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Martin L. M. Wong
and
Johnny C. L. Chan

Abstract

Numerical experiments are performed with the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) to study the effects of surface-moisture flux and friction over land on the movement of tropical cyclones (TCs). On an f plane, the TCs are initially placed 150 km due east of a north–south-oriented coastline in an atmosphere at rest. It is found that a TC could drift toward land when the roughness length is 0.5 m over land, with an average drift speed of ∼1 m s−1. Friction, but not surface-moisture flux over land, is apparently essential for the movement toward land. The friction-induced asymmetry in the large-scale flow is the primary mechanism responsible for causing the TC drift. The mechanism responsible for the development of the large-scale asymmetric flow over the lower to midtroposphere (∼900–600 hPa) appears to be the creation of asymmetric vorticity by the divergence term in the vorticity equation. Horizontal advection then rotates the asymmetric vorticity to give a northeasterly flow in the TC periphery (∼500–1000 km from the TC center). The flow near the TC center has a more northerly component because of the stronger rotation by the tangential wind of the TC at inner radii. However, the TC does not move with the large-scale asymmetric flow. Potential vorticity budget calculations indicate that while the horizontal advection term is basically due to the effect of advection by the large-scale asymmetric flow, the diabatic heating and vertical advection terms have to be considered in determining the vortex landward drift, because of the strong asymmetry in vertical motion. Two mechanisms could induce the asymmetry in vertical motion and cause a deviation of the TC track from the horizontal asymmetric flow. First, the large-scale asymmetric flow in the upper troposphere differs from that in the lower troposphere, both in magnitude and direction, which results in a vertical shear that could force the asymmetry. A vertical tilt of the vortex axis is also found that is consistent with the direction of shear and also the asymmetry in rainfall and vertical motion. Second, asymmetric boundary layer convergence that results from the internal boundary layer could also force an asymmetry in vertical motion.

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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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Kevin K. W. Cheung
and
Johnny C. L. Chan

Abstract

In Part I of this study, the technique of ensemble forecasting is applied to the problem of tropical cyclone motion prediction by perturbing the environmental flow. In this part, the focus is shifted to perturbation of the vortex structure. The same barotropic model as in Part I is used to predict 66 cases from the Tropical Cyclone Motion (TCM-90) experiment. Two series of experiments are performed. The first consists of the Monte Carlo forecast for the vortex (MCFV), lagged-average forecast for the vortex (LAFV), and breeding of growing modes for the vortex (BGMV). These are applied to the original analyses without adding any synthetic vortex, and their techniques of generating perturbations are similar to those in Part I. The second series adopts a technique that simulates uncertainties in estimating the synthetic vortex structure. The effect of adding an initial position error (IPER) is first explored. Then a set of four experiments (BETA) is carried out to study the consequence of perturbing the persistence vector and/or various parameters used to generate the β gyres in a spun-up vortex.

Response of the model forecasts to random noise added in the experiment MCFV is found to be low, and the ensemble mean is thus always close to the control forecast. The situation is similar when the rms size of the noise is slightly varied, or when its characteristic length scale is changed. The skill of the IPER experiment also differs little from the control forecast. The remaining experiments other than the MCFV and IPER show a similar average skill to one another when verified both under the perfect model assumption (PMA) and by the best tracks. In the PMA verification, potential improvement over the control forecast can be obtained by the ensemble mean in the LAFV, BGMV, and some sets of the BETA experiments. However, some failure cases are found in the LAFV and BGMV experiments when the vorticity center cannot be well identified during the model integration.

When compared with the best tracks, a portion of the cases can still outperform the control in the LAFV, BGMV, and BETA experiments. Since different control forecasts are used in different experiments, the forecast errors are scaled to the same benchmark before they are compared. It is found that among the BETA experiments, the configuration with the best performance is to perturb the parameters for generating the β gyres and persistence vector simultaneously. It can outperform LAFV after 48 h and has comparable performance with BGMV. This set of BETA ensemble may therefore be suitable for substituting BGMV when a synthetic vortex is necessary. Another implication is that since the persistence vector represents an improved large-scale flow, potential skill should exist for combining the perturbed β gyres with the environmental perturbations used in Part I.

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Johnny C. L. Chan
and
Roger H. F. Kwok

Abstract

The physical processes responsible for tropical cyclone genesis over the western North Pacific are investigated using the operational analyses of the U.K. Meteorological Office global model for the years 1992–93. The analyses are divided into two groups depending on whether a particular analysis led to the prediction of a real genesis in the atmosphere. Composites of the winds at 850 and 200 hPa from the analyses that fall into each of the two groups (the successful and failed predictions) are then made and compared.

