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2003 Vol. 20, No. 3

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Analysis and Comparison of Mesoscale Convective Systems over the Qinghai-Xizang (Tibetan) Plateau
ZHU Guofu, CHEN Shoujun
2003, 20(3): 311-322. doi: 10.1007/BF02690789
A series of mesoscale convective systems (MCSs) occurred daily over the Qinghai-Xizang Plateau during25-28 July 1995. In this paper, their physical characteristics and evolutions based on infrared satelliteimagery, their largescale meteorological conditions, and convective available potential energy (CAPE) areanalyzed. It is found that similar diurnal evolution is present in all these MCSs. Their initial convectiveactivities became active at noon LST by solar heating, and then built up rapidly. They formed andreached a peak in the early evening hours around 1800 LST and then abated gradually. Among them, thestrongest and largest is the MCS on 26 July, which developed under the conditions of the great upper-levelnearly-circular Qinghai-Xizang anticyclonic high and driven by the strong low-level thermal forcing andconditional instability. All these conditions are intimately linked with the thermal effects of the plateauitself. So its development was mainly associated with the relatively pure thermal effects peculiar to theQinghai-Xizang Plateau. The next strongest one is the MCS on 28 July, which was affected notably by thebaroclinic zone linked with the westerly trough. There are different features and development mechanismsbetween these two strongest MCSs.
A Preliminary Study of the Dynamics of Eastward Shifting Cyclonic Vortices
CHEN Lianshou, LUO Zhexian
2003, 20(3): 323-332. doi: 10.1007/BF02690790
The dynamics of eastward shifting cyclonic vortices are investigated in terms of a barotropic primitiveequation model, and six experiments are performed. Both the interaction of a cyclonic vortex with vorticitylumps and the interaction of the vortex with the shearing basic flow may induce the strengthening of thevortex in a short period, however, the vortex intensity still shows a general decreasing tendency over thewhole integration time period. The interaction among the shearing basic flow, cyclonic vortex, and multiplevorticity lumps can change the tendency. The merging of the cyclonic vortex with vorticity lumps in theshearing basic flow of positive vorticity is directly responsible for the maintenance and development of thecyclonic vortex.
The Water Vapor Transport Model at the Regional Boundary during the Meiyu Period
XU Xiangde, MIAO Qiuju, WANG Jizhi, ZHANG Xuejin
2003, 20(3): 333-342. doi: 10.1007/BF02690791
The water vapor transport model at the regional boundary in the Meiyu period is put forward throughdiagnostic analysis. The numerical simulation on the water vapor transport at the boundary of China inthe heavy rainfall period during June-July 1998 shows that the feature of water vapor transport in June isdifferent from that in July. The main body of the water cycle that forms the torrential rain in the Yangtze River Valley is made up of water vapor transport at the western and southern boundaries of the Chinaregion in June, whereas the water vapor flow at the western boundary in middle Tibet turns out to bethe main body of water vapor sources in July. The water vapor transport at the western boundary ofthe Tibetan Plateau and the southern boundary of China plays an important role in the torrential rainin the Yangtze River Valley. The temporal and spatial distribution characteristics of water vapor flow atthe regional boundary and their theoretical model would provide the scientific proof for the heavy rainforecasts in the Yangtze River Valley.
Characteristics of the Seasonal Variation of the Surface Total Heating over the Tibetan Plateau and Its Surrounding Area in Summer 1998 and Its Relationship with the Convection over the Subtropical Area of the Western Pacific
LI Wei, CHEN Longxun
2003, 20(3): 343-348. doi: 10.1007/BF02690792
Using the dataset of 1998 TIPEX, the data of 6 automatic heat balance observational stations (AWS)from May to August 1998, a dataset of 52 surface observational stations over the Tibetan Plateau (TP)and its adjacent region, the daily rainfall amounts from about 300 stations in China, the outgoing longwaveradiation (OLR) data received by the National Satellite Meteorological Center(NSMC) of China, and TBBdata from GMS remote sensing of Japan, the characteristics of the seasonal variation of the surface totalheating over TP and its surrounding area in summer 1998 and its relationship with the convection overthe subtropical area of the western Pacific is studied in this paper. The results show that the surface totalheating over TP had a close relationship with the onset of the rainy season, and after the onset of the rainyseason, the regional mean surface total heating over TP decreased distinctly. Furthermore, the regionalmean surface total heating over TP had very good negative correlation with TBB over the subtropicalarea of the western Pacific along 20-30°N, which shows that the surface total heating over TP was able toaffect the convection over the subtropical area of the western Pacific.
