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- Author or Editor: Jian Li x
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Abstract
In this study, late-summer rainfall over eastern contiguous China is classified according to hourly intensity and the changes of moderate, intense, and extreme precipitation in response to variation of surface air temperature are analyzed. The e-folding decay intensity (I mi) derived from the exponential distribution of rainfall amount is defined as the threshold that partitions rainfall into moderate and intense rainfall, and the double e-folding decay intensity (Ie ) is used as the threshold to pick out extreme cases. The mean values of I mi and Ie are about 12 and 24 mm h−1, respectively. Between the two periods, 1966–85 and 1986–2005, the ratio between moderate and intense rainfall has experienced significant changes. And the spatial pattern of changes in the percentage of moderate rainfall presents a direct relation with that of the surface air temperature. Based on temperature changes, three regimes, regime N (north China), regime C (central eastern China), and regime S (southeastern coastal area of China), are defined. In warming regimes (regimes N and S), the percentage of moderate rainfall exhibits a decreasing trend. In regime C, where the temperature has fallen, the percentage of moderate rainfall increased prominently. In all three regimes there are significant negative (positive) correlations between the percentage of moderate (intense) rainfall and the temperature. The relation between the extreme rainfall and the surface air temperature is far more regionally dependent. With plenty of water supply and little change in relative humidity, the extreme rainfall increased in regime S. Although regime N also shows strong warming trends, there is no significant trend in extreme precipitation due to the lack of water vapor transportation.
Abstract
In this study, late-summer rainfall over eastern contiguous China is classified according to hourly intensity and the changes of moderate, intense, and extreme precipitation in response to variation of surface air temperature are analyzed. The e-folding decay intensity (I mi) derived from the exponential distribution of rainfall amount is defined as the threshold that partitions rainfall into moderate and intense rainfall, and the double e-folding decay intensity (Ie ) is used as the threshold to pick out extreme cases. The mean values of I mi and Ie are about 12 and 24 mm h−1, respectively. Between the two periods, 1966–85 and 1986–2005, the ratio between moderate and intense rainfall has experienced significant changes. And the spatial pattern of changes in the percentage of moderate rainfall presents a direct relation with that of the surface air temperature. Based on temperature changes, three regimes, regime N (north China), regime C (central eastern China), and regime S (southeastern coastal area of China), are defined. In warming regimes (regimes N and S), the percentage of moderate rainfall exhibits a decreasing trend. In regime C, where the temperature has fallen, the percentage of moderate rainfall increased prominently. In all three regimes there are significant negative (positive) correlations between the percentage of moderate (intense) rainfall and the temperature. The relation between the extreme rainfall and the surface air temperature is far more regionally dependent. With plenty of water supply and little change in relative humidity, the extreme rainfall increased in regime S. Although regime N also shows strong warming trends, there is no significant trend in extreme precipitation due to the lack of water vapor transportation.
Abstract
The climatic features of the distinctive cold season precipitation over the Yungui Plateau of China and the corresponding circulation background are investigated. From daily rainfall data observed with a high-density station network, it is found that the highest rainfall frequency in southern China during November–February appears over the Yungui Plateau. The rainfall intensity in this region is fairly low, and there is no remarkable rainfall-amount maximum. In comparison with the rainfall in southeastern China, the precipitation over the Yungui Plateau is more concentrated in weak events, with 85.9% of rainfall days having daily precipitation amounts of less than 3 mm. By regressing the circulation field on the rainfall frequency index, a favorable climatic background for high rainfall frequency is explored. In high-rainfall-frequency years, the surface wind exhibits southwesterly wind anomalies west of 104°E and cold air penetrates from the north on the eastern side. These two branches converge on the eastern edge of the Hengduan Mountains. In the lower troposphere, southwesterly winds prevail and anomalous water vapor fluxes converge over the Yungui Plateau. In the middle and higher troposphere, the westerly zonal wind strengthens and leads to an anomalous divergence. These dynamic and moist conditions contribute to the formation of clouds and precipitation. The northward- and eastward-facing slopes of the Yungui Plateau uplift the shallow, cold air carried by the northerly and easterly winds, and the terrain effects trigger the precipitation process. The low temperature and small specific humidity over the Yungui Plateau modulate the rainfall intensity to a low level.
