Vol.19, NO.6, 2008

Display Method:
Preliminary Results on Network Observation of Greenhouse Gases at China GAW Stations
Zhou Lingxi, Liu Lixin, Zhang Xiaochun, Zhang Fang, Yao Bo, Wen Min, Xu Lin, Fang Shuangxi
2008, 19(6): 641-645.
CO2 and CH4 are key greenhouse gases regulated by the Kyoto Protocol. Comparable atmospheric CO2 and CH4 concentrations at Waliguan GAW global station (36.17°N, 100.55°E, 3816 m) in western China are validated by long-term observations since 1990 to that of other background stations in the world. The observational data from the GAW stations are widely referenced by the WMO Greenhouse Gases Bulletin and relevant scientific reports. In the period of July 2006 to June 2007, preliminary data from grab air sampling at the 4 GAW regional stations in China show higher atmospheric CO2 and CH4 concentrations at Shangdianzi (40°39′N, 117°07′E, 293.9 m), Lin' an (30°18′N, 119°44′E, 138 m), Longfengshan (44°44′N, 127°36′E, 310 m) and Jinsha (29°38′N, 114°12′E, 750 m) comparing to observed values at Waliguan. Spatial and temporal variations of greenhouse gas concentrations are resulted from nature and human activities in different regions. It is inferred that nature and human activities have relatively distinct influence on the regional background atmosphere.In the past decades, there are kinds of long-term or short period observations and research in China and they are globally conducted by different agencies. However, spatial and temporal distributions of greenhouse gases could not be effectively documented and essential constraints to the understanding of the global carbon cycle and climate change can not be provided by any of these measurements. Thus, in the near future it is essential to establish a long-term observational network at multiple sites especially in China and to carefully calibrate the internationally agreed greenhouse gases and related tracers reference scales, and it has to be quality controlled under the GAW framework.These long-term measurements are of the highest quality and accuracy possible to identify trends, seasonal variability, spatial and temporal distribution, source and sink strengths of greenhouse gases to allow climate and carbon cycle researchers to improve the under standing of the carbon cycle and predict how the atmosphere and climate evolve in the future as a result of human' s activities.
Homogeneity and Long-term Trend Analysis on Radiosonde Temperature Time Series in China During Recent 50 Years
Guo Yanjun, Ding Yinhui
2008, 19(6): 646-654.
In order to study long-term trend of upper air temperature in China, radiosonde temperature time series during 1958-2005 at 116 Chinese stations are examined.Firstly, uncertainties in the time series caused by artificial errors and breakpoints are detected by quality control and homogenization procedure. Original radionsonde temperature time series at 7 levels are adjusted by employing hydrostatic and two-phase-regression (TPR) method.The identification results show significant discontinuities in the studied time series, especially in earlier period (1960s-1970s). Parts of the break points are documented by station' s metadata, which contributes to the changes of instrument model or method.Significant impact on long-term trends of original time series is caused by the adjustments which vary with different periods. The cooling trends in mid-upper troposphere during 1958-1978 are weakened by homogenization; also, cooling trends in lower stratosphere are enhanced and warming trends at 400 hPa and 500 hPa during 1979-2005 are weakened.Missing rate, as an important factor influencing the utilization of radiosonde temperature time series in China, is a reasonable index for sampling stations to assess regional average trend. Sampling by low missing rate (high data requirement) results in reduced number of potential stations. By analysis on averaged trend profile with different minimum data requirements (MDR), it is found that warming trends in low troposphere are enhanced and cooling trends in upper troposphere and low stratosphere are weakened by the decreasing number of stations in accordance with low missing rate or high M DR of stations. Critical maximum missing rate is suggested as 30% (MDR as 70%) for examining reliability of the time series. Averaged temperature time series in China are deduced by 92 radiosonde stations, which are selected by meeting maximum missing rate (or minimum data requirement by MDR).Homogenous radiosonde temperature time series show that trends of upper air temperature in China are generally consistent with that at global scale but with some discrepancies. During 1958-2005, atmospheric temperatures in China tend to decrease in the low stratosphere and upper troposphere, and increase in the middle and low troposphere. The trends of upper air vary with different periods. For 1958-1978, cooling trends in the entire atmosphere are similar to those at global scale. During 1979-2005, obvious warming occurs in the low and mid-troposphere; the amplitude of warming trend weakens with the increase of altitude and shifts to cooling trend above 400 hPa. Analysis on decadal averaged temperatures anomalies shows that cooling in low stratosphere has occurred since 1980s and enhances in 1990s and warming in midlow troposphere below 500 hPa has occurred since 1970s and significantly warming in the layer between 700 hPa and 300 hPa has occurred since 1980s.
