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Andy Zung-Ching Goh
and
Johnny C. L. Chan

Abstract

This study describes an improved statistical scheme for predicting the annual number of tropical cyclones (TCs) making landfall along the coast of south China using data from 1965 to 2005. Based on the factors affecting TC behavior inside the South China Sea (SCS), those responsible for TCs making landfall are identified. Equations are then developed using the coefficients of empirical orthogonal functions of these factors to predict, in April, the number of these TCs in the early (May–August) and late (September–December) seasons, and in June, the number in the period between July to December. The new scheme achieves a forecast skill of 51% over climatology, or an improvement of about 11% compared to previous studies, when predicting landfalling TC for the whole season, and it seems to be able to capture the decrease in their number in the recent years. Analyses of the flow patterns suggest that the conditions inside the SCS are apparently the major factor affecting the number of landfalling TCs. In years in which this number is above normal, conditions inside the SCS are favorable for TC genesis, and vice versa. The strength of the 500-hPa subtropical high also seems to be a factor in determining whether TCs from the western North Pacific (WNP) could enter the SCS and make landfall.

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Kelvin T. F. Chan
and
Johnny C. L. Chan

Abstract

A comprehensive statistical climatology of the size and strength of the tropical cyclones (TCs) occurring over the western North Pacific (WNP; including the South China Sea) and the North Atlantic (NA; including the Gulf of Mexico and the Caribbean Sea) between 1999 and 2009 is constructed based on Quick Scatterometer (QuikSCAT) data. The size and strength of a TC are defined, respectively, as the azimuthally averaged radius of 17 m s−1 of ocean-surface winds (R17) and the azimuthally averaged tangential wind within 1°–2.5°-latitude radius from the TC center (outer-core wind strength, OCS).

The mean TC size and strength are found to be 2.13° latitude and 19.6 m s−1, respectively, in the WNP, and 1.83° latitude and 18.7 m s−1 in the NA. While the correlation between size and strength is strong (r ≈ 0.9), that between intensity and either size or strength is weak.

Seasonally, midsummer (July) and late-season (October) TCs are significantly larger in the WNP, while the mean size is largest in September in the NA. The percentage frequency of TCs having large size or high strength is also found to vary spatially and seasonally. In addition, the interannual variation of TC size and strength in the WNP correlate significantly with the TC lifetimes and the effect of El Niño over the WNP. TC lifetime and seasonal subtropical ridge activities are shown to be potential factors that affect TC size and strength.

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Kelvin T. F. Chan
and
Johnny C. L. Chan

Abstract

This paper is the second part of a comprehensive study on tropical cyclone (TC) size. In Part I, the climatology of TC size and strength over the western North Pacific (WNP) and the North Atlantic was established based on the Quick Scatterometer (QuikSCAT) data. In this second part, the mechanisms that are likely responsible for TC size changes are explored through analyses of angular momentum (AM) transports and synoptic flow patterns associated with the TC. Changes in AM transport in the upper and lower troposphere appear to be important factors that affect TC intensity and size, respectively. The change in TC intensity is positively related to the change in the upper-tropospheric AM export, while the change in TC size is positively proportional to the change in the lower-tropospheric AM import. An examination of the synoptic flow patterns associated with WNP TCs suggests that changes in the synoptic flow near the TC are important in determining the change in TC size, with developments of the lower-tropospheric anticyclonic flows (one to the east and one to the west) bordering the TC being favorable for TC growth and a weakening of the subtropical high to the southeast for TC size reduction. A recurving TC tends to grow if the lower-tropospheric westerlies to its west increase. Moreover, a northward TC movement is related to the change in TC size. For example, a higher northward-moving speed is found for a larger TC, which also agrees well with the AM transport concept.

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Patrick A. Harr
,
Russell L. Elsberry
, and
Johnny C. L. Chan

Abstract

Data obtained during two aircraft observing periods (AOP) from the TCM-93 mini field experiment are used to describe the transformation between 5° and 10°N of a large depression in the western North Pacific monsoon trough into a tropical cyclone over a 36-h period. The transformation is defined to occur in three stages. Although a large mesoscale convective system (MCS) was present along the eastern periphery of the monsoon depression during the preorganization stage characterized by observations from the first AOP, the overall convective organization of the broad circulation is weak. The structure of the MCS provided a midlevel subsynoptic contribution to the vorticity of the monsoon depression and contributed to a shift in the center of the monsoon depression circulation between 800 and 600 mb toward the MCS location. However, the presence of unsaturated downdrafts associated with the MCS perturbed the low-level thermodynamic conditions and contributed to the rapid decay of the MCS. Slow intensification of the monsoon depression circulation during the preorganization stage is primarily due to favorable interactions with large-scale mean and eddy circulations at both upper and lower levels. The overall convective signature was observed in hourly satellite imagery to become more organized during a 24-h period between the two AOPs. This organization stage was characterized by the formation of a new MCS near the midlevel circulation of the decaying MCS from the preorganization stage. Satellite imagery indicates that the broad monsoon depression began to organize around the new MCS and the outer convection started to be oriented in large principle bands. During the transformation to a tropical storm during the second AOP, the outer principal bands appear to separate the inner circulation of the monsoon depression from the large-scale monsoon trough environment. Convection rapidly develops along the periphery of the inner circulation that now contains a vigorous central updraft and high values of equivalent potential temperature that extend to the middle troposphere. Although several episodes of MCS generation and decay occurred throughout the development of the monsoon depression, it is hypothesized that the subsynoptic processes in the MCS during the first AOP and the MCSs that formed immediately following the second AOP contributed to the concentration of the monsoon depression center and transformation to a tropical cyclone.

