Search Results
You are looking at 1 - 10 of 20 items for :
- Author or Editor: Chih-Pei Chang x
- Article x
- Refine by Access: Content accessible to me x
Abstract
A new method of using satellite photographs in the study of large-scale cloud motions in the tropics is presented. Cloud clusters are seen to be propagating westward with a phase speed ∼9 m sec−1 in the summer of 1967, and can sometimes be followed all the way across the Pacific. It is suggested that this method can be used in the study of tropical wave disturbances.
Abstract
A new method of using satellite photographs in the study of large-scale cloud motions in the tropics is presented. Cloud clusters are seen to be propagating westward with a phase speed ∼9 m sec−1 in the summer of 1967, and can sometimes be followed all the way across the Pacific. It is suggested that this method can be used in the study of tropical wave disturbances.
Abstract
The solutions of the linear equatorial boundary layer are evaluated and found to be dependent on both the depth of the modelled boundary layer and the mode of symmetry about the equator. The inclusion of the frictional effect at levels higher than the usually assumed 1-km boundary layer depth is crucial to the concentration of convergence around the critical latitude. It is therefore suggested that the top of the boundary layer in tropical wave models should be properly selected in order to represent the deep convergence around the critical latitude.
Abstract
The solutions of the linear equatorial boundary layer are evaluated and found to be dependent on both the depth of the modelled boundary layer and the mode of symmetry about the equator. The inclusion of the frictional effect at levels higher than the usually assumed 1-km boundary layer depth is crucial to the concentration of convergence around the critical latitude. It is therefore suggested that the top of the boundary layer in tropical wave models should be properly selected in order to represent the deep convergence around the critical latitude.
Abstract
In this paper we deal with the interpretation of observed oscillations in the tropical troposphere and stratosphere within the framework of the equatorial wave theory. A difficulty with this problem arises when one compares the short vertical wavelength (or equivalent depth) predicted by the classical theory and the observed large vertical scales associated with the low Doppler-shifted frequencies of the tropospheric oscillations. In this analysis it is shown that the inclusion of simple linear damping, justified by budget studies which revealed the important role of cumulus momentum transport, has a strong influence at low frequencies on the forced equatorial waves and results in two types of dispersive relationships. The first type is characteristic of the regular internal gravity waves which have fast phase speeds and weak vertical attenuation. The second type is dominated by the viscous damping time scale and has slow phase speeds and strong vertical trapping. The theory predicts that the stratosphere oscillations may be identified with the first type and the tropospheric oscillations with the second. In the case of Kelvin waves the results can be used to explain consistently both the observed stratospheric Kelvin waves and the planetary-scale Kelvin-like oscillations in the troposphere including the 40–50 day oscillation and the monsoon and Walker circulations. Possible implications with respect to other waves in the tropics are also discussed.
Abstract
In this paper we deal with the interpretation of observed oscillations in the tropical troposphere and stratosphere within the framework of the equatorial wave theory. A difficulty with this problem arises when one compares the short vertical wavelength (or equivalent depth) predicted by the classical theory and the observed large vertical scales associated with the low Doppler-shifted frequencies of the tropospheric oscillations. In this analysis it is shown that the inclusion of simple linear damping, justified by budget studies which revealed the important role of cumulus momentum transport, has a strong influence at low frequencies on the forced equatorial waves and results in two types of dispersive relationships. The first type is characteristic of the regular internal gravity waves which have fast phase speeds and weak vertical attenuation. The second type is dominated by the viscous damping time scale and has slow phase speeds and strong vertical trapping. The theory predicts that the stratosphere oscillations may be identified with the first type and the tropospheric oscillations with the second. In the case of Kelvin waves the results can be used to explain consistently both the observed stratospheric Kelvin waves and the planetary-scale Kelvin-like oscillations in the troposphere including the 40–50 day oscillation and the monsoon and Walker circulations. Possible implications with respect to other waves in the tropics are also discussed.
Abstract
Solutions to the wave-CISK (conditional instability of the second kind with cumulus heating being induced by low-level internal wave convergence) system are obtained to study the vertical structure of orginally unstable waves. A diabatic heating profile is specified that resembles those observed and those theoretically derived from simple pararmeterization schemes. Upper and lower bounds for the vertical wavelength of the unstable waves under normal beating conditions are established through analysis of the frequency (stability) equation. The lower bound excludes the possibility of excitation or maintenance of short vertical wavelengths (relative to the vertical scale of heating) by wave-CISK. The calculated growth rates indicate that this result is basically insensitive to the vertical heating profile. The vertical structure of the most unstable waves is also computed and the possible roles played by CISK in large-scale tropical waves are discussed in light of these results.
