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Hung-Chi Kuo
,
R. T. Williams
, and
Jen-Her Chen

Abstract

An elliptical eye that rotated cyclonically with a period of approximately 144 minutes in Typhoon Herb 1996 was documented. The elliptical region had a semimajor axis of 30 km and a semiminor axis of 20 km. Two complete periods of approximately 144 min were observed in the Doppler radar data. The rotation of the elliptical eye in the context of barotropic dynamics at three levels were explored: linear waves on a Rankin vortex, a nonlinear Kirchhoff vortex, and with a nonlinear spectral model. The linear wave theory involves the existence of both the high (potential) vorticity gradient near the eye edge and the cyclonic mean tangential flow in the typhoon. The propagation of (potential) vorticity waves in the cyclonic mean flow makes the elliptical eye rotate cyclonically. The rotation period is longer than the period of a parcel trajectory moving in the cyclonic mean flow around the circumference, because the vorticity wave propagates upwind. The nonlinear theory stems from the rotation of Kirchhoff’s vortex. Estimates of the eye rotation period from both linear and nonlinear theories agree with observations of the eye rotation period when the observed maximum wind from Herb is used. Nonlinear numerical computations suggest the importance of the interaction of neutral vorticity waves, which determine the shape and the rotation period of the eye. The calculations also support the rotation of the eye in approximately 144 min in the presence of axisymmetrization, vorticity redistribution, wave breaking, and vortex merging processes.

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Hung-Chi Kuo
,
R. T. Williams
,
Jen-Her Chen
, and
Yi-Liang Chen

Abstract

The impact of the island topographic β effect on hurricane-like vortex tracks is studied. Both f plane and spherical geometry without a mean flow are considered. The simulations used in this study indicate the existence of a track mode in which vortices are trapped by the topography and follow a clockwise island-circulating path. The trapping of a hurricane-like vortex can be interpreted in terms of the influence of the island topographic β effect on the vortex track. Experiments on the f plane indicate that the drift speed along the clockwise path is proportional to the square root of β e v max. The applicability of the square root law on the f plane is dependent on the degree to which the local β e effect is felt by the vortex. The experiments on the sphere also demonstrate that the speed along the clockwise path is larger for a vortex with a larger maximum wind v max. The occurrence of hurricane-like vortex trapping, however, is not sensitive to the value of v max. When there is no background flow, the vortex will drift to the northwest in the presence of the planetary vorticity gradient. The β drift speed acts to keep the vortex from being trapped. The insensitivity of the vortex trapping to v max on the sphere appears to be due to the possible cancellation of stronger planetary β and topographic β effects. The experiments suggest that the topographic scale must be comparable to (if not larger than) the vortex radius of maximum wind for the trapping to occur. Nonlinear effects are important in that they hold the vortex together and keep it moving without strong dispersion in the island-circulating path. This vortex coherency can be explained with the β Rossby number dynamics. The global shallow-water model calculations used in this study indicate that the vortex trapping increases with peak height, topographic length scale, and latitude (larger topographic β effect). In general, the trapping and clockwise circulating path in the presence of a planetary vorticity gradient will occur if the scale of the topography is greater than the vortex radius of maximum wind and if the planetary β parameter is less than the topographic β parameter.

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Chun-Yian Su
,
Chien-Ming Wu
,
Wei-Ting Chen
, and
Jen-Her Chen

Abstract

This study implements the unified parameterization (UP) in the Central Weather Bureau Global Forecast System (CWBGFS) based on the relaxed Arakawa–Schubert scheme (RAS) at a horizontal resolution of 15 km. The new cumulus parameterization that incorporates the UP framework is called URAS. The UP generalizes the representation of moist convection between the parameterized and the explicitly resolved processes according to the process-dependent convective updraft fraction (σ). Short-term hindcasts are performed to investigate the impacts of the UP on the simulated precipitation variability and organized convective systems over the Maritime Continent when multiple scales of convection occurred. The result shows that σ is generally larger when convective systems develop, which adaptively reduces the parameterized convection and increases the spatial variation of moisture. In the URAS experiment, the moisture hotspots within organized convective systems contribute to the enhanced local circulation and the more significant variability of precipitation. Consequently, the URAS has a more realistic precipitation spectrum, an improved relationship between the maximum precipitation and the horizontal scale of the convective systems, and an improved column water vapor–precipitation relationship.

