ACCOMPLISHMENTS OF NOAA’S AIRBORNE HURRICANE FIELD PROGRAM AND A BROADER FUTURE APPROACH TO FORECAST IMPROVEMENT Jonathan Zawislak1,2, Robert F. Rogers1, Sim D. Aberson1, Ghassan J. Alaka, Jr.1, George Alvey1,2, Altug Aksoy1,2, Lisa Bucci1, Joseph Cione1, Neal Dorst1, Jason Dunion1,2, Michael Fischer1,2, John Gamache1, Sundararaman Gopalakrishnan1, Andrew Hazelton1,2, Heather M. Holbach1,3, John Kaplan1, Hua Leighton1,2, Frank Marks1, Shirley T. Murillo1, Paul Reasor1, Kelly Ryan1,2, Kathryn Sellwood1,2, Jason A. Sippel1, Jun A. Zhang1,2 1 National Oceanographic and Atmospheric Administration / Atlantic Oceanographic and Meteorological Laboratory / Hurricane Research Division, Miami, FL 2 University of Miami / Cooperative Institute for Marine and Atmospheric Studies, Miami, FL 3 Florida State University / Northern Gulf Institute, Tallahassee, FL Submitted to the Bulletin of the American Meteorological Society First Submission: February 11, 2021 Second Submission: May 14, 2021 1 * Corresponding author: Jonathan Zawislak, [email protected] 4301 Rickenbacker Causeway, Miami, FL, 33149 Early Online Release: This preliminary version has been accepted for publication in Bulletin of the American Meteorological Society, may be fully cited, and has been assigned DOI 10.1175/BAMS-D-20-0174.1. The final typeset copyedited article will replace the EOR at the above DOI when it is published. © 2021 American Meteorological Society Unauthenticated | Downloaded 10/06/21 02:03 PM UTC 1 ABSTRACT 2 Since 2005, NOAA has conducted the annual Intensity Forecasting Experiment (IFEX), 3 led by scientists from the Hurricane Research Division at NOAA’s Atlantic Oceanographic and 4 Meteorological Laboratory. They partner with NOAA’s Aircraft Operations Center, who maintain 5 and operate the WP-3D and G-IV Hurricane Hunter aircraft, and NCEP’s National Hurricane 6 Center and Environmental Modeling Center, who task airborne missions to gather data used by 7 forecasters for analysis and forecasting and for ingest into operational numerical weather 8 prediction models. The goal of IFEX is to improve tropical cyclone (TC) forecasts using an 9 integrated approach of analyzing observations from aircraft, initializing and evaluating forecast 10 models with those observations, and developing new airborne instrumentation and observing 11 strategies targeted at filling observing gaps and maximizing the data’s impact in model forecasts. 12 This summary article not only highlights recent IFEX contributions towards improved TC 13 understanding and prediction, but also reflects more broadly on the accomplishments of the 14 program during the 16 years of its existence. It describes how IFEX addresses high-priority 15 forecast challenges, summarizes recent collaborations, describes advancements in observing 16 systems monitoring structure and intensity, as well as in assimilation of aircraft data into 17 operational models, and emphasizes key advances in understanding of TC processes, particularly 18 those that lead to rapid intensification. The article concludes by laying the foundation for the “next 19 generation” of IFEX as it broadens its scope to all TC hazards, particularly rainfall, storm-surge 20 inundation, and tornadoes, that have gained notoriety during the last few years after several 21 devastating landfalling TCs. 22 23 24 Unauthenticated | Downloaded 10/06/21 02:03 PM UTC Accepted for publication in Bulletin of the American Meteorological Society. DOI 10.1175/BAMS-D-20-0174.1. 25 CAPSULE 26 The field program has contributed to improvements in tropical cyclone intensity forecasts through 27 aircraft observations. Future strategies will broaden these efforts by focusing on forecasts of storm 28 hazards. 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 Unauthenticated | Downloaded 10/06/21 02:03 PM UTC Accepted for publication in Bulletin of the American Meteorological Society. DOI 10.1175/BAMS-D-20-0174.1. 48 MOTIVATION AND OVERVIEW OF IFEX 49 During the past five years, the United States has experienced multiple tropical cyclone (TC) 50 landfalls that caused over a billion dollars in damage (NOAA/National Centers for Environmental 51 Information, NCEI): Hurricanes Matthew (2016), Harvey (2017), Irma (2017), Maria (2017), 52 Florence (2018), Michael (2018), Dorian (2019), Isaias (2020), Laura (2020), Sally (2020), and 53 Tropical Storm Imelda (2019). In 2020 alone, 12 named storms made landfall in the U.S., six of 54 which were hurricanes, during a record-breaking North Atlantic hurricane season. 55 Many of these TCs made landfall as major hurricanes (Categories 3–5 by the Saffir- 56 Simpson Hurricane Wind Scale, or maximum sustained surface wind speed > 95 kt, 1 kt ≈ 0.5 m 57 s-1). Storm-surge inundation, extreme rainfall, high surf, and tornadoes (Table 11) were significant 58 contributors to the damage, in addition to high winds; in fact, flooding from rainfall and surge 59 accounts for the majority of deaths in landfalling storms (Rappaport 2000). A common feature 60 among most of these TCs is that they underwent at least one period of rapid intensification (RI), 61 defined as an intensity increase of 30 kt or more in 24 h (Kaplan and DeMaria 2003). 