Abstract
Solutions to the wave-CISK (conditional instability of the second kind with cumulus heating being induced by low-level internal wave convergence) system are obtained to study the vertical structure of orginally unstable waves. A diabatic heating profile is specified that resembles those observed and those theoretically derived from simple pararmeterization schemes. Upper and lower bounds for the vertical wavelength of the unstable waves under normal beating conditions are established through analysis of the frequency (stability) equation. The lower bound excludes the possibility of excitation or maintenance of short vertical wavelengths (relative to the vertical scale of heating) by wave-CISK. The calculated growth rates indicate that this result is basically insensitive to the vertical heating profile. The vertical structure of the most unstable waves is also computed and the possible roles played by CISK in large-scale tropical waves are discussed in light of these results.
Abstract
The problem of scale-selection of Kelvin waves in the stratosphere by forcing from tropospheric heating is analyzed using a simple linear model. The effect of vertical wind shear is excluded because the phase speed of the waves is fast relative to the range of the mean zonal wind in the vicinity of the tropopause at which level the upward energy flux due to forcing is evaluated. Results of this analysis modify Holton's (1973) theory in that 1) the forcing is most efficient for the longest zonal wavelength even if the heat sources are distributed randomly, and 2) the most favored vertical wavelength of the excited waves is about twice the vertical scale of beating. The calculated vertical wavelengths exceed slightly those observed and the discrepancies are discussed.
Abstract
The problem of scale-selection of Kelvin waves in the stratosphere by forcing from tropospheric heating is analyzed using a simple linear model. The effect of vertical wind shear is excluded because the phase speed of the waves is fast relative to the range of the mean zonal wind in the vicinity of the tropopause at which level the upward energy flux due to forcing is evaluated. Results of this analysis modify Holton's (1973) theory in that 1) the forcing is most efficient for the longest zonal wavelength even if the heat sources are distributed randomly, and 2) the most favored vertical wavelength of the excited waves is about twice the vertical scale of beating. The calculated vertical wavelengths exceed slightly those observed and the discrepancies are discussed.
Abstract
A regional climate model is used to simulate the summer monsoon onset in South and Southeast Asia during the year 2000 to explore the interaction between orographic precipitation and the large-scale monsoon circulation. In the control run, the model uses the U. S. Geological Survey topography data and simulates the observed monsoon onset reasonably well. In the sensitivity tests, mountains are removed within different regions south of the Tibetan Plateau. It is found that the Indochina Peninsula monsoon onset is closely related to the local wind–terrain–precipitation interaction, while the Indian monsoon onset is more controlled by the large-scale land–sea thermal contrast.
The sensitivity tests suggest two opposite effects of high terrain on the monsoon circulation and precipitation. When the terrain height is below the lifted condensation level (LCL), the low-level westerlies and the orographic precipitation weaken with increasing terrain height due to the surface drag effect. When the terrain height is above the LCL, the positive feedback associated with the diabatic forcing of orographic precipitation is dominant, and a large mountain height leads to heavier orographic precipitation and stronger low-level westerlies. The sensitivity tests also show that the impact of orographic precipitation in the Indochina Peninsula extends up to 30° longitude upstream and affects monsoon precipitation along the western coast of India.
Abstract
A regional climate model is used to simulate the summer monsoon onset in South and Southeast Asia during the year 2000 to explore the interaction between orographic precipitation and the large-scale monsoon circulation. In the control run, the model uses the U. S. Geological Survey topography data and simulates the observed monsoon onset reasonably well. In the sensitivity tests, mountains are removed within different regions south of the Tibetan Plateau. It is found that the Indochina Peninsula monsoon onset is closely related to the local wind–terrain–precipitation interaction, while the Indian monsoon onset is more controlled by the large-scale land–sea thermal contrast.
The sensitivity tests suggest two opposite effects of high terrain on the monsoon circulation and precipitation. When the terrain height is below the lifted condensation level (LCL), the low-level westerlies and the orographic precipitation weaken with increasing terrain height due to the surface drag effect. When the terrain height is above the LCL, the positive feedback associated with the diabatic forcing of orographic precipitation is dominant, and a large mountain height leads to heavier orographic precipitation and stronger low-level westerlies. The sensitivity tests also show that the impact of orographic precipitation in the Indochina Peninsula extends up to 30° longitude upstream and affects monsoon precipitation along the western coast of India.
