Advanced Search

1999 Vol. 16, No. 3

Display Method:
Centennial Oscillations in an Ocean-ice Coupled Model
Jin Xiangze, Huang Ruixin, Yang Jiayan
1999, 16(3): 323-342. doi: 10.1007/s00376-999-0012-5
Irregular centennial oscillations, with a spectral peak at 106 years, were obtained from an ocean-ice coupled model for the North Atlantic with realistic coastline and bottom topography. The model’s thermohaline circulation is forced by mixed boundary conditions, i.e., a Haney-type relaxation condition for temperature, but an equivalent virtual salt flux condition for salinity. All forcing fields are taken from the observed monthly mean climatological wind stress and buoyancy fluxes. The oscillations appeared in the form of a surface-intensified tripole in both the sea surface temperature and salinity fields located in the vicinity of the Labrador Sea. The oscillations involve a delicate interplay between heat and fresh water advection by meridional overturning circulation, horizontal gyres, vertical convection, and the seasonal cycle. The oscillations are primarily control?led by the salinity component of the circulation; however, sea ice plays a minor role in driving the oscillations observed in the model. On the other hand, a regular seasonal cycle in the forcing fields is an important ingredient for the centennial oscillations.
Retrieval of Tropospheric CO Profiles Using Correlation Radiometer: I. Retrieval Experiments for a Clear Atmosphere
Wu Beiying, John Gille
1999, 16(3): 343-354. doi: 10.1007/s00376-999-0013-4
This paper discusses the retrieval scheme associated with the gas correlated radiometer- MOPITT which will be on board of EOS-AM1 to measure the global vertical profiles of car-bon monoxide. The vertical resolution and retrieval errors caused by errors in the temperature profiles and in the surface temperature have been assessed. The main results are: a. Assuming the noise equivalent radiance (NER) of 1.8 × 105 W m-2 sr-1, the surface tem?perature can be deduced from the wide band signals with uncertainly less than 1 K, and the atmospheric term of the modulated signal can be deduced with errors almost equal to the NER which does not significantly increase errors in the retrieved CO profiles. b. With typical uncertainty in temperature profiles, errors in the retrieved profiles at lati-? tudes lower than 70o are generally less than 20% with the first guess of 100 ppbv. (If a better first guess was used, the errors may decrease). c. By incorporating the total column CO amount derived from the reflected solar radiation in 2.3 μm spectral region into the retrieval, the accuracy of the retrieved CO profile below 6 km may be greatly improved. d. In the retrieval experiment with 10 CO profiles representing the typical CO profiles, the r.m.s. relative / absolute errors of the retrieved CO profiles are about 10% / 15-20 ppbv.
Retrievals of Rain-Rate over Oceans from SSM/ I Data Using SOM Model
Bian Jianchun, Chen Hongbin, Sun Haibing, Yang Peicai, Lu Daren, Zhou Xiuji
1999, 16(3): 355-360. doi: 10.1007/s00376-999-0014-3
A Self-Organizing Feature Mapping (SOM) network model was developed for the retrievals of rain-rate (RR) over oceans from SSM /I measurement, by using the SSM /I and. corresponding radar-de-rived rain-rate data provided by National Space Development Agency of Japan (NASDA). Based on the frequency-distribution of rain-rate samples, the SOM model was constructed in different rain-rate ranges. The model was first trained by five-sixths of the data, and the other data were used to test the retrieval abili-ty of the model. The retrieval results of the SOM model were compared with two statistically-based algorithms. It is shown that the SOM model provides better retrievals of rain-rate.
A Preliminary Study on the Polar Climate Predictability
Wang Huijun
1999, 16(3): 361-366. doi: 10.1007/s00376-999-0015-2
Studies have revealed that predictability of the atmospheric general circulation is generally high in the tropics throughout the year and that there is some predictability in the Northern extra-tropical winter at-mospheric circulation through some patterns of tele connection. Predictability of the general circulation at the polar regions has still remained as a ‘cold’ topic and little has been known about this question. Based on a preliminary study on the predictability by using the Institute of Atmospheric Physics (IAP) general circulation model, it is found that the SST-related predictability of the Southern winter lower atmospheric circulation in Antarctica is reasonably high and that there is some predictability in the 500 hPa and 200 hPa geopotential height fields over Europe and the Okhotsk Sea region during the Northern winter. It is sug-gested that more researches on this issue based on data analysis and model simulations are needed to obtain better understanding.