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Hung-Chi Kuo
,
Jen-Her Chen
,
R. T. Williams
, and
C-P. Chang

Abstract

The interactions between monsoon circulations and tropical disturbances in the Northwest Pacific, where the low-level mean flow is westerly in the west and easterly in the east, are studied with a barotropic model. The authors’ model results suggest that the scale contraction by the confluent background flow, the nonlinear dynamics, the β effect, and the large-scale convergence are important for the energy and enstrophy accumulation near the region where the zonal flow reverses. The energy/enstrophy accumulation can be maintained with a continuous Rossby wave emanation upstream. The largest accumulation occurs when the emanating zonal wavelength is around 2000 km. Longer Rossby waves experience less scale contraction and nonlinear effects while shorter Rossby waves cannot hold a coherent structure against dispersive effects.

The nonlinear energy/enstrophy accumulation mechanism is significantly different from previous linear energy accumulation theories. In the linear theories this is primarily accomplished by the slowdown of the Doppler-shifted group velocity through the convergence of mean zonal advection, while in nonlinear dynamics the contraction of the zonal wave scale plays the crucial role. More importantly, after the initial energy increase by the wave accumulation, linear dynamics will lead to an eventual loss of wave energy to the mean flow due to the increase of zonal wavenumber near the critical longitude. Thus, without the presence of other forcing processes such as diabatic heating, the disturbances will decay. In nonlinear dynamics, the sharpening of the vorticity gradient as the waves approach the confluence zone leads to the development of disturbance asymmetries with respect to the central latitude. This effect is through the nonlinear interaction of Rossby waves with the planetary vorticity gradient. This development leads to a pair of vorticity centers that straddles the central latitude with the cyclone (anticyclone) in the north (south), and an elongated, weak westerly flow along the central latitude. This elongated westerly flow, which possesses a zonal wavenumber smaller than that in the linear cases, reverses the sign of the Reynold’s stress and allows the energy to grow near the critical longitude, leading to intensified disturbances.

With a more realistic monsoonlike background flow, a northwestward propagation pattern with an approximately 8-day period and 3000-km wavelength is produced, in general agreement with observed disturbances in the Northwest Pacific. The intensified disturbance may disperse energy upstream, leading to a series of trailing anticyclonic and cyclonic cells along the northwestward propagation path. When an opposing current is present, the energy dispersion leads to the formation of new disturbances in the confluence zone by vortex axisymmetrization dynamics. Thus, our results indicate that the scale contraction and nonlinear effects may cause a succession of tropical disturbances to develop without disturbance-scale diabatic effects.

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Ching-Yuang Huang
,
Tzu-Chi Juan
,
Hung-Chi Kuo
, and
Jen-Her Chen

Abstract

This study applies a global model (FV3GFS) with stretched resolution of approximately 7 km for simulating Typhoon Maria (2018), which exhibited a sudden northward track deflection when approaching about 150 km northeast of Taiwan. As Maria approached land, the outer cyclonic flow at the western flank of the typhoon is split around the northern part of the Central Mountain Range (CMR) in Taiwan to converge east of Taiwan with the recirculating southerly flow around the southern corner of the CMR. Such strong convergence leads to northward deflection of the west-northwestward-moving typhoon with the stronger wind mainly east of the vortex center. The radial inflow at low levels is intensified south of the vortex center and transports larger angular momentum (AM) inward with the enhanced upward motions and vertical mean AM advection to increase the azimuthal mean tangential wind in the lower-tropospheric eyewall. A vorticity budget of wavenumber-1 decomposition indicates that the track deflection is dominated by horizontal vorticity advection in response to the intensifying flow. Numerical experiments with idealized WRF also support such northward track deflection as westward tropical cyclones approach a mountain range within an offshore meridional distance of about 200 km. The northward track deflection is only slightly amplified as the terrain height is considerably increased, consistent with the real-case simulation. However, the northward track deflection is not increased as the approaching vortex is initialized closer to the northern end of the mountain range, due to the enhanced east–west symmetry of wind structure in the inner vortex.

