共查询到20条相似文献,搜索用时 31 毫秒
1.
Comparison of different order Adams-Bashforth methods in an atmospheric general circulation model
下载免费PDF全文
![点击此处可从《Acta Meteorologica Sinica》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The Asselin-Robert time filter used in the leapfrog scheme does degrade the accuracy of calculations. As an attractive alternative
to leapfrog time differencing, the second-order Adams-Bashforth method is not subject to time splitting instability and keeps
excellent calculation accuracy. A second-order Adams-Bashforth model has been developed, which represents better stability,
excellent convergence and improved simulation of prognostic variables. Based on these results, the higher-order Adams-Bashforth
methods are developed on the basis of NCAR (National Center for Atmospheric Research) CAM 3.1 (Community Atmosphere Model
3.1) and the characteristics of dynamical cores are analyzed in this paper. By using Lorenz nonlinear convective equations,
the filtered leapfrog scheme shows an excellent pattern for eliminating 2Δt wave solutions after 20 steps but represents less computational solution accuracy. The fourth-order Adams-Bashforth method
is closely converged to the exact solution and provides a reference against which other methods may be compared. Thus, the
Adams-Bashforth methods produce more accurate and convergent solution with differencing order increasing. The Held-Suarez
idealized test is carried out to demonstrate that all methods have similar climate states to the results of many other global
models for long-term integration. Besides, higher-order methods perform better in mass conservation and exhibit improvement
in simulating tropospheric westerly jets, which is likely equivalent to the advantages of increasing horizontal resolutions.
Based on the idealized baroclinic wave test, a better capability of the higher-order method in maintaining simulation stability
is convinced. Furthermore, after the baroclinic wave is triggered through overlaying the steady-state initial conditions with
the zonal perturbation, the higher-order method has a better ability in the simulation of baroclinic wave perturbation. 相似文献
2.
A two-layer quasi-geostrophic baroclinic model in a narrow, longitudinally periodic channel on a β-plane is used, which involves near-resonant thermal forcing, frictional dissipation and a uniformly sheared basic current. By means of the multi-scale technique, a system of simplified differential equations or disturbances due to the steady thermal-forcing waves projected on the X-T plane is derived, which contains the nonlinear interactions between forced waves and free waves. The asymptotic solutions of the equilibrium states of these equations are analytically obtained by a singular perturbation method. The results show that these mul-tiple equilibrium state (MES) solutions can exist in a wide range of the parameters used. 相似文献
3.
Dr. Ch. A. Lin 《Meteorology and Atmospheric Physics》1987,37(2):72-81
Summary A nonlinear, forced, dissipative quasi-geostrophic, two-level -plane model of baroclinic instability is formulated. The model resolves a baroclinic zonal flow and a wave of arbitrary zonal scale. Multiple equilibrium solutions describing Hadley and eddy circulations coexist. Only the circulation with smaller thermal wind is stable. The most efficient eddy activity occurs at a zonal wavenumber close to the wavelength of maximum instability of linear baroclinic instability theory. For a wide range of forcing and dissipative parameters, the steady baroclinic zonal wind of the eddy regime is close to the critical shear of linear theory. Eddy statistics are obtained analytically in terms of the doparture of the zonally symmetric state from radiative equilibrium. A parameterization for the eddy heat transport is obtained.With 14 Figures 相似文献
4.
1998年夏季HUBEX/GAME期间热量和水汽收支(英) 总被引:4,自引:0,他引:4
By using the high-resolution GAME reanalysis data, the heat and moisture budgets during the period of HUBEX/GAME in the summer of 1998 are calculated for exploring the thermodynamic features of Meiyu over the Changjiang-Huaihe (CH) valley. During the CH Meiyu period, an intensive vertically-integrated heat source and moisture sink are predominant over the heavy rainfall area of the CH valley, accompanied by strong upward motion at 500 hPa. The heat and moisture budgets show that the main diabatic heating component is condensation latent heat released by rainfall. As residual terms, the evaporation and sensible heating are relatively small. Based on the vertical distribution of the heat source and moisture sink, the nature of the rainfall is mixed, in which the convective rainfall is dominant with a considerable percentage of continuous stratiform rainfall. There are similar time evolutions of the main physical parameters(〈Q1〉,〈Q2〉,and vertical motion ω at 500 hPa).The time variations of〈Q1〉and〈Q2〉are in phase with those of -ω500, and have their main peaks within the CH Meiyu period. This shows the influence of the heat source on the dynamic structure of the atmosphere. The wavelet analyses of those time series display similar multiple timescale characteristics. During the CH Meiyu period, both the synoptic scale(~6 days) and mesoscale (~2 days and ~12 hours) increase obviously and cause heavy rainfall as well as the appearances of the maxima of the main physical parameters. Among them, the mesoscale systems are the main factors. 相似文献
5.
