应用气象学报

The Atmospheric Circulation Features of Two Persistent Heavy Rainfalls over Southern China in the Summer of 2010

Niu Ruoyun, Zhang Zhigang, Jin Ronghua

2012, 23(4): 385-394.

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Abstract:
The large scale rainstorm flooding induced by the persistent heavy rainfall in summer is the main meteorological disaster in China. Still more efforts are needed to improve the quantitative and refined forecast ability on the characteristic quantity of the persistent heavy rainfall, such as starting and ending time, drop location and the intensity of precipitation. By using daily intensive observed precipitation data and NCEP/NCAR reanalysis data, two persistent heavy rainfalls in Southern China is analyzed and compared to investigate the spatial-temporal evolutioning features and the influencing mechanism of the large scale key influence systems, and atmospheric circulation features of the two processes are summarized as below.During the periods of persistent heavy rainfalls over Southern China in the summer of 2010, the westerly trough over coastland reformed and strengthened frequently and the mid-latitude frontal zone maintained steadily, the low level southwest jet strengthened repeatedly and the strong gradient belt of meridional wind on the left side of the low level southwest jet axis are relatively stable. Meanwhile the latitude location of the subtropical westerly jet, the ridge of South Asia high and the west Pacific subtropical high are also relatively stable in East Asia. Under the joint influences of the large scale key influence systems mentioned above, vapor convergence in the low layer and divergence in the upper layer occurs repeatedly, and the vertical ascending motion strengthens over the heavy rainfall belt, which leads to the formation of the persistent heavy rainfall over Southern China.Before the persistent heavy rainfall in Southern China, the trough in western Siberian keeps reforming and strengthening, shifting to east of Mascarene high and its western high. The subtropical upper westerly jet is also established and maintains in East Asian.The heavy rainfall belt of the persistent heavy rainfall lies in these areas: The strong gradient convergence belt of meridional wind on the left side of the low level southwest jet axis, the strong divergence zone between the south boundary of subtropical upper westerly jet and the ridge axis of South Asia high, the belt of high updraft vertical velocity in the middle layer in East Asian, the south margin of the middle-latitude frontal zone in the middle-upper troposphere, and the north boundary of the west Pacific subtropical high.

Calibrating 2 m Temperature Forecast for the Regional Ensemble Prediction System at NMC

Wang Min, Li Xiaoli, Fan Guangzhou, Li Zechun

2012, 23(4): 395-401.

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Abstract:
It's known that ensemble forecasts provide a flow-dependent sample of the probability distribution of possible future atmospheric states instead of the single and deterministic prediction. Ideally, the probability of any event could be skillfully estimated directly from the relative event frequency in the ensemble. Unfortunately, although the quality of ensemble prediction systems (EPS) has been improved greatly, the direct output of EPS generally is subject to the systematic deficiencies, especially for surface variables. They are under-dispersive and lack of reliability. Therefore, statistical post-processing methods have been developed to improve direct model output. The nonhomogeneous Gaussian regression (NGR) is used to calibrate 2 m temperature forecast of the regional EPS at NMC/CMA. The NGR is the statistical correction method with the first and the second moment (mean bias and dispersion) for Gaussian-distributed continuous variable. This method is based on the multiple linear regression technique and provides a predictive probability density function (PDF) in terms of the normal distribution. The method of minimum continuous ranked probability score (CRPS) estimation is used to fit the regression coefficients of PDF. It can be found that NGR method can greatly improve 2 m temperature forecast compared with the raw ensemble output, and the improvement is as follows: The mean bias is reduced and the spread of ensemble members is increased reasonably; the L-shaped Talagrand diagram of the direct ensemble output has been improved and the calibration reduces the number of outliers, especially in the 9th bin; the probabilistic scores (the brier score, continuous ranked probability score, area under relative operating characteristic curves) all show the significant forecast skill improvement in the calibrated forecasts.In addition, the sensitive study is performed to investigate the effect of the training length, and the results show that the training length plays a minor role, at least for the chosen verification period. Finally, the comparison by using the time-decaying average bias correction method and NGR is performed, showing that NGR not only has advantages in reducing ensemble mean bias and increasing ensemble spread, but improves the forecast skill in terms of probabilistic scores.

Inter-decadal Shift of East Asian Summer Monsoon in the Early 1990s

Tang Jia, Wu Bingyi

2012, 23(4): 402-413.