Open access
Ling-Feng Hsiao
,
Chi-Sann Liou
,
Tien-Chiang Yeh
,
Yong-Run Guo
,
Der-Song Chen
,
Kang-Ning Huang
,
Chuen-Teyr Terng
, and
Jen-Her Chen

Abstract

This paper introduces a relocation scheme for tropical cyclone (TC) initialization in the Advanced Research Weather Research and Forecasting (ARW-WRF) model and demonstrates its application to 70 forecasts of Typhoons Sinlaku (2008), Jangmi (2008), and Linfa (2009) for which Taiwan’s Central Weather Bureau (CWB) issued typhoon warnings. An efficient and dynamically consistent TC vortex relocation scheme for the WRF terrain-following mass coordinate has been developed to improve the first guess of the TC analysis, and hence improves the tropical cyclone initialization. The vortex relocation scheme separates the first-guess atmospheric flow into a TC circulation and environmental flow, relocates the TC circulation to its observed location, and adds the relocated TC circulation back to the environmental flow to obtain the updated first guess with a correct TC position. Analysis of these typhoon cases indicates that the relocation procedure moves the typhoon circulation to the observed typhoon position without generating discontinuities or sharp gradients in the first guess.

Numerical experiments with and without the vortex relocation procedure for Typhoons Sinlaku, Jangmi, and Linfa forecasts show that about 67% of the first-guess fields need a vortex relocation to correct typhoon position errors while eliminates the topographical effect. As the vortex relocation effectively removes the typhoon position errors in the analysis, the simulated typhoon tracks are considerably improved for all forecast times, especially in the early periods as large adjustments appeared without the vortex relocation. Comparison of the horizontal and vertical vortex structures shows that large errors in the first-guess fields due to an incorrect typhoon position are eliminated by the vortex relocation scheme and that the analyzed typhoon circulation is stronger and more symmetric without distortions, and better agrees with observations. The result suggests that the main difficulty of objective analysis methods [e.g., three-dimensional variational data assimilation (3DVAR)], in TC analysis comes from poor first-guess fields with incorrect TC positions rather than not enough model resolution or observations. In addition, by computing the eccentricity and correlation of the axes of the initial typhoon circulation, the distorted typhoon circulation caused by the position error without the vortex relocation scheme is demonstrated to be responsible for larger track errors. Therefore, by eliminating the typhoon position error in the first guess that avoids a distorted initial typhoon circulation, the vortex relocation scheme is able to improve the ARW-WRF typhoon initialization and forecasts particularly when using data assimilation update cycling.

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Chi–Sann Liou
,
Jen–Her Chen
,
Chuen–Teyr Terng
,
Feng–Ju Wang
,
Chin–Tzu Fong
,
Thomas E. Rosmond
,
Hong–Chi Kuo
,
Chih–Hui Shiao
, and
Ming–Dean Cheng

Abstract

The global forecast system (GFS), which started its operation in 1988 at the Central Weather Bureau in Taiwan, has been upgraded to incorporate better numerical methods and more complete parameterization schemes. The second-generation GFS uses multivariate optimum interpolation analysis and incremental nonlinear normal-mode initialization to initialize the forecast model. The forecast model is a global primitive equation model with a resolution of 18 sigma levels in the vertical and 79 waves of triangular truncation in the horizontal. The forecast model includes a 1.5-order eddy mixing parameterization, a gravity wave drag parameterization, a shallow convection parameterization, a relaxed version of Arakawa–Schubert cumulus parameterization, grid-scale condensation calculation, and longwave and shortwave radiative transfer calculations with consideration of fractional clouds. The performance of the second-generation GFS is significantly better than the first-generation GFS. For two 3-month periods in winter 1995/96 and summer 1996, the second-generation GFS provided forecasters with 5-day forecasts where the averaged 500-mb height anomaly correlation coefficients for the Northern Hemisphere were greater than 0.6.

