应用气象学报

A Numerical Study on Effects of the Soil Moisture upon the Regional Short-term Climate

Li Qiaoping, Ding Yihui, Dong Wenjie

2007, 18(1): 1-11.

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Abstract:
The feedback process between soil and atmosphere is very important in the climate system. Most researches on the interaction depend on the model outputs due to the scarce observation dataset of the soil moisture. Heavy flood event occurs in the mid-lower Yangtze basins in 1998. In winter and spring of that year, the soil moisture in Yangtze-Huaihe River basins is significantly wet. The concerned question is whether the abnormal soil moisture in prophase could provide valuable signal to summer rainfall. By using the improved regional climate model (RegCM_NCC), modeling study is undertaken to investigate the effects of the initial abnormal soil moisture in spring in Yangtze-Huaihe River basins (27°—35°N, 110°—120°E) on the regional climate. Based on the control run with climate values of soil moisture, two sensitivity experiments are carried out with "wet" and "dry" soil in the initial integration, respectively. Results indicate that varying the initial soil moisture in the key region shows significant influence on regional rainfall, wet soil results in the increase of local rainfall, humidity and evaporation near the ground, but the surface air temperature decreases rapidly, with the maximum value of 1.5 ℃. The experiment results of dry soil are converse to that of the wet soil. The regional climate response mainly depends on the changing surface radiation equivalent and land-atmosphere energy flux. The influences of initial abnormal soil moisture are remarkable in the first month, which can persist for several months, but the intensity deceases gradually. The change of air temperature is more remarkable and can persist longer time than the rainfall. In the mean time, the regional climate response to the abnormal soil moisture isn't confined in the key region, but also affects the rainfall and temperature in other regions due to the sub-circulation and large-scale advection. It can also cause the air temperature and humidity change at upper level through turbulent transfer. Thus, soil moisture has obvious effects on the regional-scale climate, but most of the present models can not describe the real-time variation of soil moisture due to the reason that only the climate virtual value of the soil moisture are used, which may be a factor of the modeling bias of the rainfall. The role of soil moisture would be attached much attention in the regional short-term climate research and prediction.

Snow Cover Identification with SSM/I Data in China

Li Xiaojing, Liu Yujie, Zhu Xiaoxiang, Zheng Zhaojun, Chen Aijun

2007, 18(1): 12-20.

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Abstract:
Snow piling and melting dynamically couple with the hydrological and climatological. It's very important to obtain the accurate snow parameters in time for the study of hydrology and climatology. It is also useful in mitigating or preventing from snow disaster. Microwave remote sensing has the advantage to penetrate the cloud, which is superior to the optical sensor data. With the microwave data from the satellite, it could better identify the snow, even those covered by cloud, or with the higher solar zenith angle. Not only snow cover, but also snow depth and snow water equivalent (SWE) could be obtained. SSM/I is a 7-channel 4-frequency radiometer whose frequencies are 19.35, 22.235, 37.0 and 85.5 GHz respectively. All frequencies are received in dual polarization (V and H) except 22 GHz (V). So, it is very suitable for snow parameters' retrieval. Since 1987, SSM/I data nearly 20 years are collected. Meanwhile, several global snow identifying methods are developed with these data. But it is found that the snow cover is overestimated, when compared with the operational visible-infrared products of the China National Satellite Meteorological Center (NSMC), especially over the Qinghai-Xizang Plateau. Similar conclusion is drawn by Basist. Therefore, it is necessary to develop an advanced method to improve the accuracy of snow cover identification. Using SSM/I data, an improved method is proposed to identify the snow cover in China and its adjacent areas (17°—57°N, 65°—145°E).Snow, rain, cold desert and frozen ground are the scattering matters, and have the similar signatures in the microwave band. That means either the maximum one of (T_B22V-T_B85V) and (T_B19V-T_B37V), or both of them are not less than 5 K. With the snow measurements from the ground observation stations in China and the operational snow cover products of the NSMC, the cause of overestimating snow cover of the old global methods is analyzed, and the conclusion is drawn that parts of the scattering matters, especially the frozen ground, are incorrectly identified as snow, which is the main cause of snow cover overestimated in Qinghai-Xizang Plateau and Mongolia with SSM/I data. Based on the foreign global snow identifying methods and the statistic analysis around six years' measurements from the observation stations in Inner Mongolia about snow, rain and frozen soil matching with SSM/I data, an advanced method depending on five identifying parameters is put forward: a new parameter, (T_B22V-T_B85V)-(T_B19V-T_B37V), is added, which can effectively reduce the influence of the frozen ground and the other scattering matters in snow identification. Finally, the results obtained using the improved method are validated with snow measurements from ground observation stations and operational snow cover products of NSMC in two aspects: one is the temporal variation, the other is the spatial distribution of snow cover. It is found that the improved method is more accurate in snow identifying than the existing global methods.

Preprocessing Precipitation in Objective Quantitative Forecast

Zhao Shengrong, Pei Haiying

2007, 18(1): 21-28.

