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2002 Vol. 19, No. 2

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The Predictability Problems in Numerical Weather and Climate Prediction
Mu Mu, Duan Wansuo, Wang Jiacheng
2002, 19(2): 191-204. doi: 10.1007/s00376-002-0016-x
The uncertainties caused by the errors of the initial states and the parameters in the numerical modelare investigated. Three problems of predictability in numerical weather and climate prediction are proposed.which are related to the maximum predictable time, the maximum prediction error, and the maximum admissible errors of the initial values and the parameters in the model respectively. The three problems arethen formulated into nonlinear optimization problems. Effective approaches to deal with these nonlinearin dealing with these three problems.
The Mid-Holocene Climate Simulated by a Grid-Point AGCM Coupled with a Biome Model
Wang Huijun
2002, 19(2): 205-218. doi: 10.1007/s00376-002-0017-9
The climate simulation for the mid-Holocene about 6000 years before present (6 ka BP) is carried out with a grid-point atmospheric general circulation model (AGCM) coupled with a biome model. This coupled model simulation employs orbital parameters of 6 ka BP but present forcing conditions. Results show that large-scale climate differences between now and then are substantial in summer with dramatically strong African-Asian monsoon flow and precipitation during mid-Holocene. Although the results of this coupled model are qualitatively close to those of the AGCM, the coupled model shows a larger changes in both precipitation and temperature in summer over the North African monsoon area with weaker cooling in the Northern autumn.
An OGCM Simulation of Seasonal and lnterannual Variabilities in the Surface-Layer Pacific of the Equatorial Band
Boyin Huang, Zhengyu Liu
2002, 19(2): 219-235. doi: 10.1007/s00376-002-0018-8
The heat budget is analyzed in the surface-layer (0-50 m) Pacific of the equatorial band (10°S-10°N),using the simulation of an ocean general circulation model from 1945 to 1993. The analysis indicates that downward net surface heat flux from the atmosphere and ocean advective heat fluxes play distinct roles in seasonal and interannual variabilities of surface-layer ocean temperature. The surface heat flux dominantly determines the ocean temperature in the seasonal time-scale. But, it has a negative feedback to the ocean temperature in the interannual time-scale. The interannual variability of ocean temperature is largely associated with the cold advection from off-equatorial divergent flow in the central Pacific and from upwelling in the cold tongue. Both the surface heat flux and ocean advective heat fluxes are important to the ocean temperature during an El Nino event. The ocean advective heat fluxes are further associated with local westward trade wind in the central Pacific. These results are largely consistent with some regional observational analyses.
Ability of NCAR RegCM2 in Reproducing the Dominant Physical Processes during the Anomalous Rainfall Episodes in the Summer of 1991 over the Yangtze-Huaihe Valley
Luo Yong, Zhao Zongci, Ding Yihui
2002, 19(2): 236-254. doi: 10.1007/s00376-002-0019-7
The excessively torrential rainfall over the Yangtze-Huaihe valley during the summer of 1991 is simulated with an updated version of the second generation NCAR regional climate model (RegCM2) as a case factors contributing to the generation of the anomalous rainfall. This simulation is driven by large-scale atmospheric lateral boundary conditions derived from the European Center for Medium Range Weather Forecast (ECMWF) analysis. The simulation period is May to August 1991. The model domain covers East Asia and its adjacent oceanic regions. The model resolution is 60 km × 60 km in the horizontal and 23 layers in the vertical. The model can reasonably reproduce the daily precipitation events over East Asia for the summer of 1991, especially in the Yangtze-Huaihe valley where the anomalous rainfall occurred. The spatial and temporal structure of some important physical variables and processes related to the generation of the anomalous rainfall are analyzed. The time evolution of simulated upward vertical motion and horizontal convergence agrees with the five rainfall episodes over this subregion. The water vapor feeding the rainfall is mostly transported by the horizontal atmospheric motions from outside of the region rather than from local sources. The subtropical high over the western Pacific Ocean controls the progress and retreat of the summer monsoon over East Asia, and the RegCM2 can simulate the northward migration and southward retreat of subtropical high over the western Pacific Ocean. Furthermore, the model can represent the daily variation of the low level jet, which is crucial in the water vapor transport to the Yangtze-Huaihe valley.
A Proper Time Integration with Split Stepping for the Explicit Free-Surface Modeling
Zhou Weidong
2002, 19(2): 255-265. doi: 10.1007/s00376-002-0020-1
Errors due to split time stepping are discussed for an explicit free-surface ocean model. In commonly used split time stepping, the way of time integration for the barotropic momentum equation is not compatible with that of the baroclinic one. The baroclinic equation has three-time-level structure because of leapfrog scheme. The barotropic one, however, has two-time-level structure when represented in terms of the baroclinic time level, on which the baroclinic one is integrated. This incompatibility results in the splitting errors as shown in this paper. The proper split time stepping is therefore proposed in such a way that the compatibility is kept between the barotropic and baroclinic equations. Its splitting errors are shown extremely small, so that it is particularly relevant to long-term integration for climate studies. It is applied to a free-surface model for the North Pacific Ocean.
