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2000 Vol. 17, No. 3

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Streamwise Vorticity Equation
Gao Shouting, Lei Ting
2000, 17(3): 339-347. doi: 10.1007/s00376-000-0027-4
A streamwise vorticity equation is derived in generalized natural coordinates. This equation reveals that the total change and local change of the streamwise vorticity are mainly determined by the curvature of streamline, unsteady feature of streamline and magnitude of velocity. This equation enables the study of mesoscale or small-scale systems since the term associated with pressure gradient force in the original vorticity equation is replaced by terms associated with streamlines and wind speed. With this modification the wind field rather than the pressure field is used in the calculation considering that 1) the pressure field is to adapt wind field. 2) Smoother and more consecutive streamline pattern is easier to obtain either by data analysis or by the numerical simulation. From this sense, this present study suggests the application of this equation to studying the evolution of severe storm system as well as other simplified cases.
The 1997/ 98 ENSO Cycle and Its Impact on Summer Climate Anomalies in East Asia
Huang Ronghui, Zhang Renhe, Zhang Qingyun
2000, 17(3): 348-362. doi: 10.1007/s00376-000-0028-3
The observed data of the sea surface temperature (SST) anomalies and the sea temperature (ST) in the sub-layer of the equatorial Pacific, the NCEP/ NCAR reanalysis data and the data set of daily precipita-tion in China are used to analyze the characteristics of the 1997 / 98 ENSO cycle and its impact on summer climate anomalies in East Asia. The results show that the 1997/ 98 ENSO cycle, the strongest one in the 20th century, might be characterized by rapid development and decay and eastward propagation from the West Pacific warm pool. Influenced by the ENSO cycle, in 1997, the serious drought and hot summer occur-red in North China, and in the summer of 1998, the severe floods occurred in the Yangtze River valley, especially in the Dongting Lake and Boyang Lake valleys, South Korea and Japan.The analysis also shows that: influenced by the 1997 / 98 ENSO cycle, the water vapor transportation by the Asian monsoon in the summer of 1997 was very different from that in the summer of 1998. In the summer of 1997, the water vapor transportation by the Asian summer monsoon was weak in North China and the northern part of the Korea Peninsula. Thus, it caused the drought and hot summer in North China. However, in the summer of 1998, the sea temperature in the sub-layer of the West Pacific warm pool drop?ped, the western Pacific subtropical high shifted southward. Thus, a large amount of water vapor was trans?ported from the Bay of Bengal, the South China Sea and the tropical western Pacific into the Yangtze River valley of China. South Korea and Japan, and the severe flood occurred there.
Studying Air Pollution with Kitt Peak Solar Flux Atlas-Analysis Method and Results of Observation
Jianguo Niu, Hiroaki Kuze, Nobuo Takeuchi
2000, 17(3): 363-374. doi: 10.1007/s00376-000-0029-2
For the measurement of atmospheric NO2 vertical column density (VCD), Kitt Peak Solar Flux Atlas can be substituted as an extraterrestrial solar radiation. Compared with differential analysis method, the Taylor expansion of integrated transfer equation underestimates the VCD. This underestimation is as large as 35% when the amount of NO2 is 1 × 1017 cm-2 and observation is conducted with an air mass factor of 10. Even when the VCD is 2 × 1016 cm-2 and the air mass factor is 4, the relative error of the retrieved VCD is still no less than 3%. If the observation is restricted under the small air mass factor condition (≤4), with Kitt Peak Solar spectrum as an extraterrestrial solar radiation, only an atmospheric layer of 2 km thick from ground can be studied, which will make the absorption too weak to be detected by normal instruments.The VCD in winter Tokyo area was observed and analyzed by differential method, which shows a good precision even when the absorption is as low as 3%. The largest average VCD was about 1.3 × 1017 cm-2, and the lowest was about 1.3 × 1016 cm-2. The trend of its variation was almost the same as the ground level observation by Saltzman reagent method.
The Impact of Nonlinear Stability and Instability on the Validity of the Tangent Linear Model
Mu Mu, Guo Huan, Wang Jiafeng, LiYong
2000, 17(3): 375-390. doi: 10.1007/s00376-000-0030-9
The impact of nonlinear stability and instability on the validity of tangent linear model (TLM) is investigated by comparing its results with those produced by the nonlinear model (NLM) with the identical initial perturbations. The evolutions of different initial perturbations superposed on the nonlinearly stable and unstable basic flows are examined using the two-dimensional quasi-geostrophic models of double periodic-boundary condition and rigid boundary condition.The results indicate that the valid time period of TLM, during which TLM can be utilized to approxi-mate NLM with given accuracy, varies with the magnitudes of the perturbations and the nonlinear stability and instability of the basic flows. The larger the magnitude of the perturbation is, the shorter the valid time period. The more nonlinearly unstable the basic flow is, the shorter the valid time period of TLM. With the double-periodic condition the valid period of the TLM is shorter than that with the rigid-boundary condi-tion.
