2012 Vol. 18, No. 4
2012, 18(4): 403-411.
Abstract:
This study introduces a new dynamical quantity, shear gradient vorticity (SGV), which is defined as vertical wind shear multiplying the horizontal component of vorticity gradient, aiming to diagnose heavy precipitation induced by some strong convective weather systems. The vorticity gradient component can be used to study the collision or merging process between different vortexes or the deformation of a vortex with a sharp vorticity gradient. Vertical wind shear, another contributed component of SGV, always represents the environmental dynamical factor in meteorology. By the combined effect of the two components, overall, SGV can represent the interaction between the environmental wind shear and the evolution of vortexes with a large vorticity gradient. Other traditional vorticity-like dynamical quantities (such as helicity) have the limitation in the diagnosis of the convection, since they do not consider the vorticity gradient. From this perspective, SGV has the potential to diagnose some strong convective weather processes, such as Extratropical Transition (ET) of tropical cyclones and the evolution of multicell storms. The forecast performance of SGV for the numerical ET case of Typhoon Toraji (0108) has been evaluated. Compared with helicity, SGV has shown a greater advantage to forecast the distribution of heavy precipitation more accurately, especially in the frontal zone.
This study introduces a new dynamical quantity, shear gradient vorticity (SGV), which is defined as vertical wind shear multiplying the horizontal component of vorticity gradient, aiming to diagnose heavy precipitation induced by some strong convective weather systems. The vorticity gradient component can be used to study the collision or merging process between different vortexes or the deformation of a vortex with a sharp vorticity gradient. Vertical wind shear, another contributed component of SGV, always represents the environmental dynamical factor in meteorology. By the combined effect of the two components, overall, SGV can represent the interaction between the environmental wind shear and the evolution of vortexes with a large vorticity gradient. Other traditional vorticity-like dynamical quantities (such as helicity) have the limitation in the diagnosis of the convection, since they do not consider the vorticity gradient. From this perspective, SGV has the potential to diagnose some strong convective weather processes, such as Extratropical Transition (ET) of tropical cyclones and the evolution of multicell storms. The forecast performance of SGV for the numerical ET case of Typhoon Toraji (0108) has been evaluated. Compared with helicity, SGV has shown a greater advantage to forecast the distribution of heavy precipitation more accurately, especially in the frontal zone.
2012, 18(4): 412-421.
Abstract:
Many studies have explored the importance and influence of planetary boundary layer processes on tropical cyclones (TCs). However, few studies have focused on the influence of land surface processes on the activity of TCs. To test the effect of initial perturbations of land surface processes on TCs, a land surface process perturbation module is built in a global ensemble prediction system. Ensemble experiments for the TCs that occurred from 12 UTC 22 August to 18 UTC 24 November, 2006 show that consideration of the uncertainties within the land surface process could increase the predictability of the global ensemble prediction system. Detailed analysis on TC Xangsane (2006) indicates that the perturbation of land surface processes may increase the variation of sensible heat flux and latent heat flux. Meanwhile, the effect from land surface perturbation can be transferred to the upper atmosphere, which leads to better TC forecasts.
Many studies have explored the importance and influence of planetary boundary layer processes on tropical cyclones (TCs). However, few studies have focused on the influence of land surface processes on the activity of TCs. To test the effect of initial perturbations of land surface processes on TCs, a land surface process perturbation module is built in a global ensemble prediction system. Ensemble experiments for the TCs that occurred from 12 UTC 22 August to 18 UTC 24 November, 2006 show that consideration of the uncertainties within the land surface process could increase the predictability of the global ensemble prediction system. Detailed analysis on TC Xangsane (2006) indicates that the perturbation of land surface processes may increase the variation of sensible heat flux and latent heat flux. Meanwhile, the effect from land surface perturbation can be transferred to the upper atmosphere, which leads to better TC forecasts.
2012, 18(4): 422-435.
Abstract:
A heavy rainfall process, which occurred in Shanghai during 5-6 August, 2001 from a landfalling tropical depression (TD), is examined with a control numerical experiment based on MM5 model. It is found that the contours of generalized equivalent potential temperature (θ*) are almost vertical with respect to horizontal surfaces near the TD center and more densely distributed than those of equivalent potential temperature (θe). Because the atmosphere is non-uniformly saturated in reality, θ* takes the place of θe in the definition of convective vorticity vector (CVV) so that a new vector, namely the generalized convective vorticity vector (CVV*), is applied in this study. Since CVV* can reflect both the secondary circulation and the variation of horizontal moist baroclinicity, the vertical integration of vertical component of CVV* is found, in this study, to represent the rainfall areas in the TD case better than potential vorticity (PV), moist potential vorticity (MPV), generalized moist potential vorticity (Pm), and CVV, with high-value area of CVV* corresponding to heavy-rainfall area. Moreover, the analysis from CVV* implies that the Hangzhou Bay might play an important role in the heavy rain process. A sensitivity experiment without the Hangzhou Bay is then designed and compared with the control run. It is found that the CVV* becomes weaker than that in the control run, implying that the elimination of Hangzhou Bay results in reduced rainfall. Further analyses show that the Hangzhou Bay provides sufficient water vapor and surface heat flux to the TD system, which is very important to the genesis and development of mesoscale cloud clusters around the TD and the associated heavy rainfall.
