Advanced Search

1998 Vol. 15, No. 2

Display Method:
The Westerly Anomalies over the Tropical Pacific and Their Dynamical Effect on the ENSO Cycles during 1980-1994
Huang Ronghui, Zang Xiaoyun, Zhang Renhe, Chen Jilong
1998, 15(2): 135-151. doi: 10.1007/s00376-998-0035-3
In this paper, the zonal wind anomalies in the lower troposphere over the tropical Pacific during 1980-1994 are analyzed by using the observed data. The results show that during the formation of the 1982 / 83, 1986 / 87 and 1991/92 ENSO events, there were the larger westerly anomalies in the lower troposphere over the equatorial Pacific. Moreover, it is explained by using the correlation analyses that the westerly anomalies over the equatorial Pacific could cause the warm episodes of the equatorial central and eastern Pacific. A simple air-sea coupled model is used to discuss theoretically the dynamical effect of the observed westerly anomalies of wind stress near the sea surface of the equatorial Pacific on the ENSO cycle occurred in the period of 1981-1983. It is shown by using the theoretical calculations of the equatorial oceanic Kelvin wave and Rossby waves responding to the forcing of the observed anomalies of zonal wind stress near the sea surface of the equatorial Pacific that the westerly anomalies of wind stress near the sea surface of the equatorial Pacific make significant dynamical ef?fect on the ENSO cycles occurred in the period of 1982-1983.
A Diagnostic Study of Moist Potential Vorticity Generation in an Extratropical Cyclone
Zuohao Cao, G.W.K. Moore
1998, 15(2): 152-166. doi: 10.1007/s00376-998-0036-2
Moist potential voracity (MPV) and Its generation may be important in the development of mesoscale structures such as rainbands within cyclones. In an adiabatic and frictionless flow, MPV generation is possible if the flow is three-dimensional and the air is unsaturated. Moist potential vorticity can be generated through the combined effects of gradients in the potential temperature and moisture fields. The diagnosis of MPV generation in an extratropical cy-clone was performed with the ECMWF objectively analyzed fields for a system that developed during February 1992. It was found that at various stages during the development of the cyclone, negative MPV was generated: at the north end of the cold front; along the occluded front and the cold front; and in the region of the warm core. This pattern of negative MPV generation is in excellent agreement with the predictions of previous theoretical and numerical studies. After the cyclone ceased to deepen, the region of negative MPV generated in the cyclone was horizontally adverted in-to a saturated area. The area of negative MPV generated both along the occluded front in this case study and in the region of the bent-back warm front in a numerical simulation showed a mesoscale structure with a width of about 200-500 km. It was found that the intrusion of moist or dry air into baroclinic zones was important for MPV genera-tion. In addition, baroclinicity increase (adjacent to the area of condensation) in the regions of high moisture gra-dients led to significant MPV production.
Global Oceanic Climate Anomalies in 1980’s
Fu Congbin, Xie Li
1998, 15(2): 167-178. doi: 10.1007/s00376-998-0037-1
The climate in the 1980’s is characterized by the appearance of two strong ENSO events and by the warmest dec-ade in global mean temperature. Whether there is a linkage between ENSO and global warming? This paper shows the climate anomaly patterns over the global ocean in the 1980’s and their comparison with that of ENSO composite mode and that simulated by 2 × CO2, indicating the role of super ENSO in the establishment of new climate regime in the 1980’s
A Numerical Study of Geostrophic Adjustment and Frontogenesis
Majid M. Farahani, Wu Rongsheng
1998, 15(2): 179-192. doi: 10.1007/s00376-998-0038-0
Using the PSU/NCAR Mesoscale Model version four(MM4), the frontogenesis and geostrophic adjustment problem in atmosphere with imbalance initial ideal data and conditions are studied. Based on results of experiments, it is found that the objective analysis and initialization procedure of the Model are not sensitive to the initial condi?tions used in this study. The final state of atmosphere, through process of adjustment, depends on the temperature gradient intensity of initial imbalance conditions. The front can be formed with appropriate condition. The processes of the frontogenesis are studied. It is also found that the response of the model to the ideal initial data used in this in?vestigation is sensitive to the selected lateral boundary condition. The time-dependent inflow / outflow lateral boundary condition is the best implemented option for this numerical study. Energetic study of the experiments shows that the front is formed after the initial transient stage when there is no exchange of energy between the kinetic and potential energy. The time needed for the formation of the front is longer than that predicted theoretically. The ratio of kinetic energy to the released potential energy is considered. This ratio varies with the temperature gradient intensity and the type of used wind for computing kinetic energy (geostrophic or geostrophic plus ageostrophic wind). The larger temperature gradient, the larger magnitude of this ratio. A maximum value of energy in either type of computed kinetic energies (geostrophic wind kinetic energy and actual wind kinetic energy) for cases that the fronts are observed whereby, and its magnitude and occurrence time depend on initial data distribution. The variation of the computed kinetic energies with time, after transition time, is reasonable and no sig?nificant conversion of the energy between kinetic and potential energy goes on, however, stability within variables is not achieved.