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Abstract:
Using JRA-25 and NCEP/NCAR reanalysis data from 1979 to 2009, dominant modes of summer season (June—August) 850 hPa wind field variability over East Asia is revealed by means of the complex vector empirical orthogonal function method. The two reanalysis data are consistent with the description of the first East Asian summer monsoon (EASM) mode, whereas the first mode had been studied, showing that the first mode could not reflect the inter-decadal shift of Chinese summer precipitation in the early 1990s. Consequently, the inter-decadal shift feature of the second EASM mode is deeply analyzed, as well as its effect on summer precipitation in China. Moreover, the possible external forcing factors exerting effects on the inter-decadal shift of EASM are discussed.Results show that, EASM which is revealed by two sets of reanalysis data to have undergone one inter-decadal shift in the early 1990s. The inter-decadal shift time of EASM is consistent with the inter-decadal shift time of summer precipitation in China. EASM is closely related to the mid-high latitude atmospheric circulation anomalies. Corresponding anomalous 500 hPa geopotential height fields show an anomalous quasi-zonal teleconnection pattern in northern Eurasia, whereas the distribution of summer precipitation anomalies show a meridional dipole pattern. Accompanied by the inter-decadal shift of EASM, after the early 1990s, summer precipitation decreases in the majority of northern China, especially in north of the northeast and the area between the Yangtze River and the Yellow River in the vicinity of 105°E. While summer precipitation increases significantly in South China and the Huaihe River Basin. From the perspective of dynamic, the characteristics of inter-decadal shift of summer precipitation in China are described. The difference distribution of summer 500 hPa geopotential height fields between two periods (1993—2009 and 1979—1992) show northern Eurasian quasi-zonal teleconnection pattern, then the difference distribution of summer 850 hPa wind fields show the structure that there are two anomalous anti-cyclonic circulations in southeast of Lake Baikal and south of Japan, while there are two anomalous cyclonic circulations in southern China and Okhotsk Sea.The possible external forcing for the inter-decadal shift of EASM are various, including summer sea surface temperature (SST) in the northwestern Pacific, north Indian Ocean and the part of high latitude ocean (North Atlantic and North Pacific), as well as changes of spring Eurasia snow water equivalent in the early 1990s, inter-decadal shift of spring Arctic sea ice in the early 1990s, especially the high-latitude forcing factor. The role of these external forcing in inter-decadal shift of EASM is unclear and further study is essential.

Reconstruction and Application of the Monthly Western Pacific Subtropical High Indices

Liu Yunyun, Li Weijing, Ai Wanxiu, Li Qingquan

2012, 23(4): 414-423.

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Abstract:
In order to solve the problems of the monthly Western Pacific Subtropical High (WPSH) indices used in National Climate Center (NCC) monitoring service, a series of the monthly WPSH indices, including the area index, intensity index, ridge line index and western boundary index, are redefined and reconstructed, on the basis of the monthly NCEP/NCAR reanalysis datasets from 1951 to 2010.The reconstructed area index is defined by the "real area" of the WPSH surrounded by the 588 dagpm contour over western Pacific, while the intensity index is the "volume" of the WPSH over 588 dagpm. The reconstructed western boundary index is defined as the minimal longitude degree of the 588 dagpm contour in the region between 90°E and 180°. When 588 dagpm contour disappears in some month, it is substituted by the maximum of this index in this month during 1951 to 2010. The definition of the reconstructed ridge line index has more adjustment. It is defined by the latitude position of the isoline where 500 hPa zonal wind u=0 and ∂u/∂y > 0 surrounded by 588 dagpm contour. When there is no 588 dagpm contour, the 584 dagpm contour is just considered in the definition. If even no 584 dagpm contour exists in some month, it is substituted by the minimum of this index in this month during 1951 to 2010. Both the 500 hPa geopotential height and 500 hPa zonal wind shear line are considered in the reconstructed ridge line index, and no more restricted by 588 dagpm contour, which takes full consideration of the impact of the WPSH system on the summer precipitation in East China.The reconstructed WPSH indices have enabled describing objectively characteristics of the WPSH's monthly change, and overcoming the defect of the excessive dependence of the WPSH indices using in NCC service on the data resolution. The significant correlation of the reconstructed ridge line index and the summer precipitation over the Yangtze River also confirms its rationality and objectivity. Finally, two kinds of relatively independent WPSH indices, the ridge line index and western ridge point index, are selected to combine nine classifications of WPSH, which correspond with all kinds of distributions of summer precipitation anomaly in East China at large. It provides a scientific basis to further understand the relation of the position anomaly of the WPSH and the summer main rainfall belt in East China.

