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1. The Response of the Near-Surface Tropical Cyclone Wind Field to Inland Surface Roughness Length and Soil Moisture Content during and after Landfall

   https://doi.org/10.1175/JAS-D-20-0211.1  

   Hlywiak, James; Nolan, David S. (March 2021, Journal of the Atmospheric Sciences)

   null (Ed.)

   Abstract The sensitivity of the inland wind decay to realistic inland surface roughness lengths and soil moisture contents is evaluated for strong, idealized tropical cyclones (TCs) of category 4 strength making landfall. Results show that the relative sensitivities to roughness and moisture differ throughout the decay process, and are dependent on the strength and size of the vortex. First, within 12 h of landfall, intense winds at the surface decay rapidly in reaction to the sudden change in surface roughness and decreasing enthalpy fluxes. Wind speeds above the boundary layer decay at a slower rate. Differences in soil moisture contents minimally affect intensity during the first 12 h, as the enhancement of latent heat fluxes from high moisture contents is countered by enhanced surface cooling. After TCs decay to tropical storm intensities, weakening slows and the sensitivity of the intensity decay to soil moisture increases. Increased latent heating becomes significant enough to combat surface temperature cooling, resulting in enhanced convection outside of the expanding radius of maximum winds. This supports a slower decay. Additionally, the decay of the radial wind profile by quadrant is highly asymmetric, as the rear and left-of-motion quadrants decay the fastest. Increasing surface roughness accelerates the decay of the strongest winds, while increasing soil moisture slows the decay of the larger TC wind field. Results have implications for inland forecasting of TC winds and understanding the potential for damage. 

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2. Structural Forecasting for Short-Term Tropical Cyclone Intensity Guidance

   https://doi.org/10.1175/WAF-D-22-0111.1  

   McNeely, Trey; Khokhlov, Pavel; Dalmasso, Niccolò; Wood, Kimberly M.; Lee, Ann B. (June 2023, Weather and Forecasting)

   Abstract Because geostationary satellite (Geo) imagery provides a high temporal resolution window into tropical cyclone (TC) behavior, we investigate the viability of its application to short-term probabilistic forecasts of TC convective structure to subsequently predict TC intensity. Here, we present a prototype model that is trained solely on two inputs: Geo infrared imagery leading up to the synoptic time of interest and intensity estimates up to 6 h prior to that time. To estimate future TC structure, we compute cloud-top temperature radial profiles from infrared imagery and then simulate the evolution of an ensemble of those profiles over the subsequent 12 h by applying a deep autoregressive generative model (PixelSNAIL). To forecast TC intensities at hours 6 and 12, we input operational intensity estimates up to the current time (0 h) and simulated future radial profiles up to +12 h into a “nowcasting” convolutional neural network. We limit our inputs to demonstrate the viability of our approach and to enable quantification of value added by the observed and simulated future radial profiles beyond operational intensity estimates alone. Our prototype model achieves a marginally higher error than the National Hurricane Center’s official forecasts despite excluding environmental factors, such as vertical wind shear and sea surface temperature. We also demonstrate that it is possible to reasonably predict short-term evolution of TC convective structure via radial profiles from Geo infrared imagery, resulting in interpretable structural forecasts that may be valuable for TC operational guidance. Significance Statement This work presents a new method of short-term probabilistic forecasting for tropical cyclone (TC) convective structure and intensity using infrared geostationary satellite observations. Our prototype model’s performance indicates that there is some value in observed and simulated future cloud-top temperature radial profiles for short-term intensity forecasting. The nonlinear nature of machine learning tools can pose an interpretation challenge, but structural forecasts produced by our model can be directly evaluated and, thus, may offer helpful guidance to forecasters regarding short-term TC evolution. Since forecasters are time limited in producing each advisory package despite a growing wealth of satellite observations, a tool that captures recent TC convective evolution and potential future changes may support their assessment of TC behavior in crafting their forecasts. 