Abstract
The climatic features of the distinctive cold season precipitation over the Yungui Plateau of China and the corresponding circulation background are investigated. From daily rainfall data observed with a high-density station network, it is found that the highest rainfall frequency in southern China during November–February appears over the Yungui Plateau. The rainfall intensity in this region is fairly low, and there is no remarkable rainfall-amount maximum. In comparison with the rainfall in southeastern China, the precipitation over the Yungui Plateau is more concentrated in weak events, with 85.9% of rainfall days having daily precipitation amounts of less than 3 mm. By regressing the circulation field on the rainfall frequency index, a favorable climatic background for high rainfall frequency is explored. In high-rainfall-frequency years, the surface wind exhibits southwesterly wind anomalies west of 104°E and cold air penetrates from the north on the eastern side. These two branches converge on the eastern edge of the Hengduan Mountains. In the lower troposphere, southwesterly winds prevail and anomalous water vapor fluxes converge over the Yungui Plateau. In the middle and higher troposphere, the westerly zonal wind strengthens and leads to an anomalous divergence. These dynamic and moist conditions contribute to the formation of clouds and precipitation. The northward- and eastward-facing slopes of the Yungui Plateau uplift the shallow, cold air carried by the northerly and easterly winds, and the terrain effects trigger the precipitation process. The low temperature and small specific humidity over the Yungui Plateau modulate the rainfall intensity to a low level.
Abstract
This study presents a method to linearly evaluate the rainfall frequency–intensity distribution, which is an important component of climatological rainfall characteristics. To grasp and represent the key information of the rainfall frequency distribution by intensity, a two-parameter double exponential function is formulated and fitted to the hourly rainfall observation at each station. The values of the two parameters are estimated by transforming the distribution to a linear pattern. The two parameters determine the location and shape of the fitted distribution curve, and they have different modulating effects in different intensity categories, one governing the low-intensity section and the other dominating the intense rainfall. Through analysis of the estimated parameters, essential features of rainfall distribution can be obtained and assessed. The proposed method is applied to analyze the climatology and long-term variation of the late-summer rainfall in China. It is found that topography and monsoon circulation are two major factors controlling the rainfall frequency–intensity distribution. At stations with high surface altitudes and complex orography, the frequency of light rain is extremely high and the number of intense rainfall events is relatively small. In the plain areas of eastern China, especially those influenced by the main monsoon rain belt, heavy rainfall is more frequent. By tracking the displacement of the parameter pairs, the decadal changes in rainfall frequency–intensity distribution can be clearly visualized and evaluated on a plane constructed by the two parameters. The southern flooding and northern drought pattern can be attributed to the changes in light and moderate rainfall, while the intense rainfall exhibits opposite trends.
Abstract
This study presents a method to linearly evaluate the rainfall frequency–intensity distribution, which is an important component of climatological rainfall characteristics. To grasp and represent the key information of the rainfall frequency distribution by intensity, a two-parameter double exponential function is formulated and fitted to the hourly rainfall observation at each station. The values of the two parameters are estimated by transforming the distribution to a linear pattern. The two parameters determine the location and shape of the fitted distribution curve, and they have different modulating effects in different intensity categories, one governing the low-intensity section and the other dominating the intense rainfall. Through analysis of the estimated parameters, essential features of rainfall distribution can be obtained and assessed. The proposed method is applied to analyze the climatology and long-term variation of the late-summer rainfall in China. It is found that topography and monsoon circulation are two major factors controlling the rainfall frequency–intensity distribution. At stations with high surface altitudes and complex orography, the frequency of light rain is extremely high and the number of intense rainfall events is relatively small. In the plain areas of eastern China, especially those influenced by the main monsoon rain belt, heavy rainfall is more frequent. By tracking the displacement of the parameter pairs, the decadal changes in rainfall frequency–intensity distribution can be clearly visualized and evaluated on a plane constructed by the two parameters. The southern flooding and northern drought pattern can be attributed to the changes in light and moderate rainfall, while the intense rainfall exhibits opposite trends.