Climatological Features of the Global Tropical Subsidence Region Based on Satellite Observations
Jiang Shangcheng, Ye Qian, Yang Xifeng, An Gang, Xiangqiang Wu
2000, 17(3): 391-402. doi: 10.1007/s00376-000-0031-8
Consisting of subtropical highs and tropical buffer zone, the global tropical subsidence region is the subsidence branches of Hadley cell, Walker circulation and monsoon circulation which are important com-ponents of the global general circulation. This region is closely connected with Asian monsoon. Based on long-term satellite observations of OLR (Outgoing Longwave Radiation) and HIRS-Tbl2 (the bright tem-perature from High-resolution Infra-red Radiation Sounder Channel 12 (6.7μm)), the climatological fea-tures over the global tropical subsidence region are studied in this paper and the main findings are as follows:1) Based on the physical meaning of satellite observed HIRS-12 and comparison with the satellite ob?served OLR and climatological maps of conventional observations of geopotential height and vertical veloc?ity at 500 hPa from NCEP / NCAR reanalysis data, it was found that HIRS-12 might be the best indicator of the global tropical subsidence region from the satellite observations.2) Using satellite observed HIRS-12, some new climatological features were revealed, especially those related with Asian monsoon.3) Comparing the climatological characteristics of tropical subsidence regions between both hemi-spheres, it was found that the circulation in the Northern Hemisphere is more complicated and much closely related with monsoon circulation than that in the Southern Hemisphere. Sudden decay of subsidence movement before monsoon onsets over the South China Sea revealed by HIRS-12 suggested that in the future study of Asian monsoon, it should be paid attention to not only the development of convection but also the variation of tropical subsidence region. It is very useful to monitor the convective and subsidence movements in the tropics by using OLR and HIRS-Tbl2 have been observed from satellite.
The Use of Dual-Doppler Radar Data in the Study of 1998 Meiyu Frontal Precipitation in Huaihe River Basin
Xu Hui, Zhang Weiping, Lang Xuxing, Guo Xia, Ge Wenzhong, Dang Renqing, TakaoTakeda
2000, 17(3): 403-412. doi: 10.1007/s00376-000-0032-7
During the Meiyu period in June and July of 1998, intensified field observations have been carried out for the project “Huaihe River Basin Energy and Water Cycle Experiment (HUBEX)”. For studying Meiyu front and its precipitation in Huaihe River basin, the present paper has performed analysis on the middle and lower level wind fields in the troposphere by using the radar data obtained from the two Doppler radars located at Fengtai district and Shouxian County. From June 29 to July 3 in 1998, the continuous heavy precipitation occurred in Huaihe River basin around Meiyu front. The precipitation process on July 2 occurred within the observation range of the two Doppler radar in Fengtai district and Shouxian County. The maximum rainfall of the Meiyu front was over 100 mm in 24 h, so it can be regarded as a typical mesoscale heavy precipitation process related to Meiyu front. Based on the wind field retrieved from the dual Doppler radar, we find that there are meso-γ scale vertical circulations in the vertical cross-section perpendicular to Meiyu front, the strong upward motion of which corresponds to the position of the heavy rainfall area. Furthermore, other results obtained by this study are identical with the results by analyzing the conventional synoptic data years ago. For example: in the vicinity of 3 km level height ahead of Meiyu front there exists a southwest low-level jet; the rainstorm caused by Meiyu front mainly occurs at the left side of the southwest low-level jet; and the Meiyu front causes the intensification of the low-level convergence in front of it.
Baroclinic Instability in the Generalized Phillips’ Model Part II: Three-layer Model
Li Yang
2000, 17(3): 413-432. doi: 10.1007/s00376-000-0033-6
The nonlinear stability of the three-layer generalized Phillips model, for which the velocity in each layer is constant and the top and bottom surfaces are either rigid or free, is studied by employing Arnol’d’s variational principle and a prior estimate method. The nonlinear stability criteria are established. For com-parison, the linear instability criteria are also obtained by using normal mode method, and the influences of the free parameter, β parameter and curvature in vertical profile of the horizontal velocity on the linear in-stability are discussed by use of the growth rate curves.The comparison between the nonlinear stability criterion and the linear one is made. It is shown that in some cases the two criteria are exactly the same in form, but in other cases, they are different. This phenom-enon, which reveals the nonlinear property of the linear instability features, is explained by the explosive resonant interaction (ERI). When there exists the ERI, i.e., the nonlinear mechanisms play a leading role in the dynamical system, the nonlinear stability criterion is different from the linear one; on the other hand, when there does not exist the ERI, the nonlinear stability criterion is the same as the linear one in form.
