应用气象学报

Advances in the Numerical Method and Technology of Multi scale Numerical Prediction

Peng Xindong, Li Xingliang

2010, 21(2): 129-138.

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Abstract:
With the advances of computer and computational technique, multi scale multi task numerical model is being developed for fine prediction in a global scope. The difference between a general circulation model and a mesoscale model becomes unclear except for the forcing arrangement, and a global model tends to be used for severe storm prediction and study of mesoscale convective systems. Development of a multi scale model can be stated in improving the universality of model dynamics, discretion scheme, computational algorithm and physics, and the key issue is improving the multi scale properties of each parts. It calls for no approximation to the dynamics of the real atmosphere and detailed arrangement of the topography. Numerical schemes and the computational algorithm should be flexible, accurate and stable, so that can be used to various grid spacing systems. Considering the spherical mesh, grid structure and dynamical feature of discretion, the quasi uniform mesh with fine numerical dispersion feature is preferred in the recent development of numerical models. Reduced grid, cubed grid, icosahydron grid, Yin Yang grid and Voronoi grid are discussed. In addition to detailed model physics, non hydrostatic model with global and regional configuration and unified frame is the fashion of multi scale model. The vertical coordinate and terrain arrangement are important for proper description of dynamical and thermo dynamical processes. Vertical z coordinate and “shaved cell” topography have boosted the unified model, especially in high resolution case. Besides, numerical method should describe the intrinsic nature of dynamical processes, and provides sub grid information during the temporal integration. Conservation of the main model properties is known as the key factor for long term integration. High performance numerical scheme favors the multi scale application of a model and is of great attraction. The pioneer researches over the world are summarized, which provides reference for the development of the multi scale model system, GRAPES, in China.

Compression for Freedom Degree in Numerical Weather Prediction and the Error Analogy

Zheng Zhihai, Feng Guolin, Chou Jifan, Ren Hongli

2010, 21(2): 139-148.

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Abstract:
A substantial improvement in the skill of numerical weather prediction has been achieved in recent years. Useful skill typically extends numerical weather prediction from present day to about 6 days. But for longer timescale, such as 6—15 days, the forecast quality is poor. It is necessary to improve the quality of the results. Utilizing useful information in historical data to improve the prediction skill of numerical model has been considered as a goal. The analogue dynamical method is one of these optimum methods to combine dynamical methods and statistical methods together. However, the analogue selection is very hard due to broad freedom in traditional analogue selection methods. So an analogue selection method under small degree of freedom is proposed. By the method degree of freedom on low dimensional climate attractor is reduced, and small scale components are filtered. This method is based on the theory that the high dimensional phase space in a forced dissipative nonlinear system eventually evolves into a low dimensional attractor, while small scale high frequency patterns are dissipated. Based on compression for degree of freedom of the initial field, an analogue index is defined. In order to further develop the analogue dynamical method, the regularity of the error similarity is investigated. Analyses show that similar initial conditions lead to analogical characteristics of forecast errors. Compared with the system error, the error under analogical initial condition is more close to actual forecast error. Meanwhile, in the spatial scale, forecast errors between analogical initial conditions are well consistent. The preliminary results of the forecast experiments on a complicated T63 Dynamical Extended Range Forecasting model of NCC/CMA show that the analogue dynamical method can effectively reduce the model error. Results provide support for further developing the analogue dynamical method. Evidently, further theoretical and more prediction experiments are necessary for improving the performance of analogue dynamical method. Meanwhile, the model predictable components should be reasonably obtained. These problems will be studied in further work.

Guide Star Sub catalog Method for FY 4 Meteorological Satellite

Wang Sujuan, Guo Qiang, Xu Jianmin

2010, 21(2): 149-156.

