2016 Vol. 22, No. 2
2016, 22(2): 109-117.
doi: 10.16555/j.1006-8775.2016.02.001
Abstract:
This study focuses on the decadal variability of tropical cyclones (TC) over the Western North Pacific (WNP) and how these changes are related to the Madden–CJulian Oscillation (MJO). It was done with the help of the Real-time Multivariate MJO index from the Australian Government Bureau of Meteorology of the Centre for Australian Weather and Climate Research, TC data from the Joint Typhoon Warming Center best track datasets, and daily and monthly datasets from the NCEP/NCAR reanalysis center. The results show that the TC frequency in the WNP exhibited a statistically significant decrease during 1998–C2010 compared to during 1979–C1997. The decrease in TC frequency in the WNP mainly occurred during MJO active phases (i.e., phases 4, 5, 6, and 7). Further investigation of the climate background and the propagation differences of the MJO between 1979–C1997 and 1998–C2010 was performed. The La Niña-like tropical sea surface temperature cooling caused stronger Walker circulation and thus induced unfavorable atmosphere conditions for WNP TC genesis including a low-level easterly anomaly, a negative relative vorticity anomaly, an increase in sea-level pressure, and stronger vertical wind shear. Moreover, shortening of the MJO cycle, decline in the duration of the active phases in the WNP, and easterly anomaly and shrinkage of the convection area during MJO active phases may also partly explain the decadal variation of TC.
This study focuses on the decadal variability of tropical cyclones (TC) over the Western North Pacific (WNP) and how these changes are related to the Madden–CJulian Oscillation (MJO). It was done with the help of the Real-time Multivariate MJO index from the Australian Government Bureau of Meteorology of the Centre for Australian Weather and Climate Research, TC data from the Joint Typhoon Warming Center best track datasets, and daily and monthly datasets from the NCEP/NCAR reanalysis center. The results show that the TC frequency in the WNP exhibited a statistically significant decrease during 1998–C2010 compared to during 1979–C1997. The decrease in TC frequency in the WNP mainly occurred during MJO active phases (i.e., phases 4, 5, 6, and 7). Further investigation of the climate background and the propagation differences of the MJO between 1979–C1997 and 1998–C2010 was performed. The La Niña-like tropical sea surface temperature cooling caused stronger Walker circulation and thus induced unfavorable atmosphere conditions for WNP TC genesis including a low-level easterly anomaly, a negative relative vorticity anomaly, an increase in sea-level pressure, and stronger vertical wind shear. Moreover, shortening of the MJO cycle, decline in the duration of the active phases in the WNP, and easterly anomaly and shrinkage of the convection area during MJO active phases may also partly explain the decadal variation of TC.
2016, 22(2): 118-126.
doi: 10.16555/j.1006-8775.2016.02.002
Abstract:
Combined with TRMM products and Tropical Cyclone (TC) best track data in Northwest Pacific from 1 January 2003 to 31 December 2009, a total of 118 TCs, including 336 instantaneous TC precipitation observations are established as the TRMM TC database, and the database is stratified into four intensity classes according to the standard of TC intensity adopted by China Meteorological Administration (CMA): Severe Tropical Storm (STS), Typhoon (TY), Severe Typhoon (STY) and Super Typhoon (SuperTY). For each TC snapshot, the mean rainfall distribution is computed using 10-km annuli from the TC center to a 300-km radius, then the axisymmetric component of TC rainfall is represented by the radial distribution of the azimuthal mean rain rate; the mean rain rates, rain types occurrence and contribution proportion are computed for each TC intensity class; and the mean quadrantal distribution of rain rates along TCs motion is analyzed. The result shows that: (1) TCs mean rain rates increase with their intensity classes, and their radial distributions show single-peak characteristic gradually, and furthermore, the characteristics of rain rates occurrence and contribution proportion change from dual-peak to single-peak distribution, with the peak rain rate at about 5.0 mm/h; (2) Stratiform rain dominate the rain type in the analysis zone, while convective rain mainly occurred in the eye-wall region; (3) The values of mean rain rate in each quadrant along TCs motion are close to each other, relatively, the value in the right-rear quadrant is the smallest one.
