ORCID Profile
0000-0003-4553-4172
Current Organisation
National Taiwan University
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Publisher: American Meteorological Society
Date: 11-2010
Abstract: A total of 40 out of 531 tropical cyclones that formed in the western North Pacific during 1986–2005 have accompanied trade wind surges located 5°–15° latitude to the north of the pretropical cyclone disturbance centers. Composite and empirical orthogonal function analyses indicate that the trade wind surges are related to a midlatitude eastward-moving high pressure system often found during the East Asian winter monsoon. Therefore, these trade wind surge tropical cyclones tend to occur in late season (with one-third of them in December), and at lower latitudes (7° latitude lower than the climatological average formation position). The evolution of mesoscale features during formation of trade wind surge tropical cyclones is examined. Various satellite datasets show similar mesoscale patterns during their formations. A few convective lines form by convergence between the trade wind surges and the strengthening cyclonic circulation associated with incipient vortex within the 24 h before formation. Some mesoscale convective systems are embedded in the convective line with lifetimes of about 5 h, and these are illustrated through case studies. Formations usually occur when the trade winds start to decrease in magnitude and a short period after the major episodes of convection in the convective lines and mesoscale convective systems. The relationships between the temporal variability of synoptic-scale trade wind surges, the mesoscale features, and associated tropical cyclone formations are discussed.
Publisher: American Meteorological Society
Date: 12-2012
Abstract: This study focuses on the synoptic and dynamical characteristic of compact and incompact tropical cyclones (TCs) in the western North Pacific. To identify the distinct mechanisms related to the development and maintenance of these two categories of TCs, the Weather Research and Forecasting Model (WRF) is used to simulate the compact Typhoon Yutu (2007) and the incompact Typhoon Manyi (2007). Simulation results of Yutu show that the wind speed increases primarily in the inner-core region, where strong relative vorticity and high inertial stability is also located. Comparatively, the inertial stability for Typhoon Manyi is much weaker, which makes it more susceptible to influences from external low-level forcing. Diagnoses of the numerical simulations as well as examination of the synoptic environments that embed the two TCs suggest that compact TCs mainly develop through internal dynamics, whereas incompact TCs are usually driven by external forcing. Several sets of sensitivity experiments are designed to determine the relative roles of initial vortex structure, environmental flow, and humidity in subsequent TC structural evolution. Results show that in an environment that favors compact TCs, initial vortex largely determines the later structural development. However, vortex development is quite sensitive to its initial intensity and the radius of maximum wind (RMW) under environmental flow that favors incompact TCs. Results of the experiments on environmental humidity show that a humid environment generates large vortex structural changes in incompact TCs. A relatively dry environment brings minimal impacts to the originally compact TC, but can increase the compactness of the originally incompact ones.
Publisher: American Meteorological Society
Date: 02-2011
Abstract: In this study, a tropical cyclone (TC) is considered to be compact if 1) the radius of maximum wind or the maximum tangential wind is smaller than what would be expected for an average tropical cyclone of the same intensity or the same radius of maximum wind, and 2) the decrease of tangential wind outside the radius of maximum wind is greater than that of an average TC. A structure parameter S is defined to provide a quantitative measure of the compactness of tropical cyclones. Quick Scatterometer (QuikSCAT) oceanic winds are used to calculate S for 171 tropical cyclones during 2000–07. The S parameters are then used to classify all of the cases as either compact or incompact according to the 33% and 67% percentiles. It is found that the early intensification stage is favorable for the occurrence of compact tropical cyclones, which also have a higher percentage of rapid intensification than incompact cases. Composite infrared brightness temperature shows that compact tropical cyclones have highly axisymmetric convective structures with strong convection concentrated in a small region near the center. Low-level synoptic patterns are important environmental factors that determine the degree of compactness however, it is believed that compact tropical cyclones maintain their structures mainly through internal dynamics.
Publisher: American Meteorological Society
Date: 06-2008
Abstract: The mesoscale features of 124 tropical cyclone formations in the western North Pacific Ocean during 1999–2004 are investigated through large-scale analyses, satellite infrared brightness temperature (TB), and Quick Scatterometer (QuikSCAT) oceanic wind data. Based on low-level wind flow and surge direction, the formation cases are classified into six synoptic patterns: easterly wave (EW), northeasterly flow (NE), coexistence of northeasterly and southwesterly flow (NE–SW), southwesterly flow (SW), monsoon confluence (MC), and monsoon shear (MS). Then the general convection characteristics and mesoscale convective system (MCS) activities associated with these formation cases are studied under this classification scheme. Convection processes in the EW cases are distinguished from the monsoon-related formations in that the convection is less deep and closer to the formation center. Five characteristic temporal evolutions of the deep convection are identified: (i) single convection event, (ii) two convection events, (iii) three convection events, (iv) gradual decrease in TB, and (v) fluctuating TB, or a slight increase in TB before formation. Although no dominant temporal evolution differentiates cases in the six synoptic patterns, evolutions ii and iii seem to be the common routes taken by the monsoon-related formations. The overall percentage of cases with MCS activity at multiple times is 63%, and in 35% of cases more than one MCS coexisted. Most of the MC and MS cases develop multiple MCSs that lead to several episodes of deep convection. These two patterns have the highest percentage of coexisting MCSs such that potential interaction between these systems may play a role in the formation process. The MCSs in the monsoon-related formations are distributed around the center, except in the NE–SW cases in which clustering of MCSs is found about 100–200 km east of the center during the 12 h before formation. On average only one MCS occurs during an EW formation, whereas the mean value is around two for the other monsoon-related patterns. Both the mean lifetime and time of first appearance of MCS in EW are much shorter than those developed in other synoptic patterns, which indicates that the overall formation evolution in the EW case is faster. Moreover, this MCS is most likely to be found within 100 km east of the center 12 h before formation. The implications of these results to internal mechanisms of tropical cyclone formation are discussed in light of other recent mesoscale studies.
