Search Results

You are looking at 1 - 9 of 9 items for

  • Author or Editor: Yi-Hsuan Huang x
  • All content x
Clear All Modify Search
Yi-Hsuan Huang, Michael T. Montgomery, and Chun-Chieh Wu

Abstract

In 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 therein leads to an eruption of air from the BL to support convection outside the primary eyewall in a favorable thermodynamic/kinematic environment.

The results suggest that the unbalanced response in the BL serves as an important mechanism for initiating and sustaining a ring of deep convection in a narrow supergradient wind zone outside the primary eyewall. This progressive BL control on SEF suggests that the BL scheme and its coupling to the interior flow need to be adequately represented in numerical models to improve the prediction of SEF timing and preferred location.

Full access
Guanghua Chen, Chun-Chieh Wu, and Yi-Hsuan Huang

Abstract

The effects of convective and stratiform diabatic processes in the near-core region on tropical cyclone (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 has little effect on 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 the significantly outward expansion of the inner-core size. The removal of both stratiform heating and cooling generates the strongest storm with the structure and intensity similar to those in the experiment with stratiform cooling halved. When both stratiform heating and cooling are doubled, the storm first decays rapidly, followed by the vertical connection of the updrafts at mid- to upper levels in the near-core region and at lower levels in the collapsed eyewall, which reinvigorates the eyewall convection but with a large outward slope.

Full access
Yi-Hsuan Huang, Chun-Chieh Wu, and Michael T. Montgomery

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 winds are supergradient. About two-thirds of the rapid increase in tangential wind tendencies immediately prior to SEF are attributed to agradient wind tendencies. This suggests the importance of nonlinear, unbalanced dynamical processes in SEF in addition to the classical axisymmetric balanced response to forcings of heating and momentum. In the SEF region, analyses show two distinct responsible processes for the increasing azimuthal tangential wind in two vertical intervals. Within the boundary inflow layer, the competing effect between the mean radial influx of absolute vorticity and deceleration caused by surface friction and subgrid diffusion yields a secondary maximum of positive tendency. Analyses further demonstrate the major impact of the mean radial influx of absolute vorticity on SEF. Above the boundary inflow layer, the vertical advection acts to vertically extend the tangential wind jet via the lofting of the enhanced tangential momentum farther upward. The roles of the nonlinear unbalanced dynamics in these two processes are discussed in this paper. From a Lagrangian perspective, the persistently increasing agradient force outweighs the frictional loss, effectively decelerating boundary layer inflowing air across the SEF region. This explains the sharpening of the radial gradient of boundary layer inflow, which is shown to be responsible for the buildup of a zone with concentrated boundary layer convergence. The previously proposed unbalanced dynamical pathway to SEF is elaborated upon and supported by the current results and discussion.

Full access
Yi-Hsuan Chen, Xianglei Huang, Xiuhong Chen, and Mark Flanner

Abstract

This study quantifies the impact of the inclusion of realistic surface spectral emissivity in the Sahara and Sahel on the simulated local climate and beyond. The surface emissivity in these regions can be as low as 0.6–0.7 over the infrared window band while close to unity in other spectral bands, but such spectral dependence has been ignored in current climate models. Realistic surface spectral emissivities over the Sahara and Sahel are incorporated into the Community Earth System Model (CESM) version 1.1.1, while treatments of surface emissivity for the rest of the globe remain unchanged. Both the modified and standard CESM are then forced with prescribed climatological SSTs and fixed present-day forcings for 35-yr simulations. The outputs from the last 30 years are analyzed. Compared to the standard CESM, the modified CESM has warmer surface air temperature, as well as a warmer and wetter planetary boundary layer over the Sahara and Sahel. The modified CESM thus favors more convection in these regions and has more convective rainfall, especially in the Sahara. The moisture convergence induced by such inclusion of surface spectral emissivity also contributes to the differences in simulated precipitation in the Sahel and the region south to it. Compared to observations, inclusion of surface spectral emissivity reduces surface temperature biases in the Sahara and precipitation biases in the Gulf of Guinea but exacerbates the wet biases in the Sahara. Such realistic representation of surface spectral emissivity can help unmask other factors contributing to regional biases in the original CESM.

Full access
Chun-Chieh Wu, Tsung-Han Li, and Yi-Hsuan Huang

Abstract

Observations 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 flow forms as a result of the wind speeds restrained east of the storm center and winds enhanced/maintained west of the storm center. In all, this study highlights a new mechanism that contributes to the terrain-induced southward track deflection in addition to the traditional channeling effect.

Full access
Chun-Chieh Wu, Yi-Hsuan Huang, and Guo-Yuan Lien

Abstract

Typhoon 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’s secondary eyewall formation, while others starting the simulation earlier with less data assimilated are not. This dataset provides a unique opportunity to study the dynamical processes of concentric eyewall formation in Sinlaku. In Part I of this work, results from the data assimilation and simulations are presented, including concentric eyewall evolution and the precursors to its formation, while detailed dynamical analyses are conducted in follow-up research.

Full access
Yi-Hsuan Huang, Chun-Chieh Wu, and Yuqing Wang

Abstract

High-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 deflected to the south before making sharp turns to the north, forming a kinked track pattern prior to and during landfall. This storm movement is consistent with the observed looping cases in Taiwan.

Both real-case and idealized simulations show strong channel winds enhanced between the storm and the terrain when deflection occurs. Backward trajectory analyses support the concept of the channeling effect, which has been previously found to be crucial to the looping motion of Typhoon Haitang (2005) as well. However, the inner-core asymmetric ventilation flow does not match the movement of a deflected typhoon perfectly, partly because the steering flow is not well defined and could not completely capture the terrain-induced deflection in the simulation and in nature.

Full access
Chun-Chieh Wu, Tzu-Hsiung Yen, Yi-Hsuan Huang, Cheng-Ku Yu, and Shin-Gan Chen

Abstract

This 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 cumulative frequency obtained solely from Con-ST is not representative. As compared to the results from All-ST, the extreme rainfall assessed from Con-ST is, on average, underestimated by 23%–44% for typhoons approaching different portions of Taiwan. The uneven distribution of Con-ST, with only three stations located in the mountains higher than 1000 m, is likely to cause significant biases in the interpretation of rainfall patterns. This study illustrates the importance of the increase in the number of available stations in assessing the long-term rainfall characteristic of typhoon-associated heavy rainfall in Taiwan.

Full access
Michael T. Montgomery, Sergio F. Abarca, Roger K. Smith, Chun-Chieh Wu, and Yi-Hsuan Huang
Full access