The Impact of Mesoscale Gravity Waves on Homogeneous Ice Nucleation in Cirrus Clouds

Abstract

Effects of a spectrum of mesoscale gravity waves on homogeneous aerosol freezing in midlatitude cirrus are studied by means of parcel model simulations that are driven by random vertical wind speeds constrained by balloon measurements. Stochastic wave forcing with mean updraft speeds of 5–20 cm/s leads to substantial nucleated ice crystal number concentrations (ICNC) of 0.1–1 cm−3 in situations with slow large‐scale cooling, which by itself would generate fewer ice crystals. The stochastic nature of wave‐driven air parcel temperatures enhances ICNC even further, but the times required to reach freezing conditions unsupported by large‐scale cooling may vary widely. In the presence of wave forcing, ice crystals with low ICNC (<1–10 L−1) are also generated by homogeneous freezing, albeit only rarely. Comparisons with aircraft measurements suggest significant influences of heterogeneous ice‐nucleating particles and ice crystal sedimentation on ICNC, but quantifying their individual contributions remains elusive.

Process‐Based Simulation of Aerosol‐Cloud Interactions in a One‐Dimensional Cirrus Model

“…Effects of (i) ice crystal surface kinetics on homogeneous freezing and depositional growth (Zhang & Harrington, ) and (ii) temperature fluctuations induced by mesoscale gravity waves on homogeneous freezing (Kärcher et al, ) have been studied with parcel models. These studies indicate that (i) surface kinetic limitations do not significantly impact the number of homogeneously nucleated ice crystals owing to the high ambient supersaturation in which homogeneous freezing takes place and (ii) gravity waves significantly enhance the mean, homogeneously nucleated ice numbers by exposing air to a wide range of cooling rates in excess of synoptic values.…”

A Stochastic Representation of Temperature Fluctuations Induced by Mesoscale Gravity Waves

“…First, while in the case of Brownian motion, t c —set by the time between molecular collisions—is very much shorter than any associated macroscopic time scale, values should not be neglected in microphysical applications of equation . The short autocorrelation time is responsible for the high frequency variability in CHR influencing ice crystal formation (Kärcher et al., ). Essentially, non‐zero t c ‐values from equation produce a short‐term MTF autocorrelation that results in an additional cutoff of S T ( ω ) at high frequencies.…”

“…Those might be superimposed, for example, onto a synoptic trajectory, . A range of mesoscale frequencies can be identified visually, the highest frequencies dominating cooling rates locally within short periods of time on the order of 1/ N =1.39 min, with important ramifications for ice crystal nucleation in cirrus (Kärcher et al., ). According to the statistical law of low leads, only a small subset of MTF paths behave like the black curve oscillating closely around zero net temperature excursion.…”

“…Kärcher et al. () report a first attempt to tackle this issue. Second, small‐scale turbulence is associated with entrainment and mixing of air parcels.…”

“…First, detailed microphysical ensemble simulations are needed to assess how mesoscale CHR variability affects the ice formation process-for example, in terms of expectation values for nucleated ice crystal numbers-that will differ from those obtained with a purely deterministic approach. Kärcher et al (2019) report a first attempt to tackle this issue. Second, small-scale turbulence is associated with entrainment and mixing of air parcels.…”