应用气象学报

The Effects of Radiative Parameterization Scheme on Simulation of Cloud and Radiation in an AOGCM

Wang Fang, Ding Yihui, Xu Ying

2007, 18(3): 257-265.

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Abstract:
Radiation is an important process in atmosphere. Currently the radiative parameterization schemes used in climate models are mainly simplified arithmetic, which show great uncertainty in climate change projection. To better understand the effects of radiative parameterizations on cloud and radiation simulation, a new parameterization scheme, Fu_Liou scheme, is introduced into NCC/IAP T63 AOGCM to replace the old scheme, Morcrette scheme. NCC/IAP T63 AOGCM is a spectral model with triangular truncation at wave number 63. The horizontal resolution is 1.875°×1.875° for both atmosphere and ocean component, and there are 16 and 30 vertical layers for atmosphere and ocean respectively. The main differences between two radiative schemes are in such aspects as division of waveband, approximation for radiative transfer, and treatment of cloud layer overlap in radiation calculation. In Fu_Liou scheme, the waveband is divided into 18 sub-wavebands, 6 for shortwave and 12 for long wave, while there are only 8 sub-wavebands (2 for shortwave and 6 for long wave) in Morcrette scheme. Fu_Liou scheme adopts δ-four-stream approximation which has high calculating accuracy, and uses the binary cloud when calculating the cloud effects on radiation. Two 20-year integrations are processed by use of the two schemes respectively. The first 10 years are used for coupling adjustment, and the last 10 years for analysis. Two types of monthly mean data, 40-year reanalysis data of European Centre for Medium-range Weather Forecasts (ECMWF)(ERA-40) and data from Earth Radiation Budget Experiment (ERBE), are used to verify the results. The effects of the two schemes on the simulation of cloud and radiation are analysed in detail. The main results are as follows:Firstly, the incident radiation at TOA simulated by two schemes differs obviously, which is mainly caused by the difference in radiative arithmetic. Secondly, in clear sky the shortwave absorption of FuLiou scheme is generally lower than Morcrette scheme, especially in middle and high latitudes of winter hemisphere. The planetary albedo simulated by Fu_Liou scheme is closer to ERBE data than Morcrette scheme between 60°S and 60°N, mainly due to the improvement of simulation on clear-sky albedo. Thirdly, the simulated net radiation at TOA in Fu_Liou scheme is improved obviously as compared with Morcrette scheme except for part of the east Pacific and east Atlantic where the cloud cover decreases remarkably in winter. Finally, the low stratocumulus simulated by Fu_Liou scheme decreases remarkably, which is caused by the weakening of static stability over ocean. The high cloud increases obviously, mainly due to the intensifying of convection in tropical region, as compared with Morcrette scheme. Although the cloud cover decreases, the use of binary cloud increases the cloud shortwave absorption in Fu_Liou scheme which improves the weak absorption in Morcrette scheme in some degree.

Remote Sensing and Detection of Dust Storm in China Using the Thermal Bands of Geostationary Meteorological Satellite

Hu Xiuqing, Lu Naimeng, Zhang Peng

2007, 18(3): 266-275.

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Abstract:
The earth observation system from space includes two kinds of platform:Polar orbit and geostationary satellite. Optical sensors onboard polar satellite have the advantage of high spatial resolution and more spectral bands in visible to infrared regions such as AVHRR/NOAA, MODIS/EOS, MVIS/FY-1C/1D etc. But it can conduct only twice a day. It is not enough for hazardous dust weather whose time scale is very short and moves quickly. It is difficult to understand dust moving and evolution as a whole using polar sensors' observation. Geostationary Meteorological Satellite such as GMS-5, FY-2, GOES and Meteosat can observe the earth continuously all daytime and night at high temporal resolution. Developing an algorithm for remote sensing and monitoring dust event will be very useful for forecast model, environment and climate monitoring and scientific research. Dust cloud is not easy to be discriminated like other strong weather phenomena such as typhoon. It is to be understood of the optical and radiative mechanism of airborne dust. The base theories of remote sensing of airborne dust will be introduced using the thermal and other bands of geostationary sensors. Observation signal of thermal infrared window (8—12 μm) bands have almost no sensitivity to general aerosols with small and thin particle sizes. There are some sensitivity to large and strong dust particles, especially in dust storm or heavy dust storm. The airborne dust can exert two kinds of features on thermal infrared observation signals. Firstly the infrared radiance of ground target into space will be reduced by dust layer and the brightness temperature of the observed underlying target be decreased. This kind of temperature reduction is called infrared difference dust index (IDDI). Secondly the emissivities of airborne dust are different in these two split window bands and produce the negative brightness temperature difference for dust targets. Based on these above theories and the traditional sophisticated multispectral classification technique, a set of algorithms for automatically detecting dust storm is developed using observation data of geostationary meteorological satellite. The first step of this algorithm is to extract the data of all bands from normalized disk image of observation and to conduct the interested region projection and calibration processing. The data reading is not only from present time but also from previous ten days for integrating background brightness temperature image. It is ready for IDDI image integration. The second step is cloud mask processing which is very important for dust discrimination. And then dust determination is conducted using above mentioned theories and methods. It is the key part of this algorithm. The last step is the production output including several types of dust remote sensing. This algorithm can obtain ideal result of dust storm detection and the product of IDDI. This algorithm has already been experimentally run in National Satellite Meteorological Center since 2001. It is not only used for dust detection from data of Japanese GMS-5, but also becomes a useful operational production of the new geostationary meteorological satellite FY-2C in orbit in 2005. In addition, it provides extended potential of quantitative or semi-quantitative remote sensing of dust storm.