Characteristics of PM_2.5 at Lin'an Regional Background Station in the Yangtze River Delta Region

Meng Zhaoyang, Jia Xiaofang, Zhang Renjian, Yu Xiangming, Ma Qianli

2012, 23(4): 424-432.

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Abstract:
A ground-based observation of fine particles (PM_2.5) is conducted in 2010 at Lin'an regional background station in the Yangtze River Delta region. Daily PM_2.5 samples are collected by Mini-Vol portable aerosol sampler on 47 mm quartz filters with the flow of 5 L/min. A total of 223 valid PM_2.5 samples are collected in 2010. The concentrations of Na⁺, NH₄⁺, K⁺, Mg²⁺, Ca²⁺, F^-, Cl^-, NO₃^- and SO₄^2- in PM_2.5 are determined by using Dionex 600 Ion Chromatography. EC and OC in PM_2.5 are determined by DRI Model 2001A Thermal/Optical Carbon Analyzer. The mass concentration of PM_2.5 at Lin'an Stationranges from 1.4 to 442 μg·m^-3, with the annual average concentration (58.2 ± 50.8) μg·m^-3 during 2010. Seasonal variation of PM_2.5 concentrations is significant. The concentrations of PM_2.5 are 70.3, 28.9, 66.2 μg·m^-3 and 77.4 μg·m^-3 in spring, summer, autumn and winter, respectively. To gain an insight into the impact of transport on PM_2.5 levels at Lin'an, air mass backward trajectories are calculated and analyzed in combination with corresponding pollutants concentrations using the HYSPLIT4 model. The results indicate that the site is under significant regional-scale influence of the long-range transport from the Yangtze River Delta region and northern China. The annual concentration of total water-soluble inorganic ions is (28.5±17.7) μg· m^-3, contributing an average of 47% of PM_2.5 mass concentrations. In PM_2.5, the concentrations of the most abundant ionic species following the order of SO₄^2-, NO₃^-, NH₄⁺, Cl^-, Na⁺, Ca²⁺, K⁺, F^- and Mg²⁺. Three major ions SO₄^2-, NO₃^- and NH₄⁺ account for 69% of the total water-soluble inorganic ions. The annual mean concentrations of OC and EC are (10.1±6.7) μg·m^-3 and (2.4±1.8) μg·m^-3, respectively. The concentrations vary in ranges with 0.8—29.8 μg·m^-3 for OC and 0.03—8.6 μg·m^-3 for EC. The average concentrations of OC and EC are highest in autumn and lowest in summer. The concentrations of OC and EC are (15.3±6.5) μg·m^-3 and (3.6±1.5) μg·m^-3 in autumn, while (4.8±2.2) μg·m^-3 and (1.2±0.6) μg·m^-3 for OC and EC in summer, respectively. OC and EC show a significant correlation, indicating that OC and EC are mainly from the same sources.

Inter-comparision and Application of Atmospheric Humidity Profiles Measured by CFH and Vaisala RS80 Radisondes

Yan Xiaolu, Zheng Xiangdong, Li Wei, Ma Jin

2012, 23(4): 433-440.