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Abstract:
Quantitative precipitation forecast is much difficult in objective forecast. It is because that the precipitation is not a continuous variable, either not a normal distribution variable. Except that the precipitation has an obviously different characteristic to other elements that zero precipitation include many different situations. For example, black clouds blot out the sky but precipitation amount is zero. There are not many clouds on the sky and precipitation amount is also zero. These two situations are different, but the difference is not exhibited in the amount of precipitation. So the effectiveness of the forecast model is affected.Preprocessing has important effects on forecast results. For the purpose of improving quantitative precipitation forecast effectiveness, a reasonable preprocessing scheme needs to be developed. Precipitation observation is preprocessed using relative humidity before modeling. The amount of precipitation is changed to different negative values when relative humidity is smaller than usual and precipitation amount equals zero. Based on the summer data of 2003 and 2004, forecast model of preprocessing precipitation and direct precipitation are developed respectively.BP network method is used in the study, which is a kind of artificial neural network. The BP network is a back propagation network. It contains input layer, implicit layer and output layer. There can be one or more implicit layers on the BP network. Joint on the implicit layer is named implicit node. Input signals propagate to implicit nodes. Then signals of implicit nodes propagate to the next layer after the disposal of weights and operating functions. At last, the value on the output nodes is gotten. BP networks can be considered as a nonlinear projection from input layer to output layer. Sigmond function is always taken as the operating function. Network weights of different nodes are obtained by training. The BP algorithm is used in training process.Predictors are corresponding to precipitation amount forecast of operational global model of China National Meteorological Center, operational models of German Meteorological Administration and Japan Meteo rological Agency. In order to avoid errors caused by interpolating, to every model, precipitation forecast on four grids around the stations are used as predictors instead of interpolated model precipitation forecasts at stations.Test of different forecast results during 2005 summer of Beijing and other 5 stations indicate that precipitation forecasts are improved after preprocessing. Especially absent forecasts are reduced. So it can be concluded that this reprocessing method is simple and effective. But it only provides some ideas. In real settings, different element also can be used to reprocess precipitation amount according to different areas or different seasons in a year.

Meso-scale and Small-scale Circulation Characteristics Analysis of "0185" Heavy Rainfall in Shanghai

Chen Yonglin, Yang Yinming, Cao Xiaogang, Wang Zhi

2007, 18(1): 29-35.

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Abstract:
Heavy rainfall happens on August 5, 2001 in Shanghai, especially in the central urban area with 75.4 mm of 1-hour maximum precipitation and 264 mm of 12-hour cumulated precipitation at Xujiahui. Analyses on the conventional observations, automatic stations data, GMS-5 satellite cloud images and Doppler radar data show that it is closely related with a mesoscale convective compound (MCC) in Shanghai. The number 9 tropical depression moves slowly right to the north of Shanghai because of the adjustment of the long-wave system in the middle latitude and suddenly weakening of the northwest Pacific subtropical high. The MCC is triggered in the shear line between southwesterly current in the southeast of the tropical depression and the strong southeasterly current in Shanghai and Zhejiang, when the tropical depression moves to the north of the Taihu Lake.A slowly eastward-moving circle zone without any echo is found near Wuxi at the night of August 5 in the base reflectivity products of Doppler radar. Some spiral echo bands rotate counterclockwise around the circle zone in the east. The spiral echo bands in the southeast moves to Shanghai under the effect of the shear line and the converge counterclockwise to form a strong echo group of several coordinate meso-β-scale echoes in the central urban zone of Shanghai. This strong echo group in radar products is correspondent to the mesoscale convective cloud above Shanghai, which forms in the northwest of Shanghai and Suzhou initially, and begins to weaken soon after its formation. Subsequently the mesoscale convective cloud develops strongly to be a typical mesoscale convective compound (MCC) about 102600 km² in the northwest of Shanghai. Temperature of the cloud top bright core reaches -81 ℃, and the convective clouds with top temperature of -63 ℃ cover the whole of Shanghai.A couple of positive and negative speed centers about -20 and 20 m/s respectively are located in the central urban zone of Shanghai in the radial speed products of Doppler radar. The circle zone without echo in Wuxi is corresponding to the surface convergent center in the stream field, which shows that the circle zone is just the tropical low center. The strong echo group rotating counterclockwise in the central urban zone in Shanghai and the couple of speed centers are also corresponding to another meso-β-scale surface convergent center in Shanghai, which is a meso-β-scale convergent system, named mesoscale cyclone, and is located in the core of the MCC.The meso-β-scale cyclone circulation, as well as the process of its formation and development in the core of the MCC leading to the heavy rainfall at August 5, 2001 in Shanghai, is testified through analyses of automatic weather stations data and Doppler radar data.

Distribution of Carbon Monoxide from MOPITT of 2000—2004 and Comparisons with Surface Measurements in Mt. Waliguan Station

Zhao Chunsheng, Fang Yuanyuan, Yang Jie, Zheng Xiangdong, Zhou Guangqiang

2007, 18(1): 36-41.