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William T. Curry
,
Russell L. Elsberry
, and
Johnny C-L. Chan

Abstract

A technique is developed and tested for estimating objectively the location of a tropical cyclone from a variety of fixes. The western North Pacific climatology and persistence (WPCLPR) track forecast technique is used to generate a potential track from each fix. A tentative warning position is interpolated from a smooth curve that is fit to the future and past positions. When multiple fixes are available, weighting functions are applied to account for the expected accuracy and the timeliness of each fix. Several empirical factors are determined by sensitivity tests with a dependent sample of eight storms that includes 226 warning positions. An independent sample of 22 storms with 610 warning positions is used to demonstrate that the accuracy of the objective technique is not significantly different from the official Joint Typhoon Warning Center (JTWC) warning positions during 1981–83. The short-term track forecast accuracy with WPCLPR is essentially the same whether the JTWC or the objective warning positions are used. Thus, the objective technique provides an efficient tool for the forecaster to use in establishing the present location of the tropical cyclone.

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Thomas B. Schott
,
Johnny C-L. Chan
, and
Russell L. Elsberry

Abstract

A physical basis is provided for representing the large-scale operationally analyzed wind fields around western North Pacific tropical cyclones by use of empirical orthogonal functions (EOFs). The synoptic differences in the environmental flow are demonstrated for cyclones having different initial directions and translation speeds. It is also shown that the wind-based EOF coefficients may be used to differentiate between future tracks of cyclones that have a similar initial direction of motion. Thus, the small sets of EOF coefficients that are used in statistical regression techniques for track prediction by Peak et al. have physical meaning and are not statistical artifacts.

A new application of the EOF coefficients is to post-process the track predictions from the One-way influence Tropical Cyclone Model (OTCM), which is presently the best objective aid at the Joint Typhoon Warning Center (Guam). A 30% reduction in the forecast error at 72 h is achieved in the dependent sample. Thus, the synoptic influences represented by the EOF coefficients can differentiate situations in which the dynamical model (OTCM) is likely to provide poor guidance.

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Johnny C-L. Chan
,
Brian J. Williams
, and
Russell L. Elsberry

Abstract

A detailed analysis of the performance of the U.S. Navy Nested Tropical Cyclone Model (NTCM) for western North Pacific tropical cyclones is made based on five storm-related factors: latitude, longitude, intensity, 12-h intensity change and size (radius of 15 m s−1 (30 kt) winds). The error measures used to assess the accuracy of the NTCM forecasts include: mean and median forecast errors, the systematic errors in the zonal and meridional directions, and the cross-track and along-track components relative to a climatology-persistence (CLIPER) track. These different measures provide insights into the different characteristics of the NTCM forecasts. Although the mean forecast errors are widely reported, they provide no indication of directionality. The zonal and meridional systematic errors provide additional information, but are difficult to interpret since both eastward and westward moving storms are included. Referencing the cross-track and along-track components to a standard forecast technique (CLIPER) provides directions information on the NTCM forecast errors that will be useful to the forecaster.

The analyses based on the latitude and longitude stratifications suggest that the NTCM predictions are most accurate for low-latitude storms and those in the western region of the western North Pacific. However, the model does not perform very well for storms north of 17° or east of 129°E. The NTCM provides better guidance when the observed intensity is close to that of the bogus vortex inserted in the NTCM. For cyclones with a radius of 15 m s−1 winds ≥ 389 km, the NTCM forecasts do not have good skill relative to those from the CLIPER scheme. Intensity changes in the past 12 h do not appear to affect to affect significantly the performance of the NTCM. Most of the results from these analyses may be attributed to a slow bias in the NTCM. Other potential sources of error include the fixed intensity and size of the bogus vortex and the domain size of the nested grid.

The study is intended as a prototype for evaluating an objective track forecast aid based on storm-related factors. Results from such an evaluation can be used not only by the forecasters but also in future modifications of the forecast aid. The results of this study indicate strongly a need to improve specification of the initial conditions in the NTCM, and especially to introduce a bogus storm that is more representative of the storm.

