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- Author or Editor: Ming-Cheng Yen x
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Abstract
Active wild fires over continental landmasses occur in the warm, dry summer. In contrast, in a study by Yen and Chen it was observed that the fire occurrence in Taiwan exhibits an annual variation with a peak in the cool, dry winter. Analysis of the fire-disaster reports released recently by the National Fire Administration of Taiwan shows that fire-related house damage, property loss, and human casualties, overall, exhibit annual variations in concert with the annual variation in the fire-occurrence frequency. The fire-disaster statistics support Yen and Chen’s observation of the annual variation of the fire-occurrence frequency on a subtropical maritime island.
Abstract
Active wild fires over continental landmasses occur in the warm, dry summer. In contrast, in a study by Yen and Chen it was observed that the fire occurrence in Taiwan exhibits an annual variation with a peak in the cool, dry winter. Analysis of the fire-disaster reports released recently by the National Fire Administration of Taiwan shows that fire-related house damage, property loss, and human casualties, overall, exhibit annual variations in concert with the annual variation in the fire-occurrence frequency. The fire-disaster statistics support Yen and Chen’s observation of the annual variation of the fire-occurrence frequency on a subtropical maritime island.
Abstract
Previous studies examining the seasonal contrast and spatial structure of the interdecadal variation in the Southern Hemisphere (SH) atmospheric circulation and the relationship between this interdecadal variation and cyclone activity have been extended using the SH data generated by the Australian Bureau of Meteorology for 1972–92. The major findings of this study are the following:
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In addition to the deepening of the circumpolar trough and the increase of midlatitude sea level pressure (SLP) in winter, a zonal wavenumber 3 pattern appears in the high middle latitudes. During summer, the interdecadal change is dominated by three latitudinal zones of SLP anomalies with negative values in low and high latitudes and positive values in midlatitudes.
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A zonal wavenumber 3 pattern that develops during the winter persists into early spring, while the three latitudinal SLP anomaly zones that emerge in late spring and early summer continue on through the summer.
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An enhancement of cyclone activity is associated with the deepening of the SH circumpolar low centers.
Abstract
Previous studies examining the seasonal contrast and spatial structure of the interdecadal variation in the Southern Hemisphere (SH) atmospheric circulation and the relationship between this interdecadal variation and cyclone activity have been extended using the SH data generated by the Australian Bureau of Meteorology for 1972–92. The major findings of this study are the following:
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In addition to the deepening of the circumpolar trough and the increase of midlatitude sea level pressure (SLP) in winter, a zonal wavenumber 3 pattern appears in the high middle latitudes. During summer, the interdecadal change is dominated by three latitudinal zones of SLP anomalies with negative values in low and high latitudes and positive values in midlatitudes.
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A zonal wavenumber 3 pattern that develops during the winter persists into early spring, while the three latitudinal SLP anomaly zones that emerge in late spring and early summer continue on through the summer.
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An enhancement of cyclone activity is associated with the deepening of the SH circumpolar low centers.
Abstract
Twelve years (1985–96) of monthly house fire reports for 22 districts in Taiwan, a maritime subtropical island of east Asia, were analyzed to characterize its fire nature. The major effort focused on the identification of temporal variation signals and their possible links with meteorological variables. Two significant modes of house fires were identified: annual and diurnal. As revealed from the power spectral analyses of fire time series in every fire district, a pronounced annual cycle peak emerges, with a peak phase in December and a minimum phase in June. In contrast to the warm and dry summer fire season of three continental landmasses (i.e., the U.S. West, the Northwest Territories of Canada, and the large wildland of Australia), an active fire season appears during the cool, dry winter in Taiwan. The fires on this island are highly correlated with several hydrometeorological variables; a decrease (increase) in rainfall in the dry (wet) cool (warm) environment with strong (weak) winds facilitates (hinders) fire occurrence. Under the modulation of the annual variation, two distinct fire regimes are identified in the diurnal variation of fire occurrence over the entire year: midnight–early morning and late morning–night. A sharp increase in fire occurrence occurs in the midmorning after a phase of constant fire occurrence frequency in the first regime and a gradual reduction over the nighttime hours in the second regime. Although fire occurrence is significantly suppressed by rainfall during the warm wet summer, an inverse relationship between fire occurrence and relative humidity for both annual and diurnal variations in Taiwan suggests that relative humidity plays a crucial role in fire occurrence.
