Vol.17, NO.2, 2006

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A Simple Parameterization Scheme of Snow-particle Radiative Properties and Effect on Mesoscale Precipitation
Zhou Guangqiang, Zhao Chunsheng, Qin Yu, Lou Xiaofeng
2006, 17(2): 129-137.
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Abstract:
Snow-particle, which is produced in Bergeron process, is a kind of solid particles in cloud. Snow-particle takes part in several microphysical processes, so it presents in different shapes. The multi-shape feature makes it difficult to develop a direct and detailed parameterization of the snow-particle radiative properties. At present, the water content and number concentration of snow water are predictable in newly developed dual-parameterized explicit moisture schemes, such as Reisner scheme and CAMS scheme. Consequently, the development of a simple parameterization scheme of snow-particle radiative properties becomes possible.An individual parameterization scheme, in which all snow radiative properties are determined by effective size, is set up and implemented in MM5 V3 mesoscale model. A study on the effect of snow radiation on mesoscale precipitation is carried out using a South China severe storm case on June 8th, 1998. Four numerical experiments are designed to represent no snow radiative effect, assuming snow-particle as a part of graupel and ice crystal, and to calculate snow radiation using the algorithm set up respectively.The distribution of snow water and its effect on the rainfall pattern, domain averaged integration rainfall and rain rate are investigated. The numerical experiment results show that the effect of snow radiation on mesoscale precipitation is obvious:① Snow water plays an important role in the atmospheric water substances, it has the largest water content and the second largest number concentration; ② Snow radiation distinctly modifies the local properties of precipitation, especially the rain rate and position of rainfall center, though it has little effect on overall precipitation pattern; ③ The effect of snow radiation on precipitation during the daytime is much larger than that during the nighttime; ④ Distinct difference among the different experiment results indicates the necessity of establishment of an independent snow-particle radiative properties parameterization.The discussion on the influences of snow radiation on precipitation shows that the indirect effect, which is the convective enhancement due to the relative snow radiative heating, contributes to the major rainfall variation. The radiative absorption of snow water heats the middle and upper atmosphere, causes stronger convection and produces more precipitation as a result. While the direct decreasing effect of snow radiative heating by microphysics is relatively neglectable.In general, the effect of snow-particle radiation on mesoscale precipitation is obvious and an independent snow radiative properties parameterization is much necessary for the improvement of the ability of mesoscale model on the precipitation prediction.
Relationship Between Upper Troposphere Circulation and Rainfall Band in China During Summer Revealed by Cloud Motion Vectors
Hou Qing, Xu Jianmin
2006, 17(2): 138-144.
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Abstract:
National Satellite Meteorological Center has retrieved Atmospheric Motion Vectors from GMS-5 since 1997. This data provide quite detailed information on the wind field at the upper-level troposphere over east Asia, east Indian Ocean and west Pacific. Comprehesive analysis between the cloud motion vectors and observed precipitation data from 1998 to 2002 is made, which verifies that circulations in upper troposphere of summertime rainfall events in China can be concluded into three typical patterns:① The first pattern is often seen over the Changjiang river region and in the southeastern part of China from June to July whose rain belt is associated with an anticycloneridge at the upper troposphere, with precipitation cells in the diverging area between the subtropical westerly jet and the ridge or in the velocity diverging area of the tropical easterlies north of the ridge; ② The second pattern often occurrs in the southern part of China (Hainan, Guangdong and Guangxietc.) from July to September with the rainfall center locating on the right side of the upper deformation field and the west of the asymmetry anticyclone; ③ The third pattern happens frequently to the north of 30°N in China from July to August with the rainfall center lies in the southwesterly in front of the upper level westerly trough which occurrs primarily in north China. Evident indicates that, firstly air divergence at the upper troposphere is commonly seen in these three circulations; secondly, having a small wind component inwards, the area at the upper troposphere corresponding to the precipitation areas in the first and second patterns has anticyclone vorticity, while the precipitation areas in the third pattern are under the positive vorticity advection zone in front of upper westerly trough, which has a large component inwards. Besides, compared to the first one, happening around higher latitudes, with prominent jets and northerly component on the ridge, the second one occurs around lower latitudes, with indistinctive jets and southerlies on the ridge. The first circulation pattern has a close relationship with the location, movement and maintenance of the rainbelt in the flood season in China; the second one is related to the precipitation brought by the typhoon and the tropical weather system while the third one is a typical mid-latitude upper trough cold front precipitation. They are different in the persistence, influencing ranges and mechanism.
Analysis on Spherical Function Structures of Climatic Variability for Global Atmospheric Geopotential Height Fields
Yan Jusheng, Wang Panxing, Duan Mingkeng, Li Qiaoping, Ye Lingping
2006, 17(2): 145-151.
