2012 Vol. 29, No. 2
Display Method:
2012, 29(2): 217-226.
doi: 10.1007/s00376-011-1095-3
Abstract:
The modulation of the relationship between the Arctic Oscillation (AO) and the East Asian winter climate by the 11-year solar cycle was investigated. During winters with high solar activity (HS), robust warming appeared in northern Asia in a positive AO phase. This result corresponded to an enhanced anticyclonic flow at 850 hPa over northeastern Asia and a weakened East Asian trough (EAT) at 500 hPa. However, during winters with low solar activity (LS), both the surface warming and the intensities of the anticyclonic flow and the EAT were much less in the presence of a positive AO phase. The possible atmospheric processes for this 11-year solar-cycle modulation may be attributed to the indirect influence that solar activity induces in the structural changes of AO. During HS winters, the sea level pressure oscillation associated with the AO became stronger, with the significant influence of AO extending to East Asia. In the meantime, the AO-related zonal-mean zonal winds tended to extend more into the stratosphere during HS winters, which implies a stronger coupling to the stratosphere. These trends may have led to an enhanced AO phase difference; thus the associated East Asian climate anomalies became larger and more significant. The situation tended to reverse during LS winters. Further analyses revealed that the relationship between the winter AO and surface-climate anomalies in the following spring is also modulated by the 11-year solar cycle, with significant signals appearing only during HS phases. Solar-cycle variation should be taken into consideration when the AO is used to predict winter and spring climate anomalies over East Asia.
The modulation of the relationship between the Arctic Oscillation (AO) and the East Asian winter climate by the 11-year solar cycle was investigated. During winters with high solar activity (HS), robust warming appeared in northern Asia in a positive AO phase. This result corresponded to an enhanced anticyclonic flow at 850 hPa over northeastern Asia and a weakened East Asian trough (EAT) at 500 hPa. However, during winters with low solar activity (LS), both the surface warming and the intensities of the anticyclonic flow and the EAT were much less in the presence of a positive AO phase. The possible atmospheric processes for this 11-year solar-cycle modulation may be attributed to the indirect influence that solar activity induces in the structural changes of AO. During HS winters, the sea level pressure oscillation associated with the AO became stronger, with the significant influence of AO extending to East Asia. In the meantime, the AO-related zonal-mean zonal winds tended to extend more into the stratosphere during HS winters, which implies a stronger coupling to the stratosphere. These trends may have led to an enhanced AO phase difference; thus the associated East Asian climate anomalies became larger and more significant. The situation tended to reverse during LS winters. Further analyses revealed that the relationship between the winter AO and surface-climate anomalies in the following spring is also modulated by the 11-year solar cycle, with significant signals appearing only during HS phases. Solar-cycle variation should be taken into consideration when the AO is used to predict winter and spring climate anomalies over East Asia.
2012, 29(2): 227-235.
doi: 10.1007/s00376-011-0168-7
Abstract:
A large number of experimental and theoretical investigations of carbon dioxide (CO2) spectra have been conducted since the most recent update of the High-Resolution Transmission Molecular Absorption (HITRAN) database. To maintain optimal parameters, the HITRAN 2004 CO2 line list has been completely replaced by HITRAN 2008 data in the near-infrared region from 4300 cm-1 to 7000 cm-1. To examine the effect of this change on the retrieval of CO2 vertical column data from reflected sunlight spectra in the 1.61-m spectral window, synthetic measurements for a given atmospheric state and instrument setup were generated and compared using radiative transfer model with the line-transition parameters from the HITRAN 2004 and 2008 databases. Simulated retrievals were then performed based on the optimal estimation retrieval theory. The results show that large systematic errors in atmospheric CO2 column retrievals were induced by the differences in the HITRAN laboratory line parameters in the 1.61-m region. The retrieved CO2 columns were underestimated by 10 ppm using the HITRAN 2004 data, and improvements resulting from the use of the improved HITRAN database were more pronounced at a higher spectral resolution.
A large number of experimental and theoretical investigations of carbon dioxide (CO2) spectra have been conducted since the most recent update of the High-Resolution Transmission Molecular Absorption (HITRAN) database. To maintain optimal parameters, the HITRAN 2004 CO2 line list has been completely replaced by HITRAN 2008 data in the near-infrared region from 4300 cm-1 to 7000 cm-1. To examine the effect of this change on the retrieval of CO2 vertical column data from reflected sunlight spectra in the 1.61-m spectral window, synthetic measurements for a given atmospheric state and instrument setup were generated and compared using radiative transfer model with the line-transition parameters from the HITRAN 2004 and 2008 databases. Simulated retrievals were then performed based on the optimal estimation retrieval theory. The results show that large systematic errors in atmospheric CO2 column retrievals were induced by the differences in the HITRAN laboratory line parameters in the 1.61-m region. The retrieved CO2 columns were underestimated by 10 ppm using the HITRAN 2004 data, and improvements resulting from the use of the improved HITRAN database were more pronounced at a higher spectral resolution.