Characteristics of GRAPES Dynamical Core in Long Term Integration

Hu Jianglin, Shen Xueshun, Zhang Hongliang, Yang Xuesheng, Wang Panxing

2007, 18(3): 276-284.

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Abstract:
In order to evaluate the characteristics of the dynamical core of GRAPES (Global/Regional Assimilation and PrEdiction System) and to verify whether the GRAPES dynamical core can be taken as a framework of AGCM, the long time integration has been carried out with GRAPES dynamical core followed by the benchmark test similar to the technique introduced by Held and Suarez. The GRAPES has a finite-difference dynamical core with fully compressible, non-hydrostatic framework and semi-implicit and semi-lagrangian (SISL) time integration scheme using latitude and longitude horizontal coordinate as well as following terrain height vertical coordinate. The computational design uses two simple physical processes to represent physics processes in the model during the time integration. The first process is to relax the model variant, potential temperature, to a prescribed potential temperature field which is a function of latitude and height during the model time integration. Following Held and Suarez, the second one is the momentum drag in the lower troposphere to consume the kinetic energy in the model atmosphere. The GRAPES dynamical core runs with 31 levels in vertical direction and 3 horizontal resolutions:1.25°, 2.5°and 5.0°latitude or longitude. The benchmark tests integrate for 1460 days under each horizontal resolution and the last 1000 days data are analyzed by discarding the previous 460 day results. The statistic results show that the GRAPES dynamical core can reproduce basic features of atmospheric circulation, including reasonably realistic zonal mean temperature and its eddy variance as well as zonal-mean wind and its eddy variance. A single westerly jet is generated with maximum strength of roughly 28 m/s near 45°latitude in the tropopause. And eddy temperature variance shows two middle latitude maxima, one in the low troposphere and the other above tropopause. The GRAPES dynamical core also shows stability for the energy and momentum as well as vertical wind speed during the long term integration although SISL scheme does not observe the conservations. The GRAPES dynamical core has convergence features along with increasing resolution, although 5.0° resolution in latitude and longitude may be too coarse to represent the circulation details. Finally, the sensitivity to non-hydrostatic is discussed. The results indicate that it is feasible using the GRAPES frame as a dynamical core for AGCM and climate investigation. The numerical experiments provide clues and evidences for GRAPES dynamical core improvement also.

The Asymmetric Wind Structure of Typhoon Aere Detected by Doppler Radar

Wei Yingzhi, Tang Dazhang, Xu Jianmin, Wu Chenfeng

2007, 18(3): 285-294.

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Abstract:
On August 25, 2004, after typhoon Aere approaches the ocean regions northeast of Taiwan, China, its track path turns left twice consecutively, i.e., it turns firstly from northwest to west, and then from west to southwest, which differs from the normal cases and seldom occurs in history. According to the observations of Aere both by single Doppler radar and double Doppler radars, the maximum wind speed always appears in the front right side of Aere forward direction, which means the wind speed in the northwest quadrant is the maximum among all the quadrants. Furthermore, the above results further prove that the wind distributions of Aere follow the one wave number asymmetry. And it is also confirmed by other dataset analyses, i.e., the radar data from two radars situated in Changle of Fuzhou and Xiamen which are distributed in different quadrants, regular data with both ground-based observation and radiosonde observation, and NCEP reanalysis data. Among these dataset, it is revealed by the detected results with Doppler radars in both Changle of Fuzhou and Xiamen that the fundamental causes for the phenomenon that the variation of extreme radial velocity accompanies with that of the relative positions of radar and typhoon, are the unsymmetrical structure of typhoon wind. And because of that, the wind speed in the northwest quadrant is the maximum compared with the others. When the relative position of radar to that of typhoon center changes, different extreme radial velocity in different quadrants can be detected by radar. This research indicates that when Aere locates to the southeast of the continent high, there exists an environmental guide-airflow around it with velocity of about 7 m/s. Therefore, the unsymmetrical distribution with the largest maximum wind speed occurring in the northwest quadrant may be caused by both the environmental wind and Aere itself, and such asymmetry is in favor of maintaining the southw estward movement of Aere. In the coastal regions of the southeast of China, the maximum wind speed of typhoon in the northwest, northeast and southwest quadrants can not be detected simultaneously by single Doppler radar, it can be done by the double Doppler radars and the multiple Doppler radars. Therefore, when forecasting the turning tendency of typhoon track path based on the possible unsym metrical distributions of maximum wind speed, the significance for the enhancements of capability in monitoring and forecasting typhoon is great by using the double Doppler radars or multiple Doppler radars. At the same time, methods of calculating unsym metrical Rankine wind speed with the observations of Doppler radar are discussed. The calculated results show that the variation characteristics of the direction and distance of Aere unsymmetrical wind speed are very clear, and they are identical to the observations of Doppler radars. Additionally, the calculation process is simple, and to some extent, it can work as a reference to the operational forecast. The final aim is that with the detected wind velocity structure of typhoon by Doppler radars, especially the unsymmetrical structure, the possible mechanism for the sudden turn of typhoon track path can be presented, and the beneficial clues can be the further supplied for its forecast.

Application of Radar Rainfall Estimates to Runoff Simulation in Foziling Basin

Zhang Yaping, Cheng Minghu, Xu Hui, Wang Jiatao

2007, 18(3): 295-305.

