Abstract:Based on the monthly mean NCEP/NCAR reanalysis dataset and the observed surface air temperature and precipitation from 160 China stations, the seasonal march of the North Pacific Oscillation (NPO) and its association with the interannual variations of China's climate in boreal winter and spring are investigated in this paper. By employing the Empirical Orthogonal Function (EOF) analysis method, the NPO is identified as the second EOF mode of the monthly mean Sea Level Pressure (SLP) field over the North Pacific. The seasonal mean NPO indices are then defined as the average of the monthly mean indices for each season. Wavelet analysis reveals that the significant periods of the NPO indices are below 8 years for all the four seasons, indicating strong interannual variability and weak interdecadal variability of the NPO. Besides, the winter mean NPO index experiences significant linear trend towards its positive polarity. For all the seasons, NPO is featured with a large-scale meridional dipole in the SLP field over the North Pacific region and resembles the western Pacific pattern in the middle troposphere. The two surface centers are located around Aleutian Islands and the northwest of Hawaii, respectively. Their positions vary a little with season. Comparatively the south center experiences more zonal movement and the north center bears more meridional movement. Vertically the NPO is featured with an equivalent barotropic structure in summer and tilts a little westward with height for the rest three seasons. To put the NPO jet fluctuation in perspective, the positive phase of NPO is characterized by a northward shift and downstream extension of the East Asian jet stream especially in the jet exit region. The NPO variability is influential for the China's climate. Regression analysis indicate that during boreal winter the positive phase of NPO favors significant southerly anomalies along the coasts of East Asia, which may bring warm and moisture air from the south. Consequently, significant warming is observed over most areas of eastern China as well as the eastern part of the Tibetan Plateau. Meanwhile, more precipitation is observed over southeastern China, the Huaihe River valley, and several stations of northwestern China. During boreal spring, the influence of NPO is mainly confined to North China. More precipitation and weak cooling can be observed over this region in the positive phase of NPO. Such changes may be accounted for by the NPO-associated anomalous low-level wind convergence and the secondary circulation around the entrance of East Asian jet stream. In addition to the simultaneous influences, it is further found that the wintertime NPO is closely related to the temperature and precipitation of the following spring. If the NPO phase is positive in the preceding winter, significant warming will be observed over northern China and southwestern China in the following spring. Meanwhile, about 20% more-than-normal precipitation will be observed over southwestern China. Therefore, the wintertime NPO may act as a potential predictor for the climate of the following spring in China.

Abstract:The authors propose a method to separate the truncation error and the round-off error from the numerical solution. The analytical truncation error formulas of a partial differential equation are given for the upstream scheme and the centered difference scheme, respectively. The reference solution method is then introduced to separate these two types of errors for more general equations. A scheme based on the reference solution is used to obtain the approximate truncation error. Comparing the results for the upstream scheme and the centered difference scheme, the authors find that：1) the approximate truncation error is highly consistent with the analytical one. 2) The truncation errors of 1-D wave equations for the two schemes both show wavy periodicities with amplitudes being related to the parameters of computation. 3) The analytical error is suitable for the analysis of any slice of t, while the approximate one is only suitable for the analysis of a certain time range. However, the approximate error can be more easily obtained for general differential equations without a complex theoretical deduction.

Abstract:Climatic characters and changes of ice-freezing days in China are analyzed based on the daily rime and glaze observational dataset at 743 stations in China from 1954 to 2009. It is found that ice-freezing occurs mainly in Xinjiang, eastern Northwest China, eastern Inner Mongolia, Northeast China, North China, the Huaihe River valley and the region to the south of the Yangtze River, with the rime being almost in the southern part of China while the glaze in the northern part. Annual ice-freezing day generally ranges from 1 to 5 days and heavy ice-freezing regions scatter in northern Xinjiang, southern Shannxi, central Northeast China, eastern North China, Qinling mountain range, northeastern Yunnan, Guizhou, and so on. During 1954 to 2009, the ice-freezing days show a significant decreasing trend and an abrupt change point is observed in the early 1990s, with the decreasing amplitude of rime days greater than that of glaze days. This decrease in ice-freezing days has close relevance with the obvious increase in the temperature and decrease in the relative humidity and the wind speed. Additionally, the ice-freezing days are closely related to the extent and intensity of Asian polar vortex, the intensity of the Ural blocking high and the Baykal blocking high, as well as the extent, intensity and location of the western Pacific subtropical high. Further, during 1954－2009, the extent and the intensity of Asian polar vortex decreased, and the Ural and Baykal blocking highs intensified, the extent and the intensity of the western Pacific subtropical high decreased while the subtropical high shifted more westward. These changes in general circulation may cause the significant decrease in ice-freezing (glaze, rime) days in China.

