Transcript Cantrell
Production of ice in tropospheric clouds: A review Will Cantrell and Andrew Heymsfield Bulletin of the American Meteorological Society, in press Homogeneous nucleation ● What role do collective fluctuations in water play? Do water molecules make the transition from liquid to nascent crystal one-by-one or in groups? -3 -1 Nucleation rate (cm s ) 1018 1014 What is the structure of the ice embryo and where does it form? Classical nucleation theory assumes that the ice embryo forms in the bulk liquid and has the structure of the bulk crystal. Both assumptions have been challenged ● 1010 Jeffery and Austin, 1997 Hagen et al., 1981 Stockel, 2001 DeMott and Rogers, 1990 Wood et al., 2002 (20 m drops) Wood et al., 2002 (33 m drops) Krämer et al, 1999 Shaw and Lamb, 1999 Duft and Lesiner, 2004 Pruppacher and Klett, 1997 106 102 -44 -43 -42 -41 -40 -39 -38 Is freezing only a function of the water activity? Koop et al.'s (2000) conjecture holds for a wide variety of substances. Ammoniated compounds seem to be an exception. Is this a peculiarity of ammonia or does it indicate a flaw in the model? ● -37 -36 Temperature (ºC) -35 -34 -33 “...we have made significant progress in several areas, most notably homogeneous nucleation...” Heterogeneous nucleation ● What are the most important properties of the heterogeneous nucleator? Lattice match, defects, type of bond that the adsorbing water molecules form with the substrate, type of atoms exposed on the substrate...?? -21.0 Freezing point (ºC) -21.5 -22.0 Ash intersecting the surface of the water droplet -22.5 Ash completely immersed in the water droplet -23.0 -23.5 -24.0 What is the mechanism underlying contact nucleation? None of the mechanisms put forward to explain contact nucleation explain the data shown to the left. ● What is the mechanism underlying evaporation nucleation and is it important in the atmosphere? ● -24.5 -25.0 0 10 20 30 40 Trial number 50 60 70 What role do organic compounds play in ice nucleation? Some organic compounds are very effective ice nucleators. There is evidence that organic compounds inhibit ice nucleation. What is their role in the atmosphere? What is the variation with time and space? ● “...though there have been advances made in identifying important sources of ice nuclei (e.g. desert dust), progress in heterogeneous nucleation is desperately needed.” Freezing of solutions catalyzed by (C25H51OH) as a function of water activity Heterogeneous nucleation 0 (NH4)2SO4 NaCl Temperature (ºC) -5 Melting point -10 -15 Δaw = aw – aw,i = 0.075 -20 -25 1.00 0.98 0.96 0.94 0.92 0.90 0.88 0.86 Water activity Is there a unified framework we can use to interpret and compare measurements of heterogeneous nucleation? Secondary production ● Are there secondary production mechanisms which operate in the presence of liquid water other than riming-splintering? 250 -1 Rate of crystal porduction (s ) Target velocity 200 3 m s-1 2.4 m s-1 1.8 m s-1 1.4 m s-1 150 Are there significant secondary production mechanisms which do not require the presence of liquid water? Crystals (especially dendrites) will break up in certain conditions, but not in sufficient numbers to produce rapid ice multiplication. Are there other mechanisms which have not been identified? ● 100 50 How are ice particle size distributions maintained? Size distributions of ice particles consistently show more small particles than large ones. This implies that either small particles are continually being created or that they are prevented from growing. What mechanism(s) sustains the small mode? ● 0 -2 -3 -4 -5 -6 Cloud temperature (ºC) -7 -8 “...we have made significant progress in several areas... and, to a lesser extent, secondary production of ice.”