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2000 Vol. 17, No. 2

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A survey of Unbalanced Flow Diagnostics and Their Application
Zhang Fuqing, Steven E. Koch, Christopher A. Davis
2000, 17(2): 165-183. doi: 10.1007/s00376-000-0001-1
This paper presents an extensive survey of the most commonly used tools for diagnosing unbalanced flow in the atmosphere, namely the Lagrangian Rossby number, Psi vector, divergence equation, nonlinear balance equation, generalized omega-equation, and departure from fields obtained by potential vorticity (PV) inversion. The basic theory, assumptions as well as implementation and limitations for each of the tools are all discussed. These tools are applied to high-resolution mesoscale model data to assess the role of unbalanced dynamics in the generation of a mesoscale gravity wave event over the East Cost of the United States. Comparison of these tools in this case study shows that these various methods agree to a large extent with each other though they differ in details.
The Response of Climatic Jump in Summer in North China to Global Warming
Huang Jiayou
2000, 17(2): 184-192. doi: 10.1007/s00376-000-0002-0
To reveal climatic variation over North China, the climatic jumps in summer in Beijing are analyzed using the data of precipitation of summer (June, July, August) during the period of 1841-1993, in which those missed before 1950 were reconstructed by the stepwise regression method with minimum forecast error. The climatic jumps at different scales are analyzed using different diagnostic methods with different decade (10-100 years) windows. Some new methods and ideas are proposed. The variance difference, the linear tendency difference, and the difference of power spectral distribution between the samples before and after the period at the moving point in the center of the series are compared with other methods (for example, Mann-Kendall test, t-test, and accumulative anomaly etc.). Considering the differences among the statistics above, a synthetic jump index is also proposed in order to get the definite jump points in the moving series. The results show that the climatic jumps in the area occurred in the 1890’s, the 1910s and the 1920s, and mostly in the 1920s, which suggests that the local climatic jumps in North China have a simultaneous response to the global warming in the hundred-year scales.
On the Onset of the South China Sea Summer Monsoon in 1998
Li Chongyin, Wu Jingbo
2000, 17(2): 193-204. doi: 10.1007/s00376-000-0003-z
Through analyzing the NCEP/NCAR reanalysis data, the satellite observational data and the ATLAS-2 mooring buoy observational data, it is shown that May 21 is the onset data of the South China Sea summer monsoon in 1998. There were abrupt variations in the general circulation pattern at the lower troposphere and the upper troposphere, in upper jet stream location and in the convection and rainfall over the South China Sea region corresponding to the outbreak of the South China Sea summer monsoon. It is also indicated that there was rainfall in the southern China coastal region before onset of summer monsoon, but it resulted from the (cold) front activity and cannot be regarded as the sign of summer monsoon outbreak in the South China Sea.
Anomalies in the Tropics Associated with the Heavy Rainfall in East Asia during the Summer of 1998
Lu Riyu
2000, 17(2): 205-220. doi: 10.1007/s00376-000-0004-y
The summer of 1998 was characterised by a severe flood in East Asia. The possible linkages were examined among the anomalies in the tropics that may be associated with the severe flood. The anomalies of 1998 are obtained by removing the climatology, which is the average of the ECMWF (European Centre for Medium Range Weather Forecasts) Re-Analysis (ERA) data over 15 years from 1979 to 1993, from the corresponding fields of 1998, which are obtained from the ECMWF operational analyses.In comparison to the results of Nitta (1990), it was found that there are considerable similarities in the atmospheric circulation anomalies between the summers of 1998 and 1988, in the tropics as well as in middle-high latitudes. It was shown that the atmospheric convection is slightly suppressed over the tropical western Pacific. In general, the suppressed convection corresponds to a negative anomaly of SST in the warm pool region. In the summers of 1998 and 1988, however, there are positive anomalies of SSTs in the tropical western Pacific, corresponding to the suppressed convection over there. This slightly suppressed convection may not provide a viable forcing mechanism for the severe flood in East Asia. It was postulated that the zonal wind anomalies in the tropics, in addition to the atmospheric convection over the tropical western Pacific, influence the position and intensity of the North Pacific subtropical high.In both summers of 1998 and 1988, while the stronger convection occurs over the warmer tropical Indian Ocean, the suppressed convection corresponds to the positive anomalies of SSTs in the tropical western Pacific. A possible explanation was given for the broken relationship between SSTs and OLR, (Outgoing Longwave Radiation) by analyzing the large-scale atmospheric circulation anomalies in the tropics.The heat fluxes at the surface in the warm pool of the tropical western Pacific and tropical Indian Ocean were also examined by using the ERA-15 data. To avoid the inconsistency between the ERA-15 and the operational analyses, the anomalies of the heat fluxes at the surface in the warm pool region in the summer of 1988, instead of the summer of 1998, were examined. The anomalous latent heat flux and the net solar radiation flux are the main reason for the positive anomalies of SSTs in the tropical Indian Ocean and in the tropical western Pacific, respectively. The suppressed convection over the tropical western Pacific allows more solar radiation fluxes downward at the surface, which would increase the SSTs.