2012, 29(2): 236-248.
doi: 10.1007/s00376-011-0215-4
Abstract:
The accuracies of three instruments in measuring atmospheric column humidity were assessed during an upper troposphere and lower stratosphere observation campaign conducted from 7 to 13 August 2009 in Kunming, China. The three instruments are a cryogenic frost-point hygrometer (CFH), a Vaisala RS80 radiosonde (RS80), and a GTS1 radiosonde (GTS1). The accuracy of relative humidity (RH) measurements made by the CFH, GTS1, and RS80 was similar between the surface and 500 hPa (～5.5 km above sea level). However, above 500 hPa, the errors in RH measurements made by the RS80, relative to measurements made by the CFH, are much less than those detected with the GTS1. Three different retrieval methods for determining cloud boundaries from CFH, RS80, and GTS1 measurements were developed and take into account the differences in accuracy among the three instruments. The method for the CFH is based on RH thresholds at all levels, which demands high accuracy. Given that the accuracy of RH measurements decreases at higher altitudes, the cloud detection methods for RS80 and GTS1 are different for different altitude ranges. Below 5 km, the methods for the RS80 and the GTS1 are similar to that of the CFH; above 5 km, the methods for the RS80 and the GTS1 are both developed based on the second-order derivatives of temperature and RH with respect to height, but with different criteria applied. Comparisons of cloud-layer retrievals derived from the three measurements are also made.
The accuracies of three instruments in measuring atmospheric column humidity were assessed during an upper troposphere and lower stratosphere observation campaign conducted from 7 to 13 August 2009 in Kunming, China. The three instruments are a cryogenic frost-point hygrometer (CFH), a Vaisala RS80 radiosonde (RS80), and a GTS1 radiosonde (GTS1). The accuracy of relative humidity (RH) measurements made by the CFH, GTS1, and RS80 was similar between the surface and 500 hPa (～5.5 km above sea level). However, above 500 hPa, the errors in RH measurements made by the RS80, relative to measurements made by the CFH, are much less than those detected with the GTS1. Three different retrieval methods for determining cloud boundaries from CFH, RS80, and GTS1 measurements were developed and take into account the differences in accuracy among the three instruments. The method for the CFH is based on RH thresholds at all levels, which demands high accuracy. Given that the accuracy of RH measurements decreases at higher altitudes, the cloud detection methods for RS80 and GTS1 are different for different altitude ranges. Below 5 km, the methods for the RS80 and the GTS1 are similar to that of the CFH; above 5 km, the methods for the RS80 and the GTS1 are both developed based on the second-order derivatives of temperature and RH with respect to height, but with different criteria applied. Comparisons of cloud-layer retrievals derived from the three measurements are also made.
2012, 29(2): 249-262.
doi: 10.1007/s00376-011-1080-x
Abstract:
Tropical cyclone (TC) genesis over the South China Sea (SCS) during 1965--2004 was analyzed. The locations of TC genesis display evident seasonal changes, with the mean position of formation situated north of 15oN in summer (June--July--August) and south of 15oN in autumn (September--October--November). The TC genesis in summer underwent dramatic interdecadal variations, with more and less TC frequency during 1965--1974/1995--2004 and 1979--1993, respectively. In contrast, a significant interannual variation of TC genesis with a period of ～4 years was observed in autumn.
Tropical cyclone (TC) genesis over the South China Sea (SCS) during 1965--2004 was analyzed. The locations of TC genesis display evident seasonal changes, with the mean position of formation situated north of 15oN in summer (June--July--August) and south of 15oN in autumn (September--October--November). The TC genesis in summer underwent dramatic interdecadal variations, with more and less TC frequency during 1965--1974/1995--2004 and 1979--1993, respectively. In contrast, a significant interannual variation of TC genesis with a period of ～4 years was observed in autumn.
2012, 29(2): 263-273.
doi: 10.1007/s00376-011-1119-z
Abstract:
This study investigates the use of dynamic a priori error information according to atmospheric moistness and the use of quality controls in temperature and water vapor profile retrievals from hyperspectral infrared (IR) sounders. Temperature and water vapor profiles are retrieved from Atmospheric InfraRed Sounder (AIRS) radiance measurements by applying a physical iterative method using regression retrieval as the first guess. Based on the dependency of first-guess errors on the degree of atmospheric moistness, the a priori first-guess errors classified by total precipitable water (TPW) are applied in the AIRS physical retrieval procedure. Compared to the retrieval results from a fixed a priori error, boundary layer moisture retrievals appear to be improved via TPW classification of a priori first-guess errors. Six quality control (QC) tests, which check non-converged or bad retrievals, large residuals, high terrain and desert areas, and large temperature and moisture deviations from the first guess regression retrieval, are also applied in the AIRS physical retrievals. Significantly large errors are found for the retrievals rejected by these six QCs, and the retrieval errors are substantially reduced via QC over land, which suggest the usefulness and high impact of the QCs, especially over land. In conclusion, the use of dynamic a priori error information according to atmospheric moistness, and the use of appropriate QCs dealing with the geographical information and the deviation from the first-guess as well as the conventional inverse performance are suggested to improve temperature and moisture retrievals and their applications.
This study investigates the use of dynamic a priori error information according to atmospheric moistness and the use of quality controls in temperature and water vapor profile retrievals from hyperspectral infrared (IR) sounders. Temperature and water vapor profiles are retrieved from Atmospheric InfraRed Sounder (AIRS) radiance measurements by applying a physical iterative method using regression retrieval as the first guess. Based on the dependency of first-guess errors on the degree of atmospheric moistness, the a priori first-guess errors classified by total precipitable water (TPW) are applied in the AIRS physical retrieval procedure. Compared to the retrieval results from a fixed a priori error, boundary layer moisture retrievals appear to be improved via TPW classification of a priori first-guess errors. Six quality control (QC) tests, which check non-converged or bad retrievals, large residuals, high terrain and desert areas, and large temperature and moisture deviations from the first guess regression retrieval, are also applied in the AIRS physical retrievals. Significantly large errors are found for the retrievals rejected by these six QCs, and the retrieval errors are substantially reduced via QC over land, which suggest the usefulness and high impact of the QCs, especially over land. In conclusion, the use of dynamic a priori error information according to atmospheric moistness, and the use of appropriate QCs dealing with the geographical information and the deviation from the first-guess as well as the conventional inverse performance are suggested to improve temperature and moisture retrievals and their applications.