Abstract
Current skill in the seasonal prediction of the Asian monsoon falls rapidly north of 40°N, where the Siberian high (SH) is a prominent manifestation of the East Asian winter monsoon (EAWM). Variations in the SH are closely related to winter weather over a large latitudinal span from northern Asia to the equator. Here it is shown that during the three recent decades the SH had an intraseasonal variation that tended to be seasonally synchronized, which produced an out-of-phase relationship between November and December/January. This implies a special intraseasonal predictability that did not exist in the two previous decades. If this relationship continues, the EAWM will be the only known major circulation system whose intensity can be predicted to reverse from the previous month. It is hypothesized that this predictability is related to the reduced frequency of blocking events during the positive phase of the Arctic Oscillation (AO). While this suggests the predictability may diminish if the AO phase is reversed, it may become more prevalent in the future if the prediction of more frequent positive AO-like patterns in a warming world forced by greenhouse gases is borne out.
Abstract
Current skill in the seasonal prediction of the Asian monsoon falls rapidly north of 40°N, where the Siberian high (SH) is a prominent manifestation of the East Asian winter monsoon (EAWM). Variations in the SH are closely related to winter weather over a large latitudinal span from northern Asia to the equator. Here it is shown that during the three recent decades the SH had an intraseasonal variation that tended to be seasonally synchronized, which produced an out-of-phase relationship between November and December/January. This implies a special intraseasonal predictability that did not exist in the two previous decades. If this relationship continues, the EAWM will be the only known major circulation system whose intensity can be predicted to reverse from the previous month. It is hypothesized that this predictability is related to the reduced frequency of blocking events during the positive phase of the Arctic Oscillation (AO). While this suggests the predictability may diminish if the AO phase is reversed, it may become more prevalent in the future if the prediction of more frequent positive AO-like patterns in a warming world forced by greenhouse gases is borne out.
Abstract
The highest frequency of late-winter cold-air outbreaks in East and Southeast Asia over 50 years was recorded in 2005, when three strong successive cold surges occurred in the South China Sea within a span of 30 days from mid-February to mid-March. These events also coincided with the first break of 18 consecutive warm winters over China. The strong pulsation of the surface Siberian Mongolia high (SMH) that triggered these events was found to result from the confluence of several events. To the east, a strong Pacific blocking with three pulses of westward extension intensified the stationary East Asian major trough to create a favorable condition for cold-air outbreaks. To the west, the dominance of the Atlantic blocking and an anomalous deepened trough in the Scandinavian/Barents Sea region provided the source of a succession of Rossby wave activity fluxes for the downstream development. An upper-level central Asian anticyclone that is often associated with a stronger SMH was anomalously strong and provided additional forcing. In terms of the persistence and strength, this central Asian anticyclone was correlated with the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) only when SMH is weak (warm winters). During strong SMH seasons (cold winters) the correlation vanishes. However, during late winter 2005 the central Asian anticyclone was strengthened by the Atlantic blocking through both the downstream wave activities and a circulation change that affected the Atlantic and west Asian jets. As a result, the period from mid-February to mid-March of 2005 stands out as a record-breaking period in the Asian winter monsoon.
Abstract
The highest frequency of late-winter cold-air outbreaks in East and Southeast Asia over 50 years was recorded in 2005, when three strong successive cold surges occurred in the South China Sea within a span of 30 days from mid-February to mid-March. These events also coincided with the first break of 18 consecutive warm winters over China. The strong pulsation of the surface Siberian Mongolia high (SMH) that triggered these events was found to result from the confluence of several events. To the east, a strong Pacific blocking with three pulses of westward extension intensified the stationary East Asian major trough to create a favorable condition for cold-air outbreaks. To the west, the dominance of the Atlantic blocking and an anomalous deepened trough in the Scandinavian/Barents Sea region provided the source of a succession of Rossby wave activity fluxes for the downstream development. An upper-level central Asian anticyclone that is often associated with a stronger SMH was anomalously strong and provided additional forcing. In terms of the persistence and strength, this central Asian anticyclone was correlated with the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) only when SMH is weak (warm winters). During strong SMH seasons (cold winters) the correlation vanishes. However, during late winter 2005 the central Asian anticyclone was strengthened by the Atlantic blocking through both the downstream wave activities and a circulation change that affected the Atlantic and west Asian jets. As a result, the period from mid-February to mid-March of 2005 stands out as a record-breaking period in the Asian winter monsoon.
Abstract
One of the most persistent rain-making events over East Asia is the development of an early summer monsoon trough (Mei-Yu) which extends from southeastern China to southern Japan. This work studies the structure and vorticity budget of a Mei-Yu system for the period 10–15 June 1975.
Subjectively analyzed grid-point data are time composited with respect to the trough axis along three cross sections over southeastern China (western section), southern East China Sea (central section) and southern Japan (eastern section), respectively, during the mature and decaying stages of the trough. The results indicate that the structure of the eastern and central sections resembles a typical midlatitude baroclinic front with strong vertical tilt toward an upper level cold core and a strong horizontal temperature gradient. On the other band, the western section resembles a semitropical disturbance with an equivalent barotropic, warm core structure, a weak horizontal temperature gradient, and a rather strong horizontal wind shear in the lower troposphere.
