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Y. S. Li
and
Johnny C. L. Chan

Abstract

This study attempts to investigate the linear momentum budget responsible for tropical cyclone (TC) recurvature. Using the operational analyses from the U.K. Meteorological Office global model, the environmental flow associated with recurving TCs over the western North Pacific for the years 1991–95 is composited. In general, the dominant features include an eastward-retreating subtropical ridge (STR) and an approaching westerly trough during the recurvature. While the southeasterly flow associated with the STR changes to southerly in the northeast quadrant of the TC in the mid- to upper troposphere, the southwesterly flow associated with the trough penetrates into the northwest quadrant of the TC, especially in the upper troposphere. These changes of the environmental flow throughout the recurvature alter the linear momentum of the TC from southeasterly to southwesterly.

To understand the dynamical processes responsible for the linear momentum changes, individual momentum tendency terms are calculated. In the zonal direction, throughout the recurvature, the southerly component from the environment is found to play an important role for the increase of the net zonal momentum tendency through the earth momentum advection and the zonal component of the Coriolis force. The contribution of the environmental westerlies through the advection of westerly momentum is very small and is eventually cancelled by the negative advection associated with the southerly flow. The net positive meridional force appears to be the main contributor of the meridional tendency component. The negative meridional momentum advection term during and after recurvature only weakens the net meridional momentum tendency at the later stage.

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K. S. Liu
and
Johnny C. L. Chan

Abstract

The sizes of the tropical cyclones (TCs) occurring over the western North Pacific (WNP) and the North Atlantic between 1991 and 1996 are estimated to establish a database for the study of the climatology of TC size and the physical processes responsible for the size changes of TCs. Wind data from the scatterometer onboard the European Remote-Sensing Satellites 1 and 2 (ERS-1 and ERS-2) form the data source for defining the TC size. The size of a TC is defined as the mean radius at which the relative vorticity decreases to 1 × 10−5 s−1. The mean TC size is found to be 3.7° lat for WNP TCs and 3.0° lat for those in the North Atlantic. Such a difference in size between the two basins is statistically significant at the 95% confidence level. The mean TC size in the WNP is also found to vary seasonally, with a value larger in the late season (October and November) than in midsummer (July and August). These results generally agree with those from previous studies using other measures of size. The size changes (increasing or decreasing) of some TCs are also identified. The high-resolution surface wind data from the ERS satellites are shown to be a valuable tool in the study of TC sizes.

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K. S. Liu
and
Johnny C. L. Chan

Abstract

This study investigates the synoptic flow patterns associated with small and large tropical cyclones (TCs) that occurred over the western North Pacific between 1991 and 1996. The size of a TC is defined as the azimuthally averaged radius from the TC center at which the relative vorticity decreases to 1 × 10−5 s−1. Calculation of the relative vorticity is based on the satellite-derived surface winds of the European Remote Sensing Satellites 1 and 2 (ERS-1 and ERS-2). Operational analyses of the U.K. Met Office are employed to identify the synoptic patterns around the TCs.

Characteristic synoptic patterns at 850 hPa can be identified with TCs of different sizes. The southwesterly surge and late-season patterns are related to large TCs while the dominant ridge and monsoon-gyre patterns are associated with the occurrence of a small TC. A case study of Typhoon Bart demonstrates the time evolution of the synoptic pattern and its relationship with the TC size change. Bart exhibited a distinct transition from the dominant ridge synoptic pattern to the southwesterly surge synoptic pattern and, correspondingly, the size of Bart increased significantly.

