Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
Atmos. Chem. Phys., 15, 2313-2326, 2015
https://doi.org/10.5194/acp-15-2313-2015
© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
03 Mar 2015
Persistent after-effects of heavy rain on concentrations of ice nuclei and rainfall suggest a biological cause
E. K. Bigg1, S. Soubeyrand2, and C. E. Morris3,4 111 Wesley St., Elanora Heights, NSW 2101, Australia
2INRA, UR546 Biostatistics and Spatial Processes, 84914 Avignon, France
3INRA, UR0407 Pathologie Végétale, 84143 Montfavet, France
4Dept. Plant Sciences and Plant Pathology, Montana State University, Bozeman, MT 59717-3150, USA
Abstract. Rainfall is one of the most important aspects of climate, but the extent to which atmospheric ice nuclei (IN) influence its formation, quantity, frequency, and location is not clear. Microorganisms and other biological particles are released following rainfall and have been shown to serve as efficient IN, in turn impacting cloud and precipitation formation. Here we investigated potential long-term effects of IN on rainfall frequency and quantity. Differences in IN concentrations and rainfall after and before days of large rainfall accumulation (i.e., key days) were calculated for measurements made over the past century in southeastern and southwestern Australia. Cumulative differences in IN concentrations and daily rainfall quantity and frequency as a function of days from a key day demonstrated statistically significant increasing logarithmic trends (R2 > 0.97). Based on observations that cumulative effects of rainfall persisted for about 20 days, we calculated cumulative differences for the entire sequence of key days at each site to create a historical record of how the differences changed with time. Comparison of pre-1960 and post-1960 sequences most commonly showed smaller rainfall totals in the post-1960 sequences, particularly in regions downwind from coal-fired power stations. This led us to explore the hypothesis that the increased leaf surface populations of IN-active bacteria due to rain led to a sustained but slowly diminishing increase in atmospheric concentrations of IN that could potentially initiate or augment rainfall. This hypothesis is supported by previous research showing that leaf surface populations of the ice-nucleating bacterium Pseudomonas syringae increased by orders of magnitude after heavy rain and that microorganisms become airborne during and after rain in a forest ecosystem. At the sites studied in this work, aerosols that could have initiated rain from sources unrelated to previous rainfall events (such as power stations) would automatically have reduced the influences on rainfall of those whose concentrations were related to previous rain, thereby leading to inhibition of feedback. The analytical methods described here provide means to map and delimit regions where rainfall feedback mediated by microorganisms is suspected to occur or has occurred historically, thereby providing rational means to establish experimental set-ups for verification.

Citation: Bigg, E. K., Soubeyrand, S., and Morris, C. E.: Persistent after-effects of heavy rain on concentrations of ice nuclei and rainfall suggest a biological cause, Atmos. Chem. Phys., 15, 2313-2326, https://doi.org/10.5194/acp-15-2313-2015, 2015.
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Short summary
We show that atmospheric load of ice nuclei is enhanced for up to 20 days after key rainfall events. The rate of enhancement decreases exponentially with time. Rainfall quantity and frequency are increased for a similar duration and with similar exponential decreases thereby supporting the notion of rainfall feedback. We reveal series of significant feedback in rainfall patterns across Australia over the past century and marked changes in feedback patterns, and we indicate their locations.
We show that atmospheric load of ice nuclei is enhanced for up to 20 days after key rainfall...
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