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Abstract:
It is well acknowledged that the accuracy of stream flow predictions from a hydrologic model is heavily dependent on the accuracy of the precipitation inputs. Particularly, high variability of rainfall exists both in time and space, and mountainous basins have in general fast response time. Therefore, hydrological models taking into account the rainfall variability should play an important role in flood alert systems in mountainous basins. In this sense, weather radar inform ation is a key element in flood forecasting. The studied closed basin, Foziling (1813 km²), has a basin-to-radar distance of 100 km from southwest of the Hefei CINRAD/SA radar (31.866°N, 117.257°E). Comparisons of raingage-based and radar-gage-based simulated discharges using TOPMODEL (TOPography based hydrological MODEL) are performed for the Foziling basin and its 6 subcatchments. Rainfall observations of the basin are available from 12 raingauges operated by Bureau of Hydrology, Huaihe River Commission, Ministry of Water Resources PRC. In order to utilize true independent data sources for verification purposes, 6 gauges are withheld from the estimation scheme and used for verification instead. The radar data are selected from an S-band Doppler weather radar located at Hefei, Anhui Province throughout the period from June 20 to July 12, 2003. The TOPMODEL rainfall-runoff model used in this study is a semi-distributed watershed model that simulates the variable-source-area mechanism of storm runoff generation and incorporates the effects of topography on flow paths. For the application of the TOPMODEL, the topographic index is computed based on 1:250000 DEM (Digital Elevation Model). To some extent, whether the merged radar-gage estimates are better than the gage-only estimates or not is relative to the density and representativeness of the raingage network; even when the raingage-only and radargage mean areal rainfall estimates show nearly the same value for the Foziling basin, there is discrepancy between raingage-only and radar-gage estimates of mean areal rainfall for each subcatchment; for certain subcatchment, the greater the discrepancy between the raingage-only and radar-gage mean areal rainfall estim ates, the bigger the divergence between the corresponding simulated runoff depths from TOPMODEL. Therefore, even when a relatively dense raingauge network exists, the rain gauge data alone do not provide an initial rainfall state that is detailed enough for accurate hydrologic simulation, and radar information is essential to provide accurate flow estimates using a rainfall-runoff model. In fact, a density of about 1 raingauge per 300 km², which is the case of the Foziling basin after the 6 verification raingauges are withheld, is insufficient to reproduce the spatial precipitation pattern of the event studied. Conclusions from the study may be specific to the target case, which is based on the characteristics of the QPE (Quantitative Precipitation Estimation) inputs and the TOPMODEL hydrologic model, or specific hydrologic characteristics of the Foziling basin. In the future, additional events, other hydrological models and some more robust radar-raingauge correction procedures will be investigated.

The Upright Motion Structure in Convection Cloud by Dual-Doppler Radar

Yuan Ye, Yang Guang, Hu Wen, Zeng Guangping

2007, 18(3): 306-313.

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Abstract:
The convection cloud is the main precipitation system in south China, having abundant water resource. And it is the main modification object in south China too. A convective cloud development is closely correlated with the structure of the upright wind field, but it is hardly observed directly for the equipment limit. With the quick development in multiple Doppler radar synchronously observation and retrieved technique of the wind field, analysis is possible in the upright wind structure by dual-Doppler radar. The dual-Doppler radar data are retrieved for the upright wind structure analysis in such three different phases as the development, the intensification and the decaying in a convection cloud on July 31, 2004. In the development stage, a whole secondary circulation is found in 5—10 km height, and its center is at about 8 km height. In this phase, the updraft is more than downdraught obviously. All current is updraft under the secondary circulation, at about 5 km height. The convergence area on the VCS of Hefei Doppler radar is under 6 km height, the area of retrieved updraft. The transformation area from convergence to divergence is about 6—9 km height, which corresponds with the secondary circulation. In this phase, the heavy echo concentrates near the secondary circulation, and the heaviest echo at about 5—8 km height is underside of the center of the secondary circulation. In this phase, the echo dimension is small, about 10 km in longitude direction and 20 km in latitude direction. In the intense stage, the secondary circulation is strong. Its location is at 4—10 km height, near the center of cloud. The downdraught is almost under the secondary circulation and the updraft is above the secondary circulation. In this phase, the velocity of downdraught and updraft are bigger than the development stage, and their values are almost equal which is different from the dominant updraft in the development stage. The convergence area on VCS of Hefei Doppler radar is still at the bottom, and the wind tow ards Hefei radar is in higher layer corresponding with the retrieved result. In this phase, the heavy echo area is near the lower cloud and the heaviest echo is about 3 km height under the secondary circulation. The area of echo is obviously bigger than the development stage, about 30 km in longitude direction and 30 km in latitude direction. In the decaying stage, the secondary circulation has disappeared, that means the convection in cloud has become weak. The velocities of downdraught and updraft are both slow, and the strong out-flux is in the east and the north of the cloud top. The horizontal velocity is faster than in the intense stage but the upright velocity is slower than in the intense's. At this time, the horizontal radiation in cloud is increased. The wind direction on VCS of Hefei Doppler radar is toward the radar station in lower layer and outward in higher layer. And the observation and the retrieved result are similar. The echo in this phase is obviously weaker than in the development stage and in the intense stage. The heavy echo area is near the lower cloud, and the dimension of echo area is bigger than in the intense stage, about 50 km in longitude direction and 40 km in latitude direction. Result shows that it is possible and workable on the structure analysis in wind upright fields by dual-Doppler radar.