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Abstract:
A method of selecting guide star for star tracker of FY 4 meteorological satellite is proposed. This method is based on the J1991.25 Hipparcos catalog and Pulkovo Compilation of Radial Velocities for Hipparcos Stars, through the rigorous treatment of epoch transformation, the star's position of the FY 4 runtime epoch can be determined. In order to search more accurate guide star, the stars whose magnitudes are brighter than 6.5 of J1991.25 Hipparcos catalog are chosen, and the J2000.0 Hipparcos sub catalog is got after the rigorous epoch transformation. With the help of Tycho 2 star catalog and SAO star catalog at the same epoch, the threshold of position precision is introduced, which can meet the precision of attitude determination in arc second level. The additional guide star (with star flag 5) is brought into use for improving the accuracy and speed of star identification. After the cases analysis of unsuccessful star identification, the guide star which always cause false star identification is given a star flag(such as 6), which denotes that the star is a redundant guide star and can't take part in the star identification processing. Through the introduction of redundant guide star the accuracy of star identification is improved. The 189 additional guide stars distribute all over the whole sky, even the star sparse area. And 34.4% of them are of a magnitude brighter than 3, which can improve star identification. Inaccurate reference guide star causes attitude error inevitably. Comparing with the guide star position of catalog epoch, using the guide star position of FY 4 runtime epoch through rigorous epoch transformation under the limit of the threshold of position precision is a good method to improve the precision of attitude determination.

Lightning Activities in China Offing Sea Area Observed by Satellite based Lightning Imaging Sensor

Wang Yan, Zhang Yijun, Ma Ming

2010, 21(2): 157-163.

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Abstract:
Using observation data of the optical transient detector and the lightning imaging sensor on satellites from April 1995 to April 2006, the spatial and temporal distribution of lightning activities in China offing sea area (including the Bohai Sea, the Yellow Sea, the East China Sea, the South China Sea and some parts of the Pacific in China's terrestrial sea) are statistically examined. The relationship between the lightning activities and the sea temperature is also studied by combing those data with the reanalysis data from the NOAA optimum interpolation SST. The result indicates that the average flash densities in China offing sea area is 3.39 fl·km^-2·a^-1, which is about 5 times higher than the global ocean average value. The high flash density areas are mostly in the boundary of the land and the sea, especially in the sea areas around the islands, indicating the difference of lightning activity between the sea alongside land and the ocean. The lightning activities in the Bohai Sea and the South China Sea are more frequent than those in the Yellow Sea and the East China Sea. There is a clear tendency that the flash density in China offing sea area decreases gradually with the increasing of latitude and the distance off the coast. The flash densities in the sea around the Taiwan Island and the Bohai Sea are the highest. The lightning activity is most frequent in summer, then spring, and in winter it is rare. The sea temperature in the black tide area is higher in winter and spring, and the lightning activity is more frequent than the East China Sea and parallel ocean sea areas, indicating that the black tide area is a strong lightning activity area. The annual and diurnal change of lightning activities in China offing sea area take on a similar changing trend as that on the land of the same latitude. For the lightning activities of the Bohai Sea and the East China Sea, there's one single peak, while in the South China Sea and the Yellow Sea, lightening activities take on the similar double peak trends as those in the land of subtropical zone. The lightning activities' annual change is obviously positively correlated to the sea temperature in China offing sea area. But the flash activity is insensitive to the annual change of the sea temperature, indicating that the sea temperature's annual change isn't the main cause for lightning activities' annual change.

Meso scale Weather Systems During the Opening Ceremony of Beijing Olympic Games

Yang Bo, Sun Jisong, Wei Dong

2010, 21(2): 164-170.

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Abstract:
The Opening Ceremony is one of the most influential events during 2008 Beijing Olympic Games. And the meteorological service of the Opening Ceremony is of great importance. The weather situation is very complicated, on one hand, analysis of upper air and surface layer circulation at 20：00 8 August suggests there is a great possibility of precipitation, on the other hand, the strong radar echo belts of the southeast area developed at around 21：20 begin to constantly move northward, which may combine with the strong meso scale radar echoes from northeast, and lead to heavy rainfall in the urban area. But in fact, medium and heavy rainfall occurs in the southwest and northeast suburb, but almost no precipitation occurs in the urban area. The fact is beyond the explanation of conventional meteorological data. So high resolution ground based meteorological data observed by auto weather stations (AWS), wind and temperature data from the surface layer to the tropopause observed by thee wind profilers located at Beijing Weather Observatory, Haidian Station and Shangdianzi Station are used. The possible mechanism of the meso scale weather system developing and weakening which affects the precipitation in Beijing urban areas are discussed considering the influences of the environmental wind field, topography and urban heat island effect. It is concluded that the impact of secondary circulation loop formed by the heat island and topography effect to the south urban area is different from that to the north. In the south part of the city, the secondary circulation weakens the southerly, and its subsiding air weakens the northward branch of meso scale system. But in the north part, the secondary circulation strengthens the southerly, and the air convergence will become stronger, promoting the development of meso scale system. When weak southerly environmental wind exists, the secondary circulation loop will lead to air convergence below 1500 meters high. But the air divergence above the height of 1500 meters is unfavorable for the meso scale system to develop northward and bring heavy rainfall to Beijing urban area.