It is found that about 3 days prior to genesis, the low-level trades north of the pregenesis vortex begin to strengthen. One day later, an upper-level westerly trough and/or the Tropical Upper-Tropospheric Trough (TUTT) starts to encroach toward the pregenesis cluster. The low-level trades near the cluster continue to intensify and a surge of southwesterly winds occurs to the south of the cluster. On the day before genesis, the southwesterlies become the dominant low-level feature. At the upper levels, the TUTT has moved to the northeast of the pregenesis vortex at this time. Most of these features are found to occur in a high percentage of the individual cases. Comparisons with the analyses that led to failed predictions also reveal these features to be statistically significant.

Calculations of the angular momentum (AM) fluxes suggest that at the low levels, the enhancement of the trades and the surge in the southwesterlies contribute to the spinup of the vortex primarily through two processes:the symmetric import of planetary AM and the asymmetric import of relative AM, with the former being more dominant. As the genesis time approaches, the contribution to the latter process by the trades becomes smaller relative to that by the southwesterlies. At the upper levels, the role of the TUTT or westerly trough is to reduce the export of planetary AM by the symmetric outflow. However, the contribution by the upper-level flow toward the genesis process appears to be minimal until the day before genesis. Comparisons with the analyses in the failed prediction category using individual cases again suggest that differences between the results in these two categories are statistically significant.

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Kevin K. W. Cheung
and
Johnny C. L. Chan

Abstract

The technique of ensemble forecasting is applied to the problem of tropical cyclone motion prediction. Three methods of generating perturbations for the environmental flow, Monte Carlo forecast (MCF), lagged-average forecast (LAF), and the breeding of growing modes (BGM), are tested with a barotropic model using 66 cases from the Tropical Cyclone Motion (TCM-90) Experiment. For the MCF, the ensemble mean forecast is almost identical to that without any perturbation. The other two methodologies are verified both under the perfect model assumption and using the best tracks. On average, in about half of the cases improvement in forecast can be demonstrated in the former verification. A high degree of correlation (with linear correlation coefficient >0.9) is also found between the spread of the ensemble and the root-mean-square forecast error. In the best-track verification, improvement in forecasts can also be obtained in 36% (42%) of all the cases using the LAF (BGM) technique. The spread-skill correlation is still significant (correlation coefficients vary from ∼0.4 to 0.7 for different forecast times). An examination of the synoptic flow associated with cases in which the forecast is improved suggests some favorable conditions for the application of ensemble forecasting. These include a tropical cyclone (TC) making a transition from one synoptic region to another, an apparent break in the subtropical ridge (STR), a rapid strengthening/weakening of the STR, recurvature of a TC, and multiple-TC situations.

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Yimin Liu
,
Johnny C. L. Chan
,
Jiangyu Mao
, and
Guoxiong Wu

Abstract

Assimilated analysis fields from the South China Sea Monsoon Experiment and the outgoing longwave radiation data from the National Center for Atmospheric Research (NCAR) have been employed to describe the large-scale and synoptic features of the subtropical circulation during the Bay of Bengal (BOB; 6°–20°N, 80°–100°E) and South China Sea (SCS; 7°–20°N, 110°–120°E) monsoon onsets in 1998. The results show that the Asian monsoon onset during May 1998 exhibited a typical eastward development from the BOB region to the SCS domain. The weakening and retreat of the subtropical anticyclone from the SCS were preceded by the intrusion of westerlies and the development of convective activities over the northern part of the SCS (NSCS; 15°–20°N, 110°–120°E). As the vertical shear of zonal wind changes in sign, the ridge surface of the subtropical anticyclone tilted northward and the summer pattern was established over the SCS. Based on these observational results, version 4 of the NCAR climate model (CCM3) is used to investigate the physical link between the convection associated with the BOB monsoon vortex and the SCS summer monsoon onset, as well as the mechanism of the evolution of the low-level subtropical anticyclone over the SCS.

Introduction of heating over the BOB results in vigorous convection over the BOB, and the BOB monsoon onset, as well as the development of westerlies and vertical ascent over the NSCS region due to an asymmetric Rossby wave response. Together with the low-level moisture advection, convection is induced over the NSCS. It is the condensation heating over the NSCS that causes the overturning of the meridional gradient of temperature over the SCS. Consequently the subtropical anticyclone in the lower troposphere over the SCS weakened gradually. Eventually as convection develops over the entire SCS domain, the subtropical anticyclone moves out of the region.

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Joey H. Y. Kwok
and
Johnny C. L. Chan

Abstract

The influence of a uniform flow on the structural changes of a tropical cyclone (TC) is investigated using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5). Idealized experiments are performed on either an f plane or a β plane. A strong uniform flow on an f plane results in a weaker vortex due to the development of a vertical wind shear induced by the asymmetric vertical motion and a rotation of upper-level anticyclone. The asymmetric vertical motion also reduces the secondary circulation of the vortex.

On a β plane with no flow, a broad anticyclonic flow is found to the southeast of the vortex, which expands with time. Similar to the f-plane case, asymmetric vertical motion and vertical wind shear are also found. This beta-induced shear weakens the no-flow case significantly relative to that on an f plane. When a uniform flow is imposed on a β plane, an easterly flow produces a stronger asymmetry whereas a westerly flow reduces it. In addition, an easterly uniform flow tends to strengthen the beta-induced shear whereas a westerly flow appears to reduce it by altering the magnitude and direction of the shear vector. As a result, a westerly flow enhances TC development while an easterly flow reduces it.

