Yi-Hsuan Huang

Concentric Eyewall Formation in Typhoon Sinlaku (2008) Part II: Axisymmetric Dynamical Processes

AbstractIn Part I of this study, the association between the secondary eyewall formation (SEF) and the broadening of the outer swirling wind in Typhoon Sinlaku (2008) was documented. The findings from Part I help lay the groundwork for the application of a newly proposed intensification paradigm to SEF. Part II presents a new model for SEF that utilizes this new paradigm and its axisymmetric view of the dynamics.The findings point to a sequence of structure changes that occur in the vortex’s outer-core region, culminating in SEF. The sequence begins with a broadening of the tangential winds, followed by an increase of the corresponding boundary layer (BL) inflow and an enhancement of convergence in the BL where the secondary eyewall forms. The narrow region of strong BL convergence is associated with the generation of supergradient winds in and just above the BL that acts to rapidly decelerate inflow there. The progressive strengthening of BL inflow and the generation of an effective adverse radial force ...

Concentric Eyewall Formation in Typhoon Sinlaku (2008). Part I: Assimilation of T-PARC Data Based on the Ensemble Kalman Filter (EnKF)

AbstractTyphoon Sinlaku (2008) is a case in point under The Observing System Research and Predictability Experiment (THORPEX) Pacific Asian Regional Campaign (T-PARC) with the most abundant flight observations taken and with great potential to address major scientific issues in T-PARC such as structure change, targeted observations, and extratropical transition. A new method for vortex initialization based on ensemble Kalman filter (EnKF) data assimilation and the Weather Research and Forecasting (WRF) model is adopted in this study. By continuously assimilating storm positions (with an update cycle every 30 min), the mean surface wind structure, and all available measurement data, this study constructs a unique high-spatial/temporal-resolution and model/observation-consistent dataset for Sinlaku during a 4-day period. Simulations of Sinlaku starting at different initial times are further investigated to assess the impact of the data. It is striking that some of the simulations are able to capture Sinlaku...

The Influence of Island Topography on Typhoon Track Deflection

AbstractHigh-resolution simulations for Typhoon Krosa (2007) and a set of idealized experiments are conducted using a full-physics model to investigate the eminent deflection of typhoon track prior to its landfall over mountainous island topography. The terrain height of Taiwan plays the most important role in Typhoon Krosa’s looping motion at its landfall, while the surface properties, details in the topographic shape of Taiwan, and the cloud microphysics are shown to be secondary to the track deflection. A simulation with 3-km resolution and realistic model settings reproduces the observed Krosa’s track, while that with 9-km resolution fails, suggesting that high resolution to better resolve the typhoon–terrain interactions is important for the prediction and simulation of typhoon track deflection prior to landfall. Results from idealized experiments with model configurations mimicking those of Supertyphoon Krosa show that vortices approaching the northern and central topography are significantly deflec...

Comments on “How Does the Boundary Layer Contribute to Eyewall Replacement Cycles in Axisymmetric Tropical Cyclones?”

Abstract : In a recent paper, Kepert (2013, hereafter K13) investigated the theoretical role of the boundary layer in eyewall replacement cycles. Specifically, he used a family of steady-state, axisymmetric hurricane boundary layer models to examine the boundary layer response to an imposed radial profile of tangential winds with two wind maxima. Based on these solutions, he proposed a new feedback mechanism for secondary eyewall formation (SEF) and pointed to the role of the underlying boundary layer dynamics in this process. Specifically, he proposed (abstract) that the boundary layer contributes to the formation of outer eyewalls through a positive feedback among the local enhancement of the radial vorticity gradient, the frictional updraft, and convection, and concluded (section 6) that supergradient flow . . . is not essential to SEF.

Influence of Mesoscale Topography on Tropical Cyclone Tracks: Further Examination of the Channeling Effect

AbstractObservations have documented typhoons experiencing pronounced track deflection before making landfall in Taiwan. In this study, idealized full-physics model experiments are conducted to assess the orographic influence on tropical cyclone (TC) track. An intense and westward-moving TC is simulated to approach the bell-shaped terrain imitating the Taiwan topography. Sensitivity numerical experiments are carried out to evaluate the topographic effect under different flow regimes and parameters, such as TC intensity, terrain height, and incident angle of the TC movement toward the topography. All the presented simulated storms experience southward track deflection prior to landfall. Different from the mechanism related to the channeling-effect-induced low-level northerly jet as suggested in previous studies, this study indicates the leading role of the northerly asymmetric flow in the midtroposphere in causing the southward deflection of the simulated TC tracks. The midtropospheric northerly asymmetric...