2012, 29(2): 274-284.
doi: 10.1007/s00376-011-1034-3
Abstract:
In this paper, the possible reason of Tibetan Plateau (TP) temperature increasing was investigated. An increase in Tmin (minimum temperature) plays a robust role in increased TP temperature, which is strongly related to SST over the warm pool of the western Pacific Ocean, the subtropical westerly jet stream (SWJ), and the tropical easterly upper jet stream (TEJ), and the 200-hPa zonal wind in East Asia. Composite analysis of the effects of SST, SWJ, and TEJ on pre- and post-abrupt changes in Ta (annual temperature) and Tmin over the TP shows remarkable differences in SST, SWJ, and TEJ. A lag correlation between TaTmin, SST, and SWJ/TEJ shows that changes in SST occur ahead of changes in Ta/Tmin by approximately one to three seasons. Partial correlations between Ta/Tmin, SST, and SWJ/TEJ show that the effect of SWJ on Ta/Tmin is more significant than the effect of SST. Furthermore, simulations with a community atmospheric model (CAM3.0) were performed, showing a remarkable increase in Ta over the TP when the SST increased by 0.5oC. The main increase in Ta and Tmin in the TP can be attributed to changes in SWJ. A possible mechanism is that changes in SST force the TEJ to weaken, move south, and lead to increased SWJ and movement of SWJ northward. Finally, changes in the intensity and location of the SWJ cause an increase in Ta/Tmin. It appears that TP warming is governed primarily by coherent TEJ and SWJ variations that act as the atmospheric bridges to remote SSTs in warm-pool forcing.
In this paper, the possible reason of Tibetan Plateau (TP) temperature increasing was investigated. An increase in Tmin (minimum temperature) plays a robust role in increased TP temperature, which is strongly related to SST over the warm pool of the western Pacific Ocean, the subtropical westerly jet stream (SWJ), and the tropical easterly upper jet stream (TEJ), and the 200-hPa zonal wind in East Asia. Composite analysis of the effects of SST, SWJ, and TEJ on pre- and post-abrupt changes in Ta (annual temperature) and Tmin over the TP shows remarkable differences in SST, SWJ, and TEJ. A lag correlation between TaTmin, SST, and SWJ/TEJ shows that changes in SST occur ahead of changes in Ta/Tmin by approximately one to three seasons. Partial correlations between Ta/Tmin, SST, and SWJ/TEJ show that the effect of SWJ on Ta/Tmin is more significant than the effect of SST. Furthermore, simulations with a community atmospheric model (CAM3.0) were performed, showing a remarkable increase in Ta over the TP when the SST increased by 0.5oC. The main increase in Ta and Tmin in the TP can be attributed to changes in SWJ. A possible mechanism is that changes in SST force the TEJ to weaken, move south, and lead to increased SWJ and movement of SWJ northward. Finally, changes in the intensity and location of the SWJ cause an increase in Ta/Tmin. It appears that TP warming is governed primarily by coherent TEJ and SWJ variations that act as the atmospheric bridges to remote SSTs in warm-pool forcing.
2012, 29(2): 285-294.
doi: 10.1007/s00376-011-1112-6
Abstract:
Data collected in the surface layer in a northern suburban area of Nanjing from 15 November to 29 December 2007 were analyzed to examine the Monin-Obukhov similarity for describing the turbulent fluctuations of 3D winds under all stability conditions and to obtain the turbulence characteristics under different weather conditions. The results show that the dimensionless standard deviations of turbulent velocity components (σu/u*, σv/u*, σw/u*) and dimensionless turbulent kinetic energy (TKE) can be well described by ``1/3'' power law relationships under stable, neutral, and unstable conditions, with σu/u*>σv/u*>σw/u*. Land use and land cover changes mainly impact dimensionless standard deviations of horizontal component fluctuations, but they have very little on those of the vertical component. The dimensionless standard deviations of wind components and dimensionless TKE are remarkably affected by different weather conditions; the deviations of horizontal wind component and dimensionless TKE present fog day > clear sky > overcast > cloudy; the trend of the vertical wind component is the reverse. The surface drag coefficient at a Nanjing suburban measurement site during the observation period was obviously higher than at other reported plains and plateau areas, and was approximately one order larger in magnitude than the reported plains areas. Dimensionless standard deviation of temperature declined with increasing |z'/L| with an approximate ``-1/3'' slope in unstable stratification and ``-2/3'' slope in stable stratification.
Data collected in the surface layer in a northern suburban area of Nanjing from 15 November to 29 December 2007 were analyzed to examine the Monin-Obukhov similarity for describing the turbulent fluctuations of 3D winds under all stability conditions and to obtain the turbulence characteristics under different weather conditions. The results show that the dimensionless standard deviations of turbulent velocity components (σu/u*, σv/u*, σw/u*) and dimensionless turbulent kinetic energy (TKE) can be well described by ``1/3'' power law relationships under stable, neutral, and unstable conditions, with σu/u*>σv/u*>σw/u*. Land use and land cover changes mainly impact dimensionless standard deviations of horizontal component fluctuations, but they have very little on those of the vertical component. The dimensionless standard deviations of wind components and dimensionless TKE are remarkably affected by different weather conditions; the deviations of horizontal wind component and dimensionless TKE present fog day > clear sky > overcast > cloudy; the trend of the vertical wind component is the reverse. The surface drag coefficient at a Nanjing suburban measurement site during the observation period was obviously higher than at other reported plains and plateau areas, and was approximately one order larger in magnitude than the reported plains areas. Dimensionless standard deviation of temperature declined with increasing |z'/L| with an approximate ``-1/3'' slope in unstable stratification and ``-2/3'' slope in stable stratification.