A heavy rainfall process, which occurred in Shanghai during 5-6 August, 2001 from a landfalling tropical depression (TD), is examined with a control numerical experiment based on MM5 model. It is found that the contours of generalized equivalent potential temperature (θ*) are almost vertical with respect to horizontal surfaces near the TD center and more densely distributed than those of equivalent potential temperature (θe). Because the atmosphere is non-uniformly saturated in reality, θ* takes the place of θe in the definition of convective vorticity vector (CVV) so that a new vector, namely the generalized convective vorticity vector (CVV*), is applied in this study. Since CVV* can reflect both the secondary circulation and the variation of horizontal moist baroclinicity, the vertical integration of vertical component of CVV* is found, in this study, to represent the rainfall areas in the TD case better than potential vorticity (PV), moist potential vorticity (MPV), generalized moist potential vorticity (Pm), and CVV, with high-value area of CVV* corresponding to heavy-rainfall area. Moreover, the analysis from CVV* implies that the Hangzhou Bay might play an important role in the heavy rain process. A sensitivity experiment without the Hangzhou Bay is then designed and compared with the control run. It is found that the CVV* becomes weaker than that in the control run, implying that the elimination of Hangzhou Bay results in reduced rainfall. Further analyses show that the Hangzhou Bay provides sufficient water vapor and surface heat flux to the TD system, which is very important to the genesis and development of mesoscale cloud clusters around the TD and the associated heavy rainfall.
2012, 18(4): 436-444.
Abstract:
The asymmetric distribution of convection associated with tropical cyclones making landfall on the east China coast is studied with black-body temperature (TBB) data from Fengyun-2 (FY-2) geostationary weather satellite. The convection in various quadrants of the TCs is examined for the period of -24 to 6 h relative to landfall. The convection to the southern side of the TCs was much more intense than that to the northern side during the whole landfall period. The convection to the western side of the TCs was stronger than that to the eastern side for the time -8 h before and at the landfall. After landfall, the situation reverses. The asymmetric convection of the TCs was partly due to the vertical wind shear and storm motion, and partly because the process of landfall restrained the convection in relevant quadrants. Besides, the orographic uplift along the east of China was favorable to the enhancement of convection in the eastern side of the TCs. From the characteristics of convective asymmetry of the TCs landing on the south and east of China, it is known that their main difference might be the included angle between the TC path and the coastline as well as the terrain along the coast.
The asymmetric distribution of convection associated with tropical cyclones making landfall on the east China coast is studied with black-body temperature (TBB) data from Fengyun-2 (FY-2) geostationary weather satellite. The convection in various quadrants of the TCs is examined for the period of -24 to 6 h relative to landfall. The convection to the southern side of the TCs was much more intense than that to the northern side during the whole landfall period. The convection to the western side of the TCs was stronger than that to the eastern side for the time -8 h before and at the landfall. After landfall, the situation reverses. The asymmetric convection of the TCs was partly due to the vertical wind shear and storm motion, and partly because the process of landfall restrained the convection in relevant quadrants. Besides, the orographic uplift along the east of China was favorable to the enhancement of convection in the eastern side of the TCs. From the characteristics of convective asymmetry of the TCs landing on the south and east of China, it is known that their main difference might be the included angle between the TC path and the coastline as well as the terrain along the coast.
2012, 18(4): 445-456.
Abstract:
Two land surface models, Community Land Model (CLM3.5) and NOAH model, have been coupled to the Weather Research and Forecasting (WRF) model and been used to simulate the precipitation, temperature, and circulation fields, respectively, over eastern China in a typical flood year (1998). The purpose of this study is to reveal the effects of land surface changes on regional climate modeling. Comparisons of simulated results and observation data indicate that changes in land surface processes have significant impact on spatial and temporal distribution of precipitation and temperature patterns in eastern China. Coupling of the CLM3.5 to the WRF model (experiment WRF-C) substantially improves the simulation results over eastern China relative to an older version of WRF coupled to the NOAH-LSM (experiment WRF-N). It is found that the simulation of the spatial pattern of summer precipitation in WRF-C is better than in WRF-N. WRF-C also significantly reduces the summer positive bias of surface air temperature, and its simulated surface air temperature matches more closely to observations than WRF-N does, which is associated with lower sensible heat fluxes and higher latent heat fluxes in WRF-C.
