Abstract:East Asian monsoon precipitation and circulation have experienced an interdecadal change in the last 50 years that was concurrent with the changes in global SST and recent global warming. What gives rise to this long term change has been an outstanding issue. One possible cause is the change over the Tibetan Plateau. To understand the possible impact of the Tibetan Plateau surface conditions on the downstream East Asian monsoon, a suite of sensitivity experiments were performed with Hamburgs atmospheric general circulation model (ECHAM). The land surface albedo was changed in two sets of sensitivity tests so that the Tibetan Plateau land surface temperature was changed accordingly.  The results show that a warmer condition over the Tibetan Plateau tends to enhance the upper tropospheric South Asia high and the westerly jet stream to its north and the Indian monsoon to its south, meanwhile the moisture transport toward East Asia increases. The changes in the precipitation pattern are featured by increasing rainfall over northwestern India and Meiyu and decreasing rainfall in the regions under the control of Pacific subtropical high and the Bay of Bengal.  Preliminary diagnostic analysis suggests that the plateau warming has initially increased sensible heating and convective heating locally, which then enhanced the low-level southwest monsoon transport toward down stream of the plateau, resulting in increased precipitation and latent heat release in eastern China. The changes in the overall diabatic heating strengthen the upper-tropospheric South Asia high and downstream subtropical trough. This result may have meaningful implications for understanding the interdecadal change and future change of the East Asian summer monsoon.

Abstract:This study analyses the influence of greenhouse warming on eastern China climate simulated by a high resolution regional climate model. The regional model RegCM3 is nested in one-way mode within a NASA/NCAR finite volume element AGCM (FvGCM). Two sets of multi-decadal simulations at 20-km grid spacing for present day (1961－1990) and future climate (2071－2100, IPCC A2 emission scenario) are conducted. Firstly the FvGCM and RegCM3s simulations of present climate in eastern China are analyzed. Results show that FvGCM is able to reproduce some basic features of the observed mean surface climate, including surface air temperature and precipitation, over eastern China. Comparing with FvGCM, results from RegCM3 simulations show that the high resolution regional model further improves the simulations of both surface air temperature and precipitation climatologies. It can reproduce more detailed structures of surface temperature distribution and reduce, in some extent, cold bias in the FvGCM simulations. Furthermore, it reveals that the observed heavy precipitation center in southeastern China is better simulated by RegCM3 although it may overestimate rainfall induced by topographic forcing. FvGCM gives a wrong location of the high rainfall center with its heavy rainfall belt over the Yangtze River basin.  Significant warming is simulated by RegCM3 at the end of the 21st century under IPCC A2 scenario. The warming of annual temperature ranges from 2.7℃ to 4.2℃, with greater warming in the north of the domain. Greater warming is also simulated in winter with a strong latitudinal gradient. A higher rise of the minimum temperature than the maximum temperature is simulated in the winter half of the year, which leads to a decrease in temperature diurnal variation. This feature is more significant in the northern part of the domain. Increase in the maximum temperature and the minimum temperature may cause more hot spell days in summer and less cold spell days in winter, as well as warmer winter and warmer night. General increase in annual mean precipitation is simulated by RegCM3, in a range of less than 10% to exceeding 30%. Higher increase is found to the north of the Yellow River and in South China. While a reduction of precipitation is found in broad areas in winter, in summer an increase in rainfall dominates the regions. Significant increase in summer precipitation can be found along the Yangtze River, in the southern part of Hebei Province and in South China. Increase of heavy precipitation days are simulated in the whole domain, with substantial increase found in the west and the south of Shanxi Province, the south of Hebei Province and in the north of Fujian Province, the west of Hunan Province. This can consequently lead to a higher frequency of floods in the future.