A Simple Method of Calculating the Optimal Step Size in 4DVAR Technique
Wang Yunfeng, Wu Rongsheng, Wang Yuan, Pan Yinong
2000, 17(3): 433-444. doi: 10.1007/s00376-000-0034-5
In four-dimensional variational data assimilation (4DVAR) technology, how to calculate the optimal step size is always a very important and indeed difficult task. It is directly related to the computational effi-ciency. In this research, a new method is proposed to calculate the optimal step size more effectively. Both nonlinear one-dimensional advection equation and two-dimensional inertial wave equation are used to test and compare the influence of different methods of the optimal step size calculations on the iteration steps, as well as the simulation results of 4DVAR processes. It is in evidence that the different methods have different influences. The calculating method is very important to determining whether the iteration is convergent or not and whether the convergence rate is large or small. If the calculating method of optimal step size is prop-erly determined as demonstrated in this paper, then it can greatly enlarge the convergence rate and further greatly decrease the iteration steps. This research is meaningful since it not only makes an important im-provement on 4DVAR theory, but also has useful practical application in improving the computational effi-ciency and saving the computational time.
The Seasonal Climate and Low Frequency Oscillation in the Simulated Mid-Holocene Megathermal Climate
Wang Huijun
2000, 17(3): 445-457. doi: 10.1007/s00376-000-0035-4
The mid-Holocene climate about 6000 years ago was simulated by using the atmospheric general circu-lation model. The orbital parameters for 6 ka BP (before present) were prescribed and other forcing factors were set in the modern conditions. Results show that the large-scale climate change in the African-Asian monsoon areas during the summer-time is strongly compared to the present climate, while the changes in other seasons and regions are generally weak. The results also revealed the change of the low frequency oscillation in the atmosphere.
The Influences of Boundary Layer Parameterization Schemes on Mesoscale Heavy Rain System
Xu Liren, Zhao Ming
2000, 17(3): 458-472. doi: 10.1007/s00376-000-0036-3
The mesoscale numerical weather prediction model (MM4) in which the computations of the turbulent exchange coefficient in the boundary layer and surface fluxes are improved, is used to study the influences of boundary layer parameterization schemes on the predictive results of the mesoscale model. Seven different experiment schemes (including the original MM4 model) designed in this paper are tested by the observational data of several heavy rain cases so as to find an improved boundary layer parameterization scheme in the mesoscale meteorological model. The results show that all the seven different boundary layer parameterization schemes have some influences on the forecasts of precipitation intensity, distribution of rain area, vertical velocity, vorticity and divergence fields, and the improved schemes in this paper can im-prove the precipitation forecast.
Response of IAP/ LASG GOALS Model to the Coupling of Air-Sea Fresh Water Exchange
ZhouTianjun, Zhang Xuehong, Yu Yongqiang, Yu Rucong, Liu Xiying, Jin Xiangze
2000, 17(3): 473-486. doi: 10.1007/s00376-000-0037-2
The process of air-sea fresh water exchange is included successfully in the Global-Ocean-Atmosphere-Land-System model developed at the State Key Laboratory of Atmospheric Sciences and Geophysical Fluid Dynamics (LASG). The results of the coupled integration show that the climate drift has been controlled successfully. Analyses on the responses of ocean circulation to the changes of surface fresh water or salinity forcing show that the ocean spin-up stage under flux condition for salinity is the key to the implementation of air-sea fresh water flux coupling. This study also demonstrates that the Modified-Monthly-Flux-Anomaly coupling scheme (MMFA) brought forward by Yu and Zhang (1998) is suitable not only for daily air-sea heat flux coupling but also for daily fresh water flux coupling.
A Simulating Study on Resolvable-Scale Microphysical Parameterization in a Mesoscale Model
Lin Wenshi, Fong Soikun, Wu Chisheng, Ku Chimeng, Wang Anyu, Yang Yan
2000, 17(3): 487-502. doi: 10.1007/s00376-000-0038-1
The Penn State/ NCAR Mesoscale Model (MM5) is used to simulate the precipitation event that oc-curred during 1-2 May 1994 to the south of the Yangtze River. In five experiments the Kain-Fritsch scheme is made use of for the subgrid-scale convective precipitation, but five different resolvable-scale microphysical parameterization schemes are employed. They are the simple super-saturation removal scheme, the warm rain scheme of Hsie et al. (1984), the simple ice scheme of Dudhia (1989), the complex mixed-phase scheme developed by Reisner et al. (1993). and the GSFC microphysical scheme with graupel. Our interest is how the various resolvable-scale schemes affect the domain-averaged precipitation, the pre-cipitation distribution, the sea level pressure, the cloud water and the cloud ice.Through a series of experiments about a warm sector rainfall case, results show that although the dif-ferent resolvable-scale scheme is used, the differences of the precipitation characteristics among all five runs are not very obvious. However, the precipitation is over-predicted and the strong mesoscale low is produced by the simple super-saturation removal scheme. The warm rain scheme with the inclusion of condensation and evaporation under-predicts the precipitation and allows the cloud water to reach the 300 hPa level The scheme of the addition of graupel increases the resolvable-scale precipitation by about 20%–30%. The inclusion of supercooled liquid water in the grid-scale scheme does not affect significantly the results.