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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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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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Wei Zhang
,
Bing Fu
,
Melinda S. Peng
, and
Tim Li

Abstract

This study investigates the classification of developing and nondeveloping tropical disturbances in the western North Pacific (WNP) through the C4.5 algorithm. A decision tree is built based on this algorithm and can be used as a tool to predict future tropical cyclone (TC) genesis events. The results show that the maximum 800-hPa relative vorticity, SST, precipitation rate, divergence averaged between 1000- and 500-hPa levels, and 300-hPa air temperature anomaly are the five most important variables for separating the developing and nondeveloping tropical disturbances. This algorithm also unravels the thresholds of the five variables (i.e., 4.2 × 10−5 s−1 for maximum 800-hPa relative vorticity, 28.2°C for SST, 0.1 mm h−1 for precipitation rate, −0.7 × 10−6 s−1 for vertically averaged convergence, and 0.5°C for 300-hPa air temperature anomaly). Six rules are derived from the decision tree. The classification accuracy of this decision tree is 81.7% for the 2004–10 cases. The hindcast accuracy for the 2011–13 dataset is 84.6%.

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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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Hao Jin
,
Melinda S. Peng
,
Yi Jin
, and
James D. Doyle

Abstract

A series of experiments have been conducted using the Coupled Ocean–Atmosphere Mesoscale Prediction System–Tropical Cyclone (COAMPS-TC) to assess the impact of horizontal resolution on hurricane intensity prediction for 10 Atlantic storms during the 2005 and 2007 hurricane seasons. The results of this study from the Hurricane Katrina (2005) simulations indicate that the hurricane intensity and structure are very sensitive to the horizontal grid spacing (9 and 3 km) and underscore the need for cloud microphysics to capture the structure, especially for strong storms with small-diameter eyes and large pressure gradients. The high resolution simulates stronger vertical motions, a more distinct upper-level warm core, stronger upper-level outflow, and greater finescale structure associated with deep convection, including spiral rainbands and the secondary circulation. A vortex Rossby wave (VRW) spectrum analysis is performed on the simulated 10-m winds and the NOAA/Hurricane Research Division (HRD) Real-Time Hurricane Wind Analysis System (H*Wind) to evaluate the impact of horizontal resolution. The degree to which the VRWs are adequately resolved near the TC inner core is addressed and the associated resolvable wave energy is explored at different grid resolutions. The fine resolution is necessary to resolve higher-wavenumber modes of VRWs to preserve more wave energy and, hence, to attain a more detailed eyewall structure. The wind–pressure relationship from the high-resolution simulations is in better agreement with the observations than are the coarse-resolution simulations for the strong storms. Two case studies are analyzed and overall the statistical analyses indicate that high resolution is beneficial for TC intensity and structure forecasts, while it has little impact on track forecasts.

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Weiwei Li
,
Zhuo Wang
,
Melinda S. Peng
, and
James A. Ridout

Abstract

Navy Operational Global Atmospheric Prediction System (NOGAPS) analysis and operational forecasts are evaluated against the Interim ECMWF Re-Analysis (ERA-Interim; ERAI) and satellite data, and compared with the Global Forecast System (GFS) analysis and forecasts, using both performance- and physics-based metrics. The NOGAPS analysis captures realistic Madden–Julian oscillation (MJO) signals in the dynamic fields and the low-level premoistening leading to active convection, but the MJO signals in the relative humidity (RH) and diabatic heating rate (Q1) fields are weaker than those in the ERAI or the GFS analysis. The NOGAPS forecasts, similar to the GFS forecasts, have relatively low prediction skill for the MJO when the MJO initiates over the Indian Ocean and when active convection is over the Maritime Continent. The NOGAPS short-term precipitation forecasts are broadly consistent with the Climate Prediction Center (CPC) morphing technique (CMORPH) precipitation results with regionally quantitative differences. Further evaluation of the precipitation and column water vapor (CWV) indicates that heavy precipitation develops too early in the NOGAPS forecasts in terms of the CWV, and the NOGAPS forecasts show a dry bias in the CWV increasing with forecast lead time. The NOGAPS underpredicts light and moderate-to-heavy precipitation but overpredicts extremely heavy rainfall. The vertical profiles of RH and Q1 reveal a dry bias within the marine boundary layer and a moist bias above. The shallow heating mode is found to be missing for CWV < 50 mm in the NOGAPS forecasts. The diabatic heating biases are associated with weaker trade winds, weaker Hadley and Walker circulations over the Pacific, and weaker cross-equatorial flow over the Indian Ocean in the NOGAPS forecasts.

