应用气象学报

A PLAM Index Forecast Method for Air Quality of Beijing in Summer

Yang Yuanqin, Wang Jizhi, Hou Qing, Wang Yaqiang

2009, 20(6): 649-655.

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Abstract:
For a regional air pollution event in a synoptic scale, such as typical stabilized summer weather over North China, a key scientific issue is to identify the possible impacts of pollutants on mega-cities like Beijing and on the zones between the cities and their adjacent areas, from emissions by human activities, e.g., industrial and agricultural emissions as well as traffic exhaustions in the so-called capital economic belt, covering Beijing, Tianjin and a number of surrounding provinces, including Hebei and Shanxi provinces. For this special target as well as the assignment of the air quality predicting service in 2008 Beijing Olympic Games, a parameterized method is developed to predict the air quality in Beijing and its surrounding areas in summer.On the basis of the relationship between summer daily PM₁₀ in Beijing Observatory and key meteorological data for July to September during 2000-2007, analyzing the sensitive meteorological parameters which are relevant closely with high concentration of PM₁₀, an index method of PLAM (parameter linking air-quality and meteorology) is given, which is derived to forecast the stabilized weather conditions of North China for 2008 Beijing Olympic Games. For different PLAM value there is a weight given according to the wind direction of the site pointing what extent to Beijing, as well as its wind speed. So the PLAM index can also predict quantitatively the weather conditions in surrounding areas of Beijing, showing whether or not the meteorological conditions in these areas are favorable for pollutants transport to Beijing. It is found that the poor air quality days with elevated PM₁₀( > 150μg/m³) in summer are associated with higher PLAM values, featuring high temperature, high humidity, lower wind velocity and higher stability.A satisfying result of PLAM index forecast timely for the 2008 Beijing Olympic Games is achieved.PLAM parameter forecast method provided valuable 48 hours services with an interval of 12 hours for the day of opening ceremony and 24-72 hours prediction of air-quality for subsequent events during the whole Beijing Olympic Games. It is also found that the good air qualities are associated with the traffic control measures taken by the municipal government in reducing the vehicle exhaustions during Beijing Olympic Games.The forecast verifying results of PLAM index are highly correlated with daily real time PM₁₀ data collected by Beijing Weather Observatory, indicating that the PLAM index has a remarkable effect.

Impacts of Air Parcel Transport on the Concentrations of Trace Gases at Regional Background Stations

Xu Xiaobin, Liu Xiwen, Lin Weili

2009, 20(6): 656-664.

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Abstract:
To investigate the impacts of air parcel transport on the concentrations of trace gases (SO₂, NO_x, CO and O₃) at regional background stations, backward air parcel trajectories from July 2005 to June 2007 are calculated for Lin' an, Shangdianzi, and Longfengshan stations using the H YSPLIT4 model and the NC EP reanalysis meteorological data. Four trajectories are calculated for each day, with starting time at 02: 00, 08: 00, 14: 00 and 20: 00 Beijing Time and a travelling time of 72 hours. Individual trajectories are clustered to obtain the mean trajectories and monthly statistics are made for different clusters of trajectories. Statistics of trace gas concentrations observed at the stations during the same period are conducted, corresponding to different clusters of trajectories. However, these statistics do not facilitate interpreting relative influences of different trajectories on the pollutants concentrations at the stations, because both trajectories and pollutants concentrations vary seasonally.Therefore, seasonal trends in the time series of the O₃, SO₂, NO_x and CO concentrations are filtered by 30-day smooth-averaging to obtain the short-term variations of these gases, which is nearly independent of seasonal variations. The impacts of air parcel transport over different areas on the pollutants concentrations are studied by combining the trajectories and the corresponding short-term variation data of the gases. The results show that air parcels originating from higher altitudes and transported at higher speeds can effectively remove gaseous pollutants from surface air over the stations. However, the surface O₃ level may occasionally be enhanced by downward transport of O₃-rich air from near the top of boundary layer. About 50 %-60 % of the backward trajectories for the stations are related to the transport of polluted air parcels, leading to increases in the concentrations of the gaseous pollutants. The polluted air parcels are mainly from the S-SW and N-SE sectors for Lin' an, from the SE-SW sector for Shangdianzi, and from the S-WSW sector for Longfengshan. On the average, air parcels from the S-SW sector can enhance the concentrations of SO₂, CO and O₃ at Lin' an by 1.2 ×10^-9, 57.5 ×10^-9 and 4.6 ×10^-9, respectively, while those from the E-SE sector can abate concentrations of SO₂, CO and O₃ by 0.7 ×10^-9, 5.0 ×10^-9 and 9.5 ×10^-9, respectively. For Shangdianzi, air parcels from the SE-SW sector can increase the concentrations of SO₂, NO_x, CO and O₃ by 3.6 ×10^-9, 2.8×10^-9, 406×10^-9 and 1.8×10^-9, respectively, while those from the high altitudes in the NW sector can reduce concentrations of SO₂, NO_xand CO by 8.4×10^-9, 10×10^-9 and 599×10^-9, respectively. For Longfengshan, air parcels from the SW sector can increase the concentrations of SO₂, NO_x, CO and O₃ by 1.3 ×10^-9, 0.4×10^-9, 53×10^-9 and 1. 4×10^-9 respectively, while those from the other sectors decrease the concentrations of SO₂, NO_x and O₃ by about 0.3 ×10^-9-5.9×10^-9, and that of CO by about 24×10^-9-100 ×10^-9.