Bright Band Identification from CINRAD-SA/SB

Zhang Lejian, Cheng Minghu, Tao Lan

2010, 21(2): 171-179.

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Abstract:
The bright band caused by aggregated snow melting has increased the uncertainties in radar based quantitative precipitation estimation. However, the height of the bright band can indicate the 0℃ isotherm and can be useful in identifying areas of potential icing for aviation and in the data assimilation for numerical weather predictions. Furthermore, the region of the bright band can also indicate the region of transition from convective to stratiform cloud which is useful for meso scale weather analysis. Based on extensive analysis, an automated method to identify bright band with CINRAD SA/SB Doppler radar based on vertical profile of reflectivity (VPR) and 3 dimension characteristics (horizontal, reflectivity gradient and vertical reflectivity lapse) of radar (3DVPR BBID) is developed. The 3DVPR BBID algorithm can determine from a volume scan mean VPR and a background freezing level height from soundings if a bright band exists. To evaluate the performance of the 3DVPR BBID, radar data from 22 June to 11 July 2003 and from 1 to 31 July 2007 in Hefei and from 1 to 30 June 2008 in Guangzhou are utilized, and a method which identifies bright band based only on VPR is also implemented for comparison. The results show that 3DVPR BBID can identify the existence of bright band exactly and give the height of 0℃ isotherm. The accuracy rate for VPR of bright band with 3DVPR BBID is higher than that with VPR BBID. The probabilities of identification between VPR BBID and 3DVPR BBID are 94.0% and 95.7% from 22 June to 11 July 2003 separately, 85.4% and 92.3% in July 2007, 81.0% and 89.8% in June 2008. The height of 0℃ isotherm derived from 3DVPR BBID and VPR BBID are close to the sounding observation. Furthermore, the performance of 3DVPR BBID is better than VPR BBID. In most cases, the evaluation of 3DVPR BBID is better than VPR BBID based on mean absolute error (MAE) of different distance from radar. Since the time interval of soundings is longer, the algorithm can give higher time resolution data of the 0℃ isotherm. When calculating the height of bright band with radar data, it's estimated that radar wave transmits with the standard atmosphere refraction. Another limitation is that VPR can only be used in certain parts of the radar detected regions. And there are some uncertainties with VPR in the distance far away from radar.

Structural Characteristics of Atmospheric Relative Humidity During Lightning Activity in Yunnan Province

Zhang Tengfei, Xu Yingjie, Zhang Jie, Duan Xu, Yin Liyun

2010, 21(2): 180-188.

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Abstract:
Using monitored cloud ground lightning data of lightning detection system in Yunnan Province and relative humidity parameter in NCEP/NCAR data from 1 June to 31 August in 2007, structural characteristics of atmospheric relative humidity during lightning activity are analyzed. The results show that lightning activity in Yunnan is most frequent in summer because of high temperature and high humidity, and there's a daily apex which appears from 16:00 to 17:00 and a valley which appears from 08:00 to 11:00 because of daily variation of solar radiation. The lightening activity is uneven and discontinuous on day to day variation because of environmental conditions of atmospheric relative humidity. Some specific environmental humidity conditions are necessary for lightening activity. Too high humidity may restrain ascending motion of convective development, while low humidity is insufficient for convective cloud to develop. Moreover, convective development and intensification are aroused, and convective cloud developing upwards is urged by some dynamic triggering condition when unstable energy runs up to a certain extent. When electric field intensity reaches to a certain extent, the thunder phenomenon occurs. Lightning activity in Yunnan usually happens in the typical environmental atmosphere with certain vertical configuration: The relative humidity is not high at 850 hPa, but at 700 hPa it's very humid and then becomes dry at upper layers. The relative humidity increases with height below the 700 hPa, forming high moist region at the middle layer and deceases above it. This environmental condition of relative humidity is in favor of thunderstorm weather occurrence and thunder formation. First, relative humidity increase below 700 hPa is favorable for ascending motion of water vapor and cloud formation. Second, dry atmosphere at upper layers and moist atmosphere at lower layers cause convective instability so that convective cloud may be urged to develop and promote lightning activity. When lightening occurs, relative humidity at low layer is about 40%—75%, at middle layers of 600—700 hPa it is above 80% and usually reaches up to 90%—95%, and the humidity decreases to about 35%—60% at upper layers of 250—400 hPa. It is also found that deeper middle moist layer will probably result in stronger thunder process.

