应用气象学报

Characteristics of Aerosol Optical Properties During Haze and Fog Episodes at Shangdianzi in Northern China

Yan Peng, Liu Guiqing, Zhou Xiuji, Wang Jingli, Tang Jie, Liu Qiang, Wang Zhenfa, Zhou Huaigang

2010, 21(3): 257-265.

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Abstract:
The Northern China is one of the regions with the high incidence of fog and haze. With the rapid development of economy in the recent years, the anthropogenic pollutants, such as aerosol and its precursors have brought about serious visibility degradations in the regions. In the fall of 2004, several heavy fog and haze episodes with visibility less than 1 km are observed at Shangdianzi (SDZ) Atmospheric Background Station. During the fog and haze periods, aerosol scattering and absorption coefficients are measured simultaneously at the Station with Nephelometer (M9003, ECOTECH) and 7 band Aethalometer (AE31, Magee Scientific). The results of aerosol optical properties indicate that aerosols over the region are strongly influenced by weather conditions. Under the influence of fog and haze, aerosols at the Station show higher values of scattering and absorption coefficient and higher aerosol single scattering albedo (ω), while relatively lower aerosol scattering coefficient, absorption coefficient and ω are observed after the haze and fog episodes. For the 4 fog/haze episodes, the aerosol single scattering albedo decreases by about 0.1, with the value of 0.94-0.97 during the fog/haze influencing periods and the lower value of 0.84-0.86 during the cleaner periods after the fog/haze process. Through a detailed analysis of the time series of the surface aerosol optical properties for a heavy fog and haze case, combined with the column AOD and weather condition changes, the evolution of aerosol optical properties are discussed. It is found that the fog and haze event takes on significant regional effects, air masses from polluted directions leads to higher ω, higher scattering and absorption coefficients of aerosols. This result suggests that the haze and fog weather conditions favor the accumulation of aerosol pollutants, especially favor the production of the light scattering aerosols, which reflect that the formation of secondary aerosols and their contribution to the light extinctions might be more significant during the haze and fog influencing periods.

Variation Characteristics of Thermal Resources in Northeast China in Recent 50 Years

Liu Shi, Wang Yong, Miao Qilong, Ding Yuanyuan

2010, 21(3): 266-278.

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Abstract:
Using the daily average air temperature data from 100 meteorological stations in Northeast China from 1951 to 2005, the variation of accumulated temperature steadily above 10 ℃ and 0 ℃, their lasting days, the beginning and ending date is summarized, and then the variation characteristics of thermal resources under the background of climate warming are analyzed. The air temperature increases continuously in Northeast China, and the climatic trend rate of the annual average air temperature is 0.4 ℃/10 a, more significant than any other region in China. And it is well correlated with the thermal resource indexes such as the accumulated temperature steadily above 10 ℃ and 0 ℃, and their lasting days, and the beginning and ending date. Climate warming has made the accumulated temperature steadily above 10 ℃ and 0 ℃ increase significantly in Northeast China widespread. The accumulated temperature steadily above 10 ℃ has also increased significantly by more than 40 ℃·d/10 a. The lasting days of accumulated temperature steadily above 10 ℃ and 0 ℃ increase significantly too. The increasing amplitudes are more than 2 d/10 a on average and reach up to more than 4 d/10 a in the north region of Northeast China. For some regions of Inner Mongolia close to Mongolia, southern Liaoning Province, southern Heilongjiang Province, Changbai Mountains and the west part of Jilin Province, the lasting days increase by 2-4 d/10 a. The contours of accumulated temperature steadily above 10 ℃ and 0 ℃ and their lasting days move greatly northward in Northeast China Plain and relatively flat Inner Mongolia Plateau, and an uplift trend from mountain areas to high altitude areas is found on the contours. The beginning dates of accumulated temperatures steadily above 10 ℃ has advanced by 0.1-2.9 d/10 a, and at some stations of Northeast China Plain, the advancing trends are more than 1.0 d/10 a. On the other hand, the ending dates has delayed to a less obvious extent by 0.0-2.1 d/10 a, and the stations with the delaying trends more than 0.8 d/10 a centralize in Liaoning Province and Changbai Mountains in Jilin Province. The beginning dates of accumulated temperatures steadily above 0 ℃ have advanced by 0.8-4.3 d/10 a, while the ending dates have delayed by 0.0-2.1 d/10 a, and the stations of the significant delayed ending dates centralize in the north region of Northeast China with the trend more than 1.0 d/10 a. These changes in thermal resources are conductive to increase food production and maintain the stability of food production in Northeast China.

