1. February 12, 2004 Recent observational studies on ice nuclei and ice formation in clouds Paul J. DeMott Colorado State University

2. February 12, 2004 Overview Focus primarily on some examples of studies ice and mixed-phase clouds in last 10 years in which IN and ice concentrations were measured. Some inferences about our understanding of upper tropospheric ice formation from studies of natural IN and clouds at low temperatures. A general realization/validation: mineral dust is an important source of atmospheric IN. A few thoughts on future needs.

3. February 12, 2004 Wave cloud studies (1990-present) Heymsfield and Milosevich 1993-1995 papers suggest that there are few heterogeneous IN in upper troposphere and that ice formation by homogeneous freezing dominates in cold wave clouds Some during WISP studies (1993-1994): IN collected from around clouds and processed in CFDC and controlled expansion cloud chamber. IN concentrations reasonably consistent with ice in clouds, but method not sufficient to explain variability in time and space. Tests for evaporation IN in controlled expansion cloud chamber find no more than 2-3 enhancement. Wave clouds below -40C during SUCCESS (1996) show total ice concentrations consistent with homogeneous freezing, but also evidence consistent with presence of IN up to 100 per liter at low temperatures. Such high IN not always there and sometimes few deposition nuclei present. U.K. studies (Field, Cotton, et al.) using SID show evidence of strong ice formation mechanism in evaporation portion of modestly supercooled wave clouds. Does not always occur. WAVEICE studies (2000): Little apparent evidence for enhanced ice formation in downstream portion of modestly supercooled wave clouds. IN upstream of wave cloud are reasonably consistent with ice formed in cloud. Some evidence for springtime dust impacts on cloud ice formation.

4. Some early optimism that IN measurements are meaningful: Winter Icing in Storms Project WISP 1994 –NCAR Electra, Wyoming KingAir – Six wave clouds –Upwind-downwind penetrations –Ice concentration from PMS 2DC and 1DC – Upwind aerosol bag samples, analyzed at CSU lab with CFD & Dynamic Cloud Chamber

5. February 12, 2004 WAVEICE 2000: March 17, 2000 Wind 12-25 m s -1

6. February 12, 2004 WAVEICE 2000 Ice Concentrations in 35 Cloud Passes - March 17 200X probe

7. Parcel model uses IN and CCN measurements: Data from Wyo. KA; March 17, 2000 Inference that, in absence of secondary processes, and at T > -38°C: [IN] = [Ice]

8. February 12, 2004 A number of observations have been made of enhanced ice formation in evaporation region of waves Cooper (1995, AMS Cloud Physics Conf.). See also Cotton and Field (2002, QJRMS)

9. February 12, 2004 Preliminary Inferences from AIRS-2 Studies on the Role of IN in the Evolution of Mixed Phase Clouds Extremely inhomogeneous spatial distributions of IN may exist in the atmosphere prior to winter storms. This heterogeneity is reflected by the IN detected from cloud particle residuals (sampled by CVI) but the interpretation of these data may not be straightforward.

10. February 12, 2004 November 14: Lower clouds with and without ice, deeper clouds and cirrus in some areas. Were in process of transitioning CFDC conditions to equal those in lower clouds at this time

11. February 12, 2004 November 14: Distinct layers with high IN aloft, sometimes reaching down to lower clouds. Some clouds had ice and some not. Likewise, some had IN, some not.

12. [IN] in deep precipitating cloud system on November 19, 2003 and relation to cloud residual aerosol CFDC processing T ≈ -12.5°C; RH w = 102%

13. February 12, 2004 Other Mixed Phase Cloud Studies Including IN Measurements LAKE-ICE: IN relate to cloud ice in lake-effect systems FIRE-ACE/SHEBA: Generally lower IN in Arctic, possible sources from open ocean leads, silicate/sulfur chemistry of IN (Rogers et al. 2001). North Dakota Tracer Experiment: Bag samples from cumulus cloud base levels and surface sampling suggests agreement between IN and young updraft ice (Stith et al. 1994; DeMott et al. 1995)

14. February 12, 2004 Free tropospheric sampling of concentration and composition of nuclei for cirrus formation Storm Peak Laboratory (3220 m MSL; Steamboat Springs, CO, USA) Aerosol processing methodology (Spring 2004) Cziczo et al. 2003, AS&T INSPECT: Nov. 2001