Ozone Budget in the Lower and Middle Troposphere over North China

Peng Li, Lin Yunping, Zhou Guangqiang, Zhao Chunsheng, Geng Fuhai

2009, 20(6): 665-672.

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Abstract:
Photochemical reaction, transportation and deposition are the main processes that effect ozone concentrations in troposphere.Quantitative estimation of troposphere ozone budget and the effect of intercontinental transport are very important in order to reveal ozone formation mechanism and the impacts of transport on troposphere ozone over North China. MOZART-2, a global chemical transport model (model of ozone and related tracers, Version 2) is used to assess physical and chemical processes that influence the budget of lower and middle troposphere ozone in North China.Ozone sonde data obtained by TAPTO (The Transport of Air Pollution and Troposphere Ozone over China) field campaign are compared with model results for reference. The comparison shows that MOZART-2 represents the vertical distributions of ozone in the lower and middle troposphere over North China very well, while for the upper troposphere, ozone concentrations are overestimated by the model. Budget analysis indicates that in the lower troposphere over North China, photochemical production (41.5 Tg) contributes about 58.3 % of the total ozone sources, and oxidation of NO by HO₂ is the largest contributor especially. The largest consumption process in lower troposphere is dry deposition, accounting for about 43.2 % of the total ozone sinks. Ozone chemical budget varies with seasons notably. It reaches its maximum in summer due to strong photochemical reactions, while the minimum chemical budget occurs in winter because of low temperature and weak reactions.The chemical production of ozone is more than the chemical loss in the lower troposphere in the whole year. But for middle troposphere, chemical loss exceeds chemical production all year except in summer. In summer, ozone precursors in the boundary layer can be transported to upper level of troposphere due to the active convection, and the photochemical reactions of more ozone precursors lead to more ozone production. Net horizontal transport plays an important role in ozone budget in the low and middle troposphere over North China. About 41.6 % ozone in lower troposphere comes from net horizontal transport. As the height increases, wind speed rises, and the influence of net horizontal transport enhances. Nearly 81.5 % ozone in the middle troposphere comes from net horizontal transport. Constrained by simulating ability for sub-grid processes, the model tends to overestimate ozone concentration in upper troposphere and near the tropopause over middle to high northern latitudes. Modeling for sub-grid processes such as stratosphere-troposphere exchange (STE) should be improved to comprehend mechanism of ozone in the whole troposphere in depth.

Cloud Properties Analysis and Its Application in FY-2 Cloud Detection

Liu Jian

2009, 20(6): 673-681.

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Abstract:
One pixel is cloud or not dependent on cloud detection scheme. Cloud detection approach includes histogram analysis, threshold detection, deviation analysis and so on. Cloud threshold is an important factor for cloud detection scheme. Now, cloud threshold usually can be gotten by one time and multi channel data in operational cloud detection method. But if surface is covered by snow or ice, or cloud covers one area for a long time, dynamic cloud threshold method always fails. So it should depend on clear temperature background. On the other hand, climate mean surface temperature can help to distinguish the dynamic cloud threshold' s validity. ISCCP data are proved by many researches that it is one of the best satellite cloud climate data in the world now. ISCCP data are used to analyze properties of cloud and clear temperature over China and neighboured areas. Cloud can be divided into three kinds, such as low, middle and high cloud. Different kind of cloud has different distribution property of cloud top temperature on a seasonly or daily basis along the latitude. Different kinds of cloud top temperature basically show strip distribution along longitude. The lower latitude, the higher cloud top temperature is. Along the same latitude, different area has different cloud properties during one day or one year.The analysis shows that the low cloud top temperature increases progressively in the fall and winter seasons from north to south, and has obvious linear increase tendency with reduced latitude. The middle level cloud top temperature distribution has distinct properties. Except July, August and September, the distribution of middle level cloud top temperature presents the meridional strip basically.The middle level cloud top temperature gradient is large in fall and winter, which is small in spring. The high cloud top temperature presents the meridional strip distribution in fall, winter and spring. The high cloud top temperature is lower than 240 Kand it' s latitudinal gradient is small to the north of 35°N. The high cloud top temperature meridional gradient increases and it' s diurnal variation is small to the south of 35°N. The temperature difference between clear surface and the warmest cloud top has stabile daily and seasonly change in the southeast part of China. At the same time, in the north part of China, the temperature difference between clear surface and the warmest cloud top has distinct daily and seasonly change. Above analyzed properties and mean clear temperature can be used as background information to distinguish the dynamic cloud detection threshold value' s validation and offer cloud threshold for areas that are covered by low cloud for a long time. An example is showed to testify mean clear temperature and temperature difference between clear and the warmest cloud can offer great information for cloud detection scheme.

