Distributions of ice supersaturation and ice crystals from airborne observations in relation to upper tropospheric dynamical boundaries

Abstract

Ice supersaturation (ISS) is the prerequisite condition for cirrus cloud formation. To examine multiscale dynamics' influences on ISS formation, we analyze in situ aircraft observations (~200 m scale) over North America in coordinates relative to dynamical boundaries in the upper troposphere and lower stratosphere. Two case studies demonstrate that ISS formation is likely influenced by mesoscale uplifting, small-scale waves, and turbulence. A collective analysis of 15 flights in April-June 2008 shows that the top layers of ISS and ice crystal distributions are strongly associated with thermal tropopause height. In addition, the average occurrence frequencies of ISS and ice crystals on the anticyclonic side of the jet stream are~1.5-2 times of those on the cyclonic side. By defining five cirrus evolution phases based on the spatial relationships between ice-supersaturated and in-cloud regions, we find that their peak occurrence frequencies are located at decreasing altitudes with respect to the thermal tropopause: (phase 1) clear-sky ISS around the tropopause, (phase 2) nucleation phase around 2 km below the tropopause, (phases 3 and 4) early and later growth phases around 6 km below the tropopause, and (phase 5) sedimentation/sublimation around 2-6 km below the tropopause. Consistent with this result, chemical tracer correlation analysis shows that the majority (~80%) of the earlier cirrus phases (clear-sky ISS and nucleation) occurs inside the chemical tropopause transition layer, while the later phases happen mostly below that layer. These results shed light on the role of dynamical environment in facilitating cirrus cloud formation and evolution.

CALIPSO observations of the dependence of homo- and heterogeneous ice nucleation in cirrus clouds on latitude, season and surface condition

“…Thereafter the cirrus can exist for long periods during their sedimentation stage. To summarize, with regards to in situ cirrus clouds at mid-latitudes, our results appear consistent with the findings of Diao et al (2013Diao et al ( , 2014Diao et al ( , 2015 and do not imply the existence of pre-existing ice.…”

“…Again, this supports the cirrus formation mechanisms suggested by Diao et al (2015). During summer (DJF) over land, the pattern is again similar Atmos.…”

“…In addition to this study, the observational findings of Diao et al (2015) described in Sect. 5.3 suggest that the pre-existing ice assumption may not be appropriate for in situ cirrus since the nucleation zone is near cloud top where little ice surface area exists to reduce RHi.…”

“…For example, based on 15 flights measuring cirrus clouds over the continental USA, Diao et al (2015) found that the ice nucleation zone was generally near cloud top near the thermal tropopause. They point out that this may occur for two reasons: (1) abrupt changes of temperature and humidity near the tropopause, and (2) frequent occurrence of clear-air turbulence around 25 the jet-stream.…”

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

“…The purpose of this study is to evaluate the frequency and magnitude of the simulated ISS in the upper troposphere in the Weather Research and Forecasting (WRF) model and compare results with in situ observations. Previously, Diao et al [2015] examined in situ airborne observations of RHi in the upper troposphere/lower stratosphere over North America and found the average occurrence frequencies of ISS to be~1.5-2 times greater on the anticyclonic side of the polar jet stream compared with the cyclonic side. This study aims to similarly examine the occurrence and magnitude of RHi in convective regions.…”