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Abstract:
The anomaly of height fields is set up based on the temporal decomposition of these fields, and even continuations from hemisphere to globe at certain level in certain month are made. During the study, two kinds of monthly averaged geopotential fields dataset is used, namely 2.5° by 2.5° resolution even rectangular grid data and spherical function coefficient data derived from 1958—1997 NCEP/NCAR reanalyzed geopotential height fields dataset. Firstly based on the previously defined intensity index of climate anomaly fields (Ia), it demonstrates the intensity variations of hemispheric anomaly fields at the different height and seasons, and the differences between Northern and Southern Hemisphere are also investigated. Then according to the low-dimension and low-order characteristics of the spherical function spectrum in the hemispherical geopotential height anomaly fields, hemispherical circulation anomaly is divided into four types:hemispherical homogeneous anomaly, zonal homogeneous anomaly, ultra-long and long wave scale anomaly. The corresponding variance contributions (R′00, R′0, R′ul, R′l) are also calculated respectively by using spherical function coefficient data with the wavenumber of m≥0, k≤6. Finally the variations and their hemispheric differences of the four anomalies above as the function of height and seasons are discussed in detail.The main results are as follows:① Ia shows a yearly periodic oscillation and is stronger in winter than in summer. The index always increases with the height, especially in winter. However in summer Ia has a weak high (low) value center near the tropopause (the upper stratosphere) respectively. The difference between the two hemispheres is that the seasonal variation of Ia in the Southern is weaker than the Northern at the troposphere. ② For the spectral structure of the anomaly circulations, there are some obvious changes from the troposphere to the stratosphere. In the troposphere the ultra-long wave circulation is predominant. Hemispherical homogeneous anomaly (Long wave scale anomaly) is not important in the troposphere (stratosphere) respectively. ③ The characteristics of the variance contributions of the Northern Hemisphere can be concluded that in January, R′00 at all the levels are very little (less than 10%); R′l is only significant in the troposphere; R′0 and R′ul predominate at all the levels, whereas R′ul(R′0) is more important in the troposphere (stratosphere), and in July, R′00 dominates in the medium and upper stratosphere; R′0 takes second place; R′ul and R′l are predominant in the troposphere with equivalent magnitude. ④ For the Southern Hemisphere, in July, R′00(R′l) in the troposphere (stratosphere) is very little; just as the January's situation in the Northern Hemisphere, R′0 and R′ul are dominant through the atmosphere; R′ul is a little larger. In January, the characteristics in the stratosphere are similar to July's situation in Northern Hemisphere. The difference is that R′ul and R′0 dominate through the troposphere with the comparable im portance.
Application of High Resolution Numerical Model to Wind Energy Potential Assessment
Mu Haizhen, Xu Jialiang, Ke Xiaoxin, Tang Lin, Chen Deliang
2006, 17(2): 152-159.
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Abstract:
In view of the sparse distribution of weather observation stations, particularly in coastal and riverbank zones by now, the meteorological component of TAPM (The Air Pollution Model) is utilized to assess wind energy resources, which is an incompressible, non-hydrostatic, primitive equation model with a terrain-following vertical coordinate for three-dimensional simulations. In the simulation scheme, Xujiahui station (31°12′N, 121°26′E) is selected as the center of the modeling region, the number of horizontal grids is 50 by 50, the number of vertical levels is 25. The model nests with outer grid resolution of 10 km and inner grid resolution of 3 km, the inner region ranges from 30°33′N to 31°55′N and from 120°43′E to 122°13′E, the acreage of inner region is 150 km by 150 km which covers Shanghai and parts of Jiangsu and Zhejiang Province.The model is integrated month-long for January, April, July and October in selected years, the initial data and boundary conditions needed for driving the model are obtained from attached synoptic scale analysis dataset. At first the model input parameters such as surface vegetation type, soil moisture and sea surface temperature are altered according to actual conditions of selected modeling region, then the wind field data is obtained from model output results, which agrees well with the observation data although there are slightly differences between them. The causes of model results error are discussed. In order to minimize the error, using the method of linear regression and considering impact of changes of environment around observation site on observation data quality, the reexplanation of wind speed and the third power of wind speed over different types of surface (land or water) are conducted by making use of synchronous observation data, the verification results show that these equations can be used in other years and the application range of these correction equations is wide, which will lead to improvement of accuracy and reliability of wind energy assessment result. Finally the distribution pattern of wind and wind energy density of Shanghai are obtained at the resolution of 3 km in 2002, this information, especially for detailed wind energy distribution information over coastal and riverbank regions, is impossible to be obtained by analysis of observational network data.The results of this study provide scientific basis for the work of wind energy resources assessment, planning and location-selecting of wind farm, as well as indicate that the numerical model results can be utilized in the evaluation of wind energy resources. At the same time the work of this study is primary, improvement is needed to obtain more precise results as the method of reexplanation is quite simple; the model needs to be run for long period to get more universal conclusion.
Dynamic and Thermal Features of a Sustained Heavy Fog Event in Huabei Plain
He Lifu, Li Feng, Li Zechun
2006, 17(2): 160-168.