Scenario Analyses on Future Changes of Extreme Temperature Events over China
Zhang Yong, Cao Lijuan, Xu Yinlong, Dong Wenjie
2008, 19(6): 655-660.
A regional climate model system, PRECIS, which is developed at the UK Met Office Hadley Centre for Climate Prediction and Research, and nested in one-way mode within the HadAM3P, a higher-resolution version of the atmospheric component of the Hadley Centre climate model HadCM3, is employed to verify the ability of the model to simulate the present (1961-1990) extreme temperature events and to project the future (2071-2100) extreme temperature events changes relative to the present in China under the IPCC SRES B2 scenario. It is indicated from the comparison of the simulated present results with observations that the localized fine-scale distribution characteristics of extreme temperature events in China can be simulated well by PRECIS. Under the IPCC SRES B2 scenario, the changes show there is an increase in hot days in most areas in China and the increase exceeds 500% for Northeast China, North China, Northwest China and Southwest China, while the changes also show that there is a decrease in frost days in most areas in China. The decreasing amplitude in northern China is smaller than that in southern China. There is an overall increase of the simulated future warm-spell days over most areas of China, an obvious increase occurs in Northeast China, North China, the middle and west of Northwest China and southeast coastal areas. There is a decrease of the simulated future cold-spell days over most areas of China. The decreasing amplitude is projected to reach or exceed 90% for Northeast China, North China, Northwest China, Inner Mongolia and the Tibetan Plateau.
Characteristics of Frost Changes from 1961 to 2007 over China
Ye Dianxiu, Zhang Yong
2008, 19(6): 661-665.
Based on the daily minimum temperature data at 577 meteorological stations over China from 1961 to 2007, the first frost date, the last frost date and the frost-free days are calculated by using the common climatic index on frost day. The results show that the standard deviation of the first frost date, the last frost date and the frost-free days is less in the north than in the south of China, which indicates that they are more stabile in the north than in the south. The inter-annual variation of the last frost date is larger than those of the first frost date, and the inter-annual variation of the frost-free days is larger than those of the last frostdate in most areas of China. It can be seen from the linear change trend that the last frost date is 2.0 d/10a ahead of time, the first frost date has 1.3 d/10a delay and the frost-free days prolong 3.4 d/10a over the past 47 years. From the climate trend it can be found that the range of the ahead last frost date is more than that of the delayed first frost date, which implies that the main cause of the prolonged frost-flee days is the ahead of last frost day. The first-frost date obviously delays since 1990s. The last frost date is obviously ahead of time and the frost-free days obviously prolong since 1980s. The ahead of last frost date is earlier than the first frost date delay in China from the inter-annual variation.
Characteristics of Wind Direction Change in China During Recent 50 Years
Jiang Ying, Luo Yong, Zhao Zongci
2008, 19(6): 666-672.
With the development of wind power industries, much attention is paid to the issues of the wind power's sustainable utilization against the global warming background. The stability of wind directions is very important to the effective availability of wind energy resources. The stability of the prevailing wind directions is not only able to enhance the availability rate of the wind energy, but also the life of wind turbogenerators can be prolonged. Therefore, the characteristics of the wind direction changes for the last 50 years in China are focused on. Based on the observation data sets of both wind speed and wind direction in the baseline and standard stations of China of four times per day, the wind data of 488 stations during 1956--2005 are chosen, the changes of wind directions in China for the last 50 years are analyzed. 16 wind directions are used. The statistic methods are employed to analyze the characteristics of the seasonal and annual wind directions, maximum wind directions and prevailing wind directions at each station in 12 sub-regions in China.It is found that the frequencies of the annual prevailing wind directions have decreasing trends, especially in North China, northwestern and southwestern China. There are slightly increasing trends in a few areas in West China. The wind speeds corresponding to the annual prevailing wind directions in most regions of China decrease much more obviously than the decreasing trends of the annual mean wind speeds. It indicates that the reducing annual mean wind speeds are mainly caused by the decreasing annual prevailing wind speeds. Both northerly wind (winter) and southerly (summer) prevailing winds in China decrease significantly. The reducing prevailing wind in China is lead to by the decreasing Asian winter and summer monsoon.