Two land surface models, Community Land Model (CLM3.5) and NOAH model, have been coupled to the Weather Research and Forecasting (WRF) model and been used to simulate the precipitation, temperature, and circulation fields, respectively, over eastern China in a typical flood year (1998). The purpose of this study is to reveal the effects of land surface changes on regional climate modeling. Comparisons of simulated results and observation data indicate that changes in land surface processes have significant impact on spatial and temporal distribution of precipitation and temperature patterns in eastern China. Coupling of the CLM3.5 to the WRF model (experiment WRF-C) substantially improves the simulation results over eastern China relative to an older version of WRF coupled to the NOAH-LSM (experiment WRF-N). It is found that the simulation of the spatial pattern of summer precipitation in WRF-C is better than in WRF-N. WRF-C also significantly reduces the summer positive bias of surface air temperature, and its simulated surface air temperature matches more closely to observations than WRF-N does, which is associated with lower sensible heat fluxes and higher latent heat fluxes in WRF-C.
2012, 18(4): 457-472.
Abstract:
Mesoscale convective systems (MCSs) are severe disaster-producing weather systems. Previous attempts of MCS census are made by examining infrared satellite imageries artificially, with subjectivity involved in the process unavoidably. This method is also inefficient and time-consuming. The disadvantages make it impossible to do MCS census over Asia and western Pacific region (AWPR) with an extended span of time, which is not favorable for gaining a deeper insight into these systems. In this paper, a fire-new automatic MCS identification (AMI) method is used to capture four categories of MCSs with different sizes and shapes from numerical satellite infrared data. 47,468 MCSs are identified over Asia and western Pacific region during the warm season (May to October) from 1995 to 2008. Based on this database, MCS characteristics such as shape, size, duration, velocity, geographical distribution, intermonthly variation, and lifecycle are studied. Results indicate that the number of linear MCSs is 2.5 times that of circular MCSs. The former is of a larger size while the latter is of a longer duration. The 500 hPa steering flow plays an important role in the MCS movement. MCSs tend to move faster after they reach the maximum extent. Four categories of MCS have similar characteristics of geographical distribution and intermonthly variation. Basically, MCSs are zonally distributed, with three zones weakening from south to north. The intermonthly variation of MCSs is related to the seasonal adjustment of the large-scale circulation. As to the MCSs over China, they have different lifecycle characteristics over different areas. MCSs over plateaus and hill areas, with only one peak in their lifecycle curves, tend to form in the afternoon, mature at nightfall, and dissipate at night. On the other hand, MCSs over plains, which have several peaks in their lifecycle curves, may form either in the afternoon or at night, whereas MCSs over the oceans tend to form at midnight. Affected by the sea-land breeze circulation, MCSs over coastal areas of Guangdong and Guangxi always come into being at about 1500 or 1600 (local time), while MCSs over the Sichuan Basin, affected by the mountain-valley breeze circulation, generally initiate nocturnally.
Mesoscale convective systems (MCSs) are severe disaster-producing weather systems. Previous attempts of MCS census are made by examining infrared satellite imageries artificially, with subjectivity involved in the process unavoidably. This method is also inefficient and time-consuming. The disadvantages make it impossible to do MCS census over Asia and western Pacific region (AWPR) with an extended span of time, which is not favorable for gaining a deeper insight into these systems. In this paper, a fire-new automatic MCS identification (AMI) method is used to capture four categories of MCSs with different sizes and shapes from numerical satellite infrared data. 47,468 MCSs are identified over Asia and western Pacific region during the warm season (May to October) from 1995 to 2008. Based on this database, MCS characteristics such as shape, size, duration, velocity, geographical distribution, intermonthly variation, and lifecycle are studied. Results indicate that the number of linear MCSs is 2.5 times that of circular MCSs. The former is of a larger size while the latter is of a longer duration. The 500 hPa steering flow plays an important role in the MCS movement. MCSs tend to move faster after they reach the maximum extent. Four categories of MCS have similar characteristics of geographical distribution and intermonthly variation. Basically, MCSs are zonally distributed, with three zones weakening from south to north. The intermonthly variation of MCSs is related to the seasonal adjustment of the large-scale circulation. As to the MCSs over China, they have different lifecycle characteristics over different areas. MCSs over plateaus and hill areas, with only one peak in their lifecycle curves, tend to form in the afternoon, mature at nightfall, and dissipate at night. On the other hand, MCSs over plains, which have several peaks in their lifecycle curves, may form either in the afternoon or at night, whereas MCSs over the oceans tend to form at midnight. Affected by the sea-land breeze circulation, MCSs over coastal areas of Guangdong and Guangxi always come into being at about 1500 or 1600 (local time), while MCSs over the Sichuan Basin, affected by the mountain-valley breeze circulation, generally initiate nocturnally.
2012, 18(4): 473-484.