Abstract:Based on the NCEP/NCAR reanalysis monthly data and daily precipitation data at 20 stations in eastern Southwest China from 1959 to 2006, the features of atmospheric heat source over the Tibetan Plateau and the key area associated with the drought/flood in summer over eastern Southwest China have been analyzed. Results show that there is a close relationship between the flood/drought over eastern Southwest China and the intensity of the atmospheric heat source over southeastern Tibetan Plateau (key area), i.e., the rainfall is more than normal while the heat source is stronger. It is propitious to appearance of circulation making for the summer rainfall in eastern Southwest China; furthermore, the vapor transport and convergent upward velocity are strengthened, the East Asian summer monsoon is weaker, and the ridge positions of the western Pacific subtropical high and South Asia high lean to south when the intensity of the atmospheric heat source over eastern Tibetan Plateau (key area) is stronger than normal, vice versa.

Abstract:This is a study to analyze the temporal-spatial differences of extreme precipitation events across the low-latitude highlands over the past 50 years, based on the daily precipitation data at 94 weather stations in the area during the flood seasons of 1961－2008. Meanwhile, the relationship between the interdecadal variation of heavy precipitation events and atmospheric circulations is examined in a preliminary manner using the NCEP/NCAR reanalysis atmospheric circulation data, the NOAA OLR data, and the National Climate Center (NCC) circulation indices data over the same period. Results indicate that the heavy precipitation events occurring in the flood season over the low-latitude highland areas are spatially distributed in a U shape, in the context of mean climate variation, with most heavy precipitation events appearing in the west, south, and east parts of the low-latitude highland areas. The heavy precipitation events occurring in the flood season over the areas exhibite a noticeable interdecadal variation, not only in the context of total occurrences, but also in the context of the density of occurrence. The 1960s and the period from the end of the 20th century to the early 21st century are the active periods of heavy precipitation events, and the period from the 1970s to the early 1990s is the less active period. At least one more heavy precipitation event occurs during the active periods than during the less active period. The annual frequencies that heavy precipitation events occur for 15－20 stations/times and over 20 stations/times are 13.0% and 4.4%, respectively, during the active periods, and 0% during the less active period. The condition of the western Pacific warm pool and the eastern Indian Ocean warm pool plays a major role in the occurrences of heavy precipitation events in the flood season over the low-latitude highlands.

Abstract:The northwestern Pacific is the only region where tropical cyclones (TCs) are generated all year round. China is one of the countries which are badly affected by TCs. A air－sea coupled model has been developed, focusing on the source region of the northwestern Pacific western boundary current system. Here, typhoon Kaemi is taken as an example to study air－sea interaction. The results show that: The SST cooling induced by the air－sea interaction weakens the typhoon, and the model results are consistent with observation during early simulation. However, 42 h later, the simulated typhoon is weaker than the observation. The typhoon track is scarcely influenced by the air－sea interaction. The SST cooling extent is determined by the relationship between typhoon moving speed and intensity and the maximum cooling is more than 4℃. The SST cooling zone drops behind the region of maximum wind speed along the typhoon track. The pumping and mixing induced by winds around the typhoon center bring cold water up and get the SST cooling. Such cold wakes last more than 54 hours. The changing of SST determines heat fluxes transport. Once the SST is constant, the heat fluxes are determined by the low-lying wind. Vertical convection is closely related to the typhoon intensity. Before 42 h, the ascending convection is the reason of typhoon intensification. Hereafter, the downdraft results in typhoon weakening. The coupling adds asymmetry of typhoon vorticity, and prevents its trend of ascending much higher. Maybe this is the mechanism of thermal factor affecting the dynamical structure.