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R. T. Williams
and
Johnny C-L. Chan

Abstract

The motion of tropical vortices in east–west mean flows is studied with the barotropic vorticity equation on the beta plane. The vorticity equation is integrated numerically from an initially symmetric vortex embedded in (i) a linear shear flow or (ii) a parabolic jet. The first experiment with flow (i) has β = 0 and it is linearized about the mean flow. The vortex is distorted by the mean flow so that the even Fourier components around the vortex grow, but the vortex does not move. When nonlinear effects are included the distortion is damped in the inner part of the vortex, but wavenumber two grows in the outer region. The addition of the beta effect causes the vortex to move in the same direction as the no mean flow solution provided the mean flow advection is removed from the trajectories. The trajectory for the anticyclonic mean flow is significantly longer than the cyclonic and no mean flow trajectories, which are about equal. For mean flow (ii), with the same absolute vorticity gradient as β but on an f plane, the vortex has a much shorter trajectory and a more westerly direction of movement than the no mean flow solution with beta. This effect comes from the advective distortion of the vortex, which projects onto wavenumber one in the disturbance vorticity equation. It is shown with other experiments that beta has a stronger effect on vortex motion than the relative vorticity gradient.

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Johnny C. L. Chan
and
R. T. Williams

Abstract

The β-effect on tropical cyclone motion is studied using an analytical as well as a numerical model in a nondivergent barotropic framework. The analytical model and the linear version of the numerical model give essentially the same result: the linear β-effect causes a westward stretching of the model vortex but no significant movement of the vortex center. An east-west asymmetry in the meridional wind field is also created. It is the inclusion of the nonlinear term that produces the northwestward movement of the vortex previously found by other investigators (e.g., Kitade, 1981). This northwestward movement increases with both the maximum wind speed and the radius of maximum wind in a constant-shape vortex. A wind maximum is also found to the northeast of the vortex, which appears to be consistent with the observational findings of Shea and Gray. This asymmetry plays an important role in the vortex motion.

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Kin Sik Liu
and
Johnny C. L. Chan

Abstract

Tropical cyclone (TC) activity over the western North Pacific (WNP) exhibits a significant interdecadal variation during 1960–2011, with two distinct active and inactive periods each. This study examines changes in TC activity and atmospheric conditions in the recent inactive period (1998–2011). The overall TC activity shows a significant decrease, which is partly related to the decadal variation of TC genesis frequency in the southeastern part of the WNP and the downward trend of TC genesis frequency in the main development region.

The investigation on the factors responsible for the low TC activity mainly focuses on the effect of vertical wind shear and subtropical high on multidecadal time scales. A vertical wind shear index, defined as the mean magnitude of the difference of the 200- and 850-hPa horizontal zonal winds (10°–17.5°N, 150°E–180°) averaged between June and October, is highly correlated with the annual TC number and shows a significant interdecadal variation. Positive anomalies of vertical wind shear are generally found in the eastern part of the tropical WNP during this inactive period. A subtropical high area index, calculated as the area enclosed by the 5880-gpm line of the June–October 500-hPa geopotential height (0°–40°N, 100°E–180°), shows a significant upward trend. A high correlation is also found between this index and the annual TC number, and a stronger-than-normal subtropical high is generally observed during this inactive period. The strong vertical wind shear and strong subtropical high observed during 1998–2011 together apparently lead to unfavorable atmospheric conditions for TC genesis and hence the low TC activity during the period.

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Johnny C. L. Chan
and
Kin Sik Liu

Abstract

Based on results from climate model simulations, many researchers have suggested that because of global warming, the sea surface temperature (SST) will likely increase, which will then lead to an increase in the intensity of tropical cyclones (TCs). This paper reports results of a study of the relationship between SST and observed typhoon activity (which is used as a proxy for the intensity of TCs averaged over a season) over the western North Pacific (WNP) for the past 40 yr. The average typhoon activity over a season is found to have no significant relationship with SST in the WNP but increases when the SST over the equatorial eastern Pacific Ocean is above normal. The mean annual typhoon activity is generally higher (lower) during an El Niño (La Niña) year. Such interannual variations of typhoon activity appear to be largely constrained by the large-scale atmospheric factors that are closely related to the El Niño–Southern Oscillation (ENSO) phenomenon. These large-scale dynamic and thermodynamic factors include low-level relative vorticity, vertical wind shear, and moist static energy. Such results are shown to be physically consistent with one another and with those from previous studies on the interannual variations of TC activity. The results emphasize the danger of drawing conclusions about future TC intensity based on current climate model simulations that are not designed to make such predictions.