Abstract
Twelve years (1985–96) of monthly house fire reports for 22 districts in Taiwan, a maritime subtropical island of east Asia, were analyzed to characterize its fire nature. The major effort focused on the identification of temporal variation signals and their possible links with meteorological variables. Two significant modes of house fires were identified: annual and diurnal. As revealed from the power spectral analyses of fire time series in every fire district, a pronounced annual cycle peak emerges, with a peak phase in December and a minimum phase in June. In contrast to the warm and dry summer fire season of three continental landmasses (i.e., the U.S. West, the Northwest Territories of Canada, and the large wildland of Australia), an active fire season appears during the cool, dry winter in Taiwan. The fires on this island are highly correlated with several hydrometeorological variables; a decrease (increase) in rainfall in the dry (wet) cool (warm) environment with strong (weak) winds facilitates (hinders) fire occurrence. Under the modulation of the annual variation, two distinct fire regimes are identified in the diurnal variation of fire occurrence over the entire year: midnight–early morning and late morning–night. A sharp increase in fire occurrence occurs in the midmorning after a phase of constant fire occurrence frequency in the first regime and a gradual reduction over the nighttime hours in the second regime. Although fire occurrence is significantly suppressed by rainfall during the warm wet summer, an inverse relationship between fire occurrence and relative humidity for both annual and diurnal variations in Taiwan suggests that relative humidity plays a crucial role in fire occurrence.
Abstract
Previous studies have shown diagnostically and statistically that the interannual variation of the Indian monsoon is closely correlated with the tropical Pacific sea surface temperature (SST). It seems likely that the interannual variation of the Indian monsoon results from the response of this monsoon system to the interannual variations of the Pacific SST. This hypothesis has not been substantiated in the past. In order to test it, Version 1 of the National Center for Atmospheric Research Community Climate Model (CCM) was used to perform two parallel climate simulations: a control run using the 12 calendar month climatological SST and a run using real-time Pacific SST. The SST data used in this study are derived from the Comprehensive 0cean-Atrnosphere Data set. Significant interannual variations of the Indian monsoon circulation are generated by the real-time Pacific SST experiment, but not the clmatological SST control experiment in real-time Pacific SST simulation weakened during the 1982 and 1987 summers and intensified in the 1984 and 1988 summers. The interannual variation of the model monsoon circulation resembles the observed in many ways. According to the linear theory of Matsuno and Gill, summertime stationary eddies are generated by steady tropical forcing. Because the Indian monsoon is a part of summertime stationary eddies, interannual variation of steady tropical heating induced by interannual Pacific SST anomalies results in interannual variations of summertime stationary eddies and the associated monsooon. Various diagnostic analyses are thus engaged to illustrate this explanation of the monsoon interannual variation.
Abstract
Previous studies have shown diagnostically and statistically that the interannual variation of the Indian monsoon is closely correlated with the tropical Pacific sea surface temperature (SST). It seems likely that the interannual variation of the Indian monsoon results from the response of this monsoon system to the interannual variations of the Pacific SST. This hypothesis has not been substantiated in the past. In order to test it, Version 1 of the National Center for Atmospheric Research Community Climate Model (CCM) was used to perform two parallel climate simulations: a control run using the 12 calendar month climatological SST and a run using real-time Pacific SST. The SST data used in this study are derived from the Comprehensive 0cean-Atrnosphere Data set. Significant interannual variations of the Indian monsoon circulation are generated by the real-time Pacific SST experiment, but not the clmatological SST control experiment in real-time Pacific SST simulation weakened during the 1982 and 1987 summers and intensified in the 1984 and 1988 summers. The interannual variation of the model monsoon circulation resembles the observed in many ways. According to the linear theory of Matsuno and Gill, summertime stationary eddies are generated by steady tropical forcing. Because the Indian monsoon is a part of summertime stationary eddies, interannual variation of steady tropical heating induced by interannual Pacific SST anomalies results in interannual variations of summertime stationary eddies and the associated monsooon. Various diagnostic analyses are thus engaged to illustrate this explanation of the monsoon interannual variation.