Characteristics of First/Last Frost Date Events and Frost-free Period in Dalian Area During Recent 46 Years
Wang Xiuping, Ren Guoyu, Zhao Chunyu, Yu Dehua
2008, 19(6): 673-678.
Based on the frost index defined by the daily minimum air temperature ≤ 0℃, and the method of climate diagnosis, the climatic features of the first/last frost dates and frost-free period changes are analyzed with 3 national stations data of daily minimum temperatures in Dalian area from 1961 to 2006. Results show that the first frost dates are postponed, while the last frost dates occur earlier, leading to the significant lengthening of frost-free periods at both Dalian and Wafangdian stations during recent 46 years, with the change being more significant at Wafangdian station. The tendency of the first/last frost dates is statistically significant at Dalian and Wafangdian stations, but it is not for Zhuanghe station, though the increase in frost-free periods at the station can still pass the confidence test. On the other hand, the characteristics in 1960s and 1970s are that the first frost dates comes earlier, the last frost dates ends later so that the frost-free period is short at each station. There is a trend that the first frost dates come later and the last frost dates appear earlier from 1960s to 1980s. The first frost dates are the latest, the last frost dates end the earliest and the frost-free period is the longest in early 1990s at Dalian station. It is notable that, in the past 6 years, the first frost dates appear much later and the last frost dates end much earlier at Wafang-dian and Zhuanghe stations, but the dates tend to be stable near the normal values at Dalian station. The change of extreme frost dates for the study area is also analyzed. It is found that the frequency of extreme early first-frost events ranges from 1 to 3 times, with Dalian station witnessing the highest value, and Wafangdian station the lowest. The frequency of extreme late last-frost events is from 1 to 4 times, but it is also the highest at Dalian station and the lowest at Wafangdian station. In addition, the variability of first frost dates indicates 2-year and 11-year cycle and that of last frost dates bears 22-year remarkable quasi-periods at Dalian station, while the first frost dates and frost-free periods at Zhuanghe station have undergone 11-year quasi-periods during recent 46 years. The frequency of anomalous frost events is the highest in 1960s at Dalian and Zhuanghe stations, and the lowest in early 21 century, but it is the highest in early 21 century and the lowest in 1980s at Wafangdian station.
Preliminary Studies on Variations in Droughts over China During Past 50 Years
Zou Xukai, Zhang Qiang
2008, 19(6): 679-687.
After preliminary quality control such as removing missing values, daily precipitation and temperature data of 606 observatory stations across China are selected from the original 731 station dataset to calculate drought index in different regions. Variations in droughts are analyzed based on the calculation results of the compound drought index IC. The drought index IC is recommended as an effective operational drought monitoring index from National Standard. IC has been applied by National Climate Center to monitor drought conditions across the whole country in recent years. The drought areas of different regions and the whole country during 1951-2006 are estimated by transferring the station observations into grid values. In order to discuss changes of drought areas in different regions during the past half century, Kendall'stau method is used to compute the trends of drought conditions.Results indicate that for the country as a whole, there is no obvious increasing or decreasing trend in drought area during past 50 years. Changes in drought areas are not even in different regions. Significant increases in drought area are found in North China and Northeast China with trends of 2.50%/10a and 2.31%/10a respectively. During the period of late 1990s to early of the 21st century, some regions in North China and Northeast China experience unprecedented drought conditions since 1951. There is no evident increasing or decreasing trend in drought areas in the mid-lower Basins of the Yangtze, South China and Southwest China. Decreases are found in Northwest China and the Tibetan Plateau since late 1980s. In China, the longest drought centers mainly occurred in western part of Northeast China, central North China and Yunnan Province etc. These extreme droughts often last for more than 4 months. Statistically most of the longest droughts occur in the period after the year 1980 during past 50 years in Northeast China, North China, eastern of Northwest China, South China and Sichuan Basin etc. Based on the calculations of drought days between the period of 1951-1978 and the period of 1979-2006, it suggests that droughts become much frequent in eastern and southern parts of Northeast China, North China, eastern Northwest China, South China and southern part of Yunnan Province in the later 28 years. While in the mid-lower Basins of the Yangtze and some regions of Southwest China, drought days decrease. No evident increases or decreases of drought days are found in other regions of the country.In addition, the drying trends in China such as North China, Northeast China and some regions of the eastern of Northwest China are mainly detected in the places where temperatures are increasing much more evidently across the country, which indicates that both high temperature and increased drying are produced by regional warming progress.