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Abstract:
The observation study of a persistent heavy fog event occurred in Huabei Plain from 29 Nov to 3 Dec in 2004 is undertaken using the dense observation data and NCEP/NCAR reanalysis data. It reveals the physics mechanism of its formation and maintenance by means of analyzing the large scale dynamic and thermal features of the event. The results show that the warm high ridge at middle and low troposphere and the persistent surface cold high over Huabei Plain provide an appropriate background condition for the prolonged dense fog. The warm high ridge is prone to the formation of the inversion temperature layer in boundary layer, and the dominant weak easterly winds in the central and south of the surface cold high transmit the water vapor from the east ocean area to the Huabei Plain. During the fog period, the surface wind velocity is about 2—4 m/s and the domineat wind directions are east and southeast. The bias between surface temperature and dew point is less than 2 ℃, but enlarges sharply when the dense fog process ends. By analyzing the atmospheric sounding data it demonstrates that the atmosphere over Huabei Plain dense fog region indicates convective stability and exists a downward current and an inversion layer above the fog level. The humid layer locates between groun level and 900 hPa, and becomes deeper in the late of the fog event. A cooling process at ground layer produced by net surface radiation fluxes can trigger and strengthen the dense fog event. The subsidence flow at middle and lower troposphere is important to the establishmnent and maintenance of stable stratification at boundary layer and is helpful to the weak breeze wind at ground layer over the fog region; the transportation of weak warm advection in boundary layer and weak lift motion at ground layer play a very important role in the long time maintenance of this fog event. The intrusion of north wind accompanied by cold advection is a main dynamic factor of the fog dissipation. It needs to be pointed out that although these conclusion are meaningful for recognizing this fog event, the physical mechanism on its happening and development remains unclear, especially the influence of the temperature advection process and the low cloud process. The Huabei Plain fog event exhibits a radiation fog in the beginning and early phase, but it shows more advection than radiation cooling in the later phase when the daily temperature variation is considerably weak. It is important to acquire more and higher resolution observation data especially the data in the boundary layer in order to deeply analyze the effect of warm temperature advection during the late dense fog event. The numerical simulation study by connection of two-dimension cloud model and meso-scale model and more dynamical diagnoses will need to be done in the near future.
Climate Variation Feature and Its Effect on Environment Change in Central Tibet from 1961 to 2000
Bian Duo, Du Jun
2006, 17(2): 169-175.
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Abstract:
Climate change is a global issue. It's very difficult to distinguish the climatic impacts from the variation of atmospheric constituents by human activities among human activities influences on climate. In order to demonstrate the environmental effect over Tibetan agricultural integrated development area (central Tibet) from 1990 to 2000, using meteorological factors such as precipitation, temperature, evaporation, accumulated days of gale and sandstorm in the recent 40 years and environmental integrated assessment index, it investigates the variation of regional climate feature and the environmental effects over Tibetan agricultural integrated development area.The analysis shows that the annual temperatures and precipitation exhibit increasing tendency over central Tibet, the annual mean temperature increases 0.24 ℃/10a; the precipitation decreases 2.9 mm in the earlier 20 years, but it shows distinct increasing tendency from 1981 to 2000 and the average increase is 9.6 mm/a; evaporation quantity, strong wind days and sandstorm days exhibit decreasing trend. In the target region, the evaporation quantity decreases averagely 24.4 mm/10a before 1983, and then decreases 22.1 mm/a; the accumulated days of gale present a decreased tendency, with the average of 3 d/a from 1990s. In the same way, the accumulated days of sandstorm has decreased 1—2 d/a, in which Zedang station has no record of sandstorm after 1995. Comparing with the earlier 30 years, the sandstorm days decrease by 22—31 d in 1990s.Considering arid and semi-arid river valley area, the variation trend of warm and humid climate is beneficial to environment improvement, the regional integrated environmental assessment index increases 5.24% during 1990—2000. But natural environment lose and desertification are getting worse because of the frequent natural disasters like flood, hail and human activities. Especially human activities accelerate the development of land desertification. The main problem is that human activities such as farmland expansion and deforestation caused by the population growth and the lack of fuel material, over breeding of livestock, which lead to the expanding of bare land and the acceleration of soil erosion in the remote area far from the river valley. Facing the vulnerable environment, with the strict controlling of the population growth, it is a powerful approach of agricultural integrated development and an important act to achieve the goal of sustainable development of region's society and economy that the government ought to actively adjust the structure, develop highly effective and advanced industry, attempt to strengthen the ability of the exploitation of renewable energy such as wind and solar energy.
Relationship of the SSTA Temporal Variation in the Middle and East Equatorial Pacific in Spring and Summer to East Asia Summer Monsoon
Li Xiuping, Luo Yong, Guo Pinwen, Wang Shaowu, Xie Zhihui
2006, 17(2): 176-182.