15. February 12, 2004 DeMott et al. 2003, December, PNAS Homogeneous freezing Heterogeneous ice nucleation Cirrus ice formation conditions and ice concentrations nucleated on ambient tropospheric aerosol particles

16. February 12, 2004 Inferences based on atmospheric observations of RH-T conditions required for cirrus formation Homogeneous freezing of pure sulfates from Chen et al. (2000) or Koop et al. (2000) NASA-SUCCESS RH i inside/outside cirrus, |w|<|1m/s (Jensen et al., JGR, 2001) water saturation Ice saturation

17. February 12, 2004 Heterogeneous nucleation at low temperatures on ambient tropospheric aerosol particles suggest the range of cirrus types impacted Gierens (2003): “critical” concentration of heterogeneous IN triggering a switch of predominant mechanism from homogeneous freezing to heterogeneous nucleation, as a function of T and updraft speed Synoptic lifting and Subvisual cirrus Smaller scale wave forcing and anvil cirrus w DeMott et al. 2003, PNAS

18. IN formulations for numerical modeling: need for better parameterizations and more fundamental understanding Meyers et al. INSPECT (>-35C) INSPECT (<-38C)

19. February 12, 2004 Do [IN] relate to aerosol particle concentrations in a certain size range? T = -42 to -46°C; RH w = 90-92%

20. February 12, 2004 What is the composition of heterogeneous ice nuclei active under cirrus conditions? Statistics of PALMS cluster analyses of particle types 20% 80% (1/4 with any detectable S) Note: Untold story about organic aerosol components and ice nucleation

21. February 12, 2004 Many recent results concerning cirrus from INCA Atmos. Chem. Phys., 3, 1791-1806, 2003 Freezing thresholds and cirrus cloud formation mechanisms inferred from in situ measurements of relative humidity W. Haag, B. Kärcher, J. Ström, A. Minikin, U. Lohmann, J. Ovarlez, and A. Stohl The analysis of field data taken at northern and southern midlatitudes in fall 2000 reveals distinct differences in cirrus cloud freezing thresholds. Homogeneous freezing is found to be the most likely mechanism by which cirrus form at southern hemisphere midlatitudes. The results provide evidence for the existence of heterogeneous freezing in cirrus in parts of the polluted northern hemisphere, but do not suggest that cirrus clouds in this region form exclusively on heterogeneous ice nuclei. Atmos. Chem. Phys., 3, 1807-1816, 2003 Cirrus cloud occurrence as function of ambient relative humidity: a comparison of observations obtained during the INCA experiment J. Ström, M. Seifert, B. Kärcher, J. Ovarlez, A. Minikin, J.-F. Gayet, R. Krejci, A. Petzold, F. Auriol, W. Haag, R. Busen, U. Schumann, and H. C. Hansson Discusses the cloud presence fraction (CPF) defined as the ratio between the number of data points determined to represent cloud at a given ambient relative humidity over ice (RHI) divided by the total number of data points at that value of RHI. The CPFs taken at Southern Hemisphere (SH) and Northern Hemisphere (NH) midlatitudes differ from each other. Above ice saturation, clouds occurred more frequently during the NH campaign. Clouds during the SH campaign formed preferentially at RHIs between 140 and 155%, whereas clouds in the NH campaign formed at RHIs somewhat below 130%. Observed distributions of cloud water content differ only slightly between the NH and SH campaigns and seem to be only weakly, if at all, affected by the freezing aerosols. Atmos. Chem. Phys., 3, 1037-1049, 2003 In-situ observations of aerosol particles remaining from evaporated cirrus crystals: Comparing clean and polluted air masses M. Seifert, J. Ström, R. Krejci, A. Minikin, A. Petzold, J.-F. Gayet, U. Schumann, and J. Ovarlez In-situ observations of aerosol particles contained in cirrus crystals are presented and compared to interstitial aerosol size distributions (non-activated particles in between the cirrus crystals). Size distribution measurements of crystal residuals show that small aerosol particles (Dp< 0.1 um) dominate the number density of residuals. On average the residual size distributions were shifted towards larger sizes and the calculated particle volume was three times larger in the Southern Hemisphere. The form of the residual size distribution did not depend on temperature as one might have expected considering different modes of nucleation. The observations of ambient aerosol particles were consistent with the expected higher pollution level in the Northern Hemisphere. The fraction of residual particles only contributes to approximately a percent or less of the total number of particles.

