In this article, the effects 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.
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. Plain Language Summary Spontaneous freezing of airborne, water-containing particles below −38 ◦C is a fundamental pathway to form ice crystals in high-altitude cirrus clouds. This ice formation process has been well researched and was the first represented in weather forecast and climate models to advance cirrus predictions. One key characteristic is its strong dependence of the number of ice crystals formed on the cooling rate of air. Recent observations show that rapid cooling rates are generated by ubiquitous gravity waves. Here, we explore the rich suite of phenomena taking place during cirrus formation caused by a spectrum of gravity waves. We find that wave effects should be considered in future model simulations, when comparing model results with observations, and in parameterizations of cloud ice crystal formation.
TL;DR: In this paper, the composition of the residual particles within cirrus crystals after the ice was sublimated was determined in situ, showing that mineral dust and metallic particles are the dominant sources of residual particles, whereas sulfate and organic particles are underrepresented, and elemental carbon and biological materials are essentially absent.
TL;DR: In this article, the authors make use of long-duration balloon observations to devise a probabilistic model describing mesoscale temperature uctuations (MTF) away from strong wave sources and show that MTF are subject to damping at a rate near the Coriolis frequency when the vertical wind speed fluctuations are autocorrelated over a fraction of a Brunt Väisälä period.
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Q1. What are the contributions in "The impact of mesoscale gravity waves on homogeneous ice nucleation in cirrus clouds" ?
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. This ice formation process has been well researched and was the first represented in weather forecast and climate models to advance cirrus predictions. The authors find that wave effects should be considered in future model simulations, when comparing model results with observations, and in parameterizations of cloud ice crystal formation. 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. 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.
Q2. What are the future works in "The impact of mesoscale gravity waves on homogeneous ice nucleation in cirrus clouds" ?
Given the rather long times required for stochastic trajectories to trigger freezing even in air that is already ice supersaturated, future studies should investigate in which situations large-scale cooling, local relative humidity, and INP conditions are most relevant for atmospheric applications. While the effect of such damping is unimportant for the present study, its effect on first freezing times for less ice supersaturated initial conditions and weak or absent large-scale forcing should be studied in future work. The overall similarity of the shapes of simulated and analytical statistics suggests that wave-driven dynamical forcing and homogeneous freezing play important roles in in situ cirrus formation, inasmuch as ICNC values > 10 cm−3 have been measured in cirrus. More importantly, differences between measured and simulated ICNC statistics are consistent with the potential importance of ice crystal sedimentation and INP in cirrus as observed during an aircraft campaign over the continental United States.