Recognizing Low-altitude Wind Shear and Convergence Line with Doppler Radar

Wang Nan, Liu Liping, Xu Baoxiang, Zhang Jianyun

2007, 18(3): 314-320.

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Abstract:
An algorithm for detecting low-altitude wind shear and convergence line with Doppler radial velocities is developed. The algorithm includes two parts:data pretreatment and shear calculating. Pretreatment is in order to smooth some noise in records, make them consecutive and keep the med-scale information meanwhile. Median filtering firstly and then moving average is performed for pretreatment in the algorithm. According to meteorology, convergence is defined radial grads of radial velocity and shear is defined azimuthal grads. Least square method is adopted to calculate shear. Besides of that, the algorithm gives vertical shear also.The number of treating data represents the space related to treatment. Therefore, the effect of number on results of pretreatment and shear calculating is separately analyzed. Both in median filter and moving average, the smoothing effect is more obviously while more data are used. But there are still some differences between two methods. Although median filter do better in smoothing noise and keep med-scale trend of data, the sequent is that data become discontinuous. With median filtering means, the best result is btained that the meso-scale movement of data is slippery, consecutive and preserving. Just as pretreatment, different number will lead different outcome of shear calculating. The value of shear will become large while the amount of data calculated is small. And the shear line or convergence line will become narrow. Furthermore, a gate value of result show must be set to help forecaster recognize the line easily, which is detected by the algorithm. In the end, only when the amounts of both pretreatment and shear calculating are all suit with scale of the line which will be detected, the production will be perfect. After theoretical discussion, the algorithm is tested on some heavy rain practically. The first one is caused by a squall line, the second is about of a strong convective line, last is a precipitation of mixing cloud. In all of these cases, the algorithm woks very well. First, both magnitude and position of wind shear or convergence line which are detected are all consistent with the actual. Secondly, high shear value of shear or convergence line is corresponding to reflectivity of heavy rain. Thirdly, Different gate is necessary for different rain. Fourthly, vertical shear exhibits the variety of velocity fields in different elevation. As a conclusion, it is feasible to detect low-altitude wind shear or convective line using Doppler radial velocities. The production will provide an important and objective evidence for weather forecasting and alarming.

Dynamic Diagnosis of an Infrequent Squall Line in Guangdong on March 22, 2005

Xie Jianbiao, Lin Liangxun, Yan Wensheng, Liang Qiaoqian, Liu Yan

2007, 18(3): 321-329.

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Abstract:
A dynamic diagnosis and meso-scale analysis of an infrequent squall line on March 22, 2005 in Guangdong is made using the routine observations, automatic weather station data, NCEP/NCAR reanalysis data and Doppler radar images. It's found that the squall line occurs under unstable stratification of environmental conditions and unstable physical mechanism, such as the trough at 500 hPa affecting the region where Guangdong borders Guangxi ahead of the trough at 700 hPa, so the acclivitous trough may be the dynamical trigger mechanism of the squall line's occurrence; the intrusion of the dry and cold air down from the upper troposphere affords the thermal instability field; the westerly jet is overlapped with the low southwest jet over the Guangdong and Guangxi, which brings the strong vertical wind shear, and the squall line develops strongly along the exit area of the low level jet after it is formed at the entrance of the upper jet above the low level jet; in the north and south of the squall line there are many new convection cells which keep building and tend to be combined towards the middle of the bow-shaped echo when the squall line grows; the squall line tends to have a dissy mmetrical structure rather than a symmetrical structure when it weakens, and the comma head and tail of the squall line, which cause disasters, are still growing respectively; the echo's channel of weak reflectivity factor at the rear end of the bow-shaped echo, namely the mesosphere-influx mouth of trough, comes forth, which is a sign to the change when the squall line has turned to dissymmetrical structure from symmetrical structure, and the squall line develops to the most powerful stage; in this process it shows some features such as the bow-shaped echo, the V-shaped mouth of trough, MARC and "long convection line" in front of the squall line. The strong inflow center, velocity convergence zone and small vortex velocity feature from the meridional velocity are also found. It is a good sign to identify the change of multi-cell storms that the long convection line of images occurs and develops all along the squall line; diagnostic analysis shows that the physical measures such as the vertical speed, wet static energy and CAPE can reflect effectively where the squall line has been born and where it has moved to.

Interdecadal Variabilities of SCS Summer Monsoon Intensity

Li Xia, Liang Jianyin, Zheng Bin

2007, 18(3): 330-339.

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Abstract:
In order to diagnose the interdecadal variabilities of South China Sea (SCS) summer monsoon activities and reveal the inner relationships of the sea surface temperature and the general circulation anomalies to the interdecadal variabilities of SCS summer monsoon activity, by using NCEP/NCAR reanalysis grid data, the interdecadal phases of SCS summer monsoon are divided, the differences of the average fields in the different interdecadal phases and their causes are studied with composite and comparison analysis. The results show that the intensity of SCS summer monsoon is characterized by intedecadal variability that has an abrupt jump occurring around 1976. It can be divided into two phases. The first phase is from 1960 to 1976 and the second phase is from 1980 to 1998. Comparing the southwesterly over SCS in the second phase with that in the first phase, it is found that the average intensity is weaker, the amplitude of annual variation is greater, the periods are shorter and the ascending flow over the south and center of SCS is stronger in the second phase than that in the first phase. The temperatures in whole troposphere in summer drop over the China continent and rise over the surrounding ocean, leading in the increase of geopotential height in lower and middle levels in troposphere over China continent, and it is more than that over the ocean by the thermodynamic forcing process. The weakening of the pressure gradient between the continent and ocean can result in the enhancement of an anticyclonic anomalous circulation over the continent, leading to the weakening of the southwesterly over the north and center SCS. The temperatures in the troposphere in summer drop over the China continent and rise over the surrounding ocean, leading in geopotential height in lower and middle levels of troposphere over China continent increasing more than that over ocean through the thermodynamic forcing process. The weakening of pressure gradient between the continent and ocean can trigger an anticyclonic anomalous circulation over the continent, leading to the weakening of the southwesterly over the north and center SCS. As shown in the distribution of divergence wind field and vertical motion, it implies that the remarkable increase of sea surface temperature over the eastern equatorial Pacific Ocean may play an important role in enhancing the ascending motion over southern and center of SCS.