Statistical Characteristic of Heavy Rainfall Associated with Typhoons near Taiwan Based on High-Density Automatic Rain Gauge Data

AbstractThis study utilizes data compiled over 21 years (1993–2013) from the Central Weather Bureau of Taiwan to investigate the statistical characteristics of typhoon-induced rainfall for 53 typhoons that have impacted Taiwan. In this work the data are grouped into two datasets: one includes 21 selected conventional weather stations (referred to as Con-ST), and the other contains all the available rain gauges (250–500 gauges, mostly automatic ones; referred to as All-ST). The primary aim of this study is to understand the potential impacts of the different gauge distributions between All-ST and Con-ST on the statistical characteristics of typhoon-induced rainfall. The analyses indicate that although the average rainfall amount calculated with Con-ST is statistically similar to that with All-ST, the former cannot identify the precipitation extremes and rainfall distribution appropriately, especially in mountainous areas. Because very few conventional stations are located over the mountainous regions, the ...

Secondary eyewall formation in tropical cyclones

Secondary eyewall formation (SEF), and the subsequent eyewall replacement cycle, are often observed in intense tropical cyclones (TCs), and its association with short-term changes in TC intensity and structure has been widely documented from aircraft observations and satellite imagery (Willoughby et al., 1982, Black and Willoughby, 1992, Willoughby and Black, 1996, Houze et al., 2006, 2007, Hawkins and Helveston, 2008, Kossin and Sitkowski, 2009, Kuo et al., 2009, Didlake and Houze, 2011, Sitkowski et al., 2011, Bell et al., 2012, Hence and Houze, 2012). A double-eyewall TC contains two concentric quasi-circular deep convective rings (inner and outer TC eyewalls) with a nearly cloud-free region (moat) between them. In most such cases, the outer eyewall is established later, with characteristics similar to the inner eyewall. A localized maximum swirling wind is often present in the outer eyewall, with its scope confined to the lower troposphere. Taking the example of a model simulation of Typhoon Sinlaku (2008), constructed by Wu et al. (2012), Fig. 13.1 demonstrates such flow characteristics in a concentric eyewall TC. For cases undergoing an eyewall replacement cycle, during which a TC usually weakens and enlarges, the inner eyewall gradually dissipates, while the outer eyewall later becomes the new primary eyewall. More recently, it has been shown that SEF is preceded by a broadening tangential wind field with small radial gradients in the storm’s outer-core region, serving as a precursory flow characteristic for SEF (Wu et al., 2012, Huang et al., 2012; hereafter WH12). Considering such temporary, but pronounced, changes in storm intensity and structure, and the lack of skill in predicting concentric-eyewall events, SEF remains an important research issue and forecast priority for the understanding of TC intensity/structure evolution. This review provides an updated summary and discussion of the literature concerned with our current understanding of the favorable conditions for, and potential mechanisms of, SEF. Because of limited or discontinuous spatial/temporal coverage of observations, studies of SEF mechanisms have mostly been based on numerical simulations. The remainder of this article is organized as follows. Section 13.2 describes the possible roles of various environmental conditions in SEF. Having considered the environmental factors that are conducive to SEF, Section 13.3 introduces a variety of internal dynamical processes suggested for SEF, such as the axisymmetrization process, energy accumulation through vortex Rossby wave activities, beta-skirt-induced energy cascade, unbalanced responses to boundary layer dynamics, and balanced response to convective heating. Finally, in Section 13.4, the merits and caveats of the various dynamical interpretations are discussed, and the remaining unresolved issues are addressed to provide guidance for future SEF research.

The role of near-core convective and stratiform heating/cooling in tropical cyclone structure and intensity

AbstractThe effects of convective and stratiform diabatic processes in the near-core region on TC structure and intensity change are examined by artificially modifying the convective and stratiform heating/cooling between 40- and 80-km radii. Sensitivity experiments show that the absence of convective heating in the annulus can weaken TC intensity and decrease the inner-core size. The increased convective heating generates a thick and polygonal eyewall, while the storm intensifies more gently than that in the control run. The removal of stratiform heating can slow down TC intensification with a moderate intensity, whereas the doubling of stratiform heating affects little the TC evolution compared to the control run. The halved stratiform cooling facilitates TC rapid intensification and a compact inner-core structure with the spiral rainbands largely suppressed. With the stratiform cooling doubled, the storm terminates intensification and eventually develops a double eyewall-like structure, accompanied by ...

Concentric Eyewall Formation in Typhoon Sinlaku (2008). Part III: Horizontal Momentum Budget Analyses

Abstract This is a follow-up work to two prior studies examining secondary eyewall formation (SEF) in Typhoon Sinlaku (2008). This study shows that, in the SEF region, the majority of the elevated ...