The Effect of Water Content on the Simulation of Satellite Microwave Observation in Cloudy and Rainy Area

Dong Peiming, Wang Haijun, Han Wei, Wang Deying

2009, 20(6): 682-691.

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Abstract:
Satellite data contribute to the improvement of numerical weather forecast accuracy greatly.In most current data assimilation systems, only clear-sky satellite data are used, but the observations in cloudy and rainy area have crucial information for the development and forecast of the weather system. Using satellite data affected by cloud and precipitation will be one of the effective methods to improve the accuracy of numerical forecast continuously. To achieve this target, the radiant effect modules of water content are being developed in both RTTOV and CRTM, which are two popular rapid radiant transfer models developed by EUMETSAT Satellite Application Facility on Numerical Weather Prediction (NWP SAF) and USA Joint Center for Satellite Data Assimilation (JCSDA), respectively. Using rapid radiant transfer model CRTM, the radiant effect of water content on the simulation of satellite microwave observations in cloudy and rainy area is analyzed.The water content input of CRTM is taken from the output of the regional mesoscale model WRF. Under the circumstances that the disturbance of water content output corresponds well with the weather system and physical characteristics, the simulation of satellite observations are greatly improved by the consideration of the radiant effect of water content. Deliberating on the physical mechanism of satellite observations, the effects of different kinds of water content on the simulation of different satellite observation channels are investigated. Quantitative statistics of the bias is performed and the influencing weight of each kind of water content on each channel of NOAA 16 AMSUA/B satellite is also calculated. These results are expected to facilitate understanding the error characteristics of simulated satellite microwave observations in cloudy and rainy area and accelerate using satellite data affected by cloud and precipitation in numerical weather forecast.

Preliminary Study on the Scoring Methods of Cloud-free Rainfall/Snowfall and Air Temperature Forecasts

Zhang Qiang, Xiong Anyuan, Zhang Jinyan, Feng Mingnong, Wang Bomin

2009, 20(6): 692-698.

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Abstract:
Scientific and reasonable forecast scoring method is fundamental for evaluating the efficiency of the weather forecast objectively.By comparing forecast results with the corresponding observed data of 373 stations in China, the problems in existing forecast scoring method of the cloud-free rainfall/snow fall and temperature are investigated. Experimental amendments are made to the method, too. As a widely used method, forecast accuracy of the cloud-free rainfall/snowfall proves to be simple and practical in judging the effects of forecast to some extent.But without considering influence of rainfall probability, the forecast accuracy cannot distinguish blind prediction or persistent prediction accuracy effectively, and may bring abnormal high value even some mistakes.Skill-score of the cloud-free rainfall/snowfall forecast accuracy is positively correlated to the no-rainfall frequency at a single station during the months when regional rainfall probability difference is significant. In terms of temperature forecast, daily variation of the air temperature is a significant factor that affects the skill-score.When adopting 1℃ or 2℃ as the absolute standard value threshold, the skill-score of temperature forecasting is negatively correlated to the daily temperature variation.P_F method and Index Threshold method are proposed in order to reduce the influences of rainfall probability and daily temperature variation.The results indicate the forecast score of precipitation by P_F method is not closely correlated with rainfall frequency at single station during the months when regional rainfall probability difference is dominant in China. When adopting 2/3 or 1/2 as the index threshold, the linear regression coefficient between the daily temperature variation and the temperature forecast score can be significantly reduced.The correlation coefficient also decreases obviously with the value down to below 0.15, which is clearly lower than adopting the absolute standard value threshold. In other words, skill-score of temperature forecast using Index Threshold method is less sensitive to the daily temperature variation than using forecast accuracy method. Above all, the new methods proposed can effectively reduce the influences of rainfall probability and daily temperature variation on the skill-score of the cloud-free rainfall/snowfall and air temperature. It also improves the comparability of the weather forecast scores in the regions with different climate background. Therefore, it can be applied to the quality test and assessment on the weather forecast.