The Relationship Between Rainfall in Yunnan Summer and the Circulation in Preceding Winter

Yan Hongming, Wang Ling, Zhou Guolian, Lu Yabin

2007, 18(3): 340-349.

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Abstract:
At present, in view of the limitation of the understanding of the atmospheric movement, the climate change projection depending completely on general circulation model (GCM) is in an immature period, and relying on the anomalous characteristic of early general circulation is still one of the main methods, particularly in provincial and regional meteorological observatory. So investigating on the prediction clue from early circulation anomalies is very important work, which may provides some scientific evidences to accurately predict the following climate anomalies. A lot of previous researches show the close inter-season linkage between early wintertime circulation and the following summer climate anomalies, the precipitation variations in different regions are not only the significant difference, but also the relationships between them, and early circulation anomalies are distinctly different which needs to be further investigated. Yunnan Province is located in Asian monsoon region and is commonly affected by East Asian monsoon and South Asian monsoon. Its climate anomaly is very complicated and different from those in other regions. Further study on the relationship between early circulation anomalies and Yunnan summer precipitation will be beneficial to improving the forecast accuracy and providing the accurate weather information for government's decision-making to prevent and mitigate the damages resulting from disastrous weather. Based on the monthly NCEP/NCAR reanalysis dataset from 1948 to 2004, 124 stations rainfall from 1961 to 2004 and sea surface temperature of the UK Hadley Center from 1948 to 2003, the temporal and spatial variation features of Yunnan summer (JJA) precipitation are firstly analyzed by using EOF and Morlet wavelet analysis method. Secondly, the relationship between the rainfall and preceding general circulation, air temperature is further investigated by using correlation and composite analysis methods. Yunnan summer precipitation in different regions trends to have the similar variation, and has a significant inter-annual variability and period circle feature with 2 and 4 years. The circulation anomalies in mid-high latitude region of Eastern Asia and atmospheric thermal situation from low-high layer in tropical India Ocean and Pacific Ocean in preceding December and January, particularly in January are significant signals indicating the following climate variation of Yunnan. The rainfall increases when East Asian trough deepens and East Asian winter monsoon (EAWM) strengthens, air temperature from low-high layer is cold. The opposite situation occurs when early East Asian trough weakens, EAWM weakens, and air temperature from low-high layer is warm. Furthermore, the anomalous characteristics of SST in tropical Ocean and its possible effect on the linkage relationship between East Asian winter monsoon and summer monsoon are also investigated. The results show that SSTA in tropical Indian Ocean and western Pacific Ocean keep unchanged from winter to summer, which will impact possibly EASM and rainfall in Yunnan summer by changing land-sea thermal contrast.

The Relationship Between Road Traffic Crashes and Meteorological Condition with Construction of Its Road Weather Warning Model

Luo Hui, Li Liangxu, Hu Sheng, John Nairn, Liu Yu, Zheng Lei

2007, 18(3): 350-357.

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Abstract:
In order to construct a valid road weather warning model, the effect of weather on the daily number of all reported successive traffic accidents day by day from January 2002 to December 2004 (1096 valid samples) in the area of Xi'an city is examined. Thirteen meteorological elements sampled over the same area day by day are applied for the corresponding three years. The 1096 sample days are divided into two half-year parts given different seasonal regimes of the meteorological elements. Data are processed using statistical product and service solution (SPSS) version 12.0.Factor analysis methods are utilized to summarize four public factors, which are F_i(i=1, …, 4) and H_i(i=1, …, 4), integrated by temperature, visibility, relative humidity and rainfall in spring-summer half-year period and temperature, visibility, rainfall and pressure respectively in autumn-winter half-year period. Multi-collinearity is effectively overcome by this process and the meteorological variables are reduced from thirteen elements to four public factors.Logistic regression analysis based on these four public factors is applied. Initially the model is calculated, the number in the set that occurs most frequently in all traffic crashes data is obtained. Then it is supposed that if the raw crash data are greater than the mode value 1 is set to this variable and targets the high crashes frequency; if the raw data are smaller than the mode value 0 is applied to this variable and targets low crashes frequency. Binary logistic by entrance option is applied to get the road weather warning model over the two half-year periods. But F₁ (temperature factor in spring-summer period) and H₄(pressure factor in autumn-winter period) are shown to be insignificant, so they are abandoned when the logistic regression equations are constructed.The model is evaluated for preferable prediction accuracy through samples tests. The results show that the meteorological factors used to indicate their influences and forecast traffic crashes in Xi'an city result in a suitable and effective road weather warning model.In summary, according to significance level in the logistic regression equation and the value of factor loadings, the factors are respectively integrated by visibility, relative humidity and rainfall in spring-summer half-year period and temperature, visibility and rainfall in autumn-winter half-year period.

