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2002 Vol. 19, No. 6

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On the Forced Tangentially-Averaged Radial- Vertical Circulation within Vortices Part Ⅰ: Concepts and Equations
Yuan Zhuojian, Jian Maoqiu
2002, 19(6): 953-964. doi: 10.1007/s00376-002-0058-0
A linear partial differential equation is derived in cylindrical-isobaric coordinates on the earth for the diagnostic study of the tangentially-averaged radial-vertical circulation within translating vortices. In the hydrodynamic stable atmosphere, the circulation will be forced through many dynamic and thermodynamic processes. These processes are associated with frictional torque, inertial torque, the horizontal and vertical divergence of eddy angular momentum, diabatic heating, adiabatic heating, and eddy temperature advection. For a given forcing, the intensity of circulation will increase with the decrease of static, inertial,and baroclinic stabilities. This paper also presents an explanation on the data interpolation from the latitude-longitude grid to the vortex volume gird and a brief discussion on the forcing processes.
Ocean Data Assimilation Using Intermittent Analyses and Continuous Model Error Correction
Bohua Huang, James L. Kinter III, Paul S. Schopf
2002, 19(6): 965-992. doi: 10.1007/s00376-002-0059-z
A new data insertion approach is applied to the Derber and Rosati ocean data assimilation (ODA) system, a system that uses a variational scheme to analyze ocean temperature and provide ocean model corrections continuously. Utilizing the same analysis component as the original system, the new approach conducts analyses to derive model corrections intermittently at once-daily intervals. A technique similar to the Incremental Analysis Update (IAU) method of Bloom et al. is applied to incorporate the corrections into the model gradually and continuously. This approach is computationally more economical than the original.A 13-year global ocean analysis from 1986 to 1998 is produced using this new approach and compared with an analysis based on the original one. An examination of both analyses in the tropical Pacific Ocean shows that they have qualitatively similar annual and interannual temperature variability. However, the new approach produces smoother monthly analyses. Moreover, compared to the independent observations from current meters, the new equatorial currents are significantly better than the original analyses, not only in maintaining the mean state but also in capturing the annual and interannual variations.
A Preliminary Study on the Global Land Annual Precipitation Associated with ENSO during 1948-2000
Shi Neng, Chen Luwen, Xia Dongdong
2002, 19(6): 993-1003. doi: 10.1007/s00376-002-0060-6
The global land monthly precipitation data (PREC / L) are used to investigate the relation between the global land annual precipitation and ENSO during 1948-2000, and the results of composite analysis are tested with Monte Carlo simulations. Results indicate that the global land annual precipitation was significantly reduced in large scale areas in warm event years;, the areas were the equatorial West Pacific, North China;equatorial Central America; North Bengal Bay and Nepal; East Australia; West India and South Pakistan;the district east of the Lena River; West Europe; and Wilkes Land of Antarctica. In contrast to this, the areas where precipitation increased in warm event years were less, and mainly in Chile and Argentina of South America; Somali, Kenya, and Tanzania of East Africa; Turkey, Iraq, and Iran of the Middle East; Libya and Nigeria of North Africa; Namibia of Southwest Africa; and Botswana and Zimbabwe of southern Africa. Statistical tests show that in warm event years, the area where the land annual precipitation was reduced was larger than the area where the annual precipitation increased, and the reduction in precipitation was more significant than the increase. The results in this paper are compared with previous studies. It is also pointed out that the interdecadal change of ENSO had no significant effect on the interdecadal variation of precipitation in the above regions. However, the warm events of El Nino affected the droughts in East Australia and North China more after the 1980s than before.
