By Ben Houser
So far, the 2020 Atlantic hurricane season has been unusually active with fifteen named storms, five of which have reached hurricane strength. Typically, an average hurricane season will produce twelve named storms between June and November; 2020’s season has surpassed this average before the season’s half-way point. At the beginning of August, NOAA predicted an “extremely active” season was possible. Tropical cyclones are one of SPoRT’s most prominent research focuses, and SPoRT scientists are leveraging satellite remote sensing instruments such as the GOES-R Series Geostationary Lightning Mapper (GLM), the Global Precipitation Measurement (GPM) Mission’s GPM Microwave Imager, and the JPSS Series Cross-track Infrared Sounder (CrIs) and Advanced Technology Microwave Sounder (ATMS) to make new and unique observations about the tropical cyclone environment and behavior. SPoRT researchers focus on the diurnal cycle of tropical cyclones, the behavior of lightning in tropical cyclones, and the behavior and structure of cyclones’ powerful winds. These observations are being applied to improve operational models and forecasting tools, minimizing the risk that tropical cyclones pose to life and property. SPoRT is also preparing for the launch of The NASA Time-Resolved Observations of Precipitation structure and storm Intensity with a Constellation of Smallsats (TROPICS) mission, which will provide observations of the tropics with groundbreaking observational frequency.
Tropical cyclones exhibit a distinct diurnal cycle. In other words, tropical cyclones go through predictable changes every day, in which the storm structure is affected by the day / night cycle of solar radiation. However, according to SPoRT tropical cyclone researcher Dr. Erika Duran, the exact mechanisms of these changes are still unknown. “Given the impacts on storm structure, this phenomenon is important for both forecasters and researchers to understand. Especially when such changes may occur close to a tropical cyclone landfall, there is potential for the improvement of wind and rainfall forecasts as more about this cycle is understood,” said Dr. E. Duran. A scientific understanding of the tropical cyclone diurnal cycle has the potential for improved forecasts and emergency response, so SPoRT is leveraging its expertise with satellite observations to explore this phenomenon. “SPoRT is currently using a number of satellite missions and instruments to observe and analyze the diurnal cycle of tropical cyclones. For example, precipitation data from the GPM IMERG product are currently being used to estimate the diurnal cycle of rainfall in Hurricane Dorian (2019). Lightning data from GLM on GOES-E is also being used to analyze the diurnal cycle in Hurricane Dorian,” said Dr. E. Duran. Dr. E. Duran is also exploring the possibility of using NUCAPS satellite sounding retrievals to track the tropical cyclone diurnal cycle. Research into the diurnal cycle will be bolstered by the launch of the TROPICS mission in 2022, which will make it possible to observe tropical cyclone storm structure in great detail over the storm’s entire lifecycle.
The patterns and behavior of the wind produced by a tropical cyclone is called its wind field, and, according to SPoRT tropical researcher Dr. Patrick Duran, is an important element of forecasting tropical cyclone’s impact. “The size and structure of a tropical cyclone’s wind field has important implications for storm surge, wave height, and how destructive the storm’s winds will be at landfall. Operational models do not always accurately predict the size of a tropical cyclone’s wind field, however,” Dr. P. Duran said. “Storms with larger wind fields can do more widespread wind damage at landfall because damaging winds extend over a larger area.” These wind fields are difficult to predict because they are influenced by several internal and external factors. The storm interacts with the atmosphere and climate around it; these interactions can impact the wind field but are difficult to forecast with great detail. Events within the storm can also impact the wind field but are similarly hard to anticipate. Despite these difficulties, satellite observations are powering SPoRT’s efforts to understand and track wind fields. According to Dr. P. Duran, “satellite platforms like the Soil-Moisture Active-Passive (SMAP) instrument and the Cyclone Global Navigation Satellite System (CYGNSS) constellation measure wind speed by detecting how rough the ocean surface is. SPoRT combines these observations with other similar instruments operated by EUMETSAT that can show how a hurricane’s winds evolve with time.” SPoRT also uses satellite observations to relate tropical cyclone wind fields to the processes taking place within the storms. So far, SPoRT scientists have shown that a storm’s wind field can expand and contract throughout the day, indicating a diurnal cycle. This cycle is able to be tracked using widely accessible GOES imagery, and so forecasters may be able to track diurnal wind field changes without access to wind speed data, resulting in improved forecasts.
