Dynamical condition of ice supersaturation and cirrus clouds in the extratropical upper troposphere and lower stratosphere

Cirrus clouds are one of the largest sources of uncertainties in predicting future climate. Ice nucleation and ice crystal growth inside cirrus clouds require ice supersaturation (ISS). Previously, two case studies analyzed the mesoscale (~100 km) processes that contribute to ISS formation in the extratropical upper troposphere and lower stratosphere (Spichtinger et al., 2005 a and b). However, it is still not well understood as for the dynamical processes and conditions that contribute to ISS formation from the microscale (~100 m) to the mesoscale. In this work we present the analyses of ISS (temperature ≤ -40 °C) at 1 Hz resolution (~200 m) using the water vapor measurement by the Vertical Cavity Surface Emitting Laser (VCSEL) Hygrometer (Zondlo et al., 2010) on the NSF Gulf-stream V (GV) research airplane. The analyses are based on observations from the NSF Stratosphere Troposphere Analyses of Regional Transport (START08) Campaign, which sampled chemical and microphysical variables under various dynamical conditions near the extratropical tropopause and upper level jets.

One of the key analyses is on the location of subtropical jet stream and extratropical thermal tropopause and their influences on the formation of ice supersaturated regions (ISSRs) and ice crystal regions (ICRs). Most of the ISSRs were observed below the thermal tropopause on the pole-ward side of the jet. In addition, based on the O3-CO tracer correlation, the dynamical conditions of two different schemes for ISS are compared: clear-sky ISS versus ISS with ice crystals. The clear-sky ISSRs show influences of mixing from both stratospheric-tropospheric and tropospheric-tropospheric origins, while the ISSRs coexisting with ice crystals show influence of mixing mostly from tropospheric-tropospheric origins.

In order to understand the convective influence on ISSR formation, back trajectory data were used to assess fast updraft during the evolution of air parcels. The results show that some but not all of the ISSRs have experienced convective influence. Case studies are used to further compare the specific dynamical conditions for ISS and ice particles with and without convective influence.