During the XXII Italian expedition in Antarctica, in the summer of 2007, severe weather conditions associated with deep convective
instability and heavy coastal precipitation occurred around the Terra Nova Bay area in the presence of an upper level trough
and energetic katabatic winds flowing from the Ross Ice Shelf over the open sea. In this case study we document an example
of boundary-layer frontal movement across the Ross Sea and mesocyclone development in conjunction with the frontal movement.
A westward fast moving boundary-layer front, generated by the leftward turning katabatic airstream to the east of Ross Island,
was observed propagating as a baroclinic wave disturbance in an easterly direction across the Ross Sea, merging later with
a mesocyclone approaching Terra Nova Bay from offshore. The observed inertial trajectory and an estimation of the radius of
curvature suggest that the vigorous katabatic airstream was sustained by the strengthening of a surface mesocyclonic circulation
settled over the north-eastern Ross Ice Shelf, triggered by a sub-synoptic upper level trough passing over the area. We hypothesise
that baroclinic instability in the low levels plays an important role in the development of a mesoscale vortex and for triggering
convective precipitation. 相似文献
6.
S. Dutta 《Meteorology and Atmospheric Physics》2005,90(3-4):139-152
Summary In this paper, an attempt has been made to examine the effect of static stability on the pattern of three dimensional (3-D)
baroclinic lee wave across a meso-scale elliptical barrier. For this purpose first a 3-D meso scale lee wave model has been
developed. Then the model is applied to the Western Ghats (WG) using real time radio sonde data of Santacruz (19°7′N, 72°51′E)
(here after SCZ), a station on the windward side of WG, on the days when dynamic and thermodynamic conditions of the atmosphere
were favourable to generate lee waves.
It is found that the pattern of 3-D baroclinic lee wave is very much sensitive to the value of the static stability parameter
N2. It is found that during southwest monsoon season trapped lee waves are convergent type (contours of perturbation vertical
velocity w′ are crescent shaped convex down wind) and during winter they are divergent type (contours of w′ are crescent shaped concave down wind). The study shows that for a given profile of wind, the value of N2 must exceed certain threshold value to obtain divergent type lee wave, otherwise convergent type lee waves are found.
It is also found that in the southwest monsoon season, when atmosphere is neutrally stratified, a single divergent lee wave
corresponds to a single transverse lee wave, whereas in the winter season, when atmosphere is strongly stratified, a single
divergent lee wave corresponds to a number of transverse lee wave. Furthermore, in the former case long (or short) divergent
lee wave corresponds to short (or long) transverse lee wave, whereas in the later case long (or short) divergent lee wave,
in general, corresponds to long (or short) transverse lee wave.
This revised version was published online in November 2004 with corrected captions of Figs. 1 and 2. 相似文献
7.
J.-P. Boulanger 《Climate Dynamics》2001,17(2-3):159-173
Zonal advection by long equatorial waves has been shown to be an important process in the evolution of sea surface temperature
in the central Pacific on ENSO time scales. The present study aims at investigating how well an oceanic model whose dynamics
are based on long equatorial waves can simulate the large-scale surface zonal current variability. Thus an ocean linear model
which can be run with two or three layers is validated against several sets of observations in the Pacific ocean (TOPEX/POSEIDON
sea level, TAO zonal currents, surface current climatology). The surface layer (mixed-layer) has a constant depth. Therefore
the layer model is equivalent to considering a shear layer solution and either one or two baroclinic modes. It allows evaluation
of the impact of adding a second baroclinic mode on the simulation of surface currents. This evaluation is done for different
friction parametrizations: a weak linear Rayleigh friction (24 months−1), a strong linear Rayleigh friction (6 months−1), and a new parametrization using quadratic friction in the momentum equation only. It is shown in all simulations using
various Rayleigh friction parametrizations that the addition of a second baroclinic mode always improves the simulation of
both the sea level and the surface currents, especially in the central western Pacific. In that region, there is a reduction
of the propagating long Rossby waves whose amplitude is much too large when only one baroclinic mode is used. Despite this
reduction, the use of a weak friction (24 months−1) always yields results which compare only poorly to observations confirming results from previous studies. The use of strong
friction (6 months−1) improves the model simulation, but surface current variability still remains too large. Finally, the use of quadratic friction
as proposed in the present study considerably improves the simulation of zonal currents and its comparison to all data sets.
