Unique Turbulence Dynamics in Hurricane Boundary Layers and Improving Their Parameterizations in Numerical Weather Prediction Models

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Hurricanes have been the costliest natural disaster in US history thus far by causing billions of dollars in damage. Ocean warming and climate change can exacerbate tropical cyclone destruction by increasing the frequency and intensity of future major hurricanes. Only four recent hurricanes — Katrina, Sandy, Maria, and Harvey — resulted in more than $450B in damages and about 5,000 fatalities. Thus, it is imperative for the scientific community to better understand and forecast hurricane dynamics and its turbulent winds in order to effectively mitigate their economic ramifications. Although turbulence plays a significant role in hurricane evolution, it is neither thoroughly understood nor parameterized in hurricane flows. Given the remarkable impacts of future hurricanes on humans and the lack of a reliable turbulence scale model for such rotating flows, a high-fidelity hurricane model is now essential. This project aims to address this knowledge gap using a combination of numerical weather prediction (NWP) models and observations to thrust forward the understanding of hurricane turbulence, and to develop practical methodologies for improving hurricane forecasts in NWP models.The research provides pathways to new frontiers in turbulence theory and modeling of hurricane flows. In particular, the driving hypothesis of the project is “turbulence dynamics in hurricane boundary layers (HBLs) are significantly different from typical atmospheric boundary layers (ABLs) due to rotation in HBLs and their large Rossby number (centrifugal/Coriolis force); therefore, existing turbulence models in NWPs limit the accuracy of hurricane forecasts.” This hypothesis will be tested by answering these open research questions 1) How do hurricanes modulate the characteristic mixing length scales and turbulence dynamics in the HBL? and 2) How should the horizontal and vertical turbulent fluxes of an HBL be parameterized in NWPs compared to typical ABLs? To answer these questions, a unique combination of high-fidelity large-eddy simulations (LESs), NWPs, and observations will be employed. The preliminary results support the project’s central hypothesis by demonstrating remarkably different turbulence structures and energy spectra in HBLs when compared to typical ABLs, and substantial improvements in NWP’s hurricane forecasts when current turbulence models are altered. Hence, addressing the above questions will advance the field of physical and dynamic meteorology by elucidating the distinctive turbulence mechanisms in hurricanes compared to conventional much-studied ABLs. Other notable expected outcomes of the project include an extensive dataset of high-resolution LESs of HBLs, new physics-based turbulence closures with rotation correction that are specifically designed for real hurricanes, and a dataset of improved hurricane simulations.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.