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- Author or Editor: Johnny C. L. Chan x
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Abstract
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Abstract
The flow patterns at various levels in the atmosphere around northwest Pacific tropical cyclones are studied using objectively-analyzed wind fields produced by the United States Navy. The results show large differences in the flow fields among groups of cyclones moving in different directions. Appreciable baroclinity is found in cyclones moving northward or northeastward. The results also demonstrate the impotence of stratifying cyclones by their characteristics and synoptic environment in the study and prediction of tropical cyclone motion
The relationships between tropical cyclone motion and the midtropospheric flow averaged around the 5–7° latitude radial band are also investigated using both the composite and individual cases. The composite results are generally consistent with those obtained from individual cases. In most cases, these relationships also agree with those derived in previous studies from rawinsonde composites and objectively-analyzed height fields. Since the objectively-analyzed wind fields used in this study are available for individual cases, the results suggest possible application of these fields to additional research studies of tropical cyclone motion as well as to development of short-term prediction techniques.
Abstract
The flow patterns at various levels in the atmosphere around northwest Pacific tropical cyclones are studied using objectively-analyzed wind fields produced by the United States Navy. The results show large differences in the flow fields among groups of cyclones moving in different directions. Appreciable baroclinity is found in cyclones moving northward or northeastward. The results also demonstrate the impotence of stratifying cyclones by their characteristics and synoptic environment in the study and prediction of tropical cyclone motion
The relationships between tropical cyclone motion and the midtropospheric flow averaged around the 5–7° latitude radial band are also investigated using both the composite and individual cases. The composite results are generally consistent with those obtained from individual cases. In most cases, these relationships also agree with those derived in previous studies from rawinsonde composites and objectively-analyzed height fields. Since the objectively-analyzed wind fields used in this study are available for individual cases, the results suggest possible application of these fields to additional research studies of tropical cyclone motion as well as to development of short-term prediction techniques.
Abstract
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Abstract
The interannual variations in tropical cyclone activity in the northwest Pacific (NWPAC) and their relationships with the El Niño/Southern Oscillation (ENSO) phenomenon were studied using the method of spectral analyses. Time series of a Southern Oscillation Index (SOI, defined as the sea-level pressure difference between Easter Island and Darwin) and tropical cyclone activity in the entire (NWPAC) ocean basin as well as in different regions of the NWPAC were analyzed. Two spectral peaks are apparent in all these time series. One corresponds to the generally accepted Southern Oscillation with a period of ∼3 to 3.5 years and another at the quasi-biennial oscillation (QBO) frequency. Cross-spectral analyses between the SOI and tropical cyclone activity show significant coherence in these two spectral peaks. The dominant peak is at the Southern Oscillation frequency with the SOI leading typhoon activity by almost a year. At the QBO frequency, the two series are almost in phase. Cyclone activity in the eastern part of NWPAC, however, is ∼180° out of phase with the SOI series at the Southern Oscillation frequency.
It appears that fluctuations of cyclone activity at the dominant Southern Oscillation frequency may be explained in terms of the change in the horizontal and vertical circulations in the atmosphere during periods of low SOI. The establishment of an anomalous Walker Circulation shifts areas of enhanced or suppressed convection, leading to the observed variations in cyclone activity.
Abstract
The interannual variations in tropical cyclone activity in the northwest Pacific (NWPAC) and their relationships with the El Niño/Southern Oscillation (ENSO) phenomenon were studied using the method of spectral analyses. Time series of a Southern Oscillation Index (SOI, defined as the sea-level pressure difference between Easter Island and Darwin) and tropical cyclone activity in the entire (NWPAC) ocean basin as well as in different regions of the NWPAC were analyzed. Two spectral peaks are apparent in all these time series. One corresponds to the generally accepted Southern Oscillation with a period of ∼3 to 3.5 years and another at the quasi-biennial oscillation (QBO) frequency. Cross-spectral analyses between the SOI and tropical cyclone activity show significant coherence in these two spectral peaks. The dominant peak is at the Southern Oscillation frequency with the SOI leading typhoon activity by almost a year. At the QBO frequency, the two series are almost in phase. Cyclone activity in the eastern part of NWPAC, however, is ∼180° out of phase with the SOI series at the Southern Oscillation frequency.
It appears that fluctuations of cyclone activity at the dominant Southern Oscillation frequency may be explained in terms of the change in the horizontal and vertical circulations in the atmosphere during periods of low SOI. The establishment of an anomalous Walker Circulation shifts areas of enhanced or suppressed convection, leading to the observed variations in cyclone activity.
Abstract
The upper-level features associated with the two kinds of winter monsoon surges over south China are studied: the easterly surge (ES) and the northerly surge (NS). The study is similar to that used by Wu and Chan, except that a broader region (0°–60°N, 70°–160°E) is considered.