A Neural Network Approach to Predict Tropical Cyclone Intensity

Huang Xiaogang, Fei Jianfang, Chen Peiyan

2009, 20(6): 699-705.

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Abstract:
An artificial neural network (ANN) technique is used to predict tropical cyclone intensity change in the Western North Pacific basin and the efficacy is examined. The intensity change forecasts produced by the ANN model are compared to the results of a model developed using linear regression by National Hurricane Center (NHC) at 12-hour,24-hour,36-hour, 48-hour, 72-hour forecast periods. The date, location, track, intensity and the intensity change similarities is used to identify a historical analog tropical cyclone to the current tropical cyclone.Once the analog tropical cyclone are identified, the climatology and persistence variables, such as the previous 6-hour intensity change, location of the tropical cyclone center, current intensity at the time of the observation, are computed from the China Meteorological Administration (CMA) best-track dataset. The synoptic and dynamics variables, such as vertical sheer, sea surface temperature are computed from the National Centers for Environmental Prediction (NCEP) reanalysis data and the weekly sea surface temperature dataset. These variables and intensity of the tropical cyclone are combined as a training set to identify the variables that are best correlated with tropical cyclone intensity. The variables are used to train a neural network which uses BP algorithm as a learning rule to get the best forecast model.26 tropical cyclone processes in Western North Pacific for 2004-2005 are used to compare the ANN model with linear regression. The number of forecast cases at 12-hour, 24-hour, 36-hour, 48-hour, 72-hour forecast periods is 634, 582, 530, 478, 426 respectively.The preliminary results suggest that, errors of the ANN model are significantly smaller comparing to linear regression for the 12-hour, 24-hour, 36-hour, 48-hour, 72-hour forecast periods. This improvement is the result of the analog tropical cyclones selection and variables filter for a given tropical cyclone. Several case studies show that the ANN model is able to reproduce the processes of super typhoon and tropical cyclones with fast-enhancing intensity and regrowing cases.The results also show that the most important variables for tropical cyclone predicting are the center pressure, the intensity change, the center location (longitude and latitude) and the vertical wind sheer. But the contributions of the other variables also cannot be ignored. So the tropical cyclone intensity change is complex and nonlinear.

Multi-model Superensemble Forecasts of the Surface Temperature Using the TIGGE Data

Lin Chunze, Zhi Xiefei, Han Yan, Wang Jingyu

2009, 20(6): 706-712.

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Abstract:
Based on the TIGGE data, the ensemble mean outcomes of the 24-168 hours ensemble forecasts for the surface temperature in the Northern Hemisphere during the summer of 2007 are provided by ECMWF, JMA, NCEP and UKMO. Root-mean-square errors (RMSE) of the results are examined.The multi-model ensemble forecasts of the surface temperature for the forecast period from August 1 to 31 of 2007 are conducted by utilizing the superensemble, the multi-model ensemblemean, and the bias-removed ensemble mean.The forecast skills of these multi-model ensemble methods are investigated.The results show that the JMA model performs best for 24-hour and 48-hour forecasts, but for 72-hour to 168-hour forecasts the ECM WF model is the one with the highest skill. Forecast skills of these four models of different operational forecast centers are quite different over various regions.The JMA model performs best in the United States and Europe, while in China the UKMO model is the best one. Actually, none of the four models is perfect for all regions in the Northern Hemisphere.Therefore, it is necessary to utilize the multi-model superensemble methodology to improve the forecast skill.The multi-model superensemble and the bias-removed ensemble mean reduce the RMSE of the surface temperature forecast considerably.Both methods show an improvement on the forecast skill over the best single model forecast and the multimodel ensemblemean.In early (late) phase of the forecast period, the multi-model superensemble produces a smaller (larger) RMSE than the bias-removed ensemble mean for the 24-hour and 48-hour forecasts.For the 72-hour to 168-hour forecasts, both the multi-model superensemble and the the bias-removed ensemble mean lead to smaller RMSE than the multi-model ensemble mean and single models during the first two weeks of the forecast period.Never theless, two weeks later the RMSE of the superensemble forecast increases rapidly, and exceeds that of the bias-removed ensemble mean.The RMSE of the multi-model superensemble forecast may even exceed that of the single model forecast at the end of the forecast period.Weight coefficients of multi-model superensemble methodology remain unchanged during the fixed training period. It's expected that an improved multi-model superensemble methodology with flexible training period when statistical weights of the multi-models vary with time may further improve the forecast skill.