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Eric A. Hendricks
,
Wayne H. Schubert
,
Yu-Han Chen
,
Hung-Chi Kuo
, and
Melinda S. Peng

Abstract

A forced shallow-water model is used to understand the role of diabatic and frictional effects in the generation, maintenance, and breakdown of the hurricane eyewall potential vorticity (PV) ring. Diabatic heating is parameterized as an annular mass sink of variable width and magnitude, and the nonlinear evolution of tropical storm–like vortices is examined under this forcing. Diabatic heating produces a strengthening and thinning PV ring in time due to the combined effects of the mass sink and radial PV advection by the induced divergent circulation. If the forcing makes the ring thin enough, then it can become dynamically unstable and break down into polygonal asymmetries or mesovortices. The onset of barotropic instability is marked by simultaneous drops in both the maximum instantaneous velocity and minimum pressure, consistent with unforced studies. However, in a sensitivity test where the heating is proportional to the relative vorticity, universal intensification occurs during barotropic instability, consistent with a recent observational study. Friction is shown to help stabilize the PV ring by reducing the eyewall PV and the unstable-mode barotropic growth rate. The radial location and structure of the heating is shown to be of critical importance for intensity variability. While it is well known that it is critical to heat in the inertially stable region inside the radius of maximum winds to spin up the hurricane vortex, these results demonstrate the additional importance of having the net heating as close as possible to the center of the storm, partially explaining why tropical cyclones with very small eyes can rapidly intensify to high peak intensities.

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Melinda S. Peng
,
John H. Powell
,
R. T. Williams
, and
Bao-Fong Jeng

Abstract

A hydrostatic, primitive equation model with frontogenetical deformation forcing is used to study the effects of surface friction on fronts passing over a two-dimensional ridge. Surface friction is parameterized using a K-theory planetary boundary layer (BL) parameterization with implicitly defined diffusion coefficients, following Keyser and Anthes. Previous studies without surface friction, such as Williams et al., show that a cold front weakens on the upwind slope and intensifies on the lee slope. This is in part due to a superposition effect of mountain flow where colder temperatures exist over the crest and in part due to the divergence pattern caused by the basic flow over the mountain (divergence on the upwind slope and convergence on the lee slope). In Williams et al., the final intensity of a front after passing a symmetric mountain is the same as a front moving over flat land. For no-mountain simulations, the inclusion of the BL results in a more realistic frontal structure and the frontal intensity is weaker than for the frictionless front because weaker temperature gradients are created through vertical mixing. The same type of mixing acts to strengthen a cold front on the upwind slope and weaken it on the downwind slope. The divergence forcing is also frontogenetic on the upwind slope and frontolyic on the lee slope within the BL. The vertical mixing forcing is strongest near the top of BL and weaker within the BL due to weak temperature gradient within the BL. The divergent forcing is strongest within the BL and weak at the top. When BL effects are included, the final intensity of a front passing over a mountain is weaker than the front over flat topography.

The translation of the front is slightly slower with the BL because of the overall reduced cross-frontal speed by surface friction. When moving over a mountain, a front with the BL has a more uniform speed than the frictionless front due to a more uniform flow within the BL.

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Jan-Huey Chen
,
Melinda S. Peng
,
Carolyn A. Reynolds
, and
Chun-Chieh Wu

Abstract

In this study, the leading singular vectors (SVs), which are the fastest-growing perturbations (in a linear sense) to a given forecast, are used to examine and classify the dynamic relationship between tropical cyclones (TCs) and synoptic-scale environmental features that influence their evolution. Based on the 72 two-day forecasts of the 18 western North Pacific TCs in 2006, the SVs are constructed to optimize perturbation energy within a 20° × 20° latitude–longitude box centered on the 48-h forecast position of the TCs using the Navy Operational Global Atmospheric Prediction System (NOGAPS) forecast and adjoint systems. Composite techniques are employed to explore these relationships and highlight how the dominant synoptic-scale features that impact TC forecasts evolve on seasonal time scales.

The NOGAPS initial SVs show several different patterns that highlight the relationship between the TC forecast sensitivity and the environment during the western North Pacific typhoon season in 2006. In addition to the relation of the SV maximum to the inward flow region of the TC, there are three patterns identified where the local SV maxima collocate with low-radial-wind-speed regions. These regions are likely caused by the confluence of the flow associated with the TC itself and the flow from other synoptic systems, such as the subtropical high and the midlatitude jet. This is the new finding beyond the previous NOGAPS SV results on TCs. The subseasonal variations of these patterns corresponding to the dynamic characteristics are discussed. The SV total energy vertical structures for the different composites are used to demonstrate the contributions from kinetic and potential energy components of different vertical levels at initial and final times.

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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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