Combined with TRMM products and Tropical Cyclone (TC) best track data in Northwest Pacific from 1 January 2003 to 31 December 2009, a total of 118 TCs, including 336 instantaneous TC precipitation observations are established as the TRMM TC database, and the database is stratified into four intensity classes according to the standard of TC intensity adopted by China Meteorological Administration (CMA): Severe Tropical Storm (STS), Typhoon (TY), Severe Typhoon (STY) and Super Typhoon (SuperTY). For each TC snapshot, the mean rainfall distribution is computed using 10-km annuli from the TC center to a 300-km radius, then the axisymmetric component of TC rainfall is represented by the radial distribution of the azimuthal mean rain rate; the mean rain rates, rain types occurrence and contribution proportion are computed for each TC intensity class; and the mean quadrantal distribution of rain rates along TCs motion is analyzed. The result shows that: (1) TCs mean rain rates increase with their intensity classes, and their radial distributions show single-peak characteristic gradually, and furthermore, the characteristics of rain rates occurrence and contribution proportion change from dual-peak to single-peak distribution, with the peak rain rate at about 5.0 mm/h; (2) Stratiform rain dominate the rain type in the analysis zone, while convective rain mainly occurred in the eye-wall region; (3) The values of mean rain rate in each quadrant along TCs motion are close to each other, relatively, the value in the right-rear quadrant is the smallest one.
2016, 22(2): 127-135.
doi: 10.16555/j.1006-8775.2016.02.003
Abstract:
Based on the Joint Typhoon Warning Center (JTWC) best-track dataset between 1965 and 2009 and the characteristic parameters including tropical cyclone (TC) position, intensity, path length and direction, a method for objective classification of the Northwestern Pacific tropical cyclone tracks is established by using K-means Clustering. The TC lifespan, energy, active season and landfall probability of seven clusters of tropical cyclone tracks are comparatively analyzed. The characteristics of these parameters are quite different among different tropical cyclone track clusters. From the trend of the past two decades, the frequency of the western recurving cluster (accounting for 21.3% of the total) increased, and the lifespan elongated slightly, which differs from the other clusters. The annual variation of the Power Dissipation Index (PDI) of most clusters mainly depended on the TC intensity and frequency. However, the annual variation of the PDI in the northwestern moving then recurving cluster and the pelagic west-northwest moving cluster mainly depended on the frequency.
Based on the Joint Typhoon Warning Center (JTWC) best-track dataset between 1965 and 2009 and the characteristic parameters including tropical cyclone (TC) position, intensity, path length and direction, a method for objective classification of the Northwestern Pacific tropical cyclone tracks is established by using K-means Clustering. The TC lifespan, energy, active season and landfall probability of seven clusters of tropical cyclone tracks are comparatively analyzed. The characteristics of these parameters are quite different among different tropical cyclone track clusters. From the trend of the past two decades, the frequency of the western recurving cluster (accounting for 21.3% of the total) increased, and the lifespan elongated slightly, which differs from the other clusters. The annual variation of the Power Dissipation Index (PDI) of most clusters mainly depended on the TC intensity and frequency. However, the annual variation of the PDI in the northwestern moving then recurving cluster and the pelagic west-northwest moving cluster mainly depended on the frequency.
2016, 22(2): 136-144.
doi: 10.16555/j.1006-8775.2016.02.004
Abstract:
The timing of the South Asian High (SAH) establishment over the Indochina Peninsula (IP) from April to May and its relations to the setup of the subsequent tropical Asian summer monsoon and precipitation over eastern-central China in summer are investigated by using NCEP/NCAR daily reanalysis data, outgoing longwave radiation (OLR) data and the daily precipitation data from 753 weather stations in China. It is found that the transitions of the zonal wind vertical shear and convection establishment over tropical Asia are earlier (later) in the years of early (late) establishment of SAH. In the lower troposphere, anti-cyclonic (cyclonic) anomaly circulation dominates the equatorial Indian Ocean. Correspondingly, the tropical Asian summer monsoon establishes earlier (later). Furthermore, the atmospheric circulation and the water vapor transport in the years of advanced SAH establishment are significantly different from the delayed years in Asia in summer. Out-of-phase distribution of precipitation in eastern-central China will appear with a weak (strong) SAH and western Pacific subtropical high, strong (weak) ascending motion in the area south of Yangtze River but weak (strong) ascending motion in the area north of it, and cyclonic (anti-cyclonic) water vapor flux anomaly circulation from the eastern-central China to western Pacific. Accordingly, the timing of the SAH establishment at the upper levels of IP is indicative of the subsequent onset of the tropical Asian summer monsoon and the flood-drought pattern over eastern-central China in summer.
The timing of the South Asian High (SAH) establishment over the Indochina Peninsula (IP) from April to May and its relations to the setup of the subsequent tropical Asian summer monsoon and precipitation over eastern-central China in summer are investigated by using NCEP/NCAR daily reanalysis data, outgoing longwave radiation (OLR) data and the daily precipitation data from 753 weather stations in China. It is found that the transitions of the zonal wind vertical shear and convection establishment over tropical Asia are earlier (later) in the years of early (late) establishment of SAH. In the lower troposphere, anti-cyclonic (cyclonic) anomaly circulation dominates the equatorial Indian Ocean. Correspondingly, the tropical Asian summer monsoon establishes earlier (later). Furthermore, the atmospheric circulation and the water vapor transport in the years of advanced SAH establishment are significantly different from the delayed years in Asia in summer. Out-of-phase distribution of precipitation in eastern-central China will appear with a weak (strong) SAH and western Pacific subtropical high, strong (weak) ascending motion in the area south of Yangtze River but weak (strong) ascending motion in the area north of it, and cyclonic (anti-cyclonic) water vapor flux anomaly circulation from the eastern-central China to western Pacific. Accordingly, the timing of the SAH establishment at the upper levels of IP is indicative of the subsequent onset of the tropical Asian summer monsoon and the flood-drought pattern over eastern-central China in summer.