An Automatic Ground-based Cloud Detection Method Based on Adaptive Threshold

Yang Jun, Lǜ Weitao, Ma Ying, Yao Wen, Li Qingyong

2009, 20(6): 713-721.

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Abstract:
Clouds affect the energy balance of the earth by means of absorbing and scattering radiation, and they have notable influences on global climate. It is very important to monitor clouds and there are several meteorological satellites providing sky-based large-scale scope clouds observations round-the-clock. However, the ground-based cloud observations mainly depend on visual judgments of the meteorological observers, which have become a bottleneck of automatic meteorological observation.Computing cloudage automatically is very important, and cloud detection is the basis of cloudage computation.Up to now, the mature ground-based cloud detection methods are still based on thresholds.For instance, the threshold 1.3 is recommended for ratio of ground-based blue band and red band observations. However, considering the complexity of clouds, a fixed threshold obviously cannot obtain satisfactory detection effect for different types of clouds. In clear sky, clouds are white against the blue background, so an automatic ground-based clouds detection method can be established in terms of the maximum interclass variance adaptive threshold selection. Ratio, difference, normalized difference of blue band and red band are calculated respectively, then bimodal distributing can be found in the image gray histogram, and then an adaptive threshold can be obtained using the maximum interclass variance method. Compare the band operation result with the adaptive threshold pixel by pixel, a pixel whose gray value is less than the adaptive threshold can be regarded as cloud pixel, else non-cloud pixel.Using this rule, the cloud regions can be separated from the sky background.Three different types of ground-based clouds are analyzed, and the proposed adaptive threshold method performs more appropriately comparing with fixed threshold cloud detection method.The blue band and red band ratio method can detect the maximum cloudage, and the difference operation method can detect the minimum cloudage. But quantitative assessment results show that, the ratio method mistakes many non-cloud pixels into clouds, while the norm alized difference processing is satisfactory both in correctness and accuracy. It should be pointed out that the proposed method applies only to clear sky, as for other weather conditions, more researches are needed.

Characteristic Analysis of Ice Accumulation on Transmission Lines and Simulation Based on ANN Model over China

Yin Shuiqing, Zhao Shanshan, Wang Zunya, Zhang Qiang, Tang Weian

2009, 20(6): 722-728.

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Abstract:
An extreme frozen ice and snow disaster brought severe losses in early 2008 to China. A large number of overhead transmission lines are destroyed seriously during this event. It is critical to investig ate the characteristics of ice accumulation and develop appropriate models to estimate transmission line icing in China.Using the observation data of ice accumulation on transmission lines and meteorological data collected from over 600 meteorological stations of China, the characteristics of ice accumulation are analyzed and an ANN model for predicting ice accumulation depth grades is developed. The results mainly include four aspects. First, the area with ice accumulation extremes over 10 mm is mainly located in the southeastern part of Northeastern China, the northeastern part of Inner Mongolia, the middle part of North China, the southern part of Gansu Province and a east-west zone to the south of the Yangtze River. Second, there are 50 stations (1/6 of totalice accumulation observation stations) expe riencing extreme ice accumulation in 2008. Third, trends for yearly maximum of ice accumulation depth and ice accumulation days increase for Huangshan station in Anhui Province and Lushan station in Jiangx iProvince, while decrease for Emeishan station in Sichuan Province and Xifengzhen station in Gansu Province. Finally, an ANN model based on three layer BP network is developed to predict ice accumulation thickness grades. It is used to predict ice accumulation thickness grades in recent 10 years, and the accuracy rate is 81.3%comparing with obser-vation. For 7.5% of the samples, prediction result is one grade higher than observation and for 11.3% it is one to four grades lower. The ANN model underestimates extreme high values seriously, which suggests it needs improving.Besides, the data from weather observation stations might be different from the field surroundings. Nevertheless, in the areas with high risks of freezing hazard, the model can provide information for electricity suppliers to optimize the design of reliable equipments and to take preventive actions avoiding serious damage by ice-snow frozen weather.

Estimation Models of Potential Evapotranspiration in Sichuan Province

Zhang Shunqian, Ma Zhenfeng, Zhang Yufang

2009, 20(6): 729-736.