Identification on the Beginning Date and End Date of Rainy Season over North China and Their Climatology
Liu Haiwen, Ding Yihui
2008, 19(6): 688-696.
Semi-objective analysis method designed by Samel et al, Mann-Kendall test method and moving t-test technique are used based on the understanding of rainy season in order to investigate the beginning and end dates of rainy season over North China.The yearly initial date and final date of rainy season over North China are set up, and their climatological dates are further identified. The results show that rainy season in North China has an average duration of 50 days, starting on Jun 30 and ending on Aug 18 from climatology aspect. The period of heavy rainfall for the rainy season in North China is from Jul 19 to Aug 14, which is also the mature period of summer monsoon.The initial date, end date and the duration of rainy season and the frequency of no rainy season have regional features over North China. Rainy season has the earliest staring dates, late ending dates, longer duration days and the lowest frequency of no rainy season in the mountainous areas on the north of Hebei Province. Late starting dates and higher frequency of none rainy season are in the Loess Plateau while the latest ending dates and the longest duration days are on the north of the Loess Plateau. The earlier starting date, the earliest ending date, the shortest duration days and higher frequency no rainy season are in the mid-lower Basins of Yellow River, and the latest starting date, late ending date, longer duration days and highest frequency of none rainy season are in the west of Hebei plain. There are different trends for the initial dates, end dates, duration days of rainy season and frequency of none rainy season over North China. Duration days of rainy season have the most significant decreasing trend and the end dates also have obvious decreasing trend, while the total stations have significant increasing trend and the initial dates also have a slight increasing trend. All of these trends are of benefit to the decrease of summer rainfall over North China. There are close relationships between the rainfall of rainy season over North China and the initial dates, duration days of rainy season, the total stations of none rainy season. When rainy season begins earlier(later), duration days of rainy season last longer(shorter), the total stations are more(less), rainfalls of rainy season over North China are more(less).When positive anomalies of geo-potential height over the west Pacific exit, meanwhile negative anomalies of geo-potential height in the 500 hPa filed exit over North China, and the pattern is an anomalies of "east high and west low" between the Chinese mainland and west Pacific in the surface, the results are that the anomalies of south wind reach 30°N, the rainy season begins over North China. However, when smaller positive anomalies of geo-potential height in the 500 hPa field exit over North China, positive anomalies of geo-potential height in the 500 hPa field exit over Japanese sea, and the pattern is anomalies of "west high and east low" between the Chinese mainland and west Pacific, the results are that the anomalies of north wind influence Eastern China, the rainy season ends in North China.
Sources and Transfer of High Isentropic Potential Vorticity During Meiyu Period
Zhao Liang, Ding Yihui
2008, 19(6): 697-709.
The climatological mean sources and evolutions of isentropic potential vorticity(IPV)during Meiyu period are studied. Compared to the factors in the isobaric coordinate system during the Meiyu period, potential vorticity on the isentropic surface shows clearer "trough" and "ridge", and it is found that there are two high IPV "tongue" areas near the east of Lake Baikal and the southeast of Karafuto in the lower troposphere during the Meiyu period. And pentad IPV evolutions show that the regions where the meridional IPV gradient(MIPVG)weakens in the upper troposphere could be the entrance of high IPV air invading. Before the Meiyu rainy season the MIPVG obviously weakens in upper troposphere over East Asia, then high IPV contours begin to extend equatorward forming "tongue" area. Simultaneously, in lower troposphere MIPVG appears to reverse, shaping local north south dipole between low and high MIPVG. Evident potential vorticity transports and mass exchanges exist at tropopause near 40°N, 120°E during Meiyu period where the tropopause easily folds. By using 10—90 day bandpass and lead/lag correlation analysis, the sources and paths of high IPV anomalies are further investigated and traced. The results show that high IPV anomalies originate from the lower stratosphere and upper troposphere of the high latitude, and the maximum correlation coefficients between IPV anomalies and rainfall anomaly on 345 K isentropic surface before, during and after Meiyu period appear when the former leads the latter by about 10 days, near 55°N, 130°E in the east of Lake Baikal which is an important source of high IPV influencing the rainfall during Meiyu period. On June 10 before Meiyu, the high IPV air is transferred primarily southward along the NE—SW direction at 2 PVU surface from the high IPV source and accumulates near the steepest areas of this surface, developing upward, then crosses downward to the tropopause, and partly invades the south of 40°N when the IPV anomalies fields are typical longitudinal mode. However, in the troposphere it becomes a distribution of latitudinal mode spreading out like a fan. On 315 K isentropic surface, high IPV anomalies invade later than those in the upper troposphere, and are also transferred southward along the NE—SW direction from the high IPV source region near 45°N in the lower troposphere to the Meiyu areas since about June 18. As a result, the region on the east of Lake Baikal is possibly a main source area of cold air influencing Meiyu and key region to make the medium term forecast of Meiyu precipitation.
