More Like this

1. A New Time-Dependent Theory of Tropical Cyclone Intensification

   https://doi.org/10.1175/JAS-D-21-0169.1  

   ( December 2021 , Journal of the Atmospheric Sciences)

   In this study, the boundary layer tangential wind budget equation following the radius of maximum wind, together with an assumed thermodynamical quasi-equilibrium boundary layer, is used to derive a new equation for tropical cyclone (TC) intensification rate (IR). A TC is assumed to be axisymmetric in thermal-wind balance, with eyewall convection coming into moist slantwise neutrality in the free atmosphere above the boundary layer as the storm intensifies, as found recently based on idealized numerical simulations. An ad hoc parameter is introduced to measure the degree of congruence of the absolute angular momentum and the entropy surfaces. The new IR equation is evaluated using results from idealized ensemble full-physics axisymmetric numerical simulations. Results show that the new IR equation can reproduce the time evolution of the simulated TC intensity. The new IR equation indicates a strong dependence of IR on both TC intensity and the corresponding maximum potential intensity (MPI). A new finding is the dependence of TC IR on the square of the MPI in terms of the near-surface wind speed for any given relative intensity. Results from some numerical integrations of the new IR equation also suggest the finite-amplitude nature of TC genesis. In addition, the newmore »IR theory is also supported by some preliminary results based on best-track TC data over the North Atlantic Ocean and eastern and western North Pacific Ocean. As compared with the available time-dependent theories of TC intensification, the new IR equation can provide a realistic intensity-dependent IR during weak intensity stage as seen in observations.

   « less
2. Observed Kinematic and Thermodynamic Structure in the Hurricane Boundary Layer during Intensity Change

   https://doi.org/10.1175/MWR-D-18-0380.1  

   Ahern, Kyle ; Bourassa, Mark A. ; Hart, Robert E. ; Zhang, Jun A. ; Rogers, Robert F. ( July 2019 , Monthly Weather Review)

   The axisymmetric structure of the inner-core hurricane boundary layer (BL) during intensification [IN; intensity tendency ≥20 kt (24 h)−1, where 1 kt ≈ 0.5144 m s−1], weakening [WE; intensity tendency <−10 kt (24 h)−1], and steady-state [SS; the remainder] periods are analyzed using composites of GPS dropwindsondes from reconnaissance missions between 1998 and 2015. A total of 3091 dropsondes were composited for analysis below 2.5-km elevation—1086 during IN, 1042 during WE, and 963 during SS. In nonintensifying hurricanes, the low-level tangential wind is greater outside the radius of maximum wind (RMW) than for intensifying hurricanes, implying higher inertial stability (I2) at those radii for nonintensifying hurricanes. Differences in tangential wind structure (and I2) between the groups also imply differences in secondary circulation. The IN radial inflow layer is of nearly equal or greater thickness than nonintensifying groups, and all groups show an inflow maximum just outside the RMW. Nonintensifying hurricanes have stronger inflow outside the eyewall region, likely associated with frictionally forced ascent out of the BL and enhanced subsidence into the BL at radii outside the RMW. Equivalent potential temperatures (θe) and conditional stability are highest inside the RMW of nonintensifying storms, which is potentially related to TCmore »intensity. At greater radii, inflow layer θe is lowest in WE hurricanes, suggesting greater subsidence or more convective downdrafts at those radii compared to IN and SS hurricanes. Comparisons of prior observational and theoretical studies are highlighted, especially those relating BL structure to large-scale vortex structure, convection, and intensity.

   « less
3. Can Existing Theory Predict the Response of Tropical Cyclone Intensity to Idealized Landfall?

   https://doi.org/10.1175/JAS-D-21-0037.1  

   Chen, Jie ; Chavas, Daniel R. ( October 2021 , Journal of the Atmospheric Sciences)