Numerical Simulation of a Sea Breeze Event After the Passage of a Cold Front

Sheng Chunyan, Shi Qian, Gao Shouting, Guo Junjian

2010, 21(2): 189-197.

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Abstract:
A sea breeze event occurs on 21 August 2006 when a cold front passes Shandong Province and the synoptic scale wind is northerly. In order to analyze the development of the sea breeze, the feature of the sea breeze is analyzed with 123 AWS data in Shandong Province, over 30 AWS station data in Qingdao area, in the island and three buoys in the sailing spots. As a result, the detailed AWS station data in the coastal area and buoys can give more detailed and new feature of the sea breeze. Further more, these data are assimilated into the ARPS (the Advanced Regional Prediction System) model. The controlled experiment uses 2 level 1 way nested grids, with the horizontal resolutions of 30 km and 6 km respectively. In vertical, a scheme consisting of 53 sigma z levels with a minimum thickness layer of about 20 m near the surface and vertically stretched grids extending to model tops at about 20 km height is used. Compared with the observation, the model can simulate development of the sea breeze reasonably. With the observation and the simulation, the detailed feature of the boundary wind and 3 dimension structure of the sea breeze are analyzed, indicating that the sea land temperature difference of around 2 ℃ causes the sea breeze in this case. Under the northerly wind, the surface temperature to the south of Qingdao rises faster than that on the north land, thus the sea breeze mainly occurs in the southeast coastal region. The sea breeze begins offshore and then develops into the land and the far ocean widely. The vertical height of the sea breeze circulation is as shallow as below 500 m. Due to the stronger offshore synoptic scale wind the sea breeze can only spread to very limited area along the coastal line. The northerly synoptic wind passes over the Bohai Sea and causes cold surge to land in Shandong Province, which is adverse for the sea breeze maintaining, as a result, the sea breeze in the southeast coastal region of Shandong Province dies earlier at around 17:00. When the sea breeze begins the reverse circulation forms around 1500—2500 m height at the same time. But the vertical circulation doesn't develop clearly until the evening and the height of it is below 1000 m. The vertical circulation and the reverse circulation can last 3 hours after the sea breeze dies.

Characteristics of the Climate Change in West China in Recent 50 Years

Yang Ming, Li Weiliang, Liu Yu, Xu Haiming

2010, 21(2): 198-205.

