Preprints
https://doi.org/10.5194/acp-2022-519
https://doi.org/10.5194/acp-2022-519
 
29 Jul 2022
29 Jul 2022
Status: this preprint is currently under review for the journal ACP.

Examination of Aerosol Indirect Effects during Cirrus Cloud Evolution

Flor Vanessa Maciel1, Minghui Diao1, and Ryan Patnaude1,a Flor Vanessa Maciel et al.
  • 1Department of Meteorology and Climate Science, San Jose State University, San Jose, 95192, USA
  • acurrent affiliation: Department of Atmospheric Science, Colorado State University, Fort Collins, 80521, USA

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. 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.

Flor Vanessa Maciel et al.

Status: open (until 09 Sep 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Flor Vanessa Maciel et al.

Flor Vanessa Maciel et al.

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Short summary
Aerosol indirect effects on cirrus clouds are investigated during cirrus evolution, using global-scale in-situ observations and climate model simulations. As cirrus evolves, the mechanisms to form ice crystals also change with time. Both small and large aerosols are found to affect cirrus properties. Southern Hemisphere cirrus appears to be more sensitive to additional aerosols. The climate model underestimates ice crystal growth, likely due to biases of relative humidity and vertical velocity.
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