Climatic Features of Intraseasonal Oscillations of Summer Rainfalls over Mid-lower Reaches of the Yangtze River in the Flood and Drought Years
Wang Zunya, Ding Yihui
2008, 19(6): 710-715.
In order to understand further climatic characters of the intraseasonal oscillation and its relationship with the interannual variation of summer rainfalls over the mid-lower Reaches of the Yangtze River, the composite comparison analysis is applied to the intraseasonal oscillation in between the flood and drought years.Firstly, eight flood years and 7 drought years are identified by using 740 stations daily rainfall datasets in China from 1951 to 2004.And then differences in the period, the intensity and the phase cycle of the intraseasonal oscillation of summer rainfalls in between the drought years and the flood years over the mid-lower Reaches of the Yangtze River are discussed.The main results are as follows.The period of the intraseasonal oscillation of summer rainfalls over the mid-lower Reaches of the Yangtze River is relatively longer in the flood years, with the dominating 30 —60-day oscillation.While it is relatively shorter against a droughty interannual background, mainly being 10—30-day.Generally, the 10—30-day oscillation has greater amplitude than the 30—60-day oscillation in both flood and drought years, but the situation is opposite in certain areas over the lower Reaches of the Yangtze River or in some years.As for the intraseasonal oscillation, the amplitude is greater in the flood years than in the drought years.In the drought years, significant northward propagation can be observed to reach 50°N or so.But in the flood years, both the northward propagation from the low latitude and the weak southward propagation from the mid-high latitudes are obvious.And two branches of intraseasonal oscillations merge over the mid-lower Reaches of the Yangtze River and form strong oscillation centers.The phase composite analysis of the intraseasonal components of summer rainfalls in China and the corresponding low-level circulation show that the modes of the low-frequent anomalous circulation influencing the low-frequent summer rainfalls in China are similar in flood and drought years.However, the anomalous rainfalls and circulations on the intraseasonal scale are stronger under the flood background than the drought background, being an important cause for more rainfalls in flood years than in drought years.In conclusion, the intraseasonal oscillation has an important impact on the annual variation in aspects such as the period, the propagation, the intensity, the phase cycle and so on.
Characteristics of Vegetation Growth in the Farming-pastoral Zone over the North Parts of China Based on MODIS Data
He Yong, Dong Wenjie, Yan Xiaoyu
2008, 19(6): 716-721.
The farming-pastoral zone in the north parts of China lies in the west of Northeast China, the north of North China and the west of Northeast China, the total area is about 1.3 million km2.It is an important ecological vulnerable zone in China, and sensitive to the climate change.The research relative to the vegetation ecosystem in this area is important to the ecological construction and sustainable development.The NDVI of NOAA/AVHRR remote sensing data are usually used to reconstruct the ecological condition in recent studies in this area, and the MODIS and SPOT remote sensing data are less used.The MOD17A2 data from 2000 to 2006 with 2004 missing, the Gross Primary Productivity (GPP) of the land product from NASA of USA are used to asses the vegetation growth and its relationship with the climate in the farmingpastoral zone over the north parts of China because of their high spatial resolution.The highest GPP value of every 8 days in the studied area appears in June and the lowest in February, and the annual GPP changes from 0 to 835.4 gC ·m -2 with the average of 234.45 gC · m -2.Generally the GPP which is higher than 300 gC · m -2 distributes in the northeastern area and the forest area.The low GPP, which is lower than 300 gC ·m -2, appears in the middle and southwestern area, the grassland and shrub area.The correlation analysis between the GPP of every 16 days and the climate factor, temperature and precipitation at the same time shows that temperature is the main factor affecting the vegetation growth in farming-pastoral zone over the north parts of China, also, precipitation plays an important role.The phonological data of pasture, the emergence date, flowering date and withering data, from three agro-meteorological observation stations, Xilinhaote, Erguna and Yanchi from 2000 to 2006 and its corresponding GPP values are used to make assessment quantitatively on the growth stages of pasture.The GPP of every 8 days in the three stations from 2000 to 2006 shows the annual variation from 2000 to 2006, similar to the variation of the whole farming-pastoral zone over the north parts.The GPP values of the three stations are different, the GPP in Erguna is the highest which shows the regional vegetation growth characteristic in the farming-pastoral zone over the north parts of China.For the average phonological condition of the three stations, the GPP value at the start of flowering stage is the highest, ranging from 10 to 30 gC ·m -2, and the GPP values of the emergence stage and withering stage are similar of less than 10 gC ·m -2.For the same phonological stage the GPP value is different at different observation stations.Based on the results the MOD17A2 is a useful tool to the vegetation growth research in the farming-pastoral zone over the north parts of China.