Abstract
An effort is made to explore the interannual variation of stationary eddies for the past 13 summers (1979–1991). Equatorial λ − t diagrams of sea surface temperature (SST), outgoing longwave radiation (OLR), the potential function of OLR (χOLR), and velocity potential (χ) derived from National Meteorological Center (NMC) analyses reveal the existence of a coherent and pronounced interannual variation, particularly over the Pacific. Performing regular empirical orthogonal function (EOF) analyses of eddy streamfunction and velocity potential departures from their multiple-summer averages at 200 mb [ψ E (200 mb)] and χ E (200 mb)] and correlating the distributions between these two variables, it is possible to identify two interannual variation modes of summertime stationary eddies. The horizontal structure of these two interannual variation modes, which differs from the Pacific-North American (PNA) teleconnection pattern, resembles the Matsuno-Gill type of tropically trapped modes. It is inferred from the coherent interannual variation of the tropical Pacific SST and ψ E (200 mb) anomalies that a dynamic coupling exists between interannual variations of the Pacific SST and summertime stationary eddies. The coherent eigencoefficient time series of χOLR, the χ E (200 mb) and ψ E (200 mb) anomalies, the EOF analysis of the ψ E (200 mb) budget, and numerical simulations of the response of summertime stationary eddies to the interannual variation of tropical forcing all reveal that the aforementioned dynamic coupling is accomplished through the interannual variation of the global-scale divergent circulation.
Abstract
An effort is made to explore the interannual variation of stationary eddies for the past 13 summers (1979–1991). Equatorial λ − t diagrams of sea surface temperature (SST), outgoing longwave radiation (OLR), the potential function of OLR (χOLR), and velocity potential (χ) derived from National Meteorological Center (NMC) analyses reveal the existence of a coherent and pronounced interannual variation, particularly over the Pacific. Performing regular empirical orthogonal function (EOF) analyses of eddy streamfunction and velocity potential departures from their multiple-summer averages at 200 mb [ψ E (200 mb)] and χ E (200 mb)] and correlating the distributions between these two variables, it is possible to identify two interannual variation modes of summertime stationary eddies. The horizontal structure of these two interannual variation modes, which differs from the Pacific-North American (PNA) teleconnection pattern, resembles the Matsuno-Gill type of tropically trapped modes. It is inferred from the coherent interannual variation of the tropical Pacific SST and ψ E (200 mb) anomalies that a dynamic coupling exists between interannual variations of the Pacific SST and summertime stationary eddies. The coherent eigencoefficient time series of χOLR, the χ E (200 mb) and ψ E (200 mb) anomalies, the EOF analysis of the ψ E (200 mb) budget, and numerical simulations of the response of summertime stationary eddies to the interannual variation of tropical forcing all reveal that the aforementioned dynamic coupling is accomplished through the interannual variation of the global-scale divergent circulation.
Abstract
An inverse annual variation is observed between surface pressure on the highest mountain, which has an elevation of approximately 4000 m, and in the lowlands of Taiwan (a subtropical island in east Asia). This inverse annual variation in surface pressure of high and low elevation in low latitudes reflects, essentially, a vertical phase reversal of the tropical circulation, which is illustrated with the annual variation in the vertical structure of tropical geopotential height.
Abstract
An inverse annual variation is observed between surface pressure on the highest mountain, which has an elevation of approximately 4000 m, and in the lowlands of Taiwan (a subtropical island in east Asia). This inverse annual variation in surface pressure of high and low elevation in low latitudes reflects, essentially, a vertical phase reversal of the tropical circulation, which is illustrated with the annual variation in the vertical structure of tropical geopotential height.