Cumulus convection activity south of the 850 mb trough is significant in all three sections and contributes substantially to the thermally direct secondary circulation, but the large-scale organizing mechanism differs from one section to another. In the eastern and central sections it is mainly due to differential vorticity advection while in the western section it is due to Ekman pumping (CISK). The generation of cyclonic vorticity is counteracted by cumulus damping in the eastern section and by boundary layer friction in the mountainous western section.
Abstract
One of the most persistent rain-making events over East Asia is the development of an early summer monsoon trough (Mei-Yu) which extends from southeastern China to southern Japan. This work studies the structure and vorticity budget of a Mei-Yu system for the period 10–15 June 1975.
Subjectively analyzed grid-point data are time composited with respect to the trough axis along three cross sections over southeastern China (western section), southern East China Sea (central section) and southern Japan (eastern section), respectively, during the mature and decaying stages of the trough. The results indicate that the structure of the eastern and central sections resembles a typical midlatitude baroclinic front with strong vertical tilt toward an upper level cold core and a strong horizontal temperature gradient. On the other band, the western section resembles a semitropical disturbance with an equivalent barotropic, warm core structure, a weak horizontal temperature gradient, and a rather strong horizontal wind shear in the lower troposphere.
Cumulus convection activity south of the 850 mb trough is significant in all three sections and contributes substantially to the thermally direct secondary circulation, but the large-scale organizing mechanism differs from one section to another. In the eastern and central sections it is mainly due to differential vorticity advection while in the western section it is due to Ekman pumping (CISK). The generation of cyclonic vorticity is counteracted by cumulus damping in the eastern section and by boundary layer friction in the mountainous western section.
Abstract
Taiwan, which is in the middle of one of the most active of the western North Pacific Ocean’s tropical cyclone (TC) zones, experienced a dramatic increase in typhoon-related rainfall in the beginning of the twenty-first century. This record-breaking increase has led to suggestions that it is the manifestation of the effects of global warming. With rainfall significantly influenced by its steep terrain, Taiwan offers a natural laboratory to study the role that terrain effects may play in the climate change of TC rainfall. Here, it is shown that most of the recently observed large increases in typhoon-related rainfall are the result of slow-moving TCs and the location of their tracks relative to the meso-α-scale terrain. In addition, stronger interaction between the typhoon circulation and southwest monsoon wind surges after the typhoon center moves into the Taiwan Strait may cause a long-term trend of increasing typhoon rainfall intensity, which is not observed before the typhoon center exits Taiwan. The variation in the location of the track cannot be related to the effects of global warming on western North Pacific TC tracks reported in the literature. The weaker steering flow and the stronger monsoon–TC interaction are consistent with the recently discovered multidecadal trend of intensifying subtropical monsoon and tropical circulations, which is contrary to some theoretical and model projections of global warming. There is also no evidence of a positive feedback between global warming–related water vapor supply and TC intensity, as the number of strong landfalling TCs has decreased significantly since 1960 and the recent heavy rainfall typhoons are all of weak-to-medium intensity.
Abstract
Taiwan, which is in the middle of one of the most active of the western North Pacific Ocean’s tropical cyclone (TC) zones, experienced a dramatic increase in typhoon-related rainfall in the beginning of the twenty-first century. This record-breaking increase has led to suggestions that it is the manifestation of the effects of global warming. With rainfall significantly influenced by its steep terrain, Taiwan offers a natural laboratory to study the role that terrain effects may play in the climate change of TC rainfall. Here, it is shown that most of the recently observed large increases in typhoon-related rainfall are the result of slow-moving TCs and the location of their tracks relative to the meso-α-scale terrain. In addition, stronger interaction between the typhoon circulation and southwest monsoon wind surges after the typhoon center moves into the Taiwan Strait may cause a long-term trend of increasing typhoon rainfall intensity, which is not observed before the typhoon center exits Taiwan. The variation in the location of the track cannot be related to the effects of global warming on western North Pacific TC tracks reported in the literature. The weaker steering flow and the stronger monsoon–TC interaction are consistent with the recently discovered multidecadal trend of intensifying subtropical monsoon and tropical circulations, which is contrary to some theoretical and model projections of global warming. There is also no evidence of a positive feedback between global warming–related water vapor supply and TC intensity, as the number of strong landfalling TCs has decreased significantly since 1960 and the recent heavy rainfall typhoons are all of weak-to-medium intensity.