Abstract:
The establishment of the South-Asian high (SAH) in April and May over the Indochina Peninsula (IP) is investigated based on the ERA-40 reanalysis data. The result shows that the SAH is generated and strengthened over the IP locally, rather than moving westward to the IP from the Western Pacific. After the SAH establishment the tropical upper tropospheric trough (TUTT) forms above the ocean to the east of the Philippines. We have found that the principal triggering factor of both the SAH construction and the TUTT formation is the variation in the Southern Asian atmospheric diabatic heating regime. In late April, both the climbing effect of Shan Plateau and the local surface sensible heating contribute to local rainfall over the IP. Then the local updraft and upper-air divergence are strengthened, being responsible for the SAH formed in the southern part of the IP. As convection moves northward along the Australian-Asian “maritime continent” and the Bay of Bengal (BoB) summer monsoon begins, the convection is intensified in May on the eastern BoB. The strong convection results in the SAH enhancing and expanding westward, accompanied by reinforced meridional flow to the east of SAH, where responses of the circulation to diabatic heating arrive at a quasi-steady state. Meanwhile, because of the positive geopotential vorticity advection resulting from upper equatorward flow, the local positive relative vorticity increases over the ocean to the east of the Philippines, making the tropical upper tropospheric trough (TUTT) form around 150°E.
The establishment of the South-Asian high (SAH) in April and May over the Indochina Peninsula (IP) is investigated based on the ERA-40 reanalysis data. The result shows that the SAH is generated and strengthened over the IP locally, rather than moving westward to the IP from the Western Pacific. After the SAH establishment the tropical upper tropospheric trough (TUTT) forms above the ocean to the east of the Philippines. We have found that the principal triggering factor of both the SAH construction and the TUTT formation is the variation in the Southern Asian atmospheric diabatic heating regime. In late April, both the climbing effect of Shan Plateau and the local surface sensible heating contribute to local rainfall over the IP. Then the local updraft and upper-air divergence are strengthened, being responsible for the SAH formed in the southern part of the IP. As convection moves northward along the Australian-Asian “maritime continent” and the Bay of Bengal (BoB) summer monsoon begins, the convection is intensified in May on the eastern BoB. The strong convection results in the SAH enhancing and expanding westward, accompanied by reinforced meridional flow to the east of SAH, where responses of the circulation to diabatic heating arrive at a quasi-steady state. Meanwhile, because of the positive geopotential vorticity advection resulting from upper equatorward flow, the local positive relative vorticity increases over the ocean to the east of the Philippines, making the tropical upper tropospheric trough (TUTT) form around 150°E.
2012, 18(4): 485-493.
Abstract:
With daily precipitation records at 586 stations in China for 1960-2004, this study investigates the spatio-temporal variation of the number of extreme wet days (NEWD) for each season in China and its relationship with SST anomalies and associated atmospheric circulation anomaly patterns, in which a threshold of extreme precipitation for a season and a station is defined as the value of the 90th percentile when the precipitation records for wet days during the season are ranked in an increasing order. Results show that there are significant increases of the NEWD along the Yangtze River valley during winter and summer, in North China during winter, in South China during spring, in Northeast China during winter and spring, and in Northwest China throughout the seasons, while there is a remarkable decrease in North China during summer. Besides the linear trend, the NEWD also exhibits considerable interannual and interdecadal variabilities. After eliminating the linear trend, the NEWD anomalies show distinct seasonal patterns. The NEWD anomalies are characterized by a “dipole” mode with opposite phases between northern and southern China in spring and autumn, a “tri-pole” mode with opposite phases between Yangtze River valley and southern and northern China in summer, and a “monopole” mode with the same phase over most of China in winter. The relationship of the NEWD anomalies in China with the SST anomalies in Indian and Pacific Oceans is found to be mainly dependent on the ENSO, and associated atmospheric circulation anomaly patterns for the ENSO’s impact on the NEWD in China are identified.
With daily precipitation records at 586 stations in China for 1960-2004, this study investigates the spatio-temporal variation of the number of extreme wet days (NEWD) for each season in China and its relationship with SST anomalies and associated atmospheric circulation anomaly patterns, in which a threshold of extreme precipitation for a season and a station is defined as the value of the 90th percentile when the precipitation records for wet days during the season are ranked in an increasing order. Results show that there are significant increases of the NEWD along the Yangtze River valley during winter and summer, in North China during winter, in South China during spring, in Northeast China during winter and spring, and in Northwest China throughout the seasons, while there is a remarkable decrease in North China during summer. Besides the linear trend, the NEWD also exhibits considerable interannual and interdecadal variabilities. After eliminating the linear trend, the NEWD anomalies show distinct seasonal patterns. The NEWD anomalies are characterized by a “dipole” mode with opposite phases between northern and southern China in spring and autumn, a “tri-pole” mode with opposite phases between Yangtze River valley and southern and northern China in summer, and a “monopole” mode with the same phase over most of China in winter. The relationship of the NEWD anomalies in China with the SST anomalies in Indian and Pacific Oceans is found to be mainly dependent on the ENSO, and associated atmospheric circulation anomaly patterns for the ENSO’s impact on the NEWD in China are identified.
2012, 18(4): 494-502.
