HR: 14:15h
AN: A33E-03    [Abstracts]
TI: Ice Nuclei Variability and Ice Formation in Mixed-phase Clouds
AU: * DeMott, P J
EM: pdemott@lamar.colostate.edu
AF: Colorado State University, Department of Atmospheric Science, Fort Collins, CO 80523-1371 United States
AU: Twohy, C H
EM: twohy@coas.oregonstate.edu
AF: Oregon State University, Department of Atmospheric and Oceanic Sciences, Corvallis, OR 97331-5503 United States
AU: Prenni, A J
EM: prenni@lamar.colostate.edu
AF: Colorado State University, Department of Atmospheric Science, Fort Collins, CO 80523-1371 United States
AU: Kreidenweis, S M
EM: sonia@atmos.colostate.edu
AF: Colorado State University, Department of Atmospheric Science, Fort Collins, CO 80523-1371 United States
AU: Brooks, S D
EM: sbrooks@ariel.met.tamu.edu
AF: Texas A&M University, Department of Atmospheric Sciences, College Station, TX 77843 United States
AU: Rogers, D C
EM: dcrogers@ucar.edu
AF: National Center for Atmospheric Research, EOL/RAF, Broomfield, CO 80021 United States
AB: While it is expected that ice nuclei impose a critical role in ice initiation in clouds, there are relatively few validations of direct relations between ice nuclei concentrations and ice crystal concentrations. Further, very little is known about the spatial and temporal distribution of ice nuclei, let alone their sources. Such knowledge is critical for understanding precipitation formation, cloud lifetimes, the existence of aircraft icing hazards, and the impacts of changing atmospheric aerosol particle concentrations and compositions on cold cloud processes. In this study, we document measurements of ice nuclei in relation to the presence and concentrations of ice crystals in modestly supercooled clouds and also consider the implications of differences in ice nuclei concentrations measured at different locations and times during several studies. In the first part of this presentation, we show results from measurements made in the Alliance Icing Research Study II, conducted in late Fall 2003 over the Northeast U.S. and Eastern Canada. A counterflow virtual impactor was used for selectively sampling cloud particles during aircraft measurements of clouds. Measurements were made on the evaporated residual aerosol particles, including re-processing at controlled temperatures and relative humidities to determine their ice nucleating behavior for conditions of direct relevance to the clouds using a continuous flow ice-thermal diffusion chamber (CFDC). Comparing to measurements of ice crystals in clouds, a clear correlation between the presence or absence of ice nuclei and ice crystals was demonstrated in some cases. However, the concentrations of the two populations did not correlate as well. Reasons for this may reflect different (or not assessed) ice formation processes, redistribution of ice in clouds, and potential artifacts of the sampling procedure. Since these results and those of Prenni et al. (this meeting), describing the vital role of ice nuclei in affecting Arctic cloud persistence and radiative properties, both support the critical impact of ice nuclei concentrations on mixed-phase cloud properties, we investigate climatological analyses of CFDC ice nuclei data collected at different locales and seasonally over the past several years. Aside from temperature and relative humidity control on ice nuclei concentrations, these analyses suggest regional and seasonal variability of ice nuclei concentrations, likely tied to meteorological control on transport of ice nuclei from sources (e.g., mineral dust) that may have direct implications on cold cloud processes.
DE: 0300 ATMOSPHERIC COMPOSITION AND STRUCTURE
DE: 0305 Aerosols and particles (0345, 4801, 4906)
DE: 0320 Cloud physics and chemistry
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005