Numerical Experiments for the influence of the Transition Zone Migration on Summer Climate in North China
Zheng Weizhong, Ni Yunqi
1999, 16(3): 367-377. doi: 10.1007/s00376-999-0016-1
As the position of the transition zone changes obviously, that is, as the transition zone migrates to the north or the south from present position, it affects water or heat balance between the land and the atmosphere in a considerable degree and has a profound influence on climate in North China. The experiment re-sults in this paper indicate whether in the dry case or in the wet case of the large-scale climatological back-ground field, the surface air temperature in a wide range of the transition zone migration and its sur-rounding decreases as the transition zone migrates northward. Moreover, the net upward fluxes of the sur-face long wave radiation and the sensible heat decrease, and the evaporation to the atmosphere increases. As the transition zone migrates southward, the results are opposite. This kind of significant thermal forcing between the land and the atmosphere can excite secondary circulation or circulation cells, which interact with the large-scale circulation systems, changing the atmospheric motion, affecting the water vapor transporta-tion and consequently having an effect on the precipitation.
The Variation of Warm Pool in the Equatorial Western Pacific and Its Impacts on Climate
Li Chongyin, Mu Mingquan, Zhou Guangqing
1999, 16(3): 378-394. doi: 10.1007/s00376-999-0017-0
The variation of warm pool ocean temperature in the equatorial western Pacific and its im-pacts on climatic change are studied in the present paper. The SSTs in the warm pool region not only have seasonal variation but also have interannual variation more clearly; The influence of anomalies of SST in the warm pool region on the East Asian monsoon is studied with data analysis; And the impact of SSTA in the warm pool region on the teleconnection (wave-train) in the atmospheric circulation is still investigated. The influence of ocean temperature anomalies in the warm pool subsurface on the occurrence of ENSO is also discussed by using data analysis and modelling with CGCM. All above-mentioned studies show that the situation of ocean temperature in the warm pool region in the equatorial western Pacific plays an important role in the global cli?matic variation.
Numerical Simulations of Intense Meiyu Rainfall in 1991 over the Changjiang and Huaihe River Valleys by a Regional Climate Model with p-б Incorporated Coordinate System
Liu Huaqiang, Qian Yongfu
1999, 16(3): 395-404. doi: 10.1007/s00376-999-0018-z
Based on the primitive equation model with p-σincorporated coordinate system originally developed by Qian et al., a one-way nested fine mesh limited area model is developed. This model is nested with ECMWF T42 data to simulate the extra-intensive rainfall event occurring in the Changjiang and Huaihe River valleys in summer of 1991. The results show that the model has cer-tain capacity to fairly reproduce the regional distribution and the movement of the main rainfall belts. Therefore it can be used as a regional climate model to simulate and predict the short-range regional climate changes.
A Hybrid Coupled Ocean-Atmosphere Model and ENSO Prediction Study
Wu Aiming, Ni Yunqi
1999, 16(3): 405-418. doi: 10.1007/s00376-999-0019-y
A hybrid coupled ocean-tmosphere model is designed, which consists of a global AGCM and a simple anomaly ocean model in the tropical Pacific。 Retroactive experimental predictions in-itiated in each season from 1979 to 1994 are performed. Analyses indicate that: (1) The overall predictive capability of this model for SSTA over the central-eastern tropical Pacific can reach one year, and the error is not larger than 0.8?C. (2) The prediction skill depends greatly on the season when forecasts start. However, the phenomenon of SPB (spring prediction barrier) is not found in the model. (3) The ensemble forecast method can effectively improve prediction results. A new initialization scheme is discussed.
Application of Variational Algorithms in Semi-Lagrangian Framework
Wang Yunfeng, Wu Rongsheng, Wang Yuan, Pan Yinong
1999, 16(3): 419-430. doi: 10.1007/s00376-999-0020-5
The variational data assimilation scheme (VAR) is applied to investigating the advective effect and the evolution of the control variables in time splitting semi-Lagrangian framework. Two variational algorithms are used. One is the conjugate code method-direct approach, and another is the numerical backward integration of analytical adjoint equation-indirect approach.Theoretical derivation and sensitivity tests are conducted in order to verify the consistency and inconsistency of the two algorithms under the semi-Lagrangian framework. On the other hand, the sensitivity of the perfect and imperfect initial condition is also tested in both direct and indirect approaches. Our research has shown that the two algorithms are not only identical in theory, but also iden?tical in numerical calculation. Furthermore, the algorithms of the indirect approach are much more feasible and efficient than that of the direct one when both are employed in the semi-Lagrangian framework. Taking advantage of semi-Lagrangian framework, one purpose of this paper is to illustrate when the variational assimilation algorithm is concerned in the computational method of the backward integration, the algorithm is extremely facilitated. Such simplicity in indirect approach should be meaningful for the VAR design in passive model. Indeed, if one can successfully split the diabatic and adiabatic process, the algorithms represented in this paper might be easily used in a more general vision of atmospheric model.