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Abstract:
East Asia summer monsoon has a distinct interannual variability, and the external forcing is one of main causes resulting in the interannual difference of East Asia summer monsoon, but the effect of the tropical sea surface temperature is the most prominent. Though SST is not the only factor that affects the weather and climate anomaly in China summer season, the tropical SSTA must be considered. Some previous researches only consider the influence of SSTA on East Asia summer monsoon in the different phases of warm and cold episodes. From the point of temporal variation, the study suggests a new point of view that the temporal variation of SSTA in the Middle and East Equatorial Pacific in spring-summer season has an effect on summer rainfall of China and East Asia summer monsoon. By using SSTA index of Niño1+2, 3, 4, 3.4 from 1951 to 2003, the analysis on the relationship between each of four regions temporal variation of SSTA from March to August and China summer precipitation is carried out. In the years of positive temporal variation of SSTA, there is less summer precipitation to the south of the Yangtze, North China and east part of Northwest China. The opposite situation occurred in the years with the negative temporal variation. Particularly the correlation coefficient is the highest between the temporal variation of SSTA of Niño3.4 region and summer precipitation. Thus, some further researches have been conducted regarding the temporal variation of SSTA of Niño3.4 region as a prediction signal. By means of calculating the correlation between the temporal variation of SSTA in Niño3.4 region and the vertical shear of zonal wind in East Asia, it is found that there is a significant relationship between the temporal variation of SSTA of Niño3.4 region and circulation pattern of East Asia summer monsoon. Furthermore, the composite analysis is done for the years of positive and negative temporal variation of SSTA respectively. The result shows that there are obvious differences in the 500 hPa height anomaly, 850 hPa zonal wind anomaly, 200 hPa zonal wind anomaly etc, especially to the south of the Yangtze River, South China Sea and Northeast China. The temporal variation of SSTA of Niño3.4 region in spring and summer can provide a prediction clue to the prediction of East Asia summer monsoon and the summer precipitation in China.
Preliminary Study on Circulation Characteristics and Mechanism of Rainstorm in Spring and Autumn of Shandong
Yang Xiaoxia, Wan Feng, Liu Huanzhu, Xue Deqiang, Hua Yan
2006, 17(2): 183-191.
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Abstract:
The climate characteristics and the influencing systems associated with the heavy rain in spring and autumn in Shandong Province are analyzed, and the mean circulation charts are also given. The circulation characteristics and the weather systems causing the two large scale rainstorm events in 2003 spring and autumn are undertaken with the thermal characteristics and water vapor transfer mechanism during the rainstorm discussed. The mechanisms of the two heavy rain processes are studied by using the k-helicity and slantwise vorticity development theory. The result shows that all the rainstorms in spring are caused by cyclones, and the majority of rainstorms in autumn are caused by cold front. The heavy rain during 17—18 April 2003 is caused by cyclone and that during 10—12 Oct 2003 is caused by wind shear line and cold front. There is a low-level southerly jet which transports the watervapor to the storm area, and the temperature and the humidity increase rapidly, also the convective instability and the moist baroclinity increase in heavy rain area. The strong cold front triggers the release of the convective instability energy which leads to the heavy rain. The low-level positive k-helicity develops impetuously during the two events. The warm and humid air in low-level flows into the rain storm area cyclonicly and goes up in spiral way, and flows out anticyclonicly in high-level. The positive k-helicity center in cyclone rainstorm appears in the east of low-level eddy and moves towards the northeast, but that in cold front rainstorm occurs near wind shear line which locates in the north of Shandong Province and moves to the southeast. The rainstorms occur in the moist baroclinic atmosphere and the development of slantwise vorticity is very important in the two cases. The wet potential vorticity augments intensively during the rainstorm period. The southward moving cold air encounters the warm air, which makes the convective instability and moist baroclinic are both prone to the forming of the rainstorm. The MPV1 < 0 and MPV2 > 0 in the warm areas ahead of cold front, namely, the convective instability and ageostrophic balance, are in favor of the vertical vorticity development and the ascending motion, triggering convective instability energy release, and strong precipitation by the moist potential vorticity (MPV) equation theory. When the cold front moves to the rainstorm areas, the equal θse lines in the front areas near 850 hPa are cragged and dense, the atmosphere is near neutrally or stability stratified and the positive MPV1 value augments intensively. The horizontal gradient of the θse in the front area and the wind vertical shear near the front in the low-level augments, the negative MPV2 value augments too. Equally by the MPV equation theory, in stable atmosphere, the augmentation of MPV1 positive value and MPV2 negative value indicates the development of the vertical vorticity, the updraft's augmentation and rainstorms occurrence. The MPV1 < 0 and MPV2 > 0 in the early phase of the heavy rain, MPV1 > 0 and MPV2 < 0 in the later phase of the heavy rain are both of advantages to slantwise vorticity development, and speed up ascending movement.
Data Quality Analysis of 3836 C-Band Dual-linear Polarimetric Weather Radar and Its Observation of a Rainfall Process
Cao Junwu, Liu Liping, Chen Xiaohui, Chen Gang
2006, 17(2): 192-200.