Abstract:Cold frontal precipitation case occurring during 31 Mar－1 Apr 2004 in South China is studied with CAMS mesoscale cloud resolving model. The simulation results are verified with observational surface rainfall, satellite and radar data. The occurring time, location, shape and orientation of simulated cloud bands and their evolution are consistent with the satellite observation. The distribution of simulated radar echoes is in agreement with radar observation. The southwest－northeast band echoes mainly occurred at surface frontal line and post-frontal part. The difference of individual echoes in different parts of the cold front is great. The distribution and amount of simulated surface rainfall during the main precipitating period coincide with the observations. The mesoscale southwest－northeast rain band moves southeasterly with cold front moving. There are four strong precipitating centers in the rain band, whose lifetimes are beyond 3 hours, and move mostly eastward, which agrees with the moving of radar echoes. The vertical motion of cold frontal clouds is deep and corresponding to the cloud area. The clouds appear in the areas of convergence, positive vorticity in the lower level, divergence in the upper level, and higher equivalent potential temperature. Near the surface front line, the updraft velocity is great, and the mass content of cloud water is great, then the processes of ice particles riming and collection of cloud droplets by raindrops are the main microphysical processes. However, in the wide rain band areas, updrafts only lie in upper layers, downdrafts are in lower layers, and there are mainly supercooled cloud water, graupels and snow crystals. Snow crystals are the main source term for graupel growing, precipitation is mainly formed by graupels melting, cold rain process is more important.

Abstract:基于与NCEP资料结果的比较，研究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发展的大气环流模式SAMIL对夏季南亚季风区季节内振荡（ISO）向北传播特征的模拟，并结合目前对ISO北传机制的理解对模拟结果进行了分析讨论。SAMIL在夏季南亚地区模拟出相当强度的季节内振荡的活动，并且模拟的ISO也表现出与NCEP资料相似的从赤道向北传播的特征，但传播的速度要慢于NCEP资料的结果。模拟的北传ISO具有与NCEP资料相似的结构特征，涡度和水汽场明显的呈经向不对称，涡度和水汽的正异常位于ISO对流的北面，最大的上升运动和最强的行星边界层辐合也位于ISO对流的北面。ISO 结构的经向不对称性正是模式模拟的ISO具有向北传播特征的原因；而模式对夏季南亚季风区高低层风场和行星边界层水汽的合理模拟起了关键的作用。同时，根据关于ISO北传机理的已有研究，模式的结果也表明南亚地区夏季风场的垂直结构是那里ISO向北传播的重要机制。

Abstract:By means of LDEO sea surface temperature (SST) and NCEP/NCAR reanalysis wind from 1982 to 1999, the relations of the eastern Pacific warm pool (EPWP) and abnormal meridional wind to ENSO events are analyzed. The results show that the EPWP climatologic SST has a clear intraseasonal variability, which is very similar to El Niño event growth process, i.e., occurring in spring, developing in summer, maturing in autumn and decaying in winter, meanwhile, the meridional wind anomaly and its divergence both are closely related to ENSO events. Based on synchronous considerations, a concept model of the EPWP and the meridional wind anomaly (north wind anomaly and convergence of meridional wind anomaly divergence) affecting the genesis and development of ENSO events is suggested, specifically, the north wind anomaly transports EPWP warm water to the equatorial neighborhood by producing southward ocean current, which, in turn, is instrumental to Niño3 SST increase, almost in the meantime, EPWP equatorial abnormal meridional wind convergence not only causes the warm water from north bank to pile up near the equator but also suppresses the cold upwelling around the equator, which, in turn, are favorable to Niño3 SST increase. Such factors are (not) favourable to genesis and development of (La Niña) El Niño events. Further analysis indicates that EPWP and abnormal meridional wind only play a motivating (suppressing) not decisional role in the genesis and development of El Niño (La Niña) events. Considering EPWP and abnormal meridional wind with inclusion of the western Pacific warm pool and abnormal west wind perfects further genesis mechanism for El Niño events. Finally, the characteristics difference of El Niño events occurring between in the 1980s and in the 1990s is simply examined.

Abstract:To test the applicability of the footprint model over complex surface, heat flux data measured from Beijing Xiao Tang Shan heterogeneous field experiment in May 2005 are used. The data from eddy covariance measurements and Large Aperture Scintillometer (LAS) are processed and analyzed by a footprint model, respectively. Because of heterogeneity, the measurement of eddy covariance method seems to be ambiguous, which contains turbulence information from other land covers. The LAS data have a problem of underestimation due to the blending height and ‘source area’ effect. After corrected by the footprint method, they are in good agreement. The results demonstrate that the footprint model is an effective and practical tool to solve the heterogeneous problem in the turbulence flux study.