Numerical Simulations of Anomalies of Precipitation and Surface Air Temperature in China in the Summer of 1997
Qian yongfu, Wang Qianqian
2000, 17(2): 221-233. doi: 10.1007/s00376-000-0005-x
The anomalies of precipitation and surface air temperature in the summer (June to August) of 1997 are simulated by use of a global spectral numerical climate model (L9R15) developed in Australia originally and modified in LASG. The purpose of this paper is to study the effect of the El Nino event that happened in that year on the anomalies. The results show that the 1997 El Nino event does have a lot of influences on the climatic anomaly in that summer, however, the effect is not the same as pointed out by statistical studies. Therefore, the effects of the El Nino events are of uncertainties. The effects of the El Nino events on the re-gional climate in China might be different due to the different SSTA distributions over the western and northwestern Pacific in the El Nino years. It is likely more important to pay attention to the SSTA distribu-tive patterns and values in the Chinese adjacent oceans. Besides the El Nino event there might be other fac-tors such as the South Asia high at the 100 hPa level which has more direct impact on the climatic anomaly in China and can be taken as another strong signal of the climatic change in the atmosphere.
An Effective Method for Correcting the Seasonal-Interannual Prediction of Summer Climate Anomaly
Wang Huijun, Zhou Guangqing, Zhao Yan
2000, 17(2): 234-240. doi: 10.1007/s00376-000-0006-9
An effective method was proposed for correcting the seasonal-interannual prediction of the summer climate anomaly. The predictive skill can be substantially improved by applying the method to the seasonal-interannual prediction carried out by a coupled ocean-atmosphere model. Thus the method has the potential to improve the operational summer climate predictions.
A Numerical Study on Effects of Land-Surface Heterogeneity from ‘Combined Approach’ on Atmospheric Process Part II: Coupling-Model Simulations
Zeng Xinmin, Zhao Ming, Su Bingkai
2000, 17(2): 241-255. doi: 10.1007/s00376-000-0007-8
Two land surface schemes, one the standard Biosphere / Atmosphere Transfer Scheme Version le (B0Z) and the other B1Z based on B0Z and heterogeneously-treated by ‘combined approach’ , were coupled to the meso-scale model MM4, respectively. Through the calculations of equations from the companion paper, parameters representing land surface heterogeneity and suitable for the coupling models were found out. Three cases were simulated for heavy rainfalls during 36 hours, and the sensitivity of short-term weath-er modeling to the land surface heterogeneity was tested. Through the analysis of the simulations of the three heavy rainfalls, it was demonstrated that B1Z, compared with B0Z, could more realistically reflect the features of the land surface heterogeneity, therefore could more realistically reproduce the circulation and precipitation amount in the heavy rainfall processes of the three cases. This shows that even short-term weather is sensitive to the land surface heterogeneity, which is more obvious with time passing, and whose influence is more pronounced in the lower layer and gradually extends to the middle and upper layer. Through the analysis of these simulations with B1Z, it is suggested that the bulk effect of smaller-scale fluxes (i.e., the momentum, water vapor and sensible heat fluxes) near the significantly-heterogeneous Land surface is to change the larger-scale (i.e., meso-scale) circulation, and then to influence the development of the low-level jets and precipitation. And also, the complexity of the land-atmosphere interaction was shown in these simulations.
Evolution and Frontogenesis of an Imbalanced Flow —the Influence of Vapor Distribution and Orographic Forcing
Wang Yunfeng, Wu Rongsheng, Pan Yinong
2000, 17(2): 256-274. doi: 10.1007/s00376-000-0008-7
If the initial fields are not in geostrophic balance, the adjustment and evolution will occur in the stratified fluid, and the frontogenesis will occur under suitable conditions. The evolution is studied here with a nonhydrostatic fully compressible meso-scale model (Advanced Regional Prediction System, ARPS). Four cases are designed and compared: (i) control experiment; (ii) with different initial temperature gradient; (iii) with vapor distribution; (iv) with orographic forcing. The results show that: (1) there is an inertial oscillation in the evolution of the imbalanced flow with the frequency of the local Coriolis f, and with its amplitude de-creasing with time. The stationary balanced state can only be approached as it cannot be reached in the limit duration of time, The energy conversion ratio varies in the range of [0, 1 / 3]; (2) the stronger initial tempera-ture gradient can make the final energy conversion ratio higher, and vice versa; (3) suitable vapor distribu-tion is favorable for the frontogenesis. It will bring forward the time of the frontogenesis, strengthen the in-tensity of the cold front, and influence the final energy conversion ratio; (4) the orographic forcing has an ev-idently strengthening effect on the frontogenesis. The strengthening effect on the frontogenesis and the influ-ence on the final energy conversion ratio depend on the relative location of the mountain to the cold front.
