Unexpected events

The preparation for 3-IARC (aka IARC 2020) was initiated in the second semester of 2019 by forming its Scientific Steering Group (SSG). This was composed of 15 researchers (the coauthors of this paper) from the Americas (Chile, United States, Argentina) and Europe (Spain, Portugal, Norway, and Russia), including the four chairs of the conference: Marty Ralph, René Garreaud, Anna Wilson, and Alexandre Ramos. The conference was scheduled for the first week of October 2020 and would be held at a campus of Universidad de Chile in downtown Santiago.

And then came COVID-19. We all have been affected by this pandemic and we sympathize with those suffering the most. By May 2020 we realized that an in-person conference was not possible during 2020 and the SSG revised the options. Considering both the objectives of IARCs and that postponing 3-IARC for a year or more would leave our community without its forum, it was decided to hold a virtual symposium by the IARC community during the first week of October 2020.

Running a virtual meeting has an overall production cost lower than an in-person event but still requires significant financial resources provided by CW3E and CR2. The design and implementation of the web page and the book of abstracts were handled by members of the SSG (https://cw3e.ucsd.edu/iarc/). As a result, the registration and participation in the Virtual Symposium was free of charge for all participants, lifting an important restriction, especially during pandemic times.

Asking for contributions

Once the SSG defined the dates of the virtual symposium and redesigned the web page, we began the promotion of the event in May 2020. We asked for abstracts (500-word limit) on the following topics:

• dynamical and physical processes in ARs;

• impacts of ARs;

• AR modeling and forecasting;

• identification, tracking, and inventories;

• ARs in the past and the future;

• regional flavors; and

• emerging topics.

We were delighted to receive 121 abstracts from authors on all continents (Fig. 1), a continued growth in presentations from IARC meetings in 2016 (75 abstracts) and 2018 (100 abstracts). In addition to the first authors, we received 389 registrations, tallying more than 500 people participating in this event. These impressive numbers reflect the work of a vibrant community that keeps advancing the science and applications of atmospheric rivers despite the extraordinary circumstances under which we were living. The ease to connect and participate in the event across the globe and the lack of a registration fee were also elements that helped to build the large audience in our week-long virtual symposium. Notably, more than 50% of the participants declared this symposium as the first IARC event they attended.

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Number of participants by country.

Citation: Bulletin of the American Meteorological Society 103, 7; 10.1175/BAMS-D-22-0023.1

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Virtual symposium program and structure

Back in 2020, not much experience was available in our scientific community for conducting full online events. Concerns about the number of attendees, multiple time zones, and virtual logistics were present. We asked for advice from organizers of virtual meteorological events held earlier. The SSG discussed and planned for broad issues and small details, resulting in a program and structure that we describe next.

Following a typical in-person conference, our virtual symposium was held for a whole week (Monday–Friday, 5–9 October 2020), but we knew that following online for a full working day would be difficult for many people. We thus limited the plenary sessions to 3 h, plus an extra hour of discussion, every day. The question, then, was how to accommodate 120 presentations in 3.5 meeting hours with no poster session. The “talks” were 5 min long and the “lightning presentations” were 2 min long. Participants were asked for their preference and almost everyone got their selection: 50 abstracts were presented in the talk format and 70 abstracts as lightning presentations.

The final program was crafted with the full picture of abstracts and presentation format and provided to the community in early September. The themes that gathered more abstracts were Dynamical and Physical Processes (21), Impacts of ARs (28), and Modeling and Forecasting (22). Accordingly, those themes were covered in dedicated sessions the first three days of the symposium. Thursday included the sessions on AR Identification, Tracking and Inventories (8) and Regional Flavors (18). The sessions on ARs in the Past and the Future (16) and Emergent Topics (6) were conducted the last day.