Abstract
Upstream nonoscillatory (UNO) advection schemes are derived by optimizing existing classical advection schemes and combining them in different monotonic zones to avoid flux limiters for simplicity. The UNO schemes are extended to irregular grids in the form of upstream midflux linear interpolation with symmetrical gradients and are adapted to multidimensions with an advective–conservative operator. They are given in finite-volume flux form and hence are consistent and conservative. They also preserve constancy and linear correlation. Implicit numerical diffusivity of these schemes is also derived and used as a guideline for the selection of advection schemes. One- and two-dimesional tests are used for comparisons with their classical counterparts. Multiple-cell grids are used to test the irregular grid formulation and demonstrate their performance. The simple second-order UNO2 scheme may be accurate enough when the physical diffusion or numerical smoothing term is larger than the numerical diffusion. The third-order UNO3 scheme has very small self-constrained numerical diffusion and is suitable for general atmospheric and oceanic tracer advection.
Abstract
Upstream nonoscillatory (UNO) advection schemes are derived by optimizing existing classical advection schemes and combining them in different monotonic zones to avoid flux limiters for simplicity. The UNO schemes are extended to irregular grids in the form of upstream midflux linear interpolation with symmetrical gradients and are adapted to multidimensions with an advective–conservative operator. They are given in finite-volume flux form and hence are consistent and conservative. They also preserve constancy and linear correlation. Implicit numerical diffusivity of these schemes is also derived and used as a guideline for the selection of advection schemes. One- and two-dimesional tests are used for comparisons with their classical counterparts. Multiple-cell grids are used to test the irregular grid formulation and demonstrate their performance. The simple second-order UNO2 scheme may be accurate enough when the physical diffusion or numerical smoothing term is larger than the numerical diffusion. The third-order UNO3 scheme has very small self-constrained numerical diffusion and is suitable for general atmospheric and oceanic tracer advection.
Abstract
Second- and third-order upstream nonoscillatory (UNO) advection schemes are applied on a spherical multiple-cell (SMC) grid for global transport. Similar to the reduced grid, the SMC grid relaxes the Courant–Friedrichs–Lewy (CFL) restriction of the Eulerian advection time step on the conventional latitude–longitude grid by zonally merging cells toward the poles. Round polar cells are introduced to remove the polar singularity of the spherical coordinate system. The unstructured feature of the SMC grid allows unused cells to be removed out of memory and transport calculations. Solid-body rotation and deformation flow tests are used for comparison with other transport schemes. Application on the global ocean surface is used to demonstrate the flexibility of the SMC grid by removing all land points and making possible the extension of global ocean surface wave models to cover the Arctic in response to the retreating sea ice in recent summers. Numerical results suggest that UNO schemes on the SMC grid are suitable for global transport.
Abstract
Second- and third-order upstream nonoscillatory (UNO) advection schemes are applied on a spherical multiple-cell (SMC) grid for global transport. Similar to the reduced grid, the SMC grid relaxes the Courant–Friedrichs–Lewy (CFL) restriction of the Eulerian advection time step on the conventional latitude–longitude grid by zonally merging cells toward the poles. Round polar cells are introduced to remove the polar singularity of the spherical coordinate system. The unstructured feature of the SMC grid allows unused cells to be removed out of memory and transport calculations. Solid-body rotation and deformation flow tests are used for comparison with other transport schemes. Application on the global ocean surface is used to demonstrate the flexibility of the SMC grid by removing all land points and making possible the extension of global ocean surface wave models to cover the Arctic in response to the retreating sea ice in recent summers. Numerical results suggest that UNO schemes on the SMC grid are suitable for global transport.