Publisher: American Meteorological Society
Date: 10-2006
DOI: 10.1175/MWR3221.1
Abstract: This study examines the 119 tropical cyclone (TC) formations in the South China Sea (SCS) during 1972–2002, and in particular the 20 in May and June. Eleven of these storms are associated with the weak baroclinic environment of a mei-yu front, while the remaining nine are nonfrontal. Seven of the 11 initial disturbances originated over land and have a highly similar evolution. Comparison of the frontal and nonfrontal formation shows that a nonfrontal formation usually occurs at a lower latitude, is more barotropic, develops faster, and possibly intensifies into a stronger TC. Six nonformation cases in the SCS are also identified that have similar low-level disturbances near the western end of a mei-yu front but did not develop further. In the nonformation cases, both the northeasterlies north of the front and the monsoonal southwesterlies are intermittent and weaker in magnitude so that the vorticity in the northern SCS does not spin up to tropical depression intensity. Because of the influence of a strong subtropical high, convection is suppressed in the SCS. The nonformation cases also have an average of 2–3 m s−1 larger vertical wind shear than the formation cases. A conceptual model is proposed for the typical frontal-type TC formations in the SCS that consists of three essential steps. First, an incipient low-level disturbance that originates over land moves eastward along the stationary mei-yu front. Second, the low-level circulation center with a relative vorticity maximum moves to the open ocean with the stationary front. Last, with strengthened northeasterlies, cyclonic shear vorticity continues to increase in the SCS, and after detaching from the stationary front, the system becomes a tropical depression.
Publisher: Copernicus GmbH
Date: 23-12-2008
DOI: 10.5194/NHESS-8-1463-2008
Abstract: Abstract. Due to the Central Mountain Range with an elevation up to about 4 km, the amount and distribution of rainfall in Taiwan associated with typhoons or tropical cyclones (TCs) depends not only on the distribution of convection within the TCs (internal structure) and influences from monsoon-scale environmental flow, but also on the orographic effect. This study analyzes the spatial and temporal characteristics of rainfall associated with 62 TC cases that affected Taiwan by using observations from the 371 automatic rain stations available in the period 1989–2002. It is found from the climatology maps that highly different rainfall distributions occurred for TCs that approached the Taiwan area from different directions. By performing objective clustering analysis of the rainfall time series of all the rain gauges, several characteristic temporal rainfall profiles are obtained. The geographic distribution of rain gauges that possess a particular temporal profile is also consistent with the possible TC track types that bring maximum rain to the Taiwan area at different times. Based on data in the 1989–2002 period, the development of a TC rainfall climatology-persistence (CLIPER) model is described. CLIPER is an optimized combination of climatology and persistence with different relative weighting for different forecast periods. Independent cases (other than the model development database) during 2003–2004 are used to validate the model. Objective measures like equitable threat score and bias score show that CLIPER's skill is acceptable for practical applications for 24-h rain threshold below 100 mm. However, the underestimation bias for more heavy rainfall is serious and CLIPER seems to have better performance for the northwestern Taiwan than for the other locations. Future directions for improvement of the CLIPER model are discussed.
Publisher: American Meteorological Society
Date: 08-2010
Abstract: A tropical cyclone (TC) size parameter, which is defined here as the radius of 15 m s−1 near-surface wind speed (R15), is calculated for 145 TCs in the western North Pacific during 2000–05 based on QuikSCAT oceanic winds. For the 73 TCs that intensified to typhoon intensity during their lifetimes, the 33% and 67% respective percentiles of R15 at tropical storm intensity and at typhoon intensity are used to categorize small, medium, and large TCs. Whereas many of the small TCs form from an easterly wave synoptic pattern, the monsoon-related formation patterns are favorable for forming medium to large TCs. Most of these 73 TCs stay in the same size category during intensification, which implies specific physical mechanisms for maintaining TC size in the basin. The 18 persistently large TCs from the tropical storm to the typhoon stage mostly have northwestward or north-northwestward tracks, while the 16 persistently small TCs either move westward–northwestward in lower latitudes or develop at higher latitudes with various track types. For the large TCs, strong low-level southwesterly winds exist in the outer core region south of the TC center throughout the intensification period. The small TCs are more influenced by the subtropical high during intensification. The conclusion is that it is the low-level environment that determines the difference between large and small size storms during the early intensification period in the western North Pacific.
No related grants have been discovered for Cheng-Shang Lee.