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Abstract:
Using the surface observational data and upper air data from 1951 to 2000 at 194 stations in China, the inter annual and inter decadal characteristics of climate in West China are analyzed. The results show that in the middle of 1970s, an evident climate sudden change happens to yearly average surface temperature, air temperature, precipitation, total cloud amount, low cloud amount and solar radiation. There is a rising trend of the annual mean temperature in West China. The most evident warming occurs in Hetao and Xinjiang regions; in Tibet and Hexi regions warming is also evident, and in Southwest China the warming trend is less evident. The surface temperature changes in the same trend of the air temperature, and its variation is more significant. Particularly, the surface temperature in West China is getting higher since the middle of the 1970s, but it doesn't reach the level of that in 1950s, therefore shows a dropping trend in linear from 1951 to 2000. Annual mean precipitation takes on a rising trend all the regions in West China except for Hetao region, and the largest increase of rainfall occurs in Xinjiang and Tibet regions. Both the annual mean and season mean rainfall increases significantly in Xinjiang and Tibet regions, and the annual mean precipitation in 1990s increases 50% than that in 1950s. The precipitation in Xinjiang region keeps increasing in every season particularly in winter. Actually the precipitation is nearly zero in winter before 1978(except 1950s), and it begins to grow after that. The total cloudiness and the low cloudiness decrease linearly in West China in recent 50 years. The decreasing trends of annual mean low cloudiness are observed in all the areas, with the most remarkable decreasing rate in Hetao region and Southwest China, especially in Hetao region where the rate is -1.7%/10 a. In Xinjiang region the cloudiness hasn't decreased much. The total radiation drops over the west regions, particularly in Tibet region. The change of solar direct radiation in the whole areas in West China is similar with that of total radiation. The southwest region experiences an increasing trend in solar diffuse radiation, while a dropping trend in the other four areas and the most evident decreasing occurs in Xinjiang and Tibet regions. The cloud amount and aerosol consistency have a close connection with solar diffuse radiation. In Southwest China, the solar diffuse radiation decreases as the cloud amount increases, so it's guessed that the decreasing of solar diffuse radiation is caused by the increasing of aerosol consistency.

Introduction and Quality Analysis of Chinese Aircraft Meteorological Data

Liao Jie, Xiong Anyuan

2010, 21(2): 206-213.

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Abstract:
Meteorological data observed on civil airplanes in China in recent years is all the AMDAR reports spark plugged to obtain by the AMDAR Program of WMO. For a comprehensive understanding of the current situation and quality of Chinese aircraft meteorological data, the temporal and spatial distribution character of Chinese AMDAR data from 2003 to 2009 are summarized, synthesizing the AMDAR reports of these six years observed through aircrafts taking off and landing phase to aircraft observational profiles and analyzing its usability. Moreover, in reference to the aircraft data quality control principium used by NCEP, the quality of global AMDAR data and Chinese AMDAR data from May 2003 to April 2009 are checked with a quality control method, which consists of six steps: Metadata check, validity check of air temperature and wind, aviation track organization, position consistency check, temporal consistency check of air temperature and attitude, and comprehensive analysis of quality control code. Based on the result of quality control, the temporal and spatial distribution characteristic of error data is analyzed. The statistical result shows that, the amount of Chinese AMDAR reports has been increasing year by year. In 2003, there are about 50000 files per month. In 2009, the number is about 200000 per month. Two thirds of the reports concentrate in take off and landing phase. The analysis result shows that, all of the aircraft observational profiles cover over 110 airports, which are mainly distributed in the east central areas, especially some large and medium cities. The vertical resolution of almost all the aircraft observational profiles is higher than 10 hPa. With the development of Chinese AMDAR program, the proportion of profiles with high resolution increases year by year. In 2008, there are about 78.9 percent aircraft observational profiles whose vertical resolution is higher than 2 hPa. Case study result shows that, although the vertical trend is very close, there is a little difference at low level between the temperature observed by aircraft and by radiosonde. In addition, there is also a little difference among the temperature observed at the same height by different aircrafts. The result of quality control shows that the temperature error rate of Chinese AMDAR data is lower than the global level, but the wind error rate is to the contrary. The error data is mainly concentrated in the near surface level. The quality of Chinese AMDAR data has improved significantly compared with previous years. From January 2008 to April 2009, the monthly error rate of wind speed is less than 1.3 percent, and the monthly error rate of temperature is less than 0.2 percent. In conclusion, the source and quality of Chinese aircraft meteorological data is stable，and as an important part of upper air observation data, aircraft meteorological data is of great application value.

Comparison of Temperature and Geopotential Height Records Between 59 Type and L band Radiosonde Systems

Ma Ying, Yao Wen, Huang Bingxun

2010, 21(2): 214-220.