Change of Urban Heat Island Intensity and Its Effect on Surface Mean Air Temperature Records in Southwest China
Tang Guoli, Ren Guoyu, Zhou Jiangxing
2008, 19(6): 722-730.
The surface air temperature records are obviously affected by the urbanization in China.The changes of the surface air temperature in Southwest China lag behind the countrywide changes, and air temperature records at some stations even show a dropping trend.It is important to understand the detailed features of surface air temperature change trends and the effect on them of urban development for different stations in that region.Using a data set of monthly mean temperature from 322 stations and corresponding population data, the surface air temperature change trends and the effect of urbanization development on mean surface air temperature records in towns and cities in Southwest China during 1961—2004 are analyzed.Quality control and inhomogeneity adjustment are made for air temperature data.On the basis of calculating air temperature change trends of towns and cities at national reference/baseline stations and rural stations, the extent and relative contribution proportion of urbanization effect are obtained by comparing the differences between the change trends of various stations and rural stations.Results show that there are warming trends of surface air temperature for each of the station groups.The warming rates in towns and cities, and national stations are greater than those in rural areas.In annual mean surface air temperature records in towns and cities, and national stations, urban warming rates are estimated as 0.086 ℃/10a and 0.052 ℃/10a respectively, and their contributions to overall annual mean temperature change are 57.6% and 45.3% respectively.Compared with the other regions in China, the warming rates of temperature and the urban warming rates in Southwest China are relatively low.Although the change of the average heat island intensity in this region is smaller than those in quite a number of regions in China, the contributions of urban warming to the overall mean temperature change trends are generally larger as a result of this feature.In addition, there are obvious season changes in urban warming rates, with the urban warming rates in autumn, spring or winter being the largest and second respectively, and that in summer the smallest. However, the largest contributions (100%) of urban warming to mean temperature in seasons are in spring, second to which are the contributions in summer of 73% and above.Those in autumn and winter are relatively smaller.
Relationship Between Rossby Wave Propagation in Southern Branch of Westerlies and the Formation of the Southern Branch Trough in Wintertime
Suo Miaoqing, Ding Yihui, Wang Zunya
2008, 19(6): 731-740.
Based on the calculation of wave number of Rossby wave and flux of wave activity, the characteristics of Rossby wave propagation in southern branch of westerlies are analyzed, and the relationship between Rossby wave propagation in westerly jet stream and the formation mechanism of the wintertime southern branch trough in the subtropical westerlies is also investigated by using 58-year monthly and daily NCEP/ NCAR reanalysis data with the aid of one point correlation, EOF, harmonic and composition analysis.Results show that the reare three westerly disturbances under the subtropical westerly jet stream in Arabian Sea, the Bay of Bengal, and South China of Northern Hemisphere during the winter half year. Southern branch trough over the Bay of Bengal is a semi-permanent trough due to the smallest variability. A teleconnection wave train with negative, positive and negative centers, migrating from North Africa to the Bay of Bengal via Arabian Sea, suggests that southern branch trough is positively related with the trough over North Africa.In the propagation process from North Africa to the Bay of Bengal, the westerly disturbances usually pause or strengthen in the Arabian Sea.The propagation from west to east takes about 20 days, appearing remarkable quasi-biweekly oscillation.There are waveguide regions of wave number 6—8 along the subtropical westerly jet stream in the half year of winter.Because jet stream waveguide is the strongest and the stationary Rossby wave energy in the lower troposphere coming from North Africa is transported to the Bay of Bengal, southern branch trough becomes deepest in February.Bay of Bengal is also a main source place of southern branch trough in the winter half year, and the development of trough over South China mainly connects with the eastern propagation of southern branch trough.Moreover, the activity of the cold air along the eastern and western sides of the the Plateau plays an important role in the formation of southern branch trough.After the breaking down of cold air, it first arrives at Bay of Bengal along the southern sides of the Plateau and then departs from the western sides of the Tibetan Plateau, and makes the southern branch trough gradually deepened.The eastern cold air of the Tibetan Plateau breaks down in the wake of cold surge breaking down in East Asia.Cold air at surface layer from north-east to south-west disperses to the region of India and Burma, which is another important factor of development of southern branch trough.
