应用气象学报

Differences in Ozone Distribution Patterns of East and West Parts in the North of China Based on Satellite Data

Yang Jingmei, Qiu Jinhuan

2009, 20(1): 1-7.

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Abstract:
The focus of this study is to better understand the characteristics and differences in total ozone and ozone vertical distribution patterns of east and west parts in the north of China. It is acknowledged that ozone vertical distribution feature in China not only has common behaviors, which is in line with many other regions of the same latitudes, but also has particular characteristics. It is worth noting that during the winter and spring, the monthly-mean maps of total ozone display a prominent maximum off the Asian east coast, centered at about 50°-70°N, 120°-150°E, caused by the vertically propagation of the quasi-stationary planetary waves. Affected by this high ozone concentration region, the column total ozone in the north of China displays substantial deviations from zonal symmetry, and ozone vertical distribution pattern in the east part is remarkably different from that of the west.The Stratospheric Aerosol and Gas Experiment Ⅱ (SAGE Ⅱ)(version 6.2), Halogen Occultation Experiment (HALOE)(version 19), and Total Ozone Mapping Spectrometer (TOMS)(version 8) satellite data sets from March 1997 to February 2002 are used to derive seasonally averaged ozone profiles and total ozone. Seasonal, latitudinal and comparative studies of total ozone and ozone vertical distributions are performed for the regions covering latitude bands of 45°-55°N and 35°-45°N, and longitude ranges of 75°-105°E (west part) and 105°-135°E (east part). And the standard t test method is used to determine the significance of differences in seasonal average total column ozone, 16-30km column ozone, and column ozone below 16 km between the east and the west part.During winter, spring and autumn, the total ozone values of the east part are considerably higher than those of the west. At the altitudes of ozone maximum density and below, seasonal average ozone amounts in the east part are obviously higher than those of the west, especially during winter and spring. But at the altitudes above 30 km, there are small differences between the east and the west.During winter and autumn, the east-west total ozone differences decrease with the decreasing latitude.But during spring, the east-west total ozone difference over the 35°-45°N band is nearly the same as that of the 45°-55°N band. During the summertime, the east-west total ozone difference is small in the latitude band of 50°±5°N, and the ozone profile of east region is very similar to that of the west. But for the latitude band of 35°-45°N, there are still significant ozone distribution differences between the east and the west:The east-west total ozone difference becomes as large as 20.6 DU, and the derived 0-16 km total column ozone difference is 12.8 DU. It suggests that in the latitude band of 35°-45°N, the east-west ozone differences mainly exist in the lowermost stratosphere and troposphere during the summertime.

The Application of Monitoring Sea Fog in Taiwan Strait Using MODIS Remote Sensing Data

Zhang Chungui, Cai Yiyong, Zhang Jiachun

2009, 20(1): 8-16.

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Abstract:
The sea fog is a frequent severe weather phenomenon, there are many advantages in monitoring the temporal and spatial change of sea fog using remote sensing technology. The visible light channel data and infrared channel data of new MODIS sensors are used to identify and monitor sea fog in Taiwan Strait. Spectrum radiation characteristics of MODIS data on various earth surface, such as ocean, middle-high-level cloud, low-level cloud and sea fog are analyzed, the results show that there are notable spectrum radiation characteristics differences between the sea fog, low-level cloud and ocean, middle-high-level cloud in the Taiwan Strait region.There are also notable spectrum radiation characteristics distinguish in some channels of visible light and middle infrared bands between sea fog and low-level cloud. On the basis of results, the MODIS channels which are sensitive to sea fog are chosen, and the sea fog remote sensing monitoring model in Taiwan Strait region is established by using compositive method, meanwhile the channels combination and digital monitoring indexes which are fit for the sea fog in Taiwan Strait region are validated.The sea fog events in Taiwan Strait region during 2004-2007 are monitored by using the model which has been established, and the sea fog monitoring results are verified and analyzed using the visibility data measured in five meteorological observing stations in Fujian coastal region.The results show that the veracity of sea fog monitoring model is 83.3 % in the mass, and the monitoring model veracity of strong sea fog is better than that of weak sea fog. The monitoring model veracity of night sea fog is relatively low because there are fewer remote sensing channels in the night on MODIS. After the inconsistent samples between remote sensing monitoring and meteorological observing are analyzed, the result shows that the existence of multi-layer cloud has notable influence to the veracity of sea fog. Research result shows:In the view of the sea fog monitoring results which are verified and analyzed between meteorological observing and remote sensing monitoring, the sea fog remote sensing monitoring model in Taiwan Strait region which is established using MODIS data can reasonably describe the distribution status of sea fog, and can dynamically monitor the distribution and developing process of sea fog, the model is very suitable to apply in the future operational work. But because the fog and cloud remote sensing monitoring is very complex, the variety of digital monitoring indexes of sea fog must be noticed in the practical operational work.