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Abstract:
Vertical profiles of atmospheric humidity simultaneously measured by balloon-borne Cryogenic Frostpoint Hygrometer (CFH) and Vaisala RS80 radiosonde in Tengchong, Yunnan in August 2010 are analyzed. Currently, CFH is the reference instrument in the measurement of atmosphere water vapor profile. RS80 radiosonde is ever extensively used in the world before the middle of 1990s. The humidity data measured by CFH is used to assess the quality of RS80 radiosonde humidity data. The difference of RS80 radiosonde humidity data in day and night time respectively compared to CFH data is also given in individual inter-comparison. The results have revealed there is a large dry bias produced by the RS80 humidity sensor with average of (23.7±18.5)%, and the daytime dry bias is (13.5±14.8)% larger than that in the nighttime owing to solar radiation heating on the humidity sensor. In addition, RS80 radiosonde is almost incapable of measuring the valuable humidity data in the transition region from upper troposphere to lower stratosphere. For the integrated precipitable water (PW) amounts from the profiles of GTS1, RS80, CFH and their comparisons with GPS measurements, CFH integrated PW is (4.3±2.0) mm (number of samples is 11) higher than that of GPS because that CFH tends to be saturation at moist condition, especially when passes through cloud in lower troposphere, while the PW differences of RS80, GTS1 from the GPS measurements are (0.2±1.4) mm (number of samples is 12) and (-0.2±2.2) mm (number of samples is 43) respectively. The value of GPS PW is not sensitive to the humidity variations in the altitudes above upper troposphere. CFH is demonstrated as an effective instrument measuring water vapor concentration in the circumstance with lower temperature as well as lower humidity, such as in the upper troposphere and lower stratosphere. Owing to dry bias, RS80 radiosonde detects less middle or high clouds than CFH does, especially in the detection of high clouds above 6000 m where the low humidity value from RS80 radiosonde almost cannot indicate the occurrence of cloud. Therefore, the occurrence frequency and altitude of high cloud would be much underestimated if RS80 radiosonde water profiles are used.

Ground-based Dual-band Cloud Observing System and Its Comparative Experiments

Hu Shuzhen, Ma Shuqing, Tao Fa, Qin Yong, Guo Wei, Wen Xiangang

2012, 23(4): 441-450.

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Abstract:
Clouds affect the energy balance of the earth by means of absorbing and scattering radiation, and they have influences on global climate. Precipitation of clouds is the most familiar way for the earth water circulation. In the macroscopic parameters of clouds, cloud cover and cloud base height are the primary elements in the observation of clouds, where the cloud base height condition determines cloud classification and precipitation probability. However, artificial visual observation is the main method used in China at present, which are not objective and waste a mass of work force. Therefore, the research and development of an automatic measuring cloud device is extremely necessary.A ground-based cloud observing system consists of an infrared temperature measuring sensor and a double location digital ceilometer. The double location digital ceilometer calculates the cloud height of zenith by geometry method at regular time, and the infrared temperature measuring sensor acquires brightness temperature of atmospheric radiation in real time. Based on the different characteristics of visible and infrared cloud images, the height of clouds is derived and tested by the double location digital ceilometer, combined with real-time observation of zenith single-point infrared brightness temperature and ground environmental parameters. The influence of sub cloud air from ground to clouds bottom on infrared brightness temperature is analyzed, showing the feasibility of cloud base height remote sensing by ground-based sky infrared brightness temperature. The result shows that for low and middle clouds, the sky radiation brightness temperature of ground-based observation is relatively sensitive to the variation of cloud base height, therefore it can be used to estimate cloud base height. On the basis of decreasing principle of tropospheric vertical temperature, the temperature decreasing gradient parameter K from cloud base to the ground is defined and according to the decreasing gradient parameter inverse the cloud height of zenith direction. The algorithm does not depend on the air sounding data, and the formula of cloud base height is derived in practice. Double location digital ceilometer can measure a cloud height only when the bottom of the cloud has texture, and then K can be calculated, therefore the double location digital ceilometer is a calibration device in the ground-based measuring cloud system.Experimental observation has been performed using dual-band cloud observation system in the synthesize experiment base of CMA Meteorological Observation Center since July 2010, and cloud height data of the zenith is acquired each minute. Through the comparative data analysis with Vaisala CL31, ground-based dual-band cloud observation systems can provide results with high accuracy.

Development and Verification of a Numerical Forecast Model for Road Meteorological Services

Meng Chunlei, Zhang Chaolin

2012, 23(4): 451-458.