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Kin Sik Liu
and
Johnny C. L. Chan

Abstract

This study examines the interdecadal variability of the tropical cyclone (TC) tracks over the western North Pacific (WNP) during the 1960–2005 period. An empirical orthogonal function analysis of the 10-yr Gaussian-filtered annual frequency of TC occurrence shows three leading modes of TC occurrence patterns. The first mode is related to the variation of TC activity in the areas near Japan and its east. The second mode is characterized by a northeast–southwest dipole of TC occurrence anomalies along the southeast coast of China and an east–west dipole near Japan and its east. The third mode is similar to the second mode, except for the absence of the east–west dipole. These patterns are shown to be related to the decadal changes in the prevailing TC tracks.

Two characteristic flow patterns related to the first and third modes of TC occurrence pattern are identified. The first pattern is characterized by a north–south dipole of 500-hPa geopotential anomalies over the WNP. Such a pattern may affect the strength and westward extension of the subtropical high and the midlevel steering flow and hence the TC occurrence pattern. The Pacific decadal oscillation (PDO) is found to display a similar dipole-like structure. The decadal variability of TC tracks may therefore be partly attributed to the PDO signal. The second characteristic pattern shows a series of anomalous midlevel atmospheric circulations extending from the sea east of Japan to the south coast of China, which may explain the other part of the decadal variations.

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K. S. Liu
and
Johnny C. L. Chan

Abstract

This paper presents the important climatological features of the tropical cyclones making landfall along the South China coast and proposes a statistical scheme for the prediction of the annual number of such tropical cyclones. This number is found to have a large variation, which is mainly due to the occurrence or nonoccurrence of the El Niño–Southern Oscillation (ENSO) phenomenon. A strong El Niño event is found to reduce the number of landfalling tropical cyclones whereas more tropical cyclones tend to make landfall in years associated with La Niña events. Such variations are more prominent in some seasons. The late season (October–November) activity is generally suppressed (enhanced) in El Niño (La Niña) years whereas the chance of a tropical cyclone striking the South China coast increases (decreases) significantly in the early season (May and June) after the mature phase of a La Niña (El Niño) event. These anomalous activities are apparently linked to the ENSO-induced anomalies in the low- and midlevel large-scale circulation.

Based on the ENSO-related indices such as the Niño-3.4 sea surface temperature anomaly and the equatorial Southern Oscillation index, a statistical prediction scheme for the annual number of such landfalling tropical cyclones by 1 April is developed using the projection–pursuit regression technique. This scheme provides a 40% skill improvement in root-mean-square error with respect to climatology. A real-time prediction made in 2001 gave reasonable results.

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Johnny C. L. Chan
and
Wai-Kau Kay

Abstract

A detailed evaluation of the performance of the United Kingdom Meteorological Office Global Model (UKMO) in predicting the movement of 15 tropical cyclones (TCs) that occurred over the western North Pacific during 1987 is presented. The evaluation is based on the methodology used by Chan et al. That is, in addition to the usual mean forecast error, the following error measures are employed: systematic zonal and meridional errors, cross-track, and along-track error components relative to the climatology and persistence (CLIPER) track, and the M score. To identify further the strengths and weaknesses of the model, the forecasts are also evaluated according to four storm-related parameters: latitude, longitude, intensity, and 12-h intensity change.

The analyses for the entire sample show that the skill of the UKMO generally increases with forecast intervals, as in the case of other numerical prediction models. However, the short-term forecasts are worse than those of CLIPER, and a substantial error exists in the initial position of the tropical cyclone in the model. The UKMO also has a tendency to overpredict recurvature for westward-moving TCs, and acceleration for recurved TCs.

From the analyses of the subsamples stratified based on the storm-related parameters, the UKMO is found to have the best performance for TCs north of approximately 20°N and east of approximately 140°E. Tropical cyclones in the lower latitudes or in close proximity to large landmasses are usually poorly predicted by the model. The more intense the cyclone, the better the UKMO forecast. However, the model makes good predictions for TCs that are weakening.

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