2012, 29(2): 295-306.
doi: 10.1007/s00376-011-1141-1
Abstract:
This study examined the variability in frequency of tropical night occurrence (i.e., minimum air temperature 25oC) in Beijing, using a homogenized daily temperature dataset during the period 1960--2008. Our results show that tropical nights occur most frequently in late July and early August, which is consistent with relatively high air humidity associated with the rainy season in Beijing. In addition, year-to-year variation of tropical night occurrence indicates that the tropical nights have appeared much more frequently since 1994, which can be illustrated by the yearly days of tropical nights averaged over two periods: 9.2 days of tropical nights per year during 1994--2008 versus 3.15 days during 1960--1993. These features of tropical night variations suggest a distinction between tropical nights and extreme heat in Beijing. We further investigated the large-scale circulations associated with the year-to-year variation of tropical night occurrence in July and August, when tropical nights appear most frequently and occupy 95% of the annual sum. After comparing the results in the two reanalysis datasets (NCEP/NCAR and ERA-40) and considering the possible effects of decadal change in the frequency of tropical nights that occurred around 1993/94, we conclude that on the interannual time scale, the cyclonic anomaly with a barotropic structure centered over Beijing is responsible for less frequent tropical nights, and the anticyclonic anomaly is responsible for more frequent occurrence of tropical nights over Beijing.
This study examined the variability in frequency of tropical night occurrence (i.e., minimum air temperature 25oC) in Beijing, using a homogenized daily temperature dataset during the period 1960--2008. Our results show that tropical nights occur most frequently in late July and early August, which is consistent with relatively high air humidity associated with the rainy season in Beijing. In addition, year-to-year variation of tropical night occurrence indicates that the tropical nights have appeared much more frequently since 1994, which can be illustrated by the yearly days of tropical nights averaged over two periods: 9.2 days of tropical nights per year during 1994--2008 versus 3.15 days during 1960--1993. These features of tropical night variations suggest a distinction between tropical nights and extreme heat in Beijing. We further investigated the large-scale circulations associated with the year-to-year variation of tropical night occurrence in July and August, when tropical nights appear most frequently and occupy 95% of the annual sum. After comparing the results in the two reanalysis datasets (NCEP/NCAR and ERA-40) and considering the possible effects of decadal change in the frequency of tropical nights that occurred around 1993/94, we conclude that on the interannual time scale, the cyclonic anomaly with a barotropic structure centered over Beijing is responsible for less frequent tropical nights, and the anticyclonic anomaly is responsible for more frequent occurrence of tropical nights over Beijing.
2012, 29(2): 307-319.
doi: 10.1007/s00376-011-9234-4
Abstract:
Synoptic patterns identified by an automated procedure employing principal- component analysis and a two-stage cluster analysis, and backward trajectory analysis clustered by the HYSPLIT4.9 model were used to examine air quality patterns over Urumqi, China, one of the most heavily polluted cities in the world. Six synoptic patterns representing different atmospheric circulation patterns and air-mass characteristics were classified during the winter heating periods from 2001 to 2008, and seven trajectory clusters representing different paths of air masses arriving at Urumqi were calculated during the winter heating periods from 2005 to 2008. Then air quality was evaluated using these two approaches, and significant variations were found across both synoptic patterns and trajectory clusters. The heaviest air-pollution episodes occurred when Urumqi was either in an extremely cold, strong anticyclone or at the front of a migrating cyclone. Both conditions were characterized by with light winds, cold, wet surface air, and relatively dry upper air. Urumqi was predominately influenced by air masses from the southwest and from local areas. Air pollution index (API) levels were highest for air masses originating from the southwest with a longer path or for the local area, because of transport from semi-desert/desert regions by strong winds and because of local heavy pollution emissions, respectively. The interactions between these two analytical approaches showed that poor diffusion conditions, together with local circulation, enhanced air pollution, besides, regional air-mass transport caused by strong winds contributed to serious air quality under relatively good diffusion conditions.
Synoptic patterns identified by an automated procedure employing principal- component analysis and a two-stage cluster analysis, and backward trajectory analysis clustered by the HYSPLIT4.9 model were used to examine air quality patterns over Urumqi, China, one of the most heavily polluted cities in the world. Six synoptic patterns representing different atmospheric circulation patterns and air-mass characteristics were classified during the winter heating periods from 2001 to 2008, and seven trajectory clusters representing different paths of air masses arriving at Urumqi were calculated during the winter heating periods from 2005 to 2008. Then air quality was evaluated using these two approaches, and significant variations were found across both synoptic patterns and trajectory clusters. The heaviest air-pollution episodes occurred when Urumqi was either in an extremely cold, strong anticyclone or at the front of a migrating cyclone. Both conditions were characterized by with light winds, cold, wet surface air, and relatively dry upper air. Urumqi was predominately influenced by air masses from the southwest and from local areas. Air pollution index (API) levels were highest for air masses originating from the southwest with a longer path or for the local area, because of transport from semi-desert/desert regions by strong winds and because of local heavy pollution emissions, respectively. The interactions between these two analytical approaches showed that poor diffusion conditions, together with local circulation, enhanced air pollution, besides, regional air-mass transport caused by strong winds contributed to serious air quality under relatively good diffusion conditions.