Activities of Low-Frequency Waves in the Tropical Atmosphere and ENSO
Li Guitong, Li Chongyin
1998, 15(2): 193-203. doi: 10.1007/s00376-998-0039-z
ENSO, particularly the occurrence of ENSO is still an important research object in climatic variation. Using the ECMWF data, the relationship between ENSO and the activities of low-frequency waves in the tropical atmosphere is analyzed in this paper. It is shown that the occurrence of ENSO is closely related to the intraseasonal oscillation and the quasi-stationary waves (the period > 90 days) in the tropical atmosphere. Associated with the occurrence of El Nino event, the kinetic energy of low-frequency waves has obvious varia?tion: the kinetic energy of atmospheric intraseasonal (30-60 days) oscillation (ISO) decreases abruptly and the kinetic energy of quasi stationary waves increases abruptly. Moreover, the ISO and quasi-stationary waves propagate eastward clearly corresponding to El Nino; but they clearly propagate westward in La Nina cases.
The Sensitivity of Numerical Simulation of the East Asian Monsoon to Different Cumulus Parameterization Schemes
Cheng Anning, Chen Wen, Huang Ronghui
1998, 15(2): 204-220. doi: 10.1007/s00376-998-0040-6
In this paper, a 5-level spectral AGCM is used to examine the sensitivity of simulated East Asian summer mon-soon circulation and rainfall to cumulus parameterization schemes. From the simulated results of East Asian mon-soon circulations and rainfalls during the summers of 1987 and 1995, it is shown that the Kuo’s convective parameterization scheme is more suitable for the numerical simulation of East Asian summer monsoon rainfall and circulation. This may be due to that the cumulus in the rainfall system is not strong in the East Asian monsoon region.
The Water-Bearing Numerical Model and Its Operational Forecasting Experiments Part I: The Water-Bearing Numerical Model
Xia Daqing, Xu Youping
1998, 15(2): 221-232. doi: 10.1007/s00376-998-0041-5
In first paper of articles, the physical and calculating schemes of the water-bearing numerical model are des-cribed. The model is developed by bearing all species of hydrometeors in a conventional numerical model in which the dynamic framework of hydrostatic equilibrium is taken. The main contributions are: the mixing ratios of all species of hydrometeors are added as the prognostic variables of model, the prognostic equations of these hydrometeors are in-troduced, the cloud physical framework is specially designed, some technical measures are used to resolve a series of physical, mathematical and computational problems arising from water-bearing; and so on. The various problems (in such aspects as the designs of physical and calculating schemes and the composition of computational programme) which are exposed in feasibility test, in sensibility test, and especially in operational fore?casting experiments are successfully resolved using a lot of technical measures having been developed from researches and tests. Finally, the operational forecasting running of the water-bearing numerical model and its forecasting system is realized stably and reliably, and the fine forecasts are obtained. All of these mentioned above will be described in second paper.
Frontogenesis, Evolution and the Time Scale of Front Formation
Fang Juan, Wu Rongsheng
1998, 15(2): 233-246. doi: 10.1007/s00376-998-0042-4
Observational study shows that, in some cases, the frontal structure displays the features of gravitative flows. It seems shat the formation of discontinuity is an important problem in the study of the frontogenesis which is usually defined as an increasing of the scalar gradient. In this paper, the characteristic features of air flow with initial imbalance between the wind and the density fields are studied. Much attention is paid on the condition for the forma-tion of discontinuity and its time scale. It is found that the initial distribution of density plays an important role in the formation of the discontinuity which happens in short time duration.
