OCTOBER 2020 H O L L A N D E T A L . 1923 Two Ensemble Approaches for Forecasting Sulfur Dioxide Concentrations from Kılauea Volcano LACEY HOLLAND AND STEVEN BUSINGER Department of Atmospheric Sciences, University of HawaiʻiatManoa, Honolulu, Hawaii TAMAR ELIAS U.S. Geological Survey, Hawaiian Volcano Observatory, Hilo, Hawaii TIZIANA CHERUBINI Department of Atmospheric Sciences, University of HawaiʻiatManoa, Honolulu, Hawaii (Manuscript received 18 September 2019, in final form 13 June 2020) ABSTRACT Kılauea volcano, located on the island of Hawaii, is one of the most active volcanoes in the world. It was in a state of nearly continuous eruption from 1983 to 2018 with copious emissions of sulfur dioxide (SO2) that affected public health, agriculture, and infrastructure over large portions of the island. Since 2010, the University of HawaiʻiatManoa provides publicly available vog forecasts that began in 2010 to aid in the mitigation of volcanic smog (or ‘‘vog’’) as a hazard. In September 2017, the forecast system began to produce operational ensemble forecasts. The months that preceded Kılauea’s historic lower east rift zone eruption of 2018 provide an opportunity to evaluate the newly implemented air quality ensemble prediction system and compare it another approach to the generation of ensemble members. One of the two approaches generates perturbations in the wind field while the other perturbs the sulfur dioxide (SO2) emission rate from the volcano. This comparison has implications for the limits of forecast predictability under the particularly dynamic conditions at Kılauea volcano. We show that for ensemble forecasts of SO2 generated under these conditions, the uncertainty associated with the SO2 emission rate approaches that of the uncertainty in the wind field. However, the inclusion of a fluctuating SO2 emission rate has the potential to improve the pre- diction of the changes in air quality downwind of the volcano with suitable postprocessing. 1. Introduction the other Hawaiiian islands when the predominant northeasterly trade wind regime is interrupted. Oahu, The longest eruptive episode in recorded history for the most heavily populated island, regularly experi- Kılauea volcano on the island of Hawaii ended in early enced aerosol impacts from Kılauea, despite its loca- August 2018. Kılauea’s recent episode was an effusive tion more than 300 km away. Volcanic emissions reach (nonexplosive) eruption that resulted in a continuous Oahu during episodes of southeasterly surface winds source of volcanic gas emissions for the 35 years since associated with precold frontal conditions, upper- 1983. The eruption ended with eruptive fissures in level disturbances, and Kona low conditions (Tofte Kılauea’s lower east rift zone (LERZ). The LERZ et al. 2017). eruption was an extreme event that had impacts on Although the most recent eruption has ended for visibility as far away as the Mariana Islands, more Kılauea, the state of Hawaii contains multiple active than 6000 km away (Guam Homeland Security 2018). volcanoes that will erupt again in the near future, if past The prevailing northeasterly trade wind regime that activity is any guide. In July 2019, the U.S. Geological dominates the weather of Hawaii advects volcanic Survey’s (USGS) Hawaiian Volcano Observatory emissions from Kılauea to the southwest of the island increased the alert level for Mauna Loa volcano to chain. Volcanic emissions from Kılauea can reach ‘‘advisory’’ (USGS 2019a). Historically, eruptions from Mauna Loa differ from those of Kılauea in duration Corresponding author: Lacey Holland, [email protected] and vigor. During its 1984 eruption, emissions from DOI: 10.1175/WAF-D-19-0189.1 Ó 2020 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). Unauthenticated | Downloaded 10/02/21 10:29 AM UTC 1924 WEATHER AND FORECASTING VOLUME 35 Mauna Loa affected most of the state (USGS 2019b). model guidance to forecast the reduced visibility that Future Mauna Loa eruptions are expected to repeat impacts the aviation and marine weather communities this pattern (Pattantyus et al. 2018a). (R. Ballard 2018, personal communication). The vog The noxious haze that arises from volcanic gas and model uses state-of-the-science regional weather fore- aerosol emissions is known as ‘‘vog’’ or ‘‘volcanic smog.’’ casts with real-time volcanic SO2 emission rates that Vog originates from the gases dissolved within magma. are input to a custom implementation of a Lagrangian As magma rises within the earth and approaches the dispersion model [Hybrid Single-Particle Lagrangian surface, substantial amounts of gas are released into Integrated Trajectory model (HYSPLIT; Draxler and the atmospheric environment surrounding