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R. T. Williams
,
Melinda S. Peng
, and
D. A. Zankofski

Abstract

The hydrostatic Boussinesq equations are used to simulate the passage of fronts over a two-dimensional mountain in a cyclic domain. The fronts are forced by a confluent, periodic deformation field that moves with the uniform mean flow over the mountain. The initial conditions are selected to give a cold front confined to the lower part of the domain. Fourth-order diffusion terms are included in the numerical model to control energy cascade to the grid size scale. A numerical frontogenesis experiment with no topography produces a realistic surface front in about two days. Numerical solutions for flow over the mountain with no front are found by integrating the equations from the initial conditions, which are semigeostrophic steady-state solutions. Various mountains are considered that have the same height but different widths. The numerical solutions for wide mountains remain close to the semigeostrophic initial conditions, while for narrower mountains vertically propagating waves and a hydraulic jump develop on the lee side of the mountain. The frontal solution and the mountain solution are combined to produce the initial conditions for the basic experiments. The numerical solutions show reduced frontogenesis on the upwind slope and increased frontogenesis on the lee slope. This behavior is caused by the mountain-forced divergence on the upwind side and convergence on the lee side in agreement with the semigeostrophic solution of Zehnder and Bannon. Further experiments with no deformation forcing are carried out to correspond to the semigeostrophic passive scalar studies of Blumen and Gross. A passive scalar that represents the perturbation potential temperature is advected with the mountain solution. The frontal scale, based on the tracer field, increases on the upwind side until it reaches a maximum at the top and then decreases on the lee side, back to its original value as the front moves away from the mountain. The numerical solutions for the interactive potential temperature field have a similar behavior, although some additional blocking effects are present. For the narrower mountains the frontal structure is distorted by the gravity waves on the lee side of the mountain. These solutions resemble those of Schumann for smaller-scale mountains.

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Gan Zhang
,
Zhuo Wang
,
Melinda S. Peng
, and
Gudrun Magnusdottir

Abstract

This study investigates the characteristics of extratropical Rossby wave breaking (RWB) during the Atlantic hurricane season and its impacts on Atlantic tropical cyclone (TC) activity. It was found that RWB perturbs the wind and moisture fields throughout the troposphere in the vicinity of a breaking wave. When RWB occurs more frequently over the North Atlantic, the Atlantic main development region (MDR) is subject to stronger vertical wind shear and reduced tropospheric moisture; the basinwide TC counts are reduced, and TCs are generally less intense, have a shorter lifetime, and are less likely to make landfalls. A significant negative correlation was found between Atlantic TC activity and RWB occurrence during 1979–2013. The correlation is comparable to that with the MDR SST index and stronger than that with the Niño-3.4 index. Further analyses suggest that the variability of RWB occurrence in the western Atlantic is largely independent of that in the eastern Atlantic. The RWB occurrence in the western basin is more closely tied to the environmental variability of the tropical North Atlantic and is more likely to hinder TC intensification or reduce the TC lifetime because of its proximity to the central portion of TC tracks. Consequently, the basinwide TC counts and the accumulated cyclone energy have a strong correlation with western-basin RWB occurrence but only a moderate correlation with eastern-basin RWB occurrence. The results highlight the extratropical impacts on Atlantic TC activity and regional climate via RWB and provide new insights into the variability and predictability of TC activity.

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Mengyuan Ma
,
Melinda S. Peng
,
Tim Li
, and
Lijuan Wang

Abstract

The unusual movement of Typhoon Lionrock (2016) that posed great challenges for operational numerical predictions was investigated. Analysis of the steering flow at different levels shows that Lionrock’s southwestward motion before 25 August was mainly controlled by the upper-level steering, and the dominant steering shifted to lower levels as the storm turned northeastward abruptly afterward. To examine the influence of the environmental flow on this major turning of Lionrock, three numerical simulations are conducted using the Weather Research and Forecasting (WRF) Model with different starting times. The study indicates that the initial southwestward movement of Lionrock is attributed to the westward extension of the mid- to upper-level subtropical high, and the later turning to northeast is caused by the low-level southwesterly flow associated with the monsoon gyre northeast of Lionrock. In an experiment in which the monsoon gyre is removed from the initial and boundary fields, Lionrock continues its southwestward movement without turning northeastward. This result suggests that the transition of the steering from high to low levels plays a crucial role in the major turning of Lionrock. More sensitivity experiments with modifications of the initial and/or the boundary conditions indicate a low predictability of Lionrock’s major turning.

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Melinda S. Peng
,
B-F. Jeng
, and
C-P. Chang

Abstract

A limited-area numerical model designed specifically for forecasting typhoon tracks has been operational at the Central Weather Bureau (CWB) in Taipei, Taiwan, since January 1990. It is a primitive equation model with nine σ levels and a grid size of 70 km. The model domain of 8500 km × 6000 km is centered near Taiwan, and covers the western part of the Pacific Ocean and southeast China. A model-balanced vortex is bogussed into the analysis to initialize the forecast. To ensure the maintenance of the vortex circulation throughout the forecast period, artificial heating options are incorporated to supplement the Kuo-type cumulus parameterization in the model.

