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Eric K. W. Ng
and
Johnny C. L. Chan

Abstract

In this paper, the application of partial wavelet coherence (PWC) and multiple wavelet coherence (MWC) to geophysics is demonstrated. PWC is a technique similar to partial correlation that helps identify the resulting wavelet coherence (WTC) between two time series after eliminating the influence of their common dependence. MWC, akin to multiple correlation, is, however, useful in seeking the resulting WTC of multiple independent variables on a dependent one. The possible El Niño–Southern Oscillation–related impact of the large-scale atmospheric factors on tropical cyclone activity over the western North Pacific is used as an example. A software package for PWC and MWC has been developed. It also includes modified software that rectified the bias in the wavelet power spectrum and wavelet cross-spectrum. The package is available online (see http://www.cityu.edu.hk/gcacic/wavelet).

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Johnny C. L. Chan
,
William M. Gray
, and
Stanley Q. Kidder

Abstract

An analysis of tropical cyclone forecast track errors shows that the largest errors are typically associated with storm undergoing turning motion. This paper presents results obtained from a composite study of tropical cyclones occurring in the West Indies during 1961–77. Storms which underwent a left or right turn or moved straight for a period of at least 36 h were studied. Wind fields at different levels in the atmosphere around these storms were investigated.

When a storm begins to turn, the environmental flow at 500 mb and the average tropospheric wind (between 200 and 900 mb) around the cyclone at 5–11° latitude radius is cyclonic for a left turning and anticyclonic for a right turning storm. At 24-–36 h before a storm makes a left turn, there exists a large positive vertical wind shear around the cyclone between the upper (200 mb) and lower (900b mb) troposphere in the direction of storm motion, while the opposite occurs with a right-turning storm. Straight-moving cyclones do not show such a shear pattern. Statistical tests show that these results am significant at the 95% level or higher.

Tropospheric mean temperature fields around 13 tropical cyclone turning cases in the Atlantic and Pacific Oceans derived from the Nimbus 6 microwave sounder data during 1975 were also studied. Temperature gradients across these storms indicate (through the the thermal wind relationship) vertical wind shear profiles similar to those obtained from the composite.

These results suggest or verify previous Ideas that 1) by measuring certain parameters around a storm (sense of surrounding wind rotation, vertical wind shear between 200 and 900 mb, or gradient of tropospheric mean temperature) one may he able to make a better 24–36 h forecast of cyclone turning motion; 2) the turning motion of tropical cyclones is controlled by large-scale flow fields surrounding them; and 3) them seems to be a time lag between the changes in the environment and the response of the storm center to such changes.

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Dongliang Wang
,
Xudong Liang
,
Yihong Duan
, and
Johnny C. L. Chan

Abstract

The fifth-generation Pennsylvania State University–National Center for Atmospheric Research nonhydrostatic Mesoscale Model is employed to evaluate the impact of the Geostationary Meteorological Satellite-5 water vapor and infrared atmospheric motion vectors (AMVs), incorporated with the four-dimensional variational (4DVAR) data assimilation technique, on tropical cyclone (TC) track predictions. Twenty-two cases from eight different TCs over the western North Pacific in 2002 have been examined. The 4DVAR assimilation of these satellite-derived wind observations leads to appreciable improvements in the track forecasts, with average reductions in track error of ∼5% at 12 h, 12% at 24 h, 10% at 36 h, and 7% at 48 h. Preliminary results suggest that the improvement depends on the quantity of the AMV data available for assimilation.

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Johnny C. L. Chan
,
Jiu-en Shi
, and
Kin Sik Liu

Abstract

A recent scheme to predict tropical cyclone (TC) activity over the western North Pacific partially failed in 1997 and 1998, during which a warm and a cold event of the El Niño–Southern Oscillation (ENSO) occurred, respectively. This paper presents results of two approaches to improve on such predictions. The first is to include new predictors that are related to ENSO based on some recent research, and the second is to provide an updated prediction by incorporating monthly values of predictors in April and May of the current year.

The results suggest that new predictors related to ENSO can indeed be identified, which include temporal changes in the Southern Oscillation index, strength of the Australian monsoon, and intensity of the subtropical high in the South Pacific. These predictors, together with those selected from the original prediction scheme, are combined to form a modified scheme that in general gives better forecasts of TC activity. The updated scheme that includes April and May predictors further improves the accuracy of the predictions. Real-time predictions from both schemes for the year 2000, which were made in April and June, are found to be largely accurate. Both schemes show better skill compared with the original one.