Abstract:
With the ERA40 reanalysis daily data for 1958-2001, the global atmospheric seasonal-mean diabatic heating and transient heating are computed by using the residual diagnosis of the thermodynamic equation. The three-dimensional structures for the two types of heating are described and compared. It is demonstrated that the diabatic heating is basically characterized by strong and deep convective heating in the tropics, shallow heating in the midlatitudes and deep cooling in the subtropics and high-latitudes. The tropical diabatic heating always shifts towards the summer hemisphere, but the midlatitude heating and high-latitude cooling tend to be strong in the winter hemisphere. On the other hand, the transient heating due to transient eddy transfer is characterized by a meridional dipole pattern with cooling in the subtropics and heating in the mid- and high-latitudes, as well as by a vertical dipole pattern in the midlatitudes with cooling at lower levels and heating in the mid- and higher-levels, which gives rise to a sloped structure in the transient heating oriented from the lower levels in the high latitudes and higher levels in the midlatitudes. The transient heating is closely related to a storm track along which the transient eddy activity is much stronger in the winter hemisphere than in the summer hemisphere. In Northern Hemisphere, the transient heating locates in the western oceanic basin, while it is zonally-oriented in Southern Hemisphere, for which the transient heating and cooling are far separated over South Pacific during the cold season. The transient heating tends to cancel the diabatic heating over most of the globe. However, it dominates the mid-tropospheric heating in the midlatitudes. Therefore, the atmospheric transient processes act to help the atmosphere gain more heat in the high-latitudes and in the mid-troposphere of midlatitudes, reallocating the atmospheric heat obtained from the diabatic heating.
With the ERA40 reanalysis daily data for 1958-2001, the global atmospheric seasonal-mean diabatic heating and transient heating are computed by using the residual diagnosis of the thermodynamic equation. The three-dimensional structures for the two types of heating are described and compared. It is demonstrated that the diabatic heating is basically characterized by strong and deep convective heating in the tropics, shallow heating in the midlatitudes and deep cooling in the subtropics and high-latitudes. The tropical diabatic heating always shifts towards the summer hemisphere, but the midlatitude heating and high-latitude cooling tend to be strong in the winter hemisphere. On the other hand, the transient heating due to transient eddy transfer is characterized by a meridional dipole pattern with cooling in the subtropics and heating in the mid- and high-latitudes, as well as by a vertical dipole pattern in the midlatitudes with cooling at lower levels and heating in the mid- and higher-levels, which gives rise to a sloped structure in the transient heating oriented from the lower levels in the high latitudes and higher levels in the midlatitudes. The transient heating is closely related to a storm track along which the transient eddy activity is much stronger in the winter hemisphere than in the summer hemisphere. In Northern Hemisphere, the transient heating locates in the western oceanic basin, while it is zonally-oriented in Southern Hemisphere, for which the transient heating and cooling are far separated over South Pacific during the cold season. The transient heating tends to cancel the diabatic heating over most of the globe. However, it dominates the mid-tropospheric heating in the midlatitudes. Therefore, the atmospheric transient processes act to help the atmosphere gain more heat in the high-latitudes and in the mid-troposphere of midlatitudes, reallocating the atmospheric heat obtained from the diabatic heating.
2012, 18(4): 503-511.
Abstract:
Temporal and spatial evolution characteristics of the 30-60 day oscillation (intraseasonal oscillation, ISO) of summer rainfall in China and the effects of East Asian monsoon on the rainfall ISO are analyzed in this paper. Results show that the annual and decadal variations of the oscillation exist between 1960 and 2008, and the intensity is weakest in the late 1970s and early 1980s. In the typical strong years of the rainfall ISO obtained from empirical orthogonal functions (EOF mode 1), an anticyclone is in northwestern Pacific and a cyclone is in the east of China. In the typical weak years, the wind ISO is much weaker. The low-frequency zonal wind and water vapor transport from the low latitudes to mid-latitudes in the typical strong years, and the oscillation strength of diabatic heating is much stronger than that in the weak years of the rainfall ISO. The anomaly characteristics of the rainfall ISO show anti-phases between the Yangtze River basin and south of China. As for the typical strong years of the rainfall ISO in the Yangtze River basin (EOF mode 2), the main oscillation center of water vapor is in the east of China (20-30°N, 110-130°E). In the peak (break) phase of the rainfall oscillation, a low-frequency cyclone (anticyclone) is in the Yangtze River basin and an anticyclone (cyclone) is near Taiwan Island. In addition, the peak rainfall corresponds to the heat source in the Yangtze River basin and the heat sink in the Qinghai-Tibet Plateau. As for the typical strong years of the rainfall ISO in the south of China, the main oscillation center of water vapor is south of 20°N. In the peak (break) phase of the rainfall ISO, a low-frequency cyclone (anticyclone) is in the south of China and an anticyclone (cyclone) is in the Philippines. The peak rainfall corresponds to the heat source in the south of China and the South China Sea, and the heat sink in the west of Indochina.