A Dynamic Similitude Scheme for Tropical Cyclone Quantitative Precipitation Forecast

Zhong Yuan, Yu Hui, Teng Weiping, Chen Peiyan

2009, 20(1): 17-27.

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Abstract:
It is difficult to make tropical cyclone quantitative precipitation forecast for a specified location.The forecasts from numerical weather models are generally on latitude/longitude grids and extra errors can be brought when these predictions are interpolated to a location not on the grids. A prediction scheme based on dynamic similarity principle is proposed for tropical cyclone quantitative precipitation forecast at several hundred stations in China.Tropical cyclone precipitation generates in a certain synoptic situation. As a matter of fact, a similar tropical cyclone and interaction with environmental fields usually bring similar rainfall process.The key point of the scheme is to constitute prediction scheme with NWP product to seek similar process. For the sake of looking for similarity from more past records, put to use some key technique.Using the objective criterions which are made up of initial parameters of tropical cyclone and of the surround field and the numerical prediction products the similar samples are found out from past tropical cyclones by computer.The scheme is to constitute reasonable criteria for selecting tropical cyclones similar to the predicted one in both the storm feature itself and the large scale atmospheric environment interacting with the storm. Factors considered include initial tropical cyclone parameters, initial and historical large scale atmospheric environment, and future trend of large scale atmospheric environment. A similarity index is set up using a successive-dynamic method, which is non-linear in nature and can quantitatively describe the degree of similarity. By this index, several historical tropical cyclones are selected as cases similar to the predicted one to different extent. A consensus prediction for the precipitation at each station in future 48-hour at 6-hour intervals is then made using the index as weight coefficients for the precipitation of historical cases. Independent tests are carried out for tropical cyclones causing precipitation different in both strength and time duration to China and promising results are obtained. In these tests, the numerical predictions from global circulation models are used to describe the future trend of large scale atmospheric environments. The tropical cyclone quantitative precipitation forecast is forecasted by assessment upon similarity and weight of effects on account of local surround. Using the dynamic similitude scheme for tropical cyclone quantitative precipitation forecast to different rainfall intensity and last time tests is made. Tests demonstrate that this technique has positive prediction skill comparing to the climatology and persistency method.

Characteristics of Heavy Fog in Dalian

Liang Jun, Zhang Shengjun, Sui Hongqi, Zou Yaoren

2009, 20(1): 28-35.

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Abstract:
Two heavy-fog processes occurred in February and April, 2007 of Dalian and its coastal area are analyzed using the sounding data from GTS1 digital radiosonde, conventional observation data, and NCEP/NCAR global 1°×1°reanalysis data. The diagnostic analysis of environment and its dynamic and thermodynamic effects on the heavy fog indicates that high-level zonal warm and dry airflow at high latitude and southwest warm and moist airflow in mid-lower troposphere supply water vapor and wind for the formation of fog. The existence of air steady layer at lower-layer and the superposition between fog layer and the warm, dry air are of benefit to the maintenance of heavy fog.Furthermore, sea surface temperature on Yellow Sea and Bohai Sea induces the further warming and transformation of surface cold high in winter, which is favorable to the formation and development of radiation fog. As a result, the warm air-mass cools and condenses in spring, which causes maintenance of the advection cooling fog. And the north wind followed by the southeast moving of cold convection is the dynamic factor to dissipate the persistent heavy fog.