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Abstract:
Carbon monoxide (CO) is one of the main pollutants produced by incomplete combustion processes, such as the burning of fossil fuels in urban and industrial areas as well as by biofuel and biomass burning. CO has long been recognized for its critical role in tropospheric chemistry. Coupled with a one to three month lifetime, the wide variety and seasonal variation of sources makes CO an excellent tracer of atmospheric motions. Surface CO measurements which have generally been limited to surface or boundary layer measurements often substantially impacted by local pollution can not provided a global synoptic view of CO on a daily basis. To better understand global CO cycles, observations from the space are necessary. The Measurements of Pollution in the Troposphere (MOPITT) instrument, funded by the Canadian Space Agency and manufactured by COM DEV of Cambridge, Ontario, is launched onboard the NASA Earth Observing System (EOS) Terra satellite in December, 1999. MOPITT offers the first daily, global synoptic observations of CO since March 2000. The MOPITT data set contains CO total column amount, CO mixing ratios at six altitudes (850 hPa, 700 hPa, 500 hPa, 350 hPa, 250 hPa, 150 hPa) and the corresponding location and time along the track. Here Level 2, Version 3 MOPITT CO data are used which includes the latitude and longitude of MOPITT at nadir, the corresponding CO column and six layers of CO mixing ratio. The data are aggregated and interpolated to achieve grid data with a resolution of 1°×1° at 3-day average. Distribution properties and trend of CO from MOPITT of March 2000 to May 2004 are analyzed and comparisons with CMDL/NOAA surface CO measurements in Mt. Waliguan station are made. The results show that there is a large variation for global CO distribution. On the average, CO in Northern hemisphere is higher than that in Southern hemisphere. CO peak centers are located in East Asia, West Europe and North America in Northern hemisphere while in Middle West Africa and tropic regions of South America in Southern hemisphere. There is a significant seasonal variation for CO with a peak concentration in spring time in Northern hemisphere and in Autumn in Southern hemisphere. CO concentrations are high over coast regions of China and Japan all along a year. CO at Sichuan Basin which is located in the east of Qinghai-Tibet Plateau are higher than that of its surrounding regions. Trends analysis of Beijing and Mt. Waliguan suggests that CO concentrations over these two regions increase during 2000—2004. Comparisons with CMDL/NOAA surface CO measurement in Mt. Waliguan shows that the variations of these two datasets agree well and there is a significant correlation between MOPITT CO column and CMDL surface measurements. The increasing trend for CO during 2000—2004 obtained from these two datasets is at magnitude of a few thousandth.

Radar Data Analysis and Numerical Simulation of a Squall Line

Mu Xiyu, Dang Renqing, Chen Qiuping, Fang Dexian, Ge Wenzhong

2007, 18(1): 42-49.

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Abstract:
A squall line attacks Jiangxi, Zhejiang and Fujian provinces on April, 12 2003. The severe convective system leads to heavy damage, producing wind gust as large as 32 m/s, 30 mm diameter large hails in the affected area. This process is observed by the CINRAD in Jianyang City. The whole process of this convective system, including the initial stage, the developing stage, the mature stage and the dissipating stage, is analyzed. Many convective structures are observed in the process. At the location of the strong updraft, base reflectivity observed by mid-level elevation is stronger than the low-level and the high-level. This structure is called weak echo area. Corresponding to weak echo, there is a positive velocity area within a large negative velocity area in the mid-level. This structure is judged as mesoscale vortex sometimes by WSR-98D PUP. In fact, this phenomenon means the convergence in the target area, which is known as MARC (mid-level radial convergence). Strong line echoes are observed in squall line by radar. It is well known that bow echoes that develop within a squall line are referred to as line echo wave patterns. In order to analyze the interior structure of the convective system, mesoscale numerical model (MM5V3) is used to simulate this process. In the control numerical simulation, reanalysis NCEP data (1°×1°) is used as the initial conditions. Radar observation data and simulation result are used to analyze the structure and the evolvement. Numerical simulation result confirms that the convective system takes place at the meeting of the northeaster and the southwester. There are many convective vortices and super cells within the 400 km system. Bow echoes appear near the vortices. There are obvious cyclones at the forehead of the line echo and divergence at the tail. Weisman indicates that in a mature bow echo cyclonic and anticyclonic vortices develop north and south respectively, in the channel of rear-to-front flow. These results resemble Weisman's study about squall lines in American. In the period from 0758UTC to 0840UTC, there are respective bow echoes in the north and west, seen from the reflectivity PPI. These results confirm that there are segments of mesoscale vortices and bow echoes in squall lines. Jianyang city is to the east of Wuyi Mountain, and there is a bell-mouthed mountain to the west of Jianyang city, so the influence of the topography should be considered. In order to do that, a sensitive numerical simulation is designed. In the sensitive numerical simulation, the altitude of the mountains near Jianyang is factitiously set equal to the ground. The results of the control and sensitive numerical simulation are compared. It shows that the uplifted velocity in 500 hPa level of control simulation is greater than the velocity of sensitive simulation, and the uplifted area accords with the strong reflectivity echo. These results refer that the windward mountain and bell-mouthed mountain can trigger new convective cells or enhance existing ones.

The Retrieval and Variational Assimilation Experiment on Wind Profile of Doppler Radar

Li Huahong, Xue Jishan, Wang Man, Yang Ming

2007, 18(1): 50-57.