Interpolation Methods of China Daily Precipitation Data

Shen Yan, Feng Mingnong, Zhang Hongzheng, Gao Feng

2010, 21(3): 279-286.

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Abstract:
Precipitation is one of the most important meteorological factors. Its spatial and temporal distribution is critical in climate diagnostics, numerical models verifications and hydrological models initiation. Substantial progress has been made in the last two decades in quantitatively documenting global precipitation. The development and operation of precipitation analysis products over China are relatively lagging behind other countries. Through the cooperation with the Climate Prediction Center (CPC) affiliated to National Oceanic and Atmospheric Administration (NOAA), the real time operational system of daily precipitation analysis over China is established in National Meteorological Information Center (NMIC). Based on the daily precipitation observations of over 2419 gauges, a system with the resolution of 0.5°×0.5° is established. It is consisted of four modules: Data obtaining, quality controlling, data processing and products serving. The kernel interpolation algorithm of this system is introduced from the CPC known as the climatological optimal interpolation (OI) method which can reduce the analysis error substantially arising from the large spatial discontinuity of precipitation. Daily climatology is defined for each station as the summation of the first 6 harmonics for the 365 calendar day time series of the mean daily precipitation, and then climatology fields are created by interpolating the truncated station climatology through the algorithm of Shepard. These fields are then adjusted by the PRISM (Parameter elevation Regressions on Independent Slopes Model) monthly precipitation climatology data, and the temporal variation patterns in the original daily climatology time series which also reflect orographic effects reasonably are retained. Analyzed fields of the ratio between daily precipitation and climatology are derived by interpolating the corresponding station values through the OI technique. Analyses of total daily precipitation are finally calculated by multiplying the daily climatology with the daily ratio. This system runs stably with reasonable products and users can get the products in three formats after 09:20 everyday on CDC website. It is an integral part of China real time and fine resolution precipitation analyses system under construction. Further improvements of the gauge based analysis will be implemented to correct the bias of wind effects. Collaborations in improving this system by refining the algorithm and merging with the satellite based precipitation products are welcome.

Relationship Between Lightning Activities and Surface Precipitation in Thunderstorm Weather in Beijing

Zheng Dong, Zhang Yijun, Meng Qing, Lti Weitao

2010, 21(3): 287-297.

