Examination of aerosol indirect effects during cirrus cloud evolution
Authors: Maciel Flor Vanessa , Diao Minghui , Patnaude Ryan 
Published: January 2023

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.