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Directory of participants in the network on biological ice nucleators and cloud processes


From the core of participants in the 2006 meeting in Avignon (described in the previous posting), the resulting network has grown and will continue to grow. Given that much of the communication among us is via the internet, it is becoming increasingly difficult to associate names with faces and expertise. Here is a directory of affiliations, addresses and expertise for participants in the network willing to provide such information.  This might help to keep us aware of who our colleagues are and their competences.


If you would like your name, photo, contact information and description of your expertise included in the directory, please let me know.


A decade of interdisciplinarity


Ten years ago this month, about 25 scientists from the Life Sciences and Earth Sciences convened in Avignon, France for the interdisciplinary meeting that set into motion the current international research dynamics on the role of microorganisms in atmospheric processes.  The meeting, funded by the European Science Foundation program for Exploratory Workshops, was romantically entitled Microbiological Meteorology: Working at the Intersection of Biology, Physics and Meteorology to Understand and Regulate the Microbial Component of Weather. (click here for the program, presentations and report).


Participants of the 2006 ESF workshop on Microbiological Meteorology.

Front row (from the left): Zev Levin, Gabor Vali, David Sands, Cindy Morris, Christel Leyronas, Tom Hill. Back row: Ottmar Moehler, Laurent Huber, Dominique Courault, Roland Psenner, Paul DeMott, Ulirich Pöschl, Ruprecht Jaenicke, Dimitri Georgakopoulos, Heidi Bauer, Berhard Vogel (peeking over Heidi’s head), Mihaly Posfai, Janine Fröhlich-Nowoisky, Marc Bardin, Laurent Deguillaume, Bruce Moffett.


This meeting was followed by a flush of other meetings that consolidated this initial group of researchers and drew in others. This included a session on Biological Ice Nucleators in the Atmosphere – at the Crossroads of Physics and Biology at the General Assembly of the IUGG (Int. Union of Geodesy and Geophysics) in Perugia, Italy in 2007, a special session on Biological Aerosols at the International Conference on Nucleation and Atmospheric Aerosols in Prague in 2009 and another session on Biological Aerosols in the Earth System at the IUGG General Assembly in Melbourne in 2011. Since those initial meetings, and as a consequence of the collaborations and publications that resulted, the surge of interest in this field has been remarkable.

What has this decade of effort to build an international and interdisciplinary network really changed concerning the tools for exploring the influence of microorganisms on atmospheric processes and the knowledge resulting from this exploration? And what are the major challenges that should be a priority for the next decade? To put together answers to these questions, I combined my own opinions with those of some of the scientists who have the most active hindsight on this subject – in particular Gabor Vali, Zev Levin and Keith Bigg.


We have expanded the scope of ice nucleation active particles of biological origin and confirmed their ubiquity. Various microorganisms, and especially numerous fungal species, have been added to the list of efficient (“warm-temperature”) ice nucleators. But, in addition, numerous studies have shown that particles with ice nucleation activity can be composed of various organic matter absorbed to inert mineral particles.  Rich, organic soils are one source of such particles.  This has led us to alter our vocabulary to adapt more generic expressions, such as Ice Nucleating Particles (INPs), to account for the variability in the nature of the particles. Furthermore, bio-INPs are ubiquitous: on plants, in fresh and marine waters, in precipitation, in leaf litter, in soils, and in clouds.

There has been a surge of proxies to identify INPs of biological origin. Based on the frequent use of heat sensitivity of ice nucleation activity in papers on biological INPs, it is clear that this has become the technical criterion for defining biological INPs. In parallel, particle fluorescence in UV-LIF spectrometers (such as the WIBS) has become a widely deployed criterion to identify biological particles in aerosols and in ice crystals. Probes (primers) for molecular detection (PCR) of the gene for the INA protein in Pseudomonas syringae and related bacteria have also been developed. But the sensitivity of this latter tool does not yet rival classical microbiological approaches.

Biological INPs have new rivals in the mineral world for efficient ice nucleation activity. Recently, the ice nucleation activity of feldspar was revealed, with the upper limits of the temperature of activity comparable to that for bio-INPs (near -5°C) making it the only mineral that rivals bio-INPs for this upper limit. However, in terms of the density of ice nucleation sites per surface of particles, bacterial INPs – and especially Pseudomonas syringae – still pack the greatest punch by several orders of magnitude.

Biological INPs have been “injected” into the realm of math and physics. Numerous models of cloud processes have included parameters that permit simulations of the effect of INPs having activities and abundances corresponding to those of biological INPs. This has led to the assessment of effects on cloud life-time, lightning, and precipitation.  Biological INPs have also been literally injected into cloud simulation chambers in order to catch these INPs in the act of inciting glaciation. The Snomax® product has proven to be a useful tool to allow scientists without access to microbiological labs to conduct such experiments.

Atmospheric microorganisms are also important in processes other than ice nucleation. The metabolic impact of microorganisms in the atmosphere has been a hotly debated subject because of the overriding importance of light-driven reactions in the chemistry of the atmosphere.  But, it has become established that there are conditions under which microorganisms surpass photochemistry or catalyze reactions not typical of photochemical processes.