Abstract:A new retrieval method for deducing path-integrated cloud liquid water content (LWC) by airborne upward-looking microwave radiometer is introduced in this paper. The coefficients of retrieval equation are obtained through the statistic regression method like the former method, but the statistical samples are produced by the one-dimensional stratiform cloud model. The new method is compared with the former one in which historic radiosonde data are employed to determine the vertical distribution of LWC by testing relative humidity above some threshold value. Besides, the detection errors are estimated through factor analysis and numerical simulation tests. The case on 8 July 2001 in Jilin Provence shows that the retrieval errors are diminished to some extent because the new statistical samples are consistent with the synoptic observations compared with the former method. The numerical simulation results indicate that the statistical relative errors decrease to 9.5%－12.7% and the retrieval accuracy is better than the former method at all altitudes. The factor analysis indicates that the errors caused by the uncertainty of the vertical distribution of LWC cannot be ignored, compared with the errors caused by the background and the instrument drift errors. In order to improve the retrieval accuracy, the authors use selected cloud samples which are supplied by a cloud model to regress retrieval coefficients according to the synoptic observation. The numerical simulation tests indicate that the retrieval accuracy can be improved at all altitudes.

Abstract:The atmosphere circulation variability is one of the fundamental problems in the researches on the tropical Indian Ocean Dipole (IOD) mode event. The authors analyze the variability of the wind vorticity field and divergence field during IOD event based on the observation and the numerical model. The result indicates that both the divergent and rotational parts of the wind field present the first order baroclinic pattern in the vertical direction and symmetric pattern in the horizontal direction respectively. In the lower atmosphere, the rotational part shows a pair of symmetric anticyclones around the equator during the positive phase of IOD event and the divergent part shows the divergence in the eastern Indian Ocean and the convergence in the western Indian Ocean. Both the divergent part and the rotational part could generate zonal easterly wind above the equator region in the tropical Indian Ocean, which supports the air－sea positive feedback mechanism. The upper circulation is just reversed with the lower one.

Abstract:Analysis of cloud water path (CWP) data over China available by the International Satellite Cloud Climatology Project (ISCCP) is performed for the period 1984－2004. The climatology, trends, and variability of CWP are examined. The climatological distribution and variation of CWP are dependent on the circulation, especially the monsoon circulation, topography and atmospheric moisture. Influenced by the Asia monsoon, China's CWP exhibits very large seasonal variations. All-China average shows the maximum CWP in June and the minimum CWP in October. Under the influences of the Tibetan Plateau and the westerly flow, the largest CWP is found in winter and early spring in southeastern China. Linear regression analysis is used to characterize seasonal and annual trends in CWP. Increasing trends in CWP are observed over most of China. Northwestern China, especially over the Tibetan Plateau, and the Inner Mongolia show significant increases of CWP. The largest increase in CWP is in winter and the increasing trend is weakest in spring. These increases in CWP are primarily dependent on the enhanced updraft deduced by the variation of circulation, including the weakening of the summer monsoon system. Interannual variation and trends in CWP are closely correlated to the rise of temperature in China, confirming the enhanced hydrological cycle under the background of global warming.

Abstract:Matsa (0509) is an extratropical transition (ET) typhoon which made landfall in Liaodong Peninsula in 2005. The PSU/NCAR non-hydrostatic mesoscale model MM5 is used to simulate the heavy rainfall process in Shandong Peninsula and Liaodong Peninsula during 8－9 August 2005. Several sets of numerical simulations are performed to study the impact of cold air on the rainfall of landfall tropical cyclone (TC) over Liaodong Peninsula. Results indicate that the intensity of convective cloud clusters, vertical upward motion and precipitation in Liaodong Peninsula are increased obviously when the cold air is invading the external area of TC. Both of the weak cold air in the lower and middle troposphere and the rather strong cold air in the upper troposphere can lead to notable intensification of radar reflectivity and ascending motion in Liaodong Peninsula, which is favorable for enhancement of precipitation there. The latter effect is more significant than the former one. Without the impact of cold air in the lower and middle troposphere, the heavy rainfall center would occur in the north of Liaodong Peninsula. The severe cold air in the lower and middle troposphere can result in the weakening of vertical velocity and TC circulation over Liaodong Peninsula, thus TC rainfall decreases remarkably in that area.

