Abstract
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.
Highlights
Unraveling the basis of land–atmosphere interactions with feedbacks on rainfall (Pielke et al, 2007; Morris et al, 2014) is increasingly important in the context of climate change
By comparing the patterns of the increase of ice nuclei (IN) after rainfall to those of the quantity and frequency of rainfall following a heavy fall of rain, we showed a similarity of these patterns: rainfalls are increased relative to the preceding days for a similar length of time and with similar exponential decreases with time as the changes in IN concentrations
The rain derived from feedback is small compared to total rainfall, it may be an underestimate because the analysis does not lend itself to dealing with the effects of more frequent but lighter falls of rain than those considered here
Summary
Unraveling the basis of land–atmosphere interactions with feedbacks on rainfall (Pielke et al, 2007; Morris et al, 2014) is increasingly important in the context of climate change. Rain influences a wide range of the earth’s surface characteristics including soil moisture, plant proliferation, etc. These characteristics could in turn promote or reduce subsequent rainfall, thereby leading to positive or negative feedbacks, respectively. Positive short-term feedback effects on rainfall have been predicted and attributed to changes in surface albedo and Bowen ratio (the ratio of sensible to latent heat flux) by soil moisture (Eltahir, 1998). Rosenfeld et al (2001) described a potential feedback effect involving suppression of Published by Copernicus Publications on behalf of the European Geosciences Union Global scale observational analysis of the coupling between soil moisture and precipitation found no evidence for feedback due to soil moisture (Taylor et al, 2012). Rosenfeld et al (2001) described a potential feedback effect involving suppression of Published by Copernicus Publications on behalf of the European Geosciences Union
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