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Abstract:
The radiosonde records such as upper air temperature, pressure and humidity are the fundamental data for weather forecasting and climate analysis, and especially for the latter, a long term continuity and a good consistency of these records are also required. The radiosonde systems used in China have experienced three generations since 1950s. Improvements and updates of the equipment have significantly increased the sounding data acquisition rate, accuracy and reliability, but inconsistencies of historical data arise simultaneously. In order to analyze system deviation between the records obtained from the upper air sounding records before and after applications of new equipment and method, comparison observations between the L band radiosonde systems with 59 ones at 80 upper air stations are conducted in 2002, and totally 60 records are obtained at each station using 60 normal operational 59 radiosonde systems and 60 L band ones which are used for training. From those stations, the sounding data of 70 stations are selected and analyzed. The records of temperature and geopotential height of mandatory level obtained from two types radiosonde systems are compared, and the average deviations between the records of different systems are adopted to show the difference. Considering the data error, which is related to three manufacturers of 59 radiosonde, different solar altitude and different temperature profile, the records are classified for comparison by manufacturer of 59 type, observation time and launching site, etc. On the whole, below the 100 hPa height, the temperature difference is less than 0.3 ℃ and the geopotential height difference is less than 7 meters. Therefore, before and after changing the sounding systems, the radiosonde records below the 100 hPa height do not exhibit obvious change in terms of the overall average. However, according the records higher than the 70 hPa height, the mandatory level temperature measured by 59 radiosonde is 0.1—0.7 ℃ lower than that measured by L band radiosonde, which results in a continuous cumulation of geopotential height difference with increasing height. For example, at the height of 20 hPa, the difference can reach about 30 meters, which is obvious. In summer, compared with the records of L band radiosonde, those of 59 radiosonde made by Taiyuan factory show a significantly higher temperature in troposphere, and a significantly lower temperature beyond the top of troposphere, which brings on a higher geopotential height in troposphere (as much as 20 meters on average), and a lower geopotential height in stratosphere (as much as 18 meters on average). The records of 59 radiosonde made by Shanghai factory exhibit a little lower temperature and geopotential height in both troposphere and stratosphere (with a maximum difference of 37 meters). In winter, 59 radiosondes made by the two factories exhibit the same behavior as in summer respectively but with obvious small amplitudes.

Drought Monitoring in Heilongjiang Province Based on NDVI-Ts Space

Yu Min, Cheng Minghu

2010, 21(2): 221-228.

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Abstract:
Combining the spectral vegetation index with thermal infrared information, the triangular space of NDVI Ts based on satellite remote sensing data provides an effective method for monitoring surface dryness at large spatial scales. Vegetation Temperature Condition Index (VTCI) based on the triangular space of MODIS NDVI Ts is applied to monitoring summer drought in Heilongjiang Province in 2007. The VTCI is based on satellite derived information only, and therefore the index is potential for operational application. The Terra MODIS NDVI(MOD13A2) V005, LST(MOD11A2) V005 and the land cover product(MOD12Q1) V004 products, at 1 kilometer spatial resolution, are used as the indicators for the monitoring and analysis. The monitoring period is from 10 June to 12 August 2007. It's found that the space of NDVI Ts for the whole study area is typically triangular because the monitored area is large enough to make the value of NDVI and soil moisture vary in a very large scope. Then, a linear regression analysis is conducted to get the equations of the dry and wet line from the triangular space for the whole area, and the VTCI for the Heilongjiang Province is extracted. Combined with MODIS land cover product IGBP and frequency distribution of the VTCI, the spatial pattern and temporal evolution of the drought are also analyzed. At last, the ground measured precipitation data at the same periods are used to validate the drought monitoring measurements. The result shows that the drought widely distributed in spatial scale and is very serious. It is more serious in main crop producing areas, especially in the western Songnen Plain and the west of eastern Sanjiang Plain. From 10 June to 25 June the drought is heavier than the other 3 monitoring periods. Some fluctuations occur in July, then the drought weakens in most areas in August, except the Daxinganling Mountains in northwest where the drought enhances to some extent. The correlation analysis between VTCI and ground based measured precipitation indicates that there is a significant linear correlation between VTCI and total monthly precipitation. The VTCI and cumulative total monthly precipitation are also linearly correlated from the monitoring month to the previous months. Obvious correlation is also found between VTCI and the percentage of departure from normal monthly precipitation in the monitoring month, and between VTCI and the cumulative percentage of departure from normal monthly precipitation from the monitoring month to the previous months. The result suggests that VTCI is not only correlated with recent rainfall but also correlated with past cumulative rainfall. So it is a reasonable simplification of the NDVI Ts space and can monitor drought instantly, which is proved adaptive in Heilongjiang Province.