A Dynamic Study of Ekman Characteristics by Using 1998 SCSMEX and TIPEX Boundary Layer Data
ZHANG Guangzhi, XU Xiangde, WANG Jizhi
2003, 20(3): 349-356. doi: 10.1007/BF02690793
A dynamic study on Ekman characteristics by using 1998 SCSMEX and TIPEX boundary layerdata is made. The results are as follows: (1) Similar dynamical Ekman characteristics are observed in theTibetan Plateau and in the South China Sea and its surrounding area. (2) The thickness of the boundarylayer is about 2250 m over the Tibetan Plateau, and considering its variation, the thickness could be up to2250-2750 m. In the tropical southwest Pacific, the thickness of the boundary layer is about 2000 m, andthe variation is smaller; a smaller thickness of the boundary layer is in the plain area of the Bohai Sea.(3) Because of the difference in elevation between the Tibetan Plateau and the tropical ocean area, theinfluence of the boundary layer on the atmosphere is quite different although the two areas have almostthe same thickness for the boundary layer, the height where the friction forcing occurs is quite different.(4) The vertical structure of turbulence friction is quite different in the Plateau and in the tropical oceanarea. Calculations by 1998 SCSMEX and TIPEX boundary layer data indicate that even in the lowestlevels, eddy viscosity in the Tibetan Plateauan can be 2.3 times than in the tropical ocean area.
Discussion on Calculation Methods of Sensible Heat Flux during GAME/Tibet in 1998
GAO Zhiqiu, BIAN Lingen, WANG Jinxing, LU Longhua
2003, 20(3): 357-368. doi: 10.1007/BF02690794
Based on previous research on sensible heat flux, we investigate it from different aspects usingGAME/Tibet data measured during 6 June-13 September, 1998. This work consists of the derivationof the surface heat flux equation, analysis on counter-gradient heat transference, comparison betweentwo different methods to compute the sensible heat flux, and investigation on the calculation scheme ofsensible heat flux in the Simple Biosphere model 2 (SiB2) with relevant simulation. By improving twoprevious formulations, an integrated formulation for calculating surface heat flux is given. Secondly, usingthe measured data, the counter-gradient heat flux is clarified, leading to the fact that buoyancy plays animportant role in the sensible heat transfer process. It is concluded that (1) energy imbalance is a commonphenomenon resulting from the use of the traditional closure scheme on the heterogeneous underlyingsurface because the measured ensemble heat fluxes by eddy correlation contain the effect of nonlocal parcelmovements; and (2) nonlocal parcel movement deserves more attention in any future heat flux study.
Analyses of Turbulence Parameters in the Near-Surface Layer at Qamdo of the Southeastern Tibetan Plateau
BIAN Lingen, XU Xiangde, LU Longhua, GAO Zhiqiu, ZHOU Mingyu, LIU Huizhi
2003, 20(3): 369-378. doi: 10.1007/BF02690795
The characteristics of the turbulence spectrum, turbulence variance, and turbulence flux at Qamdo overthe Southeastern Tibetan Plateau measured during the TIPEX experiment from 18 May to 30 June 1998are analyzed by the eddy correlation method. During periods of intense convection, most of the spectraof the 3D winds, temperature, and humidity follow a power law with a slope of -2/3. The normalizedvariances of the 3D winds in relation to z/L satisfy the similarity law but the normalized variances oftemperature and humidity are related to z/L by a -1/3 power law only in unstable conditions. In near-neutral stratification, σu/u* and σy/u* at Qamdo and Gerze are nearly constant in rugged terrain andσw/u* at Qamdo and Gerze is similar to the value found in plains, which indicates that the, effects oflandform on vertical turbulence is not significant over the southeast and western Tibetan Plateau. Duringthe dry period, the sensible heat dominates, comprising 81% of the heat intensity, with the other 19%being latent heat; during the wet period on the other hand, the latent heat accounts for about 64% ofthe available energy and the sensible heat only around 36%. The maximum intensity of the heating of airoccurs in the middle of the Plateau during the summertime.
