Looking Back: An Account of How Ice Nucleation by Bacteria Was Discovered (1963 to about Mid-1980s). Part I: The Basics

An overview is given of the path of research that led from asking how hailstones originate to the discovery that ice nucleation can be initiated by bacteria and other microorganisms at temperatures as high as -2 [degrees]C. The major steps along that path were finding exceptionally effective ice nucleators in soils with a high content of decayed vegetative matter, then in decaying tree leaves, and then in plankton-laden ocean water. Eventually, it was shown that Pseudomonas syringae bacteria were responsible for most of the observed activity. That identification coincided with the demonstration that the same bacteria cause frost damage on plants. Ice nucleation by bacteria meant an unexpected turn in the understanding of ice nucleation and of ice formation in the atmosphere. Subsequent research confirmed the unique effectiveness of ice nucleating particles (INP) of biological origin, referred to as bio-INPs, so that bio-INPs are now considered to be important elements of lower-tropospheric cloud processes. Nonetheless, some of the questions which originally motivated the research are still unresolved, so that revisiting the early work may be helpful to current endeavors. Part I of this manuscript summarizes how the discovery progressed. Part II (Schnell and Vali) shows the relationship between bio-INPs in soils and in precipitation with climate and other findings. The online supplemental material contains a bibliography of recent work about bio-INPs. SIGNIFICANCE STATEMENT: The story here recounts how the authors found, in the 1970s, that bacteria from soils and vegetation when lifted into clouds as small aerosol particles are very efficient in initiating ice crystals for snow, hail, and rain. Curiously, the same bacteria are also involved in causing frost damage to plants. The importance of this finding for weather and climate is underscored by the knowledge accumulated over the years about this connection between the biosphere and the atmosphere. KEYWORDS: Atmosphere; Cloud microphysics; Biosphere atmosphere interaction; History
1. Introduction Ice formation in the atmosphere is crucial for precipitation formation and is a large factor in Earth's radiation budget. At Earth's surface, it influences diverse processes and has [...]