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1. Combined Assimilation of Doppler Wind Lidar and Tail Doppler Radar Data over a Hurricane Inner Core for Improved Hurricane Prediction with the NCEP Regional HWRF System

   https://doi.org/10.3390/rs14102367  

   Li, Xin; Pu, Zhaoxia; Zhang, Jun A.; Emmitt, George David (May 2022, Remote Sensing)

   Accurate specification of hurricane inner-core structure is critical to predicting the evolution of a hurricane. However, observations over hurricane inner cores are generally lacking. Previous studies have emphasized Tail Doppler radar (TDR) data assimilation to improve hurricane inner-core representation. Recently, Doppler wind lidar (DWL) has been used as an observing system to sample hurricane inner-core and environmental conditions. The NOAA P3 Hurricane Hunter aircraft has DWL installed and can obtain wind data over a hurricane’s inner core when the aircraft passes through the hurricane. In this study, we examine the impact of assimilating DWL winds and TDR radial winds on the prediction of Hurricane Earl (2016) with the NCEP operational Hurricane Weather Research and Forecasting (HWRF) system. A series of data assimilation experiments are conducted with the Gridpoint Statistical Interpolation (GSI)-based ensemble-3DVAR hybrid system to identify the best way to assimilate TDR and DWL data into the HWRF forecast system. The results show a positive impact of DWL data on hurricane analysis and prediction. Compared with the assimilation of u and v components, assimilation of DWL wind speed provides better hurricane track and intensity forecasts. Proper choices of data thinning distances (e.g., 5 km horizontal thinning and 70 hPa vertical thinning for DWL) can help achieve better analysis in terms of hurricane vortex representation and forecasts. In the analysis and forecast cycles, the combined TDR and DWL assimilation (DWL wind speed and TDR radial wind, along with other conventional data, e.g., NCEP Automated Data Processing (ADP) data) offsets the downgrade analysis from the absence of DWL observations in an analysis cycle and outperforms assimilation of a single type of data (either TDR or DWL) and leads to improved forecasts of hurricane track, intensity, and structure. Overall, assimilation of DWL observations has been beneficial for analysis and forecasts in most cases. The outcomes from this study demonstrate the great potential of including DWL wind profiles in the operational HWRF system for hurricane forecast improvement. 

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2. A Review and Evaluation of Planetary Boundary Layer Parameterizations in Hurricane Weather Research and Forecasting Model Using Idealized Simulations and Observations

   https://doi.org/10.3390/atmos11101091  

   Zhang, Jun A.; Kalina, Evan A.; Biswas, Mrinal K.; Rogers, Robert F.; Zhu, Ping; Marks, Frank D. (October 2020, Atmosphere)

   null (Ed.)

   This paper reviews the evolution of planetary boundary layer (PBL) parameterization schemes that have been used in the operational version of the Hurricane Weather Research and Forecasting (HWRF) model since 2011. Idealized simulations are then used to evaluate the effects of different PBL schemes on hurricane structure and intensity. The original Global Forecast System (GFS) PBL scheme in the 2011 version of HWRF produces the weakest storm, while a modified GFS scheme using a wind-speed dependent parameterization of vertical eddy diffusivity (Km) produces the strongest storm. The subsequent version of the hybrid eddy diffusivity and mass flux scheme (EDMF) used in HWRF also produces a strong storm, similar to the version using the wind-speed dependent Km. Both the intensity change rate and maximum intensity of the simulated storms vary with different PBL schemes, mainly due to differences in the parameterization of Km. The smaller the Km in the PBL scheme, the faster a storm tends to intensify. Differences in hurricane PBL height, convergence, inflow angle, warm-core structure, distribution of deep convection, and agradient force in these simulations are also examined. Compared to dropsonde and Doppler radar composites, improvements in the kinematic structure are found in simulations using the wind-speed dependent Km and modified EDMF schemes relative to those with earlier versions of the PBL schemes in HWRF. However, the upper boundary layer in all simulations is much cooler and drier than that in dropsonde observations. This model deficiency needs to be considered and corrected in future model physics upgrades. 

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3. A modified vertical eddy diffusivity parameterization in the HWRF model based on large eddy simulations and its impact on the prediction of two landfalling hurricanes

   https://doi.org/10.3389/feart.2023.1320192  

   Li, Xin; Pu, Zhaoxia; Zhang, Jun A; Zhang, Zhan (November 2023, Frontiers in Earth Science)

   Vertical eddy diffusivity (VED) in the planetary boundary layer (PBL) has a significant impact on forecasts of tropical cyclone (TC) structure and intensity. VED uncertainties in PBL parameterizations can be partly attributed to the model’s inability to represent roll vortices (RV). In this study, RV effects on turbulent fluxes derived from a large eddy simulation (LES) by Li et al. (Geophys. Res. Lett., 2021, 48, e2020GL090703) are added to the VED parameterization of the PBL scheme within the operational Hurricane Weather Research and Forecasting (HWRF) model. RV contribution to VED is parameterized through a coefficient and varies with the RV intensity and velocity scale. A modification over land has also been implemented. This modified VED parameterization is compared with the original wind-speed-dependent VED scheme in HWRF. Retrospective HWRF forecasts of Hurricanes Florence (2018) and Laura (2020) are analyzed to evaluate the impacts of the modified VED scheme on landfalling hurricane forecasts. Results show that the modified PBL scheme with the RV effect leads to an improvement in 10-m maximum wind speed forecasts of 14%–31%, with a neutral to positive improvement for track forecasts. Improved wind structure and precipitation forecasts against observations are also noted with the modified PBL scheme. Further diagnoses indicate that the revised PBL scheme enhances moist entropy in the boundary layer over land, leading to improved TC intensity prediction compared to the original scheme. 