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Abstract:
Penman-Monteith formula is a standard method recommended by FAO-56 to calculate potential evapotranspiration. But it is difficult to apply in practical operation as not all related meteorological elements can be obtained in reality, so estimation models are used instead. In order to apply compound drought index in operation, Thornthwaite method and Hargreaves method for yearly and monthly potential evapotranspiration of Sichuan Basin and West Sichuan Plateau are studied.The observation data of 156 meteorological stations in Sichuan Province from 1971 to 2000 are investigated including minimum temperature, maximum temperature, wind speed, relative humidity, and sunshine hours.Using result of Penman-M onteith form ula as the ET0 standard value, the precisions of the two E_T0 estimation methods are analyzed.Comparing wind speed, relative humidity, Hargreaves method and Penman-Monteith method statistically, an operational Hargreaves method is established. These Hargreaves method and model have been applied to monitor serious summer drought in 2006 in Sichuan Province.The results show: As far as annual variation is concerned, annual value fluctuation estimated by Thornthwaite method is small so that it cannot reflect the E_T0 annual variation. However, Hargreaves method and PM method show basically consistent changing tendency and can reflect the E_T0 annual variation characteristics better. As for the annual mean, result of Thornthwaite method is significantly smaller.The error in the Basin is -10 %--20 % and -40 %--60 % on Plateau.Result of Hargreaves method is relatively smaller in Plateau and larger in Basin.But its error is 10 %-15 %, better than Thornthw aite's. As for the seasonal variation, error of Thornthwaite method is larger in winter but smaller in summer.Due to the influence of relative humidity and wind speed, seasonality of Hargreaves method's error is not obvious. The error of Hargreaves is smaller than that of Thornthwaite especially in Plateau region except for in summer.Results of Penman-Monteith formula and Hargreaves method show good linear relation.With the recovery factors air velocity and relative humidity introduced, the error of Hargreaves correction value can be controlled within 10%. It can meet the demand of practical operation for precision.The compound drought index calculated by this E_T0 estimation model has strong capability for drought monitoring in Sichuan Province.

Characteristics and Statistical Model of Road Surface Temperature on Huning Expressway

Tian Hua, Wu Hao, Zhao Linna, Chen Hui, Li Kunyu, Yang Xiao dan

2009, 20(6): 737-744.

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Abstract:
The relationship between road surface temperature and weather conditions is a key problem for road surface temperature prediction which can prevent and reduce traffic accidents effectively. Many countries such as Canada, Denmark, and Germany have established road weather information systems and road surface temperature numerical forecast models while China is somew hat behind. From July 2006 to June 2007, observations are carried out at Meicun and Xianrenshan stations along the Huning Expressway when road surface temperature, airtem perature, humidity, wind direction, wind speed, and rainfall are recorded minutely. These data are analyzed to study the daily characteristics of expressway surface temperature for different seasons and weather conditions. On the basis of that, statistical models of minimal and maximal road surface temperature are established by stepwise regression for Meicun and Xianrenshan. It shows that daily variation characteristics of road surface temperature and air temperature are obviously different. The road surface temperature increases rapidly after sunrise and goes beyond air temperature when it is sunny or cloudy. While the discrepancy between road surface temperature and air temperature is smaller in raining days or at noon.In July, the discrepancy is extremely obvious. The simulated minimal road surface temperature is close to observations and errors are within ±2 ℃, so the model is applicable in practice. The errors for maximal road surface models are relatively large because of data limitation and not considering solar radiation impacts.The models established using statistical methods are local and need modifying when used in other regions. If more data are obtained, further research will be carried out to disting uish different seasons or weather conditions and the impact of solar radiation should be considered.

The Spatial Distribution and Temporal Trend of Surface Aridity Index in Recent 40 Years in Hebei Province

Yang Binyun, Wu Rongjun, Yang Baodong, Niu Yingjie

2009, 20(6): 745-752.