2016, 22(2): 145-158.
doi: 10.16555/j.1006-8775.2016.02.005
Abstract:
The impact of strong (weak) intraseasonal oscillation (ISO) over South China Sea (SCS) and South Asia (SA) in summer on the SCS and SA summer monsoon and the summer rainfall in Eastern China are studied by using the NCEP-NCAR analysis data and the rainfall data of 160 stations in China from 1961 to 2010. It is found that the impacts are significantly different in different months of summer. The study shows that in June and July cyclonic (anticyclonic) atmospheric circulation over SCS and SA corresponds to strong (weak) ISO over SCS. In August, however, strong (weak) ISO over SCS still corresponds to cyclonic (anticyclonic) atmospheric circulation over SA. In June and August cyclonic (anticyclonic) atmospheric circulation over South Asia corresponds to strong (weak) ISO over SA while a strong (weak) ISO corresponds to anticyclonic (cyclonic) atmospheric circulation over SA in July. Besides, in June the strong (weak) ISO over SA corresponds to cyclonic (anticyclonic) atmospheric circulation over SCS, while in July and August the atmospheric circulation is in the same phase regardless of whether the ISO over SA is strong or weak. The impacts of the strong（weak）ISO over SCS on the rainfall of eastern China are similar in June and July, which favors less (more) rainfall in Yangtze-Huaihe Rivers basin but sufficient (deficient) rainfall in the south of Yangtze River. However, the impacts are not so apparent in August. In South Asia, the strong (weak) ISO in July results in less (more) rainfall in the south of Yangtze River but sufficient (deficient) rainfall in Yangtze-Huaihe Rivers basin. The influence on the rainfall in eastern China in June and August is not as significant as in July.
The impact of strong (weak) intraseasonal oscillation (ISO) over South China Sea (SCS) and South Asia (SA) in summer on the SCS and SA summer monsoon and the summer rainfall in Eastern China are studied by using the NCEP-NCAR analysis data and the rainfall data of 160 stations in China from 1961 to 2010. It is found that the impacts are significantly different in different months of summer. The study shows that in June and July cyclonic (anticyclonic) atmospheric circulation over SCS and SA corresponds to strong (weak) ISO over SCS. In August, however, strong (weak) ISO over SCS still corresponds to cyclonic (anticyclonic) atmospheric circulation over SA. In June and August cyclonic (anticyclonic) atmospheric circulation over South Asia corresponds to strong (weak) ISO over SA while a strong (weak) ISO corresponds to anticyclonic (cyclonic) atmospheric circulation over SA in July. Besides, in June the strong (weak) ISO over SA corresponds to cyclonic (anticyclonic) atmospheric circulation over SCS, while in July and August the atmospheric circulation is in the same phase regardless of whether the ISO over SA is strong or weak. The impacts of the strong（weak）ISO over SCS on the rainfall of eastern China are similar in June and July, which favors less (more) rainfall in Yangtze-Huaihe Rivers basin but sufficient (deficient) rainfall in the south of Yangtze River. However, the impacts are not so apparent in August. In South Asia, the strong (weak) ISO in July results in less (more) rainfall in the south of Yangtze River but sufficient (deficient) rainfall in Yangtze-Huaihe Rivers basin. The influence on the rainfall in eastern China in June and August is not as significant as in July.
2016, 22(2): 159-171.