Running the virtual symposium

The week before the symposium the presenters received the last instructions and conference access code and were asked to submit their presentation (PDF/PPT) with a limit of 5 slides for talks and 3 slides for lightnings presentations. This allowed organizers to create a master deck with all the presentations for a given session. Knowing that 5-min talks and 2-min lightning presentations are barely sufficient to show the main ideas, results, and findings, participants were able to add extra material in free format (slides, videos, animations) offline. This optional material was available to all the audience the week before the virtual symposium.

The virtual symposium was inaugurated with a welcome greeting and introductory remarks by Marty Ralph and René Garreaud. Every day followed a similar structure, with a 3-h plenary including keynote, regular, and lightning talks (Fig. 2). During this period the online audience fluctuated between 150 and 200 people. We used Zoom, a simple and reliable online meeting platform that most users were familiar with, but the moderators displayed the presentation from the master deck (along with activating the voice and video of the speaker), avoiding any loss of time in the transition among speakers. Overall, the sessions flowed smoothly and participants respected their allocated time sharply. In a post-symposium survey, 85% of the respondents (N = 71) manifested no technical problems.

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Summary of the final program of the IARC 2020 Virtual Symposium. The different blocks are keynote presentations (blue), regular talks (5 min each, light blue), discussion (yellow), break (green), lightning presentation (2 min each, orange), and breakout discussion (60 min each, rose).

Citation: Bulletin of the American Meteorological Society 103, 7; 10.1175/BAMS-D-22-0023.1

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• Download figure as PowerPoint slide

Interaction among the members of the community is an essential aspect that we seek to foster in IARCs. During an in-person conference, informal talks happen in the poster session and other social instances. Although during the plenaries we had periods of 20–25 min for Q&A (conducted by the moderators), this was clearly insufficient to provide and receive feedback, comments, and suggestions. To alleviate this problem, we had an extra hour for discussion at the end of each day’s plenaries. These focused discussions on specific topics were conducted in 4–5 parallel breakout rooms. The presenters were committed to stay in their preassigned room for most of the time; the rest of the audience could move freely among the rooms, and we had between 10 and 20 people in each of them. The discussion in these small groups, assisted by members of the SSG and other volunteers, was very active and was very well evaluated by the participants.

Keynote talks are a powerful attractor to scientific events that can be facilitated by the virtual format. All participants in our virtual symposium benefited from keynote presentations (20 min each) held by leaders in our field that kindly offered their insights on the state-of-the-art in the theme of the day. Beginning on Monday, Heini Wernli (ETH Zurich, Switzerland) provided a dynamical perspective on the relationship of ARs to warm conveyor belts. On Tuesday, Tom Corringham (UCSD/CW3E) illustrated the impacts of ARs from an economic perspective. On Wednesday, Vijay Tallapragada (NWS/NCEP) presented the latest advances in AR observations, modeling, and data assimilation at NCEP. During Thursday we had two keynote talks. The first, presented by Jon Rutz (NWS) and Christine Shields (NCAR), reviewed the status of the Atmospheric River Tracking Method Intercomparison Project, and later on, Irina Gorodetskaya (University of Aveiro) discussed climate change and ARs in the polar regions. Closing the week, Florian Pappenberger (ECMWF) presented recent progress in AR forecasting at ECMWF.

Science outcomes

After two years from the last IARC (2018), enough data, ideas, and findings were accumulated among the members of our community, as reflected in the large number of abstracts submitted to the virtual symposium, as well as the expansion of AR sciences in scope (e.g., economic and environmental impacts) and geography (Middle East, Indian Ocean, tropical regions). Even after the formal definition of ARs (Ralph et al. 2018), we continue refining our understanding of the way in which they evolve in the large scale, their relationship with other atmospheric features (e.g., explosive cyclogenesis, terrain trapped airflows), and their different moisture sources. A few studies delved into the mesoscale structure and microphysical features of ARs. The impacts of AR were documented by case studies and climatologies on all continents (including Antarctica), especially regarding heavy, hazardous precipitation events but also their beneficial side as “drought-busters” in semiarid regions. Some studies directly linked strong ARs with major flooding in mountainous terrain. In addition to the effect of rainfall/snowfall, the moist, temperate environment of ARs is also capable of altering the energy (and mass) balance of glaciers and icefields, as documented in Greenland, Patagonia, and Antarctica.