The Difference and Relativity Between Rainfall by Automatic Recording and Manual Observation

Ren Zhihua, Feng Mingnong, Zhang Hongzheng, Ju Xiaohui, Wang Ying

2007, 18(3): 358-364.

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Abstract:
Automatic Surface Observation System (ASOS) is set up by China Meteorological Administration (CMA) about 2000. Main surface meteorological parameters start to be automatically measured at about 600 base stations, 1200 ordinary stations in CMA by the end of 2005. Since both field intercomparison experiment results and parallel comparison observation results show that the difference is much great between the amount of rainfall measured by commonly used rain gauges (manual observation) and that measured by tipping-bucket rain gauges (automatic recording), commonly used rain gauges are still kept on rainfall measurement manually twice a day in China surface meteorological observation operation, though automatic measurements of rainfall have been implemented to record hourly precipitation totals at a large number of stations at present. What the difference and relativity are between the amount of rainfall measured by automatic recording and by manual observation in observational operation remains a question to many meteorologists. Based on 55641 operational rainfall observations respectively by automatic way and by manual way at 627 national base stations in 2005, various analyses are carried out on the rainfall observation difference, the difference and relativity between above two observation methods.In the rainy season, when there is no rain by manual observation, there is no rain by automatic recording at each station. The average daily rain amount observed by automatic way is 0.12 mm (or 1.42%) more than that by manual way. The standard deviation of the difference is 0.70 mm. In the 55641 rainfall comparison observations, 26.2% daily data observed in the two ways is identical, 30.8% daily data observed by automatic way are less, 43.0% daily data observed by automatic way are more than the manual way. The annual rainfall measured by automatic recording is more than that by manual observation at 70% base stations. There are 80% stations whose difference values in annual rainfall respectively observed in the two ways are less than 5%, and the difference values are above 10% at 4% stations. Comparing to field experiment results in the two observation methods in early 1990s, the difference of operational annual rainfall observations respectively in the two ways is obviously improved at the 627 base stations in 2005. In general, the more the difference value in annual rainfall, the less the annual rainfall in a station. The distribution difference of rainfall in space, the difference in measurement time of the two observation ways at 20:00 and other urgent events can result in the rainfall difference in daily data, but they have no systematic effect on accumulative total. Different observation instruments and likely low quality can lead to the systematic deviation in above comparing observations. The daily rainfall by automatic recording is obviously linearly related with that of manual observation and the correlation coefficient is 0.9988.

Quality Assessment of the Observational Data of Automatic Precipitation Stations in China

Zhang Qiang, Tu Manhong, Ma Shuqing, Yang Zhibiao, Luo Yongchun

2007, 18(3): 365-372.

[FullText HTML] (728) [PDF 2093KB](1997)

Abstract:
Using the observational data of 19 different types of automatic precipitation stations located in Beijing, Yichang and Nanjing from June to September in 2005, the method of quality assessment of precipitation data is studied. Different methods can lead to different assessment results. It is mainly discussed by the present study how to choose standard precipitation in the assessment. The results show that it may affect the veracity of the assessment more or less due to the uncertainty of standard precipitation when exploiting the Automatic Weather Stations (AWS) data and timed manned observation record as standard precipitation to examine the automatic precipitation stations. On the basis of extensive experiments, a new method to define standard precipitation as fitting precipitation derived from the automatic precipitation stations data is put forward. Moreover, the feasibility, applicability and computation method of fitting precipitation are also addressed in the study.Comparison results show that when employing the observed precipitation obtained from AWS as a standard to assess the automatic precipitation stations, the errors of monthly precipitation in the respective automatic precipitation stations are generally higher (above 8%) and the differences of the assessment results are larger because of the uncertainty of the observed precipitation in AWS. In comparison to AWS, the uncertainty in the timed manned observations is low and the veracity of data is high. It is therefore more reliable for the estimate results to use the timed manned observation as standard precipitation for comparison assessment. However, low temporal precision of the timed manned observational data restricts its role in the assessment. The observed rainfall can be estimated by fitting precipitation relative accurately under the conditions of more samples. Time series of precipitation based on the fitting precipitation have high credibility as well as temporal precision for comparison. The assessment results using the fitting precipitation as standard precipitation to evaluate automatic precipitation stations are basically consistent with those utilizing timed manned observations as a standard. The consistency between them demonstrates their respective credibility.The fitting precipitation is based on the observational results of different automatic precipitation stations under the same conditions, therefore, it is much more reliable. The fitting method is considered to be used in the setting of standard series under the conditions of more samples. The present study shows a new idea of quality assessment of automatic meteorological instruments data.

Forecasting Model of Snow Calamity and GIS Application of Spacial Analysis Technology in Qinghai Plateau

Zhou Bingrong, Li Fengxia, Shen Shuanghe, Song Liming, Li Hongmei

2007, 18(3): 373-379.

