Evolution of ice crystal regions on the microscale based on in situ observations

Abstract

[1] Microphysical properties of cirrus clouds largely influence their atmospheric radiative forcing. However, uncertainties remain in simulating/parameterizing the evolution of ice crystals. These uncertainties require more analyses in the Lagrangian view, yet most in situ observations are in the Eulerian view. Here we demonstrate a new method to separate out five phases of ice crystal evolution, using the horizontal spatial relationships between ice supersaturated regions (ISSRs) and ice crystal regions (ICRs). Based on global in situ data sets, we show that the samples of clear-sky ISSRs, ice crystal formation/growth, and evaporation/sedimentation are~20%, 10%, and 70% of the total ISSR + ICR samples, respectively. In addition, the variance of number-weighted mean diameter (Dc) becomes narrower during the evolution, while the distribution of ice crystal number density (Nc) becomes wider. The new method helps to understand the evolution of ICRs and ISSRs on the microscale by using in situ Eulerian observations.

Effects of pre-existing ice crystals on cirrus clouds and comparison between different ice nucleation parameterizations with the Community Atmosphere Model (CAM5)

“…Including the latter may lead to a much stronger effect and coupling between different nucleation events. The diagnosed f hom is in general less than 20 %, consistent with the work of Diao et al (2013). We note that the uncertainty caused by f hom is moderate because the effect of f hom on ice number concentration is weaker than the PREICE effect.…”

“…Third, the upper limit (0.2 m s −1 ) of W sub is also removed because updraft velocities measured from several aircraft campaigns show frequent occurrence of larger values (> 0.2 m s −1 ; Zhang et al, 2013b). Finally, in situ observations of cirrus clouds show that only a small fraction of in-cloud S i data surpasses the homogeneous freezing saturation threshold (S hom ; Diao et al, 2013). So, we assume that the homogeneous freezing takes place only in a fraction of cirrus clouds (f hom ) where in-cloud S i > S hom .…”

“…Both modeling and observational analyses suggest that f hom in the tropical regions is larger than that in mid-latitudes. Diao et al (2013) analyzed the evolution of ice crystals based on in situ observations over North America. They found that ice crystal formation/growth is ∼ 20 % of total analyzed samples.…”

Direct comparisons of ice cloud macro- and microphysical properties simulated by the Community Atmosphere Model version 5 with HIPPO aircraft observations

“…Currently there are still large unknowns about the types of aerosol, modes of action (e.g., immersion/condensation, deposition, and contact), and the efficiencies of heterogeneous nucleation in the atmosphere (Hoose and Möhler, 2012). Other ice microphysical processes (e.g., ice aggregation, deposition/sublimation, and sedimentation) as well as interactions among cirrus microphysical properties, macroscopic properties (e.g., spatial extent), and meteorological fields could further render the interpretation of observed ice cloud properties challenging (Diao et al, 2013;Krämer et al, 2016).…”

“…The observed microphysical properties of cirrus clouds vary significantly in time and space (e.g., Hoyle et al, 2005;Diao et al, 2013Diao et al, , 2014aJensen et al, 2013), associated with variability in relative humidity (RH), temperature, and vertical wind speed. The spatial extent of clouds is represented in GCMs by diagnosing the cloud fraction in individual model grid boxes using a parameterization.…”