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Abstract:
Accurate road meteorology forecast and road traffic information are very important to road transportation security. Road surface temperature is a crucial parameter in traffic weather forecast. Now there are three main kinds of road surface parameters forecast model: Statistical model, GIS-based model and physical model. Physical model is widely used and it mainly considers the road surface energy balance model and the effect of anthropogenic heat. In 2008, based on the rapid update cycling forecast system (BJ-RUC), the road weather information system is developed and run operationally by the Institute of Urban Meteorology. Since 2007, Beijing Meteorological Bureau has established 18 weather stations along the express way using the apparatus manufactured by ROSA Vaisala in Finland, and established 8 visibility observation stations using the digital visibility sensor. These all make the fine traffic weather forecast and operational run possible.A fine numerical model for urban road surface temperature (RST), snow depth and ice depth prediction (BJ-ROME) is developed based on Common Land Model (CoLM). The model is developed according to characteristics of the road surface, and based on the data of express way weather observation and fine land surface data of Beijing. The model is forced by the meteorological data output from BJ-RUC, and the forecast and update time span is 24 hours and 3 hours, respectively. The model is validated using in-situ observation data measured by the ROSA road weather stations of Vaisala Company, Finland. The sensitivity analysis is also implemented.Nine sites are chosen to validate the RST prediction results of BJ-ROME. The validation time is during 9—24 Aug 2009, when the RST is very high in Beijing. Four sites, i.e., Xihongmen, Wenyuhe, Xiguan and Lugouqiao are chosen to validate the snow depth prediction results of BJ-ROME. The validation time is during 2—5 Jan 2010, when a big snowfall happens in Beijing. The validation results indicate that BJ-ROME can successfully simulate the diurnal variation and maximum value of RST both under clear-sky and rainfall conditions. The validation results also indicate that BJ-ROME can successfully simulate the accumulation time and the variation and maximum value of snow depth. The results of sensitivity analysis indicate that road surface evaporation and the anthropogenic heat are very important in road surface temperature forecast. The forecast results of BJ-ROME can be used as an important reference to take measures by traffic administration department and road administration department.In the near future, BJ-ROME will be coupled in double direction with BJ-RUC to improve the forecast. The anthropogenic heat (AH) should be parameterized more precisely, and the variational assimilation algorithm should be used to assimilate the RST observations. The predicting performance of water depth of BJ-ROME should also be validated.

Response of Rise and Fall in Hulun Lake Wetland to Meteorological and Hydrological Factor Change

Gao Yonggang, Zhao Huiying, Li Chong, Song Weishi, Meng Jun

2012, 23(4): 459-466.

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Abstract:
With meteorological and hydrological data from 1961 to 2005 in Hulun Lake Wetland, based on gray correlation analysis, Mann-Kendall test, regressive statistics, and wavelet analysis methods, the response models of rise and fall in Hulun Lake Wetland are established by meteorological and hydrological factors. The response characteristics of rise and fall are analyzed under meteorological and hydrological factors change in Hulun Lake Wetland, and references are provided for Hunlun Lake Wetland protecting, resuming and utilizing.Year and summer climate are major roles for the wetland rise and fall, but the water budget of the wetland rise and fall could be reflected completely by year climate. With annual evaporation and runoff fixed, water area and water level depth will respectively increase by 2.6 km² and 1.6 cm if yearly precipitation increases by 10 mm. Runoff increase of 1×10⁸m³ will lead the water area and water level depth to increase by 4.8 km² and 3.0 cm respectively, given the evaporation and precipitation unchanged.The coincidence response characteristics are prominent between the wetland rise and fall and the continuous change process of the influential factors from 1961 to 2005.The response is more prominent after the 1990s in particular, when the water resource shortness of the wetland become serious, because the hydrologic environment of the wetland is worsening and the wetland atrophy is faster. The impacting rates of air temperature and precipitation changes on the rise and fall of water area and water level depth are respectively 33.1% and 66.9%, 22.5% and 77.5%, and the precipitation change are dominant.The response of multi-time scale periodic characteristics is prominent between the wetland rise and fall and the influential factors. Their chief periods are all 27 years on the inter-decadal timescale. At the same time, the secondary period (11—13 years) on the inter-decadal timescale and the quasi-period (5—10 years) on the inter-annual timescale are the same for annual precipitation and annual runoff, the secondary period (14—16 years) and the quasi-period (2—7 years) are the same too for annual average air temperature and annual evaporation. Under the overlying action with the chief period and the secondary period, the wetland rise and fall is displayed the period processes of twice increasings and once decreasing from 1961 to 2005. The short period response about quasi-period is prominent between the wetland rise and fall and the influential factors, the wetland rise and fall take on the wave characteristics from 1985 to 2000 in Hulun Lake Wetland.

Wind Retrieval Simulation in Tropical Cyclone for FY-3 Dual-Frequency WFR

Dou Fangli, Lu Naimeng, Gu Songyan

2012, 23(4): 467-477.