Prediction of Typhoon Tracks Using Dynamic Linear Models
Keon-Tae SOHN, H. Joe KWON, Ae-Sook SUH
2003, 20(3): 379-384. doi: 10.1007/BF02690796
This paper presents a study on the statistical forecasts of typhoon tracks. Numerical models havetheir own systematic errors, like a bias. In order to improve the accuracy of track forecasting, a statisticalmodel called DLM (dynamic linear model) is applied to remove the systematic error. In the analysis oftyphoons occurring over the western North Pacific in 1997 and 2000, DLM is useful as an adaptive modelfor the prediction of typhoon tracks.
A Numerical Case Study on a Mesoscale Convective System over the Qinghai-Xizang (Tibetan) Plateau
ZHU Guofu, CHEN Shoujun
2003, 20(3): 385-397. doi: 10.1007/BF02690797
A mesoscale convective system (MCS) developing over the Qinghai-Xizang Plateau on 26 July 1995 issimulated using the fifth version of the Penn State-NCAR nonhydrostatic mesoscale model (MM5). Theresults obtained are inspiring and are as follows. (1) The model simulates well the largescale conditionsin which the MCS concerned is embedded, which are the well-known anticyclonic Qinghai-Xizang PlateauHigh in the upper layers and the strong thermal forcing in the lower layers. In particular, the modelcaptures the meso-α scale cyclonic vortex associated with the MCS, which can be analyzed in the 500 hPaobservational winds; and to some degree, the model reproduces even its meso-β scale substructure similarto satellite images, reflected in the model-simulated 400 hPa rainwater. On the other hand, there aresome distinct deficiencies in the simulation; for example, the simulated MCS occurs with a lag of 3 hoursand a westward deviation of 3-5° longitude. (2) The structure and evolution of the meso-α scale vortexassociated with the MCS are undescribable for upper-air sounding data. The vortex is confined to thelower troposphere under 450 hPa over the plateau and shrinks its extent with height, with a diameter of4° longitude at 500 hPa. It is within the updraft area, but with an upper-level anticyclone and downdraftover it. The vortex originates over the plateau, and does not form until the mature stage of the MCS. Itlasts for 3-6 hours. In its processes of both formation and decay, the change in geopotential height fieldis prior to that in the wind field. It follows that the vortex is closely associated with the thermal effectsover the plateau. (3) A series of sensitivity experiments are conducted to investigate the impact of varioussurface thermal forcings and other physical processes on the MCS over the plateau. The results indicatethat under the background conditions of the upper-level Qinghai-Xizang High, the MCS involved is mainlydominated by the low-level thermal forcing. The simulation described here is a good indication that itmay be possible to reproduce the MCS over the plateau under certain large-scale conditions and with theincorporation of proper thermal physics in the lower layers.