doi: 10.16555/j.1006-8775.2016.02.006
Abstract:
This study aims to explore the relative role of oceanic dynamics and surface heat fluxes in the warming of southern Arabian Sea and southwest Indian Ocean during the development of Indian Ocean Dipole (IOD) events by using National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) daily reanalysis data and Global Ocean Data Assimilation System (GODAS) monthly mean ocean reanalysis data from 1982 to 2013, based on regression analysis, Empirical Orthogonal Function (EOF) analysis and combined with a 2? layer dynamic upper-ocean model. The results show that during the initial stage of IOD events, warm downwelling Rossby waves excited by an anomalous anticyclone over the west Indian Peninsula, southwest Indian Ocean and southeast Indian Ocean lead to the warming of the mixed layer by reducing entrainment cooling. An anomalous anticyclone over the west Indian Peninsula weakens the wind over the Arabian Sea and Somali coast, which helps decrease the sea surface heat loss and shallow the surface mixed layer, and also contributes to the sea surface temperature (SST) warming in the southern Arabian Sea by inhibiting entrainment. The weakened winds increase the SST along the Somali coast by inhibiting upwelling and zonal advection. The wind and net sea surface heat flux anomalies are not significant over the southwest Indian Ocean. During the antecedent stage of IOD events, the warming of the southern Arabian Sea is closely connected with the reduction of entrainment cooling caused by the Rossby waves and the weakened wind. With the appearance of an equatorial easterly wind anomaly, the warming of the southwest Indian Ocean is not only driven by weaker entrainment cooling caused by the Rossby waves, but also by the meridional heat transport carried by Ekman flow. The anomalous sea surface heat flux plays a key role to damp the warming of the west pole of the IOD.
This study aims to explore the relative role of oceanic dynamics and surface heat fluxes in the warming of southern Arabian Sea and southwest Indian Ocean during the development of Indian Ocean Dipole (IOD) events by using National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) daily reanalysis data and Global Ocean Data Assimilation System (GODAS) monthly mean ocean reanalysis data from 1982 to 2013, based on regression analysis, Empirical Orthogonal Function (EOF) analysis and combined with a 2? layer dynamic upper-ocean model. The results show that during the initial stage of IOD events, warm downwelling Rossby waves excited by an anomalous anticyclone over the west Indian Peninsula, southwest Indian Ocean and southeast Indian Ocean lead to the warming of the mixed layer by reducing entrainment cooling. An anomalous anticyclone over the west Indian Peninsula weakens the wind over the Arabian Sea and Somali coast, which helps decrease the sea surface heat loss and shallow the surface mixed layer, and also contributes to the sea surface temperature (SST) warming in the southern Arabian Sea by inhibiting entrainment. The weakened winds increase the SST along the Somali coast by inhibiting upwelling and zonal advection. The wind and net sea surface heat flux anomalies are not significant over the southwest Indian Ocean. During the antecedent stage of IOD events, the warming of the southern Arabian Sea is closely connected with the reduction of entrainment cooling caused by the Rossby waves and the weakened wind. With the appearance of an equatorial easterly wind anomaly, the warming of the southwest Indian Ocean is not only driven by weaker entrainment cooling caused by the Rossby waves, but also by the meridional heat transport carried by Ekman flow. The anomalous sea surface heat flux plays a key role to damp the warming of the west pole of the IOD.
2016, 22(2): 172-181.
doi: 10.16555/j.1006-8775.2016.02.007
Abstract:
After compositing three representative ENSO indices, El Niño events have been divided into an eastern pattern (EP) and a central pattern (CP). By using EOF, correlation and composite analysis, the relationship and possible mechanisms between Indian Ocean Dipole (IOD) and two types of El Niño were investigated. IOD events, originating from Indo-Pacific scale air-sea interaction, are composed of two modes, which are associated with EP and CP El Niño respectively. The IOD mode related to EP El Niño events (named as IOD1) is strongest at the depth of 50 to 150 m along the equatorial Indian Ocean. Besides, it shows a quasi-symmetric distribution, stronger in the south of the Equator. The IOD mode associated with CP El Niño (named as IOD2) has strongest signal in tropical southern Indian Ocean surface. In terms of mechanisms, before EP El Niño peaks, anomalous Walker circulation produces strong anomalous easterlies in equatorial Indian Ocean, resulting in upwelling in the east, decreasing sea temperature there; a couple of anomalous anticyclones (stronger in the south) form off the Equator where warm water accumulates, and thus the IOD1 occurs. When CP El Niño develops, anomalous Walker circulation is weaker and shifts its center to the west, therefore anomalous easterlies in equatorial Indian Ocean is less strong. Besides, the anticyclone south of Sumatra strengthens, and the southerlies east of it bring cold water from higher latitudes and northerlies west of it bring warm water from lower latitudes to the 15° to 25°S zone. Meanwhile, there exists strong divergence in the east and convergence in the west part of tropical southern Indian Ocean, making sea temperature fall and rise separately. Therefore, IOD2 lies farther south.