Abstract
It has been observed that the low-level monsoon circulation, especially the Somali jet, exhibits a 40–50 day oscillation and obtains its maximum intensity in this oscillation when the migrating transient monsoon trough approaches ∼20°N. The data generated by the FGGE III-b analyses of the European Centre for Medium Range Weather Forecasts for the northern summer were used to explore the air mass source of this oscillation and to explain energetically and synoptically the intensification and decay of the low-level monsoon circulation in association with the 40–50 day oscillation.
A synoptic analysis of the divergent wind fields suggests that the convergence induced by the intertropical convergence zone and the deepening of the monsoon trough over northern India supplies the air mass to the 40–50 day oscillation. The energetics analysis shows that the 40–50 day oscillation of the low-level monsoon circulation is essentially described by the rotational mode. The 40–50 day oscillation of this flow field is maintained both by the generation of the 40–50 day rotational kinetic energy, i.e., the work done by the cross contour rotational mode [G(kRν )], and by the interaction between the 40–50 day divergent and rotational modes with the former process of primary importance. It is inferred from the dominant role of [G(kRν )] that the intensity of the Somali jet is highly related to the gradients of height field over the southwest fringe of the monsoon trough over northern India. This trough is deepened when the transient migrating monsoon trough reaches ∼20°N. It is clear that the concerned gradients of height field and the monsoon trough over northern India must develop synchronously. This synoptic relationship between them explains why the Somali jet intensifies or decays with the same pace as this monsoon trough.
Abstract
It has been observed that the low-level monsoon circulation, especially the Somali jet, exhibits a 40–50 day oscillation and obtains its maximum intensity in this oscillation when the migrating transient monsoon trough approaches ∼20°N. The data generated by the FGGE III-b analyses of the European Centre for Medium Range Weather Forecasts for the northern summer were used to explore the air mass source of this oscillation and to explain energetically and synoptically the intensification and decay of the low-level monsoon circulation in association with the 40–50 day oscillation.
A synoptic analysis of the divergent wind fields suggests that the convergence induced by the intertropical convergence zone and the deepening of the monsoon trough over northern India supplies the air mass to the 40–50 day oscillation. The energetics analysis shows that the 40–50 day oscillation of the low-level monsoon circulation is essentially described by the rotational mode. The 40–50 day oscillation of this flow field is maintained both by the generation of the 40–50 day rotational kinetic energy, i.e., the work done by the cross contour rotational mode [G(kRν )], and by the interaction between the 40–50 day divergent and rotational modes with the former process of primary importance. It is inferred from the dominant role of [G(kRν )] that the intensity of the Somali jet is highly related to the gradients of height field over the southwest fringe of the monsoon trough over northern India. This trough is deepened when the transient migrating monsoon trough reaches ∼20°N. It is clear that the concerned gradients of height field and the monsoon trough over northern India must develop synchronously. This synoptic relationship between them explains why the Somali jet intensifies or decays with the same pace as this monsoon trough.
Abstract
Located in northern Taiwan, Taipei is a metropolis surrounded by hills and mountains that form a basin in which two river valleys funnel the surface airflow of this basin to the open sea. Because of the southwest monsoon, summer is a dry season in northern Taiwan but is the season of maximum rainfall in the Taipei basin. This unusual summer rainfall maximum in Taipei is largely produced by afternoon/evening thunderstorms—in particular, on the downwind side and slopes of mountains south of the city. The population in the city of Taipei and the county in which this city is located has more than tripled during the past four decades while land use for building and surface construction increased by a factor of 3. This urbanization may contribute to an increase of 1.5°C in daily mean temperature, a decrease of 1°C in daily temperature range, an increase of more than 67% in the frequency of afternoon/evening thunderstorms, and an increase of 77% in rainfall generated by thunderstorms. These findings may explain the reduction in the water supply deficit to the Taipei metropolitan area and the ground subsidence of the Taipei basin caused by the excessive use of groundwater. Results of this study also provide important information for urban planning and pollution control and for management of the increasing traffic hazards caused by the enhanced thunderstorm activity and rainfall.