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Abstract:
Tropical cyclone is one of the primary disastrous synoptic systems in China. With the continuous observation, global coverage and the ability to penetrate through precipitation layer, microwave sensors on polar orbit satellites can provide more precise observations of the tropical cyclone location and intensity for marine extreme weather forecasting, which will compensate for the shortage of conventional observations. The FY-3 satellite microwave scatterometer, named Wind Field Radar (WFR), is the only way to measure ocean vector winds (OVW).Compared with single-frequency scatterometer, dual-frequency scatterometer has advantages in higher spatial resolution and better response to winds in extreme weather conditions, where high winds are usually associated with high rain rates. The Jet Propulsion Laboratory (JPL) has developed a conceptual design for a Dual Frequency Scatterometer (DFS) in the Extended Ocean Vector Winds Mission put forward in 2007. The concept of Rotating Fan Beam Scatterometer which will be extended to dual-frequency mode has been studied under ESA. The WFR installed on FY-3 satellite which will be launched in 2016 is designed to use the dual-frequency and dual-polarization time-sharing observation pattern.Rain perturbations result from volume scattering and attenuation by precipitation in the atmosphere, as well as changes of sea surface roughness by impinging rain drops. Few studies which investigate effects of rain on Ku-band and C-band scatterometer data indicate that the impact of rain to sea surface is a complex phenomenon not yet fully understood, and it's not dominating in high wind and strong rain fall areas. Therefore, changes in surface roughness are not considered here.The purpose of this study is to investigate the potential of the WFR proposed to fly aboard FY-3 satellite to measure OVW in rain fall areas of Typhoon Ivan. A theoretical model based on radiation transfer equation including rain attenuation and scattering, has been developed to quantify the modification by rain of the measured backscatter. Based on the simulated normalize radar cross section (NRCS) dataset generated by forward model, the impact on the retrieved winds has been analyzed, and the dual-frequency retrieval algorithm has been firstly studied. Analysis shows that changes of contaminated NRCS briefly depend on relative size of volume scattering item and attenuation item. Wind vectors measured at Ku-band can be more severely altered by rain than those at C-band. Precipitation in tropical cyclone can significantly degrade the OVW accuracy. Retrieval results show that new methods combined Ku-band and C-band have higher spatial resolution than C-band retrieval, and better performance in rain fall region than Ku-band retrieval. Especially, partitioned wind retrieval technique can significantly reduce the rainfall error, is an effective way to improve the wind retrieval accuracy in tropical cyclone with the synchronous observation by microwave humidity sounder (MWHS) aboard FY-3 satellite.

Statistical Characterization for L and X-Band Meteorological Satellite Channel

Zhang Xiuzai, Guo Yecai, Chen Jinli, Yang Changjun

2012, 23(4): 478-484.

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Abstract:
Communication's environment of the meteorological satellite is the physical layer of the atmosphere. In different weather conditions, there are varying degrees of attenuation, shadowing and multipath effects in meteorological satellite channels due to the impact of cloud and rainfall, which makes the receiving signals become unstable concerning to the atmosphere state, and result in inter-symbol interference, affecting the quality of receiving meteorological satellite data, prediction of weather phenomena.In order to study the influence of the physical layer of atmospheric space on meteorological satellite communications, the weather conditions affecting meteorological satellite transmission are classified into three cases: Clear sky, cloudy and rainy weather. There are thick clouds, covering clouds and rainfall in the rainy weather, which take the whole shadow block above the ground station receiving signals of the meteorological satellite communications. In this case, the signals received by ground station are only composed of multipath scattering signals without the line of sight, and the envelope probability density function (PDF) obeys the statistical characterizations of Rayleigh. There are thick clouds and covering clouds in the cloudy weather, which form part of the shadow block over a certain range spread above the ground station. In this case, the signals received by ground station may have two situations. One circumstance, the received signals are composed of the line of sight and a certain intensity of multipath scattering signals that are diffracted, refracted and scattered, and PDF obeys the statistical characterizations of Rice. The other circumstance, the received signals are composed of the line of sight obscured by clouds, and PDF obeys the statistical characterizations of Lognormal. There are a few thin clouds in the clear sky and good visibility in the high atmosphere layer, when the signals received by ground station are composed of very weak multipath scattering signals and the line of sight, and PDF obeys the statistical characterizations of Gauss.According to theoretical analysis, the simulation models of Rayleigh, Rice, Lognormal and Gauss probability distribution are established. Through the computer calculation, the results of the simulation models show that the signals received by ground station with different composition lead to different statistical characterizations because meteorological satellite signal pass through different physical state of the atmosphere. The multipath scattering signals both exist in the Rice channel and the Rayleigh channel, however, the line of sight only exists in the Rice channel. Gauss channel model and the Rice channel model have the same structure, but the received signals in both channels have different intensity of the multipath scattering components. That explains the cause why the variety of the received signals envelope brings out different statistical characterizations in different channels. The probability density curve of the simulation model and the theoretical model match quite well, verifying the correctness and validity of the theoretical analysis, providing a theoretical guidance to calculate the data error rate of the meteorological satellite communication.