Temporal and spatial evolution characteristics of the 30-60 day oscillation (intraseasonal oscillation, ISO) of summer rainfall in China and the effects of East Asian monsoon on the rainfall ISO are analyzed in this paper. Results show that the annual and decadal variations of the oscillation exist between 1960 and 2008, and the intensity is weakest in the late 1970s and early 1980s. In the typical strong years of the rainfall ISO obtained from empirical orthogonal functions (EOF mode 1), an anticyclone is in northwestern Pacific and a cyclone is in the east of China. In the typical weak years, the wind ISO is much weaker. The low-frequency zonal wind and water vapor transport from the low latitudes to mid-latitudes in the typical strong years, and the oscillation strength of diabatic heating is much stronger than that in the weak years of the rainfall ISO. The anomaly characteristics of the rainfall ISO show anti-phases between the Yangtze River basin and south of China. As for the typical strong years of the rainfall ISO in the Yangtze River basin (EOF mode 2), the main oscillation center of water vapor is in the east of China (20-30°N, 110-130°E). In the peak (break) phase of the rainfall oscillation, a low-frequency cyclone (anticyclone) is in the Yangtze River basin and an anticyclone (cyclone) is near Taiwan Island. In addition, the peak rainfall corresponds to the heat source in the Yangtze River basin and the heat sink in the Qinghai-Tibet Plateau. As for the typical strong years of the rainfall ISO in the south of China, the main oscillation center of water vapor is south of 20°N. In the peak (break) phase of the rainfall ISO, a low-frequency cyclone (anticyclone) is in the south of China and an anticyclone (cyclone) is in the Philippines. The peak rainfall corresponds to the heat source in the south of China and the South China Sea, and the heat sink in the west of Indochina.
2012, 18(4): 512-520.
Abstract:
Based on analyses of the relationship between Pacific Meridional Mode (PMM) and number of tropical cyclones (TCs) activity over the western North Pacific, the impacts of the PMM on Tc activity over the western North Pacific are studied using numerical simulations with an Atmospheric General Circulation Model (CAM3) of National Center for Atmospheric Research (of USA). The result shows that the PMM has impacts on the large-scale generating environment of TCs, thus affecting their number and strength. The numerical simulations using the NCAR CAM3 indicate that with the inclusion of the forcing from sea surface temperature (SST) of the PMM, there appears a decreased magnitude of the vertical zonal wind shear, large proportion of relative humidity, anomalous westerly wind at low levels and anomalous easterly wind at high levels, in association with anomalous cyclonic circulation at low levels and anomalous anti-cyclonic circulation at high levels over the tropical western Pacific. Thus, the PMM provides favorable environment for the typhoon genesis. In the sensitivity experiment, TCs have larger strength, lower SST at the center, stronger tangential wind at 850 hPa and intensified warm cores at high levels. In this paper, the simulation results are similar to that in the data analyses, which reveals the important impact of the PMM on TC activity over the western North Pacific.
Based on analyses of the relationship between Pacific Meridional Mode (PMM) and number of tropical cyclones (TCs) activity over the western North Pacific, the impacts of the PMM on Tc activity over the western North Pacific are studied using numerical simulations with an Atmospheric General Circulation Model (CAM3) of National Center for Atmospheric Research (of USA). The result shows that the PMM has impacts on the large-scale generating environment of TCs, thus affecting their number and strength. The numerical simulations using the NCAR CAM3 indicate that with the inclusion of the forcing from sea surface temperature (SST) of the PMM, there appears a decreased magnitude of the vertical zonal wind shear, large proportion of relative humidity, anomalous westerly wind at low levels and anomalous easterly wind at high levels, in association with anomalous cyclonic circulation at low levels and anomalous anti-cyclonic circulation at high levels over the tropical western Pacific. Thus, the PMM provides favorable environment for the typhoon genesis. In the sensitivity experiment, TCs have larger strength, lower SST at the center, stronger tangential wind at 850 hPa and intensified warm cores at high levels. In this paper, the simulation results are similar to that in the data analyses, which reveals the important impact of the PMM on TC activity over the western North Pacific.
2012, 18(4): 521-527.
Abstract:
In this article, a new definition for the North Huaihe River rainy season (NHRS) is presented using summer daily precipitation in East China and subtropical high ridge axis at 500 hPa. By calculating the annual precipitation amounts in the NHRS and Meiyu of the Yangtze-Huaihe Rivers basin (YHMY) from 1961 to 2009, the dates of precipitation beginning and ending as well as the duration of the two rainy seasons in the 49 years are analyzed. Atmospheric circulation characteristics in positive and negative precipitation anomaly years during the NHRS are also studied. Results are shown as follows. (1) The new definition for the NHRS is much easier to use. It involves only two meteorological factors, making its application more practical. It can also distinguish two rainy periods of the NHRS more objectively. (2) The average duration of the NHRS is similar to that of the YHMY, except that its average dates of beginning and ending are about one week later than those of the YHMY. The average precipitation of the NHRS is slightly less than that of the YHMY, and the yearly precipitation variation of the two rainy seasons are similar to each other with no obvious increasing or decreasing trend in the 49 years, but with distinguished decadal and inter-annual variations. (3) In positive precipitation anomaly years, the South Asian high moves more northward and more eastward, the western Pacific subtropical high is located more northward and westward, and the summer monsoon is stronger than normal, resulting in the convergence of the warm and moist southwesterly airflow from the west side of the subtropical high and the cold air from the north side of the northeast trough in North Huaihe River basin.