Grassland Vegetation Change Based on MODIS NDVI Data and Climate Information

Chen Yanli, Long Buju, Pan Xuebiao, Mo Weihua

2010, 21(2): 229-236.

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Abstract:
Monitoring vegetation change is an important aspect to study global climate chan ge. The Normalized Difference Vegetation Index (NDVI) is the most frequently us ed vegetation index to discuss environmental change. It's common to study vegetati on change using the NOAA Advanced Very High Resolution Radiometer (AVHRR) data which are available for more than 20 years, and the EOS Moderate Resolution Im aging Spectroradiometer (MODIS) data with improved quality are also widely used now. MODIS NDVI dataset during 2000—2005 in Xilingol are analyzed. Precipitation, vapor pressure, mean temperature, max temperature, min temperatur e, hours of sunshine are taken as climatic elements to explore the relationship between MODIS NDVI and climatic factors over different types of grassland. Due t o the narrow spectra bands of infrared and near infrared satellite sensors, MOD IS NDVI is sensitive for dynamically monitoring the four types of steppe: Meadow steppe, typical steppe, sandy steppe, desert steppe. As the water conditions of these four typical grassland vary widely, their spectral responses are also sig nificantly different. The vegetation increases on the whole in Xilingol during t he period of 2000 to 2005. Desert steppe decreases while typical steppe increase s for the research area. The global warming trend is obvious in Xilingol. Althou gh the rain doesn't increase much from 2000 to 2005, the grassland grows for t he meadow steppe and the typical steppe, promoted by the increasing temperature and better water conditions. For the sandy steppe and the desert steppe where wa ter conditions are poor, as the increasing temperature accelerates water evapora tion, the vegetation degradation occurs as a result of the worse environment. MODIS NDVI series exhibit obvious correlation with climatic factors. For typical steppe, max temperature is the most related factor with NDVI, and vapor pressur e is the second. For desert steppe, max temperature is the most related factor w ith NDVI, and min temperature is in the second place. Responses of MODIS NDVI to climatic factors lag in time obviously. The lag time for all climatic factors i s -1 stage for typical steppe, and for desert steppe, the lag time for both wate r and mean temperature climatic factors is -1 stage, while the lag time is incon sistent for max temperature and min temperature, which may be caused by the diff erences of soil texture and vegetation types.

Application of Poisson Gumbel Distribution to Wind Speed Calculation for the Southeast Coastland of China

Zhang Rongyan, Zhang Xiuzhi, Cai Lianwa

2010, 21(2): 237-242.

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Abstract:
Tropical cyclone (TC) is the main cause of extreme wind in China southeast coastal, especially in coast to the south of the Yangtze River delta. The TC maximum wind speed may exceed 40 m·s^-1, raising the risks for engineering design near the coast. The 1961—2006 TC/typhoon almanacs published by Shanghai Typhoon Institute and wind data of ground stations are investigated, and the TCs that pass within 3 latitudes away from the coast are defined as influential ones. There are 428 TCs passing through this area from 1961 to 2006. Typhoons and strong tropical storms are major TCs with the percentage up to 33.6% and 25.7%, and 59.3% in total. Super strong tropical cyclones and strong tropical cyclones often cause losses to wind farm, and the percentage of them is 7.9% and 13.6%， respectively. The percentage of tropical storm and cyclone is 10.3% and 8.9%, and these TCs usually bring lots of power for coastal wind farms. The numbers, motion paths and intensity of TCs generated from Northwest Pacific Ocean are stochastic every year. Therefore, the number of influential tropical cyclones make some kind of discrete distribution in China southeast coast, obeying the Poisson distribution. But maximum wind speed under the influential tropical cyclones (TC) may satisfy some kind of continual distribution, obeying the Gumbel distribution. Based on the Poisson Gumbel compound extreme distribution, the extreme wind speeds of different time scales can be obtained using TC data of coastal weather stations for the period of 1961—2006. The result indicates that the Poisson Gumbel joint distributing algorithm is superior and gives stable and accurate results when the observed data sample sequence is short, especially for the strong weather event like TC which is very random. For 53.9% of the stations, the extreme wind speed of 50 years is less than 50 m·s^-1. The extreme wind speed of 50 years exceeds 42.5 m·s^-1 in Dacheng, Shengshan, Shanmen, Nanjishan Islands of Zhejiang Province, Beijiao coast and Taishan Island of Fujian Province, Zhelang and Shangchuan Island of Guangdong Province, and Xisha Island of Hainan Province. In these areas, the risks of tropical cyclone for wind farms construction should be paid enough attention.