Wind Structure in an Intermediate Boundary Layer Model Based on Ekman Momentum Approximation
Tan Zhemin, Wang Yuan
2002, 19(2): 266-278. doi: 10.1007/s00376-002-0021-0
A quasi three-dimensional, intermediate planetary boundary layer (PBL) model is developed by including inertial acceleration with the Ekman momentum approximation, but a nonlinear eddy viscosity (1993). The model could keep the same complexity as the classical Ekman model in numerical, but extends the conventional Ekman model to include the horizontal accelerated flow with the Ekman momentum approximation. A comparison between this modified Ekman model and other simplified accelerating PBL models is made. Results show that the Ekman model overestimates (underestimates) the wind speed and pumping velocity in the cyclonic (anticyclonic) shear flow due to the neglect of the acceleration flow, however, the semi-geostrophic Ekman model overestimates the acceleration effects resulting from the underestimating (overestimating) of the wind speed and pumping velocity in the cyclonic (anticyclonic) shear flow. The Ekman momentum approximation boundary layer model could be applied to the baroclinic atmosphere.The baroclinic Ekman momentum approximation boundary layer solution has both features of classical baroclinic Ekman layer and the Ekman momentum approximate boundary lager.
Variation Features of Total Atmospheric Ozone in Beijing and Kunming Based on Dobson and TOMS Data
Bian Jianchun, Chen Hongbin, Zhao Yanliang, Lü Daren
2002, 19(2): 279-286. doi: 10.1007/s00376-002-0022-z
About 20 years of Dobson and TOMS data are used to analyze the variation characteristics of total atmospheric ozone in Beijing (39.93°N, 116.40°E) and Kunming (25.02°N, 102.68°E). It is shown that:(1) the long-term change trends for 1979 (or 1980)-2000 period are -0.642 DU / year and -0.009 DU / year respectively in Beijing and Kunming, (2) there are strong intra-seasonal variations especially in wintertime.which are comparable to seasonal variations both in Beijing and Kunming, (3) the long-term trend deduced from shorter time period of record is significantly different from that for longer time period, both in Kunming and Beijing, (4) there are significant QBO signals both in Beijing (mid latitude) and Kunming (low latitude), (5) the inter-annual variations of atmospheric ozone in both stations are mainly composed of the long-term trend and QBO signals, and (6) our Dobson and TOMS measurements of total ozone are generally in good agreement.
The Investigation of Atmospheric Angular Momentum as a Contributor to Polar Wobble and Length of Day Change with AMIP II GCM Data
Zhong Min, Yan Haoming, Zhu Yaozhong
2002, 19(2): 287-296. doi: 10.1007/s00376-002-0023-y
The atmospheric angular momentum (AAM) functions in terms of contribution to polar wobble and length of day change, are calculated from the output data of GSM9603 global circulation model (GCM) of Japan Meteorological Agency (JMA), from the reanalysis data of the National Centers for the Environmental Prediction (NCEP) / National Center for Atmospheric Research (NCAR), and from the operational objective analysis data of JMA, respectively. The comparison shows that during the period from 1985 to 1995, the values of the pressure terms in the equatorial components of AAM functions calculated from three data sets agree with each other better along 90°E longitude than along Greenwich meridian direction. The axial component of relative AAM function estimated from GSM 9603 agrees well with those from the other two data sets in terms of seasonal variations with the moderate amplitudes, but not so well with the composite axial component of relative AAM functions estimated from 23 GCM models anticipating in the first phase of AMIP. In addition, its interannual variation from 1979 to 1996 shows the main characteristics of ENSO evolution, just as does the axial component of relative AAM function estimated from NCEP reanalysis data except for the period of anomalous ENSO from 1991 to 1993.
Possible Impacts of Winter Arctic Oscillation on Siberian High, the East Asian Winter Monsoon and Sea-Ice Extent
Wu Bingyi, Wang Jia
2002, 19(2): 297-320. doi: 10.1007/s00376-002-0024-x
Using the NCEP/NCAR reanalysis dataset covering a 40-year period from January 1958 to December 1997, sea surface temperature (1950-1992), and monthly sea-ice concentration dataset for the period from 1953to 1995, we investigate connections between winter Arctic Oscillation (AO) and Siberian high (SH), the East Asian winter monsoon (EAWM), and winter sea-ice extent in the Barents Sea. The results indicate that winter AO not only influences climate variations in the Arctic and the North Atlantic sector, but also shows possible effects on winter SH, and further influences EAWM. When winter AO is in its positive phase, both of winter SH and the EAWM are weaker than normal, and air temperature from near the surface to the middle troposphere is about 0.S-2°C higher than normal in the southeastern Siberia and the East Asian coast, including eastern China,Korea, and Japan. When AO reaches its negative phase, an opposite scenario can be observed. The results also indicate that winter SH has no significant effects on climate variations in Arctic and the ture in contrast to AO. This study further reveals the possible mechanism of how the winter AO is related to winter SH. It is found that winter SH variation is closely related to both dynamic processes and air temperature variations from the surface to the middle troposphere. The western SH variation mainly depends on dynamic processes, while its eastern part is more closely related to air temperature variation. The maintaining of winter SH mainly depends on downward motion of airflow of the nearly entire troposphere. The airflow originates from the North Atlantic sector, whose variation is influenced by the AO. When AO is in its positive (negative) phase,downward motion remarkably weakened (strengthened), which further influences winter SH. In addition, winter AO exhibits significant influences on the simultaneous sea-ice extent in the Barents Sea.