Operational Implementation of the ATOVS Processing Procedure in KMA and Its Validation
Myoung-Hwan AHN, Mee-Ja KIM, Chu-Yong CHUNG, Ae-Sook SUH
2003, 20(3): 398-414. doi: 10.1007/BF02690798
The Korea Meteorological Administration (KMA) has processed the data from the advanced TOVS(ATOVS) onboard NOAA-16 satellite since May 2001. The operational production utilizes the AAPP(ATOVS and AVHRR Processing Package) of EUMETSAT and IAPP (International ATOVS ProcessingPackage) of the University of Wisconsin. For the initial guess profiles, the predicted fields (usually 6 to 12hour forecasted fields) from the global aviation model of NOAA/NCEP are used. The average number ofprofiles retrieved from the ATOVS data is about 1,300 for each morning and afternoon orbit at about 18 and06 UTC, respectively. The retrieved temperature and dew point temperatures are provided to forecastersin real time and used for initialization of prediction models. With the advanced microwave sensor (AMSU;Advanced Microwave Sounding Unit), accuracy of the ATOVS products is expected to be better than thatof the TOVS products, especially in cloudy conditions. Indeed, the preliminary results from a validationstudy with the collocated radiosonde data during a 8-month period, from May to December 2001, for theEast Asia region show an improved accuracy of the ATOVS products for cloudy skies versus the TOVS,especially for higher altitudes. The RMS (Root Mean Square) difference between the ATOVS productsand radiosonde data is about 1.3℃ for both clear and cloudy conditions, except for near the ground and athigher altitudes, at around 200 hPa. There is no significant temporal variation of the error statistics at allpressure levels. In case of the water vapor mixing ratio, the largest difference is shown at lower altitudes,while the accuracy is much better for the clear sky cases than the cloudy sky cases. The bias and RMSEat lower altitudes is about 0.557 g kg-1 and 2.5 g kg-1 and decrease significantly with increasing altitude.
Streamflow Simulation for the Yellow River Basin Using RIEMS and LRM
ZHANG Jingyong, DONG Wenjie, FU Congbin, WU Lingyun, XIONG Zhe, MA Jun, ZHANG Kejia
2003, 20(3): 415-424. doi: 10.1007/BF02690799
The streamflow over the Yellow River basin is simulated by using the high-resolution RegionalIntegrated Environmental Model System (RIEMS), and an off-line Large-scale Routing Model (LRM).The RIEMS was designed and has been developed by the Global Change System for Analysis, Researchand Training Regional Center for Temperate East Asia (START/TEA) since 1991 and has a good capabilityto simulate the regional climate of East Asia. The LRM is based on the assumption of linearity and timeinvariance and can calculate the horizontal travel of water. The RIEMS-LRM allows the direct comparisonof predicted and observed streamflow data for large-scale rivers. The application of the RIEMS-LRM tothe upper reaches of the Yellow River verifies that the coupled model system has the capability to simulatethe streamflow over a large-scale river. Furthermore, the paper discusses the reasons leading to simulation errors.
A Monitoring Study of the 1998 Rainstorm along the Yangtze River of China by Using TIPEX Data
WANG Jizhi, YANG Yuanqin, XU Xiangde, ZHANG Guangzhi
2003, 20(3): 425-436. doi: 10.1007/BF02690800
By using data from the Secondary Tibetan Plateau Science Experiment (TIPEX) in 1998, includingenhanced soundings, surface observations, data from captive balloons, remote sensing, and Doppler radar(China and Japan cooperative study of GAME/Tibet), a monitoring study on the generation and movingtrack of the cumulus convective systems over the Tibetan Plateau is made, and the relationship between theevolution of cloud systems over the Tibetan Plateau and 1998 flooding in China is studied. The results areas follows. 1) Analyzing the image animation and Hovmoller diagram of satellite TBB data shows that therainstorms for the Yangtze River in the last ten days of July 1998 can be tracked regionally to the TibetanPlateau. 2) For the period of cloud clusters passing through the Amdo station (18-19 July), monitoringobservations by Doppler radar is made. The monitoring of radar echoes shows that the developing, eastwardmotion, and strengthening of the echoes can be frequently observed in the middle of the Tibetan Plateau.An integrated analysis and tracking of the generation, disappearance, development, and eastward motionof these convective systems by using multiple instruments is very valuable for diagnosing and predicting theinfluence of the plateau systems on the downstream weather situation. 3) The integrated analysis of space-time cross sections of the enhanced upper air and surface observations from TIPEX during the intensifiedobservation period shows that the frequent development of convective clouds over the Tibetan Plateauis related with the quasi-stationary convergence of surface winds. The dynamic convergence of surfacewinds, the vertical shear in the upper air, and transportation of water vapor due to increasing humidityover the Tibetan Plateau played an important role in the developing and strengthening of rainstorms overthe Yangtze River in 1998. 4) Meso-sale filtration analysis of the vertical distribution of water vapor overthe Tibetan Plateau indicates that alternating changes of high and low water vapor distribution over theTibetan Plateau reveals clearly that the sub-synoptic scale waves exist, whose lifetime is on the order ofthe hours. The revelation of the eastward motion of mesoscale waves from the Tibetan Plateau indicatesthat the plateau systems obviously influenced the rainstorms over the Yangtze River valley in 1998.