Abstract
Fine-scale characteristics of summer precipitation over Cang Mountain, a long and narrow mountain with a quasi-north–south orientation in Southwest China, are studied using station and radar data. Three kinds of rainfall processes are classified according to the initial stations of regional rainfall events (RREs) by utilizing minute-scale rain gauge data. RREs initiating in the western part of Cang Mountain exhibit eastward evolution and tend to reach their maximum rainfall intensity on the mountaintop. The results indicate differences in the precipitation evolution characteristics between short-duration (1–3 h) and long-duration (at least 6 h) events. Short-duration events begin farther from the mountaintop and then propagate eastward, whereas long-duration events remain longer around the mountaintop. RREs that initiate from the eastern part of Cang Mountain display westward propagation and frequently reach their maximum rainfall intensity over the eastern slope of the mountain. Among them, short-duration events tend to propagate farther west of Cang Mountain at high speeds, but the westward evolution of long-duration events is mainly confined to the eastern part of Cang Mountain. For mountaintop-originated RREs, precipitation quickly reaches its maximum intensity after it starts and then continues for a long time around the mountaintop during the period from late afternoon to early morning. These findings provide references for the fine-scale prediction of precipitation evolution in small-scale mountainous areas.
Abstract
Fine-scale characteristics of summer precipitation over Cang Mountain, a long and narrow mountain with a quasi-north–south orientation in Southwest China, are studied using station and radar data. Three kinds of rainfall processes are classified according to the initial stations of regional rainfall events (RREs) by utilizing minute-scale rain gauge data. RREs initiating in the western part of Cang Mountain exhibit eastward evolution and tend to reach their maximum rainfall intensity on the mountaintop. The results indicate differences in the precipitation evolution characteristics between short-duration (1–3 h) and long-duration (at least 6 h) events. Short-duration events begin farther from the mountaintop and then propagate eastward, whereas long-duration events remain longer around the mountaintop. RREs that initiate from the eastern part of Cang Mountain display westward propagation and frequently reach their maximum rainfall intensity over the eastern slope of the mountain. Among them, short-duration events tend to propagate farther west of Cang Mountain at high speeds, but the westward evolution of long-duration events is mainly confined to the eastern part of Cang Mountain. For mountaintop-originated RREs, precipitation quickly reaches its maximum intensity after it starts and then continues for a long time around the mountaintop during the period from late afternoon to early morning. These findings provide references for the fine-scale prediction of precipitation evolution in small-scale mountainous areas.
Abstract
The climatic features of the diurnally varying summer precipitation over and around the central Tian Shan Mountains are investigated. Both the hourly rainfall data observed at eight stations along a transect across the mountains and the convective index derived from the satellite data show that there are three distinct regimes: the early morning peak at stations to the south of the mountains, the late afternoon peak at stations on the mountains, and the night peak at stations to the north of the mountains. The relation between regimes of diurnal variation is analyzed. By defining the regional rainfall event (RRE), the initial stations of each RRE are recorded. The early morning rainfall in the southern periphery of the mountains is triggered locally in the southern basin. Both the late afternoon peak over the mountains and the night peak in the northern periphery are influenced by mountain-originated rainfall events. These rainfall events appear over the mountains in the afternoon, and some of them move northward and lead to the nocturnal rainfall in the northern basin. The triggering of convection in the afternoon over the mountains and that in the early morning in the southern basin is related to the diurnally varying wind and thermodynamic conditions over and around the mountains. Low-level convergence with thermodynamic instability appears at noon (night) over the mountains (in the southern basin) just before the start of the convection.
Abstract
The climatic features of the diurnally varying summer precipitation over and around the central Tian Shan Mountains are investigated. Both the hourly rainfall data observed at eight stations along a transect across the mountains and the convective index derived from the satellite data show that there are three distinct regimes: the early morning peak at stations to the south of the mountains, the late afternoon peak at stations on the mountains, and the night peak at stations to the north of the mountains. The relation between regimes of diurnal variation is analyzed. By defining the regional rainfall event (RRE), the initial stations of each RRE are recorded. The early morning rainfall in the southern periphery of the mountains is triggered locally in the southern basin. Both the late afternoon peak over the mountains and the night peak in the northern periphery are influenced by mountain-originated rainfall events. These rainfall events appear over the mountains in the afternoon, and some of them move northward and lead to the nocturnal rainfall in the northern basin. The triggering of convection in the afternoon over the mountains and that in the early morning in the southern basin is related to the diurnally varying wind and thermodynamic conditions over and around the mountains. Low-level convergence with thermodynamic instability appears at noon (night) over the mountains (in the southern basin) just before the start of the convection.