2012, 29(2): 320-332.
doi: 10.1007/s00376-011-0165-x
Abstract:
Assessment of the nitrogen (N) balance and its long-term trend is necessary for management practices because of the negative environmental effects caused by an imbalance of reactive N in grassland ecosystems. In this study, we designed a module for the IAP-N (Improving Anthropogenic Practices of managing reactive Nitrogen) model to enable it to assess the N budget of regional grasslands. The module was developed to quantify the individual components of the N inputs and outputs for grassland ecosystems using livestock and human populations, grassland area, and fossil-energy consumption data as the model inputs. In this paper, the estimation approaches for individual components of N budget, data acquisition, and parameter selection are described in detail. The model was applied to assess the N budget of Inner Mongolia in 2006 at the county scale. The simulation results show that the most important pathway of N outputs from the grassland was livestock intake. The N output from livestock intake was especially large in the middle of Inner Mongolia. Biological fixation, atmospheric deposition, and livestock excreta deposition were comparably important for the N inputs into the grassland. The N budget for Inner Mongolia grassland in 2006 was -1.71080.6108 kg. The case study for Inner Mongolia shows that the new grassland module for the IAP-N model can capture the characteristics of the N budget in a semiarid grassland.
Assessment of the nitrogen (N) balance and its long-term trend is necessary for management practices because of the negative environmental effects caused by an imbalance of reactive N in grassland ecosystems. In this study, we designed a module for the IAP-N (Improving Anthropogenic Practices of managing reactive Nitrogen) model to enable it to assess the N budget of regional grasslands. The module was developed to quantify the individual components of the N inputs and outputs for grassland ecosystems using livestock and human populations, grassland area, and fossil-energy consumption data as the model inputs. In this paper, the estimation approaches for individual components of N budget, data acquisition, and parameter selection are described in detail. The model was applied to assess the N budget of Inner Mongolia in 2006 at the county scale. The simulation results show that the most important pathway of N outputs from the grassland was livestock intake. The N output from livestock intake was especially large in the middle of Inner Mongolia. Biological fixation, atmospheric deposition, and livestock excreta deposition were comparably important for the N inputs into the grassland. The N budget for Inner Mongolia grassland in 2006 was -1.71080.6108 kg. The case study for Inner Mongolia shows that the new grassland module for the IAP-N model can capture the characteristics of the N budget in a semiarid grassland.
2012, 29(2): 333-343.
doi: 10.1007/s00376-011-1100-x
Abstract:
Global climate models predict that the increasing Amazonian-deforestation rates cause rising temperatures (increases of 1.8oC to 8oC under different conditions) and Amazonian drying over the 21st century. Observations in the 20th century also show that over the warmer continent and the nearby western South Atlantic Ocean, the lower-layer equatorial westerly wind (LLEWW) strengthens with the initiation of tropical cyclones (TCs). The warmer-continent-related LLEWW can result from the Coriolis-force-induced deflection of the cross-equatorial flow (similar to the well-known heat-island effect on sea breeze) driven by the enhanced land-sea contrast between the warmer urbanized continents and relatively cold oceans. This study focuses on the processes relating the warmer-continent-related LLEWW to the TC initiation and demonstrates that the LLEWW embedded in trade easterlies can directly initiate TCs by creating cyclonic wind shears and forming the intertropical convergence zone. In addition to this direct effect, the LLEWW combined with the rotating Earth can boost additional updraft vapor over the high sea-surface temperature region (factor 1), facilitating a surface-to-midtroposphere moist layer (factor 2) and convective instability (factor 3) followed by diabatic processes. According to previous studies, the diabatic heating in a finite equatorial region also activates TCs (factor 4) on each side of the Equator with weak vertical shear (factor 5). Factors 1--5 are favorable conditions for the initiation of severe TCs. Statistical analyses show that the earliest signal of sustained LLEWW not only leads the earliest signal of sustained tropical depression by >3 days but also explains a higher percentage of total variance.
Global climate models predict that the increasing Amazonian-deforestation rates cause rising temperatures (increases of 1.8oC to 8oC under different conditions) and Amazonian drying over the 21st century. Observations in the 20th century also show that over the warmer continent and the nearby western South Atlantic Ocean, the lower-layer equatorial westerly wind (LLEWW) strengthens with the initiation of tropical cyclones (TCs). The warmer-continent-related LLEWW can result from the Coriolis-force-induced deflection of the cross-equatorial flow (similar to the well-known heat-island effect on sea breeze) driven by the enhanced land-sea contrast between the warmer urbanized continents and relatively cold oceans. This study focuses on the processes relating the warmer-continent-related LLEWW to the TC initiation and demonstrates that the LLEWW embedded in trade easterlies can directly initiate TCs by creating cyclonic wind shears and forming the intertropical convergence zone. In addition to this direct effect, the LLEWW combined with the rotating Earth can boost additional updraft vapor over the high sea-surface temperature region (factor 1), facilitating a surface-to-midtroposphere moist layer (factor 2) and convective instability (factor 3) followed by diabatic processes. According to previous studies, the diabatic heating in a finite equatorial region also activates TCs (factor 4) on each side of the Equator with weak vertical shear (factor 5). Factors 1--5 are favorable conditions for the initiation of severe TCs. Statistical analyses show that the earliest signal of sustained LLEWW not only leads the earliest signal of sustained tropical depression by >3 days but also explains a higher percentage of total variance.
