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1999 Vol. 16, No. 2

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An Ensemble Forecast of the South China Sea Monsoon
T. N. Krishnamurti, Mukul Tewari, Ed Bensman, Wei Han, Zhan Zhang, William K. M. Lau
1999, 16(2): 159-182. doi: 10.1007/BF02973080
This paper presents a generalized ensemble forecast procedure for the tropical latitudes. Here we propose an empirical orthogonal function-based procedure for the definition of a seven-mem-ber ensemble. The wind and the temperature fields are perturbed over the global tropics. Although the forecasts are made over the global belt with a high-resolution model, the emphasis of this study is on a South China Sea monsoon. Over this domain of the South China Sea includes the passage of a Tropical Storm, Gary, that moved eastwards north of the Philippines. The ensemble forecast handled the precipitation of this storm reasonably well. A global model at the resolution Triangular Truncation 126 waves is used to carry out these seven forecasts. The evaluation of the ensemble of forecasts is carried out via standard root mean square errors of the precipitation and the wind fields. The ensemble average is shown to have a higher skill compared to a control exper?iment, which was a first analysis based on operational data sets over both the global tropical and South China Sea domain. All of these experiments were subjected to physical initialization which provides a spin-up of the model rain close to that obtained from satellite and gauge-based esti?mates. The results furthermore show that inherently much higher skill resides in the forecast pre?cipitation fields if they are averaged over area elements of the order of 4o latitude by 4o longitude squares.
An Unsaturated Soil Water Flow Problem and Its Numerical Simulation
Xie Zhenghui, Dai Yongjiu, Zeng Qingcun
1999, 16(2): 183-196. doi: 10.1007/BF02973081
A numerical model for the unsaturated flow equation with moisture content as prognostic variable is established in order to simulate liquid moisture flow in an unsaturated zone with homoge-neous soil, and different initial and boundary conditions. For an infiltration or evaporation prob-lem, its numerical solution by using a finite difference method is very sensitive to its upper bounda-ry condition and the related soil parameters, and using a traditional finite element method usually yields oscillatory non-physics profiles. However, we obtain a nonoscillatory solution and evade a non-physics solution for the problem by using the mass-lumped finite element method. This kind of boundary conditions is handled very well. Numerical simulations for certain soils show that the numerical scheme can be used in simulation of liquid moisture flow for infiltration, evaporation, re-distribution and their alternate appearances. It can be also applied to a high-resolution land surface model.
Fundamental Framework and Experiments of the Third Generation of IAP/ LASG World Ocean General Circulation Model
Jin Xiangze, Zhang Xuehong, Zhou Tianjun
1999, 16(2): 197-215. doi: 10.1007/BF02973082
A new generation of the IAP / LASG world ocean general circulation model is designed and presented based on the previous 20-layer model, with enhanced spatial resolutions and improved parameterizations. The model uses a triangular-truncated spectral horizontal grid system with its zonal wave number of 63 (T63) to match its atmospheric counterpart of a T63 spectral atmosphere general circulation model in a planned coupled ocean-atmosphere system. There are 30 layers in vertical direction, of which 20 layers are located above 1000 m for better depicting the permanent thermocline. As previous ocean models developed in IAP / LASG, a free surface (rather than “rigid-lid” approximation) is included in this model. Compared with the 20-layer model, some more detailed physical parameterizations are considered, including the along / cross isopycnal mixing scheme adapted from the Gent-MacWilliams scheme. The model is spun up from a motionless state. Initial conditions for temperature and salinity are taken from the three-dimensional distributions of Levitus’ annual mean observation. A preliminary analysis of the first 1000-year integration of a control experiment shows some encouraging improvements compared with the twenty-layer model, particularly in the simulations of permanent thermocline, thermohaline circu?lation, meridional heat transport, etc. resulted mainly from using the isopycnal mixing scheme. However, the use of isopycnal mixing scheme does not significantly improve the simulated equatorial thermocline. A series of numerical experiments show that the most important contribution to the improvement of equatori?al thermocline and the associated equatorial under current comes from reducing horizontal viscosity in the equatorial regions. It is found that reducing the horizontal viscosity in the equatorial Atlantic Ocean may slightly weaken the overturning rate of North Atlantic Deep Water.