Indices of the Summertime Western North Pacific Subtropical High
Lu Riyu
2002, 19(6): 1004-1028. doi: 10.1007/s00376-002-0061-5
By averaging June-July-August (JJA) mean geopotential height anomalies at 850 hPa over the specified areas, the author proposes two innovative and succinct parameters to objectively define the zonal and meridional displacements of the western North Pacific subtropical high (WNPSH) in summer, respectively.Thus, these two indices and the present results may provide a basis for validating atmospheric general circulation models simulating the WNPSH. For the zonal index, the specified area is the west edge (110°-150°E,10°-30°N) of the WNPSH. For the meridional index, the specified area is the northwest edge (120°-150°E,30°-40°N) of the WNPSH. The interannual variations of these two indices are found to be independent. The results from a composite analysis based on the meridional index are in good agreement with previous studies based on case analyses.The two indices are compared with the existing indices announced by the National Climate Center (NCC) in China, on the interannual timescale. Despite slight differences, the interannual variations of the presented indices are basically similar to those of the NCC indices, and thus the circulation and precipitation associated with the present indices exhibit similar features to those associated with the NCC indices.Furthermore, an analysis of the differences between the associations of the present indices and the NCC indices shows that the presented indices are better than the NCC indices. An important result is that the zonal index is related to a more outstanding anomaly of precipitation, especially in East Asia and the Philippine Sea,both based on the presented indices and the NCC indices.The two indices can also be used to describe the seasonal march of the WNPSH during summer,namely, the poleward and eastward shifts. It is found that climatologically, the WNPSH shifts poleward and eastward rapidly in middle July, but the amplitudes of the poleward and eastward shifts are more remarkable in the summers when the WNPSH is located poleward and eastward in average.
The Impact of Dropsonde Data on Forecasts of Hurricane Debby by the Meteorological Office Unified Model
Qu Xiaobo, Julian Heming
2002, 19(6): 1029-1044. doi: 10.1007/s00376-002-0062-4
The numerical product of hurricane tracks vastly depends on initial observation fields. However, the forecast error is very large because of lack of observational data, especially when hurricanes are over the sea.This paper shows that extra non-real-time data (dropsonde data) can improve hurricane track forecasts compared with real-time observational data, and that the wind and relative humidity components of the dropsonde data have the greatest impact on the track forecasts.
Influences of Structures on Urban Ventilation:A Numerical Experiment
Liu Huizhi, Sang Jianguo, Zhang Boyin, Johnny C.L. Chan, Andrew Y.S.Cheng, Liu Heping
2002, 19(6): 1045-1054. doi: 10.1007/s00376-002-0063-3
Numerical simulations are carried out for the disturbed flow caused by three six-story buildings and a twenty-story tall tower respectively, as well as the distribution of automobile exhaust gas from a nearby road, based on the Peking University Model of Atmospheric Environment. The results show that the ventilation is better around the tall tower than around the three six-story residential buildings for the same number of households in the same urban region.
Spatiotemporai Variations of Summer Rainfall over Eastern China during 1880-1999
Li Xiaodong, Zhu Yafen, Qian Weihong
2002, 19(6): 1055-1068. doi: 10.1007/s00376-002-0064-2
By applying rotated complex empirical orthogonal function (RCEOF) analysis on 1880-1999 summer rainfall at 28 selected stations over the east part of China, the spatio-temporal variations of China summer rainfall are investigated. Six divisions are identified, showing strong temporal variability, the middle and lower reaches of the Yangtze River, the Huaihe River, Southeast China, North China, Southwest China, and Northeast China. The locations of all divisions except Southwest China are in a good agreement with those of the rainband which moves northward from Southeast China to Northeast China from June-August. The phase relationship revealed by the RCEOF analysis suggests that rainfall anomalies in the middle and lower reaches of the Yangtze River, Southeast China, and Northeast China are all characterized by a stationary wave, while a traveling wave is more pronounced in the Huaihe River division, North China, and Southwest China. The fourth RCEOF mode indicates that rainfall anomalies can propagate from south of Northeast China across lower reaches of the Huanghe River and the Huaihe River to the lower reaches of the Yangtze River. A 20-25-year oscillation is found at the middle and lower reaches of the Yangtze River, the Huaihe River valley, North China, and Northeast China. The middle and lower reaches of the Yangtze River and Northeast China also show an approximately-60-year oscillation. Northeast China and the Hnaihe River division are dominated by a 36-year and a 70-80-year oscillation, respectively. An 11-year oscillation is also evident in North China, with a periodicity similar to sunspot activity. The interdecadal variability in the middle and lower reaches of the Yangtze River, the Huaihe River valley, and North China shows a significant positive correlation with the solar activity.