Study on a Boundary-layer Numerical Model with Inclusion of Heterogeneous Multi-layer Vegetation
Zeng Xinmin, Zhao Ming, Su Bingkai, Wang Hanjie
1999, 16(3): 431-442. doi: 10.1007/s00376-999-0021-4
On the basis of improving the algorithm of the mixing length in and above forest canopies, a PBL numerical model including the multi-layer, heterogeneous vegetation is developed. Simulations indicate that different treatments of mixing length can make a great difference in the wind field especially for dense forest, and results from the improved mixing length scheme are in better agreement with observations than those from the original scheme. It may be expected that the improved mixing length scheme can lead to more ra-tional turbulent transfer than the original one. From the sensitivity experiments, we obtain the characteris-tics of both wind and temperature profiles in and above plant canopies, e.g., during the daytime, a stable thermal stratification exists near the surface in the canopies, but a neutral or slightly unstable condition ap-pears above plant canopies, while at night the reverse situations occur; the increase of the temperature of the dense-forest case is less than that of the sparse-forest case; the windspeed is reduced within the canopy lay-er and the large wind shear occurs near the treetop, etc.
Numerical Simulation of the Regional Ocean Circulation in the Coastal Areas of China
Zhang Yaocun, Qian Yongfu
1999, 16(3): 443-450. doi: 10.1007/s00376-999-0022-3
The regional ocean circulation in the coastal areas or China (including a part of the western Pacific Ocean, the South China Sea and the Bay of Bengal el al.) is simulated by using the improved Princeton Uni-versity ocean circulation model (POM). Compared with the modeling results obtained by the large-scale ocean general circulation model (OGCM), the basic ocean circulation features simulated by the regional ocean circulation model are in good agreement with that simulated by OGCM and some detailed character-istics such as the regional ocean circulation, sea temperature, salinity and free sea surface height have also been obtained which are in good accord with the observations. These results indicate that the regional ocean circulation model has good capability to produce the regional ocean circulation characteristics and it can be used to develop coupled regional ocean-atmospheric model systems.
Nonlinear Three-Wave Interaction among Barotropic Rossby Waves in a Large-scale Forced Barotropic Flow
Luo Dehai
1999, 16(3): 451-466. doi: 10.1007/s00376-999-0023-2
In this paper, the coupling equations describing nonlinear three-wave interaction among Rossby waves including the forcing of an external vorticity source are obtained. Under certain conditions, the coupling equations with a constant amplitude forcing, the stability analysis indi-cates that when the amplitude of the external forcing increases to a certain extent, a pitchfork bifurcation occurs. Also, it is shown from numerical results that the bifurcation can lead to chaotic behavior of “strange” attractor. For the obtained three-variable equation, when the amplitude of modulated external forcing gradually increases, a period-doubling bifurcation is found to lead to chaotic behavior. Thus, in a nonlinear three-wave coupling model in the large-scale forced barotropic atmospheric flow, chaotic behavior can be observed. This chaotic behavior can explain in part 30-60-day low-frequency oscillations observed in mid-high latitudes.
Interaction of Orographic Disturbance with Front
Wang Xingbao, Wu Rongsheng
1999, 16(3): 467-481. doi: 10.1007/s00376-999-0024-1
The interaction of orographic disturbance with front is investigated with a nonhydrostatic fully compressible mesoscale model (ARPS). It is shown that the front is dominated mainly by the orographic dis-turbance if the front is weak. Firstly, because the stratified airstream is forced lo flow along the topographic surface, the topographic surface almost coincides with the lowest isentrope for the barotropic flow. The po-tential temperature gradients are opposite on upwind slope and downwind slope. As the cold front moves across the mountain, its intensity decreases on the upwind side and increases on the downwind side due to the thermal superposition. Conversely, the warm front is strengthened on the upwind slope and weakened on the downwind slope. This is the thermal superposition effect. Secondly, the mountain-forced circulation and orographic waves, which depend on the shape and size of topography and characteristics of airflow, contribute to frontogenesis and /or frontolysis. This is referred as dynamical action. For the mesoscale mountain ridge of gentle slope, the dynamical action weakens the cold front on the upwind slope, and strengthens the cold front on the lee side. While for the mesoscale mountain of steep slope, the dynamical ef-fect weakens the cold front on the upwind side and strengthens the cold front on the mountain top, the frontal intensity is decreased when front moves downslope rapidly. As front moves into the convergent zone near the mountain base, its intensity is enhanced severely. If the front is intensive, there is strong interaction between the orographic disturbance and the front. The cold front dramatically increases downslope wind and lee side gravity wave activity. And these in turn act upon the frontal intensity and frontal structure. For the baroclinic basic flow, the southerly warm advection on the upwind side makes the cold front less frontolysis; the northerly on the lee side violently intensifies the clod front.
Dynamics of Absolute Vorticity in the Boussinesq Fluid
Zuohao Cao
1999, 16(3): 482-486. doi: 10.1007/s00376-999-0025-0
The dynamics of absolute vorticity in the Boussinesq fluid is examined. It is shown that the Boussinesq approximation only captures one of the horizontal components of the solenoidal term. Based on scaling ana-lysis of typical midlatitude synoptic systems, the horizontal component of the solenoidal term neglected by the Boussinesq approximation is at least of the same order of magnitude as the one captured by the Boussinesq approximation. This leads to severe underestimation of absolute vorticity and circulation.