Impacts of Chinese Doppler Radar on the Severe Heavy Rainfall Forecast During Meiyu Season

Xu Guangkuo, Sun Jianhua, Lei Ting, Zhao Sixiong

2009, 20(1): 36-46.

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Abstract:
CINRAD/SA Doppler radar data and ARPS data analysis system (ADAS) are used to adjust the initial field, then the adjusted initial field is inputted into the mesoscale model WRF (The Weather Research and Forecasting Model) to simulate two rainstorm processes. The two rainstorm processes produce a flash flooding over Huaihe River Basin during Meiyu of 2003. According to the difference of initial data to be used in the numerical simulation, four experiments are designed and conducted in the study. The initial field obtained without use of radar data in control run (Exp CNTL), with use of radial velocity in Exp RADAR-V, with use of radar reflectivity in Exp RADAR-V, with both radar reflectivity and radial velocity in Exp RADAR-RV. The simulated outputs of these four experiments are compared and verified. The results show that after the adjustment of initial field using radar data through the ARPS complex cloud analysis scheme, both wind field and moisture field are adjusted at the region of radar observation, and the initial fields carries more information about moisture, cloud, latent heat, meso-scale circulation, and so on. At the same time, the simulation results also show that the spin-up time of meso-scale model is shortened and accuracy of rainstorm simulation is improved. Through the quantitative verification of the simulation results, both the ETS and the HSS of experiments simulating the radar data in initial fields are proved higher than those of experiments not simulating the radar data in initial fields, and the BS verification score for the former experiments is more closer to 1.Based on the comparing of the results of these four experiments, it can concluded that the impacts of Doppler radar data assimilation on initial field are different between Exp RADAR-V and RADAR-R. In ADAS system, radar radial velocity observations are used to adjust the wind field, but radar reflectivity observations are used to adjust the thermal and moisture fields. The significant differences exist between simulation results of four experiments. On the basis of the quantitative verification, the Exp RADAR-V, which uses radar radial velocity, gets the highest score and is able to simulate meso-scale convective system and its rainfall more accurately. However, it is noticed that the other experiment, such as Exp RADAR-R and Exp RADAR-RV, also can improve the accuracy of rainfall simulation at a certain extent, though not as good as the results of Exp RADAR-V. In addition, the improvements of precipitation prediction are notable in the first six-hour simulation.

The Valid Mosaic Data Region of the CINRAD Network

Yang Hongping, Zhang Peiyuan, Cheng Minghu, Li Bai, Xiong Yi, Gao Yuchun, Chen Daren

2009, 20(1): 47-55.

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Abstract:
Along the normal propagation path, the radar ray becomes higher and higher due to effects of the earth curvature and positive elevation angle of transmitter. Actually, radar waves are often blocked as they propagate through mountain areas. When radar beam has occultation, its real sample volume will be smaller than that of zero occultation, echo intensity of targets will be underestimated if it is partial occultation, and targets will be totally missed if complete occultation occurs. It is common practice now to use occultation rate for beam occultation correction. Assuming that radar waves propagate in standard atmosphere, occultation rates of radar beam herein are calculated with the high resolution digital elevation map data, and are utilized to analyze the valid mosaic data region of netted radars and to assess the beam occultation correction. Test shows that echo will be severely underestimated and waves can be considered to be completely blocked when the occultation rate is larger than 55 %. Based on that, in terms of the 14 elevation angles of two Volume Coverage Pattern modes, VCP11 and VCP12, the real detecting ranges of netted radars are obtained. The netted radars include the local CINRAD in Hunan, Jiangxi, Zhejiang, Fujian, Guangdong, Guangxi and Hainan provinces. Compared with the equivalent beam range at the same height, the valid data range of CAPPI is more suitable to represent the real detection range. Among the valid mosaic date regions at the height of 1500 m, 3000 m and 6000 m above the sea level, the largest blank area can be seen in the graph of 1500 m height, but can hardly be seen in that of 6000 m height. In the overlapping area of 6000 m height, most common grids can be detected by three radars or more, some by the maximum of six. Observations in VCP12, compared with that in VCP11 or in VCP21, are more useful to construct mosaic data not only because of its dense vertical sample in low elevation but also its larger valid data range in mountain areas. According to the definition formula of echo intensity, correction of intensities sampled in partial occultation range gates is obtained from its occultation rate. For instance, the correction value is 1.0 dB, 1.5 dB, 2.2 dB, 3.0 dB and 3.5 dB while the occultation rate is 20 %, 30 %, 40 %, 50 % and 55 % respectively. In the experiment of correction, base data are observed in the common latitude longitude grid at the same time by the two weather radars in Wenzhou and Ningbo. The common grids are separated into two groups. The group Ⅰ consists of the grids where Wenzhou radar has partial occultation and Ningbo radar has none. And the group Ⅱ consists of the grids where both two has no occultation. Therefore, the observations of Wenzhou radar are corrected in group Ⅰ, and correction is assessed by the observations of Ningbo radar. The results indicate that the correlative coefficient of corrected data is larger than that of uncorrected in group Ⅰ, and the values are close to that of group Ⅱ, the corrections are effective.