Meteorological Data Files Quick Download Service System

Gao Feng, Wang Guofu, Yu Wen, Feng Mingnong, Sun Chao, Luo Qi

2010, 21(2): 243-249.

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Abstract:
The development of meteorological data service system is summarized as 4 stages: Stand alone file retrieval stage, the interface calling stage, database system stage and distributed data service stage. The current service system can meet the needs of most users querying data, but for meteorological professional and operational users, the system has certain limitations. These users call for a new data service system that offers fast bulk download meteorological data services, which should also be established to make up for the deficiencies of existing meteorological data sharing service system. According to operational requirements, a new meteorological data service system is designed based on metadata technology, through which professional and operational users can quickly download batch of real time meteorological data. The system is based on front end application systems and background management systems to achieve a bulk download, file directory navigation, data synchronization update, multi threaded HTTP, security management, and flexible configuration capabilities. With the system, users can use third party software to download great amounts of meteorological data files. Based on the file directory navigation provided by the system, users can access the data level by level following the data classification and the time characteristics, and can flexibly select and retrieve the data. The system achieves the meteorological data collection system synchronous update, and users can access to download the latest meteorological data products. The system is a brand new design for the meteorological data services, and has been well accepted by the professional users. It has a wide range of promotional value and is good reference for future development of meteorological data service system.

Application of GIS to Tropical Cyclone Product Generation System

Zheng Weijiang, Wu Huanping, Luo Bing, Liu Zhenkun, Lü Zhongliang, Sun Lihua

2010, 21(2): 250-255.

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Abstract:
China is one of the most typhoon influenced countries in the world, so it is urgent to strengthen the capacity of typhoon forecast services, in which Geographic Information System (GIS) plays a more important role increasingly. On analyzing the operational requirement of typhoon forecast, the workflow of tropical cyclone product generation is designed with the support of historical and real time typhoon spatial database. First, visualize typhoon track and wind data. Second, smooth typhoon track, edit feature property and validate topology for spatial analysis. Then analyze the data to obtain landing area, real time and future affected area, and similar track search results. Finally, service product is generated using Microsoft Office development technology. Four key issues of GIS application on production system are discussed in details. GIS spatial analysis aims at extracting and transporting spatial information based on geospatial object location and geometry character. Using Geodatabaseand Spatial Data Engine, historical and real time typhoon database can be built and provide the ability of data record and update. ESRI representation symbolizes data using a flexible and rule based structure that is stored inside the geodatabase along with data, supporting multiple representations simultaneously. Constructing feature filling modes and color representation styles by standard rules can efficiently implement the visualization of large volume of typhoon data. In order to precaution which administrative region would be affected in the near future, the spatial overlay analysis is carried out to extract and transport spatial information. Two probability ellipse methods of typhoon track forecasting are also proposed to predict potentially infected districts ahead of schedule. With spatial interpolation method intensive typhoon forecasts are made for multiple points, and for each point a weight error radius is calculated, then the vertical line throughout the points are determined, finally the control points are linked to construct a polygon and smoothed with Bezier Curve. Dynamic buffer method is based on integral splits the typhoon tracks by kilometer unit, by the ratio of the length of typhoon track the buffer radius can be calculated, after then do dynamic buffer based on a series of radiuses and combine the buffer area to a probability ellipse. GIS provides a reliable scientific basis for typhoon forecast, and with its aid more applicable and efficient typhoon forecast models can be achieved in the near future.

Vol.21, NO.2, 2010