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Title: Examination of aerosol indirect effects during cirrus cloud evolution
Abstract. Aerosols affect cirrus formation and evolution, yet quantificationof these effects remain difficult based on in situ observations due to thecomplexity of nucleation mechanisms and large variabilities in icemicrophysical properties. This work employed a method to distinguish fiveevolution phases of cirrus clouds based on in situ aircraft-basedobservations from seven U.S. National Science Foundation (NSF) and five NASAflight campaigns. Both homogeneous and heterogeneous nucleation werecaptured in the 1 Hz aircraft observations, inferred from the distributionsof relative humidity in the nucleation phase. Using linear regressions toquantify the correlations between cirrus microphysical properties andaerosol number concentrations, we found that ice water content (IWC) and icecrystal number concentration (Ni) show strong positive correlations withlarger aerosols (>500 nm) in the nucleation phase, indicatingstrong contributions of heterogeneous nucleation when ice crystals firststart to nucleate. For the later growth phase, IWC and Ni show similarpositive correlations with larger and smaller (i.e., >100 nm)aerosols, possibly due to fewer remaining ice-nucleating particles in thelater growth phase that allows more homogeneous nucleation to occur. Both200 m and 100 km observations were compared with the nudged simulations fromthe National Center for Atmospheric Research (NCAR) Community AtmosphereModel version 6 (CAM6). Simulated aerosol indirect effects are weaker thanthe observations for both larger and smaller aerosols for in situ cirrus,while more » the simulated aerosol indirect effects are closer to observations inconvective cirrus. The results also indicate that simulations overestimatehomogeneous freezing, underestimate heterogeneous nucleation andunderestimate the continuous formation and growth of ice crystals as cirrusclouds evolve. Observations show positive correlations of IWC, Ni and icecrystal mean diameter (Di) with respect to Na in both the Northern and SouthernHemisphere (NH and SH), while the simulations show negative correlations inthe SH. The observations also show higher increases of IWC and Ni in the SHunder the same increase of Na than those shown in the NH, indicating highersensitivity of cirrus microphysical properties to increases of Na in the SHthan the NH. The simulations underestimate IWC by a factor of 3–30 in theearly/later growth phase, indicating that the low bias of simulated IWC wasdue to insufficient continuous ice particle formation and growth. Sucha hypothesis is consistent with the model biases of lower frequencies of icesupersaturation and lower vertical velocity standard deviation in theearly/later growth phases. Overall, these findings show that aircraftobservations can capture both heterogeneous and homogeneous nucleation, andtheir contributions vary as cirrus clouds evolve. Future model developmentis also recommended to evaluate and improve the representation of watervapor and vertical velocity on the sub-grid scale to resolve theinsufficient ice particle formation and growth after the initial nucleationevent. « less
Authors:
; ;
Award ID(s):
1744965 1642291
Publication Date:
NSF-PAR ID:
10400911
Journal Name:
Atmospheric Chemistry and Physics
Volume:
23
Issue:
2
Page Range or eLocation-ID:
1103 to 1129
ISSN:
1680-7324
Sponsoring Org:
National Science Foundation
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