Abstract:Cloud－climate feedback is an important process of air－sea interaction and it is a primary difficulty of climate simulation. By using results of the 20C3M coupled ocean－atmosphere models from the IPCC AR4 (Fourth Assessment Report of the Intergovernmental Panel on Climate Change) and observed data, interannual and interdecadal signals of the tropical Pacific sea surface temperature (SST) were extracted with the methods of filtering and Empirical Orthogonal Function (EOF). Then the authors calculated interannual and interdecadal cloud and heat flux feedback. Both interannual and interdecadal cloud and heat flux feedback of coupled ocean－atmosphere models are weaker than that from the reanalysis and observed data. The weakness may be caused by weaker sensitivity of tropical convection and cloud-to-SST change of coupled ocean－atmosphere models than real ocean－atmosphere. Though interdecadal thermodynamical feedback of coupled ocean－atmosphere models is weaker, SST interdecadal warming trend of coupled ocean－atmosphere models in the tropical Pacific between 10°S and 10°N is equivalent to the observed data. Only interdecadal thermodynamical feedback is not enough to explain the climate change of the tropical Pacific, the interdecadal dynamical feedback effect also must be considered.

Abstract:Based on the forcing from the NCEP (National Centers for Environmental Prediction) reanalysis data for the period 1992－2009, the newly developed MITgcm (MIT general circulation model) coupled ice-ocean model shows that the simulated variabilities of the Arctic sea ice extent/area are in good agreement with the observations derived from the SSM/I (Special Sensor Microwave Imager). On the basis of this, the ability of the MITgcm coupled ice-ocean model in forecasting the Arctic sea ice is investigated. Two cases are selected, one is during the melting period and the other is during the freezing period in 2009. Four forecasting experiments are conducted using atmospheric forcing from the NCEP reanalysis data and GFS (Global Forecast System), which use partly and entirely initialized SSM/I sea ice concentrations..The preliminary results show that the model does have the Arctic sea ice forecast capability. It demonstrates that the sea ice forecast is not very sensitive to different atmospheric forcings, whereas initialization using SSM/I sea ice concentrations can much improve the sea ice forecast. 

Abstract:By dividing the semi-arid and arid regions into three parts: arid region in the northwest, Loess Plateau, and cold region in the northeast, the regional differences in the diurnal variation of land surface radiation and energy processes in the semi-arid and arid regions are investigated using the data measured during July and September, 2008, provided by the Experimental Co-observation and Integral Research in the Semi-arid and Arid Regions over northern China. The results show that pronounced regional differences in land surface radiation and energy processes are seen in distinct climatic zones and the differences are primarily attributed to the regional differences in available water either in land or in the atmosphere. Consequently, the studies of the regional differences in land surface processes to a large extent are the analyses on how the arid climate for different intensities affects the land surface processes. For radiation components, downward short-wave radiation decreases with the latitude increasing, and the upward short-wave radiation is largest in the arid region and smallest in the cold region. The downward long-wave radiation is strongest in the cold region and weakest in the Loess Plateau while the upward long-wave radiation is obvious in the arid region and weak in the cold region. For energy components, 70% of the land surface available energy is consumed by heating the atmosphere in the arid region, and the remaining 30% is used for evaporation and heating the soil. In the Loess Plateau, however, only 30% of energy goes to atmosphere heating and 50% is lost to evaporation. Roughly half the energy is lost to evaporation in the cold region, and most of the other half is used for heating atmosphere.

Abstract:Based on the 740 stations daily rainfall datasets in China from 1954 to 2005 and the NCEP/NCAR reanalysis data, the interannual variability of the intraseasonal oscillation (ISO) of summer rainfall over the Yangtze－Huaihe River basin was analysed and the associated air－sea features were discussed. The results are as follows：(1) when the ISO of summer rainfall is more active, the 30－60-day oscillation is significant in atmosphere over the Yangtze－Huaihe River basin, the Bay of Bengal, the South China Sea and the sea area southeast of Japan. In contrast, when the ISO is inactive, the 30－60-day oscillation is not significant over the above-mentioned areas. But, the ISO in the atmosphere over the equatorial Indian Ocean and the equatorial western Pacific is more significant than that in the active years.(2) In the extreme phases of ISO active years, the low-frequency Rossby wave was very significant in the low layer, centered at the South China Sea, the sea area east of Japan, and the Gulf of Alaska. Analogously, there was a significant Rossby wave in the high layer, centered at the Ural Mountains, Lake Baikal, the Sea of Okhotsk, and Sakhalin.(3) The ISO intensity showed significant positive correlation with SST in pre-spring Kuroshio and simultaneous South China Sea. The numerical experiments showed that the positive SST anomaly in pre-spring Kuroshio (simultaneous South China Sea) could excite low-frequency responds in the atmosphere which extended via the Rossby wave train and enhanced the amplitude and phase characteristics of this wave train. Moreover, the ISO intensity of summer rainfall is more active in the year after ENSO.