The Ensemble Forecasting Verification on the Summer Eurasian Middle-high Latitude Circulation

Duan Mingkeng, Wang Panxing, Wu Hongbao, Zhu Yuejian

2009, 20(1): 56-61.

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Abstract:
The middle-range ensemble forecasting results of Eurasian middle-high latitude region (40°—80°N, 40°—150°E) in JJA 2003 are validated using NCEP EPS. The ACC and RMSE validation on the ensemble mean and deterministic forecasts shows that ensemble average forecasts are more efficient than single forecasts when the lead time is greater than 5 days. Under the same model resolution, ensemble average forecasts can extend the lead time of "usable forecasts" to more than 12 hours, even 7.5 days. The ROC analyses on the deterministic forecasts and ensemble probabilistic forecasts indicates that ensemble forecasting can provide the true probabilistic forecasts through the character of multi-members. The ensemble probabilistic forecasts have obvious advantages comparing to the single forecasts and are also better than increasing model resolution. The case study on the blocking system indicates that ensemble mean is better than single deterministic forecast. The characteristic isoline can reflect the disagreement among the ensemble members and the anomalous behavior in ensemble members. Based on those information, the probability of the target happening can be estimated, hence better forecast results can be achieved.

New Research on Cone of Influence and Computing Scheme of Mexican Hat Wavelet Transform

Gu Dejun, Wang Dongxiao, Ji Zhongping, Zheng Bin

2009, 20(1): 62-69.

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Abstract:
Using the effective domain of wavelet function, the problems of cone of influence (COI) and high-frequency distortion of Mexican Hat wavelet transform in the paper of Torrence and Compo (1998) are analyzed and the solution are explored. The analytic expressions of wavelet coefficient and global wavelet power spectrum for time series of sinusoid and the relationship between period and wavelet scale are deduced. The effective domain of Mexican Hat wavelet transform is [ b-2.12a, b+2.12a] for localized time b and wavelet scale a. The authentic cone of influence for Mexican Hat wavelet transform is 2.12a. The maximum value of wavelet scale is N/4.24 (N is the length of time series). When wavelet scale approaches zero, wavelet function is singular. So the simple sum scheme replacing the integration for wavelet coefficient does not include the singularity of wavelet function, therefore the pseudo high-frequency oscillation is deduced. A new computing scheme for wavelet coefficient is designed. The major elements include improved-resolution and located time b in the new grids. The difference of effective domain of wavelet function for small different wavelet scales can be distinguished by the improved-resolution. Located time b has great contribution to its wavelet coefficient because the wavelet function is the biggest at the located point b. The pseudo significant high-frequency oscillation that is produced by the computing scheme proposed by Torrence and Compo (1998) can be eliminated by the new computing scheme of wavelet coefficient, well using sharply descending characteristics of wavelet function and cubic spline interpolation. The rationality of new computing scheme is tested by the analytic solution of wavelet transform of sinusoid time sequence. The global wavelet power spectrum from the new computing scheme indicates that the variability of winter Niño3.4 index exhibits interdecadal oscillation of about 12 years and interannual oscillation of quasi-four years, and no significant quasi-biennial oscillation. The questions of cone of influence and pseudo high-frequency oscillation have two adverse aftermaths. First is that the correct judgment of interdecadal variation can be affected and second is that pseudo quasi-biennial variation can be deduced. Therefore, the results about COI and the new computing scheme for wavelet coefficient make the conclusions induced from wavelet analysis more credible.