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Abstract:
Study on the relationship between lightning activity and precipitation is valuable for estimating the rainfall based on lightning detection in the areas where other observation methods such as radar, precipitation gauge and so on are inapplicable. Therefore, 18 thunderstorm processes occurred in Beijing area are investigated based on radar observation and correlative radar algorithms, then the convective regions and stratiform regions are distinguished and the precipitation intensity of the thunderstorms is calculated. The total lightning data (intracloud (IC) lightning and cloud to ground (CG) lightning) of the thunderstorms detected by SAFIR3000 3 D total lightning location system is used in the analysis to discuss the relationship between the lightning activities and the amounts of precipitation in convective regions and the areas of convective regions. It is found that the precipitation per flash (PRF) ranges from 0.86×10⁷ kg/fl to 6.57×10⁷ kg/fl with the average value of 2.65×10⁷ kg/fl. Through comparing the standard deviation of the PRF calculated based on total lightning (PRF_L) and that calculated based on CG lightning (PRFCG), it shows that PRF_L is more stable than PRFCG in different thunderstorms. It demonstrates that for the time series of the peaks of lightning activities and the peaks of the amounts of precipitation in convective regions and the areas of convective regions, there are three styles of situation, i.e., ahead, behindhand and synchronous, which shows the complex relationship between them. The significant linear correlations between lightning activities and the amounts of precipitation in convective regions are found in all thunderstorms. The statistics including all samples shows that the coefficient is 0.826 and the fitting equation is M_CR=1.574×10⁷F_TL+2.956×10⁸. FTL is the frequency of the total lightning in 6 min and MCR is the amount of precipitation in convective regions in 6 min with the unit of kg/6 min. According to the situation of the time series of the peaks of lightning activities and precipitation, every thunderstorm process is divided into three stages: Rising stage, intermediate stage and falling stage. The correlation between lightning activity and the precipitation is found to be the most significant during the rising stage, followed in turn by falling stage and intermediate stage, and the corresponding coefficients are 0.859, 0.853 and 0.736, respectively. The relationships between the lightning activities and the areas of convective regions are found to be outstanding in 16 thunderstorms. When all the thunderstorms are considered, the coefficient reaches 0.846 and the fitting equation is A_CR=4.267F_TL+130.283.A_CR is the area of convective region with the unit of km² and FTL is the frequency of the total lightning in 6 min. According to the time series of the peaks of the lightning frequencies and the areas of convective regions, the thunderstorm processes are divided into three stages in the same way. It shows that all the linear correlations for three stages are outstanding, especially for rising stage. The coefficients for rising stage, intermediate stage and falling stage are 0.862, 0.834 and 0.837, respectively. Through the analysis, it is found that the relationships between the total lightning activity and the precipitation in convective region and the area of convective region are always better than those between the CG lightning activity and the precipitation and the area, indicating that the application of the total lightning data has improved the quality and reliability of the results.

Automatic Identification Forecast for Cold Wave in China Based on TIGGE Ensemble Data

Kang Zhiming, Jin Ronghua, Bao Yuanyuan, Tian Weihong, Gui Hailin

2010, 21(3): 298-306.

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Abstract:
The existing cluster products of ensemble system are produced by the classification on the ensemble members and are linked with certain weather process. An automatic identification forecast model for cold wave is proposed and the application of TIGGE data is improved. Based on the cold wave activity data provided by China Central Meteorological Office and reanalysis data from NCEP/NCAR, several different cluster analysis methods cooperated with subjective analysis have been used to find the typical synoptic pattern of 500 hPa geopotential height field of cold wave in China. When the cluster analysis is made on the 500 hPa geopotential height field, the results dont match subjective analysis. The cause is that the disturbance of 500 hPa geopotential height is weak compared to the basic circulation, so the different distance used by the cluster method mainly reflects the seasonal variation rather than the dissimilarities of the synoptic feature. Then the cluster based on the abnormal field of 500 hPa geopotential height is made to reduce the influence of seasonal variation and give prominence to the circulation disturbance of the cold wave. The cluster analysis results in three typical abnormal distributions of 500 hPa geopotential height during cold wave: Meridional positive-negative-positive distribution, zonal negative-positive-negative-positive and positive-negative-positive distribution. Based on the typical synoptic pattern and the threshold of the physical characteristic during the cold wave process, the objective cold wave forecast model is established. The capability of the objective cold wave forecast model are confirmed by the forecast experiments using the data from 1991 to 2006, although the result also shows the objective method makes false alarm by forecasting weak cold process sometimes. Finally two forecast experiments are conducted using the TIGGE ensemble data of the cold wave process in Jan 2008 and Jan 2009. The result indicates that by using the objective cold wave forecast model, the information of the ensemble products is concentrated, and the method could provide the forecaster with probability of the cold wave occurrence, and it consequently builds a direct relation between the ensemble forecast system and the cold wave in China. Due to the limitation of the TIGGE data, just two forecast experiments are carried out. More cases are needed in order to study the efficiency of the objective cold wave forecast model and the prediction performance of TIGGE ensemble data more thoroughly.

Extracting Typhoon Environmental Wind Field Using Doppler Radar Radial Velocity

Wei Yingzhi, Wu Chenfeng, Su Weidong

2010, 21(3): 307-316.