A Diagnostic Study on the Relationship between the Assembling of Low Frequency Waves in the Pacific Ocean and the Abnormality of the Subtropical High
Zhang Jianwen, Yu Shihua
1998, 15(2): 247-257. doi: 10.1007/s00376-998-0043-3
By use of the filter analysis technique, the Complex Empirical Othogonal Function (CEOF) method and the ECMWF/WMO 2.5o× 2.5o grid data of the geopotential heights during the summer months in 1988, an interseasonal process that the western Pacific subtropical high (WPSH) was anomalously far to the north in the first and second ten days of July is studied. It has been found that in the western Pacific subtropical region in the first and second ten days of July, it is the continuous assembly of low frequency geopotential waves (LFGWs) that leads to the abnormality of WPSH. This abnormality emerges with the enhancement of wave assembling and ceases while the wave assembling situation disappears. The structure of the low frequency assembling waves corresponds to the struc-ture of subtropical high in its abnormal period. The effect of the assembling waves on the abnormality of subtropical high can be considered as the accumulation of disturbance energy carried by the low frequency waves from different directions in the western Pacific region.
An Analytical Study on the Urban Boundary Layer
Lin Naishi, Zhou Zugang, Zhou Liufei
1998, 15(2): 258-266. doi: 10.1007/s00376-998-0044-2
A two-layer model based on the linearized time-independent atmospheric dynamical equations is proposed in this paper. The analytical solutions of the vertical, the horizontal motions and the potential temperature field induced by the anthropogenic source of urban surface heating are obtained, therefore the heat island circulation existing in unstable boundary layer is verified theoretically. From the analytical solutions, some conclusions can be drawn. (1) The vertical motion induced by urban heat island consists of two parts, namely, the cross-hill wave and the lee wave; (2) The cross-hill wave only exists in the unstable boundary layer, and varies with height according to exponential function law; (3) The vertical motion induced by heat island reaches the maximum at the top of the unstable boundary layer; (4) The wave generated by heat island not only propogates to the downwind district but also travels to the upwind area; (5) is not the necessary condition of the lee wave generation.
Monsoon Circulation Related to ENSO Phase-Locking
Xu Jianjun, Zhu Qiangen, Zhou Tiehan
1998, 15(2): 267-276. doi: 10.1007/s00376-998-0045-1
The relation of interannual anomaly of East Asian monsoon to the ENSO cycle is investigated in terms of even and odd symmetry analysis over a tropical heating field based on the past 30-year data. Evidence suggests that odd and even symmetry components related to the monsoon and Walker heating, respectively, effectively describe the East Asian monsoon circulation and Pacific Walker analog, with the monsoon intensity index corresponding to its heating vigor and western Pacific Walker heating vigor to ENSO phase change, both types of heating marked by pro-nounced seasonal variation and phase-locking; the key region for linking monsoon-ENSO interaction is the western Pacific warm pool; the monsoon effect upon ENSO cycle is affected jointly by the seasonal evolution and interannual anomaly of the heating components; the superimposition of an anti-Walker circulation phase produced by interannual winter monsoon perturbation upon a weaker Walker phase on a seasonal basis leads to an El Nino hap-pening in March-April and plays a significant role in maintaining a warm ENSO phase.
Estimation of Cloud Motion Using Cross-Correlation
Li Zhenjun
1998, 15(2): 277-282. doi: 10.1007/s00376-998-0046-0
This paper describes the estimation of cloud motion using lag cross-correlation. In order to compute the lag cross correlation, the Bayes Decision method is used first to identify cloud and surface or earth. Then cloud motion vectors are retrieved at a subset of points through multiple applications of a cross-correlation analysis. An objective analysis is used to define displacement at every satellite pixel throughout the domain and smooth the local inconsistencies. Cloud motions are then produced with a backward trajectory technique by using these displacement vectors.