A Rapid Optimization Algorithm for GPS Data Assimilation
KUANG Zheng, WANG Bin, YANG Hualin
2003, 20(3): 437-441. doi: 10.1007/BF02690801
Global Positioning System (GPS) meteorology data variational assimilation can be reduced to theproblem of a large-scale unconstrained optimization. Because the dimension of this problem is too large,most optimal algorithms cannot be performed. In order to make GPS/MET data assimilation able tosatisfy the demand of numerical weather prediction, finding an algorithm with a great convergence rateof iteration will be the most important thing. A new method is presented that dynamically combines thelimited memory BFGS (L-BFGS) method with the Hessian-free Newton(HFN) method, and it has a goodrate of convergence in iteration. The numerical tests indicate that the computational efficiency of themethod is better than the L-BFGS and HFN methods.
Characteristics of Hydrologic Transfer between Soil and Atmosphere over Gobi near Oasis at the End of Summer
ZHANG Qiang, SONG Lianchun, HUANG Ronghui, WEI Guoan, WANG Sheng, TIAN Hui
2003, 20(3): 442-452. doi: 10.1007/BF02690802
By utilizing the data observed at Dunhuang during August and September 2000 in the "Field Experi-ment on Interaction between Land and Atmosphere in the Arid Region of Northwest China (FEILARNC)",the characteristics of the soil moisture, temperature, and atmospheric humidity are analyzed. It is foundthat the thickness of the soil temperature active layer is about 5 cm and much thinner than is typical,that not only the atmospheric humidity gradient is often inverted but also the soil moisture gradient inthe shallow layer in the Gobi near oasis, that the diurnal variation of soil moisture can be divided into thefour stages that are called the wet stage, the losing-water stage, the dry stage, and the attaining-waterstage. It is shown that in soil moisture profiles, the depth of the soil moisture active layer is about 10 cmand soil moisture inversion is the main feature in the shallow layer during the wet stage. Such a featureas soil moisture inversion indicates that soil in the shallow layer can inhale moisture from the air throughcondensation in the nighttime and exhale moisture to the air through evaporation in the daytime. Thecondensation and evaporation constitute together the full respiration process of moisture on the ground.The formation of soil moisture inversion is related with the state of soil temperature and moisture, theintensity of atmospheric humidity inversion, and the atmospheric thermodynamic stability.
Numerical Study on the Velocity Structure around Tidal Fronts in the Yellow Sea
LIU Guimei, WANG Hui, SUN Song, HAN Boping
2003, 20(3): 453-460. doi: 10.1007/BF02690803
The velocity components across tidal fronts are examined using the Blumberg and Mellor 3-D nonlinearnumerical coastal circulation model incorporated with the Mellor and Yamada level 2.5 turbulent closuremodel based on the reasonable model output of the M2 tide and density residual currents. In the numericalexperiments, upwelling motion appears around all the fronts with different velocity structures, accountingfor surface cold water around the fronts. The experiments also suggest that the location and formation offronts are closely related to topography and tidal mixing, as is the velocity structure around the front.