After compositing three representative ENSO indices, El Niño events have been divided into an eastern pattern (EP) and a central pattern (CP). By using EOF, correlation and composite analysis, the relationship and possible mechanisms between Indian Ocean Dipole (IOD) and two types of El Niño were investigated. IOD events, originating from Indo-Pacific scale air-sea interaction, are composed of two modes, which are associated with EP and CP El Niño respectively. The IOD mode related to EP El Niño events (named as IOD1) is strongest at the depth of 50 to 150 m along the equatorial Indian Ocean. Besides, it shows a quasi-symmetric distribution, stronger in the south of the Equator. The IOD mode associated with CP El Niño (named as IOD2) has strongest signal in tropical southern Indian Ocean surface. In terms of mechanisms, before EP El Niño peaks, anomalous Walker circulation produces strong anomalous easterlies in equatorial Indian Ocean, resulting in upwelling in the east, decreasing sea temperature there; a couple of anomalous anticyclones (stronger in the south) form off the Equator where warm water accumulates, and thus the IOD1 occurs. When CP El Niño develops, anomalous Walker circulation is weaker and shifts its center to the west, therefore anomalous easterlies in equatorial Indian Ocean is less strong. Besides, the anticyclone south of Sumatra strengthens, and the southerlies east of it bring cold water from higher latitudes and northerlies west of it bring warm water from lower latitudes to the 15° to 25°S zone. Meanwhile, there exists strong divergence in the east and convergence in the west part of tropical southern Indian Ocean, making sea temperature fall and rise separately. Therefore, IOD2 lies farther south.
2016, 22(2): 182-190.
doi: 10.16555/j.1006-8775.2016.02.008
Abstract:
A new scheme that separates convective-stratiform rainfall is developed using threshold values of liquid water path (LWP) and ice water path (IWP). These cloud contents can be predicted with radiances at the Advanced Microwave Sounding Unit (AMSU) channels (23.8, 31.4, 89, and 150 GHz) through linear regression models. The scheme is demonstrated by an analysis of a two-dimensional cloud resolving model simulation that is imposed by a forcing derived from the Tropical Ocean Global Atmosphere Coupled Ocean–CAtmosphere Response Experiment (TOGA COARE). The rainfall is considered convective if associated LWP is larger than 1.91 mm or IWP is larger than 1.70 mm. Otherwise, the rainfall is stratiform. The analysis of surface rainfall budget demonstrates that this new scheme is physically meaningful.
A new scheme that separates convective-stratiform rainfall is developed using threshold values of liquid water path (LWP) and ice water path (IWP). These cloud contents can be predicted with radiances at the Advanced Microwave Sounding Unit (AMSU) channels (23.8, 31.4, 89, and 150 GHz) through linear regression models. The scheme is demonstrated by an analysis of a two-dimensional cloud resolving model simulation that is imposed by a forcing derived from the Tropical Ocean Global Atmosphere Coupled Ocean–CAtmosphere Response Experiment (TOGA COARE). The rainfall is considered convective if associated LWP is larger than 1.91 mm or IWP is larger than 1.70 mm. Otherwise, the rainfall is stratiform. The analysis of surface rainfall budget demonstrates that this new scheme is physically meaningful.
2016, 22(2): 191-199.
doi: 10.16555/j.1006-8775.2016.02.009
Abstract:
Particle number size distribution (PNSD) between 10 nm and 20 μm were measured in the Pearl River Delta(PRD) region in winter 2011. The average particle number concentration of the nucleation mode (10-20 nm), Aitken mode (20-100 nm), accumulation mode (100 nm-1 μm) and coarse mode (1-20 μm) particles were 1 552, 7 470, 4 012, and 19 cm-3, respectively. The volume concentration of accumulation mode particles with peak at 300 nm accounted for over 70% of the total volume concentration. Diurnal variations and dependencies on meteorological parameters of PNSD were investigated. The diurnal variation of nucleation mode particles was mainly influenced by new particle formation events, while the diurnal variation of Aitken mode particles correlated to the traffic emission and the growth process of nucleation mode particles. When the PRD region was controlled by a cold high pressure, conditions of low relative humidity, high wind speed and strong radiation are favorable for the occurrence of new particle formation (NPF) events. The frequency of occurrence of NPF events was 21.3% during the whole measurement period. Parameters describing NPF events, including growth rate (GR) and source rate of condensable vapor (Q), were slightly larger than those in previous literature. This suggests that intense photochemical and biological activities may be the source of condensable vapor for particle growth, even during winter in the PRD.
Particle number size distribution (PNSD) between 10 nm and 20 μm were measured in the Pearl River Delta(PRD) region in winter 2011. The average particle number concentration of the nucleation mode (10-20 nm), Aitken mode (20-100 nm), accumulation mode (100 nm-1 μm) and coarse mode (1-20 μm) particles were 1 552, 7 470, 4 012, and 19 cm-3, respectively. The volume concentration of accumulation mode particles with peak at 300 nm accounted for over 70% of the total volume concentration. Diurnal variations and dependencies on meteorological parameters of PNSD were investigated. The diurnal variation of nucleation mode particles was mainly influenced by new particle formation events, while the diurnal variation of Aitken mode particles correlated to the traffic emission and the growth process of nucleation mode particles. When the PRD region was controlled by a cold high pressure, conditions of low relative humidity, high wind speed and strong radiation are favorable for the occurrence of new particle formation (NPF) events. The frequency of occurrence of NPF events was 21.3% during the whole measurement period. Parameters describing NPF events, including growth rate (GR) and source rate of condensable vapor (Q), were slightly larger than those in previous literature. This suggests that intense photochemical and biological activities may be the source of condensable vapor for particle growth, even during winter in the PRD.