Constraint Inversion Algorithm of Lidar Equation for Deriving Aerosol Optical Property
Qiu Jinhuan
1999, 16(2): 216-228. doi: 10.1007/BF02973083
A key question of the backward integration algorithm to lidar equation is how to determine the far-end boundary value. This paper develops a Constraint Inversion Algorithm (CIA) for deriving the value and then the aerosol extinction profile from lidar signals, which uses the ground-level horizontal lidar signals as the constraint information. The smaller the wavelength is, the more sensitive to the variation of aerosol ex-tinction to backscatter ratio solved by CIA. According to the property an algorithm is further proposed to simultaneously retrieve the aerosol extinction profile, the size distribution and the imaginary part of its re-flective index from the multi-wavelength lidar observations. CIA is tested in the inversion simulations with satisfactory result.
A Diagnostic Study of the Impact of El Nino on the Precipitation in China
Zhang Renhe, Akimasa Sumi, Masahide Kimoto
1999, 16(2): 229-241. doi: 10.1007/BF02973084
The impact of El Ni?o on the precipitation in China for different seasons are investigated diagnostically. It is found that El Ni?o can influence the precipitation in China significantly during its mature phase. In the Northern winter, spring and autumn, the positive precipitation anomalies are found in the southern part of China during the El Ni?o mature phase. In the Northern summer, the patterns of the precipitation anomalies in the El Ni?o mature phase are different from those in the other seasons. The negative precipitation anomalies appear in both southern and northern parts of China, while in between around the lower reaches of the Yangtze River and the Huaihe River valleys the precipitation anomalies tend to be positive. In the Northern winter, spring and autumn, the physical process by which El Ni?o affects the precipitation in the southern part of China can be explained by the features of the circulation anomalies over East Asia during the El Ni?o mature phase (Zhang el al., 1996). The appearance of an anticyclonic anomaly to the north of the maritime continent in the lower troposphere during the El Ni?o mature phase intensifies the subtropical high in the western Pacific and makes it shift westward. The associated southwesterly flow is responsible for the positive precipitation anomalies in the southern part of China, In the Northern summer, the intensified western Pacific subtropical high covers the southeastern periphery of China so that the precipitation there becomes less. In ad?dition, the weakening of the Indian monsoon provides less moisture inflow to the northern part of China.
Modeling the Sudden Decrease in CH4 Growth Rate in 1992
Zhang Renjian, Wang Mingxing
1999, 16(2): 242-250. doi: 10.1007/BF02973085
A two-dimensional global chemistry model is developed to study the distribution and long-term trends of methane. The model contains 34 species and 104 chemical and photochemical reactions. Using the model, the long-term trends of CH4, CO and OH in at-mosphere are simulated, comparison between the model and observations shows that the simulation is successful. Experiments are done to investigate the causes of dramatic decrease in the growth rate of CH4, in 1992 such as OH increase due to stratospheric ozone depletion, decrease of tempera?ture in the troposphere due to Mount Pinatubo eruption and descendent of CH4 sources fluxes. A new explanation is proposed and verified by this model that the decrease of CO emission plays an important role for the abnormal growth rate of CH4 in 1992. We find that the de?creases of CH4 and CO emissions are the main reasons for the sudden decrease of growth rate of CH4 in 1992, which account for 73% and 27% respectively.
A Numerical Experiment Study for Effects of the Grassland Desertification on Summer Drought in North China
Zheng Weizhong, Ni Yunqi
1999, 16(2): 251-262. doi: 10.1007/BF02973086
In this paper, the summer climate of 1991 in North China is simulated by using the high- resolution regional climate model (RegCM2) and the effects of the grassland desertification on summer drought in the central and the northern parts of North China as well as Mongolia are studied. It shows that the regional climate model essentially catches the characteristics on distribution and seasonal variation of the precipitation that keep good agreement with the observation. The desertification makes precipitation in the central part of North China during its flood period de-crease obviously in July. The border of the precipitation or the soil moisture reduction in the desertification region extends about one latitude southeastward and beyond the southeast edge of the desertification. Thus, vegetation in the border region approaches desertification further. How-ever, there appears evident difference of variation of precipitation over the whole desertification region. The grassland desertification greatly changes the transfers of fluxes between land and at-mosphere. The secondary circulation or secondary circulation cells in the desertification region are excited and as a result moisture transport is changed. The variation of flux transfers between land and atmosphere as well as the vertical motion of atmosphere is closely related to that of precipita-tion.