Observational data available to the GFS are much less than those at other numerical weather prediction centers, especially in the Tropics and Southern Hemisphere. The GRID messages of 5° resolution, ECMWF 24-h forecast 500-mb height and 850- and 200-mb wind fields available once a day on the Global Telecommunications System are used as supplemental observations to increase the data coverage for the GFS data assimilation. The supplemental data improve the GFS performance both in the analysis and forecast.

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Guo-Yuan Lien
,
Chung-Han Lin
,
Zih-Mao Huang
,
Wen-Hsin Teng
,
Jen-Her Chen
,
Ching-Chieh Lin
,
Hsu-Hui Ho
,
Jyun-Ying Huang
,
Jing-Shan Hong
,
Chia-Ping Cheng
, and
Ching-Yuang Huang

Abstract

The FORMOSAT-7/COSMIC-2 Global Navigation Satellite System (GNSS) Radio Occultation (RO) satellite constellation was launched in June 2019 as a successor of the FORMOSAT-3/COSMIC mission. The Central Weather Bureau (CWB) of Taiwan has received FORMOSAT-7/COSMIC-2 GNSS RO data in real time from the Taiwan Analysis Center for COSMIC. With the global numerical prediction system at CWB, a parallel semioperational experiment assimilating the FORMOSAT-7/COSMIC-2 bending angle data with all other operational observation data has been conducted to evaluate the impact of the FORMOSAT-7/COSMIC-2 data. The first seven-month results show that the quality of the early FORMOSAT-7/COSMIC-2 data has been satisfactory for assimilation. Consistent and significant positive impacts on global forecast skills have been observed since the start of the parallel experiment, with the most significant impact found in the tropical region, reflecting the low-inclination orbital design of the satellites. The impact of the FORMOSAT-7/COSMIC-2 RO data is also estimated using the ensemble forecast sensitivity to observation impact (EFSOI) method, showing an average positive impact per observation similar to other existing GNSS RO datasets, while the total impact is impressive by virtue of its large amount. Sensitivity experiments suggest that the quality control processes built in the Gridpoint Statistical Interpolation (GSI) system for RO data work well to achieve a positive impact by the low-level FORMOSAT-7/COSMIC-2 RO data, while more effort on observation error tuning should be focused to obtain an optimal assimilation performance. This study demonstrates the usefulness of the FORMOSAT-7/COSMIC-2 RO data in global numerical weather prediction during the calibration/validation period and leads to the operational use of the data at CWB.

Open access
Hann-Ming Henry Juang
,
Tzu-Yu Wu
,
Pang-Yen Brian Liu
,
Hsin-Yi Lin
,
Ching-Teng Lee
,
Mien-Tze Kueh
,
Jia-Fong Fan
,
Jen-Her River Chen
,
Mong-Ming Lu
, and
Pay-Liam Lin

Abstract

The first version of the Taiwan Central Weather Bureau one-tier (TCWB1T) fully coupled global atmospheric and oceanic modeling forecast system had been developed and implemented as a routine operation for seasonal prediction at Central Weather Bureau (CWB) in 2017, with a minor revision in 2020. Based on NCEP CFSv1, the global atmospheric model in NCEP CFSv1 was replaced by CWB’s atmospheric global spectral model (GSM) and coupled with the GFDL MOM3. Several parameters have been tested and tuned in the CWB atmospheric GSM, achieving an optimal configuration with better sea surface temperature (SST) predictions for integration more than one year. Using NCEP CFSR as the initial condition, TCWB1T conducted hindcasts from 1982 to 2011 and forecasts from 2012 to 2019 to analyze its performance. The results of these hindcasts and forecasts show that the TCWB1T can make useful predictions as verified against the observations of OISST, ERSST, CFSR, and GPCP based on the methods of EOF, RMSE, anomaly correlation, ranked probability skill score (RPSS), reliability diagram (RD), and relative operating characteristics (ROCs). TCWB1T also has the same level of skill scores as NCEP CFSv2 and/or the ECMWF fifth-generation seasonal forecast system (SEAS5), based on EOF, anomaly pattern correlation, climatological bias, RMSE, temporal correlation, and anomaly correlation percentage of forecast skill. TCWB1T shows forecast skill that is better in winter than in summer. Overall, it indicates that TCWB1T can be used for seasonal ENSO predictions.

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