Abstract
Located in northern Taiwan, Taipei is a metropolis surrounded by hills and mountains that form a basin in which two river valleys funnel the surface airflow of this basin to the open sea. Because of the southwest monsoon, summer is a dry season in northern Taiwan but is the season of maximum rainfall in the Taipei basin. This unusual summer rainfall maximum in Taipei is largely produced by afternoon/evening thunderstorms—in particular, on the downwind side and slopes of mountains south of the city. The population in the city of Taipei and the county in which this city is located has more than tripled during the past four decades while land use for building and surface construction increased by a factor of 3. This urbanization may contribute to an increase of 1.5°C in daily mean temperature, a decrease of 1°C in daily temperature range, an increase of more than 67% in the frequency of afternoon/evening thunderstorms, and an increase of 77% in rainfall generated by thunderstorms. These findings may explain the reduction in the water supply deficit to the Taipei metropolitan area and the ground subsidence of the Taipei basin caused by the excessive use of groundwater. Results of this study also provide important information for urban planning and pollution control and for management of the increasing traffic hazards caused by the enhanced thunderstorm activity and rainfall.
Abstract
The summer monsoons in East and Southeast Asia are characterized, respectively, by the Mei-yu (in eastern China)–Baiu (in Japan) front (MBF) and by the monsoon trough stretching from northern Indochina to the Philippine Sea. These two major monsoon elements are separated by the North Pacific anticyclone. As indicated by the 850-mb zonal wind and cumulus convection over some key areas, a distinct opposite-phase intraseasonal variation exists between the two monsoon elements. Two approaches are adopted to explore the cause of this opposite-phase variation (which reflects the coupling between the two monsoon components): 1) the correlation coefficient patterns between the 850-mb zonal-wind monsoon index and the 850-mb streamfunction field and 2) the composite 850-mb streamline charts and the 120°E zonal-wind cross sections. It is shown that the opposite-phase variation between the two monsoon elements is caused by the anomalous circulation associated with the northward-migrating 30–60-day monsoon trough/ridge from the equator to 20°N and with the westward-propagating 12–24-day monsoon low–high along the latitude of ∼15°–20°N. Results obtained in this study are used to address two often discussed phenomena of the East Asian monsoon: 1) the rapid northward shift of the MBF across the Yangtze River basin during the Mei-yu onset is related to the north–south meridional oscillation of the MBF, and 2) the three longitudinally oriented location zones of extremely heavy rain events in eastern China are formed by the alternation of deep cumulus convection zones associated with the intraseasonal monsoon vortices (centered in the northern part of the South China Sea) between extreme monsoon conditions.
Abstract
The summer monsoons in East and Southeast Asia are characterized, respectively, by the Mei-yu (in eastern China)–Baiu (in Japan) front (MBF) and by the monsoon trough stretching from northern Indochina to the Philippine Sea. These two major monsoon elements are separated by the North Pacific anticyclone. As indicated by the 850-mb zonal wind and cumulus convection over some key areas, a distinct opposite-phase intraseasonal variation exists between the two monsoon elements. Two approaches are adopted to explore the cause of this opposite-phase variation (which reflects the coupling between the two monsoon components): 1) the correlation coefficient patterns between the 850-mb zonal-wind monsoon index and the 850-mb streamfunction field and 2) the composite 850-mb streamline charts and the 120°E zonal-wind cross sections. It is shown that the opposite-phase variation between the two monsoon elements is caused by the anomalous circulation associated with the northward-migrating 30–60-day monsoon trough/ridge from the equator to 20°N and with the westward-propagating 12–24-day monsoon low–high along the latitude of ∼15°–20°N. Results obtained in this study are used to address two often discussed phenomena of the East Asian monsoon: 1) the rapid northward shift of the MBF across the Yangtze River basin during the Mei-yu onset is related to the north–south meridional oscillation of the MBF, and 2) the three longitudinally oriented location zones of extremely heavy rain events in eastern China are formed by the alternation of deep cumulus convection zones associated with the intraseasonal monsoon vortices (centered in the northern part of the South China Sea) between extreme monsoon conditions.