This result gives more confidence in the choice of such a simple model to further explore the role of zonal advection by long
equatorial waves on ENSO time scales.
Received: 28 May 1999 / Accepted: 18 May 2000 相似文献
8.
By using a low-order,two-layer baroclinic quasi-geostrophic model,a nonlinear system including the interaction between a thermal forced wave,a transient wave and zonal flow is studied. Under the conditions of near-resonance and weak baroclinic instability,the features of solution in phase space are discussed with the analytical methods of multiple scale and discontinuous oscillation.The results show that the dynamic coupling between forced wave and transient wave is responsible for the physical mechanism of the non-uniform index cycle of the atmospheric circulation. 相似文献
9.
A conservation law for the Phillips model is derived. Using this law, the nonlinear saturation of purely baroclinic instability caused by the vertical velocity shear of the basic flow in the Phillips model-the case of energy-is studied within the context of Arnold's second stability theorem. Analytic upper bounds on the energy of wavy disturbances are obtained. For one unstable region in the parameter plane, the result here is a second-order correction in ε to Shepherd's; For another unstable region, the analytic upper bound on the energy of wavy disturbances offers an effective constraint on wavy (nonzonal) disturbances φ'i at any time. 相似文献
10.
It is shown by observational data and synoptic analysis that the development of strong convective echo is influenced by the
horizontally non-uniform heating, such as the one caused by lake-land distribution. In this paper, a simple linear cell-convection
model is established using an appropriate heating field, and the instability of heating convection is theoretically studied.
It is found that the heating convection development will be unstable if the heating-caused temperature gradientdT
0/dy is greater than the critical value (dT
0/dy)
c
which is approximately 0.64°C/10 km, and that the development of convective band has a preferred width of 12.5 km. It will
take 25 min for the initial disturbance to increase intensity by 10 times. All these results are in rather good agreement
with the squall line process in the lake-land region of Jiangsu Province on June 8, 1979. 相似文献
11.
K. G. Mcnaughton 《Boundary-Layer Meteorology》2006,118(1):83-107
We present a new account of the kinetic energy budget within an unstable atmospheric surface layer (ASL) beneath a convective
outer layer. It is based on the structural model of turbulence introduced by McNaughton (Boundary-Layer Meteorology, 112:
199–221, 2004). In this model the turbulence is described as a self-organizing system with a highly organized structure that
resists change by instability. This system is driven from above, with both the mean motion and the large-scale convective
motions of the outer layer creating shear across the surface layer. The outer convective motions thus modulate the turbulence
processes in the surface layer, causing variable downwards fluxes of momentum and kinetic energy. The variable components
of the momentum flux sum to zero, but the associated energy divergence is cumulative, increasing both the average kinetic
energy of the turbulence in the surface layer and the rate at which that energy is dissipated. The tendency of buoyancy to
preferentially enhance the vertical motions is opposed by pressure reaction forces, so pressure production, which is the work
done against these reaction forces, exactly equals buoyant production of kinetic energy. The pressure potential energy that
is produced is then redistributed throughout the layer through many conversions, back and forth, between pressure potential
and kinetic energy with zero sums. These exchanges generally increase the kinetic energy of the turbulence, the rate at which
turbulence transfers momentum and the rate at which it dissipates energy, but does not alter its overall structure. In this
model the velocity scale for turbulent transport processes in the surface layer is (kzɛ)1/3 rather than the friction velocity, u*. Here k is the von Kármán constant, z is observation height, ɛ is the dissipation rate. The model agrees very well with published experimental results, and provides
the foundation for the new similarity model of the unstable ASL, replacing the older Monin–Obukhov similarity theory, whose
assumptions are no longer tenable. 相似文献
12.