Atmospheric River Reconnaissance (AR Recon) activities in the northeast Pacific are now operational. Assimilation of dropsonde data and other measurements (e.g., coastal buoys) has improved AR forecasting in terms of the timing, location, and intensity of the AR landfall and associated precipitation. Several talks further discussed the ability of regional numerical models (e.g., WRF and ICON) to simulate ARs, the increasing use of ensemble forecasts, and the prospect of seasonal-to-subseasonal predictions in regions where AR are prominent. Coupled with hydrological modeling, the improved AR forecasts benefit reservoir operations and terrestrial transport, among other sectors. Yet, quantitative precipitation forecasting remains a big challenge in mountainous regions and the use of AR scales (as in Ralph et al. 2019) that convey qualitative information on the AR impacts is being adapted and adopted in many regions worldwide.

Beyond the extratropical west coasts of the continents, regional studies illuminate the occurrence of ARs in less obvious places, such as the Atacama Desert, the east coast of South America, the Yangtze Plain of China, the tropical Indian Ocean, the Middle East and North Africa, and Antarctica. Although global climatologies indicate few ARs in these regions, these new studies reveal that ARs can trigger heavy precipitation events. The AR behavior and distribution in past climates (the Holocene and Pliocene) was documented in a few presentations, and several others addressed their fate in a warmer climate. With global circulation models that are still coarse for resolving the fine structure of ARs, uncertainty remains on where and when ARs will become more frequent or intense. The varying influences of thermodynamic and dynamic changes across the planets call for regional assessment of ARs in the future, as was presented in the symposium for selected locations. The advent of higher-resolution GCMs under CMIP6 will likely boost this area of research.

Lessons for future events

The transition from traditional in-person conferences to fully virtual events left a number of lessons. The most important one was the value of providing a forum to the research community despite the uncertainty and dark passages that the global pandemic brought to our lives. The large audience and enthusiasm of all participants was a welcome sign and reflected the interest of all of us in pushing forward our science and their many applications.

Therefore, keeping a two-year periodicity for IARC events seems like a very sensible choice. We do not know yet the exact format of IARC 2022, but it will be hybrid most likely, with part of the audience attending in person and perhaps many others connecting online. In a postevent survey, each option (hybrid, virtual, in-person) received nearly 1/3 of the preferences. The in-person side benefits from the informal connection among participants when serendipity can flourish. The virtual component allows a broader audience to participate and share their ideas and results. How we offer a structure that takes the best of the two formats needs to be carefully considered. To assist in the design of the next IARC, a few logistic lessons from the October 2020 Virtual Symposium are summarized below:

• A long week with short days helps keep an active, large audience connected to the event. In our case, the 3-h plenary followed by 1-h discussion in breakout rooms worked very well. We had more than 500 people registered and each day over 200 participants connected online.

• Short presentations did work. In particular, lighting presentations were a viable form to highlight new results. In the postevent survey, more than 80% considered that the allocated time for presentation and discussion was right.

• Keep the control. Moderators should display the presentations and activate voice and video of the presenter, thus avoiding any loss of time in the transition among speakers.

• Humans and more humans. Although computers provide the basic frame for a virtual meeting, humans run the show. In each session we had two facilitators (one announcing the speakers and the other taking questions from the audience) and two people in charge of the technical aspects. Likewise, in the breakout room discussions, we had one leader and two facilitators (notetakers, rapporteur). Please see their names in the Acknowledgments section.

• Freedom to choose and more. Participants were able to add extra material in free format (slides, videos, animations). This optional material was available to the entire audience the week before the virtual symposium.

• Legacies. The virtual symposium website (https://iarc-symposium.com/) has links to the book of abstracts and the videos recording the presentations in the plenary section of each day. All these materials can be freely accessed by the broad community.