Abstract:The surface reflectance is an important parameter controlling surface energy balance. However, the heterogeneity of the distribution of urban buildings makes it difficult to collect in-situ measurements of surface reflectance. Therefore, there are few studies published on the measurements of urban reflectance (albedo) although there are a large amount researches on natural surfaces, such as crop, forest and desert. The complex urban canopy makes it difficult to simulate urban surface reflectance because of the difficulty in describing the urban configuration. Therefore, the urban areas are not well presented in the current land, atmospheric and climate models. This issue is addressed in this paper.  In this paper, a three-dimensional model is proposed, which is of high accuracy and high speed by combining the advantages of Monte-Carlo method and geometrical optical method. The model can simulate the surface reflectance over different urban configuration by incorporating “city map”. The MODIS (MODerate Resolution Imaging Spectroradiometer) bands 1－7 reflectance measurements during 2002－2004 are selected to validate the model. The MODIS TOA (top-of-atmosphere) reflectances are corrected for the atmospheric effect using the data of aerosol and water vapor collected by Beijing AERONET site with 6S (Second Simulation of Satellite Signal in the Solar Spectrum) atmospheric radiation transfer model. The validation results show that the model simulations of the surface reflectances are closely related to the MODIS measurements, with the correlation coefficients varying from 0.80 to 0.93, which means that the model proposed here can simulate the influence of solar and view angle on surface reflectance over urban areas. Finally, the influences of the urban configuration on the surface reflectance are discussed.

Abstract:Soil moisture simulated with 14 representative ocean-atmosphere-land system models is evaluated over China using observational data. The results show that the observed spatial patterns of soil moisture are captured by almost all coupled models. But the seasonal cycle and the interannual variation are poorly simulated by most coupled models. The relationship between soil moisture and precipitation is well simulated by most coupled models, but the relations are too stronger in some coupled models. The poor simulations of precipitation are one of the reasons for the poor simulations of soil moisture by coupled models. The soil moisture simulated by high-resolution models (horizontal grid systems higher than 1.875°×1.875°) is generally better than those simulated by low-resolution (horizontal grid systems lower than 1.875°×1.875°) and medium-resolution (horizontal grid systems between 1.875°×1.875°and 3.75°×3.75°) models in the spatial pattern and the interannual variation of the annual and summer means. However, the superiority of medium-resolution models relative to low-resolution models is not apparent.

Abstract:Combining the global distributions of black carbon (BC) mass concentration given by GADS (Global Aerosol Data Set), The direct radiative forcing due to BC have been calculated with an improved radiative transfer model under clear sky in both winter and summer. The results show that the radiative forcing by BC is positive at the tropopause and negative at the surface, which is different from the positive radiative forcing of greenhouse gases through the whole atmosphere. The reason is explained theoretically in this paper. The global mean forcing values are 0.085 W/m2 for winter and 0.155 W/m2 for summer at the tropopause, while they are -0.37 W/m2 for winter and -0.63 W/m2 for summer at the surface. Although the radiative forcing due to BC is highly dependent on the optical properties of itself and its concentration in the atmosphere, surface albedo and zenith angle also influence the forcing greatly. It is pointed out in this work that the absolute values of radiative forcing due to BC at both the tropopause and the surface increase linearly with the cosine of zenith angle and the surface albedo. The distribution of BC radiative forcing relies significantly on the latitude change, its maximum value is located between 30°N and 90°N for both winter and summer, which shows that anthropogenic activities are the main cause for BC radiative forcing.

Abstract:Based on the successful diagnostic study, numerical simulation was conducted for better understand of heavy rainfall in lower latitude plateau from 0000 UTC 31 May to 0000 UTC 1 June 2001. Due to successful reproducing of large-scale circulation, precipitation and rainy-cluster area, there existed the probability to study one of the rainy-clusters causing heavy rainfall by using the high resolution output data from the numerical model. It shows that the obvious mesoscale dynamical and thermodynamic structure inside the rainy-cluster defined as rainy-cluster A can be found during its development even though there was no obvious closed vortex in the wind field. The positive vortex maximum occurred earlier than rainy-cluster A, and the intensifying of ascending motion resulted from the horizontal convergence in the lower troposphere. The formation of rainy-cluster A was related not only to mesoscale convergence lines near the ground, but also to the special topography in the lower latitude plateau, especially, in Yunnan Province. The southern wind played an important role in the formation and the development of rainy-cluster A. And the appearance and evolution of rainy-cluster A was also related closely to the Indian-Myanmar trough at 500 hPa. The analysis of water vapor shows, there existed the pronounced horizontal and vertical water vapor convergence before the mature stage of rainy-cluster A, and the water vapor came mainly from the Bay of Bengal.

