Aerosol and Dynamical Impacts on Cirrus Cloud Properties Analyzed from In-situ Observations and CAM5 Simulations (Invited Presentation)

Cirrus clouds in the upper troposphere play a key role in the Earth radiation budget, and their radiative forcing depends strongly on number concentration and size distribution of ice particles. Anthropogenic emissions from fossil fuel and biomass burning activities have substantially perturbed and enhanced concentrations of aerosol particles in the atmosphere. Currently it is still unclear how the formation of cirrus clouds and their microphysical properties are influenced by anthropogenic emissions. This lack of knowledge stems from the complexity of cirrus cloud formation: cirrus cloud properties can not only be influenced by aerosols (e.g., aerosol type, concentration) on which ice crystals are nucleated, but affected or even determined by dynamical conditions (e.g., updraft velocity, ice super-saturation, and temperature) under which cirrus clouds are formed. The evolution of cirrus clouds with the growth and sedimentation of ice crystals can further mask the correlation between aerosol and cirrus cloud properties.

In this work, we use airborne in-situ observations to compare cirrus cloud properties between polluted and pristine regions. Our dataset includes: the NSF HIAPER Pole-to-Pole Observations (HIPPO) Global campaign (2009-2011), and the EU Interhemispheric Differences In Cirrus Properties from Anthropogenic Emissions (INCA) campaign (2000). The combined dataset include observations of extratropical cirrus over the Northern and Southern Hemispheres. To minimize the sampling biases over different cirrus evolution phases, we focus our analyses on the “nucleation” events (i.e., partially cloudy segments in ice supersaturated air masses), identified using the Diao et al. (2013) method.

We use the in-situ measured carbon monoxide (CO) mixing ratio, and total aerosol number and aerosol number with sizes larger than 0.5 microns as pollution indicators, respectively, and compare ice microphysical properties (i.e., ice crystal number concentration (Nc) and number-weighted mean diameter (Dc)) between air masses with higher and lower concentrations of pollution indicators. In addition to aerosol, influences of co-varying parameters such as updraft velocity, ice supersaturation and temperature are sorted out. All analyses are restricted to T ≤ -40°C. By analyzing ice crystals (Fast-2DC, 87.5-1600 µm) in HIPPO, we found that Dc decreases with increasing CO concentration at multiple constant pressure levels. In addition, analysis of INCA data shows that Nc and extinction of small ice particles (FSSP 3-20 µm) increase with increasing CO. Overall, our results suggest that extratropical cirrus are likely to have more numerous small ice particles, when sampled in the more polluted background.

Model simulations of cirrus clouds from the NCAR Community Atmosphere Model version 5 (CAM5) will be evaluated and compared with in situ observations. The model will be nudged towards the reanalysis data during the HIPPO and INCA campaign periods and output along the aircraft flight tracks to facilitate the “apple to apple” comparison with observations. Biases of the model with different configurations of ice nucleation will be assessed.