Dynamical Conditions of Ice Supersaturation and Cirrus Clouds in the Extratropical Upper Troposphere and Lower Stratosphere

Abstract:

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, remote sensing observations have shown that the locations of cirrus clouds’ cloud top height are highly correlated with the thermal tropopause height (Pan and Munchak, 2011). However, it is unclear if the initial conditions of cirrus clouds – ice supersaturated region (ISSRs) – have similar features in the extratropical upper troposphere and lower stratosphere (UT/LS). In fact, the dynamical processes and conditions that contribute to ISS formation from the microscale (~100 m) to the mesoscale (~100 km) are still not well understood.

In this work, we present in-situ observations of ISS and ice crystals with ~200 m horizontal resolution at temperatures ≤ -40°C. Our analyses are based on 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 about the influences of jet stream and extratropical thermal tropopause on the formation of ice supersaturated regions (ISSRs) and ice crystals. We found that most of the ISSRs are vertically constrained by the thermal tropopause height, agreeing with the findings of cirrus cloud top constraint (Pan and Munchak, 2011). In addition, the number of ISSRs observed on the anti-cyclonic side of the jet is ~1.6 times of that on the cyclonic side, consistent with previous model simulations (Gierens and Brinkop, 2012). Based on the O₃-CO and O₃-H₂O tracer-tracer correlations, two different air mass mixing scenarios are found: stratosphere-troposphere mixing and troposphere-troposphere mixing. These different mixing scenarios are also associated with different dynamical conditions: small-scale waves/turbulence and large scale uplifting, respectively. Understanding the dynamical conditions of ISSR and cirrus clouds and the related microphysical properties will ultimately improve the simulation of cirrus clouds and the prediction of their radiative forcing in the future climate.