Abstract
An effort was made to search for relationships between interannual variations of population, lifetime, genesis locations, and intensity of named typhoons and numbered tropical depressions in the western North Pacific during the 1979–2002 period. To support this research task, climatological relationships of tropical cyclone characteristics were also investigated for these cyclones. Major findings of this study are summarized as follows:
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Climatology: Measured by the intensity scale of the Japan Meteorological Agency, three groups of tropical cyclones were identified in terms of population versus intensity: Group 1 [tropical depression (TD) + typhoon (TY)], Group 2 (strong + very strong TY), and Group 3 (catastrophic TY). This group division coincides with that formed in terms of lifetime of tropical cyclones versus intensity. Weak cyclones (Group 1) have a larger population than strong cyclones (Group 3), while the former group has shorter lifetime than the latter group. For genesis locations, the monsoon trough is established as a favorable region of tropical cyclone genesis because it provides an environment of large vorticity. Therefore, the northward latitudinal displacement of the maximum genesis frequency in the three groups of tropical cyclones follows that of the monsoon trough.
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Interannual variation: Any mechanism that can modulate the location and intensity of the monsoon trough affects the genesis location and frequency of tropical cyclones. In response to tropical Pacific sea surface temperature anomalies, a short wave train consisting of east–west oriented cells emanates from the Tropics and progresses along the western North Pacific rim. Population of the Group-1 tropical cyclones varies interannually in phase with the oscillation of the anomalous circulation cell northeast of Taiwan and south of Japan in this short wave train, while that of Group 3 fluctuates coherently with the tropical cell of this short wave train. Because these two anomalous circulation cells exhibit opposite polarity, the out-of-phase interannual oscillation between these two cells results in the opposite interannual variation of genesis frequency between tropical cyclones of Groups 1 and 3.
Abstract
An effort was made to search for relationships between interannual variations of population, lifetime, genesis locations, and intensity of named typhoons and numbered tropical depressions in the western North Pacific during the 1979–2002 period. To support this research task, climatological relationships of tropical cyclone characteristics were also investigated for these cyclones. Major findings of this study are summarized as follows:
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Climatology: Measured by the intensity scale of the Japan Meteorological Agency, three groups of tropical cyclones were identified in terms of population versus intensity: Group 1 [tropical depression (TD) + typhoon (TY)], Group 2 (strong + very strong TY), and Group 3 (catastrophic TY). This group division coincides with that formed in terms of lifetime of tropical cyclones versus intensity. Weak cyclones (Group 1) have a larger population than strong cyclones (Group 3), while the former group has shorter lifetime than the latter group. For genesis locations, the monsoon trough is established as a favorable region of tropical cyclone genesis because it provides an environment of large vorticity. Therefore, the northward latitudinal displacement of the maximum genesis frequency in the three groups of tropical cyclones follows that of the monsoon trough.
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Interannual variation: Any mechanism that can modulate the location and intensity of the monsoon trough affects the genesis location and frequency of tropical cyclones. In response to tropical Pacific sea surface temperature anomalies, a short wave train consisting of east–west oriented cells emanates from the Tropics and progresses along the western North Pacific rim. Population of the Group-1 tropical cyclones varies interannually in phase with the oscillation of the anomalous circulation cell northeast of Taiwan and south of Japan in this short wave train, while that of Group 3 fluctuates coherently with the tropical cell of this short wave train. Because these two anomalous circulation cells exhibit opposite polarity, the out-of-phase interannual oscillation between these two cells results in the opposite interannual variation of genesis frequency between tropical cyclones of Groups 1 and 3.