Abstract
The regional differences in hourly precipitation characteristics along the western coast of South China are investigated. The coastal area can be divided into three regions: two regions with large precipitation amounts to the west and east of the Leizhou Peninsula and another region with less precipitation over the Leizhou Peninsula. The precipitation center west of the Leizhou Peninsula (Dongxing station) is dominated by heavy precipitation, which frequently peaks in the early morning and has a long duration. The precipitation center east of the Leizhou Peninsula (Yangjiang station) has a high frequency of weak precipitation. There are two kinds of precipitation that occur at Yangjiang: short-duration precipitation in the afternoon and long-lasting precipitation in the morning. Over the Leizhou Peninsula (Zhanjiang station), precipitation mainly occurs over a short duration in the afternoon. The possible reasons for the different precipitation features are discussed. The morning precipitation at Dongxing is usually caused by deep convection. A large proportion of the morning precipitation events are locally generated events, which are closely related to the southerly winds and topographic barriers. Among the many possible factors affecting the morning precipitation at Yangjiang, the convergence between the land breeze and prevailing onshore airflow is an important one. The afternoon precipitation events affecting Yangjiang mostly originate over the mountains northwest of Yangjiang. Influenced by the curvature of the coastline and the relatively flat terrain over the Leizhou Peninsula, there is no convergence and less convection in the morning. However, the Leizhou Peninsula is prone to being influenced by daytime thermal forcings, which trigger afternoon precipitation.
Abstract
The regional differences in hourly precipitation characteristics along the western coast of South China are investigated. The coastal area can be divided into three regions: two regions with large precipitation amounts to the west and east of the Leizhou Peninsula and another region with less precipitation over the Leizhou Peninsula. The precipitation center west of the Leizhou Peninsula (Dongxing station) is dominated by heavy precipitation, which frequently peaks in the early morning and has a long duration. The precipitation center east of the Leizhou Peninsula (Yangjiang station) has a high frequency of weak precipitation. There are two kinds of precipitation that occur at Yangjiang: short-duration precipitation in the afternoon and long-lasting precipitation in the morning. Over the Leizhou Peninsula (Zhanjiang station), precipitation mainly occurs over a short duration in the afternoon. The possible reasons for the different precipitation features are discussed. The morning precipitation at Dongxing is usually caused by deep convection. A large proportion of the morning precipitation events are locally generated events, which are closely related to the southerly winds and topographic barriers. Among the many possible factors affecting the morning precipitation at Yangjiang, the convergence between the land breeze and prevailing onshore airflow is an important one. The afternoon precipitation events affecting Yangjiang mostly originate over the mountains northwest of Yangjiang. Influenced by the curvature of the coastline and the relatively flat terrain over the Leizhou Peninsula, there is no convergence and less convection in the morning. However, the Leizhou Peninsula is prone to being influenced by daytime thermal forcings, which trigger afternoon precipitation.
Abstract
This study introduces a new methodology for identifying El Niño and La Niña events. Sea surface temperature (SST) anomaly patterns for El Niño and La Niña onset, peak, and end phases are classified by self-organizing maps (SOM) analysis. Both onset and end phases for El Niño and La Niña exhibit eastern Pacific (EP) and central Pacific (CP) types. The SST anomaly patterns in peak phase can be classified into EP, EP-like, and CP types for El Niño, and EP, mixed (MIX), and CP types for La Niña.
The general type of each El Niño or La Niña event is then defined according to the SST type for each of the three phases. There is no robust connection between the general types of the contiguous El Niño and La Niña except that the MIX La Niña rarely induces a subsequent CP El Niño. However, there are strong relationships between the end-phase type of El Niño and the onset-phase type of the subsequent La Niña. The EP-end-type El Niño favors transition to the CP-onset-type La Niña, while the CP-end-type El Niño favors transition to the EP-onset-type La Niña. On the other hand, the CP-end-type La Niña favors transition to EP-onset-type El Niño. Furthermore, an El Niño that occurs after the decay of La Niña favors initiating as an EP-onset type. These relationships are driven by different atmosphere–ocean dynamics, such as coupled air–sea feedback, thermocline feedback, slow SST mode, and Bjerknes feedbacks.
