven National Science Foundation aircraft campaigns, impacts of temperature, relative humidity, vertical velocity, and aerosols are individually quantified. Nonmonotonic correlations of ice water content, ice crystal number concentration (Ni), and mean diameter (Di) with respect to aerosol number concentrations (Na) are consistently seen at various conditions. Positive correlations become significant when Na > 500 nm (Na 500 ) and >100 nm (Na 100 ) are 3 and 10 times higher than average, respectively. While Na 500 are more effective at temperatures closer to −40°C with small vertical velocity fluctuations and are less sensitive to ice supersaturation, Na 100 are more effective at colder temperatures with higher updraft and higher ice supersaturation, indicating heterogeneous and homogeneous nucleation mechanisms, respectively.Plain Language Summary Cirrus clouds in the upper troposphere can either warm or cool the Earth surface. At temperatures ≤−40°C, cirrus clouds are entirely composed of ice and form under two major mechanisms, depending upon the environmental conditions such as temperature, water vapor, vertical velocity, and aerosols. Ice forms with the help of a solid particle via heterogeneous nucleation, while homogeneous nucleation directly freezes liquid aerosols. Because multiple factors can affect cirrus cloud formation, a large uncertainty in understanding the individual effect is present. This study uses a large global data set of aircraft observations to better understand cirrus cloud formation. Findings indicate that controlling environmental factors is critical before investigating the aerosol indirect effects on ice clouds. Ice crystals are more populated and larger, in the presence of more aerosols. Comparing the impacts of larger and smaller aerosols, they show different effectiveness under various environmental conditions, indicating ice formation via heterogeneous and homogeneous nucleation, respectively. These results suggest that aerosols emitted by human activities very likely modify cirrus cloud properties, but their influences are nonmonotonic and depend on environmental conditions.

Influence of temperature perturbation on moisture dynamics

“…with μ T and ν T as in equation (17). Here, q j and g  are respectively the mixing ratio of the moist air and the acceleration due to gravity of the Earth.…”

“…The transposition operation is represented by the superscript T and μ T as in equation (17). The frictional force f vis…”

“…Furthermore, clouds act as a shield against long wave radiation that is released by continents and oceans, absorbing it and re-emitting part of it back into space. This process keeps the planet's average temperature constant and produces the atmosphere's greenhouse effect [16][17][18][19].…”

Effects of Thermodynamics, Dynamics and Aerosols on Cirrus Clouds Based on In Situ Observations and NCAR CAM6 Model

“…Chylek et al (2006) showed an increase in ice crystal size during the more polluted winter months compared with cleaner summer months over the eastern Indian Ocean. Using a global-scale dataset of multiple flight campaigns, Patnaude and Diao (2020) isolated individual effects on cirrus clouds from temperature, RHi, vertical velocity (w) and aerosol number concentrations (Na). They found that when Na is 3 -10 times higher than average conditions, it shows strong positive correlations with cirrus microphysical properties such as IWC, Ni and number-weighted mean diameter (Di).…”