The Impact of the Eastward Shifting of Dipole Systems over Large-Scale Terrain on Tropical Cyclone Tracks
Chen Lianshou, Luo Zhexian
2002, 19(6): 1069-1078. doi: 10.1007/s00376-002-0065-1
With a quasi-geostrophic barotropic model on the β-plane with a topography term, 16 experiments were performed with an integration period of 6 days. The interaction between tropical cyclone tracks and 500 kin-scale vortices originating from the western part ora large-scale topography is investigated. It is suggested that this kind of interaction may have a significant impact on the moving speed and direction of tropical cyclones. Under certain conditions, this interaction may be a factor in causing an abnormal tropical cyclone track. Furthermore, the effect of large-scale topography plays an important role in the formation of unusual tropical cyclone tracks.
Nonlinear Saturation of Baroclinic Instability in the Phillips Model: The Case of Energy
Xiang Jie, Sun Litan
2002, 19(6): 1079-1090. doi: 10.1007/s00376-002-0066-0
A conservation law for the Phillips model is derived. Using this law, the nonlinear saturation of purely baroclinic instability caused by the vertical velocity shear of the basic flow in the Phillips model-the case of energy-is studied within the context of Arnold's second stability theorem. Analytic upper bounds on the energy of wavy disturbances are obtained. For one unstable region in the parameter plane, the result here is a second-order correction in ε to Shepherd's; For another unstable region, the analytic upper bound on the energy of wavy disturbances offers an effective constraint on wavy (nonzonal) disturbances φ'i at any time.
Analysis of the Zonal Mean Atmospheric Climate State in IAP/ LASG GOALS Model Simulations
Zhang Tao, Guo Yufu, Wu Guoxiong
2002, 19(6): 1091-1102. doi: 10.1007/s00376-002-0067-z
The results of four versions of IAP/LASG Global Ocean-Atmosphere-Land System Model (GOALS) are analyzed separately over the oceans and over continents, and compared with observed data.Some fundamental atmospheric variables including surface air temperature (SAT), sea level pressure (SLP) and precipitation are examined to evaluate the ability of the GOALS model to simulate the contemporary climate and climate variability. In general, all four versions of the GOALS model are capable of reproducing the main features of the mean state and seasonal variation of the observed climate with reasonable accuracy.The evaluation also reveals some weakness of the model. According to this study, we can clearly see that the essential discrepancy of global averaged SLP lies over the continents in boreal summer. The simulated higher SAT over land versus the observed is mainly due to the effect of the land surface process. It is worth noting the underestimation by simulated precipitation rates mostly appears over the oceans, yet over-land precipitation is higher in high and middle latitudes than the observed for the boreal winter.Through intercomparisons among different versions of the model, it can be clearly seen that the incorporation of the diurnal cycle of solar radiation apparently improves the simulation of SAT, especially in the low and middle latitudes over land. Also, the introduction of the diurnal cycle shows a great improvement in precipitation in tropical continents and wintertime precipitation in high and middle latitudes. Furthermore,based on the daily flux anomaly exchange scheme (DFA), the latest version of GOALS model simulated over-ocean temperature variability is improved in the low and middle latitudes.Having compared the standard deviation of the annual mean surface air temperature (SAT) simulated by the GOALS model to observation, it is found that all four versions of the GOALS model underestimate surface air temperature variability over both oceans and land relative to observations. Several factors that may contribute to these differences between simulated and observed temperature variability are identified.
Elimination of Computational Systematic Errors and Improvements of Weather and Climate System Models in Relation to Baroclinic Primitive Equations
Zhong Qing, Chen Jiatian, Sun Zuoling
2002, 19(6): 1103-1112. doi: 10.1007/s00376-002-0068-y
The design of a total energy conserving semi-implicit scheme for the multiple-level baroclinic primitive equation has remained an unsolved problem for a long time. In this work, however, we follow an energy perfect conserving semi-implicit scheme of a European Centre for Medium-Range Weather Forecasts (ECMWF) type sigma-coordinate primitive equation which has recently successfully formulated. Some real-data contrast tests between the model of the new conserving scheme and that of the ECMWF-type of global spectral semi-implicit scheme show that the RMS error of the averaged forecast Height at 850 hPa can be clearly improved after the first integral week. The reduction also reaches 50 percent by the 30th day.Further contrast tests demonstrate that the RMS error of the monthly mean height in the middle and lower troposphere also be largely reduced, and some well-known systematical defects can be greatly improved.More detailed analysis reveals that part of the positive contributions comes from improvements of the extra-long wave components. This indicates that a remarkable improvement of the model climate drift level can be achieved by the actual realizing of a conserving time-difference scheme, which thereby eliminates a corresponding computational systematic error source / sink found in the currently-used traditional type of weather and climate system models in relation to the baroclinic primitive equations.