Abstract
This study introduces a new methodology for identifying El Niño and La Niña events. Sea surface temperature (SST) anomaly patterns for El Niño and La Niña onset, peak, and end phases are classified by self-organizing maps (SOM) analysis. Both onset and end phases for El Niño and La Niña exhibit eastern Pacific (EP) and central Pacific (CP) types. The SST anomaly patterns in peak phase can be classified into EP, EP-like, and CP types for El Niño, and EP, mixed (MIX), and CP types for La Niña.
The general type of each El Niño or La Niña event is then defined according to the SST type for each of the three phases. There is no robust connection between the general types of the contiguous El Niño and La Niña except that the MIX La Niña rarely induces a subsequent CP El Niño. However, there are strong relationships between the end-phase type of El Niño and the onset-phase type of the subsequent La Niña. The EP-end-type El Niño favors transition to the CP-onset-type La Niña, while the CP-end-type El Niño favors transition to the EP-onset-type La Niña. On the other hand, the CP-end-type La Niña favors transition to EP-onset-type El Niño. Furthermore, an El Niño that occurs after the decay of La Niña favors initiating as an EP-onset type. These relationships are driven by different atmosphere–ocean dynamics, such as coupled air–sea feedback, thermocline feedback, slow SST mode, and Bjerknes feedbacks.
Abstract
Evolution characteristics of the Madden–Julian oscillation (MJO) during the eastern Pacific (EP) and central Pacific (CP) types of El Niño have been investigated. MJO activities are strengthened over the western Pacific during the predeveloping and developing phases of EP El Niño, but suppressed during the mature and decaying phases. In contrast, MJO activities do not show a clear relationship with CP El Niño before their occurrence over the western Pacific, but they increase over the central Pacific during the mature and decaying phases of CP El Niño. Lag correlation analyses further confirm that MJO activities over the western Pacific in boreal spring and early summer are closely related to EP El Niño up to 2–11 months later, but not for CP El Niño. EP El Niño tends to weaken the MJO and lead to a much shorter range of its eastward propagation. Anomalous descending motions over the Maritime Continent and western Pacific related to El Niño can suppress convection and moisture flux convergence there and weaken MJO activities over these regions during the mature phase of both types of El Niño. MJO activities over the western Pacific are much weaker in EP El Niño due to the stronger anomalous descending motions. Furthermore, the MJO propagates more continuously and farther eastward during CP El Niño because of robust moisture convergence over the central Pacific, which provides adequate moisture for the development of MJO convection.
Abstract
Evolution characteristics of the Madden–Julian oscillation (MJO) during the eastern Pacific (EP) and central Pacific (CP) types of El Niño have been investigated. MJO activities are strengthened over the western Pacific during the predeveloping and developing phases of EP El Niño, but suppressed during the mature and decaying phases. In contrast, MJO activities do not show a clear relationship with CP El Niño before their occurrence over the western Pacific, but they increase over the central Pacific during the mature and decaying phases of CP El Niño. Lag correlation analyses further confirm that MJO activities over the western Pacific in boreal spring and early summer are closely related to EP El Niño up to 2–11 months later, but not for CP El Niño. EP El Niño tends to weaken the MJO and lead to a much shorter range of its eastward propagation. Anomalous descending motions over the Maritime Continent and western Pacific related to El Niño can suppress convection and moisture flux convergence there and weaken MJO activities over these regions during the mature phase of both types of El Niño. MJO activities over the western Pacific are much weaker in EP El Niño due to the stronger anomalous descending motions. Furthermore, the MJO propagates more continuously and farther eastward during CP El Niño because of robust moisture convergence over the central Pacific, which provides adequate moisture for the development of MJO convection.