The Impact of Low-Level Cloud over the Eastern Subtropical Pacific on the "Double ITCZ" in LASG FGCM-0
DAI Fushan, YU Rucong, ZHANG Xuehong, YU Yongqiang, LI Jianglong
2003, 20(3): 461-474. doi: 10.1007/BF02690804
Like many other coupled models, the Flexible coupled General Circulation Model (FGCM-0) suffersfrom the spurious "Double ITCZ". In order to understand the "Double ITCZ" in FGCM-0, this study firstexamines the low-level cloud cover and the bulk stability of the low troposphere over the eastern subtropicalPacific simulated by the National Center for Atmospheric Research (NCAR) Community Climate Modelversion 3 (CCM3), which is the atmosphere component model of FGCM-0. It is found that the bulkstability of the low troposphere simulated by CCM3 is very consistent with the one derived from theNational Center for Environmental Prediction (NCEP) reanalysis, but the simulated low-level cloud coveris much less than that derived from the International Satellite Cloud Climatology Project (ISCCP) D2data. Based on the regression equations between the low-level cloud cover from the ISCCP data and thebulk stability of the low troposphere derived from the NCEP reanalysis, the parameterization scheme otlow-level cloud in CCM3 is modified and used in sensitivity experiments to examine the impact of low-levelcloud over the eastern subtropical Pacific on the spurious "Double ITCZ" in FGCM-0. Results show thatthe modified scheme causes the simulated low-level cloud cover to be improved locally over the cold oceans.Increasing the low-level cloud cover off Peru not only significantly alleviates the SST warm biases in thesoutheastern tropical Pacific, but also causes the equatorial cold tongue to be strengthened and to extendfurther west. Increasing the low-level cloud fraction off California effectively reduces the SST warm biasesin ITCZ north of the equator. In order to examine the feedback between the SST and low-level cloudcover off Peru, one additional sensitivity experiment is performed in which the SST over the cold ocean offPeru is restored. It shows that decreasing the SST results in similar impacts over the wide regions fromthe southeastern tropical Pacific northwestwards to the western/central equatorial Pacific as increasing thelow-level cloud cover does.
How Does the Partitioning of Evapotranspiration and Runoff between Different Processes Affect the Variability and Predictability of Soil Moisture and Precipitation?
Robert E. DICKINSON, Guiling WANG, Xubin ZENG, ZENG Qingcun
2003, 20(3): 475-478. doi: 10.1007/BF02690805
Water stored as part of the land surface is lost to evapotranspiration and runoff on different time scales,and the partitioning between these time scales is important for modeling soil water in a climate model.Different time scales are imposed on evapotranspiration primarily because it is derived from differentreservoirs with different storage capacities, from the very rapid evaporation of canopy stores to the slowremoval by transpiration of rooting zone soil moisture. Runoff likewise ranges in time scale from rapidsurface terms to the slower base-flow. The longest time scale losses of water determine the slow variationof soil moisture and hence the longer time scale effects of soil moisture on precipitation. This paper showswith a simple analysis how shifting the partitioning of evapotranspiration between the different reservoirsaffects the variability of soil moisture and precipitation. In particular, it is concluded that a shift toshorter time scale reservoirs shifts the variance of precipitation from that which is potentially predictableto unpredictable.
The Variational Assimilation Experiment of GPS Bending Angle
WANG Yunfeng, WANG Bin
2003, 20(3): 479-486. doi: 10.1007/BF02690806
More and more new types of observational data provide many new opportunities for improving numericalweather forecasts. Among these, the GPS (Global Positioning System) bending angle is undoubtedly veryimportant. There are many advantages of the GPS bending angle, such as high resolution, availability inall weather conditions, and global data coverage. Thus it is very valuable to assimilate GPS bending angledata into numerical weather models. This paper introduces how to obtain and assimilate the GPS bendingangle. There are two methods of assimilation: the indirect method and direct method, and they are bothintroduced in this paper. During the minimizing process of variational assimilation, calculation efficiencyis very important and the optimal step size greatly influences the algorithm efficiency. Based on thecharacteristics of the minimizing algorithm, we obtain an adaptive method for calculating the optimizingstep suitable for all kinds of minimization algorithms through mathematical deduction. Finally, a numericalvariational assimilation experiment is performed using the GPS bending angle data of 11 October 1995.The numerical results indicate the validity of the variational assimilation method and the adaptive methodintroduced here.