2016, 22(2): 200-207.
doi: 10.16555/j.1006-8775.2016.02.010
Abstract:
The ensemble based forecast sensitivity to observation method by Liu and Kalnay is applied to the SPEEDY-LETKF system to estimate the observation impact of three types of simulated observations. The estimation results show that all types of observations have positive impact on short-range forecast. The largest impact in Northern Hemisphere is produced by rawinsondes, followed by satellite retrieved profiles and cloud drift wind data, which in Southern Hemisphere is produced by satellite retrieved profiles, rawinsondes and cloud drift wind data. Satellite retrieved profiles influence more on the Southern Hemisphere than on the Northern Hemisphere due to few observations from rawinsondes in the Southern Hemisphere. At the level of 200 to 300 hPa, the largest impact is attributed to wind observations from rawinsondes and cloud drift wind.
The ensemble based forecast sensitivity to observation method by Liu and Kalnay is applied to the SPEEDY-LETKF system to estimate the observation impact of three types of simulated observations. The estimation results show that all types of observations have positive impact on short-range forecast. The largest impact in Northern Hemisphere is produced by rawinsondes, followed by satellite retrieved profiles and cloud drift wind data, which in Southern Hemisphere is produced by satellite retrieved profiles, rawinsondes and cloud drift wind data. Satellite retrieved profiles influence more on the Southern Hemisphere than on the Northern Hemisphere due to few observations from rawinsondes in the Southern Hemisphere. At the level of 200 to 300 hPa, the largest impact is attributed to wind observations from rawinsondes and cloud drift wind.
2016, 22(2): 208-219.
doi: 10.16555/j.1006-8775.2016.02.011
Abstract:
The present work provides a novel method for calculating vertical velocity based on continuity equations in a pressure coordinate system. The method overcomes the disadvantage of accumulation of calculating errors of horizontal divergence in current kinematics methods during the integration for calculating vertical velocity, and consequently avoids its subsequent correction. In addition, through modifications of the continuity equations, it shows that the vorticity of the vertical shear vector (VVSV) is proportional to -ω, the vertical velocity in p coordinates. Furthermore, if the change of ω in the horizontal direction is neglected, the vorticity of the horizontal vorticity vector is proportional to -ω. When ω is under a fluctuating state in the vertical direction, the updraft occurs when the vector of horizontal vorticity rotates counterclockwise; the downdraft occurs when rotating clockwise. The validation result indicates that the present method is generally better than the vertical velocity calculated by the ω equation using the wet Q-vector divergence as a forcing term, and the vertical velocity calculated by utilizing the kinematics method is followed by the O'Brien method for correction. The plus-minus sign of the vertical velocity obtained with this method is not correlated with the intensity of dBZ, but the absolute error increases when dBZ is >=40. This method demonstrates that it is a good reflection of the direction of the vertical velocity.
The present work provides a novel method for calculating vertical velocity based on continuity equations in a pressure coordinate system. The method overcomes the disadvantage of accumulation of calculating errors of horizontal divergence in current kinematics methods during the integration for calculating vertical velocity, and consequently avoids its subsequent correction. In addition, through modifications of the continuity equations, it shows that the vorticity of the vertical shear vector (VVSV) is proportional to -ω, the vertical velocity in p coordinates. Furthermore, if the change of ω in the horizontal direction is neglected, the vorticity of the horizontal vorticity vector is proportional to -ω. When ω is under a fluctuating state in the vertical direction, the updraft occurs when the vector of horizontal vorticity rotates counterclockwise; the downdraft occurs when rotating clockwise. The validation result indicates that the present method is generally better than the vertical velocity calculated by the ω equation using the wet Q-vector divergence as a forcing term, and the vertical velocity calculated by utilizing the kinematics method is followed by the O'Brien method for correction. The plus-minus sign of the vertical velocity obtained with this method is not correlated with the intensity of dBZ, but the absolute error increases when dBZ is >=40. This method demonstrates that it is a good reflection of the direction of the vertical velocity.