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Abstract:
The technical status of upgraded 3836 C-band Dual-linear Polarimetric Weather Radar (3836 radar) in Beijing Meteorological Bureau in 2004 is studied. Based on the large area stratus rainfall observation over Beijing on November 19, 2004, the 3836 radar's measurement quality and its detection ability are presented. According to the characteristics of the scattering and orientations of the precipitation particles with different phases in the space within the melting-layer (so-called bright band), a model is established, in which the bright band can be identified by applying the fuzzy logic method to three polarimetric radar observation variables and the height. The radar measurements are used to assess the outputs from the model and discuss the variation features of different phased hydrometeors within the melting-layer.The results show that: ① the phase distribution structure of hydrometeors of the storm revealed by the 3836 radar data, except the LDR non-analysed data, is reasonable and similar to those from foreign polarimetric radars. ② The result from the established model based on the 3836 radar measurements is reasonable and able to reflect the feature of melting-layer. ③ Upon the polarimetric radar measurements, the melting-layer is characterized by a drop in ρhv (0) associated with Z and ZDR peaks, the cause is that the ice particles descend below 0 ℃ height, melt into large rain drops, and the break into more rain droplets, resulting in the variation rule that the ZH and ZDR increase first then decrease while ρhv (0) is just on the contrary in this layer. In additional, the maximum Z is observed at a higher altitude than that of the maximum ZDR and minimum of ρhv (0). It indicates that the maximum eccentricity of melted hydrometeors occurs at a lower level in the atmosphere than that with the maximum size, assuming the largest particles are located near the reflectivity maximum.It is a preliminary study on examining the potential ability of polarimetric weather radar upgraded from ChIna New generation RADar (CINRAD) in detecting microphysical structure of rainfall storm, which may provide more useful help and reference to the further development of polarimetric radar in China.
Mesoscale Analysis on a Heavy Rain Associated with a Bengal Bay Storm and Cold Air in West Yunnan
Lu Yabin, Zhang Tengfei, Xu Balin, Yang Ming, Zhou Guolian, Suo Miaoqing, Cun Canqiong
2006, 17(2): 201-206.
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Abstract:
A severe tropical storm develops over the Bengal Bay at the mid May, 2004. Jointly affected by the Bangal Bay storm and cold air and wind shear, a heavy rainstorm which lasts 3 days hits Yunan Province with the rainband stretching mainly from the west to the south. From 08:00 (BT) May 18 to 08:00 (BT) May 19, a severe rainfall occurs in west Yunnan with the centers in Baoshan, Dehong and Lincang. 24-hour rainfall is over 100 mm in 6 county stations, the rainfall center is at Longling of Baoshan with the 24-hour rainfall reaching 124.3 mm. It is the first time that such a concentrated heavy rain occurrs in a circular area of 180 km radius in Yunan.The tropical storm is one of the important weather systems which cause severe rainstorm in China. There are a lot of scientific investigations on the occurrence and evolution of the tropical storm (typhoon) over the west Pacific Ocean. Thus the forecasting ability on the tropical storm (typhoon) over the west Pacific Ocean has been improved. However, researches on the tropical storms originated in the Bengal Bay are still few. In fact, the storm of the Bengal Bay has significant impacts on the rainstorm in the southwest China. To explore the physical mechanism of the heavy rain in west Yunnan occurring on May 18, 2004, the MICAPS products, FY-2 satellite images and Dehong CINRAD-CC Doppler radar volume scan data are taken into account for the investigations. Moisture flux divergence, K index, Si index and Q vector divergence show that the synoptic scale affecting systems are the storm over the Bengal Bay and the southward cold front shear. The heavy rain occurs in the favorable environment field where the centers of water vapor convergence with high instability energy and high wet, positive helicity region at 700 hPa level and convergence area of wet-Q vector are located. Also, the satellite images present that the evolutions of some meso-β scale convective systems are the key of the heavy storm. And Doppler tracking observations give the facts that the echoes are mixed wadding rain echo with intensity between 30~44 dBz, the direct factors of the process are the meso-and small-scale systems such as frequent inverse wind, low level jet and meso-scale convergence line.
An Automatic Identification Algorithm for the Removal of Bright Band from Reflectivity of CINRAD/SA
Chen Mingxuan, Gao Feng
2006, 17(2): 207-214.
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Abstract:
An automatic identification algorithm for the removal of bright band from radar reflectivity data is introduced and tested by case analysis and pre-operational run. The algorithm has good results of removal of bright band based on the radar reflectivity data which have been transformed into a regular 3-dimensional Cartesian grid while configuration and running setup are relative simple. Firstly, the algorithm performs the recognition of vertical bright band profile. The algorithm considers each point independently, and attempts to find the evidence of a bright band by using a pattern recognition approach. Based on the typical reflectivity of the vertical profile attributes in the bright band deducted from statistics, a set of ideal templates of radar reflectivity vertical profiles is established, which can resemble the actual radar reflectivity characteristics in different bright band areas at the utmost. For each horizontal grid the algorithm attempts to couple the templates to the measured vertical profile of radar reflectivity. The couple function is obtained by computing the slope and the correlation of the least squares regression between the template and the measured profile. Secondly, the algorithm finds continuous regions of bright band. Regions of bright band may be further recognized by the horizontal uniformity of the signature. The algorithm attempts to use the goodness-of-fit and height-of-best-fit field computed above to gain further clues of a bright band. A grid kernel-based approach is used, in which a kernel is composed of 5×5 grids in the field. For the grids in the kernel, the algorithm counts the number of difference between contiguous goodness-of-fit values exceeding a given threshold or the difference in the height-of-best-fit values for contiguous points exceeding a given threshold. A high interested value is assigned to points with low numbers, since this indicates uniformity. So bright band areas are identified as those fields with high interested values. Finally, the algorithm removes the bright band signature. It removes the continuous regions of bright band by correcting the reflectivity within the bright band, which employs linear or non-linear interpolation methodology based on the measured reflectivity values in the profile above the band and the measured values below the band. Some thunderstorm cases analysis and pre-operational running tests suggest that the algorithm is able to successfully identify and remove most bright band regions, which influences quantitative precipitation estimation, based on CINRAD/SA radar reflectivity located at Tanggu of Tianjin. However, the algorithm has less effect on actual storms reflectivity data. Results of the analysis also demonstrate that the bright band regions usually appear at 2.5 km over Beijing and Tianjin in early summer. Because the algorithm is relatively easy to configure and run, it is appropriate to use in real-time or operational mode. Of course the algorithm has limitations and needs to be continuously improved for actual operational applications.