A multitude of field observations have corroborated a role for biological INPs in precipitation. High altitude observatories and aviation-based sampling campaigns have provided much needed evidence that biological particles are guilty of being in the right places at the right time to be involved in processes leading to precipitation. The names of these sites and campaigns are becoming household words, such as Storm Peak, Jungfraujoch, Puy de Dôme, CalWater, etc. Results of these campaigns have revealed the possible importance of season and origin of air masses in the abundance of biological INPs.

Above all else, the atmospheric cycle of bio-INPs is now considered more seriously and a role for them in precipitation is assumed to be plausible.  A striking sign that this plausibility is being accepted is the more and more frequent use of the term “bioprecipitation” by physicists and biologists alike. The term is used mostly to describe the cycle in which microorganisms, mostly from plants, are transported to clouds, foster cloud glaciation and rainfall that deposits these microorganisms back on plants and also provides water for their subsequent multiplication. The past 10 years has seen an incredible complicity between Biology and Physics to provide evidence for the plausibility of some of the major processes of this cycle. Importantly, there have been several new reports about the rapid increase of INPs in the atmosphere shortly after rainfall and we have been reminded of the data on this phenomenon from as early as the 1950’s – all bolstering the occurrence of this important step of the bioprecipitation cycle. Mathematical tools have been made to assess the intensity of the feedback of rainfall in this cycle as a means to identify where rainfall is most sensitive to aerosols.



Another important outcome of the past decade has been the creation of a research community. This community is not only interdisciplinary but it also includes scientists from several generations of research on ice nucleation and bio-INPs. As Gabor points out, this community is critical to assure continuity in research.  The long term efforts that will be needed to attain certain goals about the role of bio-INPs in precipitation are difficult to get funded as single, full projects.  Therefore, as a community we will need to continue stitching together the patchwork of our results and to try to collectively define the subsequent goals. The visibility and interest that we have created has managed to attract young people who have smartly tackled relatively short-term projects that are contributing to the long-term goals.



The greatest challenge for the future is to provide solid, direct evidence for the importance of bio-INPs in precipitation formation. This will be very difficult mostly because of the inherent complexity of how the ice phase in general leads to precipitation in different cloud regimes. The nature and spectrum of action of bio-INPs is also highly variable and this adds on an additional dimension of complexity. Field observations have been one of the main foundations of the growing body of corroborative evidence and of the increasing plausibility of a role for bio-INPs in precipitation. But in the future will field experimentation be used to provide some of the needed solid evidence?  Would such field experimentation be another form of direct cloud seeding? Or rather, would we be able to deploy tools of modern molecular genetics and genomics to link sources, such as vegetation that harbors very specific types of microbial INPs, with these same microbial INPs in clouds, in ice crystals and in precipitation? The ubiquity of certain bio-INPs such as Pseudomonas syringae, Fusarium spp., lichens, etc. will, in fact, hinder such an approach. On the other hand, the extreme host specificity of rust fungi, that cause recurrent epidemics from clearly identifiable sources for which there are hundred-year historical records on most major continents, is one model of bio-INPs that could serve us well for this challenge. Because of the size of bio-INPs such as fungal spores, their action as giant cloud condensation nuclei (GCCN) might be a compounding factor in their role in precipitation. Keith points out that the interest in GCCN has not been proportional to their potential role in the bioprecipitation process and deserves more attention. Furthermore, ice nuclei are only important to rainfall production at temperatures below zero while GCCN can potentially stimulate precipitation in clouds at any temperature.

Another obstacle to advancing our understanding of the role of bio-INPs in the formation of precipitation is the continuing debate about the sufficient abundance of microbial INPs. Continued effort to expand the scope of biological INPs to include submicron-sized remnants of biological organisms, which are likely to be much more numerous than intact microbial cells and spores themselves, will probably greatly contribute to resolving this debate.

Unraveling the mechanisms and efficiency of transfer of bio-INPs from Earth’s surface to the atmosphere is also another important challenge. Methods to facilitate quantification of flux of bio-INPs from the ground into the atmosphere will be very welcome and will be an enormous technical leap.  In the meantime, mapping of the properties of near ground aerosols vs. the properties of bio-INPs on land surfaces across a wide variety of land covers and geographic sites might contribute to this challenge. Nevertheless, once bio-INPs are released from ground level sources, they age during transport in the atmosphere. Hence, questions about the transfer of bio-INPs into the atmosphere go beyond the basics of mass release as a function of wind, humidity and surface conditions and must address how the ice nucleation activity of bio-INPs is modified. Likewise, inert atmospheric particles with no inherent biological constituents might also pick up biological components during their transport. Zev reminds us that examples of such phenomena are starting to be suspected during the transportation of aerosols over oceans and during the flight of volcanic ash. In this light, there will be greater and greater need for techniques of detection and characterization of atmospheric INPs that can separate out the increasing range of classes of bio-INPs.