62 Although improvements in intensity forecast skill have lagged improvements in track 63 forecasts over the last 30 years, there has been a notable decrease in intensity forecast errors and 64 improved model skill over the past 10 years (Cangialosi et al. 2020). That said, forecasters continue 65 to struggle to predict the onset and magnitude of RI, a challenge that is one of the high-priority 1 The impacts from Laura (2020) will not be included in Table 1 as the Tropical Cyclone Report authored by the National Hurricane Center has not yet been released as of writing. 1 Unauthenticated | Downloaded 10/06/21 02:03 PM UTC Accepted for publication in Bulletin of the American Meteorological Society. DOI 10.1175/BAMS-D-20-0174.1. 66 objectives of the Weather Research and Forecasting Innovation Act of 20172 and NOAA’s 67 Hurricane Forecast Improvement Project (HFIP; Gall et al 2013; Marks et al. 2019). 68 Improving TC forecasts requires advancements in multiple areas, including developing 69 new observing technologies and strategies that fill existing temporal and spatial gaps in observing 70 networks, improving the assimilation of those observations into numerical models, and improving 71 the models themselves, for instance through the use of observations to evaluate and improve how 72 well a model simulates a TC. Although satellites are one of the primary platforms to observe TC 73 characteristics (Rogers et al. 2019) and improve model initialization through assimilation (e.g., 74 Goerss 2009; Christophersen et al. 2018; Magnusson et al. 2019), aircraft continue to be a 75 necessary platform for obtaining in-situ measurements at a spatial resolution adequate enough to 76 resolve structures within the inner core. 77 In 2005, the Hurricane Research Division (HRD) at NOAA’s Atlantic Oceanographic and 78 Meteorological Laboratory (AOML) began their Intensity Forecasting Experiment (IFEX), a 79 program dedicated towards using existing airborne tools with new observing strategies, numerical 80 models, and data assimilation to improve intensity prediction. Three primary IFEX goals were 81 defined as (Rogers et al. 2006): 1) collect observations that span the TC life cycle in a variety of 82 environments for model initialization and evaluation; 2) develop and refine measurement 83 strategies and technologies that provide improved real-time monitoring of TC intensity, structure, 84 and environment; and 3) improve the understanding of physical processes important for intensity 85 change for a TC at all stages of its life cycle. 2 https://www.congress.gov/bill/115th-congress/house-bill/353 - Section 104 2 Unauthenticated | Downloaded 10/06/21 02:03 PM UTC Accepted for publication in Bulletin of the American Meteorological Society. DOI 10.1175/BAMS-D-20-0174.1. 86 IFEX operates as a partnership between AOML/HRD; NOAA’s Office of Marine and 87 Aviation Operations (OMAO) Aircraft Operations Center (AOC), who maintain and operate two 88 Lockheed WP-3D Orion (P-3) turboprops and the Gulfstream IV-SP (G-IV) jet; the 53rd U.S. Air 89 Force Reserve Weather Reconnaissance Squadron (53rd WRS) who also fly TC missions, 90 NOAA/NCEP National Hurricane Center (hereafter, just NHC), who establish the priorities and 91 needs that motivate IFEX; and NOAA/NCEP Environmental Modeling Center (hereafter, just 92 EMC), who develop and implement the operational models, notably the Hurricane Weather 93 Research and Forecasting (HWRF) model. 94 IFEX also partners with HFIP, which was established in 2007 in response to the active and 95 devastating 2004 and 2005 hurricane seasons (Gall et al. 2013; Gopalakrishnan et al. 2020). HFIP 96 contributes to TC forecast improvement though the development of numerical weather prediction 97 models, e.g., HWRF, the “Basin-scale” version of HWRF (HWRF-B; Zhang et al. 2016; Alaka et 98 al. 2017, 2019), and the next-generation Hurricane Analysis and Forecast System (HAFS) (Dong 99 et al. 2020; Hazelton et al. 2020). HFIP invests in forecast improvements not only through 100 advancements of existing models and development of new models, but also in the techniques by 101 which they assimilate observations, and the post-processing, production, and visualization of 102 model products. Given the observational needs for both data assimilation and model evaluation, 103 HFIP benefits from IFEX data collection. 104 The goal of this article is not only to highlight IFEX activities since the previous summaries 105 (Rogers et al. 2006, hereafter R06; Rogers et al. 2013, R13), but to reflect on IFEX 106 accomplishments after 16 years of existence, including how the program contributes to improving 107 TC intensity forecasts through basic research and transition of that research into operations. This 108 summary article serves as a “culmination” of IFEX by reviewing the program’s invaluable 3 Unauthenticated | Downloaded 10/06/21 02:03 PM UTC Accepted for publication in Bulletin of the American Meteorological Society.