Summary Seasonal variations of gravity wave characteristics are investigated using rawinsonde data observed at Pohang observatory,
Korea (36°2′N, 129°23′E) during the one-year period of 1998. Analysis is carried out for two atmospheric layers representing
the troposphere (2–9 km) and stratosphere (17–30 km). There exist clear seasonal variations in amplitudes of temperature and
wind perturbations and wave energy in the stratosphere, with their maxima in wintertime and minima in summertime. A strong
correlation is found between the wave activity and the strength of the jet stream, but there is no clear correlation between
the wave activity and the vertical gradient of static stability. The intrinsic frequency and vertical and horizontal wavelengths
of gravity waves in the stratosphere are 2f–3f, where f is the Coriolis parameter, and 2–3 km and 300–500 km, respectively. The intrinsic phase velocity directs westward in January
and northeastward in July. The vertical flux of the stratospheric zonal momentum is mostly negative except in summertime with
a maximum magnitude in January. Topography seems to be a major source of stratospheric gravity waves in wintertime. Convection
can be a source of gravity waves in summertime, but it is required to know convective sources at nearby stations, due to their
intermittency and locations relative to floating balloons. 相似文献
13.
A double parallel-connection (DPC) Lorenz system is developed by performing spectrum truncation of the Galerkin series expansion
of the two-dimensional Rayleigh-Benard convection equation. Analyses of the equilibrium states indicate that a convective
roll stems from a flow with a given wavenumber first losing its stability for a particular aspect ratioβ after a stable laminar flow gets unstable; whenβ has the valueβ
c able to deprive synchronously two flows with different wavenumbers of stability, occurrences of convective rolls with different
wavenumbers depend entirely on the initial conditions, in good agreement with the relevant experimental results. The calculations
of the unstablized rolls show that, with a smallerβ (as compared withβ
c), the DPC Lorenz system has the same bifurcation properties as the ordinary Lorenz system; for a moderateβ, the system has very complicated periodic, quasi-periodic and phase-locking motions; for a largerβ, it results in intermittent chaos and causes mean flows with different numbers of vortices to occur alternately with time.
All these indicate thatβ has substantial effect on the two Lorenz systems coupled through parallel connection in their interaction and the results. 相似文献
14.
Based on the simulation displayed in Part I of this study, the intensification and maintenance, the relationship between deep moist mesoscale convective system (DMMCS) and instability, convective vorticity vector (CVV) are analyzed in the present paper. Results show that: (1) middle-low level convective instability is the precondition of the occurrence of DMMCS. The convergence and merger enhancement of convection cells, as well as the convective instability energy transporting from the left-front of typhoon play an essential role in the re-establishment and enhancement of convective instability. (2) Baroclinic instability and conditional symmetric instability appear not only in the middle-low level, but also are distinct in the middle-upper level of DMMCS. (3) In DMMCS, there is an alternative distribution of inertial instability column and inertial stability column. In the west and south, there are negative CVV columns, which is favorable for the burst of deep moist convection. (4) The strong slantwise convection induced by inertial instability, baroclinic instability, and conditional symmetric instability enhance the upper-level southerly component. Due to the appearance of the compensated downdraft at the low level of south side of DMMCS, the low level southerly intensified, and the enhancement of upper- and low-level cores is in favor of the development of DMMCS, which will be beneficial to the reinforcement and maintenance of inertial instability, baroclinic instability, and conditional symmetric instability. It is a positive feedback process. (5) There is a downshear circulation to the east of rainfall cell. Shallow convections near this cell absorb the vapor and instability energy coming from the south. In the meanwhile, the mesoscale convergence line and meso-β-scale vortex organize and intensify convective cells. In DMMCS, there is an alternative distribution of convergence and divergence columns, and the couple between strong divergence and vorticity columns. They are both conducive to the development of DMMCS, and the instability will be intensified and maintained for its development in depth. 相似文献
15.