The Propagation and Transport Effect of Planetary Waves in the Northern Hemisphere Winter
Chen Wen, Huang Ronghui
2002, 19(6): 1113-1126. doi: 10.1007/s00376-002-0069-x
Based on the transformed Eulerian-mean equations, the dynamics of planetary waves are discussed.Both observations and simulations indicate that in the Northern Hemisphere winter there are two waveguides for the meridional propagation of quasi-stationary planetary waves. One is the high latitude waveguide, and the other is the low latitude waveguide. These results are in good agreement with theoretical analysis. Moreover, the convergence of EP flux indicates that the stratospheric sudden warming is the result of anomalous planetary wave propagation along the high latitude waveguide and its interaction with mean flows. The tropical quasi-biennial oscillation (QBO) winds, which represent one significant variation of zonal flow in the lower stratosphere at low latitudes, can influence the low latitude waveguide of planetary wave propagation. Our results of the wave-mean flow coupled model show that these tropical winds can also modulate the high latitude waveguide significantly in the case of wave-mean flow interaction.The transport effect of planetary waves on ozone is also analyzed. The residual mean circulation forced by planetary waves indicates that there is strong transport circulation for the dissipative planetary waves.Under the forcing of northward eddy heat transport, a positive transport circulation can result which rises at low latitudes and sinks at high latitudes. At the same time, the modification of planetary wave propagation by the equatorial QBO winds is shown to have an important impact on the transport circulation. The model results indicate that the meridional transport is amplified during the easterly phase of the QBO. This mechanism may explain the interannual variability of ozone in the stratosphere at high latitudes.
ENSO-Dependent and ENSO-Independent Variability over the Mid-Latitude North Pacific: Observation and Air-Sea Coupled Model Simulation
Zhou Tianjun, Yu Rucong, Li Zhaoxin
2002, 19(6): 1127-1147. doi: 10.1007/s00376-002-0070-4
During El Nino events, the warm anomalies in the eastern tropical Pacific are seen to occur in conjunction with prominent warm anomalies in the North Pacific SSTs off the west coast of North America as well as with cold anomalies in the central North Pacific. This kind of North Pacific response to ENSO is examined in observational data and IPSL air-sea coupled model simulations. Analyses based on observational data and the model output data both support the hypothesis of an "atmospheric bridge concept", i.e., the atmospheric response to ENSO, in turn, forces the extra-tropical SST anomalies associated with the El Ninno event, thereby serving as a bridge between the tropical and extra-tropical Pacific. Regarding the mechanism responsible for this, the ocean dynamical response to the atmospheric forcing is suggested to be active, while the contribution of latent heat flux is also significant. The role of solar radiation, longwave radiation, and sensible heat flux are of minor importance however, as indicated in the model.Further analysis shows that the North Pacific mode, which is linearly independent of ENSO, resembles the El Nino-type SST mode in the northern Pacific, i.e. both take the pattern ora zonally-oriented dipole in the subtropical Pacific, though differ slightly in the location of the anomaly center. The coupling between the North Pacific mode and the atmosphere is found to be mainly via air-sea heat flux exchange in the model. Both solar radiation and longwave radiation play important roles, while the contribution of latent heat flux is nearly negligible.
Modeling and Diagnostic Studies on the Variations of the Subtropical High over the Western Pacific from 1880 to 1999
Wang Shaowu, Cai Jingning, Mu Qiaozhen, Xie Zhihui, Zhu Jinhong, Gong Daoyi
2002, 19(6): 1148-1152. doi: 10.1007/s00376-002-0072-2
Index series of Subtropical High over the western Pacific was extended to AD 1880 by using of statistical and modeling method. Reconstructed indices by both methods show good accordance each other. Association of the indices to the rainfall patterns over eastern China indicated the robustness of the reconstructions.