Abstract:Based on the measured and retrieved products of the TMI (TRMM Microwave Imager) on board TRMM (Tropical Rainfall Measuring Mission) and combined with the microwave radiative transfer model, the simulation capability of hydrometeor variables of typhoon Chaba (0417) in the AREM (Advanced Regional Eta-coordinate Model) and the WRF (Weather Research and Forecasting) model is verified. First, the simulated track and rain of typhoon are studied and they are consistent with the best track and retrieved rain from the TMI, respectively. Then the simulated hydrometeor variables with the AREM and the WRF model are used as the input data of the microwave radiative transfer model to calculate microwave TBs (brightness temperatures). By comparing the simulated and measured TBs, the hydrometeor structures from the AREM and the WRF model are verified indirectly. Moreover, the hydrometeor structures are verified with the retrieved data from the TMI directly. The results show that, the content of cloud liquid water is overestimated by the AREM while the content of precipitation ice is underestimated. The WRF model underestimates the area of liquid hydrometeors, but the WRF model has better performance than AREM in the forecasting of frozen hydrometeors. Both the models perform well in the simulation of water vapor.

Abstract:The nowcasting method of precipitation based on the maximum overlap rate is an important part of short-term forecast radar system. On the basis of recalling the development of the radar nowcasting technology of precipitation, considering the merits of a new generation of weather radar with high spatial and temporal resolution, this paper proposes that within a short time interval, the moving direction and speed of the two adjacent radar echos with the greatest overlap rate are also the moving direction and speed of the precipitation system. The current radar echoes are translated by calculating the moving direction and speed, and are revised by use of nine-point average change amounts of echo dissipation model to form extrapolation forecasting; by use of the time weights synthesis method, the extrapolation forecasting every six minutes is synthesized to form the precipitation nowcasting field. In order to test the above-mentioned methods, the data at Hefei, Nanchang, Xiamen radar stations are used to carry out the calculation and prediction. The results show that: the moving direction and speed with the maximum overlap rate at two moments may reflect the overall movement characteristics of precipitation system, the calculation result is consistent with the subjective forecasting of forecasters and moreover it is objective, fine, and stable; using the growth and decay model to revise the translated echo can reflect the actual precipitation change; the 1-hour precipitation distribution which is synthesized by using precipitation forecasting every six minutes with the time weights synthesis method is in good agreement with the actual one.

Abstract:本文分析了中国科学院大气物理研究所大气科学和地球流体力学数值模拟国家重点实验室 (LASG) 发展的快速耦合模式FGOALS_gl对低纬太平洋区域水汽温室效应 (Ga) 和云辐射强迫特征的模拟能力, 讨论了模拟偏差的成因。结果表明, FGOALS_gl能合理再现Ga、 云辐射强迫的气候态空间分布特征, 但也存在明显的偏差。模式低估了冷舌区和西北太平洋Ga的强度, 原因在于表层海温 (SST) 和对流层 (特别是中层) 水汽的模拟偏差。模式总体上高估了云长波辐射强迫(Cl) 和云短波辐射强迫 (Cs) 的强度, 未能合理再现副热带北太平洋的Cl低值中心和东南太平洋的Cs大值中心。模式对Cl(Cs) 的模拟偏差主要来自高云量 (总云量) 的偏差, 云辐射强迫的模拟偏差还与云垂直结构和光学厚度的模拟偏差有关。从对El Niño型海温强迫的反馈来看, 模式能基本再现低纬太平洋区域Ga正反馈、Cl正反馈和Cs负反馈的空间分布, 但较之观测, 反馈大值区过于西伸。Ga正反馈的偏差主要源自低纬太平洋SST年际异常的强度和空间型, 及水汽对ENSO响应的偏差。高云量 (总云量) 对ENSO响应的偏差是导致Cl正反馈 (Cs负反馈) 的主要原因, 同时, 在部分区域云顶高度和云光学厚度对云辐射反馈的模拟偏差亦有影响。