Abstract:Based on the analysis of the point-to-point correlation coefficient of the monthly mean 500-hPa geopotential height in the Northern Hemisphere (north of 20°N) from 1958 to 2002, the persistence of circulation is investigated. It is shown that there is a significant persistent area in middle latitudes of Asia, and this area is only one in the extratropics of the Northern Hemisphere. The central area is located in (30°N－50°N, 70°E－110°E), which is over the Tibetan Plateau, the Mongolian Plateau and the Loess Plateau . The persistence varies clearly with season. The persistence between February and March is little, between March and April is little too. Then persistence between April and May increases sharply, the inter-monthly correlation coefficient reaches 0.6; between May and Jun, it increases to 0.8. For summer to early autumn (June－September) every inter-monthly correlation coefficient remains over 0.8 stably. In autumn and winter the persistence weakens month by month, until early spring, reaches the lowest point.  The analysis is made from May to September during the most significant positive persistent seven years and negative persistent seven years in 1958－2002. It is shown that in the most significant positive persistent years circulation pattern “west-high－east-low” appears in middle latitudes of East Asia in summer; in the most significant negative persistent years circulation pattern “west-low－east-high” appears. In the positive persistent years eastern China summer rainfall is “north-below average－south-abundant”; in the negative persistent years summer rainfall is “north-abundant－south-below average”. The mean 500-hPa height anomaly changes from negative persistence to positive persistence during the 45 years, and correspondingly the eastern China summer rainfall pattern changes from “north-abundant－south-below average” to “north-below average－south-abundant”. The day-to-day recognition for weather system shows that high and ridge are prevailing over the middle part of Asia, trough and convergence zone systems are prevailing over the middle and lower reaches of the Yangtze River during the summer of positive persistent years; low and trough are prevailing over the middle part of Asia, subtropical high and southwest pattern are prevailing during the summer of negative persistent years. It is shown that there are two kinds of climatic courses and prevailing weather processes for summer drought and water logging in China. So, this work may be beneficial to predicting summer rainfall trend in China and summer circulation over East Asia in May and June.

Abstract:The relationships between summer rainfall in Xinjiang and Rossby wave activity (RWA) along West Asia subtropical westerly jet stream (WASWJS) and the North Atlantic Oscillation (NAO) are examined by using the NCEP/NCAR reanalysis dataset and 75 stations monthly precipitation data in Xinjiang from 1961 to 2003. The results show that Rossby wave train along Scandinavian Peninsula－middle Europe－WASWJS in summer connects NAO and RWA along WASWJS and summer rainfall variations in Xinjiang. Based on Eliasson-Palm flux (EPF) diagnoses, the characteristics and difference of stationary waves propagation in positive/negative NAO phase years are discussed．Not only does summer NAO impact current regions climate, but also WASWJS and Xinjiang climate in its lower areas. It is found that the strong divergence center of EPF over Scandinavian Peninsula is more westward (eastward) and stronger (weaker) than normal in positive (negative) NAO phase years. There are two horizontal wave activity propagation paths from the strong EPF divergence center over Scandinavian Peninsula in positive NAO phase years. First, the quasi-stationary wave from the EPF divergence center propagates eastward, turns southeastward near the Ural Mountains, enters subtropical westerly jet stream over the Caspian Seap－the Aral Sea, and continues propagating eastward, this wave propagation is stronger than normal. Second, the quasi-stationary wave from the strong EPF divergence center directly propagates southeastward and enters subtropical westerly jet stream near eastern Mediterranean－the Black Sea, so this quasi-stationary wave propagation along WASWJS is stronger than normal in positive NAO phase years. While in negative NAO phase years the quasi-stationary wave from the strong EPF divergence center propagates eastward in high latitude areas, and the quasi-stationary wave from the second strong EPF divergence center propagates southeastward and enters WASWJS near eastern Mediterranean－the Black Sea, the quasi-stationary wave propagation along WASWJS is weaker than normal. As a result, summer NAO activity anomalies cause EPF divergence intensification and position variations over Scandinavian Peninsula, and variations of wave eastward propagation in mid-high latitudes and the quasi-stationary wave activity along WASWJS are induced by these divergence variations, so summer rainfall anomalies in Xinjiang are affected.