Applicability Evaluation of Surface Pressure for Several Reanalysis Datasets over China

Zhao Tianbao, Hua Lijuan

2009, 20(1): 70-79.

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Abstract:
The evaluation of applicability of the reanalysis datasets is crucial to their application. The reliability of surface pressure in China obtained respectively from the reanalysis datasets is focused on, i. e. , ERA-40, NCEP/NCAR and NCEP/DOE. The analysis is carried out in different regions, different seasons and different periods with compared to the station observations over China.The results indicate that mean values of the three reanalysis products could basically display spatio-temporal consistence with observation. However, the differences found among them are more obvious in the western China than in the eastern China, and is more significant in summer than in winter. At the same time, the differences among the ERA-40, NCEP/NCAR and the observations are stronger before 1970s, and the significant systematic bias could be detected from the mean value of NCEP/DOE and the observations, but the ERA-40 is closer than the NCEP/NCAR and NCEP/DOE to the observations over most parts of China, especially in the west of China.The results also indicate that three reanalysis products all can represent the interannual variations and the linear climate trends displayed by the observations in most regions of China to a certain extent, but the discrepancies among them still has a certain regional and seasonal dependency. The most obvious differences among them can be detected in the west areas, especially the Tibet Plateau and the surrounding regions in sumer. In terms of the agreement with the observations, the ERA-40 is more consistent with observations, but the NCEP/NCAR overestimate interdecadal variations for most areas of China in summer due to the tremendously "low" value before 1970s.In conclusion, the reliability and applicability the surface pressure of ERA-40 are better than those of NCEP/NCAR and NCEP/DOE in most areas of China. The possible causes may be that more observing data, such as surface air temperature, surface pressure and so on, have been used in the ERA-40 assimilation systems, while the data used by the NCEP assimilation systems only include the observed wind field.

Responses of the Electric Power Consumption to Climate Change in Guangzhou City

Duan Hailai, Qian Huaisui

2009, 20(1): 80-87.

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Abstract:
The earth's climate is now experiencing significant change characterized by global warming. Profoundinfluence has been or will be brought by the global warming to the world. Exploring and assessing climatechange and its impacts are one of the hot issues in the fields of climate change.The electric power consumption of Guangzhou City which lies in the low latitude belt is sensitive to climate change. The trend ofthe electric power consumption changes with different paces and in different forms in Guangzhou City because of the global climate warming. In order to rationalize the allocation of the electric power resource andsave the electric power, it is necessary to study the responses of the electric power consumption to the climate warming. A research method is introduced which is used to study climate change impact on agriculture yield and energy sources, the dynamic assessment model for the influence intensity of the climaticchange in the electric power consumption and the model of cooling degree-day are established respectively.The responses of the electric power consumption to climate change in Guangzhou City are analyzed, andthe trend of the electric power consumption in Guangzhou City is forecasted. Firstly, the correlation between the electric power consumption of Guangzhou City and weather factors is studied, it is found thatthe main weather factors which affect the electric power consumption of Guangzhou City are the temperature, humidity and wind speed, but the temperature is the key factor. Secondly, the stability of climatechange is a very important factor for the electric power consumption besides weather factors. A dynamicassessment model including mean influence intensity and deviation are established. Considering all the climate factors and the stability of climate change, in the late fifty years, influence intensity of the climaticchange to electric power consumption has been increasing steadily, the probability of positive influence intensity has an ascending trend, rising at the speed of 10% every ten years, especially significant in recenttwenty years. Thirdly, through the analysis of cooling degree-day of Guangzhou City, it is found that themain cooling time is from May to October. The intensity of cooling degree-day is influenced by the increaseof the temperature greatly, reaching up to 46.6%/ ℃. At the same time, by analyzing the variability of thelength of cooling period, it shows that the length of cooling period has a gradual increasing trend in Guangzhou City. Therefore, the increase of cooling degree-day caused by temperature increasing has great effectson the electric power consumption.Finally, in the future scenery of climate warming, when the averagemaximum temperature increases 1 ℃ in summer, the unit industrial production value electric power consumption of the whole year will increase 2.02%. When the average temperature from May to October increases 1 ℃, the percentage of electric power consumed by residents will increase 1.25%. So in Guangzhou City, the pressure of the electric power consumption will be continuously increasing in the future because of the climate warming.