Impact of Climate Change on Runoff in the Huaihe River Basins
Gao Ge, Chen Deliang, Xu Ying
2008, 19(6): 741-748.
The assessment about future climate change on runoff of the Huaihe River Basins in China is focused. The Xin'anjiang monthly distributed hydrological model is used to simulate the past and future runoffs. The model is run with observed monthly precipitation and air temperature during 1961—2000 and the simulated runoff is verified with the corresponding hydrological observations.The verifications show that annual and monthly runoff are simulated well and the Nash efficiency coefficients range from 60% to 80% for the monthly simulations but above 80% for annual runoff at major hydrological stations such as Wangjiaba, Lutaizi and Bengbu.For runoff average in many years, the simulation is better.The correlation coefficient between simulations and observations reaches 0.99 and the mean absolute relative error is about 10%.The verified model is then forced with future climate scenarios based on three emission ones (B1, A2 and A1B) during 2011—2040 period from four coupled atmospheric and oceanic general circulation models to simulate future changes in the runoff.In the simulated future, the climate of Huaihe River Basins will be warmer and more humid compared with the climate of the base period of 1961—1990.Future annual runoff in the periods would have decreasing trends, which poses a challenge to the sustainable development of the Huaihe River Basins and water management of the East Route in the South-north Division Project.Monthly runoff would decrease in January and from July to December with high certainty and to increase during April to June with high uncertainty in most area of the Huaihe River Basin.During February and March, the monthly runoff would increase in the north of the Basin and to decrease in the south area to the stem of the Huaihe River as well as the Hongze Lake and the plain area of lower reaches of the Huaihe River.
Validation of Simulation on Surface Shortwave Radiation over East Asia by Global Climate Models
Wang Fang, Ding Yihui
2008, 19(6): 749-759.
Surface shortwave radiation (SSR) plays an important role in surface energy balance.The ability to simulate the disposition and variation of SSR in CGCMs has direct effects on climate projection for the future.Especially the simulation on East Asia monsoon may be affected by the impacts of SSR on surface thermal condition.So it is necessary to validate the ability of CGCMs to simulate SSR in East Asia.By use of 18 CGCMs output provided by WCRPCMIP3 and ERA40 reanalysis data, the ability of GCMs to simulate East Asia SSR is validated through ensemble analysis.The simulation shows remarkable differences among models.The simulated SSR and clear-sky SSR are generally higher than ERA40, while the effect of cloud on SSR (SSCRF) is generally weak.Multi-model ensemble is simulated at about 8.7 W/m2 and 3.4 W/m2 for SSR and clear-sky SSR respectively, and about 5.3 W/m2 lower for SSCRF as compared to ERA40.The standard deviation (STDEV) among different models is 9.6, 7.8 W/m2 and 8 W/m2 for SSR, clear-sky SSR and SSCRF, respectively.The phase characteristics are simulated well for seasonal variation of zonally-averaged SSR, although there is a great gap in magnitude.The simulation is obviously higher in the south of 30°N, especially in summer hemisphere, which is 30—50 W/m2 higher, mainly due to combined effects of low simulation on SSCRF and overestimation of clear-sky SSR both of which have a positive deviation on the SSR simulation. In the north of 30°N, SSR is mainly low by simulation due to the higher simulation of SSCRF.The root mean square deviation (RMSD) in summer is higher than in winter.Multi-model ensemble of RMSD is 34.7, 17.1 W/m2 and 29.1 W/m2 for SSR, clear-sky SSR and SSCRF, respectively, which shows the great effect of cloud on SSR modeling.The STDEV of different model RMSD is 12.5, 11.3 W/m2 and 10.2 W/m2, respectively, showing little difference.The linear decrease in annual downscaling SSR can be simulated rather well by multi-model ensemble. However, for clear-sky downscaling SSR and downscaling SSCRF, the simulation is not good, in other words, the decrease trend of clear-sky downscaling SSR is overestimated by models, while the adverse trend of downscaling SSCRF is given as compared to ERA40.