一次大范围暴雨过程的诊断分析 总被引:1,自引:1,他引:0
用ECMWF 0.75°×0.75°, 6 h间隔再分析资料、地面加密观测资料、Micaps资料和云图TBB资料, 对2012年8月20日一次大范围暴雨过程进行诊断分析。结果表明:本次大暴雨过程是在高层急流入口辐散和中低层的低槽切变线的耦合作用以及台风的间接影响使得低槽系统移动缓慢和提供水汽的有利条件下产生的。暴雨带中水汽主要来源于南海和东海。从等熵位涡、湿位涡和总能量分析说明这次暴雨和大暴雨是在水汽条件充沛条件下, 对流不稳定叠加斜压不稳定和对称不稳定等共同作用下, 产生暴雨-大暴雨。另外, 南北两支气流在暴雨区强烈辐合(南侧为上升运动, 北侧为下沉运动)也起到了重要作用, 且总能量垂直廓线与雨团中心对流强度和强降水时段对应较好。低层东海东南暖湿气流和干冷的东北气流对本次大范围暴雨过程的产生起触发作用。 相似文献
16.
In this paper, we analyze the effects of radiative cooling on the pure baroclinic low-frequency waves under the approximation of equatorial β-plane and semi-geostrophic condition. The results show that radiative cooling does not, exclusively, provide the damping effects on the development of low-frequency waves.Under the delicate radiative-convective equilibrium, radiative effects will alter the phase speed and wave period,and bring about the broadband of phase velocity and wave period by adjusting the vertical profiles of diabaticheating. When the intensity of diabatic heating is moderate and appropriate, it is conductive to the development and sustaining of the low-frequency waves and their broadband phenomena, not the larger, the better. The radiative cooling cannot be neglected in order to reach the moderate and appropriate intensity of diabatic heating. 相似文献
17.
We examine the unsteady response of a neutral atmospheric boundary layer (ABL) of depth h and friction velocity u * when a uniform surface heat flux is applied abruptly or decreased rapidly over a time scale t<inf>θ</inf> less than about h /(10u *). Standard Monin–Obukhov (MO) relationships are used for the perturbed eddy viscosity profile in terms of the changes to
the heat flux and mean shear. Analytical solutions for changes in temperature, mean wind and shear stress profile are obtained
for the surface layer, when there are small changes in h /|LMO| over the time scale tMO~|L MO|/(10u*) (where L MO and t MO are the length and time scales, respectively). They show that a maximum in the wind speed profile occurs at the top of the
thermal boundary layer for weak surface cooling, i.e. a wind jet, whereas there is a flattening of the profile and no marked
maximum for weak surface heating. The modelled profiles are approximately the same as those obtained from the U.K. Met Office
Unified Model when operating as a mesoscale model at 12-km horizontal resolution. The theoretical model is modified when strong
surface heating is suddenly applied, resulting in a large change in h /|L MO| (>>1), over the time scale t MO. The eddy structure is predicted to change significantly and the addition of convective turbulence increases the shear turbulence
at the ground. A low-level wind jet can form, with convective turbulence adding to the mean momentum of the flow. This was
verified by our laboratory experiment and direct numerical simulations. Additionally, it is shown that the effects of Coriolis
acceleration diminish (rather than as suggested in the literature, amplify) the formation of the wind jets in the situations
considered here. Hence, only when the surface heat flux changes over time scales greater than 1/f (where f is the Coriolis parameter) does the ABL adjust monotonically between its equilibrium states. These results are also applicable
to the ABL passing over spatially varying surface heat fluxes. 相似文献
18.
19.
Summary The effects of surface temperature anomalies (STAs) upon frontal cyclones are investigated with a nonlinear model. The model used is a modified version of the NCAR Community Climate Model (CCM 1). The experiments are run with hemispheric domain and R 30 (rhomboidal) truncation. The present study isolates the effects of sensible heating. Topography and latent heating are excluded from this model. The initial data are created from a solution (normal mode) to the linear eigenvalue problem. Six experiments use various locations and intensities for dipole-shaped STA; one control case is run without STA. The intensity is either ±5 or ±10°K and the anomalies ae located at 40°N, 50°N, or 35°N. The jet is centered at 40°N. All cases are run for 20 days. Nonlinear, time-dependent, growth rate and phase frequency are derived and compared to the linear (eigenvalue) amounts.The resulting waves grow primarily by baroclinic instability. Perturbation fields at higher levels grow faster before they mature (occlude) and decay faster afterward, than do lower level fields. The baroclinic conversion of energy lessens as the perturbations mature. The principal hypothesis tested is that: the STA alters the static stability which in turn modulates the baroclinic instability. Over warm anomalies the static stability should be reduced, enhancing baroclinic instability. Over cold anomalies the opposite may happen. The nonlinear simulations confirm this hypothesis in part. In the present study, the intensity of the warm anomaly produces greater growth rate during and after the storm's mature state. Larger STA intensity increases the maximum amplitude of the perturbation in a roughly linear fashion. However, the STA effects are nonlinear after maximum amplitude is reached: during decay, the difference in amplitude between the control case and the 10°K STA case is more than twice the difference between the control and 5°K case. In contrast, little deviation from the control case is found for perturbations over the cold anomaly, indicating a nonlinear link between STA and wave growth. The latitudinal variation used of the surface temperature anomaly centers had no significant influence on the baroclinic growth. Secondary growths of storms after 10 days are more commonly seen in cases with STA. 相似文献
20.