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Abstract:
The information of wind fields is an important parameter in weather forecast. At present, a great deal of information from CINRAD is not really applied to numerical weather prediction (NWP), domestic studies on the application of wind information from Doppler radar in NWP are not many. In order to apply observational wind data of Doppler radar to numerical weather prediction models better, wind profiles are retrieved from single Doppler radar observation by using VAD method (Velocity Azimuth Display), the VAD method has developed for thirty years and now becomes mature. Further more the wind profile data are processed as the normal form of radiosonde observation to make up the deficiency of radiosonde observation in space-time resolution. And then the retrieved wind data are assimilated by the 3D-VAR system of GRAPES-Meso (Global and Regional Assimilation and PrEdiction System-Meso) model. With different initial fields a torrential rain occurring in the Changjiang River valley from 1200UTC on June 27, 2005 to 1200UTC on June 28, 2005 is numerically simulated and comparatively analyzed. The results indicate that the retrieved wind profile corresponds to the radiosonde observation profile very closely and accurately reflects the status and location of the wind shears at low levels. The retrieved wind profile not only has higher space-time resolution but also remedies the lack of radiosonde observation in this case. It reflects that using VAD method to retrieve wind profile from single Doppler radar is viable and of advantages. With the assimilation of the retrieved wind profile the initial wind fields of numerical experiments are remarkably improved. In the improved initial wind fields vapor transportation becomes stronger where the observed torrential rain occurs. Moreover, the intensity and areas of forecasted precipitation are improved in some ways. As a whole the amendment of the 6-hour precipitation forecast is better than that of the 24-hour. With assimilated initial fields the numerical experiments successfully simulate a rainfall center of 10—25 mm in the 6-hour forecast, which indicates that using VAD method to retrieve wind profile from single Doppler radar has a prospective future in NWP once more. On the other hand, the contrast analysis of the numerical results indicates that a difference of assimilation frequency and assimilation time window will have a different influence on a short time torrential rain forecast in the numerical assimilation experiments. In this case, the 6-hour rain forecast by assimilating the retrieved wind profile once an hour is better than the forecasts once three hours or once six hours; the 6-hour rain forecast by assimilating the retrieved wind profile for three hours is better than the forecast assimilating for six hours. That is to say, a higher assimilation frequency of retrieved wind data will lead to a better forecast, but a longer assimilation time not always leads to a better simulated result in NWP, which has a valuable guidance effect on the assimilation operation of wind data from Doppler radar.

Binomial Climate Prediction Model of Tropical Cyclone Frequency

Liang Jian, Lin Yongtang, Xie Dingsheng

2007, 18(1): 58-63.

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Abstract:
There are about 28 tropical cyclones per year over the Northwest Pacific Ocean and 7 tropical cyclones per year which make landfall or have significant impacts on China. Tropical cyclones lead to huge damages and economic losses. Accurate prediction of the tropical cyclone (TC) in advance would be a powerful tool in disaster preparedness and prevention.Based on the monthly mean 500 hPa geopotential height in Northern Hemisphere during 1951—2005 of 10°×10° grid points between 10°N and 85°N, monthly mean sea-surface temperature (SST) of the Pacific Ocean during 1949—2005 of 5°×5° grid points between 10°S and 50°N, monthly and annual numbers of TC over the Northwest Pacific Ocean, South China Sea, and TCs make landfalls or generate significant impacts on China or Guangdong Province during 1951—2005, the correlation coefficients between them are calculated. By analyzing the spatial distribution of the correlation coefficients between the TC frequency and the 500 hPa geopotential height and SST respectively, several key regions exist (over significant level of 5%). Those highly significant region's geographical locations are stable. The synoptic climatologically significance and the physical characteristics are investigated. Several high correlated factors are selected and combined and used to construct the binomial prediction equations to predict the TC's monthly and annual frequency in the Northwest Pacific Ocean, South China Sea, and landing ones in China and Guangdong Province respectively.The operational binomial climate prediction system is constructed and verified using the last 11-year data. The accuracy of the TC annual frequency predicted using the 500 hPa and SST is 79.6% and 77.3% respectively. The accuracy is 75.8% by using the preceding 500 hPa in November to predict the TC monthly frequency that lands in China from July to September during last 11 years.The results are as follows: Select the same sign factors to combine in the same group when two to four predictors are chosen. By analyzing high significance regions, it is found that the combined predictor method is preferable to the single sea analysis. The weighted regression ensemble analysis is superior to the general statistical methods, such as the stepwise regression analysis of principle components and empirical orthogonal function. The method has the statistical and physical significance. The prediction of the multinomial prediction equation is better than the linear prediction equation. In practice the binomial prediction equation is the best. The Binomial Climate Prediction Model can be used for many purposes and performs well.

Contrast Analysis of 4 Heavy Snow Events in Yunnan Since 2000

Zhang Tengfei, Lu Yabin, Zhang Jie, Lu Guiming, Zhou Guolian

2007, 18(1): 64-72.