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Abstract:
Aridity index, used as an important parameter to measure regional dry and wet conditions, has become an important climatic indicator in global change research, especially widely applied in the climate change, drought and desertification research. Dry and wet changes of regional climate affect the local socio-economic and agricultural production severely, therefore attracted extensive attention and study. Hebei Province locates in the semi-arid areas, researching the temporal and spatial variation of surface aridity index there, determining the detailed climatic regionalization, will be helpful to mitigating the adverse effects of droughts and floods. The surface aridity index in Hebei Province is calculated from 1970 to 2007 on the basis of meteorological data in 48 stations, so as to investigate the impacts of climate change on surface dry-wet status. The aridity index is defined as a ratio of potential evapotranspiration to precipitation, and potential evapotranspiration is computed using FAO Penman-Monteith model. The spatial characters of aridity index are addressed by adopting Kriging interpolation, providing some decision-making evidences for the local government of Hebei Province.The results indicate that the variation of annual aridity index, potential evapotranspiration and precipitation could be divided into two periods: 1970-1985 and 1986-2007. Values of aridity index decrease generally in the period from 1970 to 1985. The decrease in potential evapotranspiration is greater than the reduction in precipitation, induced by a decrease of wind speed and hours of sunshine, particularly. The climatic tendency of aridity index is negative (-0.1/10 a), and its average value is 2.34. By contrast, the values of aridity index increase generally in the period from 1986 to 2007, which is attributed to the increase of potential evapotranspiration, caused by the significant increase of air temperature. The climatic tendency of aridity index is 0.14/10a, and its ave rage value is 2.32. Obviously, the ascent rate of yearly aridity index after 1986 is greater than decline rate of that before 1986.From 1970 to 1985, the higher values region of aridity index is located in Sangyang Basin and plateau of western dam in Zhangjiakou, mainly involving Xuanhua, Yangyuan, Huaian, Wanquan, Huailai and Zhuolu County. The range of aridity index is 2.81-3.39, and the higher values center is located in Yangyuan and Huailai County. Inversely, the lower values center of that is located in southwestern Chengde, northern Tangshan and middle-northern Qinhuangdao, mainly involving Luan county, Qian'an, Qianxi, Zunhua, Qinglong, Funing, Lulong and Xinglong county. The range of aridity index is 1.27-1.60. The overall trend of aridity index gradually increase from eastern to western, indicating that the surface wetting conditions of the eastern region is better than those of the western. Compared with the situations during 1970-1985, the increase and decrease trend of surface aridity index in Hebei Province is similar in 1986-2007. The decline region of aridity index is located in the majority of north-eastern part of Hebei Province. The decline trends of aridity index of sourthern Fuping and Laiyuan County in Taihang Mountain are significant, where the reduction value is 0.3. Inversely, the aridity index in the Kangbao, Shangyi, Zhangbei and Guyuan County in plateau of dam increase by 0.1, and it increase 0.1-0.2 in the region from southern Qinhuangdao to southern Handan, in eastern and southern Hebei respectively.

The Evolution Features of Precipitable Water Vapor Derived from Ground-based GPS During Autumn Rain Weather Process in West China

Chen Jiaona, Li Guoping, Huang Wenshi, Liu Biquan, Yang dong

2009, 20(6): 753-760.

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Abstract:
Based on the principle of deriving precipitable water vapor with ground-based GPS, the estimates of total zenith delay are calculated using ZTD data from the ground-based GPS network in Chengdu Plain during the period of September to November 2007. Precipitable water vapor (PWV) derived from GPS are obtained at 30-minute interval combining meteorological data from automatic weather stations. The autumn rain is classified as showery rain and continuous precipitation in Chengdu Plain, and the relationship between GPS-PWV and autumn rain is analyzed. It shows that precipitation always happens in high value phase of water vapor, so the high precipitable water vapor is necessary for rain in most cases. Precipitation happen when the PWV anomaly is positive, and the PWV anomaly is always higher than 1 when a rainstorm occurs. The variation range of showery in autumn is large. The GPS-PWV always increases 12 hours before the precipitation. When GPS-PWV is higher than the base value of the month or acutely increases in the adjacent time, a shower is likely to occur. High GPS-PWV level and weak updrafts just lead to small rain. But the increasing of precipitable water with strong ascending motion and the decreasing of temperature always causes shower. During continuous precipitation in autumn in Chengdu Plain, the accu-mulation of water vapor is very important, strong precipitation often happens when water vapor rises once again. If the precipitable water vapor maintains the high level, it may rain within 12 hours. If the water vapor falls to the base value of the month then rise to above that within 12 hours, it indicates the beginning of another phase of precipitation. GPS-PWV variation range, extremum level and duration are different in different rain process. These results may be referential for applying precipiatable production derived from ground-based GPS network in precipitation forecast.

Comparisons of Two Calibration Methods for the Net Pyrradiometer

Yang Yun, Lv Wenhua, Fu Xigui, Wang Dong, Ding Lei

2009, 20(6): 761-766.

