Chanh Kieu

Genesis of Tropical Storm Eugene (2005) from Merging Vortices Associated with ITCZ Breakdowns. Part III: Sensitivity to Various Genesis Parameters

Abstract In this study, a series of sensitivity simulations is performed to examine the processes leading to the genesis of Tropical Storm Eugene (2005) from merging vortices associated with the breakdowns of the intertropical convergence zone (ITCZ) over the eastern Pacific. This is achieved by removing or modifying one of the two vortices in the model initial conditions or one physical process during the model integration using the results presented in Parts I and II as a control run. Results reveal that while the ITCZ breakdowns and subsequent poleward rollup (through a continuous potential vorticity supply) provide favorable conditions for the genesis of Eugene, the vortex merger is the most effective process in transforming weak tropical disturbances into a tropical storm. The sensitivity experiments confirm the authors’ previous conclusions that Eugene would not reach its observed tropical storm intensity in the absence of the merger and would become much shorter lived without the potential vorticit...

Large-scale control of the lower stratosphere on variability of tropical cyclone intensity

Recent studies of tropical cyclones (TC) have suggested intricate impacts of the lower stratosphere layer (LSL) on TC development at the high-intensity limit. This study examines the potential realization of the impacts of the LSL interannual variability on TC intensity. By minimizing the effects of sea surface temperature and outflow temperature based on the potential intensity framework, partial correlation analyses show a negative correlation between the power dissipation index (PDI) for intense TCs and the tropopause height in the northwestern Pacific basin, but a weaker relationship in the north Atlantic basin. Similar analyses for the LSL stratification also reveals signals of negative correlations between the LSL stratification and PDI in both basins, corroborating recent modeling studies about the impacts of the LSL on TC development. Due to the complexity of the relationships and data limitations, however, all correlations are statistically insignificant, thus rendering the impacts of LSL on TC intensity inconclusive at present.

The Control of Environmental Stratification on the Hurricane Maximum Potential Intensity

This study examines the dependence of the hurricane maximum potential intensity (MPI) on environmental stratification beyond the traditional MPI framework. Unlike the previous formulation in which MPI is a function of the convective available potential energy in the eyewall only, a new MPI formulation is introduced herein that explicitly incorporates the effects of environmental stratification. The new formulation is examined within an axisymmetric modeling framework, using various initial vertical thermodynamic structures. Results show the strong dependence of the model simulated maximum hurricane intensity on environmental stratification, with a lower maximum intensity for a more stable troposphere. Given the growing evidence from recent studies showing that a warmer sea surface temperature would induce a more stable troposphere, our finding suggests a smaller change in the maximum hurricane intensity in the future warming climate than that estimated from the current MPI framework. The new formulation highlights the importance of environmental stratification in hurricane development and the long-term variability of hurricane intensity, a complete understanding of which is still elusive at present. Plain Language Summary This study presents a significant advance in understanding the maximum potential intensity (MPI) that a hurricane can attain in a given stratified environment. Through a series of idealized simulations in an axisymmetric framework, it is shown that environmental stratification plays a more important role in determining the MPI of hurricanes than previously thought. This finding suggests that the hurricane MPI statistics in the future warming climate is significantly overestimated, if the variation of environmental stratification is not taken into account. The opposing role of environmental stratification presented in this study provides new insights into the long-term variability of hurricane intensity beyond the traditional sea surface or upper outflow temperature proxy.

Comments on “Revisiting the Relationship between Eyewall Contraction and Intensification”

AbstractThis comment presents some concerns with the study of Stern et al. and their misinterpretation of the contraction of the radius of the maximum wind (RMW) in tropical cyclones. It is shown that their geometrical RMW contraction model provides little dynamical understanding of the RMW contraction during tropical cyclone intensification, and it differs fundamentally from the RMW contraction model of Willoughby et al. that was derived from the directional derivative concept. Moreover, it is demonstrated that Stern et al. were mistaken in commenting on the derivation of the governing equation for the RMW contraction in Kieu.