2012, 29(2): 344-358.
doi: 10.1007/s00376-011-1063-y
Abstract:
Constructing β-mesoscale weather systems in initial fields remains a challenging problem in a mesoscale numerical weather prediction (NWP) model. Without vertical velocity matching the β-mesoscale weather system, convection activities would be suppressed by downdraft and cooling caused by precipitating hydrometeors. In this study, a method, basing on the three-dimensional variational (3DVAR) assimilation technique, was developed to obtain reasonable structures of β-mesoscale weather systems by assimilating radar data in a next-generation NWP system named GRAPES (the Global and Regional Assimilation and Prediction System) of China. Single-point testing indicated that assimilating radial wind significantly improved the horizontal wind but had little effect on the vertical velocity, while assimilating the retrieved vertical velocity (taking Richardson's equation as the observational operator) can greatly improve the vertical motion. Experiments on a typhoon show that assimilation of the radial wind data can greatly improve the prediction of the typhoon track, and can ameliorate precipitation to some extent. Assimilating the retrieved vertical velocity and rainwater mixing ratio, and adjusting water vapor and cloud water mixing ratio in the initial fields simultaneously, can significantly improve the tropical cyclone rainfall forecast but has little effect on typhoon path. Joint assimilating these three kinds of radar data gets the best results. Taking into account the scale of different weather systems and representation of observational data, data quality control, error setting of background field and observation data are still requiring further in-depth study.
Constructing β-mesoscale weather systems in initial fields remains a challenging problem in a mesoscale numerical weather prediction (NWP) model. Without vertical velocity matching the β-mesoscale weather system, convection activities would be suppressed by downdraft and cooling caused by precipitating hydrometeors. In this study, a method, basing on the three-dimensional variational (3DVAR) assimilation technique, was developed to obtain reasonable structures of β-mesoscale weather systems by assimilating radar data in a next-generation NWP system named GRAPES (the Global and Regional Assimilation and Prediction System) of China. Single-point testing indicated that assimilating radial wind significantly improved the horizontal wind but had little effect on the vertical velocity, while assimilating the retrieved vertical velocity (taking Richardson's equation as the observational operator) can greatly improve the vertical motion. Experiments on a typhoon show that assimilation of the radial wind data can greatly improve the prediction of the typhoon track, and can ameliorate precipitation to some extent. Assimilating the retrieved vertical velocity and rainwater mixing ratio, and adjusting water vapor and cloud water mixing ratio in the initial fields simultaneously, can significantly improve the tropical cyclone rainfall forecast but has little effect on typhoon path. Joint assimilating these three kinds of radar data gets the best results. Taking into account the scale of different weather systems and representation of observational data, data quality control, error setting of background field and observation data are still requiring further in-depth study.
2012, 29(2): 359-368.
doi: 10.1007/s00376-011-1045-0
Abstract:
The objective of this study was to reconstruct light extinction coefficients (bext) according to chemical composition components of particulate matter up to 2.5 m in size (PM2.5). PM2.5 samples were collected at the monitoring station of the South China of Institute of Environmental Science (SCIES, Guangzhou, China) during January 2010, and the online absorbing and scattering coefficients were obtained using an aethalometer and a nephelometer. The measured values of light absorption coefficient by particle (bap) and light scattering coefficient by particle (bsp) significantly correlated (R20.95) with values of bap and bsp that were reconstructed using the Interagency Monitoring of Protected Visual Environments (IMPROVE) formula when RH was 70%. The measured bext had a good correlation (R20.83) with the calculated bext under ambient RH conditions. The result of source apportionment of bext showed that ammonium sulfate [(NH4)2SO4] was the largest contributor (35.0%) to bext, followed by ammonium nitrate (NH4NO3, 22.9%), organic matter (16.1%), elemental carbon (11.8%), sea salt (4.7%), and nitrogen dioxide (NO2, 9.6%). To improve visibility in Guangzhou, the effective control of secondary particles like sulfates, nitrates, and ammonia should be given more attention in urban environmental management.
The objective of this study was to reconstruct light extinction coefficients (bext) according to chemical composition components of particulate matter up to 2.5 m in size (PM2.5). PM2.5 samples were collected at the monitoring station of the South China of Institute of Environmental Science (SCIES, Guangzhou, China) during January 2010, and the online absorbing and scattering coefficients were obtained using an aethalometer and a nephelometer. The measured values of light absorption coefficient by particle (bap) and light scattering coefficient by particle (bsp) significantly correlated (R20.95) with values of bap and bsp that were reconstructed using the Interagency Monitoring of Protected Visual Environments (IMPROVE) formula when RH was 70%. The measured bext had a good correlation (R20.83) with the calculated bext under ambient RH conditions. The result of source apportionment of bext showed that ammonium sulfate [(NH4)2SO4] was the largest contributor (35.0%) to bext, followed by ammonium nitrate (NH4NO3, 22.9%), organic matter (16.1%), elemental carbon (11.8%), sea salt (4.7%), and nitrogen dioxide (NO2, 9.6%). To improve visibility in Guangzhou, the effective control of secondary particles like sulfates, nitrates, and ammonia should be given more attention in urban environmental management.
2012, 29(2): 369-380.
doi: 10.1007/s00376-011-1049-9
Abstract:
Global teleconnections associated with tropical convective activities were investigated, based on monthly data of 29 Northern Hemisphere winters: December, January, February, and March (DJFM). First, EOF analyses were performed on the outgoing long-wave radiation (OLR) data to characterize the convective activity variability in the tropical Indian Ocean and the western Pacific. The first EOF mode of the convective activity was highly correlated with the ENSO. The second EOF mode had an east--west dipole structure, and the third EOF mode had three convective activity centers. Two distinct teleconnection patterns were identified that were associated, respectively, with the second and third EOF modes. A global primitive equation model was used to investigate the physical mechanism that causes the global circulation anomalies. The model responses to anomalous tropical thermal forcings that mimic the EOF patterns matched the general features of the observed circulation anomalies well, and they were mainly controlled by linear processes. The importance of convective activities in the tropical Indian Ocean and western Pacific to the extended- and long-range forecasting capability in the extratropics is discussed.