Analysis on Strong Hail Storm Three-body Scattering Signature Using Doppler Weather Radar Data
Zhu Minhua, Yu Xiaoding, Xia Feng, Zhou Honggen, Wang Meng
2006, 17(2): 215-223.
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Abstract:
Based on the scattering characters of dry ice-ball and water-ball, the formation mechanisms of three-body scattering and the major factors determining its shape and size of strong hail-storm are discussed. A three-body scattering hail-storm case occurrs on the 26th June 2004, is analyzed by means of Lianyungang S-band Doppler weather radar data, and the application of TBSS (three body scattering spike) in operational hail warnings is studied. The results show that: for dry ice-ball and water-ball, the backwards scattering ability is stronger with longer diameter. The backwards scattering ability of the large hails is much stronger than the raindrops and the small hails. Three-body scattering is caused by the hail-storm's lateral scattering of radar wave to the ground, the scattered by the ground surface to the hailstorm again, and the hailstorm scatters the radar wave back to radar. The main factors influencing the three-body scattering observation include the position and intensity of the hail-storm, radar elevation, and the characteristics of the underlying surface. The best radar elevation to observe three-body scattering are 4.3°, 5.3°and 6.2°. But the three-body scattering can be observed from all radar elevations under a certain conditions. The distance between the three-body scattering and 60 dBz strong reflection region depends on the intensity of the strong reflection region, radar elevation and underlying surface characteristics. The "long spike" appearance depends on the ratio of radial length and tangential length of the strong echo region. The three-body scattering can last more than 1 hour for the base reflectivity, radial velocity and spectrum width. To produce TBSS, the intensity of reflectivity core must be larger than 60 dBz. The intensity of three-body scattering spike is less than 18 dBz, its length is usually less than 14 km, radial velocity is very low, towards the radar direction, its spectrum width can be over 13 m/s. The TBSS is clearer in spectrum width and velocity than in reflectivity maps. When the three-body scattering at the upper air is the strongest, the falling hail of the surface is not always the strongest. The large hails descend to the ground when the threebody scattering weakens suddenly. The highest altitude of the three-body scattering is at 11.8 km with the corresponding temperature of -45 ℃ in the target case. The spectrum width can show the real size of the TBSS, for TBSS has a much higher spectrum width than the real storm, while the TBSS on reflectivity or velocity maps can be partly obscured by the real storms. When the TBSS is first identified, large hail can be predicted in downstream area over 20 minutes. These results provide a new clue for forecasting the hail based on TBSS.
Observational Analysis on a Cold Front Heavy Rain Using Doppler Radar Data
Zhang Jiaguo, Wu Cuihong, Wang Yu, Wan Yufa
2006, 17(2): 224-231.
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Abstract:

Forced by cold front and low-level jet, a regionally torrential rainfall occurs in the east of Hubei Province during June 22—23, 2003. On 22 June, a synoptic-scale cyclone is developing over the reaches of the Changjiang and Huaihe River. Leaded by the deepening cyclone, the ground cold front moves southward and triggers meso-scale convective system (MCS) when it confronts with southwest low-level jet. The MCS perches on the tail of the comma-pattern cloud system in infrared weather satellite image. The MCS moves southward, strengthens and results in heavy rain in the east of Hubei Province.Wuhan Doppler radar collects the data set during the heavy rain period. The radial velocity and spectrum width features of the cold front and the low-level jet on Doppler radar are analyzed in detail using base velocity, velocity cross section, base spectrum width data collected by Wuhan Doppler radar every six minutes. Also, combining analysis on radar base reflectivity and surface rain-gages data, the synoptic relationships between the spread of the MCS and the cold front, low-level jet are discussed briefly. The results are as follows: ① There is a convergence line corresponding to cold front on base velocity product. Because Doppler radar only detects radial velocity of the wind, the convergence line gets short gradually when it moves towards radar station. Spectrum width is high near the cold front for relatively high vertical wind shear in low level of the troposphere, which helps to determine the position of the cold front. ② The salient features of the meso-scale low-level jet on base velocity product include big radial velocity and big tangent gradient of the radial velocity, and the farther the gale core is apart from radar station, the clearer the big tangent gradient behaves, which is called "great gale core" detected by Doppler radar many times. ③ The MCS that brings regional torrential rainfall is jointly resulted from cold front and low-level jet. At the intense developing stage of the MCS, the strong low-level jet climbs along cold surface front quickly and steeply. At weakening stage of the MCS, rainfall intensity decreases because of lessened gradient of the cold front though there is a divergence flow in troposphere aloft. In addition, the heavy rain occurs near the join point of the cold front and low-level jet and on the left of the jet. As the direction and position of the low-level jet is fixed, the join point can shift to west when cold front moved to south. Accordingly, MCS strengthens in the west of the join point for horizontally cyclonic vortex on the left of the jet, and weakens in the east of it for horizontally anti-cyclonic vortex on the right of it.
