During the past five years, the United States has experienced multiple tropical cyclone (TC) landfalls that caused over a billion dollars in damage [National Centers for Environmental Information (NCEI); NCEI 2020]: Hurricanes Matthew (2016), Harvey (2017), Irma (2017), Maria (2017), Florence (2018), Michael (2018), Dorian (2019), Isaias (2020), Laura (2020), Sally (2020), and Tropical Storm Imelda (2019). In 2020 alone, 12 named storms made landfall in the United States, 6 of which were hurricanes, during a record-breaking North Atlantic hurricane season.
Many of these TCs made landfall as major hurricanes (categories 3–5 on the Saffir–Simpson hurricane wind scale, or maximum sustained surface wind speed > 95 kt; 1 kt ≈ 0.5 m s−1). Storm-surge inundation, extreme rainfall, high surf, and tornadoes (Table 1) were significant contributors to the damage, in addition to high winds; in fact, flooding from rainfall and surge accounts for the majority of deaths in landfalling storms (Rappaport 2000). A common feature among most of these TCs is that they underwent at least one period of rapid intensification (RI), typically defined as an intensity increase of 30 kt or more in 24 h (Kaplan and DeMaria 2003).
A summary of the hazards experienced during recent landfalling TCs. Landfall locations are provided for each storm, as well as the peak rainfall measured, observed or estimated maximum storm-surge inundations, the best track maximum sustained wind speed at landfall, and the number of tornadoes observed in the United States. All information is taken from NHC tropical cyclone reports. In “rainfall,” boldface text indicates > 20 in., while in “wind” boldface text indicates major hurricane landfalls and italic text further indicates a category 5.
Although improvements in intensity forecast skill have lagged improvements in track forecasts over the last 30 years, there has been a notable decrease in intensity forecast errors and improved model skill over the past 10 years (Cangialosi et al. 2020). That said, forecasters continue to struggle to predict the onset and magnitude of RI, a challenge that is one of the high-priority objectives of the Weather Research and Forecasting Innovation Act of 20171 and NOAA’s Hurricane Forecast Improvement Project (HFIP; Gall et al. 2013; Marks et al. 2019).
Improving TC forecasts requires advancements in multiple areas, including developing new observing technologies and strategies that fill existing temporal and spatial gaps in observing networks, improving the assimilation of those observations into numerical models, and improving the models themselves, for instance through the use of observations to evaluate and improve how well a model simulates a TC. Although satellites are one of the primary platforms to observe TC characteristics (Rogers et al. 2019) and improve model initialization through assimilation (e.g., Goerss 2009; Christophersen et al. 2018; Magnusson et al. 2019), aircraft continue to be a necessary platform for obtaining in situ measurements at a spatial resolution adequate enough to resolve structures within the inner core.
In 2005, the Hurricane Research Division (HRD) at NOAA’s Atlantic Oceanographic and Meteorological Laboratory (AOML) began their Intensity Forecasting Experiment (IFEX), a program dedicated toward using existing airborne tools with new observing strategies, numerical models, and data assimilation to improve intensity prediction. Three primary IFEX goals were defined as follows (Rogers et al. 2006, hereafter R06): 1) collect observations that span the TC life cycle in a variety of environments for model initialization and evaluation; 2) develop and refine measurement strategies and technologies that provide improved real-time monitoring of TC intensity, structure, and environment; and 3) improve the understanding of physical processes important for intensity change for a TC at all stages of its life cycle.
IFEX operates as a partnership between AOML/HRD; NOAA’s Office of Marine and Aviation Operations (OMAO) Aircraft Operations Center (AOC), who maintain and operate two Lockheed WP-3D Orion (P-3) turboprops and the Gulfstream IV-SP (G-IV) jet; the 53rd U.S. Air Force Reserve Weather Reconnaissance Squadron (53rd WRS), who also fly TC missions; NOAA/NCEP National Hurricane Center (NHC), who establish the priorities and needs that motivate IFEX; and NOAA/NCEP Environmental Modeling Center (EMC), who develop and implement the operational models, notably the Hurricane Weather Research and Forecasting (HWRF) Model.
IFEX also partners with HFIP, which was established in 2007 in response to the active and devastating 2004 and 2005 hurricane seasons (Gall et al. 2013; Gopalakrishnan et al. 2020). HFIP contributes to TC forecast improvement though the development of numerical weather prediction models, e.g., HWRF, the “basin-scale” version of HWRF (HWRF-B; Zhang et al. 2016; Alaka et al. 2017, 2019), and the next-generation Hurricane Analysis and Forecast System (HAFS) (Dong et al. 2020; Hazelton et al. 2020). HFIP invests in forecast improvements not only through advancements of existing models and development of new models, but also in the techniques by which they assimilate observations, and the postprocessing, production, and visualization of model products. Given the observational needs for both data assimilation and model evaluation, HFIP benefits from IFEX data collection.
The goal of this article is not only to highlight IFEX activities since the previous summaries (R06; Rogers et al. 2013a, hereafter R13), but to reflect on IFEX accomplishments after 16 years of existence, including how the program contributes to improving TC intensity forecasts through basic research and transition of that research into operations. This summary article serves as a culmination of IFEX by reviewing the program’s invaluable contributions to understanding and predicting TCs. It will conclude with the path forward, introducing the next generation of NOAA’s airborne hurricane field program as it evolves and broadens to meet the future needs of the TC forecasting community, and how it will contribute in a new era of HFIP.
While the global models (e.g., GFS and ECMWF) assimilate some reconnaissance data, the focus here will be on the impact of reconnaissance data in HWRF, which accepts nearly the complete suite of real-time reconnaissance data.
Data collection for the hurricane field program would not have been possible without the incredible and heroic efforts of all the pilots, flight directors, navigators, engineers, technicians, mechanics, program managers, and leadership at NOAA/OMAO and their Aircraft Operations Center (AOC). We especially want to acknowledge the late Jim McFadden, former Chief of Programs at AOC, whose decades of leadership and dedication paved the way for the success and accomplishments of this program. IFEX also appreciates the courageous and tireless efforts of 53rd Air Force Reserve Weather Reconnaissance Squadron and the data they provide each season, as well as the staff at the unit of the Chief, Aerial Reconnaissance Coordination, All Hurricanes (CARCAH). Michael Brennan, James Franklin, and Ed Rappaport at NHC and Avichal Mehra and Vijay Tallapragada at EMC have also been instrumental and supportive partners who guide the priorities of this program and have allowed IFEX to benefit through piggybacking research on operationally tasked flights. Many scientists both within and outside of NOAA—too many to fairly convey here—have played integral roles in the development, execution, and success of IFEX. Several funding sources have provided support to this project: NOAA base funds (AOC base funds for flight hours and AOML base funds for HRD manpower and travel), CIMAS, NOAA’s Joint Hurricane Testbed (JHT), HFIP (for flight hours, expendables, and travel), and the 2018 and 2019 Hurricane Supplementals (for flight hours and expendables). We thank Michael Brennan (NHC), Ricardo Domingues (UM/CIMAS, AOML/PhOD), Ed Zipser, Jeff Halverson, and an anonymous reviewer for their helpful comments that improved this manuscript.