The Performance of CERES-Wheat Model in Wheat Planting Areas and Its Uncertainties

Xiong Wei

2009, 20(1): 88-94.

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Abstract:
Crop models, coupling with climate data from climate models(GCMs, RCMs), are often employed to assess the impacts of climate change on crop production. However, there is a systematic mismatch of resolutions between climate models and crop models. Scaling up the crop model to regional scale is an appropriate method to resolve this problem. CERES Wheat crop model is used to simulate the wheat yields of 1981—2000 at 50 km×50 km grid scale. Performances of this simulation in wheat planting areas are evaluated based on the comparison of simulated yields to census values. The relative root mean square error(RMSE)between simulated and census yields for whole China is 27.9%, and the agreement index is 0.75. Of 2206 simulation units(50 km×50 km grid), 59.2% show relative RMSE less than 30%, in which 26.3% less than 15%. The performances differ among regions. Smallest bias occurs in agro-ecological zone 2(the largest wheat planting areas accounting for 39.9% of China's wheat planting area), with relative RMSE of 16.6% and D=0.68. To sum up, CERES Wheat crop model is able to produce reasonable results temporally and spatially. It can provide simulation information for policy making at macro scale despite existing uncertainties. The uncertainties of this regional simulation are ascribed to simplification and limitations of crop models, the aggregated inputs in wheat planting area, and errors in dataset etc, which need to be addressed in future.

MODIS in Monitoring the Chlorophyll-a Concentrations of Chaohu Lake

Xun Shangpei, Zhai Wuquan, Fan Wei

2009, 20(1): 95-101.

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Abstract:
Using remote sensing data for large area water quality survey is a typical water environment monitoring method. The Chaohu Lake reflectance spectra are measured using ASD field spectrometer and the MODIS data collected during the same period. After the MODIS data are preprocessed, the spectral characteristics of Chaohu Lake are analyzed and the spectral is compared with references. Then the relation between the measured water reflectance and chlorophyll a is fitted. The fitting formula is consistent with previous research results and the correction coefficient is 0.75. But after analyzing the apparent reflectance of the MODIS L1B and the measured concentration of the chlorophyll-a, the ideal inversion lake model can not be obtained because of the influence of the atmosphere and other factors. After a special atmospheric correction for MODIS data, the ground reflectance spectra are obtained in the channels 1—7. The relation between the ground reflectance and the chlorophyll a is fitted and the correction coefficient is improved. At last, the optimum bands combination are chosen after analyzing the relation between the reflectance spectra and the corresponding moment of the concentration of the chlorophyll a, using EOS/MODIS data after atmospheric correction. The chlorophyll a index I_Chla is defined. Therefore the inversion model for chlorophyll a concentration of Chaohu Lake is achieved, and the correction coefficient is 0.5079. This correction coefficient is not as good as the result(0.75)obtained by the hyperspectral data measured by the field spectrometer. There are 3 possibilities that may cause the errors. First, the bands selection of the MODIS data are not cover exactly the spectral sensitive of the concentration of the chlorophyll a(685—715 nm). Second, the narrowband of the field spectrometer and broadband of the MODIS data is not very exact. Third, the atmospheric correction errors above Chaohu Lake may be too big.