Abstract:Although observations suggest there is a close relationship among the western Pacific tropical cyclones (TC), the monsoon troughs and the mesoscale convective systems (MCS), few real case numerical simulations associated with MCS have been performed. In order to further examine the contribution of middle tropospheric mesoscale convective vortex (MCV) forced convectively by MCS to the TC genesis, a real case numerical simulation of typhoon Durian (2001) is completed using the PSU/NCAR nonhydrostatic mesoscale model (MM5) with a high resolution (6 km). The results show: MM5 captures successfully the location of initial formation of Durian, which has a connection with the location of MCS as indicated by previous observations; the simulated track and evolvement of intensity resemble those of the best observation report, furnishing an accurate forecast of the landfall location, and revealing the slow descending and the sharp descending in the change of central minimum pressure; the simulated radar reflectivity reproduces well the boundary between the TC cloud system and the MCS cloud system in the monsoon trough during the genesis stage, also the eye and spiral cloud bands during the mature stage; the information of MCS included in the initial conditions of MM5 is crucial to this successful simulation. Moreover, the simulation reveals the presence of middle tropospheric MCV that has a 200 km horizontal extent and is located between 800 hPa and 400 hPa with a warm core, which is similar to observations. The verifications enhance the reliability for using the high resolution simulation results to investigate the role of MCV in TC genesis.

Abstract:Mass exchange between the layers in a numerical model should follow the mass conservation principle when dry convection happens in atmosphere. By use of this principle, the dry convection adjustment scheme in thep－σ regional climate model is improved. The improvement is mainly to adjust specific humidity and wind at the same time when temperature is adjusted. By use of the 1979－1995 NCEP/NCAR reanalysis monthly mean data, two simulations of the climate characteristics in January and July are conducted, one is to use the original adjustment scheme and the other the newly developed scheme in this paper. The modeled results are compared with each other. Furthermore, the F and t tests are utilized to examine the confidence levels of their differences between the new and the old methods. The results show that in both months, the adjusted areas are not evidently affected by the new scheme, but the times of adjustment increases in the new scheme. The F and t tests indicate that the differences between the two simulations are more obvious in July than in January. Moreover, the general mean climates of precipitation and specific humidity are not obviously different, but the differences of their variances are much more remarkable. As far as the temperature and the wind are concerned, the general mean climates and the differences of their variances are all very prominent. The sensitivity experiment with the new scheme shows that the simulation of wind field from 20°N to 35°N at 500 hPa in July is improved outstandingly; as far as the simulation of the precipitation field in July, the areas with false large-value precipitation in the north of Xinjiang, the vicinity of Hohhot, the east of Jilin, Hubei and Guangdong are eliminated.

Abstract:Six-station measurements on electric field change of lightning flashes were carried out in the summer of 2002 in Qinghai area ［ (37°3.795′N, 101° 34.94′E), 2553.32 m above sea level］, the northeastern verge of the Tibetan Plateau. By using the point charge model, the height and magnitude of charge neutralized by return strokes have been fitted, with the nonlinear least-square method, based on the electric field changes of cloud-to-ground (CG) flashes observed in a typical thunderstorm with larger-than-usual lower positive charge center (LPCC) occurring in the afternoon of 4 August 2002. A total of 65 return strokes in 16 negative CG and 2 positive CG flashes has been analyzed. The results show that the height of charge region neutralized by negative CG flashes ranges from 3 km to 5 km above the ground. For the two positive CG flashes, the neutralized charge region is located in a height range from 5 km to 6 km. It shows that the thunderstorm has a triple charge structure. The average charges neutralized by a negative return stroke and a positive return stoke are about 1.48 C and 2.02 C, respectively.