The upcoming generations of environmental scientists will be under considerable influence from the concern over climate change and its numerous ramifications. Sources of funding will probably mirror this concern with the likely consequence that research will be focused on macroscale processes. As much as such approaches are the path to direct evidence for the importance of bio-INPs in precipitation formation, understanding microscale processes will be needed to make better detection tools and for modeling. Gabor suggests that understanding the nature of nucleating sites and their permanent vs. transitory features is one aspect of microscale processes that should not be forgotten, in particular because of the dependence of quantitative laboratory analyses of ice nucleation activity on these features.

Future research should build on the past. Through online discussions in this network we have realized that there is a gold mine of research publications and competence from about 30-50 years ago that we probably have not build upon as best as we could have. Likewise for contemporary research findings, we need to use our convivial and dynamic network to anchor them into our collective knowledge, to frankly identify their strengths and deficiencies, and thereby continue building toward our ambitious long term goals.




Taming microbial ice nucleators: On the rough road from academic research toward policy and action


Working for a research institute that has recently taken on the moto « Science & Impact » has heightened my interest in how academic research is transformed into tools and behaviors that are adopted by society. I see this as bona fide impact, a natural extension beyond the numerical “impact factors” whose relevance is restricted to the sphere of academia.

Think about three seemingly unlikely Nobel Peace Prizes: one in 1970 for the plant pathologist and breeder Norman Borlaug who is considered to be the founder of agriculture’s Green Revolution, one in 2004 for the biologist Wangari Maathai who founded the Green Belt Movement, and one in 2007 for the Intergovernmental Panel on Climate Change. These prizes recognized individuals and organizations who put science and its results on a course of action that has helped society be sustainably peaceful. Yet, for many scientists those courses of action seem mysterious or elusive in spite of their importance for resolving world class problems such as feeding the growing human population, halting and reversing deforestation, or slowing emissions of greenhouse gases.

In the year leading up to the recent COP21  and for a few days during the weeks of this convention in Paris, I participated in activities that are helping to advance the concept that microorganisms play a role in land-atmosphere interactions and to push it into a trajectory where it could, one day, contribute to policy and action for adapting to and/or mitigating climate change.  I felt very lucky to see this first hand, so I would like to share what I witnessed and learned – and in the hopes of igniting the same passion in some of you.

From the perspective of climate change policy, plants are basically just a sink for CO2. Although the many other services of plants for the environment are known, the policies and actions that have issued from the climate change convention have focused on CO2. In the past few years numerous organizations (NGOs, companies, foundations, etc.) have intensified their lobbying for the recognition of these additional ecosystem services of plants – and of forests in particular – in policies for adaptation to and mitigation of climate change.  In 2010, I was invited to be on the scientific advisory board of several such organizations. One of them, WeForest, is particularly intent on influencing policy at national and international levels concerning reforestation and forest management. Recently they wanted to bring scientists and policy makers together to promote a range of issues.  However, I was unsure about the consensus in the scientific community concerning the importance of other ecosystem services of plants relative to climate change.  So in early 2015 following my advice, we launched the organization of a meeting to identify the consensus.

Our 3-day conference on Why Do Forests Matter for Water and Climate? Strategies for Sustainability took place in Leuven, Belgium in June 2015. In an effort to initiate the visibility of the meeting, it was announced as a partner event of the Common Future Under Climate Change conference held in Paris in July, one of the many events leading up to the COP21.  I don’t know how our meeting obtained this recognition, but it is likely due to the other organizers whom Victoria Gutierrez, WeForest’s chief science officer, had invited to work with me (Bruno Locatelli and David Ellison).  The conference format we chose was the fruit of hours of discussions among the four of us to overcome our disciplinary obstacles (social sciences vs. environmental sciences vs. political sciences vs. life sciences), to clearly establish the objectives of the meeting and to identify the people to invite.  The main objectives we agreed upon were:

  • To elucidate the current state of knowledge on the importance of forests for water and other Earth system processes and
  • To outline a policy brief on how this role of forests is or could be addressed in existing and potential policy frameworks

The 30 people we invited to attend the meeting represented institutes for research and/or development (CIRAD, CIFOR, INRA, ICRAF); universities in North America, Europe and Australia; the private sector and policy makers (FAO). They also represented a multitude of disciplines in the Life Sciences, Earth Sciences, Math and Physics, Social Sciences and Law. By the end of the 3 day meeting we had put together the skeleton of a white paper that has since blossomed into an in-depth manuscript soon be submitted for publication in a scientific journal.  By establishing the content of this manuscript we defined our consensus on the current state of knowledge.  Writing such papers is what we scientists know how to do. But the format and writing style are a far cry from a brief policy outline that can be grasped by policy makers. Importantly, in scientific papers we are long-winded, we hedge, we are vague and we rarely commit to definitive statements about “truths” because our task is to reject hypotheses – and there are always a slew of them still awaiting a test.