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Abstract:
By using upper air observational data, surface observational data, MICAPS 1°×1° objective analysis data and Kunming Doppler Radar echo data, 4 heavy snow processes in Yunnan since 2000 are contrastively analyzed and diagnosed. The results show that in the 4 heavy snow processes in Yunnan, 2 processes aroused by the transversal trough pattern affect the western area of Ailao Mountain, the snow area is large with the strong intensity, the other 2 processes aroused by the north ridge and south trough pattern mainly affect the middle and the east Yunnan. 3 heavy snow processes accompany the southern trough. Warm and moist airflow in front of the south trough converges with the strong cold air, producing heavy snow. This is the major heavy snow pattern of Yunnan. But only one without south trough in 2005 is very special. Before and during the 4 heavy snow processes, the water vapour flux increases quickly. Most areas in Yunnan are in the big value band of the east—west water vapour flux, which indicates that plenty water vapour transports to Yunnan from the west whether the south trough exists or not. In 3 processes with active south trough, the water vapour comes from the southwest Bengal Bay and this is the main heavy snow pattern in Yunnan. In the heavy snow process without the south trough influencing, the water vapour comes to Yunnan from the west of Arabian sea along the India high pressure. The strong water vapour flux convergence belt and area can better indicate the heavy snow processes of Yunnan. The heavy snow always appears in the area of strong water vapour convergence and closes to the side of strong cold air in the east of the convergence center. Because the updraft and the downdraft in the south and the north coexist, the mesoscale vertical circulation near the front area may establish. When the cold air from the north moves to the south, the θ_se sharpness area also moves to the south, and then causes the water vapour brought by the south warm moist airflow to condense and the unstable energy to release, providing enough uplift condensation condition for the heavy snow. In the Doppler radar echo intensity field, the sheet cloud echo arouses the heavy snow, the intensity of sheet cloud echo is about 20 to 30 dBz. In the velocity field, the cold and warm advection's alternative change and the stronger southwest jet stream of the upper air in the middle and lower layer of troposphere are the major macro-scale characters of the heavy snow with the south trough in 2004. The partial east jet stream in the lower layer is the major macro-scale character of the heavy snow without the south trough in 2005. So the high and low level jet stream are the key to the heavy snow processes.

Features of Annual Temperature and Precipitation Variety with the Effects on NPP in Chongqing

Li Yonghua, Gao Yanghua, Han Fengqing, Xiang Ming, Tang Yunhui, He Yongkun

2007, 18(1): 73-79.

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Abstract:
In terms of mean temperature and precipitation data of 34 observation stations in Chongqing from 1959 to 2001 the NPP is calculated using Leith's famous Thornthwaite Memoriae model. Based on EOF and MHF wavelet the variations of temperature, precipitation, NPP and their relations are analyzed. The effects of temperature and precipitation on NPP are discussed and the prediction results in the future and possible increase extent of the NPP in Chongqing are discovered. Results show that there are consistent distributing characters among annual temperature, precipitation and NPP in Chongqing. The variation tendency of annual temperature is descend, that of precipitation is unconspicuous, and that of NPP is descend slightly during the whole times. But they all have their own obvious staggered change features, and the variation tendency of precipitation and NPP are approximate. The variation tendency of climatic productivity strongly resembles that of rainfall in multi-time scales, but it is not consanguineous to that of temperature in less than 10 years scales. The quasi-period of temperature series is main about 10 years and those of precipitation and NPP series is main about 2 years from 1959 to 2003. The interannual oscillations of precipitation and NPP are strong, and interdecadal oscillations of temperature are strong. The increase or decrease of mean temperature or precipitation result in corresponding change of the NPP. The NPP increase by degrees while only temperature or precipitation raises. The warm-wet type climate is most advantageous for NPP but cold-dry type one is most disadvantageous. The NPP will increase 11.2% or so controlled by the former climate type but lessen 12.5% or so controlled by the latter climate type. Correlative climate prediction results show that the temperature and precipitation will both increase little and so the increase scope of the NPP in Chongqing will be 0.3%—3.9% in 2010, but 6.7%—10.0% in 2030 because the temperature and precipitation will both increase much, and 4.4%—8.4% in 2050 while the temperature will continue increase but the precipitation will increase indistinctively. In the coming 50 years the climatic variation tendency will be propitious to increase of NPP and the NPP will be top in 2030 or so.

The Causes of Catastrophic Gales in Zhoushan Islands with Their Forecasting

Yang Zhong'en, Chen Shuqin, Huang Hui

2007, 18(1): 80-85.

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Abstract:
The Zhoushan islands see strong gales all the year round, which bring great dangers to sea operations. So it is very important to study the rules with the causes of strong gales, and set up methods to forecast strong gales. First the occurrence rules of strong gales of five kinds by ten years data of Shengsi station in Zhoushan city and the weather charts are studied. The results show that the gales caused by cold airs from the central area are more than the others, and the numbers have annual variation. The cold airs from the eastern areas are a little less than that from the central areas. The numbers of strong gales caused by depression in the East China Sea are much more homogeneous in each year. Diagnosis of a case of gales caused by cold airs shows that the causes are the combination of strong cold advection, upper air jet stream and momentum downward transport. Studies about gales caused by depression show that the vorticity advection, thermal advection and latent heat release make great contribution to the development of the depression. Some physical elements are selected to calculate the t statistic according to the types of strong gales and the forecast experiences. Then forecasting factors are selected based on the abnormal physical elements when gales occur. Forecasting factors are calculated by data interpolated from real station data. Then these factors are used to establish forecast models. For forecasting, the numerical production of T213 is used to calculate forecasting factors.The forecasting models are established by artificial neural networks, which have two layers of pass. The first transfer function is tangent, and the second is linear transfer function. 70% of the samples are used to train a neural network, 15% of which are used to verify the network, and the other 15% are used to test. Finally the best neural network is selected according to the results of imitation. The correlation coefficients between real gales and imitations are all above 0.80. Otherwise, absolute errors of the test samples are all less than 4 m/s.The forecasting models have been used since October 2004. Up to March 2005, all the absolute errors of the forecast samples are less than 4.5 m/s. So it can be concluded that this method has some capability to forecast the gales in Zhoushan sea area.