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James E. Peak
,
William E. Wilson
,
Russell L. Elsberry
, and
Johnny C-L. Chan

Abstract

Tropical wind fields from the U.S. Navy Global Band Analyses (GBA) are studied to depict the synoptic flow surrounding tropical cyclones. The composite fields of the zonal and meridional wind components on a grid centered on the tropical cyclone indicate physically realistic flow patterns.

Scalar empirical orthogonal function (EOF) analysis is used to represent the zonal and meridional GBA wind component fields. The representation of these components in terms of the first 35 out of a total of 527 EOF coefficients accounts for at least 80% of the total variance and eliminates much of the noise from the fields. This truncation requires only 7% of the storage needed for the original gridpoints. The eigenvectors can be interpreted as representing different synoptic flow patterns.

Statistical regression equations are derived to predict the future zonal and meridional translation of the tropical cyclone. The EOF coefficients are used as predictors to represent the synoptic information for the scheme. The track forecast errors are slightly smaller than those from the Joint Typhoon Warning Center. Further reduction in forecast errors results from stratification of the cases. Stratification by prior 12-h motion results in 72-h weighted mean forecast errors of only 481 km for the dependent sample. Stratification by synoptic situation based on the EOFs is also tested. From discriminant analysis, the second zonal eigenvector at 700 mb best relates to the 72-h zonal storm motion while the second meridional eigenvector at 250 mb has the best correlation with the meridional motion. Weighted mean forecast errors for regression equations derived within the synoptic subgroups are 484 km at 72 h. Thus, stratification by synoptic situations in terms of wind-based EOFs is as effective as stratification by past storm motion in improving track forecasts.

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Richard C. Y. Li
,
Wen Zhou
,
Johnny C. L. Chan
, and
P. Huang

Abstract

The present study investigates the modulation by the Madden–Julian oscillation (MJO) and the impact of the El Niño–Southern Oscillation (ENSO) on tropical cyclone (TC) genesis in the western North Pacific (WNP) during the period 1975–2010. Results reveal a stronger modulation of cyclogenesis by the MJO during El Niño years, while the modulations in neutral and La Niña years are comparable to each other.

The asymmetric background modification by ENSO is found to greatly affect the extent of MJO modulation under different ENSO conditions. First, MJO activity is intensified and extends farther eastward during El Niño years, instead of being confined west of 150°E as in neutral and La Niña periods. Thus, the influence of MJO is stronger and more zonally widespread in El Niño years, causing significant differences in cyclogenesis parameters in most parts of the WNP. In El Niño years, cyclogenesis is further enhanced in the active phase due to synchronization of MJO signals with favorable background ENSO conditions. While in the inactive phase, the dominance of the strong MJO signals leads to further suppression in TC formation. This leads to overall enhancement of the MJO–TC relationship during El Niño years. On the other hand, the MJO signals confined to the western region west of 150°E in neutral and La Niña years lead to changes in TC-related parameters mainly in the western region, which contribute to the comparatively weaker TC modulations. It can thus be concluded that the MJO has an asymmetric modulation on cyclogenesis in the WNP under different ENSO conditions.

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Wen Zhou
,
Johnny C. L. Chan
,
Wen Chen
,
Jian Ling
,
Joaquim G. Pinto
, and
Yaping Shao

Abstract

In January 2008, central and southern China experienced persistent low temperatures, freezing rain, and snow. The large-scale conditions associated with the occurrence and development of these snowstorms are examined in order to identify the key synoptic controls leading to this event. Three main factors are identified: 1) the persistent blocking high over Siberia, which remained quasi-stationary around 65°E for 3 weeks, led to advection of dry and cold Siberian air down to central and southern China; 2) a strong persistent southwesterly flow associated with the western Pacific subtropical high led to enhanced moisture advection from the Bay of Bengal into central and southern China; and 3) the deep inversion layer in the lower troposphere associated with the extended snow cover over most of central and southern China. The combination of these three factors is likely responsible for the unusual severity of the event, and hence a long return period.

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Johnny C. L. Chan
,
K. S. Liu
,
Simon E. Ching
, and
Edwin S. T. Lai

Abstract

This study investigates the asymmetric distribution of convection associated with tropical cyclones making landfall along the south China coast using Geostationary Meteorological Satellite 5 (GMS-5) data and Doppler radar data. Four tropical cyclones (TCs; Maggie, Sam, York, and Cam) that made direct landfall over Hong Kong in 1999, are chosen as case studies to examine the changes in the distribution of convection prior to, during, and after landfall.