A Study on the Nonlinear Stability of Fronts in the Ocean on a Sloping Continental Shelf
Li Yang, Mu Mu, Wu Yonghui
2000, 17(2): 275-284. doi: 10.1007/s00376-000-0009-6
The baroclinic nonlinear stability of fronts in the ocean on a sloping continental shelf is studied, the model equations, called the frontal geostrophic model, developed by Cushman-Roisin et al. (1992) for des-cribing the dynamics of surface density fronts in the ocean are developed and the two-layer frontal geostrophic model for fronts on a sloping continental shelf is first obtained. The nonlinear stability criteria for the fronts on a sloping bottom are obtained by using Arnol'd (1965, 1969) variational principle and a prior estimate method. It is shown that our result is better than the former works.
Diagnostic Study of Apparent Heat Sources and Moisture Sinks in the South China Sea and Its Adjacent Areas during the Onset of 1998 SCS Monsoon
Wang Shiyu, QianYongfu
2000, 17(2): 285-298. doi: 10.1007/s00376-000-0010-0
The apparent heat sources () and moisture sinks () are calculated based on the reanalyzed data of the South China Sea Monsoon Experiment (SCSMEX) from May 1 to August 31, 1998. It is found that the formation and distribution of the atmospheric heat sources are important for the monsoon onset. The earlier onset of the SCS monsoon is the result of enduring atmospheric heating in the Indo-China Pe-ninsula and South China areas. The atmospheric heating firstly appears in the Indo-China Peninsula area and the sensible heat is the major one. The 30-50 day periodic oscillation of atmospheric heat sources be-tween the SCS area and the western Pacific warm pool has a reverse phase distribution before the middle of July and the low frequency oscillation of heat sources in SCS area has an obvious longitudinal propagation. The 30-50 day low frequency oscillation has vital modificatory effects on the summer monsoon evolution during 1998.
A Study of the Relationship between Low-level Jet and Inversion Layer over an Agroforest Ecosystem in East China Plain?
Zhong Zhong, Wang Hanjie
2000, 17(2): 299-310. doi: 10.1007/s00376-000-0011-z
The relationship between the super-low-level jet (LLJ) and inversion layer over an agroforest ecosystem on the Huang-Huai-Hai plain in the eastern China is studied by means of a time-independent K-closure model. It is found that the intensified inversion near the surface of a luxuriantly growing agrofor-est ecosystem leads to the formation and development of the LLJ, the more intense the inversion, the strong-er is the LLJ. The critical value of inversion intensity index for the LLJ formation is 0.75℃/ 100 m, which relates to the necessary geostraphic wind velocity of 6.0 to 10 m/ s at the top level of the model The numer-ical calculations show that the roughness length of the underlying surface has considerable effects on the LLJ structure.
Low-Frequency CISK-Rossby Wave and Stratospheric QBO in the Tropical Atmosphere
Zhang Ren, Yu Zhihao
2000, 17(2): 311-321. doi: 10.1007/s00376-000-0012-y
Dynamic study is undertaken of the tropical atmospheric CISK-Rossby wave genesis and propagation mechanisms, the vertical structure of the low-frequency wave and the basic characteristics and constraint of the vertical transport of momentum and wave energy fluxes in relation to the quasi-biennial oscillation (QBO) of the stratospheric zonal winds over the tropics in the context of a baroclinic quasi-geoslrophic model. Results suggest that in the properly posed thermal conditions and zonal belt there exist two kinds of CISK-Rossby waves of low frequency (LF) and very low frequency (VLF), travelling zonally in opposite di-rections, which act as sources responsible for upward transferring momentum and wave energy fluxes for easterly and westerly perturbations in such a way as to provide required momentum and energy for the stratospheric QBO genesis and maintenance. The present study offers interpretations for some of the fun-damental observational facts of the QBO and proposes mew ideas of the QBO generation mechanism.
Main Features of Regional Circulation Variation during Onset of the South China Sea Monsoon in 1998
Shao Hui, Qian Yongfu
2000, 17(2): 322-338. doi: 10.1007/s00376-000-0013-x
In this work, the National Center for Environmental Prediction/ National Center for Atmospheic Re-search {NCEP/ NCAR} global daily-mean reanalysis data are used to diagnose the features of the local circulation variation during the South China Sea (SCS) monsoon in 1998. It is found that by taking the ap-pearance of southwest wind in lower layers and east wind in upper layers as the sign of the monsoon onset, the SCS monsoon starts on May 25 in 1998, which is later than that in normal years. The formation of the SCS monsoon is not a simple propagation of southwest monsoon in the north-south direction, but a process in which the southwest wind starts first over the north of the SCS. withdraws southward, and then propa-gates from south to north again. During this process, both meteorological elements and circulation fields change significantly. The outbreak of the SCS monsoon is the result of the seasonal variation of the height in lower and upper layers. The air rounding the Tibetan Plateau might be one of the dynamic reasons that make the summer monsoon start over the SCS at first. At the different stages of the monsoon, the vertical circulations as well as the lower and the upper layer divergence fields undergo evident temporal and regional changes. The SCS monsoon has the 60-day, 20-30-day and 8-15-day low frequency oscillations (LFOs), and dominant scale changes at the different stages of monsoon. The monsoon onset is related to the superimposition of the amplitudes of LFOs with different scales.