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3. Comprehensive Comparison of Seven Widely-Used Planetary Boundary Layer Parameterizations in Typhoon Mangkhut Intensification Simulation

   https://doi.org/10.3390/atmos15101182  

   Ye, Lei; Li, Yubin; Zhu, Ping; Gao, Zhiqiu; Zeng, Zhihua (October 2024, Atmosphere)

   Numerical experiments using the WRF model were conducted to analyze the sensitivity of Typhoon Mangkhut intensification simulations to seven widely used planetary boundary layer (PBL) parameterization schemes, including YSU, MYJ, QNSE, MYNN2, MYNN3, ACM2, and BouLac. The results showed that all simulations generally reproduced the tropical cyclone (TC) track and intensity, with YSU, QNSE, and BouLac schemes better capturing intensification processes and closely matching observed TC intensity. In terms of surface layer parameterization, the QNSE scheme produced the highest Ck/Cd ratio, resulting in stronger TC intensity based on Emanuel’s potential intensity theory. In terms of PBL parameterization, the YSU and BouLac schemes, with the same revised MM5 surface layer scheme, simulated weaker turbulent diffusivity Km and shallower mixing height, leading to stronger TC intensity. During the intensification period, the BouLac, YSU, and QNSE PBL schemes exhibited stronger tangential wind, radial inflow within the boundary layer, and updraft around the eye wall, consistent with TC intensity results. Both PBL and surface layer parameterization significantly influenced simulated TC intensity. The QNSE scheme, with the largest Ck/Cd ratio, and the YSU and BouLac schemes, with weaker turbulent diffusivity, generated stronger radial inflow, updraft, and warm core structures, contributing to higher storm intensity. 

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4. Why Does the Initial Wind Profile Inside the Radius of Maximum Wind Matter to Tropical Cyclone Development?

   https://doi.org/10.1029/2022JD037039  

   Li, Yuanlong; Wang, Yuqing; Tan, Zhe‐Min (August 2022, Journal of Geophysical Research: Atmospheres)

   Abstract This study examines the possible dependence of tropical cyclone (TC) development on the initial winds inside the radius of maximum wind (RMW) through ensemble axisymmetric numerical simulations. Results demonstrate that the vortex with higher initial winds inside the RMW favor larger surface enthalpy flux and thus faster moistening and earlier convective organization in the inner core, significantly shortening the initial spinup period. Higher inertial stability associated with higher winds inside the RMW implies higher eyewall‐heating efficiency, giving rise to higher intensification rate in the subsequent intensification stage but little difference in the steady‐state intensity. The results are confirmed with several sensitivity experiments using different model parameters and three‐dimensional simulations using the same model and configuration. The findings from this study strongly suggest that the realistic representation of the initial inner‐core winds is key to skillful TC intensity forecasts by numerical models and routine high‐resolution observations of the inner‐core wind structure are urged for improving TC intensity forecasts. 

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5. Impacts of an Upper-Tropospheric Cold Low on Tropical Cyclone Intensity

   https://doi.org/10.1175/MWR-D-24-0077.1  

   Li, Han; Ge, Xuyang; Peng, Melinda; Wang, Zhuo (December 2024, Monthly Weather Review)

   Abstract Following a previous study examining the influence of an upper-tropospheric cold low (CL) on the track of a nearby tropical cyclone (TC), this study investigates the impacts of a CL on TC intensity. The results suggest that the relative position and separation distance between the CL and the TC are the key factors affecting TC intensity. When located outside the CL’s radius of maximum winds (RMW) but within its circulation, TCs initially in the northwest quadrant of the CL intensify faster than those in the other quadrants. Theβeffect causes the CL to move northwestward toward the TC and enhances eddy angular momentum flux convergence. Meanwhile, the upper-level TC outflow erodes the CL and reduces the associated vertical wind shear, promoting TC intensification. In contrast, for TCs initially located southeast of the CL, the attraction of the Fujiwhara effect between the two entities counteracts the CL’sβdrift and helps to maintain their separation distance. Moreover, Rossby wave energy dispersion induces an anticyclone southeast of the CL, which transports lower-θ_eair toward the TC and hinders the TC development. Furthermore, TCs within the CL’s RMW reach a similar intensity due to their PV superposition, irrespective of their relative positions to the CL. For TCs located outside the CL circulation, the CL’s impacts are largely negligible for TCs located northwest of the CL, but TCs located southeast of the CL may still be affected by the CL-induced anticyclone. Significance StatementThis study examines the influence of an upper-tropospheric cold low on tropical cyclone intensity. The results illustrate that the relative position and separation distance between the cold low and tropical cyclone are crucial factors in determining tropical cyclone intensity. Tropical cyclones initially northwest of a cold low intensify faster than those to the southeast when located outside the cold low’s radius of maximum winds but still within its circulation. The main mechanisms are how theβsteering and interactions between the two entities act together. The midlevel intrusion of cold, dry air and Rossby energy dispersion also contribute to their complex interaction. These insights provide a guide for forecasting the tropical cyclone intensity when influenced by a nearby upper-level cold low. 

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