The Effect of Different Breeding Length upon Ensemble Forecasting Based on BGM

Yu Yongfeng, Zhang Lifeng

2007, 18(1): 86-93.

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Abstract:
Ensemble of initial conditions is the most important method in ensemble prediction, and how to generate the initial perturbations is crucial. Among all the methods to create perturbations for ensemble forecast, the "breeding of growing modes" (BGM) method has gained more and more favor for its good performance and almost "free" cost of computation. The breeding method simulates how fast-growing errors are "bred" in the analysis cycle. When a breeding cycle starts, an random initial perturbation field is added upon the control analysis. After some time's breeding, the most non-growing or decaying perturbations are filtered and the remainders can be mainly the fast-growing modes (perturbations). Accordingly, the perturbation's growth rate reaches certain value and its shape is also getting to the phase of slow change. In some sense, the perturbation in the breeding cycle reaches "saturation". This saturated status of perturbation is regarded as the estimate of the fast-growing modes in the realistic analysis error, thus is applied as the initial perturbation.From the idea of BGM method, one of the crucial questions may be when the breeding cycle ends, which is called "the breeding length" problem. In fact, the breeding length is the total time of breeding of growing modes (initial perturbations) in the ensemble prediction system. The general steps to determine the breeding length are: defining the saturated character of growing modes; investigating the increasing and saturating process of the growing modes, and defining an approximate saturated time; according to the saturated time, doing a great lot of experiments with different breeding lengths, and determining the final length by the forecast skill.The model is the global media-range forecast spectral model T63L9 and the NCEP/NCAR 6-hour reanalysis data in 1998 are used. The initial modes in the beginning of a breeding cycle are random fields with unified distribution in [-1, 1]. The value of 6-hour is chosen as the length of the integration in each cycle. Besides, "rescaling" method to make the magnitude of growing modes comparable with the initial analysis error in RMSE sense is used.Some researches show that, if an appropriate breeding scheme is adopted, the breeding of the growing modes can present a clear saturation character in both the norm (magnitude) and the form (shape) after 3—4 days of breeding. Based on these results, numerical experiments are done with breeding length of 2, 3 and 4 days. Conclusions from these experiments show that the ensemble prediction with different breeding length can all reach a certain extent improvement upon the control forecast, especially the betterments increase steadily after 4 days of forecast lead time. Results also show the differences among the three breeding lengths in the improvement upon the control forecast. It shows a better improvement in ensemble mean with a breeding length of 3 days than 2 days, but the 4-day breeding seems pretty much the same as the 3-day breeding. A pilot study on the spread and the Talagrand distribution of the ensemble members with different breeding lengths is also made. It seems that the most appropriate way is to choose 3-day as the breeding length in the system.

The Application of Spatial Structure Functions to the Design of Weather Station Networks in Beijing Area

Zhao Ruixia, Li Wei, Wang Yubin, Fu Qiang

2007, 18(1): 94-101.

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Abstract:
Using the data of daily air temperature and water-vapor pressure from 1978 to 2000 over the southeast Beijing, the spatial structure functions of the two elements are regressed for different seasons. Furthermore, based on the structure functions, the relationships between the error of linear and plane interpolation and distance are established. Finally, according to the principle that the standard error of interpolation should not exceed the standard error of observation, the maximum admissible spacing between stations of second group meteorological networks are estimated over the area.It is found that the equilateral triangle distributing is the best scheme to use, and the distance between stations should be less than 16 km. Firstly, the regressed spatial structure functions to the distances of the air temperature and water-vapor pressure are linear or close to linear. Over the southeast Beijing, the spatial structure functions increase monotonously with the distances, and have different values in different seasons. The season sequences in which the spatial structure functions of air temperature varies from largest to least are winter, autumn, spring and summer, and they are just opposite for the structure functions of water-vapor pressure. It illustrates that the temperature gradient is largest in winter, least in summer, and larger in autumn than in spring over the southeast Beijing, and the water-vapor pressure gradient varies from the largest to the least sequentially in summer, spring, autumn and winter, just oppositely to the condition of temperature gradient. The standard error of interpolation, maximum admissible error, and maximum admissible spacing all vary with seasons and distributing schemes, and the season sequences according to the standard error values is the same with that for the structure functions. When the standard error of interpolation is less than the standard error of observation, in all seasons, the equilateral triangle distributing has the least interpolation error, the largest maximum admissible error and admissible spacing between stations for the both elements. So the equilateral triangle distributing is the best scheme to use. With this distributing scheme, the distance between stations should be less than 16 km for temperature and 88 km for water-vapor pressure. So in a word, based on these two elements, the equilateral triangle distributing is the best scheme to use, and the distance between stations should be less than 16 km for the second group meteorological networks over the southeast Beijing.Further investigation shows that if consider the northwest high altitude region together with the southeast low lying region of Beijing, the equilateral triangle distributing is still the best scheme, but the distance between stations should be less than 4 km for the second group meteorological networks over the whole Beijing region. This is not consistent with the request of the second group meteorological networks. So the second group meteorological networks over the southeast and northwest region of Beijing should be designed separately, or some resource would be wasted.

