Dynamical conditions of ice supersaturation and ice nucleation in convective systems: A comparative analysis between in situ aircraft observations and WRF simulations

Abstract

Occurrence frequency and dynamical conditions of ice supersaturation (ISS, where relative humidity with respect to ice (RHi) > 100%) are examined in the upper troposphere around convective activity. Comparisons are conducted between in situ airborne observations and the Weather Research and Forecasting model simulations using four double‐moment microphysical schemes at temperatures ≤ −40°C. All four schemes capture both clear‐sky and in‐cloud ISS conditions. However, the clear‐sky (in‐cloud) ISS conditions are completely (significantly) limited to the RHi thresholds of the Cooper parameterization. In all of the simulations, ISS occurrence frequencies are higher by ~3–4 orders of magnitude at higher updraft speeds (>1 m s−1) than those at the lower updraft speeds when ice water content (IWC) > 0.01 g m−3, while observations show smaller differences up to ~1–2 orders of magnitude. The simulated ISS also occurs less frequently at weaker updrafts and downdrafts than observed. These results indicate that the simulations have a greater dependence on stronger updrafts to maintain/generate ISS at higher IWC. At lower IWC (≤0.01 g m−3), simulations unexpectedly show lower ISS frequencies at stronger updrafts. Overall, the Thompson aerosol‐aware scheme has the closest magnitudes and frequencies of ISS >20% to the observations, and the modified Morrison has the closest correlations between ISS frequencies and vertical velocity at higher IWC and number density. The Cooper parameterization often generates excessive ice crystals and therefore suppresses the frequency and magnitude of ISS, indicating that it should be initiated at higher ISS (e.g., ≥25%).

Aerosol Indirect Effects on Cirrus Clouds Based on Global Aircraft Observations

“…The impacts on Ni and Di are similar to those seen in IWC, except that Ni is less sensitive to temperature, and the maximum Di occurs at higher RHi (30% d RHi). These findings are consistent with studies that only analyzed one of these factors, such as Schiller et al () (temperature effect in their Figure 2), Diao, Zondlo, et al () (RHi effect in their Figure 3), and D'Alessandro et al () (w effect in their Figures 9 and 10). Our results demonstrate the necessity of controlling temperature, RHi, and w before investigating aerosol indirect effects, since these factors could either obscure or amplify the signal of aerosol indirect effects.…”

“…To alleviate the shattering effect and optical uncertainties, we exclude ice particles <62.5 μm (omitting the first two bins), while various cirrus cloud evolution phases can still be observed by using a size cutoff of 100 μm (Diao, Zondlo, Heymsfield, & Beaton, ). For definition of in‐cloud conditions, we follow several previous studies that analyzed the G‐V aircraft observations for clear‐sky and in‐cloud conditions (D'Alessandro et al, ; Diao, Zondlo, et al, ; Diao, Zondlo, Heymsfield, Avallone, et al, , , ) whereas in‐cloud is considered when the 1‐s sample of the Fast‐2DC probe detects at least one ice particle (i.e., >0 L −1 ), while all the remaining data are considered clear‐sky conditions.…”

“…The maximum and minimum IWC, Ni, and Di occur at higher updrafts ( d w > 3 m/s) and weak downdrafts ( d w = −0.5 m/s), respectively. The higher IWC, Ni, and Di in updrafts and downdrafts are likely due to the increasing ISS frequency in updrafts and the proximity between updrafts and downdrafts, respectively (D'Alessandro et al, ). Comparing the thermodynamic effect (RHi) and dynamic effect (w), the average IWC and Ni increase 1 order of magnitude when d RHi increases from −15% to 10%, comparable to the effect of d w increasing from 0 to 1 m/s.…”

Can Quasi‐Periodic Gravity Waves Influence the Shape of Ice Crystals in Cirrus Clouds?

“…However, to the authors' knowledge, there have been no observational studies reporting the influence of GWs on ice crystal habits. Moreover, dynamical influence on cirrus clouds is one of the major uncertainties in climate models (Boucher et al, 2013; D'Alessandro et al, 2017; Wu et al, 2017). Henceforth, we have focused on ice crystal habit variations in cirrus clouds in the presence of upward propagating GWs.…”

Respones to Referee #1 comments

“…The uncertainties are the result of several different aspects. First, our understanding of processes initiating the cirrus cloud formation is still limited (DeMott et al, 2003;Kärcher and Spitchtinger, 2009;Hoose and Möhler, 2012). Ice crystals can form via the homogeneous nucleation of soluble aerosol particles and the heterogeneous nucleation associated with insoluble or partly insoluble aerosol particles (e.g., Hagg et al, 2003;Liu and Penner, 2005;.…”