The outbreak of an NS is associated with a breakdown of the Siberia–Mongolia high. The upper-level features suggest that the evolution of the Siberia–Mongolia high in an NS can be related to an eastward passage of a short-wave trough and the polar jet based on the quasigeostrophic theory. The intensification of the Siberia–Mongolia high appears likely to be caused mainly by the adiabatic cooling due to rising motion associated with the trough and the polar jet passages. After the passage of the trough and the jet, the Siberia–Mongolia high breaks down in response to the sinking motion upstream of the trough, causing a surge of the northerly winds over the south China coast.
For the ES, the passage of an upper ridge is observed. The zonal index increases in an ES but the subtropical jet weakens. The high pressure center responsible for the ES (the Dahingganling high) is found to be a split cell from the Siberia–Mongolia high, and the splitting is related to the ridge passage. A temperature inversion is only observed when the high is not far from the parent high (i.e., Siberia–Mongolia high). Unlike the NS, an ES is found not to be associated with a strong north–south thermal contrast. Significant differences are observed when comparing the features of the ES and NS. On the whole, the results from this study and those from Wu and Chan suggest that on the synoptic scale a clear distinction exists between the ES and NS on the synoptic scale both at upper levels and the surface.
Abstract
The upper-level features associated with the two kinds of winter monsoon surges over south China are studied: the easterly surge (ES) and the northerly surge (NS). The study is similar to that used by Wu and Chan, except that a broader region (0°–60°N, 70°–160°E) is considered.
The outbreak of an NS is associated with a breakdown of the Siberia–Mongolia high. The upper-level features suggest that the evolution of the Siberia–Mongolia high in an NS can be related to an eastward passage of a short-wave trough and the polar jet based on the quasigeostrophic theory. The intensification of the Siberia–Mongolia high appears likely to be caused mainly by the adiabatic cooling due to rising motion associated with the trough and the polar jet passages. After the passage of the trough and the jet, the Siberia–Mongolia high breaks down in response to the sinking motion upstream of the trough, causing a surge of the northerly winds over the south China coast.
For the ES, the passage of an upper ridge is observed. The zonal index increases in an ES but the subtropical jet weakens. The high pressure center responsible for the ES (the Dahingganling high) is found to be a split cell from the Siberia–Mongolia high, and the splitting is related to the ridge passage. A temperature inversion is only observed when the high is not far from the parent high (i.e., Siberia–Mongolia high). Unlike the NS, an ES is found not to be associated with a strong north–south thermal contrast. Significant differences are observed when comparing the features of the ES and NS. On the whole, the results from this study and those from Wu and Chan suggest that on the synoptic scale a clear distinction exists between the ES and NS on the synoptic scale both at upper levels and the surface.
Abstract
The surface features associated with two kinds of winter monsoon surges over south China are studied: the easterly surge (ES) and the northerly surge (NS). Surface meteorological parameters over the region 15°–50°N, 90°–130°E for the surges that occurred in the three winters (October–March) from 1988 to 1991 are analyzed. For the northerly surge, the surface features found are 1) an abrupt temperature drop and wind direction turning from easterly to northerly, which can be related to the passage of a cold front; 2) an increase in the dewpoint depression; and 3) a large north–south pressure gradient. On the other hand, the easterly surge is found to be associated with strong easterly winds up to approximately 40 km h−1, little temperature or pressure change, and a southeastward motion of a high pressure center from Dahinggangling to the Yellow Sea together with a sharp pressure ridge along the east China coast. Furthermore, an ES and an NS are associated with different perturbations (anomalies) in pressure, wind, temperature, and dewpoint depression when compared with the wintertime normal condition. The results suggest a clear distinction between the two surges on the synoptic scale.
Abstract
The surface features associated with two kinds of winter monsoon surges over south China are studied: the easterly surge (ES) and the northerly surge (NS). Surface meteorological parameters over the region 15°–50°N, 90°–130°E for the surges that occurred in the three winters (October–March) from 1988 to 1991 are analyzed. For the northerly surge, the surface features found are 1) an abrupt temperature drop and wind direction turning from easterly to northerly, which can be related to the passage of a cold front; 2) an increase in the dewpoint depression; and 3) a large north–south pressure gradient. On the other hand, the easterly surge is found to be associated with strong easterly winds up to approximately 40 km h−1, little temperature or pressure change, and a southeastward motion of a high pressure center from Dahinggangling to the Yellow Sea together with a sharp pressure ridge along the east China coast. Furthermore, an ES and an NS are associated with different perturbations (anomalies) in pressure, wind, temperature, and dewpoint depression when compared with the wintertime normal condition. The results suggest a clear distinction between the two surges on the synoptic scale.
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.
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.
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.
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.
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.
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.
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.
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.