The vortex tilt and midlevel warming found in this study agree with the previous investigations of vertical wind shear. A strong uniform flow with a constant f results in a tilted and deformed potential vorticity at the upper levels. For a variable f, such tilting is more pronounced for a vortex in an easterly flow, while a westerly flow reduces the tilt. In addition, the vortex tilt appears to be related to the midlevel warming such that the warm core in the lower troposphere cannot extent upward, which leads to the subsequent weakening of the TC.

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Johnny C. L. Chan
,
Yihong Duan
, and
Lynn K. Shay

Abstract

The interaction between a tropical cyclone (TC) and the underlying ocean is investigated using an atmosphere–ocean coupled model. The atmospheric model is developed from the Pennsylvania State University (Penn State)–National Center for Atmospheric Research (NCAR) mesoscale model version 4 MM4 and the ocean model consists of a mixed layer and an inactive stagnant layer beneath. Coupling between the atmosphere and the ocean models is achieved through wind stress and surface heat and moisture fluxes that depend on the sea surface temperature (SST). In the absence of a background flow, the atmospheric component consists of only a predefined vortex with an initial central pressure and the radius of the 15 m s−1 wind. The basic control experiments demonstrate that the coupled model can simulate the development of a TC and its interaction with the ocean.

Changes in TC intensity are sensitive to those of SST and the response is almost instantaneous. An SST of ∼27°C is found to be the threshold for TC development. In addition, the initial depth of the ocean mixed layer has an appreciable effect on TC intensity, which also depends on the movement of the TC. Furthermore, the vertical structure of ocean (vertical temperature gradient in the stagnant layer and temperature differential between the two layers) plays a significant role in modulating TC intensity.

In the presence of a warm core eddy (WCE), a TC intensifies before its center reaches the edge of the WCE. Although the TC attains maximum intensity at the center of the WCE, it does not weaken to its original intensity after leaving the WCE. During the entire passage of the TC, the SST at the center of the WCE decreases by about only 1°C, and the WCE generally maintains its original characteristics. However, two cold pools are observed around its periphery. A similar intensification process occurs when a TC moves over a sharp SST gradient and a locally deep ocean mixed layer. These results are explained by the interaction between the ocean and the TC circulation as well as the change in the total surface heat flux.

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Wei Zhang
,
Yee Leung
, and
Johnny C. L. Chan

Abstract

This paper is the first of a two-part series of papers that employs the data-mining approach to analyze tropical cyclone (TC) movement in the western North Pacific Ocean. Part I unravels conditions under which TCs tend to recurve, and Part II uncovers conditions leading to TCs making landfall. Here in Part I, a detailed study is carried out into TC recurvature over the South China Sea and western North Pacific. The investigation focuses on the unraveling of rules governing TC recurvature hidden in TC data. The historical TC track database comprises recurving TCs and straight movers. Potential parameters affecting TC recurvature are categorized into three groups: large-scale circulation, circulations surrounding TCs, and variables characterizing TCs. The tree construction algorithm, C4.5, is applied to classify recurving and straight-moving TCs. Parameters measuring large-scale circulation patterns and characterizing TCs play significant roles in building the classification tree. Altogether, 18 rules are discovered from the processed database. Most of the 18 rules can be explained by existing theories and are supported by various empirical findings on TC recurvatures. Rules governing TC recurvature discovered by the present study contain quantitative descriptions of factors such as composite wind fields, geopotential heights, and deep-layer mean winds that are essential to the understanding, interpretation, and prediction of TC recurvatures.

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Wei Zhang
,
Yee Leung
, and
Johnny C. L. Chan

Abstract

This is the second paper of a two-part series of papers on the analysis of tropical cyclone (TC) tracks in the western North Pacific Ocean. In this paper, TC landfalls in the South China Sea and western North Pacific basins are investigated through the data-mining approach. On the basis of historical TC archives, the C4.5 algorithm, a classic tree algorithm for classification, has been employed to quantitatively discover rules governing TC landfall. A classification tree, with 14 leaf nodes, has been built. The path from the root node to each leaf node forms a rule. Fourteen rules governing TC landfall across the Chinese coast have been unraveled with respect to the selected attributes having potential influence on TC landfall. The rules are derived by the attributes and splitting values. From the classification tree, split values, such as 27°N latitude, 130°E longitude, 141°E in the west extension index, and 0.289 in the monsoon index have been shown to be useful for TC forecasting. The rules have been justified from the perspective of meteorology and knowledge of TC movement and recurvature (e.g., deep-layer mean winds and large-scale circulation). The research findings are also consistent with existing results concerning TC movement and landfall. Both the unraveled rules and the associated splitting values can provide useful references for the prediction of TC landfall over China.

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