So in the months following the meeting in Leuven, Victoria Gutierrez of WeForest worked with incredible persistence to bring life to a policy brief and to obtain a venue to present it. The activities that were to take place around the upcoming COP21 meeting in Dec. 2015 would provide many options for a venue – in theory.  But most of them were inaccessible due to the cost for participating (over $20000 for a stand in a Pavilion at the Global Landscape Forum, for example) and the lack of space because most of the organizations in-the-know had been positioning themselves for many months in advance.  In the summer and fall of 2015 when we were considering all these options, I was a rather silent participant in the discussion because this sphere of events was really foreign to me and I felt lost.

I was really impressed and inspired by Victoria’s dedication and persistence. But it was becoming clear that there was little initiative among the participants of the meeting to write the policy brief; none of us really had experience. We considered the option of farming it out to professionals in communication, such as those associated with certain research and development organizations, but this was costly and we risked losing control of the message. Eventually, the participant from the FAO (Elaine Springgay) mentioned that the many policy briefs she had seen through her work gave her some ideas for the organization and format of ours.  That was the trigger for me to jump on the boat. In fact I felt compelled to do this because of my responsibility as one of the organizers of the Leuven meeting, but also because the scientific consensus was really exciting – as you can see below. In one week she and I created a document and figures that were validated by the other participants of the meeting and then sent off to the CIFOR communication service for the final beautification.  Here is the final version of the policy brief (that you can also find on the WeForest website).


To write the policy brief, we condensed in very short sentences the 5 points of consensus that dominated the meeting in Leuven and that were detailed in the manuscript.  The role that plants could play in rainfall via the ice nucleation-active microorganisms that they harbor was one of the points of consensus. This idea was identified by the participants as important and credible and needed to be promoted as a potential impact of vegetation on climate – beyond its role as a sink for CO2. The participants agreed that it should be integrated into the list of ecosystem services that vegetation could offer relative to the water cycle and climate – given the appropriate knowledge base.  Keep in mind that this point of consensus was proposed by several other participants of the Leuven meeting and was not due to any strong lobbying by David Sands or myself during the meeting.  When that consensus became clear – and importantly when it became clear that other scientists perceived this – I realized that the concept that we call “bioprecipitation” had moved from a wild idea hidden in the Journal of the Hungarian Meteorological Service, to ambitious but virtually unfundable projects for interdisciplinary research, to academic publications, to something that land managers in the future might reckon with if provided the appropriate tools and information. In my opinion, to witness this transformation first hand is a very scenic and thrilling ride on the rollercoaster of a career in science.

While the policy brief was being validated, beautified and printed, Victoria was tirelessly hunting for a venue.  When she finally announced the news that we had a place at the stand of ICRAF/FAO in the Pavilion on Achieving the Sustainable Development Goals at the Global Landscape Forum (GLF) and a 1-hour slot for a presentation on 6 December, we were stunned. I asked her to summarize the obstacle course of events that led to this unexpected opportunity. In a nutshell, it was the consequence of her professional network and of the activities in which she has been engaged over the past several years. Most notably, we were offered to share the ICRAF/FAO space free-of-charge and were provided a few entry tickets to facilitate the participation of the people presenting the policy brief. I think that this generosity reflects that Victoria’s colleagues perceive her strong sense of engagement for forest management, her persistence in achieving goals, her professional competence and her capacity for interdisciplinary work.  At least this is how I see the situation and it made me feel even more dedicated to helping Victoria achieve the initial objectives that we set.  So when she asked me to help present the policy brief at the GLF, I was ecstatically and enthusiastically on board.

My enthusiasm met head-on with the reality of the Pavilions at the GLF.  They were like a handicrafts fair that is set up right next to a museum of art with well-advertised expositions by major artists.  I don’t mean to be pejorative.  But the reality was that the time of our presentation was in competition with events that involved well-known policy makers and organizations.  Furthermore, the specific presentations at the Pavilions were not announced on the program. So with shiny policy brief in hand, we made publicity plugs every time we could get the microphone in the sessions we attended on 5 December.  This meant that we needed to come up with good questions at these sessions and insist on getting the floor. We distributed lots of copies of the brief, and attracted people to our presentation. And as a side benefit, I met many interesting people with whom I would otherwise never have had the opportunity to talk such as a director of a division of the World Health Organization, journalists from the European Plant Science Organization, economists, heads of start-ups and funding initiatives, etc. Something turned off my general reticence for social interactions leading me to even participate in the evening speed-networking session.  I was armed with calling cards specifically conceived for this event and copies of publications that were pertinent to the meeting – in addition to the policy brief.

About a dozen of the thousands of participants of the GLF attended the presentation of our policy brief and participated in the discussion afterwards. But I don’t think that is the real indication of the attention that we generated.  The notion that plants can influence rainfall not only because of evapotranspiration but also via the microorganisms they emit is now out of the closet of science and is tweaking the interest of organizations involved in development. Via the network that grew from the Leuven meeting and the launching of the policy brief I am hearing about future activities including fora for advocacy and initiatives to set up field research projects that would involve the 5 consensus points of the policy brief. Furthermore, a second complementary version of the policy brief was crafted in collaboration with ICRAF and presented at the Rio Pavilion activities at COP21 (see pg. 6 of the summary of activites at the Rio Pavilion) that will likely lead to interest that we have yet to measure.