SPoRT is also exploring the role of lightning in tropical storm intensification, leveraging the state-of-the-art observational capacity of the GOES-R Series GLM. Lightning, depending on where in the storm it occurs, can be a key signal of storm intensification. SPoRT lightning scientist Dr. Chris Schultz says, “the key is where the lightning is located in the system. Stevenson et al. (2018) indicated that 92% of lighting increases within the radius of maximum wind resulted in an intensity change or maintenance. When lightning is located in the outer rainbands, tropical cyclone weakening was generally observed.” SPoRT is currently working to integrate GLM observations into the scientific understanding of tropical cyclones, which will improve forecasters’ ability to anticipate changes in storm intensity. “This involves a partnership with fellow scientists Dr. Stephanie Stevenson and Dr. Jason Dunion at NOAA’s National Hurricane Center and Hurricane Research Division,” said Dr. Schultz. “Early results indicate that the size and energy of the lightning within the eyewall can indicate the type of intensification or maintenance that is ongoing within the system.”
SPoRT scientists are preparing for the launch of the TROPICS mission in 2022, which will provide groundbreaking observations of tropical cyclone precipitation structure and thermodynamics. The satellite constellation will improve researchers’ ability to understand and monitor tropical cyclone structure throughout the storm lifecycle, and will provide more detailed observations at an extremely high observational frequency. Currently, much of SPoRT’s research into tropical cyclones is based on hyperspectral infrared sounders. However, according to SPoRT scientist Dr. Emily Berndt, these types of observations can fail when skies are cloudy. “TROPICS offers the opportunity to look at these same types of events offering all-sky observations, and the ability to diagnose dry air intrusions closer to the storm center and at higher temporal resolution than infrared sounders onboard polar-orbiting satellites,” said Dr. Berndt. TROPICS could also be helpful in uncovering the tropical cyclone diurnal cycle, as its observations will provide more details into the thermodynamic processes of tropical cyclones in comparison to current observers. Dr. Berndt is also the TROPICS Deputy Program Applications Lead and is responsible for engaging the atmospheric science community through the TROPICS Early Adopter program. The Deputy Program Applications role represents a collaboration between SPoRT and NASA Applied Sciences, and is crucial both for realizing the cross-benefit of science and applications and increasing the practical societal benefit of NASA missions. “This entails hosting quarterly telecons to foster interaction between the community of end users and the science team and making data and information available to the community to inform them about TROPICS,” said Dr. Berndt. As part of this collaboration, SPoRT has made the mission’s official proxy data available to the public, so that end-users can test mission-formatted data and ensure that their operations are ready for the launch.
Tropical cyclones are one of the most severe weather events that occur every year, and are responsible for extreme damage all over the world. While much about tropical cyclones is still unknown, SPoRT is applying its expertise in transitioning research to operations to improve the scientific understanding of these storms. SPoRT’s findings lead to improvements to forecasting tools and models, allowing forecasters to anticipate storms with more time and more accuracy. Ultimately, better forecasting leads to better emergency response, minimizing the damage of tropical cyclones and saving lives.
Stevenson, S. N., K. L. Corbosiero, M. DeMaria, and J. L. Vigh, 2018: A 10-Year Survey of Tropical Cyclone Inner-Core Lightning Bursts and Their Relationship to Intensity Change. Wea. Forecasting, 33, 23–36, https://doi.org/10.1175/WAF-D-17-0096.1.
Nolan, D. S., Atlas, R., Bhatia, K. T., and Bucci, L. R. (2013), Development and validation of a hurricane nature run using the joint OSSE nature run and the WRF model, J. Adv. Model. Earth Syst., 5, 382– 405, doi:10.1002/jame.20031.