Global teleconnections associated with tropical convective activities were investigated, based on monthly data of 29 Northern Hemisphere winters: December, January, February, and March (DJFM). First, EOF analyses were performed on the outgoing long-wave radiation (OLR) data to characterize the convective activity variability in the tropical Indian Ocean and the western Pacific. The first EOF mode of the convective activity was highly correlated with the ENSO. The second EOF mode had an east--west dipole structure, and the third EOF mode had three convective activity centers. Two distinct teleconnection patterns were identified that were associated, respectively, with the second and third EOF modes. A global primitive equation model was used to investigate the physical mechanism that causes the global circulation anomalies. The model responses to anomalous tropical thermal forcings that mimic the EOF patterns matched the general features of the observed circulation anomalies well, and they were mainly controlled by linear processes. The importance of convective activities in the tropical Indian Ocean and western Pacific to the extended- and long-range forecasting capability in the extratropics is discussed.
2012, 29(2): 381-390.
doi: 10.1007/s00376-011-1068-6
Abstract:
Improving and validating land surface models based on integrated observations in deserts is one of the challenges in land modeling. Particularly, key parameters and parameterization schemes in desert regions need to be evaluated \textit{in-situ} to improve the models. In this study, we calibrated the land-surface key parameters and evaluated several formulations or schemes for thermal roughness length (z0h) in the common land model (CoLM). Our parameter calibration and scheme evaluation were based on the observed data during a torrid summer (29 July to 11 September 2009) over the Taklimakan Desert hinterland. First, the importance of the key parameters in the experiment was evaluated based on their physics principles and the significance of these key parameters were further validated using sensitivity test. Second, difference schemes (or physics-based formulas) of z0h were adopted to simulate the variations of energy-related variables (e.g., sensible heat flux and surface skin temperature) and the simulated variations were then compared with the observed data. Third, the z0h scheme that performed best (i.e., Y07) was then selected to replace the defaulted one (i.e., Z98); the revised scheme and the superiority of Y07 over Z98 was further demonstrated by comparing the simulated results with the observed data. Admittedly, the revised model did a relatively poor job of simulating the diurnal variations of surface soil heat flux, and nighttime soil temperature was also underestimated, calling for further improvement of the model for desert regions.
Improving and validating land surface models based on integrated observations in deserts is one of the challenges in land modeling. Particularly, key parameters and parameterization schemes in desert regions need to be evaluated \textit{in-situ} to improve the models. In this study, we calibrated the land-surface key parameters and evaluated several formulations or schemes for thermal roughness length (z0h) in the common land model (CoLM). Our parameter calibration and scheme evaluation were based on the observed data during a torrid summer (29 July to 11 September 2009) over the Taklimakan Desert hinterland. First, the importance of the key parameters in the experiment was evaluated based on their physics principles and the significance of these key parameters were further validated using sensitivity test. Second, difference schemes (or physics-based formulas) of z0h were adopted to simulate the variations of energy-related variables (e.g., sensible heat flux and surface skin temperature) and the simulated variations were then compared with the observed data. Third, the z0h scheme that performed best (i.e., Y07) was then selected to replace the defaulted one (i.e., Z98); the revised scheme and the superiority of Y07 over Z98 was further demonstrated by comparing the simulated results with the observed data. Admittedly, the revised model did a relatively poor job of simulating the diurnal variations of surface soil heat flux, and nighttime soil temperature was also underestimated, calling for further improvement of the model for desert regions.
2012, 29(2): 391-406.
doi: 10.1007/s00376-011-1059-7
Abstract:
In this study, the impact of atmosphere--wave coupling on typhoon intensity was investigated using numerical simulations of an idealized typhoon in a coupled atmosphere--wave--ocean modeling system. The coupling between atmosphere and sea surface waves considered the effects of wave state and sea sprays on air--sea momentum flux, the atmospheric low-level dissipative heating, and the wave-state-affected sea-spray heat flux. Several experiments were conducted to examine the impacts of wave state, sea sprays, and dissipative heating on an idealized typhoon system. Results show that considering the wave state and sea-spray-affected sea-surface roughness reduces typhoon intensity, while including dissipative heating intensifies the typhoon system. Taking into account sea spray heat flux also strengthens the typhoon system with increasing maximum wind speed and significant wave height. The overall impact of atmosphere--wave coupling makes a positive contribution to the intensification of the idealized typhoon system. The minimum central pressure simulated by the coupled atmosphere--wave experiment was 16.4 hPa deeper than that of the control run, and the maximum wind speed and significant wave height increased by 31% and 4%, respectively. Meanwhile, within the area beneath the typhoon center, the average total upward air--sea heat flux increased by 22%, and the averaged latent heat flux increased more significantly by 31% compared to the uncoupled run.
In this study, the impact of atmosphere--wave coupling on typhoon intensity was investigated using numerical simulations of an idealized typhoon in a coupled atmosphere--wave--ocean modeling system. The coupling between atmosphere and sea surface waves considered the effects of wave state and sea sprays on air--sea momentum flux, the atmospheric low-level dissipative heating, and the wave-state-affected sea-spray heat flux. Several experiments were conducted to examine the impacts of wave state, sea sprays, and dissipative heating on an idealized typhoon system. Results show that considering the wave state and sea-spray-affected sea-surface roughness reduces typhoon intensity, while including dissipative heating intensifies the typhoon system. Taking into account sea spray heat flux also strengthens the typhoon system with increasing maximum wind speed and significant wave height. The overall impact of atmosphere--wave coupling makes a positive contribution to the intensification of the idealized typhoon system. The minimum central pressure simulated by the coupled atmosphere--wave experiment was 16.4 hPa deeper than that of the control run, and the maximum wind speed and significant wave height increased by 31% and 4%, respectively. Meanwhile, within the area beneath the typhoon center, the average total upward air--sea heat flux increased by 22%, and the averaged latent heat flux increased more significantly by 31% compared to the uncoupled run.
