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1. Examination of Aerosol Indirect Effects during Cirrus Cloud Evolution

   https://doi.org/10.5194/acp-2022-519  

   Maciel, Flor V. ; Diao, Minghui ; Patnaude, Ryan ( July 2022 , Atmospheric chemistry and physics discussion)

   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. Observations show stronger aerosol indirect effects (i.e., positive correlations between IWC, Ni and Na) in the Southern Hemisphere (SH) compared with the Northern Hemisphere (NH), while the simulations show negative correlations in the SH. 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 ice particle growth. Such 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 the competitions between 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 growth. 

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2. Effects of thermodynamics, dynamics and aerosols on cirrus clouds based on in situ observations and NCAR CAM6

   https://doi.org/10.5194/acp-21-1835-2021  

   Patnaude, Ryan ; Diao, Minghui ; Liu, Xiaohong ; Chu, Suqian ( January 2021 , Atmospheric Chemistry and Physics)

   null (Ed.)

   Abstract. Cirrus cloud radiative effects are largely affected byice microphysical properties, including ice water content (IWC), ice crystalnumber concentration (Ni) and mean diameter (Di). These characteristics varysignificantly due to thermodynamic, dynamical and aerosol conditions. Inthis work, a global-scale observation dataset is used to examine regionalvariations of cirrus cloud microphysical properties, as well as several keycontrolling factors, i.e., temperature, relative humidity with respect toice (RHi), vertical velocity (w) and aerosol number concentrations (Na).Results are compared with simulations from the National Center forAtmospheric 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 indata collection. The differences between simulations and observations arefound to vary with latitude and temperature. Comparing with averagedobservations at ∼100 km horizontal scale, simulations arefound to underestimate (overestimate) IWC by a factor of 3–10 in theNorthern (Southern) Hemisphere. Simulated Ni is overestimated in mostregions except the Northern Hemisphere midlatitudes. Simulated Di isunderestimated by a factor of 2, especially for warmer conditions(−50 to −40 ∘C), possibly due tomisrepresentation of ice particle growth/sedimentation. For RHi effects, thefrequency and magnitude of ice supersaturation are underestimated insimulations for clear-sky conditions. The simulated IWC and Ni show bimodaldistributions with maximum values at 100 % and 80 % RHi, differing fromthe unimodal distributions that peak at 100 % in the observations. For weffects, both observations and simulations show variances of w (σw) decreasing from the tropics to polar regions, but simulations show muchhigher σw for the in-cloud condition than the clear-sky condition.Compared with observations, simulations show weaker aerosol indirect effectswith a smaller increase of IWC and Di at higher Na. These findings provide anobservation-based guideline for improving simulated ice microphysicalproperties and their relationships with key controlling factors at variousgeographical locations. 

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

   https://doi.org/10.5194/acp-2020-497  

   Patnaude, Ryan ; Diao, Minghui ; Liu, Xiaohong ; Chu, Suqian ( July 2020 , Atmospheric chemistry and physics discussion)

   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). The differences between simulations and observations are found to vary with latitude and temperature. Specifically, simulations are found to underestimate IWC by a factor of 5–30 in all regions. Simulated Ni is overestimated in most regions except Northern Hemisphere midlatitude and polar regions. Simulated Di is underestimated, especially for warmer conditions (−50 °C to −40 °C) and higher Na, possibly due to less effective ice particle growth/sedimentation and weaker aerosol indirect effects, respectively. For RHi effects, the frequency and magnitude of ice supersaturation is underestimated in simulations for clear-sky conditions, and the simulated IWC and Ni show maximum values at 80 % RHi instead of 110 % as observed. For w effects, both observations and simulations show variances of w (σw) decreasing from tropics to polar regions, but simulations show much higher σw for in-cloud condition than clear-sky condition. These findings provide an observation-based guideline for improving simulated ice microphysical properties and their relationships with key controlling factors at various geographical locations. 