In this article, a new definition for the North Huaihe River rainy season (NHRS) is presented using summer daily precipitation in East China and subtropical high ridge axis at 500 hPa. By calculating the annual precipitation amounts in the NHRS and Meiyu of the Yangtze-Huaihe Rivers basin (YHMY) from 1961 to 2009, the dates of precipitation beginning and ending as well as the duration of the two rainy seasons in the 49 years are analyzed. Atmospheric circulation characteristics in positive and negative precipitation anomaly years during the NHRS are also studied. Results are shown as follows. (1) The new definition for the NHRS is much easier to use. It involves only two meteorological factors, making its application more practical. It can also distinguish two rainy periods of the NHRS more objectively. (2) The average duration of the NHRS is similar to that of the YHMY, except that its average dates of beginning and ending are about one week later than those of the YHMY. The average precipitation of the NHRS is slightly less than that of the YHMY, and the yearly precipitation variation of the two rainy seasons are similar to each other with no obvious increasing or decreasing trend in the 49 years, but with distinguished decadal and inter-annual variations. (3) In positive precipitation anomaly years, the South Asian high moves more northward and more eastward, the western Pacific subtropical high is located more northward and westward, and the summer monsoon is stronger than normal, resulting in the convergence of the warm and moist southwesterly airflow from the west side of the subtropical high and the cold air from the north side of the northeast trough in North Huaihe River basin.
2012, 18(4): 528-536.
Abstract:
By using NCEP/NCAR daily reanalysis data and daily precipitation data of 740 stations in China, relationships between the position variation of the West Pacific subtropical high (WPSH) and the diabatic heating during persistent and intense rains in the Yangtze-Huaihe Rivers basin are studied. The results show that the position variation of WPSH is closely associated with the diabatic heating. There are strong apparent heating sources and moisture sinks in both the basin (to the north of WPSH) and the north of Bay of Bengal (to the west of WPSH) during persistent and intense rain events. In the basin, Q1z begins to increase 3 days ahead of intense rainfall, maximizes 2 days later and then reduces gradually, but it changes little after precipitation ends, thus preventing the WPSH from moving northward. In the north of Bay of Bengal, 2 days ahead of strong rainfall over the basin, Q1z starts to increase and peaks 1 day after the rain occurs, leading to the westward extension of WPSH. Afterwards, Q1z begins declining and the WPSH makes its eastward retreat accordingly. Based on the complete vertical vorticity equation, in mid-troposphere, the vertical variation of heating in the basin is favorable to the increase of cyclonic vorticity north of WPSH, which counteracts the northward movement of WPSH and favors the persistence of rainbands over the basin. The vertical variation of heating in the north of Bay of Bengal is in favor of the increase of anti-cyclonic vorticity to the west of WPSH, which induces the westward extension of WPSH.
By using NCEP/NCAR daily reanalysis data and daily precipitation data of 740 stations in China, relationships between the position variation of the West Pacific subtropical high (WPSH) and the diabatic heating during persistent and intense rains in the Yangtze-Huaihe Rivers basin are studied. The results show that the position variation of WPSH is closely associated with the diabatic heating. There are strong apparent heating sources and moisture sinks in both the basin (to the north of WPSH) and the north of Bay of Bengal (to the west of WPSH) during persistent and intense rain events. In the basin, Q1z begins to increase 3 days ahead of intense rainfall, maximizes 2 days later and then reduces gradually, but it changes little after precipitation ends, thus preventing the WPSH from moving northward. In the north of Bay of Bengal, 2 days ahead of strong rainfall over the basin, Q1z starts to increase and peaks 1 day after the rain occurs, leading to the westward extension of WPSH. Afterwards, Q1z begins declining and the WPSH makes its eastward retreat accordingly. Based on the complete vertical vorticity equation, in mid-troposphere, the vertical variation of heating in the basin is favorable to the increase of cyclonic vorticity north of WPSH, which counteracts the northward movement of WPSH and favors the persistence of rainbands over the basin. The vertical variation of heating in the north of Bay of Bengal is in favor of the increase of anti-cyclonic vorticity to the west of WPSH, which induces the westward extension of WPSH.
2012, 18(4): 537-542.
