Examination of aerosol indirect effects during cirrus cloud evolution

Abstract

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.

Significant trend in vertical wind velocity variability over the last 40 years

“…Table 1]. All field data was extensively screened for quality, to ensure reliability [19,24]. For each campaign, the σ W dataset derived from MERRA-2 was interpolated and compared to the measured values along each flight track.…”

“…These environments were underrepresented in the training dataset [20]. Measurement of W in these conditions also carries a larger experimental error (∼ 0.3 m s −1 ) [24]. Nevertheless, Figure 2 shows that the B24 model performs well across diverse environments and can generalize beyond its training data.…”

A Study on the Wide Range of Relative Humidity in Cirrus Clouds Using Large-Ensemble Parcel Model Simulations

“…Multiple field campaigns have provided the microphysical properties of cirrus clouds [24,40]. The preferred choice is that of cirrus clouds which are consistent with the closed adiabatic assumption.…”

“…The in-cloud RHi dominates the deposition/sublimation process of ICs and plays an important role in cloud evolution [15][16][17]. At present, the understanding of cirrus evolution is still incomplete, and there are very few studies that have investigated the mechanisms related to the wide range of in-cloud RHi [18][19][20][21][22][23][24].…”

“…The cloud parcel model is an important tool for analyzing cloud evolution [13,[37][38][39]. Furthermore, field campaigns with aircraft measurements have provided numerous in situ observations of the in-cloud RHi, ICs' physical characteristics, and the basic atmospheric state [24,40]. Based on these observations and a few artificial settings, the impact of vertical motion and IC deposition/sublimation on in-cloud RHi can be analyzed with large-ensemble parcel model simulations.…”