Evaluation of Extreme Heavy Precipitation in Coupled Ocean-atmosphere General Circulation Models
Zhang Li, Ding Yihui
2008, 19(6): 760-769.
Based on the surface observation of daily precipitation from 1961 to 1999 in China, the simulation capabilities of 18 coupled ocean-atmosphere general circulation models for extreme heavy precipitation in China are evaluated.The analysis of annual rainy days with different categories precipitation in models shows that the annual rainy days with 1—10 mm daily precipitation in China are overestimated in all of the 18 models.The annual rainy days with 10—25 mm daily precipitation are also overestimated in most areas of China in most of the models and the high value center locates in South China in observation northward to the mid-low reaches of Yangtze River Basin in some models although those models are able to capture the high value centers to some extent.The annual rainy days with 25—50 mm daily precipitation are obviously underestimated especially in the southern China in the models and the locations of high value centers are missed in most of the models.The annual rainy days with daily precipitation greater than 50 mm are also obviously underestimated in all of the models, the days in the models except MIROC3.2(hires) are less than 2 days in south of the Yangtze River.And most models are not able to simulate the right spatial distribution of heavy rain trend in eastern China.The analysis of extreme heavy precipitation in models shows that the thresholds of extreme heavy precipitation are generally underestimated in all of the models; similar increase trend in west part of Northwest China to observation are obtained in about half of the models, and the trend coefficients in some individual model are close to the observation.While opposite linear trend in northeast area and mid-low reaches of the Yangtze River is produced by most of the models, and the " +-+-" spatial mode in Eastern China for the extreme heavy precipitation can not be caught hold by any model; close standard deviations of annual rainy days with extreme heavy precipitation with observation are obtained by most models, which is maybe mainly due to the same analysis method among the observation and the models.In conclusion, it is very necessary to further improve the simulating capabilities of the coupled ocean-atmosphere general circulation models for extreme heavy precipitation.
Summer Precipitation Change over Eastern China in Future 30 Years Under SRES A2 Scenario
Li Qiaoping, Ding Yihui, Dong Wenjie
2008, 19(6): 770-780.
By using the nested regional climate model (NCC/IAP T63-RegCM_NCC), climate change over China during the past 30 years (1961 —1990) is simulated as the control run of the climatic background.Based on it, under the IPCC SRES A2 emission scenario, projection is made over East Asia with the aim of examining climate change scenarios for the future 30 years (2001—2030).The changes of summer precipitation are analyzed, with the key focus on the eastern China monsoon regions.The results show that, due to the effect of greenhouse gas increase, the summer mean precipitation presents increasing trend over northern regions but decreasing over southern regions.The summer rain belts expressed by rainfall departure are expected to shift to the North of Yangtze River.Furthermore, the increase of the total precipitation over northern regions is mainly caused by the increase of convective precipitation, with non-convective precipitation unchanged.With regard to the climate field of 1961—1990, annual mean precipitation in North China (35°—40°N, 110°—120°E) shows less increase by 2%—3% in the next 30 years, but summer mean precipitation is expected to increase remarkably with the maximum precipitation variability of about 19%.Based on the projection, drought over North China in summer is expected to be relieved to some extent in the next 30 years.Summer mean air moisture (especially in lower-layers) is also expected to change obviously in the future, with the atmosphere becoming wetter in mid-high latitude but drier in low latitude regions. East Asian summer monsoon will be much intensive during 2001 —2030.The southwesterly airflow is expected to be enhanced in eastern monsoon regions, which would lead to more warm and wet moisture transporting to the regions of northern China and then increasing the precipitation over that region.Considering the reliability of the projection mainly lies on the capability of the global and regional models as well as the reliability of greenhouse gases concentration, further experiments and integrated comparisons need to be done in the future.Based on the constant improvement of the GCM and RCM, the optimized schemes and the appropriate emission scenarios relative to the different research aim are needed to be chosen.Furthermore, other kinds of the greenhouse gases and aerosols also should be included in the model to decrease the uncertainties in the future climate projection.