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4. Ice and Supercooled Liquid Water Distributions Over the Southern Ocean Based on In Situ Observations and Climate Model Simulations

   https://doi.org/10.1029/2021JD036045  

   Yang, Ching An ; Diao, Minghui ; Gettelman, Andrew ; Zhang, Kai ; Sun, Jian ; McFarquhar, Greg ; Wu, Wei ( December 2021 , Journal of Geophysical Research: Atmospheres)

   Three climate models are evaluated using in situ airborne observations from the Southern Ocean Clouds, Radiation, Aerosol Transport Experimental Study (SOCRATES) campaign. The evaluation targets cloud phases, microphysical properties, thermodynamic conditions, and aerosol indirect effects from −40°C to 0°C. Compared with 580‐s averaged observations (i.e., 100 km horizontal scale), the Community Atmosphere Model version 6 (CAM6) shows the most similar result for cloud phase frequency distribution and allows more liquid‐containing clouds below −10°C compared with its predecessor—CAM5. The Energy Exascale Earth System Model (E3SM) underestimates (overestimates) ice phase frequencies below (above) −20°C. CAM6 and E3SM show liquid and ice water contents (i.e., LWC and IWC) similar to observations from −25°C to 0°C, but higher LWC and lower IWC than observations at lower temperatures. Simulated in‐cloud RH shows higher minimum values than observations, possibly restricting ice growth during sedimentation. As number concentrations of aerosols larger than 500 nm (Na₅₀₀) increase, observations show increases of LWC, IWC, liquid, and ice number concentrations (N_liq, N_ice). Number concentrations of aerosols larger than 100 nm (Na₁₀₀) only show positive correlations with LWC and N_liq. From −20°C to 0°C, higher aerosol number concentrations are correlated with lower glaciation ratio and higher cloud fraction. From −40°C to −20°C, large aerosols show positive correlations with glaciation ratio. CAM6 shows small increases of LWC and N_liqwith Na₅₀₀and Na₁₀₀. E3SM shows small increases of N_icewith Na₅₀₀. Overall, CAM6 and E3SM underestimate aerosol indirect effects on ice crystals and supercooled liquid droplets over the Southern Ocean.

    

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5. The Transition from Supercooled Liquid Water to Ice Crystals in Mixed-phase Clouds based on Airborne In-situ Observations

   Flor Vanessa Maciel ; Minghui Diao ( October 2022 , Atmospheric measurement techniques Papers in open discussion)

   The on-set of ice nucleation in mixed-phase clouds determines cloud lifetime and their microphysical properties. In this work, we develop a novel method that differentiates the early and later transition phases of mixed-phase clouds, i.e., ice crystals are initially surrounded by supercooled liquid water droplets, then as they grow, pure ice segments are formed. Using this method, we examine the relationship between the macrophysical and microphysical properties of mixed-phase clouds. The results show that evolution of cloud macrophysical properties, represented by the increasing spatial ratio of regions containing ice crystals relative to the total in-cloud region (defined as ice spatial ratio), is positively correlated with the evolution of microphysical properties, represented by the increasing ice water content and decreasing liquid water content. The mass partition transition from liquid to ice becomes more significant during the later transition phase (i.e., transition phase 3) when pure ice cloud regions (ICRs) start to appear. Occurrence frequencies of cloud thermodynamic phases show significant transition from liquid to ice at a similar temperature (i.e., -17.5 °C) among three types of definitions of mixed-phase clouds based on ice mass fraction, ice number fraction, or ice spatial ratio. Aerosol indirect effects are quantified for different transition phases using number concentrations of aerosols greater than 100 nm or 500 nm (N>100 and N>500, respectively). N>500 shows stronger positive correlations with ice spatial ratios compared with N>100. This result indicates that larger aerosols potentially contain ice nucleating particles, which facilitate the formation of ice crystals in mixed-phase clouds. The impact of N>500 is also more significant on the earlier transition phase when ice crystals just start to appear compared with the later transition phase. The thermodynamic and dynamic conditions are quantified for each transition phase. The results show in-cloud turbulence as a main mechanism for both the initiation of ice nucleation and the maintenance of supercooled liquid water, while updrafts are important for the latter but not the former. Overall, these results illustrate the varying effects of aerosols, thermodynamics, and dynamics throughout cloud evolution based on this new method that categorizes cloud transition phases. 

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