2012, 29(2): 407-421.
doi: 10.1007/s00376-011-0229-y
Abstract:
Tropical cyclone (TC) Nargis (2008) made landfall in Myanmar on 02 May 2008, bringing a storm surge, major flooding, and resulting in a significant death toll. TC Nargis (2008) displayed abnormal features, including rare eastward motion in its late stage, rapid intensification before landing. Using reanalysis data and a numerical model, we investigated how a low-latitude westerly wind modulated TC Nargis' (2008) track and provided favorable atmospheric conditions for its rapid intensification. More importantly, we found a possible counterbalance effect of flows from the two hemispheres on the TC track in the Bay of Bengal. Our analysis indicates that a strong westerly wind burst across the Bay of Bengal, resulting in TC Nargis' (2008) eastward movement after its recurvature. This sudden enhancement of westerly wind was mainly due to the rapidly intensified mid-level cross-equatorial flow. Our results show that a high-pressure system in the Southern Hemisphere induced this strong, mid-level, cross-equatorial flow. During the rapid intensification period of TC Nargis (2008), this strong and broad westerly wind also transported a large amount of water vapor to TC Nargis (2008). Sufficient water vapor gave rise to continuously high and increased mid-level relative humidity, which was favorable to TC Nargis' (2008) intensification. Condensation of water vapor increased the energy supply, which eventuated the intensification of TC Nargis (2008) to a category 4 on the Saffir-Simpson scale.
Tropical cyclone (TC) Nargis (2008) made landfall in Myanmar on 02 May 2008, bringing a storm surge, major flooding, and resulting in a significant death toll. TC Nargis (2008) displayed abnormal features, including rare eastward motion in its late stage, rapid intensification before landing. Using reanalysis data and a numerical model, we investigated how a low-latitude westerly wind modulated TC Nargis' (2008) track and provided favorable atmospheric conditions for its rapid intensification. More importantly, we found a possible counterbalance effect of flows from the two hemispheres on the TC track in the Bay of Bengal. Our analysis indicates that a strong westerly wind burst across the Bay of Bengal, resulting in TC Nargis' (2008) eastward movement after its recurvature. This sudden enhancement of westerly wind was mainly due to the rapidly intensified mid-level cross-equatorial flow. Our results show that a high-pressure system in the Southern Hemisphere induced this strong, mid-level, cross-equatorial flow. During the rapid intensification period of TC Nargis (2008), this strong and broad westerly wind also transported a large amount of water vapor to TC Nargis (2008). Sufficient water vapor gave rise to continuously high and increased mid-level relative humidity, which was favorable to TC Nargis' (2008) intensification. Condensation of water vapor increased the energy supply, which eventuated the intensification of TC Nargis (2008) to a category 4 on the Saffir-Simpson scale.
2012, 29(2): 422-434.
doi: 10.1007/s00376-011-1054-z
Abstract:
The lower bound of maximum predictable time can be formulated into a constrained nonlinear optimization problem, and the traditional solutions to this problem are the filtering method and the conditional nonlinear optimal perturbation (CNOP) method. Usually, the CNOP method is implemented with the help of a gradient descent algorithm based on the adjoint method, which is named the ADJ-CNOP. However, with the increasing improvement of actual prediction models, more and more physical processes are taken into consideration in models in the form of parameterization, thus giving rise to the on--off switch problem, which tremendously affects the effectiveness of the conventional gradient descent algorithm based on the adjoint method. In this study, we attempted to apply a genetic algorithm (GA) to the CNOP method, named GA-CNOP, to solve the predictability problems involving on--off switches. As the precision of the filtering method depends uniquely on the division of the constraint region, its results were taken as benchmarks, and a series of comparisons between the ADJ-CNOP and the GA-CNOP were performed for the modified Lorenz equation. Results show that the GA-CNOP can always determine the accurate lower bound of maximum predictable time, even in non-smooth cases, while the ADJ-CNOP, owing to the effect of on--off switches, often yields the incorrect lower bound of maximum predictable time. Therefore, in non-smooth cases, using GAs to solve predictability problems is more effective than using the conventional optimization algorithm based on gradients, as long as genetic operators in GAs are properly configured.
The lower bound of maximum predictable time can be formulated into a constrained nonlinear optimization problem, and the traditional solutions to this problem are the filtering method and the conditional nonlinear optimal perturbation (CNOP) method. Usually, the CNOP method is implemented with the help of a gradient descent algorithm based on the adjoint method, which is named the ADJ-CNOP. However, with the increasing improvement of actual prediction models, more and more physical processes are taken into consideration in models in the form of parameterization, thus giving rise to the on--off switch problem, which tremendously affects the effectiveness of the conventional gradient descent algorithm based on the adjoint method. In this study, we attempted to apply a genetic algorithm (GA) to the CNOP method, named GA-CNOP, to solve the predictability problems involving on--off switches. As the precision of the filtering method depends uniquely on the division of the constraint region, its results were taken as benchmarks, and a series of comparisons between the ADJ-CNOP and the GA-CNOP were performed for the modified Lorenz equation. Results show that the GA-CNOP can always determine the accurate lower bound of maximum predictable time, even in non-smooth cases, while the ADJ-CNOP, owing to the effect of on--off switches, often yields the incorrect lower bound of maximum predictable time. Therefore, in non-smooth cases, using GAs to solve predictability problems is more effective than using the conventional optimization algorithm based on gradients, as long as genetic operators in GAs are properly configured.
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