Abstract:
The number of tropical cyclone (TC) genesis over the South China Sea and the Northwest Pacific Ocean in 2009 is significantly less than the average (27.4). However, the number of landfall TC over mainland China and its associated rainfall is more than the average. This paper focuses on the performance of numerical weather prediction (NWP) of landfall TC precipitation over China in 2009. The China Meteorological Administration (CMA) and Japan Meteorological Agency (JMA) models are compared. Although the schemes of physical processes, the data assimilation system and the dynamic frame are entirely different for the two models, the results of forecast verification are similar to each other for TC rainfall and track except for TC Goni. In this paper, a day with daily rainfall amount greater than 50 mm was selected as a storm rain day when there was a TC affecting the mainland. There are 32 storm rain days related to the landing of typhoons and tropical depressions. The rainfall forecast verification methods of National Meteorological Centre (NMC) of CMA are selected to verify the models’ rainfall forecast. Observational precipitation analyses related to TCs in 2009 indicate a U-shape spatial distribution in China. It is found that the rain belt forecasted by the two models within 60 hours shows good agreement with observations, both in the location and the maximum rainfall center. Beyond 3 days, the forecasted rainfall belt shifts northward on average, and the rainfall amount of the model forecasts becomes under-predicted. The rainfall intensity of CMA model forecast is more reasonable than that of JMA model. For heavy rain, the JMA model made more missing forecasts. The TC rainfall is verified in Guangdong, Guangxi, Fujian and Hainan where rainfall amount related to TCs is relatively larger than in other regions. The results indicate that the model forecast for Guangdong and Guangxi is more skillful than that for Hainan. The rainfall forecast for Hainan remains difficult for the models because of insufficient observation data and special tropical ocean climate.
The number of tropical cyclone (TC) genesis over the South China Sea and the Northwest Pacific Ocean in 2009 is significantly less than the average (27.4). However, the number of landfall TC over mainland China and its associated rainfall is more than the average. This paper focuses on the performance of numerical weather prediction (NWP) of landfall TC precipitation over China in 2009. The China Meteorological Administration (CMA) and Japan Meteorological Agency (JMA) models are compared. Although the schemes of physical processes, the data assimilation system and the dynamic frame are entirely different for the two models, the results of forecast verification are similar to each other for TC rainfall and track except for TC Goni. In this paper, a day with daily rainfall amount greater than 50 mm was selected as a storm rain day when there was a TC affecting the mainland. There are 32 storm rain days related to the landing of typhoons and tropical depressions. The rainfall forecast verification methods of National Meteorological Centre (NMC) of CMA are selected to verify the models’ rainfall forecast. Observational precipitation analyses related to TCs in 2009 indicate a U-shape spatial distribution in China. It is found that the rain belt forecasted by the two models within 60 hours shows good agreement with observations, both in the location and the maximum rainfall center. Beyond 3 days, the forecasted rainfall belt shifts northward on average, and the rainfall amount of the model forecasts becomes under-predicted. The rainfall intensity of CMA model forecast is more reasonable than that of JMA model. For heavy rain, the JMA model made more missing forecasts. The TC rainfall is verified in Guangdong, Guangxi, Fujian and Hainan where rainfall amount related to TCs is relatively larger than in other regions. The results indicate that the model forecast for Guangdong and Guangxi is more skillful than that for Hainan. The rainfall forecast for Hainan remains difficult for the models because of insufficient observation data and special tropical ocean climate.
2012, 18(4): 543-550.
Abstract:
The spring (March-April-May) rainfall over northern China (SPRNC) is predicted by using the interannual increment approach. DY denotes the difference between the current year and previous years. The seasonal forecast model for the DY of SPRNC is constructed based on the data that are taken from the 1965-2002 period (38 years), in which six predictors are available no later than the current month of February. This is favorable so that the seasonal forecasts can be made one month ahead. Then, SPRNC and the percentage anomaly of SPRNC are obtained by the predicted DY of SPRNC. The model performs well in the prediction of the inter-annual variation of the DY of SPRNC during 1965-2002, with a correlation coefficient between the predicted and observed DY of SPRNC of 0.87. This accounts for 76% of the total variance, with a low value for the average root mean square error (RMSE) of 20%. Both the results of the hindcast for the period of 2003-2010 (eight years) and the cross-validation test for the period of 1965-2009 (45 years) illustrate the good prediction capability of the model, with a small mean relative error of 10%, an RMSE of 17% and a high rate of coherence of 87.5% for the hindcasts of the percentage anomaly of SPRNC.
The spring (March-April-May) rainfall over northern China (SPRNC) is predicted by using the interannual increment approach. DY denotes the difference between the current year and previous years. The seasonal forecast model for the DY of SPRNC is constructed based on the data that are taken from the 1965-2002 period (38 years), in which six predictors are available no later than the current month of February. This is favorable so that the seasonal forecasts can be made one month ahead. Then, SPRNC and the percentage anomaly of SPRNC are obtained by the predicted DY of SPRNC. The model performs well in the prediction of the inter-annual variation of the DY of SPRNC during 1965-2002, with a correlation coefficient between the predicted and observed DY of SPRNC of 0.87. This accounts for 76% of the total variance, with a low value for the average root mean square error (RMSE) of 20%. Both the results of the hindcast for the period of 2003-2010 (eight years) and the cross-validation test for the period of 1965-2009 (45 years) illustrate the good prediction capability of the model, with a small mean relative error of 10%, an RMSE of 17% and a high rate of coherence of 87.5% for the hindcasts of the percentage anomaly of SPRNC.
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