While writing this post, I can hear some of you saying “Yes, but…..”  The contents of the policy brief make the subjects seem simple and resolved when there is so much that we don’t know and numerous details for which there is still discord within the scientific community.  Admittedly, in the policy brief the summary of the facts concerning the power of plant-associated microbes to influence rainfall is a short sentence that would be accompanied by much detail and several nuances if it were for an audience of scientists.  Wordsmithing for an audience other than scientists is a challenge.  But I am of the opinion that at some point we need to bring attention to novel scientific ideas when they reach a certain stage of maturity, a stage that is sort of like adolescence – with pimples but with clear potential.  When is the best time for this? If someone knows a general rule, then I would like to hear about it.

To prepare this post, I discussed with Gabor Vali who remarked that the dilemma of knowing when to talk to the public about potential applications of research in environmental sciences reminds him of the history of weather modification. Specifically he notes:

The lessons that I’d take from that, from my participation in weather modification research and policy making, are (1) that the prospect of weather modification has been the basis for getting attention and funding to much research and engineering over the past ~66 years; (2) that unrealistic promises have stained the credibility of science and scientists in serious ways, (3) that enormous amounts of money have been spent on trying to prove and/or improve the potential for weather modification, (4) that even today the degree of certainty is minimal about being able to beneficially apply cloud seeding, and (5) that the large potential benefit/cost ratio continues to give rise to both research and operational programs.

According to the notions of weather modification and of bioprecipitation, weather and climate effects are expected from influencing ice nucleation in clouds. This led Gabor to wonder if the application of knowledge about biological ice nucleating particles to land management will progress more rapidly than the application of knowledge of other INPs to cloud seeding. Cloud seeding was advocated as early as the 1950’s by Irving Langmuir (Nobel Prize in Chemistry, 1932 ), inspired by the work of Bernard Vonnegut, yet there is still important controversy. But we have the good fortune to have this historical perspective as a reference. Furthermore, the economic incentives driving the interest in weather modification via cloud seeding are very different from the interest in climate benefits expected from land management. And perhaps most importantly, we are in an epoch where we can deploy the powerful tools of the internet to facilitate a collective debate that might foster efficiency in application of knowledge, reduce controversy and inhibit overselling.

Ice nucleators from vegetation in the Sahel: the impact of overgrazing


The search for ice nuclei with remarkably efficient activity has been a pre-occupation of the atmospheric sciences long before the recent interest in biological ice nucleators propelled this search into the limelight.  Some of the data and observations that constitute the collective knowledge about these ice nucleators are not available in accessible publications.  Russ Schnell has contributed some more of the gems from the collection of observations that he has amassed during his long career.  (For more information about Russ, see the post from 2015/03/04). The following observations concern sources of ice nuclei in the Sahel.

In 1973 at the height of the great Sahel Drought of the early 1970s, Russ noticed a satellite photo that showed a fenced area of some 50,000 hectares in central Niger, Africa, (black & white photo, lower center section) where vegetation was growing much better inside the fenced area than outside.  Outside of the fence the land was heavily overgrazed and even shrubs and trees cut down to feed goats (color photo, taken southeast of the place on satellite photo labeled “photo here”) The fenced property was later identified as the “Ekrafane Ranch”.


Russ suspected that that overgrazing had removed the most active biological ice nuclei and thus reduced precipitation that in turn exacerbated the drought. To test this hypothesis he sought and obtained funding from the Rockefeller Foundation and then for about a month in August-September 1974 traveled alone across the Sahel area of Niger, often on foot, collecting vegetation and soil samples.  He tested them for ice nuclei content using the method described by Schnell and Vali in 1976 (1) (referred to as the Univ. Wyoming report AR111 in Russ’s report to the Rockefeller Foundation).

In his report to the Rockefeller Foundation (2), Russ presented a series of freezing spectra for the vegetation, litter and soil samples illustrating that vegetated areas contained more active immersion freezing ice nuclei than nearby less-vegetated areas.  Interestingly, the most active ice nuclei (active at -7° C) were from vegetation closer to the Sahara Desert where all of the cattle and goats had died the two years before and vegetation was recovering. The results in the report were based on a partial analysis of the samples; a more comprehensive analysis of the data led to the same conclusions.

The drought ended in October 1974 and the Rockefeller Foundation lost interest in the project. It was particularly frustrating for Russ to not be able to assure the Foundation that the local source of active ice nuclei was possibly important in the precipitation process. Without any assurance of this importance, the Foundation was not interested in funding any further work.  Also, soon after Russ moved to Kenya, Africa to work for the UN in a different orientation – but he has kept all of his precious log books that contain the data from this trek in the Sahel.