2016, 22(2): 220-232.
doi: 10.16555/j.1006-8775.2016.02.012
Abstract:
Using real-time data and the WRF mesoscale model, a heavy rain event in the process of Mesoscale Convective Complex (MCC) turning into banded Mesoscale Convective Systems (MCSs) during 18-19 June 2010 is simulated and analyzed in this paper. The results indicated that the formation and maintenance of a southwest vortex and shear line at 850 hPa was the mesoscale system that affected the production of this heavy rain. The low-vortex heavy rain mainly happened in the development stage of MCC, and the circular MCC turned into banded MCSs in the late stage with mainly shear line precipitation. In the vicinity of rainfall area, the intense horizontal vorticity due to the vertical shear of u and v caused the rotation, and in correspondence, the ascending branch of the vertical circulation triggered the formation of heavy rain. The different distributions of u and v in the vertical direction produced varying vertical circulations. The horizontal vorticity near the low-vortex and shear line had obvious differences which led to varying reasons for heavy rain formation. The low-vortex heavy rain was mainly caused by the vertical shear of v, and the shear line rainfall formed owing to the vertical shear of both u and v. In this process, the vertical shear of v constituted the EW-trending rain band along the shear line, and the latitudinal non-uniformity of the vertical shear in u caused the vertical motion, which was closely related to the generation and development of MCSs at the shear line and the formation of multiple rain clusters. There was also a similar difference in the positively-tilting term (conversion from horizontal vorticity to vertical positive vorticity) near the rainfall center between the low-vortex and the shear line. The conversion in the low vortex was mainly determined by ?v/?p<0, while that of the shear line by ?u/?p<0. The scale of the conversion from the horizontal vorticity to vertical vorticity was relatively small, and it was easily ignored in the averaged state. The twisting term was mainly conducive to the reinforcement of precipitation, whereas its contribution to the development of southwest vortex and shear line was relatively small.
Using real-time data and the WRF mesoscale model, a heavy rain event in the process of Mesoscale Convective Complex (MCC) turning into banded Mesoscale Convective Systems (MCSs) during 18-19 June 2010 is simulated and analyzed in this paper. The results indicated that the formation and maintenance of a southwest vortex and shear line at 850 hPa was the mesoscale system that affected the production of this heavy rain. The low-vortex heavy rain mainly happened in the development stage of MCC, and the circular MCC turned into banded MCSs in the late stage with mainly shear line precipitation. In the vicinity of rainfall area, the intense horizontal vorticity due to the vertical shear of u and v caused the rotation, and in correspondence, the ascending branch of the vertical circulation triggered the formation of heavy rain. The different distributions of u and v in the vertical direction produced varying vertical circulations. The horizontal vorticity near the low-vortex and shear line had obvious differences which led to varying reasons for heavy rain formation. The low-vortex heavy rain was mainly caused by the vertical shear of v, and the shear line rainfall formed owing to the vertical shear of both u and v. In this process, the vertical shear of v constituted the EW-trending rain band along the shear line, and the latitudinal non-uniformity of the vertical shear in u caused the vertical motion, which was closely related to the generation and development of MCSs at the shear line and the formation of multiple rain clusters. There was also a similar difference in the positively-tilting term (conversion from horizontal vorticity to vertical positive vorticity) near the rainfall center between the low-vortex and the shear line. The conversion in the low vortex was mainly determined by ?v/?p<0, while that of the shear line by ?u/?p<0. The scale of the conversion from the horizontal vorticity to vertical vorticity was relatively small, and it was easily ignored in the averaged state. The twisting term was mainly conducive to the reinforcement of precipitation, whereas its contribution to the development of southwest vortex and shear line was relatively small.
2016, 22(2): 233-242.
doi: 10.16555/j.1006-8775.2016.02.013
Abstract:
Based on observations and reanalysis data, the characteristics of the evolution of climatological spring precipitation over Southern China (SPSC) and the associated climatological intraseasonal oscillation (CISO) and atmospheric circulation are studied. Results show that SPSC increases in an oscillatory way. Although the evolution of SPSC is similar in different regions, there are also differences. In different regions of Southern China, the onset dates of the rain season are from the 12th to 24th pentad and the peak dates are after the 20th pentad. CISO is an important component of SPSC, which is not only statistically significant, but also accompanies a dynamically coherent structure. The peak wet/dry phase of each CISO cycle corresponds to a significant rainfall increasing/decreasing period and modulates the evolution of SPSC. The rainfall growth in the second half of March and mid-April is the result of the modulation. The wet/dry phase of CISO is accompanied by low-level convergent (upper-level divergent) and cyclonic (anti-cyclonic) circulation, which favors ascending motion to develop over Southern China.