Ocean Surface Non-cyclone Wind Block Ambiguity Removal Algorithm for Scatterometer

An Dawei, Gu Songyan, Yang Zhongdong, Lu Feng

2012, 23(4): 485-492.

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Abstract:
The Maximum Likelihood Estimation (MLE) algorithm for scatterometer wind vector retrieval generates several wind vector ambiguities, so a circle median filter is needed to perform the ambiguity removal. But the traditional circle median filtering method can hardly solve the block ambiguity problem. According to the spatial distribution characteristic of the most likely ambiguities in each non-cyclone wind vector cell, a new enhanced circle median filtering method for block ambiguity removal is derived and discussed theoretically, with experiments carried out to check its adaptability. This method features simple definition, low computation and easiness to converge. Using some L2 raw data from EUMETSAT to validate the method, the results indicate that under non-cyclone wind distribution condition the new method is effective in resolving the problem of block ambiguity after eliminating the cyclone wind field with other reference data.The core of the enhanced circular median filter algorithm is to initialize the non-cyclone characteristics first, which can effectively solve the problem of massive fuzzy. Thus the data which may cause circular median filtering failure and the data that affect the neighborhood will be corrected. Then divide the two-dimensional space into M rows and N columns, then calculate the wind field in the open window. The wind vector in the center of a window is solved by selecting an alternative from corresponding fuzzy solutions. And then do this with the next location iteratively, until the wind field does not change or until the times of iterations reaches a preset maximum number. Finally, defective value in the wind field is smoothed.Compared with traditional circle median filtering method, this approach is better in several ways. First, by initializing the first wind field, the fuzziness of the second wind field is reduced; while with the traditional method the block fuzzy cannot be removed. Second, the calculation process is simple and need no statistical circular histogram, nor do they need to calculate the mean. Third, the definition of circular median is only one rather than getting multiple solutions. And last, the calculation will not be interrupted by the narrow wind element values in the boundary region.The enhanced circular median filtering method based on non-cyclone wind field vector distribution characteristics can overcome the harsh conditions of the traditional method (such as wind field must be randomly distributed, non-block fuzzy), extracting the true wind vector solutions to overcome the fuzzy block. In order to apply this method widely, a crucial issue is to determine and eliminate the coverage typhoon cloud by operational satellite equipment properly. The method provides a new idea for exacting data of non-cyclone wind field on the ocean surface.

Full Spectrum Inversion of Mountain-based GPS Occultation Data

Fan Lei, Sun Shouxun, Wang Yeying, Yu Jianglong, Li Bo

2012, 23(4): 493-499.

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Abstract:
Mountain-based GPS radio occultation sounding is an economic novel technique for monitoring temporal and spatial variations of regional lower atmospheric environments. The GPS receiver, which is on a mountain top with a downward looking perspective toward the earth's limb, can track any GPS satellite as it sets or rises behind the earth's limb, therefore it collects data at both negative and positive elevations to the receiver's local horizon. By combining both the negative and the positive elevations data, a high-resolution profile of refractivity below the height of the receiver and in some case extending to 1—2 km above the receiver can be obtained.An experiment of the mountain-based GPS radio occultation is conducted on the top of Mt Wuling during 1—29 August 2005. During the experiment, the radiosonde observation is taken at Xinglong County, which is about 30 km away to the southeast of the Mt Wuling. Mountain-based occultation data and radiosonde data are successfully obtained. The traditional Geometric OPtics (GOP) inversion method are described in details, and refractivity profiles below the GPS receiver are obtained from occultation data by this method. A novel Full Spectrum Inversion (FSI) is also applied for mountain-based occultation data, which can deal with the multi-path effects frequently occurring in the lower atmosphere, while the traditional method usually becomes inefficient here. FSI provides a simple and efficient tool for deriving the instantaneous frequencies of a signal composed of several narrow banded sub-signals. When certain criteria are fulfilled, FSI is capable of resolving the frequency variation of each signal component. Since FSI is based on the Fourier transform of the entire signal, the problem that different signals get to the GPS receiver at the same time in multi-path regions is thus solved. Refractivity profiles below the receiver are also obtained by FSI. The comparisons of the refractivity obtained from the same GPS radio occultation events indicate that with FSI method, the precision has been improved by 2%, and the standard deviation is less than 3%. The refractivity differences between FSI and radiosonde are also achieved in a time window of 1.5 h and azimuth angle window of 40°, the comparison results show that the mean refractivity relative deviation is-8.15% with a standard deviation of 1.4%. The results suggest that the FSI is an efficient inversion scheme for mountain-based GPS radio occultation data. In the future, further research will be carried out on how to improve the inversion precision, and use this method in space GPS radio occultation data.