Spatial and Temporal Analysis on Annual Temperature and Precipitation During Last 40 Years in Gansu Province

Li Zhanling, Xu Zongxue

2009, 20(1): 102-107.

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Abstract:
Temporal features of annual temperature and precipitation at 28 stations in Gansu Province from 1960 to 2003 are analyzed using nonparametric Mann Kendall method and the accumulated departure approach. Spatial characteristics of these two variables in the 1960s and 1990s are detected using geostatistical method. It shows that the annual temperature has a significant increasing trend, while the precipitation has an insignificant decreasing trend at 0.1 significance level. Annual temperature experienced with low level from the 1960s to mid 1980s and with high level from 1986 to 2003, and abundant precipitation occurred in the 1970s and relative low in the 1990s at 0.05 significance level. Gaussian model is the best theoretical model for temperature semivariogram. 25% and 32% of total spatial variability in temperature is resulted from spatial autocorrelation in the 1960s and 1990s. Spherical model is the best model for precipitation semivariogram. The incidence of precipitation in the 1990s is smaller than that in the 1960s, and smaller than those of temperature in both 1960s and 1990s, indicating that the effects of local micro climate such as topography and slopes on precipitation is greater than those on temperature. 78% and 80% of total spatial variability in precipitation is attributed to spatial autocorrelation in the 1960s and 1990s, showing intensive spatial autocorrelation in precipitation in Gansu Province. Compared with the spatial variations from 1960s to 1990s, temperature increased in most of the regions, including the south of Longnan, Longdong, the east of Longzhong, Hexi Corridor plain, local area of Beishan middle mountains and the Qilian Mountains area. Precipitation tended to decrease in most of the regions, especially in Longnan area and Gannan Plateau area, while the situations in some local areas of Hexi Corridor plain and Beishan middle mountains area are on the contrary.

A Method for Estimating the Extreme 10-minute Average and 3-second Wind Speed with a Recurrence Period of 50 Years at the Different Height in a Wind Farm

Hu Jinhua, Wang Xiangming

2009, 20(1): 108-113.

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Abstract:
The extreme 10 minute average and 3 second wind speed with a recurrence period of 50 years at the hub height on a wind farm is a major index for choosing the wind turbines types. From the viewpoint of meteorology, and linking some developed examples of the wind farm projects in China, the article brings forward a sampling method of the maximum 10 minute average wind speed of 5 days from a whole year actual measured wind speed data of a mast at the wind farm. The method refers to the typeⅠ extreme value distribution and the modification matrix estimating the parameters method, for estimating the extreme 10 minute average wind speed with a recurrence period of 50 years at different heights in a wind farm. Using the same period and long term wind speed data of a nearby meteorological station, the above results can be revised. Since the distribution of wind resource has its own specialty in China, the maximum gust coefficient is reduced when the wind speed gets larger in north area of China, and is bigger than 1.4 when the wind farm is influenced by tropical storm or typhoon. It suggests that using the actual observed maximum gust coefficient of the bigger wind speed to estimate the extreme 3 second wind speed with a recurrence period of 50 years at different heights in a wind farm. This method is applied to estimate the extreme 10 minute average and 3 second wind speed with a recurrence period of 50 years at the different heights in the Wulanyiligeng wind farm 300 MW project in Bayannaoer City, Inner Mongolia. The result reveals that the method is convenient and reliable for the Wulanyiligeng wind farm projects developing.

Principal Component Analysis Method Acquiring Soil Moisture Information from MODIS Data

Zhou Bingrong, Li Fengxia, Shen Shuanghe, Hu Ling

2009, 20(1): 114-118.