Abstract:The first indirect radiative forcing and climatic effect of sulfate are investigated by using a Regional Integrated Environment Modeling System (RIEMS). The RIEMS can generally represent cloud and radiation processes by comparison with the MODIS data and the results from other models. The first indirect radiative forcing of sulfate over East Asia is negative and varies seasonally, with seasonal means being -0.88 W/m2 in winter, -2.27 W/m2 in spring, -2.41 W/m2 in summer, -1.47 W/m2 in autumn, and annual mean being -1.75 W/m2. The spatial distributions of simulated cloud droplet effective radius are similar to the MODIS data, showing larger effective radius over the sea (6－15 μm) than that over land (4－10 μm), and the decreasing trend from Southeast China to Northwest China, but the model results are systemically smaller than the MODIS data. The indirect radiative effect of sulfate generally leads to a decrease in the ground temperature, with maximum surface cooling reaching 1.5℃ in Chongqing and its vicinity in spring and autumn. The vapor decreases in winter and spring due to the ground temperature decreasing, but its changes seem much more complex in summer and fall, not only decreasing but also increasing, because the vapor transport is more complex. The changes in precipitation are consistent with those in water vapor, and show a general decrease trend over eastern China. The precipitation decreases in spring and winter over most areas of eastern China (0－60 mm), but the situation is complex in summer and autumn. In summer, the precipitation increases in the Sichuan Basin, the middle and lower reaches of the Yangtze River, and large areas south of the Yangtze River (0－150 mm), whereas in most areas of Northeast China and the North China Plain, the precipitation decreases (0－90 mm). The spatial averages of these changes over specific regions of China show that the ground temperature decreases in all regions and all seasons except Northeast China in summer where the ground temperature increases (0.2℃), the maximum decrease in the ground temperature appears in the region of Central China (as large as 0.5℃) in autumn. The changes in precipitation in these regions are smallest in winter (-0.1－0 cm), largest in summer. In summer, the decrease in regional mean accumulated precipitation can reach 1.7 cm in Northeast China, whereas the precipitation increases by about 0.2 cm in the region of Central China.

Abstract:Impact of the Madden-Julian Oscillation (MJO) on pre-flood season precipitation in South China is investigated based on a real-time MJO index and observed daily rainfall from gauge stations. It is shown that the rainfall patterns change from enhanced to suppressed in South China at intraseasonal timescales as the MJO moves from the Indian Ocean to the western Pacific. The most significant positive and negative anomalies are found in phase 4 and phase 7, respectively. The maximum positive and negative anomalies of regional mean rainfall are 17% and 11% relative to the climatological regional mean, respectively. Such different rainfall regimes are associated with the corresponding changes in large-scale background fields such as the western Pacific subtropical high (WPSH), moisture, and vertical motion. In phase 4, when the MJO is mainly over the Indian Ocean, the WPSH shifts farther westward, the moisture and upward motion in South China are increased, such environment is in favor of rainfall enhancement. In contrast, in phase 7, when the MJO enters the western Pacific, the WPSH retreats eastward, the moisture and upward motion in South China are decreased, such environment is in favor of rainfall suppression.

Abstract:The dynamic vegetation model, Interactive Canopy Model (ICM), which includes the ecosystem carbon and nitrogen cycling processes, is able to model the vegetation variations on short time scales. A reproductive organs carbon pool that constitutes flowers and fruits has been added into the original carbon and nitrogen reservoirs. It is assumed that the reproductive organs carbon pool will be a main carbon sink from the dates of vegetation flowering. A phenological model, ForcSar, is used to predict the flowering dates when the main partitioning positions of carbon and nitrogen change. The simulated LAI (Leaf Area Index) from the modified ICM is compared with the observations from satellite data in boreal middle and high latitudes where the vegetation varies the most severely all the year round. It shows that the seasonal variations of boreal vegetation can be better simulated than before. The modified ICM represents its maximal LAI in July, conforming to the reality, so that the problem of modeled vegetation growth lag in the original ICM has been rectified. The correlation coefficients of observed and simulated seasonal LAI are obviously higher than before. And the errors are decreased for all types of boreal vegetations. Besides, the correlation of observed and simulated interannual LAI is increased to some extent, even not very prominent. The changeable LAI from the ICM will alter the land surface conditions such as energy balance and water cycle. Solar radiation absorbed by vegetation, sensible heat fluxes and latent heat fluxes change significantly before the blossom of vegetation in the case of temperate deciduous broadleaf trees in eastern USA. And therefore the modeled physical properties of land surface will be influenced.