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Abstract:
It is always a challeging subject to forecast landing path of tropical cyclone and related wind distribution in operational weather forecast. Therefore, Doppler radar radial velocity is applied to the short term weather forecast as a new attempt. Rankine model is used to simulate the characteristics of the Doppler velocity (CDV) in the pure cyclone, and the impacts of both different environmental wind directions and convergent wind towards typhoon center are considered. Based on the simulation, the method of extracting typhoon environmental wind information is brought up, and then the extracted environmental wind is applied to judge the typhoon path. Comparison of radar observation and numerical simulation typhoon wind mainly focuses on three aspects: Deviation of typhoon center from zero velocity line, curvature difference of zero velocity line and difference between orders of positive and negative extreme velocities. A number of typhoon cases have been analyzed, especially during the key time of the turning of typhoon path, and it indicates that this method is good enough to judge the turning of the environmental wind of the CDV. At the same time, by comparing the Doppler radar velocity image and the simulation of different environmental wind directions, the direction and its change of the environmental air flow can be real time monitored (only 6 minutes for the interval of radar observation), so that the typhoon path can be well monitored, while this can hardly be achieved with large scale meteorological data. In particular, the turning of environmental wind occurs before the turning of typhoon path. For instance, the movement path of typhoon Aere changes from westward to southwestward and the simulation shows that there is an obvious change of the environmental wind more than 3 hours ago. For typhoon Nock Ten, when its movement path changes from northward to northwestward, the environmental wind direction changes from southward to southward by eastward at least 1 hour in advance. These results are important for the short term forecast of typhoon path both theoretically and practically. The stronger a typhoon is, the more obvious cyclonical circulation is, and thus the method performs better in estimating typhoon path, otherwise typhoon environmental wind information may affect the accuracy.

Comparative Analysis of the Precipitation Caused by Two Landing Tropical Cyclones in Yunnan

Guo Rongfen, Xiao Ziniu, Chen Xiaohua, Yang Jiakang

2010, 21(3): 317-328.

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Abstract:
Yunnan is a typical monsoon climate area located in the low latitude highland in China, affected by South Asia monsoon and East Asia monsoon at the same time. The heavy rainfall in Yunnan has peculiar area characteristic because of special geographic location and landform. In summer, the landing westbound TC from the South China Sea is one of the most important weather systems causing heavy rainfall in Yunnan. Statistics indicate the extreme heavy rainstorm in Yunnan is mostly caused by weaken landing TC, so its important to research mechanisms of landing TC heavy rainfall on the plateau. In 2006, the typhoon class TC Prapiroon, lands westbound and weakens into tropical depression in Yunnan without causing heavy precipitation. But in 2008, another TC Kammuri of the same strength causes eight days continuous precipitation, and a two day course of large-scale heavy precipitation in Yunnan Province. With the conventional observation data, best track data of CMA-STI and FY-2C satellite images and TBB, the precipitation caused by Prapiroon (0608) and Kammuri (0809) are compared on track, rainfall, speed, the circulation and hydrodynamic field. The result shows that they have similar birthplace, track, landing place, affecting time and central precipitation falling area, but Kammuri has much greater impacts on the length of time, the scope and impact of disasters. Active southwest monsoon and ITCZ, southerly low level jet, westward moving subtropical high and convergence ascending region, contribute to the major weather background of tropical cyclone precipitation in Yunnan. Effected by a variety of systems together, the typhoon circulation is very durable, resulting in the formation and maintenance of MαCS and MβCS clouds, leading to the occurrence of precipitation. Diagnosis on these physical fields shows that as a result of the active monsoon system, the Bay of Bengal and the South China Sea constitute a powerful water channel, through which instability energy and water vapor are transported to Yunnan accompanied by the establishment and enhancement of low-level jet. CSI and deep convection of positive feedback mechanisms lead to heavy precipitation in Yunnan.

Classification of Thunderstorm Processes with the Circumfluence Types and Environmental Field Characteristics in Ningxia

ji Xiaoling, Wang Shigong, Mu Jianhua, Chen Xiaojuan, Yang Kan, Xin Yaosheng

2010, 21(3): 329-334.