[FullText HTML] (642) [PDF 3385KB](1551)

Abstract:
In recent years, with the application of 3S technology in monitoring disaster field, utilizing the technology of GIS spacial analysis, the minimum unit of analytic target could become the concrete geographical unit. As a result, spacial analysis precision is improved. The foundation of the function of snow calamity fragility degrees can lead to the realization of integrated and stepwise forecasting, warning and assessment. GIS and RS technology are applied, and the result on relation between quantity of precipitation and depth of snow is used, then the functions are defined, which are the function of frangibility on livestock husbandry and function of frangibility on social economic level. The Forecasting Model of Snow Calamity on Meteorology and the Forecasting Model of Snow Calamity on Remote Sensing are established. Then, the snow calamity of the Plateau is forecasted step by step, in order to find out the mechanism of snow calamity in different areas, to improve the capability of forecasting snow calamity, and to give the way against the calamity. It is regarded that the mechanism of snow calamity in south Qinghai is different from that in areas around Qinghai Lake. Although snow is a factor causing snow calamity in every area, its effects are more severe in south Qinghai than in the areas around Qinghai Lake. Snow of similar degree may cause disaster, but it may also affect animal husbandry in areas around Qinghai Lake only slightly. Snow calamity is not the only nature disaster, the unreasonable activity of human beings also affect the occurrence and the development of snow calamity. At last, snow calamity which appears in Qinghai Plateau is forecasted using this model. Result shows the model can forecast snow calamity in the Plateau, and enhance forecasting ability of snow calamity in the Plateau.

The Design of Asymmetric Bogus Vortex Scheme and Preliminary Experiment

Qu Anxiang, Ma Suhong

2007, 18(3): 380-387.

[FullText HTML] (860) [PDF 2172KB](1997)

Abstract:
The Model for Typhoon Track Prediction (MTTP) at National Meteorological Center (NMC), which has provided operational guidance for forecasters since June 1996, has always been imposed a scale-selective sophisticated filtering to remove the incorrect shallow vortex from the first guess fields and has implemented axisymmetric bogus vortex into the initial condition. However, with the upgrade of global spectral model system at NMC (from T106 to T213), the global forecast model now has a horizontal resolution that allows the representation of the tropical cyclone (TC) circulation, although the resolution is not adequate for resolving the inner core structure. Correspondingly, some valuable information about TC physical characteristics can be provided by the first guess fields from global gridded analyses. When damping out the entire shallow vortex of first guess fields, the background circulation around TC center has already been smoothed away by sophisticated filtering scheme, especially the asymmetric wind components, and it has adverse effect on the modified background field. Nowadays, the operational TC initialization scheme is constructed from a bogus vortex based on a few parameters analyzed by forecasters (such as TC central pressure, central position and radius of 15 m/s wind) and has an axisymmetric property. Because TC vortex has an obvious character of asymmetric structure, the implementation of axisymmetric property can contribute to partial track forecast biases more or less. In order to investigate the impact of background asymmetric circulation on the TC track forecast, an experimental scheme to introduce asymmetry into TC bogus vortex is carried out as follows:Extract asymmetric component of geopotential height from first guess fields by taking the difference between the total geopotential height and its azimuthal average about TC center; derive gradient wind from an azimuthal average of the total geopotential height; extract asymmetric component of wind by taking the difference between gradient wind and total wind of first guess fields; merge asymmetric components into axisymmetric bogus vortex and produce TC initial condition of MTTP model. It is expected that the scheme can provide a significant enhancement of initial TC asymmetric circulation and structure. Using the new scheme, a total of 12 forecast experiments for typhoon Sinlaku in 2002 are performed and the track forecast skills are evaluated in comparison with the operational ones. From the study it is found that much more accurate TC bogus data are produced by the new scheme compared with operational one. The verification of the experiments also shows that mean track forecast biases can be reduced effectively by introducing asymmetric components into axisymmetric bogus vortex.

Application of Doppler Radar Data to Nowcasting of Heavy Rainfall

Wang Gaili, Liu Liping, Ruan Zheng

2007, 18(3): 388-395.

[FullText HTML] (737) [PDF 1358KB](1977)

Abstract:
Severe rainstorm, typhoon heavy rain and so on are the main precipitation system causing disastrous flood. In order to reduce the occurrence of meteorological disasters, timely forecasting and warning of severe weather are needed. The development on rainstorm nowcasting with tracking radar echoes by correlation (TREC) is described, which is also applied in Hong Kong rainstorm nowcasting system SWIRLS (Short-range Warning of Intense Rainstorms in Localized System) and NCAR's ANC (Auto-NowCast). TREC technique is applied to two successive CAPPI reflectivity fields. The first field is divided into a number of equally sized two-dimensional arrays of pixels. The arrays of reflectivity values are then cross-correlated with the arrays in the second field. The correlation coefficient R is calculated repeatedly for all possible arrays found at the second field to determine which array results in the highest correlation, and the center of this second array is the end point of TREC vector. Based on the Doppler radar data from the mosaic by Guangzhou and Meizhou radar, Wenzhou single radar, effects on the tracking result are discussed which are produced by the parameters variation including interval between two CAPPI reflectivity fields, the size of the boxes, the threshold of rain intensity etc. In order to correct noisy TREC vectors and improve the consistency of the resulting vector field, a two-step procedure is used:The purpose of the first step is to minimize the influence of apparently incorrect TREC vectors. Vectors with zero velocity (often caused by ground cluster) are replaced by vectors that represent the average of the neighboring vectors. Objective analysis is used in the second step to produce a continuous gridded vector field, which is used in rainstorm nowcasting. The nowcasting result is validated by making use of observational data of radar. The results show that the TREC vectors calculated from rain intensity CAPPI, reflectivity CAPPI and CR are consistent with each other, and they are capable of indicating the directory of rainstorm displacement. The result of extrapolation forecast from CR is slightly below the two others, and forecast accuracy of the three kinds of data are decreasing with forecast lead time, which can be improved by fitting the successive five TREC vectors. Though TREC technique segments radar imagery lacks of clear physical meaning, it is still valuable for forecasting of storm rainfall and typhoon rainstorm with complex structure and identified difficultly. Both single radar data and mosaic data are used in the research, which is helpful for forecasting severe weather utilizing Doppler weather radar observation net.