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Abstract:
Monitoring soil moisture exactly is very important. Soil moisture is one important factor of agricultural meteorology, which can reflect humid condition of soil, and is a main base to forecast drought of farmland. But it is influenced by too many factors, so it is difficult to monitor real-time soil moisture of largescale areas. General method such as soil sampling method, neutron probe method and TDR method takemuch time and efforts, and can only monitor limited spots. However, the development of remote sensingtechnology can provide assistance to monitoring real-time soil moisture of large-scale areas dynamically.Thermal inertial is a matured technological method to monitor bare soil moisture applying MODIS data. But it needs remote sensing data of both daytime and nighttime, which is difficult to obtain in practicaloperation. It inherits the idea of K-L transformation that is applied in the remote sensing system of targetclassification, uses principal component analysis, regression analysis, residual image, and relative reflection of internal average methods to correct remote sensing radiation data, and establishes soil moisturemodel of multiple-dimensional feature space based on mono temporal normalized MODIS data.Then the result from the image of monitoring is obtained and checked up. Monitored and model results of 35 spots are compared, and the accuracy of the model is 80%. It shows that the model has potentialto be applied in operation. More problems are discussed, including representative of the data, the calibration of remote sensing detector and so on.

Beam Characteristics Analysis on Phased Array Weather Radar

Yang Jinhong, Gao Yuchun, Cheng Minghu, Chai Xiumei

2009, 20(1): 119-123.

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Abstract:
The current NEXRAD system completes a volume coverage pattern in a minimum of approximately 5 minutes for 14 elevation steps. Phased array radar, with easily designed multi beams and processing of signals, offer the prospect of routinely sampling the atmosphere with volume scan rates an order of magnitude faster than the fastest(5 minutes)operational scan mode of the Doppler radars. Therefore, phased array weather radar is to form a wider transmitted lobe and receive backward radar signal using multiple narrower beams in order to save radar scanning time. The directional diagrams of the phased array antenna are derived respectively under the uniform and the non uniform circumstances, then their characteristics are analyzed and discussed. Changes of beam width and antenna gain will bring observation errors, therefore, a method of using non uniform partitioning subspace to correct the beam width and antenna gain is derived to overcome the shortcoming of phased array radar. An antenna scheme of phased array weather radar is designed using 144 array element numbers based on one dimensional linear array, meanwhile, a wider transmitted beam and multi narrow beams patterns are derived. Results show that, under the circumstances of uniform antenna caliber, side lobe level of antenna is higher. The methods of Hamming weighting can decrease the electrical level of side lobes to-25 dB, which can fundamentally meet the requirements of weather observation. The designed antenna scheme can not only save the scanning time of antenna, but also sufficiently utilize the radar resource.

Application Method of Monthly Dynamics Extended-range Forecast Based on EOF Iteration Scheme

Chi Juncheng, Shi Yinshan

2009, 20(1): 124-128.

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Abstract:
Based on EOF iteration scheme model with dynamic extended range forecast products, experiment of precipitation prediction is done for 121 station of Hebei Province from June to August in 2005—2007. It is necessary to use 500 hPa correlation area in the EOF iteration scheme model in order to improve the prediction accuracy. Experiment in different correlation areas indicates that the model performs best when the critical correlation coefficient is 0.45, correlation area is of 90 grids. The average sign coincidence rate between predicted precipitation and observed precipitation is 0.54.When critical correlation coefficient is 0.45, grid number is 90 and iteration error is 0.05, let truncation order of eigenvector equal 1, 2, …, 11 and do experiment separately. The first two eigenvectors account for about 50% of total variance explained. When truncation order equal 1 and 2, average sign coincidence rate are 0.60 and 0.58. When truncation order is greater than 2, average sign coincidence rate decreases as truncation order increases slightly until stability in 0.54—0.52.When critical correlation coefficient is 0.45, grid number is 90 and truncation order of eigenvector is 4, let iteration number equal 0, 1, …, 19 do the prediction experiment separately.The experiment shows that iteration error decreases as iteration number increases, but the decrease rate is very different. As accuracy reaches 0.05, the fastest experiment requires only one times of iterative operator and the slowest one requires 15 times. The predicted result affected by iteration error is slight. To acquire good result, many times of iteration is unnecessary, usually no more than three times. When truncating the first eigenvector without iteration, the average sign coincidence rate of monthly precipitation prediction can reach 0.63 and operational scores PS can reach 0.74.

Vol.20, NO.1, 2009