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Abstract:
Using the thunderstorm observation data of 25 weather stations in Ningxia during 1961—2005, according to the distribution of weather stations and occurring characteristics of thunderstorms, 6 thunderstorm types are defined as thunderstorm process over whole or most of Ningxia, regional thunderstorm process Ⅰ and Ⅱ, continuous thunderstorms process, local thunderstorm process and dispersed thunderstorm process. Statistics derives that during 1961—2005, there are 110 thunderstorm processes over whole or most of Ningxia, occupying 3% of the total thunderstorm processes, and 357 regional Ⅰ thunderstorm processes, occupying 9.2%, 931 regional Ⅱ thunderstorm processes, occupying 23.9%, 1039 local thunderstorm processes, occupying 26.7%, 1461 dispersed thunderstorm processes, occupying 37.5%, and 27 continuous thunderstorm processes. 263 regional thunderstorm processes occurred from March to September during 1996—2005 are analyzed using the NCEP/NCAR global reanalyzed data, indicating that there are 4 main circulation types favorable for the occurrence of thunderstorms in Ningxia: Mongolia cold vortex, cold transversal trough at the back of northeast cold vortex, Hetao cold vortex and west cold trough. The first 3 circulation conditions often lead to continuous thunderstorm. Several types of environmental field are favorable for the occurrence of thunderstorms in Ningxia, for example, weak northwest airflow under the circulation background of high in the east and low in the west at 500 hPa, ground surface air being controlled by cyclone or warm inverted trough, cold front, shear moving eastward and southward from Mongolia and Xinjiang or west Hetao, certain moisture and convergence uplifting conditions in the mid low level, and the relative humidity reaching 50%—80% at 700 hPa. From March to September, if the 4 circulation types and the environmental characteristics mentioned above occur in the area from Europe to Asia, then attentions should be paid to the possibility of thunderstorms. The relationships between circulation background and weather phenomenon should also be investigated based on analysis of the circulation evolvement characteristics of different circulation types. That will be helpful for judging the possibility of thunderstorm and its occurring area, time and intensity.

The Radiometric Characteristics of Sounding Channels for FY-3A/MWHS

Gu Songyan, Wang Zhenzhan, Li Jing, Zhang Shengwei

2010, 21(3): 335-342.

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Abstract:
The microwave humidity sounder (MWHS) is installed on new generation of Fengyun polar orbiters, FY 3A satellite, which is launched on 27 May 2008, for global all weather humidity sounding. The sounder produced by Center for Space Science and Applied Research Chinese Academy of Sciences has 5 channels in the range of 150—191 GHz. Around the center frequency of 183.31 GHz water vapor line, there are 3 sounding channels at 183.31±1, 183.31±3, and 183.31±7 GHz. All these three channels have two pass bands, and are only sensitive to vertical polarization at nadir. At 150 GHz there are 2 subsidiary window channels of V/H polarization.Every 2.667 s, MWHS scans through 98 earth views, 3 space views, and 3 internal black body target views. Each pixel has a nominal field of 1.1° (15 km on surface at nadir). One scan line could cover±53.35° from sub satellite point. The instruments are comprehensively tested to determine their actual antenna, spectral, thermal and radiometric characteristics.In order to determine the radiometric performance and verify the method for calibration of MWHS, an extensive test is performed on the flight models. The instruments are placed in a 3 m thermal-vacuum chamber where the temperature could be controlled simulating the conditions in orbit and they are used to observe an earth target and a space one. The thermal-vacuum facility is used to modify temperature circularly to test the instrument in vacuum before launch. The temperature of MWHS is controlled at 273 K, 283 K, 293 K and 303 K except in the earth and space directions where the shroud is maintained at any desired temperature between 100 K and 350 K for an earth target, and 90 K for space target. The whole targets temperature is measured to an absolute accuracy of better than 0.1 K by regularly calibrating the PRT with a standard one traceable to a national standard.The T/V test data are quantitatively analyzed, and the T_NE of MWHS three channels on 183.31 GHz water vapor line at 183.31±1, 183.31±3 and 183.31±7 GHz are calculated. All three channel T_NEare less than 0.90 K. MWHS is designed to allow a 2 point calibration, for each scan line, from the internal target and space views. The instrument response is assumed to be linear between these two calibration points. After correcting all the biases, the calibration accuracy is well below 1.05 K, and the changes of calibration results with instrument temperature are relatively small. Therefore, it's expected that temperature variations in orbit will not affect measuring brightness temperatures much.The radiometric characteristics analysis of sounding channels for FY-3A/MWHS will be the base for quantitative application in orbit. And the T/V test results will be directly used in orbit calibrating. Once MWHS is in orbit, the T_NE will not be measured directly but the variance of the internal target will provide a measurement of the noise for each channel, and the in orbit radiometric sensitivities can be monitored. The MWHS calibration test can be validated using NWP results and intensive field campaigns. The lab test results will be valuable for interpretation of the in orbit data.