22. February 12, 2004 Dust and IN: transports affect different parts of the world at different times (E.g., Asian dust in N. America) Fine (PM2.5) soil concentration at the Mt. Zirkel IMPROVE site (1993-2002). VanCuren and Cahill [JGR, 2002]. Continental transect of inferred fine Asian dust frequency (top) and concentrations (bottom) in ng m -3.

23. February 12, 2004 Cirrus forming within Asian Dust layer Sassen (2002) Polarization lidar data in Salt Lake City, UT on April 29, 2001 Considerable warmer and lower than climatological means for cirrus

24. February 12, 2004 Possible dust impacts (Sassen 2002 GRL introduced PDL evidence) – Also evidence in WAVEICE (2000) Early morning 3/25 ruby lidar relative backscattered power and linear depolarization ratio at Salt Lake City (FARS-Ken Sassen) March 25: Hazy day aloft Mt Zirkel IMPROVE network sampler indicates dust intrusion [2D-c] =16 to 206 l -1 ; T= -14 to -37°C

25. February 12, 2004 WAVEICE 2000 Ice Concentrations in 40 Cloud Passes - March 25

26. February 12, 2004 Recent Lab Studies Corroborate Ice Formation by Dust Particles (resuspended Asian dust – Cassie Archuleta thesis) 200 nm Ca, Si, S, Mg Si, Al, Fe Homogeneous freezing points of sulfuric acid aerosols Heterogeneous nucleation by dust

27. February 12, 2004 Saharan dust aerosol sampled in-situ (DeMott et al., GRL, 2003 and Sassen et al., GRL, 2003) Processing at T =-37 ° C, RH w = 86%, RH ice = 123% assured heterogeneous ice nucleation only MODIS aerosol optical depth, July 20-27; July 29, 2003 back trajectory S. Florida PDL lidar data on 7/29

28. July 29: CFDC operating mostly in expected homogeneous freezing regime at low temperature during anvil ascent profile. High IWC contentsHigh IWC contents No Citation FSSP data during periodNo Citation FSSP data during period CFDC [IN] correspond with [2D] within factor 2CFDC [IN] correspond with [2D] within factor 2 CPI concentrations also correspond well with [IN]CPI concentrations also correspond well with [IN] [IN] up to 600/liter[IN] up to 600/liter CPI data: C. Scmitt, A. Bansemer, A. Heymsfield

29. February 12, 2004 TEM analyses of IN from July 29, 2002

30. February 12, 2004 PALMS analysis of particles from high tropical cirrus during CRYSTAL-FACE (Source D.J. Cziczo, NOAA) Histogram of the area of the sodium peak in each positive polarity mass spectrum. This area can be used as a rough indicator of particle type. Most ice residue, particles outside cloud, and interstitial aerosols are sulfate / organics : this is consistent with our understanding of homogeneous freezing. Sulfates plus Organics (~70%) Mineral Dust, Fly Ash, Meteoritic, Etc. (~25%)Sea Salt (<5%)

31. February 12, 2004 PALMS analyses of anvil cirrus particles (July 29, 2002) Ice residue from July 28 -29 2002 have a much higher sodium signal than out of cloud particles or interstitial aerosol. ~20% are consistent with frozen sea salt. <10% Sulfates and organics. Most of the remainder (70%) are consistent with mineral dust or fly ash - heterogeneous freezing… Sulfates plus Organics (<10%) Mineral Dust (~70%)Sea Salt (20%)

32. February 12, 2004 Thoughts on future studies Still believe that wave clouds have much to offer in understanding ice formation mechanisms. Do not yet have IN measurements at appropriate conditions in and around cirrus (Tropical cirrus missions and high altitude capabilities coming). Need IN versus ice concentration in convective clouds. Likely missing an important ice formation mechanism. Validate the impacts of mineral dusts on clouds in programs such as AMMA? Continued need for laboratory studies of aerosol effects on ice nucleation and more fundamental work. Definition of ice versus water is still a critical issue for studies relating IN to ice formation. Need to take real-time combined IN and residual composition measurements aloft.

33. February 12, 2004 Dust versus no dust adiabatic simulation of cumuli parcels (12 m s -1 updraft; 15˚C cloud base, maritime CCN)