Risk Assessment of Frost Damage in Wheat

Zhong Xiuli, Wang Daolong, Li Yuzhong, Zhao Peng, Yan Xuyu, Sun Zhongfu

2007, 18(1): 102-107.

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Abstract:
The resistance of the winter wheat drops rapidly when the plants begin to joint in early spring, in which season air temperatures fluctuate frequently. Winter wheat plants are prone to spring frost injury once strong cold air invades. The variation in wheat jointing date and the occurrence of low temperatures for frost injury (LTFI) in Huang-Huai wheat production areas are analyzed. The jointing date is affected mainly by cultivars growth. The occurrence of LTFI is mainly determined by the local climate. The variation in probability of wheat developing into jointing stage with Gregorian day could be expressed well by normal distribution function, and the accumulative probability of the occurrence of LTFI by exponential function. Based on these analyses, the mathematical model for assessing the degree of frost injury risk (DFIR) is then established. DFIR is mainly determined by such parameters as the termination date for some frost temperature (μ), the earliest jointing date (K), the standard deviation of the jointing date (σ), and the rate of variation in the accumulative probability of frost temperature with Gregoran day proceeding (b) etc. Parameters in the assessment model differ among the counties located in Huang-Huai and around area, leading to the different DFIR. According to the DFIR function, the zone with high DFIR is from the upriver region of Hanshui River that lies to the south of the Qinling Mountain, via the western Huang-Huai plain, to the plain lying in the mid-lower region of Huaihe River. The DFIR tends to drop gradually northwards and southwards, more rapidly for the former than the latter. Some effective measures could be chosen according to the properties of the parameters in the given area. In the area with high b value and low σ value, postponing the jointing stage may reduce the DFIR effectively; while in the area with high σ value, avoiding wheat jointing particularly early, appropriate cultivars and sowing date seem to be a good strategy to reduce DFIR; for the area of high DFIR calculated from the function with combined parameters, growing the frost resistant varieties becomes the most effective measure to avoid frost injury.

The Agro-climatic Vertical Zoning Based on GIS for Shangluo District in Southern Shaanxi Province

Zhu Lin, Zhu Yannian, Chen Mingbin, Liu Minfeng, Guo Zhaoxia

2007, 18(1): 108-113.

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Abstract:
The agrotopoclimatic vertical zoning based on GIS is carried out by fuzzy comprehensive assessment. Shang-luo mountainous area is taken as an example, which is in the east of the south slope of Qinling Mountains in Shaanxi Province, and which belongs to the semi-humid mountainous climate transition from warm extratropical to subtropical zone.The heat index for vertical zoning is used to resemble the general characteristics and the regional differences by four kinds of spatial grids database. Cumulative temperature greater than 10 ℃, average extreme lowest temperatures, mean temperatures of the coldest mouth and mean temperatures of the warmest mouth are included. Before the indexes are spatially interpolated by inverse distance weight (IDW), they are emended to the same height above sea level via the lapse rates with the help of digital elevation model (DEM), which are based on sectional observation data in subtropical west mountainous regions in Qinling and Bashan.With the overlapping function of GIS, the graph of comprehensive zone of agro-climatic resources in Shangluo mountainous region based on GIS is made by superposing vertical zone grids with aridity grids in growth season and north subtropical boundary vector, using the combined function of the GIS.According to the comprehensive division graph made by GIS, three layers are divided into two climatic zones in Shangluo area:hot layer with easily drought tendency ( < 700 m), warm and semi-humid layer (750—1250 m), mild-cold and cool-cold humid layer (1250—1800 m) in subtropical zone. Warm drought layer with easily drought (800—1000 m), mild-cold and semi-humid layer (1100—1500 m), cool-cold and humid layer (1500—2500m) in warm extratropical zone. The naming for the vertical zoning reflects the horizontal and vertical climate characteristics of the Shangluo region, and the economic development direction in each layer.

Antenna Setting Altitude Design and Analysis of Side-lobe Contamination for a Bistatic Doppler Weather Radar

Mo Yueqin, Liu Liping, Xu Baoxiang

2007, 18(1): 114-118.

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Abstract:
A bistatic Doppler weather radar system consists of one conventional Doppler weather radar and one or more remote passive receivers with low-gain antennas. It is among the new techniques in the atmospheric wind field detection. The first atmospheric observation by a bistatic Doppler radar in China is done by the National Research Project, "Mechanism and Predict Theory of Disastrous Weather over China". The meteorological echo data are obtained in Hefei by a bistatic Doppler radar made in China in 2004.Antenna setting altitude design and side-lobe contamination are key problems in the design and application of the bistatic Doppler weather radar. Based on the principles of bistatic Doppler radar observation, the problems mentioned above are studied. For operational application of a bistatic Doppler weather radar, a design for radar antenna installation with parameter of target detection height is described, which is convenient in calculation and practicable in operation. For a specified detection height, the longer baseline requires the higher antenna installation of transmitter or receiver. In addition, the side-lobe contamination is an important problem in bistatic radar technique, since its receiver has wide-beamwidth antenna. Also, the main contamination causes are analyzed. A method of reducing or eliminating the contamination is proposed, by which may control the bistatic radar observation data quality.