   Abstract Tropical cyclones cause significant inland hazards, including wind damage and freshwater flooding, which depend strongly on how storm intensity evolves after landfall. Existing theoretical predictions for storm intensification and equilibrium storm intensity have been tested over the open ocean but have not yet been applied to storms after landfall. Recent work examined the transient response of the tropical cyclone low-level wind field to instantaneous surface roughening or drying in idealized axisymmetric f -plane simulations. Here, experiments testing combined surface roughening and drying with varying magnitudes of each are used to test theoretical predictions for the intensity response. The transient response to combined surface forcings can be reproduced by the product of their individual responses, in line with traditional potential intensity theory. Existing intensification theory is generalized to weakening and found capable of reproducing the time-dependent inland intensity decay. The initial (0–10 min) rapid decay of near-surface wind caused by surface roughening is not captured by existing theory but can be reproduced by a simple frictional spindown model, where the decay rate is a function of surface drag coefficient. Finally, the theory is shown to compare well with the prevailing empirical decay model for real-world storms. Overall, results indicate themore »potential for existing theory to predict how tropical cyclone intensity evolves after landfall.« less
4. The Role of Eyewall Turbulent Transport in the Pathway to Intensification of Tropical Cyclones

   https://doi.org/10.1029/2021JD034983  

   Zhu, Ping ; Hazelton, Andrew ; Zhang, Zhan ; Marks, Frank D. ; Tallapragada, Vijay ( August 2021 , Journal of geophysical research)

   In a tropical cyclone (TC), turbulence not only exists in the planetary boundary layer (PBL) but also can be generated above the PBL by the cloud processes in the eyewall and rainbands. It is found that the Hurricane Analysis and Forecast System (HAFS), a new multi-scale operational model for TC prediction, fails to capture the intense turbulent mixing in eyewall and rainband clouds due to a poor estimation of static stability in clouds. The problem is fixed by including the effects of multi-phase water in the stability calculation. Simulations of 21 TCs and tropical storms in the North Atlantic basin of 2016–2019 hurricane seasons totaling 118 forecast cycles show that the stability correction substantially improves HAFS's skill in predicting storm track and intensity. Analyses of HAFS's simulations of Hurricane Michael (2018) show that the positive tendency of vortex's tangential wind resulting from the radially inward transport of absolute vorticity dominates the eddy correlation tendencies induced by the model-resolved asymmetric eddies and serves as a main mechanism for the rapid intensification of Michael. The sub-grid scale (SGS) turbulent transport above the PBL in the eyewall plays a pivotal role in initiating a positive feedback among the eyewall convection, mean secondary overturningmore »circulation, vortex acceleration via the inward transport of absolute vorticity, surface evaporation, and radial convergence of moisture in the PBL. Without the SGS transport above the PBL, the model-resolved vertical transport alone may not be sufficient in initiating the positive feedback underlying the rapid intensification of TCs.« less
5. Uncovering the Intrinsic Intensity–Size Relationship of Tropical Cyclones

   https://doi.org/10.1175/JAS-D-22-0082.1  

   Sun, Jie ; Cai, Ming ; Liu, Guosheng ; Yan, Ruikai ; Zhang, Da-Lin ( November 2022 , Journal of the Atmospheric Sciences)

   The central theme of this study is to explore if and how the intensity of a tropical cyclone (TC) is related to its size. This subject has puzzled atmospheric scientists since the work of Deppermann, but the existence of this relationship still remains elusive. The improved understanding of the intensity–size relationship of TCs will help coastal communities to prepare for the maximum potential damage as both the intensity and size have important impacts on wind damages, storm surges, and flooding. This study considers 33 years (1988–2020) of TC records of maximum surface winds and radii of maximum and gale-force winds over the North Atlantic basin derived from the Extended Best Track Dataset. Analysis of these TC records reveals a robust positive correlation between loss of Earth and relative angular momentum. This finding together with the inspiration from the seminal work of Emanuel and his collaborators leads us to combine absolute angular momentum and its frictional loss as a radially invariant quantity, referred to as “effective absolute angular momentum” (eAAM), for radial profiles of TC surface winds. It is demonstrated that the eAAM model can reproduce the observed complex intensity–size relationship of TCs, which can be further reduced to amore »quasi-linear one after factoring out the angular momentum loss and the radius of maximum surface winds. The findings of this study would not only advance our understanding of the complex TC intensity–size relation, but also allow for operational assessments of TC severity and potential damage just using its outer wind information.

   « less