Cloud microphysics

Ice cloud formation and dynamics play a significant role in Greenhouse warming in the Arctic. Interaction with aerosols affecting the ozone balance and a strong radiative impact due to absorption of upwelling infrared radiation and reflection of sunlight back to space are only two examples that emphasize the importance of understanding the underlying physical mechanisms.

The ISOCLOUD project was initiated by researchers from Chicago (PI: E. Moyer), Karlsruhe (O. Mueller and H. Saathoff), and PTB/Darmstadt (V. Ebert), in order to study the underlying mechanisms of cloud formation in the controlled environment of the AIDA cloud simulation chamber, employing ultra-high sensitivity optical instruments for in-situ measurements of water vapor isotopologues. Our Grenoble group (thesis of Janek Landsberg) joined the project with a unique, ultra-sensitive extractive water isotope ratio laser spectrometer based on the OFCEAS detection technique. The first three ISOCLOUD campaigns yielded important knowledge about the isotopic changes (fractionation) during cloud simulations and the influence of the chamber walls. A model was developed at U. of Chicago that already describes the experiment with reasonable success.

The ISOCLOUD4 campaign focused on a systematic determination of equilibrium fractionation factors in the range from 240 K down to below 200 K. At the lower temperatures no previous data are available. Even at the higher end an important controversy exists, as very recent re-measurement of the fractionation factors by the Copenhagen group does not agree with the Merlivat-Nief and Majoube reference data from the 1970’s [Ellehoj, PhD thesis U. of Copenhagen, 2011]. A preliminary analysis of the ISOCLOUD4 data appears to support the older datasets. Results were presented during the 2013 Goldschmidt conference in sessions 17e and 20k, and in particular the invited talk by Prof. E. Moyer (Mineralogical Magazine 77(5), 1797, 2013).