Regression Models of Winter Wheat Yield Components in Zhengzhou Area

Qian Jinxia, Guo Jianpin

2012, 23(4): 500-504.

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Abstract:
Winter wheat yield prediction plays an important role in ordinary meteorological service and its accuracy is associated with forecasting techniques. It's one of main techniques analyzing relationship of yield components to meteorological elements and then establishing regression model about yield components.Winter wheat yield in Zhengzhou Area is studied based on the date of winter wheat growth stage, 1000-grain weight, spike grain number and the daily meteorological data at Zhengzhou Agro-meteorological Experimental Station from 1980 to 2006. By calculating the impact indexes of light, temperature and precipitation, the relationship between these indexes and 1000-grain weight, spike grain number are analyzed, and key factors are selected by analyzing the change of correlation coefficient to establish the prediction model of 1000-grain weight and spike grain number. The result indicates that 1000-grain weight shows a positive relationship and a negative relationship with the average temperature from the 15th day to 19th day and from the 29th day to 33rd day during the heading stage, respectively. At the same time, the average sunlight hour from the 9th day to 13th day shows a negative effect. To gain a high 1000-grain weight, the mean temperature is supposed to be between 18.8℃ and 22.8℃, and the highest temperature doesn't exceed 29.5℃. The average temperature from the 21st day to 25th day during the reviving stage shows a positive effect obviously for spike grain number. By analyzing the relationship between yield components and climatic elements respectively, the estimate of 1000-grain weight and spike grain number are possible in advance.

Reconsideration About Effects of Natural Environmental Conditions on the Calculation of Solar Energy Resources in China

Wang Bingzhong, Shen Yanbo

2012, 23(4): 505-512.

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Abstract:

In the calculation of solar energy resources, Ångström method, by means of the sunshine percentage is most commonly used. But it should be noted that this method is suitable for regions where the natural environment is relatively homogeneous. Such conditions are easy to get to meet for small countries or regions. However, for the large countries, e.g., China, it is inappropriate to make an assumption that the natural environment is homogeneous. On one hand, the impact of the terrain on radiation can't be ignored; on the other hand, other atmospheric factors that affect the radiation incident, such as the layer of atmospheric water content, content of aerosol particles and so on, also can't be ignored. Former Soviet Union is also a vast country, and its scholars exclude regions where the altitude is higher than 1500 m when they are dealing with the terrain effect. This approach doesn't resolve the radiation distribution in the mountain area, but from a scientific perspective, it is prudent and rigorous.In the previous researches, the selection of the basic radiation, such as extraterrestrial radiation, clear radiation has been paid more attentions to. This is very important for the Ångström method; however, it doesn't consider the former facts influencing solar radiation enough.The clear radiation close to the actual conditions is calculated based on SMARTS model from Gueymard. In SMARTS model, each factor influencing solar radiation is considered, such as the whole layer water content deduced by surface water content, the aerosol effect considered by horizontal visibility, and the ozone effect derived by ozone content distribution. The clear radiation calculated in this way can be used as basic radiation, which helps overcome the lack of previous work ignoring the atmospheric impact factor. Owing to lack of observation data, the clear radiation in the whole country is not calculated. Only the clear radiation at Chengdu Station and Emei Mount Station are calculated. With the previous method, the results are much the same for two stations, but with the new method, the differences are quite obvious.

Vol.23, NO.4, 2012