Easterly wave regimes and associated convection over West Africa and tropical Atlantic: results from the NCEP/NCAR and ECMWF reanalyses 总被引:1,自引:0,他引:1
NCEP/NCAR and ECMWF daily reanalyses are used to investigate the synoptic variability of easterly waves over West Africa
and tropical Atlantic at 700 hPa in northern summer between 1979–1995 (1979–1993 for ECMWF). Spectral analysis of the meridional
wind component at 700 hPa highlighted two main periodicity bands, between 3 and 5 days, and 6 and 9 days. The 3–5-day easterly
wave regime has already been widely investigated, but only on shorter datasets. These waves grow both north and south of the
African Easterly Jet (AEJ). The two main tracks, noted over West Africa at 5 °N and 15 °N, converge over the Atlantic on latitude
17.5 °N. These waves are more active in August–September than in June–July. Their average wavelength/phase speed varies from
about 3000 km/8 m s-1 north of the jet to 5000 km/12 m s-1 south of the jet. Rainfall, convection and monsoon flux are significantly modulated by these waves, convection in the Inter-Tropical
Convergence Zone (ITCZ) being enhanced in the trough and ahead of it, with a wide meridional extension. Compared to the 3–5-day
waves, the 6–9-day regime is intermittent and the corresponding wind field pattern has both similar and contrasting characteristics.
The only main track is located north of the AEJ along 17.5 °N both over West Africa and the Atlantic. The mean wavelength
is higher, about 5000 km long, and the average phase speed is about 7 m s-1. Then the wind field perturbation is mostly evident at the AEJ latitude and north of it. The perturbation structure is similar
to that of 3–5-days in the north except that the more developed circulation centers, moving more to the north, lead to a large
modulation of the jet zonal wind component. South of the AEJ, the wind field perturbation is weaker and quite different. The
zonal wind core of the jet appears to be an almost symmetric axis in the 6–9-day wind field pattern, a clockwise circulation
north of the AEJ being associated with a counter-clockwise circulation south of the jet, and vice versa. These 6–9-day easterly
waves also affect significantly rainfall, convection and monsoon flux but in a different way, inducing large zonal convective
bands in the ITCZ, mostly in the trough and behind it. As opposed to the 3–5-day wave regime, these rainfall anomalies are
associated with anomalies of opposite sign over the Guinea coast and the Sahelian regions. Over the continent, these waves
are more active in June–July, and in August–September over the ocean. GATE phase I gave an example of such an active 6–9-day
wave pattern. Considered as a sequence of weak easterly wave activity, this phase was also a sequence of high 6–9-day easterly
wave activity. We suggest that the 6–9-day regime results from an interaction between the 3–5-day easterly wave regime (maintained
by the barotropic/baroclinic instability of the AEJ), and the development of strong anticyclonic circulations, north of the
jet over West Africa, and both north and south of the jet over the Atlantic, significantly affecting the jet zonal wind component.
The permanent subtropical anticyclones (Azores, Libya, St Helena) could help initiation and maintenance of such regime over
West Africa and tropical Atlantic. Based on an a priori period-band criterion, our synoptic classification has enabled us
to point out two statistical and meteorological easterly wave regimes over West Africa and tropical Atlantic. NCEP/NCAR and
ECMWF reanalyses are in good agreement, the main difference being a more developed easterly wave activity in the NCEP/NCAR
reanalyses, especially for the 3–5-day regime over the Atlantic.
Received: 28 May 1998 / Accepted: 2 May 1999 相似文献