Vertical Profile of Radar Echo and Its Deteermination Methods
Wu Cuihong, Wan Yufa, Wu Tao, Jin Hongxiang
2006, 17(2): 232-239.
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Abstract:
Radar beam's lifting and widening in the distance and the vertical nonuniformity of the natural distribution of water content in the atmosphere are important factors of radar detecting rainfall errors. During 1980s—1990s, the importance of vertical profile of reflectivity (VPR) to radar detecting rainfall is emphasized and the study on the VPR technology is conducted. There are two major aspects on the researches of VPR, one is the determination method of VPR, the other is the technique to correct radar precipitation estimation. After comparing the major features of three existing determination methods, i.e., parameterization, averaging and identification, it is found that the mean vertical profile of reflectivity (MVPR) has the advantage of simplicity and utility. Four algorithms of MVPR are obtained and compared with emphasis on investigating the determination method for mean vertical profile (MVPR); and the different functions of determining parameters, especially for MVPR, are demonstrated. Results show that the R/G ratio can be enhanced from 0.84 (0.86) to 0.93 (0.97) and the AD of radar rainfall estimation can decrease by 4% after the MVPR correction in the two precipitation cases.Among the four algorithms of MVPR, M11 is the best one in which the ratio profile takes the near groundlevel as the datum plane so that the feasibility of radar precipitation correction can be assured, and it has better representation owing to consideration of the same weighing of each bin in domain D. The MVPR algorithms relate to 8 attribute parameters, of which some values exert significant effect on the quality of MVPR. For example, the initial distance Ds in domain D obviously affects the profile shape below 1 km, therefore 1~5 km should be adopted; the distance outside D0 domain D plays its role from near ground-level to 6 km and the smaller D0, the smaller the more smoothing due to the beam widening, so 40~50 km should be adopted; the smaller the value of the vertical piecewise-height Δh, the more detailed the profile shape, so 250 m should be adopted. The evidence also indicates that when the exponent b in the different Z-R relationships are kept constant, the slope of the profile below 1.5 km will increases and the bright band of the profile will be more distinguishable with the decrease of the coefficient A.By analyzing VPR derived from the volume data in terms of VCP21 model of CINRAD/SA in Wuhan, it is found that the bright band locates at the height of 5 km and the radar precipitation estimation is generally not affected by the bright band when using 0.5° reflectivity PPI (or hybrid scan) within radius of 200 km. When echoes of major precipitation system in domain D, Z (R) below the bright band decreases with the height (dZ/ dh < 0), and these may be the causes why radars underestimate summer rainfall (especially for long distance) in the middle reaches of the Yangtze and it could be done by means of VPR to correct the radar precipitation estimation.
Numerical Weather Prediction———Necessity and Feasibility of an Alternative Methodology
Chou Jifan, Ren Hongli
2006, 17(2): 240-244.
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Abstract:
The motivation of the study is to discuss the issue whether numerical weather prediction should be practiced in the meteorological departments at provincial, regional or city levels. It suggests that numerical weather prediction need not be operated by meteorological departments at all levels, and only need to be done by those who carry on scientific-research and urgently expecting to improve the accuracy of local high-impact weather prediction. With respect to the problem how numerical weather prediction should be put into practice, it is put forward that local meteorological departments are not fit for repeating those approaches originated from mainstream opinions, but should break a new path such as an alternative methodology.Furthermore, the content, method and meaning of such an alternative methodology are examined. The inverse problem that the information of historical data including analogue is made full use of estimating model errors is raised and an analogue errors correction method is theoretically introduced in order that one can combine effectively both statistical and dynamical methods together, and need not change the current numerical prediction models. The new alternative methodology not only adequately utilizes dynamical achievements but also can rationally extract the information of a great many analogues in historical data so as to reduce model errors and improve forecast skills. This prediction strategy can overcome the limitation of only using initial values in traditional numerical models on one hand, and on the other hand, is able to realize the pertinence of numerical model prediction for specific elements through the process of selecting their corresponding analogues.Theoretical analyses show that based on analogue correction method of errors, the conventional problem of numerical weather prediction may be transformed into the prediction problem of forecast errors. That is to say, the unknown error terms in current prediction can be mathematically estimated by means of the forecast error fields generated from historical analogue data similar to the current prediction initial values. Some related preliminary experiment results on a complicated atmospheric prediction model have documented the feasibility and validity of this method based on such alternative methodology.As are seen in related studies, in such an alternative methodology as analogue correction method of errors, it emphasizes that it is unnecessary to construct new numerical prediction model which is very difficult for local meteorological departments. In such circumstance, one only needs to operate the existing models and modify them in order to localize by using historical data associated with interesting phenomena. There for it is completely feasible in practice and has operational future. Of course, so far the selection process of historical analogues is still coarse, and especially, as to the problem how the unknown error terms in current prediction can be estimated using the forecast errors from historical analogue data requires much more work, and further researches are still needed.
