Long-term Goal of the Ice in Clouds Experiment (ICE):
To show that under given conditions, direct ice nucleation measurement(s), or other specific measurable characteristics of the aerosol, can be used to predict the number of ice particles forming by nucleation mechanisms and secondary processes in selected clouds. Improved quantitative understanding of the roles of thermodynamic pathway, location within the cloud, and temporal dependency are also sought.
More than 50% of Earth's precipitation originates in the ice phase, so having a basic understanding of the conditions of the ice forming process bears greatly on our understanding of precipitation globally. The ICE-T study will draw on lessons learned in ICE-L (2007), but will extend the investigations to towering warm cumulus clouds, where both primary and secondary ice formation processes occur. This work will be conducted on the NCAR C-130 aircraft platform, based in St Croix, US Virgin Islands for a period of time in May 2011. In-situ instrumentation will measure the chemical, physical, and cloud activating properties of aerosols, and will include both offline and real-time methods of analysis, including A-ATOFMS.
This project is a collaborative effort, led by Dr. Andy Heymsfield of the National Center for Atmospheric Research (NCAR).
Key Scientific questions:
- Can we document the observed evolution of ice formation in maritime cumulus with top temperatures warmer than -10°C, and if this is dependent upon the generation of ice by propeller aircraft sampling the clouds?
- Which types of aerosol act as CCN and IN and how do they depend on temperature, size and aging?
- How do the warm rain and primary and secondary ice processes vary as a function of cloud lifecycle and with changing environmental conditions, particularly ambient dust?
- Does primary nucleation, specifically the number concentrations of ice nuclei, explain the onset and glaciation of cumuli?
- Are secondary ice formation processes critical to the glaciation process in these clouds, what concentration of primary IN are sufficient to trigger them and how does the process work?
- Does mid-level entrainment play a role in feeding CCN and IN, particularly dust, into convective clouds?
ICE-T Mission Objectives:
- Establish which primary heterogeneous ice nucleation modes are active and important by measuring ice formation in tropical maritime cumulus clouds.
- Compare ice production in dust-free and dust-laden conditions.
- Identify ice nuclei by characterizing the physical and chemical properties of ice nuclei collected within the air ingested by the clouds.
- Characterize secondary ice production processes:
- In regions with first ice (from radar observations), measure ice particle size distributions and ice particle shapes as a function of time and with high spatial resolution.
- Quantify the drop size distributions (including drizzle) created by the warm rain process at lower altitudes in the clouds, ascending above the freezing level and compare with ice formation in mixed-phase regions.
- Determine the likely influence of aircraft produced ice on the ICE-T measurements by characterizing the nuclei of the APIP and the spread of these particles through the cloud volume using in-situ and remote sensing observation.
- Conduct laboratory experiments that characterize the properties of the nuclei of particles generated by the Hallett Mossop process.
- Improve the prediction of ice concentrations within numerical models by:
- Determining the thermodynamic history of air parcels in which ice nucleation occurs in smaller, developing mixed-phase cumulus clouds.
- Performing numerical experiments that demonstrate the importance to ice formation on dynamical processes that drive the thermodynamics, such as updrafts, downdrafts, turbulence, entrainment and cloud-edge mixing events.
- Testing the sensitivity of numerical modeling results to secondary ice nucleation production rates and mechanisms.
- Simulating the overall development of the clouds sampled during ICE-T, ranging from simpler to more complex cases.