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

Abstract

The fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) is used to simulate tropical cyclone (TC) wind distribution near landfall. On an f plane at 15°N, the effects of the different surface roughness between the land and sea on the wind asymmetry is examined under a strong constraint of a dry atmosphere and time-invariant axisymmetric mass fields. The winds are found to adjust toward a steady state for prelandfall (50, 100, and 150 km offshore), landfall, and postlandfall (50, 100, and 150 km inland) TC positions.

The TC core is asymmetric even when it lies completely offshore or inland. The surface (10 m) wind asymmetry at the core for pre- (post) landfall position is apparently related to the acceleration (deceleration) of the flow that has just moved over the sea (land) as a response to the sudden change of surface friction. For prelandfall TC positions, the resulted strong surface inflow to the left and front left (relative to the direction pointing from sea to land) also induces a tangential (or total) wind maxima at a smaller radius, about 90° downstream of the maximum inflow, consistent with the absolute angular momentum advection (or work done by pressure). The surface maximum wind is of similar magnitude as the gradient wind. There is also a small region of weak outflow just inside the wind maxima. For postlandfall TC positions, inflow is weakened to the right and rear right associated with the onshore flow. Both onshore and offshore flows affect the surface wind asymmetry of the core in the landfall case. Above the surface and near the top of the planetary boundary layer (PBL), the wind is also asymmetric and a strongly supergradient tangential wind is primarily maintained by vertical advection of the radial wind. Much of the steady-state vertical structure of the asymmetric wind is similar to that forced by the motion-induced frictional asymmetry, as found in previous studies.

The associated asymmetry of surface and PBL convergences has radial dependence. For example, the landfall case has stronger PBL convergence to the left for the 0–50-km core region, due to the radial inflow, but to the right for the 100–500-km outer region, due to the tangential wind convergence along the coastline.

The strong constraint is then removed by considering an experiment that includes moisture, cumulus heating, and the free adjustments of mass fields. The TC is weakening and the sea level pressure has a slightly wavenumber-1 feature with larger gradient wind to the right than to the left, consistent with the drift toward the land. The asymmetric features of the wind are found to be very similar to those in the conceptual experiments.

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

Abstract

The structure and intensity changes of tropical cyclones (TCs) in environmental vertical wind shear (VWS) are investigated in this study using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5). Triply nested domains of 36-, 12-, and 4-km resolution are used with fully explicit moisture physics in the 4-km domain. Idealized environments with easterly shears of 2, 4, 6, 8, and 10 m s−1 between 800 and 200 hPa are applied on an f plane. Under small values of VWS (2 and 4 m s−1), the TC intensities are similar to that of the control (CTRL; i.e., no VWS) after initial adjustments. The TCs under 6 and 8 m s−1 of VWS are not as intense, although they do not weaken during the simulation. On the other hand, the TC in 10 m s−1 of VWS weakened significantly.

Given the same VWS, the TC intensity is also found to be sensitive to TC size. Experiments with TCs with a smaller radius of 15 m s−1 wind reveal that while the TC in 2 m s−1 of VWS remains as intense as the CTRL, the TC in the 4 m s−1 VWS case weakened significantly to a minimal hurricane by the end of the simulation. A VWS of 6 m s−1 is strong enough to cause dissipation of the TC in 72 h. These results indicate that the size of a TC has to be taken into account in determining the intensity change of a TC in VWS.

In the 10 m s−1 VWS case, the average temperature over the lower half of the troposphere within 50 km from the TC surface center is higher than that of the CTRL throughout the simulation. Such a warming, though of a small magnitude, is also observed for a brief period in the upper half of the troposphere before the rapid weakening of the TC and is related to the asymmetry of temperature required for a tilt of the vortex axis. The evolution of the vortex tilt is found to be similar to the dry simulations in previous studies, with the midlevel center (σ = 0.525) located mainly in the southeast quadrant of the surface center. A tendency for the midlevel center to rotate about the surface center is also observed. These results support the idea that the resistance to vertical tilt by the mutual rotation between the low-level and midlevel centers is also valid in the moist simulations.

It is hypothesized that the secondary circulation and the associated diabatic heating reduce the vertical tilt and the weakening. Condensation heating offsets the anomalous cooling effect due to the anomalous rising motion ahead of the vortex tilt. For small VWS, the vertical motion asymmetry is not strong enough to destroy the complete secondary circulation and the eyewall. As a result, a large temperature asymmetry and the associated vortex tilt cannot develop. Furthermore, there is no entrainment of cool/dry air in the upper troposphere. Therefore, TCs under small shears can be as intense as the CTRL.

Large-scale asymmetries in the form of anticyclones found in previous studies are also observed. These asymmetries are apparently related to the change of shears near the TCs. While the shears at outer radii stay roughly constant with time, the shears near the TC centers can have large temporal fluctuations both in magnitude and orientation. This result suggests that the location at which the VWS is estimated in observational studies could be important in determining the relationship between VWS and TC intensity change.

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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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