Evaluation of the Forest Fire Areas in the Hill Region in South China on the Basis of MODIS Data

Zhang Chungui, Huang Chaofa, Pan Weihua, Lin Jing

2007, 18(1): 119-123.

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Abstract:
Based on the MODIS data which have 250 meters resolution and the ground investigation data of forest fire in 2004, the study taking Fujian Province as an example analyzes and discusses the technological method about how to assess the forest fire acreages when there is a forest fire in hills regions in the southeast China.According to a number of forest fires cases happened in 2004, 35 typical cases are selected, the corresponding MODIS data characteristics are calculated and analyzed before and after the forest fires. The results show that the reflectivity of the near infrared channel (CH2) and the value of normalized difference vegetation index (NDVI) of MODIS data before the forest fire are obviously lower than after it, whatever are the data such as the minimum value of CH2 and NDVI, the maximum value or the average value. Moreover, the average reflectivity of CH2 declines by 6.65 points, and the average value of NDVI decreases by 0.23 points. The reflectivity of the visible channel (CH1) of MODIS data before the forest fire is much lower than that after it, and the ratio of samples to the total samples is 71%, but on the opposite, the ratio of samples with higher values reaches 29%. From the analysis, the reflectivity of CH2 channel of MODIS data and the NDVI value have different characteristics obviously before and after the forest fire on the mountain areas in Southern China, which can be used as an index to evaluate the size of the fire areas after the forest fire.On the basis of analyzing the properties of MODIS data to detect the forest fire areas, the method of comparing the dispersed graph between reflectivity of the MODIS near-infrared channels and NDVI before and after the forest fires is adopted in the study, combining with the pseudo color composite image in the fire regions, some means to evaluate the forest fire acreages with the help of using ROI function to deal with MODIS data in ENVI software are found.Finally, the study shows that the overall estimate error of burned acreages is 15 hectares in the evaluated 35 forest fire sample cases in 2004. At the same time, the calculated forest fire areas by the method of analyzing the reflectivity of the near-infrared channel (CH2) of MODIS data are showed and they are close to the actual results by ground investigation, and the areas of forest fire calculated by the means of adopting the value of NDVI are near the areas of the destroyed forest from ground investigation. Moreover, the results indicate it is significant that to evaluate the forest fire areas by the means of applying the MODIS data in hills of Southern China can be used in daily operational works and can meet the needs of monitoring the forest fire.

Preliminary Application of Three Dimensions Nesting to Limited Area Model in Long-term Integral

Wang Xiaofeng, Li Yongping, Ma Xiaoxing, Zhang Ruiyi

2007, 18(1): 124-128.

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Abstract:
A new nesting technique called three dimensions nesting (3DN) is used in a regional model which is nested in a circulation model based on the complex terrain with high resolution model (YH model). A long term integral is achieved steadily which provides a useful way to process the lateral boundary conditions for regional climate model. 3DN means not only keeping horizontal lateral boundary stable but also changing ways to use GSM data in vertical dimension. The primary results show that only with the appropriate high level data that the long term integral could obtain better outcomes in 3DN model. During the prophase integral, 3DN has an obviously positive impact just on the levels lower than 500 hPa, but not for the other levels. However, with the growing process of integral period, the good effect of 3DN is exhibited gradually. The variance of each level is smaller than the one derived from the two dimensions nesting (2DN) model, particularly for the high levels compared with the result from the prophase integral. As for the mean deviation of height field at 500 hPa level during the whole integral period, the variances of these two nesting technologies have a common character that is the value of deviation decreasing from the North to the South. In the eastern integral area, the deviation of 3DN is obviously smaller than the one of 2DN and more similar to the reference field. The mean variance of the u field for each level in the whole integral time shows that 3DN is better than 2DN atthe levels which are lower than 100 hPa; while the one of the v field progresses at all levels. At 100 hPa level, the improved percentage is nearly up to 5% and much better than that of the u field. In the long integral period, dynamic energy and geopotential height keep their stableness with a small swing range, which indicates that 3DN can be run steadily in long term integral numerical experiments. Large scale precipitation, vapor flux and so on are analyzed. 3DN is obviously better than 2DN for large scale precipitation, especially in the middle of the integral term, the result of 3DN is almost the same as that of the reference field. With the increase of the integral time, the errors of both nesting methods rise at a certain degree. During the long integral term, the nesting error of vapor prediction is relatively large at the upper boundary out-going level. However, within that area, the error extremum of vapor flux from 3DN is much smaller than that from 2DN. With regard to the whole integral term, the advantage of 3DN appears especially in the mid-phase and anaphase. It is necessary to point out that there is not much processing for the large scale model data, which is likely the cause for the less satisfaction at upper levels in the integral prophase. If potential nesting is considered, which means that a remittent area will be established when the regional model vertically absorbs data from the large scale model, better result may be obtained in the future study.

Vol.18, NO.1, 2007