The statistics of track errors for all forecast cases conducted during the development and operational checkout period (before December 1989) and during 1990, the first year of real-time operation, are reported. During the operational checkout period, 12 typhoons were forecasted, with an average 48-h track error of 415 km (62 forecast cases). For the 1990 season, there were 11 typhoons, with an average 48-h error of 392 km (63 forecast cases). The errors are compared with the One-Way Interactive Tropical Cyclone Model (OTCM), which is considered as the best long-term operational numerical track model for the western Pacific, using a homogeneous sample. The results indicate that the two models have similar average errors. The model had larger errors than the climatology and persistence (CLIPER) method. However, for all three typhoons with erratic movements, the model outperformed the CLIPER.

The model was modified in several ways prior to the beginning of the 1990 season. The most beneficial modification appears to have been the enlargement of the forecast domain. However, the domain was still not large enough to cover important synoptic fields for Typhoon Marian, which was the westernmost typhoon forecasted by the model. Postoperational experiments were conducted and the forecast track of Typhoon Marian improved when the model domain was expanded to the west. Examination of the synoptic patterns indicates that the track forecast depends closely on the forecast of the subtropical high circulation.

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Bing Fu
,
Tim Li
,
Melinda S. Peng
, and
Fuzhong Weng

Abstract

High-resolution satellite data and NCEP–NCAR reanalysis data are used to analyze 34 tropical cyclone (TC) genesis events in the western North Pacific during the 2000 and 2001 typhoon seasons. Three types of synoptic-scale disturbances are identified in the pregenesis stages. They are tropical cyclone energy dispersions (TCEDs), synoptic wave trains (SWTs) unrelated to preexisting TCs, and easterly waves (EWs). Among the total 34 TC genesis cases, 6 are associated with TCEDs, 11 cases are associated with SWTs, and 7 cases are associated with EWs. The analyses presented herein indicate that the occurrence of a TCED depends on the TC intensity and the background flow, with stronger cyclones and weaker background easterlies being more likely to induce a Rossby wave train. Not all Rossby wave trains would lead to the formation of a new TC. Among the 11 SWT cases, 4 cases are triggered by equatorial mixed Rossby–gravity waves. Cyclogenesis events associated with EWs are identified by the westward propagation of the perturbation kinetic energy and precipitation fields. For all three types of prestorm disturbances, it seems that scale contraction of the disturbances and convergence forcing from the large-scale environmental flow are possible mechanisms leading to the genesis. Further examination of the remaining 10 genesis cases with no significant prior synoptic-scale surface signals suggests three additional possible genesis scenarios: 1) a disturbance with upper-tropospheric forcing, 2) interaction of a preexisting TC with southwesterly monsoon flows, and 3) preexisting convective activity with no significant initial low-level vorticity. Tropical intraseasonal oscillations have a significant modulation on TC formation, especially in 2000.

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Eric A. Hendricks
,
Melinda S. Peng
,
Xuyang Ge
, and
Tim Li

Abstract

A dynamic initialization scheme for tropical cyclone structure and intensity in numerical prediction systems is described and tested. The procedure involves the removal of the analyzed vortex and, then, insertion of a new vortex that is dynamically initialized to the observed surface pressure into the numerical model initial conditions. This new vortex has the potential to be more balanced, and to have a more realistic boundary layer structure than by adding synthetic data in the data assimilation procedure to initialize the tropical cyclone in a model. The dynamic initialization scheme was tested on multiple tropical cyclones during 2008 and 2009 in the North Atlantic and western North Pacific Ocean basins using the Naval Research Laboratory’s tropical cyclone version of the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS-TC). The use of this initialization procedure yielded significant improvements in intensity forecasts, with no degradation in track performance. Mean absolute errors in the maximum sustained surface wind were reduced by approximately 5 kt for all lead times up to 72 h.

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Yi-Ting Yang
,
Hung-Chi Kuo
,
Eric A. Hendricks
, and
Melinda S. Peng

Abstract

An objective method is developed to identify concentric eyewalls (CEs) for typhoons using passive microwave satellite imagery from 1997 to 2011 in the western North Pacific basin. Three CE types are identified: a CE with an eyewall replacement cycle (ERC; 37 cases), a CE with no replacement cycle (NRC; 17 cases), and a CE that is maintained for an extended period (CEM; 16 cases). The inner eyewall (outer eyewall) of the ERC (NRC) type dissipates within 20 h after CE formation. The CEM type has its CE structure maintained for more than 20 h (mean duration time is 31 h). Structural and intensity changes of CE typhoons are demonstrated using a T–Vmax diagram (where T is the brightness temperature and Vmax is the best-track estimated intensity) for a time sequence of the intensity and convective activity (CA) relationship. While the intensity of typhoons in the ERC and CEM cases weakens after CE formation, the CA is maintained or increases. In contrast, the CA weakens in the NRC cases. The NRC (CEM) cases typically have fast (slow) northward translational speeds and encounter large (small) vertical shear and low (high) sea surface temperatures. The CEM cases have a relatively high intensity (63 m s−1), and the moat size (61 km) and outer eyewall width (70 km) are approximately 50% larger than the other two categories. Both the internal dynamics and environmental conditions are important in the CEM cases, while the NRC cases are heavily influenced by the environment. The ERC cases may be dominated by the internal dynamics because of more uniform environmental conditions.