10-25-Day Intraseasonal Variations of Convection and Circulation Associated with Thermal State of the Western Pacific Warm Pool during Boreal Summer
Ren Baohua, Huang Ronghui
2002, 19(2): 321-336. doi: 10.1007/s00376-002-0025-9
This study focuses on the characteristics of 10-25-day oscillation associated with the interannual variability of the thermal state in the western Pacific warm pool. The time series of 10-25-day oscillation shows a distinct feature between warm (WARM case) and cold (COLD case) summers over the western Pacific warm pool. The significant negative relationship between the time series of 10-25-day convection anomalies in Warm and Cold cases appears over most of Asian-Pacific region manifesting the interactions between the convection on interannual and 10-25-day intraseasonal time scales. At the peak and trough stages of 10-25-day convection oscillation, a Gill-type low-level atmospheric circulation anomaly, cyclonic or anticyclonic, appears northwest of the convection anomaly. This relationship between the convection and circulation exists both in Warm case and in Cold case. However, at other stages rather than the peak and trough stages, there is no Gill-type circulation response, and the circulation anomaly shows a distinct feature between the Warm and Cold cases, although the convection oscillation exhibits a roughly similar feature.
Monotonic Digit Filter for Limited-Area Model
Wang Yuan, Tan Zhemin
2002, 19(2): 337-349. doi: 10.1007/s00376-002-0026-8
Numerical diffusion or filter are used in most numerical models in order to eliminate small-scale (near two-grid intervals in wavelength) waves. However, conventional diffusion or filter schemes introduce the noise, and indeed few people realized, by filters themselves. For instance, most filters are troubled when they are put to use on meteorological fields with sharp gradient or with steep slope and consequently, the recurrence of undesirable numerical high-frequent oscillations (overshooting and undershooting) seems to be inevitable. Particularly when diffusion or filter is implemented in limited-area models, serious side effects on the limited-area boundaries often contaminate the modeling results. The merits and demerits are surveyed for commonly used diffusion or filter operations. A new type of monotonic digit filter is suggested to prevent overshooting and undershooting (due to the computational shock and Gibbs oscillation) nearby the discontinuous or nearly discontinuous locations when the filtering process was carried out, meanwhile the high selective property of damping is retained. Moreover, the new filter is designed on the implicit framework so that it can easily handle the problem of boundary diminishing in limited-area modeling.
On Problem of Nonlinear Symmetric Instability in Zonal Shear Flow
Shen Xinyong, Ni Yunqi, Ding Yihui
2002, 19(2): 350-364. doi: 10.1007/s00376-002-0027-7
This paper is focused on the problem of nonlinear symmetric instability in a baroclinic basic flow. The limited amplitude characteristics of unsteady wave were investigated with the aid of equations of adiabatic,inviscid, nonlinear symmetric disturbance and a multi-scale singular perturbation technique. Evidence suggests that the limited amplitude of unsteady wave exhibits an oscillatory trend of its intensity: the amplitude of the symmetric disturbance displays periodical variation both in super- and sub-critical shear case, and the duration of the periods is related not only to the stability parameters of the basic field and wave properties but to the amplitude of initial disturbance and its time-varying change rate as well.
The Diurnal Variation of Precipitation in Monsoon Season in the Tibetan Plateau
Liu Liping, Feng Jinming, Chu Rongzhong, Zhou Yunjun, K. Ueno
2002, 19(2): 365-378. doi: 10.1007/s00376-002-0028-6
The soundings, precipitation and radar data obtained from IOP of GAME-TIBET in the summer of 1998 are used to analyze the diurnal variations of precipitation and thermodynamic variables CAPE. LCL and relationships between precipitation and thermodynamic variables in monsoon season. The diurnal variations of precipitation are obvious. Maximum precipitation appears at the same time when CAPE reaches its maximum value. The atmospheric layer between 6 km and 8.5 km is unstable in most of the time, the strong stable layers below 6 km and above 9 km from 0400 to 0800 resists the development of convective system. The diurnal variation of precipitation is related to diurnal variations of thermodynamic variables.The diurnal variation of precipitation and effects of water vapor and temperature on precipitation are simulated by a three-dimensional cloud model. The cloud model reproduces the maxima and minima of diurnal variations of precipitation and reflectivity. The high humidity in low level at night is the key factor for the precipitation.