Observation and Analysis on Ice Nucleus of Henan County of Qinghai Province in Autumn 2003
Shi Aili, Zheng Guoguang, You Laiguang
2006, 17(2): 245-249.
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Abstract:
Natural ice nucleus in atmosphere are one of the basic conditions for the ice crystal formation, which can exert a key influence on the cold cloud precipitation. In addition, many researches on ice nucleus show that ice nucleus play the same important role in the various weather processes as the cloud condensation nucleus. Besides the influence on the microphysical structure of cold cloud and precipitation, ice nucleus concentration affects the radiation characteristics of cloud and then influences the weather and climate processes accordingly. In order to understand the physical mechanism of the precipitation and microphysical background of rainfall enhancement in China, the observation on ice nucleus is carried out and a lot of useful results are gained since 1960. However, up to now, most researches focus on different areas, the continuous observations on the same area, the same period and using the same method are few, especially over Tibet Plateau. To highlight the characteristics of the ice nucleus over Tibet Plateau area (Henan County in Qinghai Province), some useful results for the further understanding on the precipitation mechanism and weather and climate processes are provided. In terms of the Bigg mixing chamber in which the valid volume of the chamber is 3.05 liter the ice nucleus are observed from October 5 to 26 in 2003. On the bottom of the chamber, there is the sugar plate whose diameter is 10 centimeter. The super cold fog is formed manually, which can last 2—3 minutes. The ice nucleus form little crystals falling down to the sugar plate, increase until they can be distinguished by eye, then the sugar plate is pulled up to count the crystals number, so the ice nucleus concentration is obtained. This method is applied in the observation of ice nucleus over Tibet Plateau in the year of 1989, 2000, 2001, 2002. Also, the observations are compared with that gained in last two years in the target area and other plateau areas. Results show that: ① The concentration of ice nucleus over the plateau is greater than previous while the distribution slope is smaller, and the distribution of ice nucleus is inclined to evenness; ② The graupel and rainfall during night contribute to the uncertainty of ice nucleus concentration; ③ The relation between the concentration of ice nucleus and the pressure is negative while the relation between the concentration of ice nucleus and humidity is positive; ④ Wind influence on concentration of ice nucleus and the variety of ice nucleus in low temperature is larger than that in the high temperature; ⑤ Human activity may have influence on the concentration of ice nucleus.
A Method of Designing a Reliable and Safe Distributed Meteorological Database System
Zhao Wei, Tuo Yufeng, Yang Yinjuan, Jiang Nan, Du Yanfu
2006, 17(2): 250-256.
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Abstract:
In the process of constructing an actual meteorological database system, it is a complicated and important task to design a reliable, safe, flexible and expandable meteorological database system which based on the characteristic of actual meteorological data and combined with some current advanced computer and software developing technology. Combined with the actual circumstances of one department's meteorological database system construction, an applied method of establishing a reliable and safe meteorological database system based on the distributed database technology is introduced. In the logical design for database, all meteorological data are divided into relation data and non-relation data. The relation data are directly stored in Microsoft SQL Server 2000, but the non-relation data are stored according to the different file index structure of Windows operating system. The technology of Microsoft cluster server (MSCS) and the redundant array of inexpensive disks (RAID) are used in the reliability design for database, the system's fault-tolerant is realized on the database server and storage device. The fault-tolerant and failover capability of the history meteorological database system are improved by using this technology. For the first time, the safe component conception and the models to design a safe database is proposed. Applying this conception to the database management module, the unsafe problems in database systems can be resolved. The Browse/Server (B/S) structure and multi-tier Client/Server (C/S) structure are used in the software architecture design of the meteorological database. The multi-tier C/S structure is used in meteorological database management software and the querying and displaying system of the meteorological graph data. The B/S structure is used in the meteorological Web applications based on the internet network. An integrated meteorological data query and display system based on the Web is developed, and a series of server-side COM component to display different kinds of meteorological data on the Web are designed, in which the COM technology and Browser/Server structure are used. Meteorological data dynamic real-time publishing and retrieving on the Web can be achieved by using this system. Since the database system is done and applied in the actual meteorological operation, it has been working well, with excellent stabilization, reliability, and quick data storage/reading speed. In the process of developing software for meteorological database, the computer technologies such as the safe component models, the COM component of server side, the structure of Browser/Server are applied very well, which is proved to be a good method for solving hidden safe trouble in the original protocol of computer network and database system, and an effect method for expanding applied field of distributed database system.
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