Evaluating the Solar Radiation Resources of China in Recent 20 Years by Meteorological Model

Wu Qizhong, Wang Zifa, Cui Yingiie

2010, 21(3): 343-351.

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Abstract:
High resolution spatial and temporal distribution of the solar radiation resources in China in the past decades is simulated using the fifth generation Penn State University/National Center for Atmospheric Research (PSU/NCAR) Mesoscale Model (MM5), and its parameterization schemes are determined according to the numerical sensitive experiments results. The daily radiation observation data from 1975 to 1997 with good quality control is used to validate and evaluate the model performance, and to improve the simulation of the solar radiation with the optimal interpolation method, after which the mean absolute normalized gross error (MANGE) has decreased from 24.4% to 8.5% and the normalized mean bias (MB) has decreased from 20.6% to 3.5%.The simulated spatial distribution of the solar radiation shows that the solar radiation of the whole country is 5648.6 MJ/m² per year on averge. The solar radiation is higher in western regions than in east, divided by the line from the middle western of the Inner Mongolia, to Ningxia, northwestern Gansu, western Sichuan and northwestern Yunnan provinces. To the west of this line, the radiation is more than 6000 MJ/m² and to the east, the solar radiation in North China is highest. The inter annual variability tendency of the radiation from 1975 to 1997 is increase—decrease—increase, with the highest annual radiation in 1978 and the lowest in 1989. The solar radiation in each province is calculated from the simulated radiation after the optimal interpolation with the Geographical Information System (GIS) tools (based on ArcGIS 8.3 desktop) and classified according to their solar radiation resources. Tibet, Qinghai, Xinjiang provinces have the most solar radiation resources in China, and the averaged solar radiation in Tibet is more than 6900 MJ/m² per year.

Cluster Extreme Events Based on Point Process Theory

Yang Ping, Feng Guolin, Liu Weidong, Hou Wei, Sun Shupeng

2010, 21(3): 352-359.

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Abstract:
Extreme weather and climate events have attracted more attention in the last few years due to the often large loss of human life and exponentially increasing costs coping with them. The assessment of extreme climate events has found that extreme events are showing the characteristic of clustering. Many natural phenomena manifest themselves as spatial point processes which produce numerous events in space, such as extreme events. For clarification, the occurrence of a phenomenon located at a single point is defined as an event in contrast to a simple geometric point. Some events assemble in a restricted region while other events are dispersed. Regarding extreme events as spatial point processes and the cluster events as the clustered points, a method is developed to delineate the cluster events from a region which is called k-th order nearest distance spatial point method. By combining spatial point theory with cluster events, the algorithm is validated with a new method which consists of four major steps. The first step is to define different kinds of extreme events including 12 kinds of extreme temperature events and 6 kinds of extreme precipitation events. The second step is to calculate the statistically weights of every station based on the frequency and strength. The third step is to give the spatial distributions of all the stations that extreme events happens and the last step is to pick out the clustered events combining the spatial theory. Based on this method, extreme events research in eastern and middle areas of China have been accomplished, including extreme temperature events and extreme precipitation events. Results of k-th order nearest distance, the material definitions of weights and the distribution charts are given, which can detect cluster extreme events effectively. Based on the given charts and two efficient indexes (one is called average of ratio which is represented by R, the other is called efficiency represented by η), all kinds of extreme climate events are tested to validate the method, finding out that the algorithm is fit for cluster extreme climate events, especially for heavier extreme events. The method performs better on temperature extreme events than precipitation extreme events. So it's concluded that cluster extreme events can be delineated not only by qualitative research, but also by quantitative research, which offers a new idea in extreme events researches.