Based on the radar data, three of the four cases (Maggie, Sam, and York) generally showed enhanced convection to the western side of the TC center starting from ∼6 to 18 h before landfall, especially in the cases of Sam and York. For Cam, a dry continental air intrusion apparently destroyed much of the western and southern portion of its eyewall. From the satellite data, convection was generally more enhanced on the southward side. In other words, enhanced convection occurred to the west of the TC in the mid- to lower troposphere and was then advected to the southward side of the upper troposphere by the cyclonic flow and rising motion. These observations are consistent with previous observational and numerical studies.

The possible influences of storm motion and vertical wind shear on the observed convective asymmetries are also examined for the cases of Sam and York. Both storms showed similar evolution of convective distribution relative to storm motion as they approached the coastline. Such storm-motion-related convective distributions may partly explain the observed convective asymmetries. The environmental vertical wind shear may be another factor contributing to the large left–right convective asymmetry in the case of Sam.

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Johnny C. L. Chan
,
Francis M. F. Ko
, and
Ying Man Lei

Abstract

This paper proposes a consistent conceptual framework to explain tropical cyclone (TC) motion based on the concept of potential vorticity tendency (PVT) and to verify this framework based on analyses of different observational datasets. The framework suggests that a TC is likely to move toward an area of maximum wavenumber-1 (WN1) PVT, which is mainly contributed by the corresponding WN1 components of potential vorticity (PV) advection and diabatic heating (DH). The PV advection process consists of advection of symmetric PV by the asymmetric flow [AASPV, which includes, but is not limited to, the environmental “steering flow” and the beta-induced circulation (the so-called ventilation flow)] and the advection of asymmetric PV by the symmetric flow (SAAPV). The asymmetric PV includes any asymmetry in the TC circulation, the beta gyres and contributions from asymmetric convective heating. The modification of PVT by the DH process depends on the vertical gradient of convective heating and the coupling between horizontal gradient of convective heating and vertical wind shear. In steady (i.e., without much change in direction or speed) TC motion, the PV advection processes are generally dominant while the contribution by DH is usually less significant. However, the latter process becomes important for irregular TC motion. Changes in TC motion are then not only caused by those in steering, but can also be induced by variations in the other processes.

Composites of the Met Office operational analyses associated with TCs that had similar and relatively steady motion are first made to verify the contribution by the advection terms. In all motion categories examined, while the magnitude of the AASPV term is found to be generally dominant, its maximum is not downstream of the TC motion. The SAAPV term also contributes to the overall PV advection. The sum of these two terms gives a maximum at a location that generally aligns with the direction of TC motion.

The contribution of the DH process to PVT, and hence TC motion, is then examined using satellite-derived temperatures from high-resolution geosynchronous satellite images for individual TCs. It is found that DH appears to be important especially for slow-moving TCs. Track oscillations as well as irregular track changes may be explained by changes in the convection pattern that lead to variations in the location of maximum WN1 DH.

The entire PVT concept is further investigated using analyses from the Tropical Cyclone Motion Experiment TCM-90 for individual TCs with different track types. The results are consistent with those from the composites (for straight-moving cases) as well as from the satellite image analyses (for the irregular-moving case). Further, in the recurving case, the locations of the maximum in the advection terms rotate ahead of the turning motion of the TC, which is consistent with previous studies of TC motion based on the concept of absolute vorticity conservation.

An integration of all these observational analyses generally verifies the validity of the proposed conceptual framework, which appears to explain most types of TC motion.

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Dasol Kim
,
Chang-Hoi Ho
,
Doo-Sun R. Park
,
Johnny C. L. Chan
, and
Youngsun Jung

Abstract

In this study, the variation of tropical cyclone (TC) rainfall area over the subtropical oceans is investigated using the Tropical Rainfall Measuring Mission precipitation data collected from 1998 to 2014, with a focus on its relationship with environmental conditions. In the subtropics, higher moving speed and larger vertical wind shear significantly contribute to an increase in TC rainfall area by making horizontal rainfall distribution more asymmetric, while sea surface temperature rarely affects the fluctuation of TC rainfall area. This relationship between TC rainfall area and environmental conditions in the subtropics is almost opposite to that in the tropics. It is suggested that, in the subtropics, unlike the tropics, dynamic environmental conditions are likely more crucial to varying TC rainfall area than thermodynamic environmental ones.

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