Based on observations and reanalysis data, the characteristics of the evolution of climatological spring precipitation over Southern China (SPSC) and the associated climatological intraseasonal oscillation (CISO) and atmospheric circulation are studied. Results show that SPSC increases in an oscillatory way. Although the evolution of SPSC is similar in different regions, there are also differences. In different regions of Southern China, the onset dates of the rain season are from the 12th to 24th pentad and the peak dates are after the 20th pentad. CISO is an important component of SPSC, which is not only statistically significant, but also accompanies a dynamically coherent structure. The peak wet/dry phase of each CISO cycle corresponds to a significant rainfall increasing/decreasing period and modulates the evolution of SPSC. The rainfall growth in the second half of March and mid-April is the result of the modulation. The wet/dry phase of CISO is accompanied by low-level convergent (upper-level divergent) and cyclonic (anti-cyclonic) circulation, which favors ascending motion to develop over Southern China.
2016, 22(2): 243-251.
doi: 10.16555/j.1006-8775.2016.02.014
Abstract:
Monthly mean surface air temperatures and precipitation at 20 meteorological stations in the Jinsha River Valley (JRV) of southwest China were analyzed for temporal-spatial variation patterns during the period 1961-2010. The magnitude of a trend was estimated using Sen’s Nonparametric Estimator of Slope approach. The statistical significance of a trend was assessed by the MK test. The results showed that mean annual air temperature has been increasing by 0.08°C /decade during the past 50 years as a whole. The climate change trend in air temperature was more significant in the winter (0.13°C/decade) than in the summer (0.03°C/decade). Annual precipitation tended to increase slightly thereafter and the increasing was mainly during the crop-growing season. Both the greatest variation of the annual mean temperature and annual precipitation were observed at the dry-hot valley area of middle reaches. Significant warming rates were found in the upper reaches whereas the dry-hot basins of middle reaches experienced a cooling trend during the past decades. Despite of the overall increasing in precipitation, more obvious upward-trends were found in the dry-hot basins of middle reaches whereas the upper reaches had a drought trend during the past decades.
Monthly mean surface air temperatures and precipitation at 20 meteorological stations in the Jinsha River Valley (JRV) of southwest China were analyzed for temporal-spatial variation patterns during the period 1961-2010. The magnitude of a trend was estimated using Sen’s Nonparametric Estimator of Slope approach. The statistical significance of a trend was assessed by the MK test. The results showed that mean annual air temperature has been increasing by 0.08°C /decade during the past 50 years as a whole. The climate change trend in air temperature was more significant in the winter (0.13°C/decade) than in the summer (0.03°C/decade). Annual precipitation tended to increase slightly thereafter and the increasing was mainly during the crop-growing season. Both the greatest variation of the annual mean temperature and annual precipitation were observed at the dry-hot valley area of middle reaches. Significant warming rates were found in the upper reaches whereas the dry-hot basins of middle reaches experienced a cooling trend during the past decades. Despite of the overall increasing in precipitation, more obvious upward-trends were found in the dry-hot basins of middle reaches whereas the upper reaches had a drought trend during the past decades.
2016, 22(2): 252-264.
doi: 10.16555/j.1006-8775.2016.02.015
Abstract:
The radiative forcing (RF) of Asian desert dust and its regional feedbacks to the East Asian summer monsoon (EASM) system are investigated with a coupled regional climate-desert dust model. The statistical significance of desert dust effects are analyzed through 20 summer seasons (1990-2009). In order to estimate the dust effects reasonably, some improvement has been achieved for the coupled model, including the updates of optical properties and desert source area distribution. We find that the desert dust can result in a roughly weakened monsoon in eastern China, Korean Peninsula, Japan and Indian Peninsula and a strengthened monsoon in Indochina Peninsula in the lower troposphere. Moreover, the precipitation comparisons between observational data and simulated patterns are also suggestive of the desert dust effect on the EASM. In the upper troposphere, the southward shift of the westerly jet (WJ) by the dust effect can be seen as an indicator of the weakened monsoon in great part of the monsoon areas. The change of the moist static energy (MSE) contrast between land and ocean is the main reason for the EASM variations.
The radiative forcing (RF) of Asian desert dust and its regional feedbacks to the East Asian summer monsoon (EASM) system are investigated with a coupled regional climate-desert dust model. The statistical significance of desert dust effects are analyzed through 20 summer seasons (1990-2009). In order to estimate the dust effects reasonably, some improvement has been achieved for the coupled model, including the updates of optical properties and desert source area distribution. We find that the desert dust can result in a roughly weakened monsoon in eastern China, Korean Peninsula, Japan and Indian Peninsula and a strengthened monsoon in Indochina Peninsula in the lower troposphere. Moreover, the precipitation comparisons between observational data and simulated patterns are also suggestive of the desert dust effect on the EASM. In the upper troposphere, the southward shift of the westerly jet (WJ) by the dust effect can be seen as an indicator of the weakened monsoon in great part of the monsoon areas. The change of the moist static energy (MSE) contrast between land and ocean is the main reason for the EASM variations.
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