Variation Characteristics of Autumn Precipitation in Shaanxi Province

Sun Xian, Wei Na, Xiao Keli

2010, 21(3): 360-365.

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Abstract:
Based on monthly rainfall data of 74 stations and NCEP/NCAR reanalysis dataset from 1960 to 2009, the variation characteristics of autumn rainfall over Shaanxi Province and the main factors influence on autumn rainfall are analyzed by the methods of Mann Kendall and REOF with a horizontal resolution of 2.5°×2.5°. It shows that the rainfall of September has decisive effect on the total autumn rainfall over Shaanxi Province. The September rainfall occupies 57.3% of the total autumn rainfall from 1960 to 2009, and from 2000 to 2009 it occupies 61.5%. The September rainfall can be divided into 2 types by spatial distribution: One is fewer in the north and more in the south, the other is more in the north and fewer in the south. The first type shows no obvious decadal variations, but the second type varies obviously in these decades as it occurs a lot from 1960 to 1985, less frequently from 1986 to 2000, and then more frequently again from 2001 to 2009. Statistics on the variation of atmospheric circulation and the second type rainfall distribution indicates that when subtropical high over western Pacific (SHWP) is more northwest (northeast and southwest) to its regular position, there are 12 years (25 years) that the September rainfall over the northern Shaanxi Province is more (less) in the past 50 years. When the zonal circulation index (ZCI) is weaker (stronger), there are 14 years (16 years) that the September rainfall over the northern Shaanxi Province is more (less) in the past 50 years. The correlations between the rainfall, the SHWP and ZCI are more obvious since the 1980s. Meanwhile the sea level pressure is low in the west and high in the east in the years when September rainfall over the northern Shaanxi Province is more, because there are more low weather systems from the northwest Xinjiang. So the east part of northwest China is controlled by the subtropical high over west Pacific and the southeast winds provides sufficient water vapor.

Application of EMD to Seasonal Precipitation Forecast in Guangxi

Bi Shuoben, Xu Yin, Qin Zhinian, Chen Xuan, Wang Biqiang

2010, 21(3): 366-371.

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Abstract:
The climate system is a high order nonlinear system with dissipation. In recent years, the BP neural network algorithm and the Support Vector Machine (SVM) algorithm are applied widely in the short range climate forecast for its superiority in handling nonlinear time series problem. Besides, the climatic time series are non stationary, so the signal needs processing to improve its predication result. The Empirical Mode Decomposition (EMD) algorithm introduced by Huang is used to stabilize the climatic time series. Combined with the SVM algorithm, it's used for short range climate forecast and applied to the seasonal precipitation forecast in Guangxi. The EMD algorithm decomposes non stationary signal into several Intrinsic Mode Functions (IMF) components and a remainder with stationary. EMD algorithm doesn't provide a good solution for the endpoints extremes problem, and the extreme extending method is adopted as the endpoints continuation method for short range climate forecast. Anomaly percentage of accumulated precipitation data are analyzed, which are observed at 88 meteorological observatories in Guangxi from June to August during 1957—2005. Using the EMD algorithm, the time series being standardized are decomposed into four IMF components and a remainder; then a SVM model is built for each component, and the forecasts are composed to the final forecast result. For comparison, BP neural network algorithm and SVM algorithm are adopted to forecast respectively without the EMD algorithm. Analysis on the predicted values and errors show that, without being processed with EMD, errors of the SVM algorithm are smaller than that of the BP neural network algorithm. So it proves that the generalization capability of BP is weaker than SVM when processing the small sample size problem, whereas SVM algorithm follows the structural risk minimization, and can coincidence the change trend better in condition of finite samples. It shows that the results of the EMD method combined with the SVM algorithm are more accurate. It illustrates that the EMD algorithm can reflect the regularity in different time scales of time series via decomposing into a collection of components with stationarity, which is more suitable for predicting with machine learning methods. The superiority of this scheme makes it widely applicable in precipitation forecast.