Almost 40 years later, drought and desertification are rampant and are threatening major world economies as well as the developing world. If, as a scientific community, we needed to provide guidance for the best ways to exploit natural reservoirs of the most active ice nucleators to help fend off disaster, what would we say?  What key observations would we need to assure ourselves and to assure the end users of such guidance?



  1. Schnell R.C. and Vali G. 1976: Biogenic Ice Nuclei: Part I. Terrestrial and Marine Sources. J. Atmos. Sci., 33, 1554–1564. (see the Mendeley data base for the link to this paper).
  2. Schnell R. 1974. Biogenic and inorganic sources for ice nuclei in the drought-stricken areas of the Sahel – 1974.  Interim report to the Directors, Rockefeller Foundation, New York

Rainfall Feedback Maps: a website to explore rainfall patterns that might involve bioprecipitation


Recent research has led to the development of a time series analysis to assess feedback in series of daily rainfall data.  This analytical tool was applied to rainfall data from Australia and revealed that rainfall on one day can be significantly correlated with the probability of rainfall on subsequent days for up to 20 days.  Patterns of positive rainfall feedback mirror the patterns of accumulation of ice nucleation active particles that have been observed to occur after a rainfall.  Taken together, these observations are consistent with the phenomena involved in bioprecipitation whereby biological ice nucleation-active particles are enriched as a consequence of rainfall and then have the possibility to influence subsequent rainfall events.

The time series analysis used for this work provides a tool to characterize sites for their propensity for bioprecipitation. Hence, it offers criteria for selecting geographical sites for research  to understand the processes that underlie bioprecipitation and rainfall feedback – either via direct field measurements or via meta-analyses of how land use and geography can influence rainfall feedback. In light of the ready availability of daily rainfall data for certain continents and the automation of the calculation and mapping procedures for rainfall feedback indices that has been achieved, we have made maps of rainfall feedback indices across 1250 sites in the western states of the USA.  The new website makes these maps available, explains how to interpret the maps, will allow you to access the data resulting from our analyses, and provides information on how to make maps of other regions.

The website address is:

I encourage you to visit the website and deploy it as much as possible.

Happy navigating

Dusting off decades-old insight about atmospheric ice nucleators



Over the past year or so we have witnessed an impressive opulence of work describing the pool of ice nucleators in the atmosphere – with special focus on those having activity at temperatures warmer than that of the dominant mineral particles in the atmosphere – and about techniques to assess this (1–13). Results from this work are contributing to a more comprehensive understanding of the diversity of ice nucleators in the atmosphere over various spatial and temporal scales.  They are also moving forward the notion that the “importance” of any type of ice nucleator is not necessarily the direct consequence of concentrations (i.e. numbers / m3), but also involves the interplay of inherent ice nucleation activity, stability, and being at the right place at the right time.

Work conducted several decades ago on ice nucleators from the environment deserves to be brought to the forefront in the context of this evolving vision of atmospheric ice nucleators. Russ Schnell, who over his career has been associated with NCAR, NOAA and the University of Colorado – Boulder among other institutes, consecrated much of his research to understanding the nature of atmospheric ice nucleators.  Inspired by the parallels he saw between the current wave of research mentioned above and earlier work in the 1970’s and 80’s, he has contributed the following paragraphs.  The reports about this work are in conference proceedings and are not easily accessible.  Therefore, you will find links to scans of these documents embedded in the text.

Among the work from the 70’s and 80’s that is pertinent to current research efforts, there are the following:


1) ” A new technique for measuring atmospheric ice nuclei active at temperatures from -20 C to approaching 0 C, with results” (Schnell, 1979) (pdf) presents a technique and results very similar those recently reported about measurements at the Jungfraujoch Observatory (4).  From Schnell 1979, Figure 2,  it may be observed that following a cold front in late August (no freezing temperatures, no snow, just wind and a frontal passage), IN active at just warmer than -2 C were detected at concentrations of 1 IN/liter active at -6 C. In November and December with snow on the ground, air immediately behind cold fronts in air masses that had been snowing along their trajectory prior to reaching Boulder, there were few IN.  The weeks following passage of the fronts, the  IN concentrations built up slowly.  In Figure 1 in this report, it is apparent that IN concentrations measured below a temperature inversion were much more active and in higher concentration than those measured 300 meters higher in altitude which was above the inversion. This suggests that the IN source was probably local, and in this case from vegetation as the sampling tower was located in the center of about 3,000 acres of grass and grain fields.

Overall, this paper suggests that the filter drop-freezing combination technique is a sensitive, simple, cheap and effective means to measure IN concentrations in the atmosphere. Furthermore, it shows that there are warm temperature IN in the atmosphere just as they are in decayed vegetation at the surface in proximity to the atmospheric measurements.