Analytic Study of Sea-Land Breezes
K. Young, Zhang Ming
1999, 16(2): 263-278. doi: 10.1007/BF02973087
An analytic study of the structure of sea-land breezes is presented, with special attention paid to the dependence on the model parameters. In this linearized model, the wind speed of the sea-land breezes is directly proportional to the difference or sea and land heating rates. For the same differential heating, the sea-land breeze is more prominent if the stratification is weakly stable, or if the frictional force is small. The horizontal penetration from the coast is also investigated, and found to be asymmetric between the land and the sea. The above results are in agreement with observation.
Bifurcation of Nonlinear Kelvin Wave-CISK with Conditional Heating in a Truncated Spectral Model: A Possible Mechanism of 30-60-Day Osculation at the Equator
Luo Dehai
1999, 16(2): 279-296. doi: 10.1007/BF02973088
In this paper, the nonlinear Kelvin wave equations with “positive-only” nonlinear (conditional) heating at the equator are reduced to a sixth-order nonlinear ordinary differential equation by using the Galerkin spectral truncated method. The stability analysis indicates that when the heating parameter increases, the supercritical pitchfork and Hopf bifurcations can occur for the prescribed three heating profiles. Numerical calculations are made with the help of the fourth-order Rung-Kutta method. It is found that the convec-tion heating-related Hopf bifurcation can lead to limit cycle and chaotic solutions. In a wide range of heat-ing parameter, the solutions possess 30-60-day periods, and are dominated by wavenumbers one and two, especially by wavenurnber-one. In addition, the zonal winds of the low-frequency solutions have a phase reversal between the upper and lower tropospheres. Thus, it appears that the convection heating-related Hopf bifurcation might be a possible mechanism of 30-60-day oscillation in the tropical atmosphere.
Calculation of Probability of Cloud-Free Lines-of-Sight at Given Heights in Foshan, China
Li Yunying, Sun Litan
1999, 16(2): 297-303. doi: 10.1007/BF02973089
Based on Lund and Shanklin's work (1972), methods of calculating Probability of Cloud-Free Lines-of-Sight (PCFLOS), Persistence Probability of Cloud-Free Lines-of-Sight (PPCFLOS) and Recur-rence Probability of Cloud-free Lines-or-Sight (RPCFLOS) at given heights are presented. PCFLOS, PPCFLOS and RPCFLOS are calculated in Foshan, China by conventional observation data from 1961 to 1990. The conclusions are: (1) The higher the elevations, the smaller the PCFLOS and the larger the view angles, the larger the PCFLOS. (2) PPCFLOS and RPCFLOS decrease with the increase of elevation and the delay of time. (3) RPCFLOS is always equal to or larger than PPCFLOS at lag times.
Simplification of Potential Vorcticity and Mesoscale Quasi-balanced Dynamics Model
Zhao Qiang, Liu Shikuo
1999, 16(2): 304-313. doi: 10.1007/BF02973090
The physical characteristics of mesoscale are analyzed, and results show that the unbalanced forced motion is the fundamental cause, which leads to the evolution of some important mesoscale weather systems. In this paper, an alternative asymptotic expansion method, which is quite differ-ent from the conventional Rossby-number expansion, is used to simplify the potential vorticity equation. And the quasi-balanced (QB) model based on nonlinear balance equation is derived. The QB model, which is in analogy with the quasi –geostrophic model, can describe the fundamen-tal characteristics of the mesoscale accurately and may be used as the basis of theoretical studies on the mesoscale atmospheric dynamics.
Study on Moist Potential Vorticity and Symmetric Instability during a Heavy Rain Event Occurred in the Jiang-Huai Valleys
Shou Shaowen, Liu Yaohui
1999, 16(2): 314-321. doi: 10.1007/BF02973091
In the light of the theory on moist potential vorticity (MPV) investigation was undertaken of the 700 hPa vertical (horizontal) component MP1 (MPV2) for the heavy rain event occurring in Ju-ly 5-6,1991. Results show that the distribution features of the two components were closely related to the development of a mesoscale cyclone as a rainstorm-causing weather system in the lower troposphere in such a way that the ambient atmosphere of which MPV1>0 and MPV2<0 with |MPVl|≥|MPV2| favored the genesis of conditional symmetric instability (CSI) and that, as in-dicated by calculations, a CSI sector was really existent in the lower troposphere during the heavy rain happening and contributed greatly to its development.