Kılauea Hess 1997; Stein et al. 2015)] to predict the impact of (Edmonds et al. 2013). Water vapor, carbon dioxide, vog on local air quality for the state of Hawaii. It now sulfur dioxide (SO2), hydrogen sulfide, and hydrogen includes a 27-member forecast ensemble that runs on a halides are among the components of the exsolved High-Performance Computing (HPC) cluster at UHM. gases (Mather et al. 2012). Attempts to develop air quality forecasts for vog Vog has appreciable and detrimental effects on hu- without numerical weather prediction (NWP) guidance man health (Longo et al. 2010; Tam et al. 2016), water have achieved varying degrees of success. One study quality, agriculture, infrastructure (Elias et al. 2009), (Michaud et al. 2007) developed statistical relationships and the local economy (Halliday et al. 2019). Vog is between meteorological variables to describe conditions particularly harmful to those with respiratory condi- that lead to poor air quality in Hawaii. The study tions such as asthma, sinusitis, and respiratory dis- suggested the impact of local wind patterns on the eases (Kleinman 1995; Ruben et al. 1995; Mannino spatiotemporal variability of SO2 near Kılauea were et al. 1995; Worth 1995; Tam et al. 2007; Longo et al. too important to ignore. Others have since succeeded 2010). Two EPA criteria air pollutants are among its in applying statistical models that generally perform components: SO2 gas and fine particulate matter of size well at short lead times (less than 6 h) and show po- 2.5 mmorless(PM2.5). The PM2.5 componentofvogis tential at longer lead times during steady, trade wind primarily sulfate aerosol (henceforth, SO4) and only on conditions (Reikard 2012). rare occasion contains ash because of Kılauea’s effusive These and other statistical forecast models that do not eruptive style. The SO4 primarily forms as a secondary simulate physical processes directly, can benefit from pollutant from the oxidation of SO2. the use of physical models (i.e., weather or air quality Kılauea was a significant source of SO2 pollution. models) to improve forecast skill. Forecast techniques Between 2014 and 2017, Kılauea averaged an SO2 emis- based on the leveraging statistical relationships are sion rate of nearly 2 million tons per year (Elias et al. known to add the most value to short-term forecasts, 2018). This number far exceeds the 1.26 million tons of while physical models generally perform better at longer SO2 emissions from all electricity generated in the United lead times. Although comparisons have been made States during 2018 (Environmental Protection Agency between these two types of models, efforts to develop 2019). As a potent, volcanic source of SO2 within a physical or statistical models should be viewed as remote, tropical environment, emissions from Kılauea complementary because they can be applied together. evolve differently than urban and industrial sources Examples include model output statistics (MOS; Glahn of SO2. Pattantyus et al. (2018b) describe some of and Lowry 1972), various ensemble MOS (Wilks and the complexities in the SO4 pathways and estimate the Hamill 2007) and analog ensemble methods (Delle rates of these reactions for Kılauea. Monache et al. 2013; Eckel and Delle Monache 2016), Air quality forecasts and warnings can mitigate the neural networks (Gardner and Dorling 1999), and blends public health risks associated with vog exposure. Efforts thereof (Larson and Westrick 2006; Giorgi et al. 2011). to develop vog forecasts at the University of Hawaiʻiat This list is by no means comprehensive but merely Manoa (UHM) began in the 1990s to meet the need for demonstrates the broad range of statistical forecast air quality guidance within the state of Hawaii (Businger techniques that can enhance the skill of physically et al. 2015; Hollingshead et al. 2003). The transport and based models or vice versa. dispersion (hereafter ‘‘vog model’’) that predicts SO2 Among the findings of statistical forecast studies of and SO4 concentrations within the state of Hawaii was vog, Reikard (2019) noted the challenges that non- developed initially as a proof-of-concept exercise. stationarity in the mean and variance of SO2 concen- The vog model is a forecast system that has operated trations introduces. He also noted the difficulty in nearly continuously since its implementation at UHM in forecasting extreme vog events, which are the most 2011. Among its users is the Honolulu National Weather critical events. Although nonstationarity is challeng- Service Forecast Office, which historically has used vog ing to address, both problems can be approached with Unauthenticated | Downloaded 10/02/21 10:29 AM UTC OCTOBER 2020 H O L L