2) “Kaolin and a biogenic ice nucleant: some nucleation and identification studies” (Schnell, 1977) (pdf1, pdf2) presented a range of measurements showing that active IN associated with a kaolin sample were in direct proportion to the organic content of the kaolin and could be removed by heating (Figures 1 and 2).  The paper also showed that the active IN in decayed tree leaf litter could be efficiently transferred to kaolin through water transfer, and then removed by heating.  Taken together, this suggests the possibility that the warmer range IN activity attributed to soil samples is of biological origin.  This would also be the case for dusts from the Sahara that originate in playas (dry lake beds and washes) that have higher biological content from vegetation that grew around and was washed into the playas.  There is little dust transported from the Sahara that is of the large grained variety from sand dunes. This work has particular application to recent papers such as those focusing on characterizing ice nuclei from ground-based measurements in the eastern Mediterranean region (2)  among many others.


3)  “Ice nucleation studies on bacteria aerosols” (Schnell et al., 1981) (pdf) shows that aerosolized IN bacteria can be captured and tested for IN activity using a filter based technique.  The aerosolized bacteria captured on the filters exhibited IN activity similar to their IN activity in water droplets tested prior to being aerosolized but that the aerosolized IN bacteria  lost activity rapidly in the relatively dry air in the aerosol tent (Figure 1).  Furthermore, the IN activity of the  Pseudomonas syringae decreases on filters rapidly over the first few hours and then much more slowly over the next 24 (Figure 2).  In some cases IN activity at -8 C was still detectable on the filters after a month of storage at room temperature.  The work on the survival and INA of bacterial aerosols in a cloud simulation chamber (14) reported somewhat analogous results but the chamber temperature and RH were different.


4)  “Ice nucleus measurement intercomparisons using three systems and three natural ice nucleants” (Schnell et al., 1982) (pdf) shows that the warmest IN in the atmosphere are the rarest but when a larger air sample is taken, more of the warm IN are captured (Figure1).  This is obviously intuitive and the data support this contention. In Figure 3, it can be seen that three different ways to measure IN (Filter-Drop-Freezing: FDF; Diffusion Chamber: DIFF; and NCAR Counter: NCAR) produce comparable results if the natural IN in a well decayed,  well aerosolized leaf  litter sample are the IN source. This paper adds support to the feasibility of measuring atmospheric IN concentrations with the Filter Drop Freezing technique in other studies on the subject.


Literature cited

You can find these and other references in the data base on biological ice nucleators and land-atmosphere feedbacks at:

  1. Atkinson JD, Murray BJ, Woodhouse MT, Whale TF, Baustian KJ, Carslaw KS, Dobbie S, O’Sullivan D, Malkin TL. 2013. The importance of feldspar for ice nucleation by mineral dust in mixed-phase clouds. Nature 498:355–8.
  2. Ardon-Dryer K, Levin Z. 2014. Ground-based measurements of immersion freezing in the eastern Mediterranean. Atmos. Chem. Phys. 14:5217–5231.
  3. Budke C, Koop T. 2015. BINARY: an optical freezing array for assessing temperature and time dependence of heterogeneous ice nucleation. Atmos. Meas. Tech. 8:689–703.
  4. Conen F, Rodriguez S, Hüglin C, Henne S, Herrmann E, Bukowiecki N, Alewell C. 2015. Atmospheric ice nuclei at the high-altitude observatory Jungfraujoch, Switzerland. Tellus B 67:25014,
  5. Conen F, Leifeld J. 2014. A new facet of soil organic matter. Agric. Ecosyst. Environ. 185:186–187.
  6. Hader JD, Wright TP, Petters MD. 2014. Contribution of pollen to atmospheric ice nuclei concentrations. Atmos. Chem. Phys. 14:5433–5449.
  7. Hiranuma N, et al. 2014. A comprehensive laboratory study on the immersion freezing behavior of illite NX particles: a comparison of seventeen ice nucleation measurement techniques. Atmos. Chem. Phys. Discuss. 14:22045–22116.
  8. O’Sullivan D, Murray BJ, Ross JF, Whale TF, Price HC, Atkinson JD, Umo NS, Webb ME. 2015. The relevance of nanoscale biological fragments for ice nucleation in clouds. Sci. Rep. 5:doi:10.1038/srep08082.
  9. O’Sullivan D, Murray BJ, Malkin TL, Whale TF, Umo NS, Atkinson JD, Price HC, Baustian KJ, Browse J, Webb ME. 2014. Ice nucleation by fertile soil dusts: relative importance of mineral and biogenic components. Atmos. Chem. Phys. 14:1853–1867.
  10. Stopelli E, Conen F, Zimmermann L, Alewell C, Morris CE. 2014. Freezing nucleation apparatus puts new slant on study of biological ice nucleators in precipitation. Atmos. Meas. Tech. 7:129–134.
  11. Tobo Y, DeMott PJ, Hill TCJ, Prenni a. J, Swoboda-Colberg NG, Franc GD, Kreidenweis SM. 2014. Organic matter matters for ice nuclei of agricultural soil origin. Atmos. Chem. Phys. Discuss. 14:9705–9728.
  12. Wex H, Augustin-Bauditz S, Boose Y, Budke C, Curtius J, Diehl K, Dreyer A, Frank F, Hartmann S, Hiranuma N, Jantsch E, Kanji ZA, Kiselev A, Koop T, Möhler O, Niedermeier D, Nillius B, Rösch M, Rose D, Schmidt C, Steinke I, Stratmann F. 2014. Intercomparing different devices for the investigation of ice nucleating particles using Snomax® as test substance. Atmos. Chem. Phys. Discuss. 14:22321–22384.
  13. Whale TF, Murray BJ, O’Sullivan D, Umo NS, Baustian KJ, Atkinson JD, Morris GJ. 2014. A technique for quantifying heterogeneous ice nucleation in microlitre supercooled water droplets. Atmos. Meas. Tech. Discuss. 7:9509–9536.
  14. Amato P, Joly M, Schaupp C, Attard E, Möhler O, Morris CE, Brunet Y., Delort A-M. 2015. Survival and ice nucleation activity of bacteria as aerosols in a cloud simulation chamber. Atmos. Chem. Phys. Discuss. 15:4055–4082.