Design and Implementation of Beijing Olympic Meteorology Forecasting Product Service System

Li Dequan, Zhou Yong, Yu Haiyan

2010, 21(3): 372-378.

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Abstract:
To meet weather service requirements of 2008 Beijing Olympic Games and Paralympic Games, a software system named OMIS is developed and deployed in Beijing Olympic Meteorological Service Centre (BOMSC), which takes responsibility for producing and sending various types of meteorological forecasting service products on time to the relevant departments of the Olympic Games. The technological framework for this meteorology forecasting service product is described in detail, including the application of TeX, relational database, and template. The subsystem implemented in this model is developed as the core of OMIS and plays a key role in many events such as the stage of preparation, rehearsal, and held for Olympic Games. To generate multiple sets of weather service products, OMIS must meet at least three strict requirements: High stability, automation and flexibility. Additionally, there are strict requirements in respect of intellectual property rights for the Olympic weather support system, particularly focusing on the copyright issues. Due to these demands and limitations, it's analyzed on how to select documents generating technical strategy. Comparisons of technological framework for generating documents between TeX with Microsoft Word are elaborated in various aspects such as opening, stability, flexibility and extensibility. Therefore, an effective framework integrated with TeX language, relational database, templates technology strategy and forecasting service product are designed. The model covers the whole process from data receiving, decoding, storing, querying, to product generating and distributing. Among technologies in this model, utilizing TeX under Linux is to meet the needs of stability and security; the MySQL database ensures the openness, storage, and automated processes; templates strategy under TeX template language is to supply documents generated by a dynamic configuration process in response to frequent changes in the document style. It is the first time to introduce TeX technology into the application of the entire weather service for Olympic Games. Besides these techniques, the core of this platform is coded in C++ with standard template library (STL), which can simplify the future migration to other operating systems, such as Linux and Windows. As a result, the OMIS shows high stability and good usability during the meteorology service in Beijing Olympic Games and Paralympic Games.

The Impact of PS Method on Operational Climate Forecast

Tian Wuwen, Wu Suliang, Wang Na

2010, 21(3): 379-384.

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Abstract:
In the operation of climate forecast, scoring methods of PS, SS, ACC are widely used. Among them, SS and ACC, known as technique scores, are defined as the similarity between forecast and realistic results when comparing with non technique forecasts or the expectations of non technique forecasts equal to zero. The technique scores are basically originated from the evaluations in the forecast results from different populations and thus provide a basis for impartial comparisons. However, PS is considered as the concordance of forecast grades and are not compared with technique scores, so PS is not technique scores. In China, PS is of greater importance in climate forecasts. The PS of short term climate forecasts by each province are performed every year in the China Meteorological Administration and the ranks of monthly precipitation, mean temperature and total forecasting ability are released. The impact of present PS method used on the climate forecast is analyzed. The results show that the theoretical PS is a score of consistence rate based on accurate forecasts on the grades; the foundational requirement of the PS inter comparison is that they share the same probability distribution in ranking. Through comparisons between the present and theoretical PS method, it's also found that the present scoring method is taken as the theoretical scoring method with two modifications which makes the present PS method actually become the concordance of weighted anomalies. The first revision has extended the range of forecast accuracy assessment, in which level 1, 2, 5 and 6 have extended to three grades, level 3 and 4 have extended to four grades, and weight coefficients are added for abnormal grades. However, only two grades are forecasted in climate operational forecast after PS method performs. Based on the examples of the annual precipitation extremes occur in Shaanxi Province, the random forecasts on level 2 and 6 are evaluated and compared. The results show that 2 level forecasts can score higher, while reducing the forecast capability. Another revision is to divide the grades based on the uniform anomalies of elements, which leads to the forecast grade distributions varying between the stations and months. The distribution of forecast grades of is the dominant factor of non technique forecasting scores. Moreover, the forecasting skill is actually determined by the differences between PS and non technique prediction scores. In the cases of different non technique forecasting scores, the foundation for comparisons becomes less stable to use PS in different months and stations. Some suggestions are introduced to the prevailing PS method to solve these problems.

Vol.21, NO.3, 2010