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Abstract:
China doesn't conduct operational observations of net radiation until the 1990s, when the remote detection for radiation is applied. Temporary reference standard for the net pyrradiometers is based on 3 CN-11 type net pyrradiometers which are imported from Japan in 1980. The structures of CN-11 net pyrradiometers and homemade pyrradiometers are of the same. The inductive parts consist of the thermopile and two black inductive surfaces. The black inductive surface can receive the whole wave-band radiation with the wavelength 0.3-100 μm. The upper inductive surface receives the whole radiation from the sky and the lower surface receives the radiation from the earth surface. The net pyrradiometer is more complex than other radiometers because of two inductive surfaces and the broad range of received radiation. To stop wind and protect the inductive surface, a polyvinyl film cover is installed. But the measuring uncertainty is increased by it because the film cover is difficult to clean and keep sealed, while it gets distorted and aging easily. According to the suggestions of WMO, two pyrgeometers (CG4) and two pyranometers (PSP) are used together as the new standard group. Under different environment conditions, six working net pyrradiometers are calibrated using four components method as well as the CN-11 type pyrradiometer standard group and the results are analyzed. Experiments are conducted to provide scientific basis for the updating of standard group, through which the relationship between the new standard group and the old standard group in the calibration environment are found out. The random error of the results using the new standard group is calculated. Analysis suggests the measuring accuracy can be improved 9.2% than before in daytime and 5.2% at night after using the new standard group. The data of the net pyrradiometers calibrated by the new standard group prove to be closer to the true value. In September 2005, two CG4 type pyrgeometers are traced to the infrared radiation standard of World Radiation Center (WRC). The new net pyrradiometer calibration regulation is established in 2006. China Meteorological Administration has taken the combination of the two pyrgeometers (CG4) and two pyranometers (PSP) as the formal standard group of net pyrradiometers since 1 January 2007. It is proved that using the new standard group could ensure the transfer of the radiation value reliable and raise the measuring accuracy for net radiation in China.

Comprehensive Analysis on Snow and Freezing-rain Events Based on Doppler Weather Radar in Ningbo

He Caifen, Huang Xuanxuan, Lu Jingjing

2009, 20(6): 767-771.

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Abstract:
Seven snow processes in Ningbo are analyzed using conventional observational data, sounding and Doppler radar data in aspects of weather background, temperature stratification, and characteristics of radar echo and so on. The necessary conditions for snowfall are summarized as follows: The temperature inversion in the lower and middle level, the northerly winds in the lower and the surface temperature is below 4 ℃. The strength and influencing ranges of southwest current determine the strength of the snowfall. When freezing rain occurs, it is above 0 ℃ in the lower-middle level forming a melting layer, and the surface temperature is below 0 ℃. During snow weather, the depth of freezing layer is much thicker than the melting layer or there is no melting layer at all. Generally speaking, the reflectivity of snow is less than 30 dBz, the horizontal and vertical gradient of reflectivity is small and the structure is uniform.The spectrum width is below 4 m/s and the echo top is basically below 6000m. It is found the height of 0 ℃-layer bright band is closely related with the precipitation nature. The distribution of the zero velocity line under the layer of 600 hPa can be divided into three types: Forward-inverse, forward-inverse-forward-inverse, and inverse-forward-inverse. And the analysis on the distribution of the zero velocity line is of great significance to forecast the end time of snow precipitation.

The Evaluation Model of Typhoon Disaster Influence on Zhejiang Province

Zhang Yongheng, Fan Guangzhou, Ma Qingyun, Li Zechun, Cao Haiwei

2009, 20(6): 772-776.

[FullText HTML] (818) [PDF 464KB](2229)

Abstract:
Based on the unceasing increase of economy and continual development of society, the loss from typhoon disaster is getting more fierce and fatal. So the scientific and objective evaluation model of typhoon disaster is very important for recovery from the disaster. Historical research shows that most evaluation methods centralize elements of natural disaster change, but ignore crucial factors about society and economy.Taking the social and economic factors into account, the Fuzzy Mathematics method is applied to analyze the disaster damage caused by 20 typhoons that have influenced on Zhejiang Province during the period from 2000 to 2006. Ten disaster impact factors are selected as model input and an index of every typhoon disaster can be obtained using the method. Meanwhile, the concept of disaster grade in Feng Lihua's work is used to translate the casualty and direct loss of every typhoon disaster into a normal index by means of numerical statistical analysis. Losses of typhoon disasters are divided into four grades as extreme great, heavy, and general by the two indexes. Comparison result shows that for 90% of the cases, the two indexes are accordant while the discrepancy for the rest 10% of cases is only one grade. So the model established can reasonably evaluate the losses caused by typhoon disaster of different density. The model is feasible in beforehand and fast evaluation of typhoon disasters based on the weather forecast and observation data as well as the economic data. It can offer directorial gist for emergency management, and support decision-making for disaster recovery.

Vol.20, NO.6, 2009