After-effects of splashing rain: insights from high-speed imaging


Rainfall can have several types of after-effects. Research on the phenomenon of bioprecipitation is based on the idea that some of these after-affects feed back into the processes involved in rainfall formation [1]. The most crucial for bioprecipitation is the generation of aerosols containing ice nuclei. Enhanced atmospheric concentrations of ice nuclei immediately after rain, and also for prolonged post-rain periods have been observed under various conditions [2, 3, 4]. Biological particles are among the aerosols generated by rain. For example, the concentration of airborne biological particles and ice nuclei in a forest eco-system increased about 10-fold during rain and up to one day thereafter in periods of extended leaf wetness [4], and the increases in ice nuclei were associated with increases in atmospheric concentrations of bacteria and fungi. Forceful impact of rain on leaves leads to rapid increases in leaf-surface population densities of the ice nucleation active bacterium Pseudomonas syringae [5] by a mechanism that has not yet been elucidated. These bacteria could contribute to atmospheric concentrations of ice nuclei when emitted into the atmosphere via the processes of aerosolization. Rain could also cause enhanced aerosol concentrations by what is generally known as splashing. Via high speed imaging, a team from the Dept. of Mechanical Engineering at MIT have provided details of aerosol generation by splashing in the case when raindrops fall on a porous surface – and on soils, in particular [6]. This team of researchers has suggested that this could be a means of release of microorganisms associated with soil into the atmosphere. But it could also provide a mechanism for soil particles themselves to become aerosols, and perhaps most importantly particles from rich organic soils, where plants are growing under conditions with sufficient rainfall, thereby increasing the atmospheric concentrations of the efficient ice nuclei associated with organic soils [7, 8].

A video of their observations can be seen at:

In reading background information about this work, I came across the history of the discovery of the nature of petrichor, the earthy smell that often follows rain [see: the original article; popular press information]. The Australian research team that discovered the chemical nature of this compound was interested in its effect on plant growth. Very unexpectedly, they found that petrichor was inhibitory to seed germination [9] – a seeming paradox if rainfall is to have a beneficial effect on sprouting of plants after dry conditions. Oddly, this work has never been followed-up (based on the lack of Web of Science citations relative to plant biology), leaving open some questions about other after-effects of rainfall.


  1. Morris C.E., Conen F., Huffman J.A., Phillips V., Pöschl U., Sands D.C. 2014. Bioprecipitation: A feedback cycle linking Earth history, ecosystem dynamics and land use through biological ice nucleators in the atmosphere. Global Change Biology 20:341-351
  2. Bigg E.K. 1958. A long period fluctuation in freezing nucleus concentration. J. Meteorol. 15:561-562
  3. Bigg E.K., Miles G.T. 1964. The results of large-scale measurements of natural ice nuclei. J. Atmos. Sci. 21:396–403
  4. Huffman J. A., et al. 2013. High concentrations of biological aerosol particles and ice nuclei during and after rain. Atmos. Chem. Phys. 13:6151–6164
  5. Hirano S.S., Baker L.S., and Upper C.D. 1996. Raindrop momentum triggers growth of leaf-associated populations of Pseudomonas syringae on field-grown snap bean plants. Applied and Environmental Microbiology 62:2560–2566
  6. Joung Y.S., Buie C.R. 2015. Aerosol generation by raindrop impact on soil. Nature Communications 6:6083 doi: 10.1038/ncomms7083
  7. Conen F., Morris C.E., Leifeld J., Yakutin M.V., Alewell C. 2011. Biological residues define the ice nucleation properties of soil dust. Atmos. Chem. Phys. 11: 9643-9648
  8. O’Sullivan D. et al. 2014. Ice nucleation by fertile soil dusts: relative importance of mineral and biogenic components. Atmos. Chem. Phys. 14:1853-1867
  9. Bear I.J., Thomas R.G. 1965. Petrichor and plant growth. Nature 207:1415-1416.