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4. The Effects of Surface Waves on Hurricane Boundary Layer Dynamics and Forecasts

   Momen, Mostafa (January 2025, American Meteorological Society)

   Hurricanes have unique dynamics when compared to regular Atmospheric Boundary Layers (ABLs). Strong winds and elevated surface waves differentiate the air-sea interactions in Hurricane Boundary Layers (HBLs) from classic marine ABLs. Although significant progress has been made in modeling hurricanes, our understanding of the turbulence dynamics of HBLs is still limited due to the lack of sufficient measurement data and high-resolution simulations. Our objective in this work is to address this knowledge gap using high-resolution Large-Eddy Simulations (LESs) that explicitly resolve hurricane turbulence (Momen et al. 2021; Sabet et al. 2022). In this presentation, we will characterize the role of surface waves in HBL mean and turbulence dynamics with the help of multiple unique LES runs in the parameter space of the problem. First, we will show the impacts of surface waves on HBL dynamics using wave-resolving LESs. It was found that the ocean waves can significantly modulate the surface layer dynamics of HBLs as shown in the attached figure. The steep waves in hurricanes were found to remarkably influence the HBL turbulence up to ~800 m away from the surface. The impacts of waves on turbulent eddies are high near the surface (up to ~100 m) as shown in the 3D spatial correlation of the attached figure. Typical low wave ages enhance surface drag and decrease the HBL wind, while higher wave ages can intensify the local surface winds. Moreover, the Turbulent Kinetic Energy (TKE) is increased by the enhanced drag of young waves, while older higher speed waves can decrease the TKE compared to the flat non-wavy case. We also found that higher wave heights, which are more prevalent in hurricanes, magnify these effects. The implications of these results on surface layer parameterizations in large-scale hurricane forecasts will also be briefly discussed using the Weather Research and Forecasting (WRF) model. We will present that the current aerodynamic roughness length parameterizations in WRF overestimate the observational estimates and theoretical hurricane intensity models for high wind regimes over the ocean (≳ 45 m/s). By adjusting the roughness length values in WRF, we were able to improve the intensity forecasts of five strong hurricane cases (category 3-5) by more than 20% on average compared to the default models (Li et al. 2023). These insights and findings can be useful for improving hurricane forecasts in numerical weather prediction models, eventually aiding in disaster preparedness efforts. References: Li, M., J. A. Zhang, L. Matak, and M. Momen, 2023: The impacts of adjusting momentum roughness length on strong and weak hurricanes forecasts: a comprehensive analysis of weather simulations and observations. Mon Weather Rev, https://doi.org/10.1175/MWR-D-22-0191.1. Momen, M., M. B. Parlange, and M. G. Giometto, 2021: Scrambling and reorientation of classical boundary layer turbulence in hurricane winds. Geophys Res Lett, 48, https://doi.org/https://doi.org/10.1029/2020GL091695. Sabet, F., Y. R. Yi, L. Thomas, and M. Momen, 2022: Characterizing mean and turbulent structures of hurricane winds via large-eddy simulations. Proceedings of the Summer Program 2022, Stanford, Center for Turbulence Research, Stanford University, 311–321. 

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5. Evaluating the impact of parameterized turbulent mixing and boundary layer structure on hurricane intensification

   Zhang, Jun A.; Rogers, Robert; Marks, Frank; Tallapragada, Vijay (June 2018, 23 Symposium on Boundary Layers and Turbulence)

   This talk presents results from the authors’ recent work on evaluating the role of turbulence and boundary-layer parameterizations on hurricane intensification. We show that observation-based modification of these physical parameterizations significantly improved the HWRF intensity forecast. Turbulent mixing in both the vertical and horizontal directions are found to be crucial for hurricane spin-up dynamics in 3D numerical simulations and HWRF forecasts. Vertical turbulent mixing regulates the inflow strength and the location of boundary-layer convergence that in turns regulates the distribution of deep convection and the intensification of the whole hurricane vortex. Convergence of angular momentum in the boundary layer that is a key component of the hurricane spin-up theory is also found to be regulated by vertical turbulent mixing in connection to the boundary layer inflow. Horizontal turbulent mixing, on the other hand, mainly influences the eddy momentum flux inside the radius of the maximum wind speed in the angular momentum budget. The effect of horizontal turbulent mixing on the convergence of angular momentum is on smoothing the radial gradient of the angular momentum when the horizontal mixing length is large. In a sheared storm, both the vertical and horizontal turbulent mixing affect vortex and shear interaction in terms of the evolution of vortex tilt and boundary-layer recovery processes. 

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