The Ice in Clouds Experiment (ICE-L) – November to December 2007

Updated, January 25, 2007

NSF C-130 Facility Request PIs: A. Heymsfield (NCAR), J. Stith (NCAR), P. DeMott (CSU)

Contributors/participants: Andy Heymsfield¹, Jeff Stith¹, Dave Rogers¹, Paul Field¹, Paul DeMott², Charles Knight¹, Joyce Penner³, Ken Sassen⁴, Greg Thompson¹, Bill Cotton², Will Cantrell⁵, Sharon Lewis⁶, Gabor Vali⁷, Al Cooper¹, Steve Platnick⁸, Jim Dye¹, George Isaac⁹, Ulrike Lohmann¹⁰, Ottmar Möhler¹¹, Axel Seifert¹², Dan Cziczo¹⁰, Raymond Shaw⁵, Brad Baker¹³, Paul Lawson¹³, Kim Prather¹⁴

Affiliations: ¹NCAR, ²CSU, ³U. Michigan, ⁴U Alaska, ⁵Michigan Tech. U., ⁶NOAA, ⁷U. Wyoming, ⁸NASA, ⁹Meteorol. Service Canada, ¹⁰ETH Switzerland, ¹¹Inst. Meteorol. Climate Res. Germany, ¹²U. Karlsruhe Germany, ¹³SPEC Inc., ¹⁴UC San Diego

Executive Summary

More than 50% of the earth’s precipitation originates in the ice phase. Ice nucleation, therefore, is one of the most basic processes that lead to precipitation. The poorly understood processes of ice initiation and secondary ice multiplication in clouds result in large uncertainties in the ability to model precipitation production and to predict climate changes. Therefore, progress in modeling precipitation accurately requires a better understanding of ice formation processes.

ICE-L (Ice in Clouds Experiment – Layer clouds) ensued as the first development of the NCAR Ice Initiative. It includes field observations, laboratory experiments, and numerical modeling of ice cloud processes. The objective of the Ice in Clouds Experiment (ICE) is to focus on the following long term scientific goal:

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 in selected clouds. We also seek improved quantitative understanding of the roles of thermodynamic pathway, location within the cloud, and temporal dependency.

This goal statement implies that ice nucleation is definable as the process responsible for at least the initial ice concentration in the selected clouds, that the specific ice nucleation path is identified, and that the parameters most important to governing the process are understood. We recognize that secondary ice formation processes occur in many clouds, subsequent to the formation of ice by nucleation. The present focus, however, is on heterogeneous nucleation in clouds where secondary processes do not occur or where they can be separated (in time or space) from the primary process.

The first step in this project is to seek cases with a strong aerosol-ice nucleation signal. It will focus on observational studies with high likelihood of showing a strong connection of aerosols to effect on ice formation. These cases occur in geographic areas that experience alternatively dust events and dust-free background. The targets are layer clouds: lenticular wave clouds, nimbostratus, and extensive altocumulus and altostratus decks. The thermodynamic and kinematic environments of lenticular wave clouds are relatively steady with lifetimes often longer than an hour, making these clouds an attractive target for study. Wave clouds provide a range of temperature, humidity, and vertical wind conditions in which first ice may form in a laboratory-like setting. Some of the conditions observed in wave clouds can be approximated in laboratory cloud chamber experiments for ice formation studies and for characterizing the performance of airborne ice nuclei instruments. An especially intriguing and important feature of wave clouds is an “evaporation glaciation signature” that is often observed in wave clouds. Observations from previous field experiments indicate that high concentrations of ice particles are nucleated near the location where supercooled liquid water evaporates in wave clouds. However, in the previous studies, ice nuclei and detailed aerosol measurements were lacking, which leaves out a critical component to understanding the nucleation process.

Key observations and flight strategies are described in detail in the scientific overview document (link below). Our groups participation in this study is via the proposal, “Ice Nuclei and Ice Initiation in Mid-Latitude Clouds in Springtime: Background and Dust-Affected” (NSF-ATM-0611936), which also funds participation in the PACDEX study. A project web site with participant and measurement details should appear following a planning workshop in Spring 2007.

ICE Scientific Overview Document (PDF)