Abstract. Cirrus cloud radiative effects are largely affected by ice microphysical properties, including ice water content (IWC), ice crystal number concentration (Ni) and mean diameter (Di). These characteristics vary significantly due to thermodynamic, dynamical and aerosol conditions. In this work, a global-scale observation dataset is used to examine regional variations of cirrus cloud microphysical properties, as well as several key controlling factors, i.e., temperature, relative humidity with respect to ice (RHi), vertical velocity (w) and aerosol number concentrations (Na). Results are compared with simulations from the National Center for Atmospheric Research (NCAR) Community Atmosphere Model version 6 (CAM6). Observed and simulated ice mass and number concentrations are constrained to ≥62.5 µm to reduce potential uncertainty from shattered ice in data collection. The differences between simulations and observations are found to vary with latitude and temperature. Comparing with averaged observations at ∼100 km horizontal scale, simulations are found to underestimate (overestimate) IWC by a factor of 3–10 in the Northern (Southern) Hemisphere. Simulated Ni is overestimated in most regions except the Northern Hemisphere midlatitudes. Simulated Di is underestimated by a factor of 2, especially for warmer conditions (−50 to −40 ∘C), possibly due to misrepresentation of ice particle growth/sedimentation. For RHi effects, the frequency and magnitude of ice supersaturation are underestimated in simulations for clear-sky conditions. The simulated IWC and Ni show bimodal distributions with maximum values at 100 % and 80 % RHi, differing from the unimodal distributions that peak at 100 % in the observations. For w effects, both observations and simulations show variances of w (σw) decreasing from the tropics to polar regions, but simulations show much higher σw for the in-cloud condition than the clear-sky condition. Compared with observations, simulations show weaker aerosol indirect effects with a smaller increase of IWC and Di at higher Na. These findings provide an observation-based guideline for improving simulated ice microphysical properties and their relationships with key controlling factors at various geographical locations.

The ice nucleating ability of sea spray aerosol (SSA) particles has been explored in recent years due to the abundance of SSAs in the atmosphere. The role of SSAs in ice nucleation extends to cirrus clouds, due to processes that loft SSAs to the upper troposphere. This is of special relevance because of the frequent occurrence of cirrus in the atmosphere, their role in the Earth's radiative balance, and uncertainties regarding how aerosols may affect their formation and evolution. In this study, a continuous flow diffusion chamber (CFDC) is used to investigate the ice nucleating ability of size-selected particle distributions of SSAs and its primary constituent sodium chloride (NaCl) at temperatures <235 K. Results show that, above ∼220 K, the majority of NaCl and SSA particles fully deliquesce and freeze via homogeneous nucleation at or below water relative humidities, RH w , of ∼95%. However, below 220 K, the onset RH w of freezing for NaCl and SSA particles is much lower, at ∼75%, where strong heterogeneous freezing of 10% of the aerosol population occurs. Similar heterogeneous freezing behavior for NaCl and SSA particles, occurring near their predicted deliquescence RH w , points toward SSA freezing at the lowest temperatures being controlled by the crystalline salts. Finally, the calculations of ice nucleation active surface site densities show that particle size does not dictate the efficiency of freezing for NaCl and SSA particles. These results indicate SSAs as a potentially significant source of ice nucleating particles at cirrus temperatures, with the ability to contribute to cirrusmediated climate impacts if sea spray emission and transport scenarios change in the future.

Abstract. Aerosols affect cirrus formation and evolution, yet quantification of these effects remain difficult based on in situ observations due to the complexity of nucleation mechanisms and large variabilities in ice microphysical properties. This work employed a method to distinguish five evolution phases of cirrus clouds based on in situ aircraft-based observations from seven U.S. National Science Foundation (NSF) and five NASA flight campaigns. Both homogeneous and heterogeneous nucleation were captured in the 1 Hz aircraft observations, inferred from the distributions of relative humidity in the nucleation phase. Using linear regressions to quantify the correlations between cirrus microphysical properties and aerosol number concentrations, we found that ice water content (IWC) and ice crystal number concentration (Ni) show strong positive correlations with larger aerosols (>500 nm) in the nucleation phase, indicating strong contributions of heterogeneous nucleation when ice crystals first start to nucleate. For the later growth phase, IWC and Ni show similar positive correlations with larger and smaller (i.e., >100 nm) aerosols, possibly due to fewer remaining ice-nucleating particles in the later growth phase that allows more homogeneous nucleation to occur. Both 200 m and 100 km observations were compared with the nudged simulations from the National Center for Atmospheric Research (NCAR) Community Atmosphere Model version 6 (CAM6). Simulated aerosol indirect effects are weaker than the observations for both larger and smaller aerosols for in situ cirrus, while the simulated aerosol indirect effects are closer to observations in convective cirrus. The results also indicate that simulations overestimate homogeneous freezing, underestimate heterogeneous nucleation and underestimate the continuous formation and growth of ice crystals as cirrus clouds evolve. Observations show positive correlations of IWC, Ni and ice crystal mean diameter (Di) with respect to Na in both the Northern and Southern Hemisphere (NH and SH), while the simulations show negative correlations in the SH. The observations also show higher increases of IWC and Ni in the SH under the same increase of Na than those shown in the NH, indicating higher sensitivity of cirrus microphysical properties to increases of Na in the SH than the NH. The simulations underestimate IWC by a factor of 3–30 in the early/later growth phase, indicating that the low bias of simulated IWC was due to insufficient continuous ice particle formation and growth. Such a hypothesis is consistent with the model biases of lower frequencies of ice supersaturation and lower vertical velocity standard deviation in the early/later growth phases. Overall, these findings show that aircraft observations can capture both heterogeneous and homogeneous nucleation, and their contributions vary as cirrus clouds evolve. Future model development is also recommended to evaluate and improve the representation of water vapor and vertical velocity on the sub-grid scale to resolve the insufficient ice particle formation and growth after the initial nucleation event.

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.

The ice nucleating ability of sea spray aerosols (SSA) has been explored in recent years due to the abundance of SSA in the atmosphere. The role of SSA in ice nucleation extends to cirrus clouds, due to processes that loft SSA to the upper troposphere. This is of special relevance because of the frequent occurrence of cirrus in the atmosphere, their role in the Earth’s radiative balance, and uncertainties regarding how aerosols may affect their formation and evolution. In this study, a continuous flow diffusion chamber (CFDC) is used to investigate the ice nucleating ability of size-selected particle distributions of SSA and its primary constituent sodium chloride (NaCl) at temperatures < 235 K. Results show that above ~220 K, the majority of SSA and NaCl particles fully deliquesce and freeze via homogeneous nucleation at or below water relative humidities, RHw, of ~ 95%. However, below 220 K, the onset RHw of freezing for NaCl and SSA is much lower, at ~75%, where strong heterogeneous freezing of 10% of the aerosol population occurs. Similar heterogeneous freezing behavior for NaCl and SSA aerosols, occurring near their predicted deliquescence RHw, points towards SSA freezing at the lowest temperatures being controlled by the crystalline salts. Finally, calculations of ice nucleation active surface site densities show that particle size does not dictate the efficiency of freezing for NaCl and SSA. These results indicate SSA as a potentially significant source of ice nucleating particles at cirrus temperatures, with the ability to contribute to cirrus-mediated climate impacts if sea spray emission and transport scenarios change in the future.