2007, 18(3): 395-395. DOI: 10.11898/1001-7313.20070334

[FullText](1722) [PDF 110KB](323)

Abstract:

Meso-scale Ensemble Forecasts on Physical Perturbation Using a Non-hydrostatic Model

Tan Yan, Chen Dehui

2007, 18(3): 396-406.

[FullText HTML] (744) [PDF 3042KB](1568)

Abstract:
The past decade has seen increasing interest in ensemble methods for operational numerical weather prediction. Ensemble forecasting is motivated by the recognition that numerical predictions always contain both initial condition uncertainties and numerical model uncertainties. It is traditionally desirable to use an ensemble method focused mainly on uncertainties in the initial conditions for the medium range forecasts. Encouraged by the success of global medium-range ensemble forecast, investigations are made in examining the short-range ensemble forecast (SREF) with meso-scale models. It is found that the ensemble approach could also improve short-range weather forecasts, especially the forecasting of quantitative precipitation, meso-scale convective systems. However, it is still challenging for meso-scale ensemble forecast due to the difficulty to generate a well spreading perturbation which triggers meso-scale and convective processes in a short time.Motivated by the previous studies on meso-scale ensemble forecasts, a meso-scale ensemble forecast system based on a limited area non-hydrostatic meso-scale NWP model (GRAPES-Meso) is carried out, to construct meso-scale ensemble, especially considering the perturbation of the physical sensitive factors and the initial conditions. As a preliminary step, the objective of the study is to understand the impact of model physics uncertainties on predicting extreme precipitation events using meso-scale ensemble forecast. A flash flood case on July 10, 2004 in Beijing is particularly chosen for the study of a 36 h meso-scale ensemble forecast. The "7.10" flash flood causes a high impact on traffic and open air activities because of the failure of the forecast. In particular, the uncertainties in convective parameterization schemes are focused on. The members of SREF are constructed by specifying the closure assumptions, triggering parameters and precipitation efficiency. Meanwhile meso-scale ensemble forecast is constructed using multi-physical parameterizations and the uncertainty of initial condition is added using the Monte Carlo method.The verification results show some characteristics of the meso-scale system could be captured by GRAPES and meso-scale ensemble is feasible to improve on the forecast site and the forecast intensify of the precipitation. Different ensemble experiment has different result, even in the same experiment, members differ with each other, and it means there is an appropriate spread among members. In spread analysis, from the surface to the middle-level of troposphere there is a maximum value area in Beijing, i.e., this area has large uncertainties. Meanwhile it is found that it's difficult to construct ensemble system only considering model uncertainties, if initial condition uncertainties are added, the results are preliminary but encouraging, it is helpful to capture more meso-scale information and construct more effective meso-scale ensemble system.

The Comparison of Six Methods to Calculate Turbulent Momentum Transfer Coefficient of Near-surface Layer

Hu Yanbing, Gao Zhiqiu, Sha Wenyu, Xiao Tao, Gao Chao

2007, 18(3): 407-411.

[FullText HTML] (714) [PDF 925KB](1535)

Abstract:
The momentum bulk transfer coefficient (C_M) is calculated by using six typical parameterization schemes and verified by the data of Naqu flux observation station of GAME (Global Energy and Water Cycle Experiment, Asian Monsoon Experiment)/Tibet Plateau Experiment. The results show obvious difference exists between results of the six schemes and the degree of difference is decided by the type of undersurface and the near surface stability. Wherein, schemes of Businger 71, Dyer 74 and B & H91 must calculate the turbulent flux transfer coefficient by iteration and waste CPU time for numerical simulation. For flux data of Naqu observation station which is covered by sparse grass is considered, when the Richardson number is less than 0.1, all the other five schemes can do better estimation on the C_M except the scheme of Businger in 1971 which has an obvious underestimation. Under unstable conditions, the scheme of Dyer in 1974 has the best estimation on the momentum bulk transfer coefficient (C_M), the schemes of Wang et al. in 2002, Launiainen in 1995 and Louis et al. in 1982 can also be used with gradually increasing error, and the scheme of Businger in 1971 has serious underestimation.

Features of the Tropical Cyclones Landing on China in 2005

Cao Xiangcun, Yuan Qunzhe, Yang Jili, Yi Haiqi

2007, 18(3): 412-416.

[FullText HTML] (803) [PDF 968KB](1460)

Abstract:
The tropical cyclone is one of the most catastrophic synoptic systems which influence the coastal regions of China. The gales, the storms, the billows as well as the storm tides, which could be brought by the tropical cyclones, can cause a great threat to the safety of the coastal people's lives and properties. Moreover, the worse disasters are always brought by the landing of the tropical cyclones. To better understand the rules of the tropical cyclones landing on China, the features of the tropical cyclones which land on China in 2005 are analyzed. It concludes that, in 2005, the landing season of the tropical cyclones is short, the landing points distributes unusually, the percentage of typhoons is unusually high and the loss is extremely serious. Furthermore, the climatic reasons of the phenomenon are discussed, and it is pointed out that the frequency of the tropical cyclones and typhoons landing on China per year is on the rise in future years.

Vol.18, NO.3, 2007