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Yi-Ting Yang
,
Eric A. Hendricks
,
Hung-Chi Kuo
, and
Melinda S. Peng

Abstract

The authors report on western North Pacific Typhoon Soulik (2013), which had two anomalously long-lived concentric eyewall (CE) episodes, as identified from microwave satellite data, radar data, and total precipitable water data. The first period was 25 h long and occurred while Soulik was at category 4 intensity. The second period was 34 h long and occurred when Soulik was at category 2 intensity. A large moat and outer eyewall width were present in both CE periods, and there was a significant contraction of the inner eyewall radius from the first period to the second period. The typhoon intensity decrease was partially due to encountering unfavorable environmental conditions of low ocean heat content and dry air, even though inner eyewall contraction would generally support intensification. The T–Vmax diagram (where T is the brightness temperature and Vmax is the best track–estimated intensity) is used to analyze the time sequence of the intensity and convective activity. The convective activity (and thus the integrated kinetic energy) increased during the CE periods despite the weakening of intensity.

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Ling-Feng Hsiao
,
Melinda S. Peng
,
Der-Song Chen
,
Kang-Ning Huang
, and
Tien-Chiang Yeh

Abstract

Tropical cyclone (TC) track predictions from the operational regional nonhydrostatic TC forecast system of the Taiwanese Central Weather Bureau (CWB) are examined for their sensitivities to initial and lateral boundary conditions. Five experiments are designed and discussed, each using a combination of different initial and lateral boundary conditions coming either from the CWB or the National Centers for Environmental Prediction (NCEP) global forecast system. Eight typhoons in the western Pacific Ocean with 51 cases in 2004 and 2005 are tested with the five designed experiments for the 3-day forecast. The average track forecasts are the best when both the initial and lateral boundary conditions are from the NCEP global forecast system. This reflects the generally superior performance of the NCEP global forecast system relative to that of the CWB. Using different lateral boundary conditions has a greater impact on the track than using different initial conditions. Diagnostics using piecewise inversion of potential vorticity perturbations are carried out to identify synoptic features surrounding the featured typhoon that impact the track the most in each experiment. For the two cases demonstrated with the largest track improvement using NCEP global fields, the diagnostics indicate that the prediction of the strength and extent of the subtropical high in the western Pacific plays the major role in affecting these storm tracks. Using the analysis and predictions of the CWB global forecast system as the initial and lateral boundary conditions produces an overintensified subtropical ridge in the regional TC forecast model. Because most of the typhoons studied are located in the southwestern peripheral of the western Pacific subtropical high, the stronger steering from the more intense and extended high system is the main cause of the poleward bias in the predicted typhoon tracks in the operational run, which uses the CWB global forecast fields. The study suggests that, when efforts are made to improve a regional TC forecast model, it is also critically important to improve the global forecast system that provides the lateral boundary and initial conditions to the regional system.

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Simon T. K. Lang
,
Sarah C. Jones
,
Martin Leutbecher
,
Melinda S. Peng
, and
Carolyn A. Reynolds

Abstract

The sensitivity of singular vectors (SVs) associated with Hurricane Helene (2006) to resolution and diabatic processes is investigated. Furthermore, the dynamics of their growth are analyzed. The SVs are calculated using the tangent linear and adjoint model of the integrated forecasting system (IFS) of the European Centre for Medium-Range Weather Forecasts with a spatial resolution up to TL255 (~80 km) and 48-h optimization time. The TL255 moist (diabatic) SVs possess a three-dimensional spiral structure with significant horizontal and vertical upshear tilt within the tropical cyclone (TC). Also, their amplitude is larger than that of dry and lower-resolution SVs closer to the center of Helene. Both higher resolution and diabatic processes result in stronger growth being associated with the TC compared to other flow features. The growth of the SVs in the vicinity of Helene is associated with baroclinic and barotropic mechanisms. The combined effect of higher resolution and diabatic processes leads to significant differences of the SV structure and growth dynamics within the core and in the vicinity of the TC. If used to initialize ensemble forecasts with the IFS, the higher-resolution moist SVs cause larger spread of the wind speed, track, and intensity of Helene than their lower-resolution or dry counterparts. They affect the outflow of the TC more strongly, resulting in a larger downstream impact during recurvature. Increasing the resolution or including diabatic effects degrades the linearity of the SVs. While the impact of diabatic effects on the linearity is small at low resolution, it becomes large at high resolution.

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