Abstract:In this study, the precipitation process of stratiform and embedded convective cloud during 5-6 October 2007 in the Beijing area was simulated using the Chinese Academy of Meteorological Sciences mesoscale cloud model. Based on observation data, the characteristics of the macro- and micro-structure of the cloud system and precipitation were analyzed. The authors also analyzed the moisture budget and precipitation efficiency of the cloud system. The results show that the stratiform and embedded convective cloud was the main precipitation cloud system in this Beijing-precipitation process. The precipitation distribution of the stratiform and embedded convective cloud was not uniform. In addition, the microphysical variables in the cloud system were not uniform in the horizontal and vertical directions. The stratiform and embedded convective cloud over the Beijing area had microphysical structural characteristics of a mixed phase cloud. The melting of snow was the main microphysical process contributing to raindrop formation. The main source region of water vapor for this precipitation process in the Beijing area was the Yellow Sea and Mongolia. Two air flows merged in the north of the Shaanxi Province forming a southwest airflow that transported water vapor to northern China. Outside the Beijing area, water vapor and hydrometeor was mainly transported to the region from the western and southern boundaries. In the main precipitation period in the Beijing area, the flux of the total water substance in the horizontal direction was the net inflow. The estimation of the water budget of the water vapor, hydrometeor, and total water substance showed that the water substance was generally balanced. The precipitation efficiency, condensation rate, deposition rate, and hydrometeor precipitation efficiency were 5.6%, 4.77%, 4.19%, and 44.9%, respectively, in the Beijing area from 2000 BJT (Beijing time) 5 October to 1400 BJT 6 October 2007.

Abstract:In this study, information transfer of the interaction between the tropical Indian Ocean and global atmosphere is examined by using sea surface temperature and geopotential height based on the definition of source/sink information. Information transfer between the tropical Indian Ocean (20°S-20°N, 50°E-100°E) and atmosphere in the tropics, Northern Hemisphere, and Southern Hemisphere is analyzed, and regional characteristics are indicated. In addition, decadal changes and seasonal differences of information transfer between the tropical Indian Ocean and atmosphere are discussed. Essentially, the ocean information source is mainly in an area between 10°S-10°N and 60°E-90°E, and the atmosphere information sink is mainly in mid-latitude, zonal distribution in both hemispheres. The atmosphere information sink in the tropics is mainly in the lower troposphere. The distribution of source/sink information in the interaction between the tropical Indian Ocean and atmosphere in different seasons shows that the atmosphere response is stronger in the winter hemisphere to the tropical Indian. Accompanied by the interdecadal shift of the large-scale general circulation pattern in the late 1970s, the changes in information transfer between the tropical ocean and atmosphere in the Northern and Southern hemispheres are opposite, i.e., the Northern Hemisphere strengthens, and the Southern Hemisphere weakens.

Abstract:By using a three-dimensional convective cloud model with the AgI seeding scheme, sensitivity simulations are conducted with various graupel densities and fall velocities. These parameters influence seven microphysics processes of graupel particles. Numerical simulations show that these parameters affect rainfall amount as much as 4.9%. The relationship between graupel fall velocities and wind updrafts are modified after seeding, and the values of collection of cloud water by graupel, collection of ice by graupel, and melting of graupel to rain water are influenced. When increasing only graupel density, graupel mixing ratios are increased significantly. Graupel density and fall speed parameter also change the rainfall efficiency of both seeding and natural clouds. Increasing the graupel density along with the fall velocity parameter results in relatively high rainfall efficiency, and their seeding effects are only 15% rather than the 25% noted in control seeding simulation. Therefore, in the simulation of the rime density of convective clouds, both the graupel density and fall velocity parameter should be increased; otherwise, the seeding effect will be significantly enlarged.

Abstract:This study projects changes in China's dry/wet climate in the 21st century using datasets from 21 climate models from the Coupled Model Intercomparison Project Phase 5 (CMIP5) under the RCP4.5 (Representative Concentration Pathway 4.5) scenario through soil moisture levels and a drought index, with the latter being calculated using eight surface meteorological variables. Results show that the models can reproduce the basic characteristics of the climatological dry/wet climate in China during 1986-2005, although there are some differences between the models and observations for the spatial pattern of dry climate over western China. Under the RCP4.5 scenario, the standardized precipitation evapotranspiration index and soil moisture levels would generally decrease across the whole of China, corresponding to an upward trend in the frequency of short- and long-term droughts, and a downward trend in the wet climate region. From 2016 to 2100, approximately 1.5 to 3.5 percent of the land area of China would change from a humid to semi-humid or semi-arid climate. With respect to the geographical distribution, the most obvious dryness would occur in northwestern and southeastern China where short- and long-term drought frequencies would increase remarkably, and the dryness would be detectable earlier than in other regions. Wetness would only occur in northeastern and southwestern China, but would be very weak. Seasonally, there would be warm season dryness in northern China and cold season dryness in southern China. The dryness trend of China would be primarily due to the decrease of available surface water as determined by precipitation and evapotranspiration levels.

Abstract:The temporal variation of ultraviolet (UV) radiation and the ratio of UV radiation to broadband solar radiation were analyzed in Yucheng using the radiation observations of 2005-2011, and the estimating equation of UV radiation was established using the data of air temperature, precipitation, and dew point temperature. The results showed that the UV radiation has consistent variation characteristics with solar radiation; the cumulative value range was 0.10-1.20 MJ m^-2 d^-1, and the average value was 0.468 MJ m^-2 d^-1. The highest and lowest values of monthly UV radiation were measured in June and January, respectively. The highest value of daily UV radiation was measured at noon, and the lowest value occurred in the morning or at night. The range of the ratio of UV radiation to solar radiation was 0.023-0.046. The seasonal highest and lowest ratios of UV radiation to solar radiation occurred in summer and winter, respectively. The ratio reduced as the clearness index increased, and the ratio was relatively stable when the clearness index was greater than 0.5. Finally, a UV radiation estimating equation was established by using temperature and precipitation. The determination coefficient R² was 0.80, and the mean bias error was 0.19. There are 95% data the difference in observed and estimated values was less than 1 level. Thus, the equation is appropriate for estimation.

Abstract:Using the daily National Centers for Environmental Prediction (NCEP) global final analysis (FNL) data and daily station rainfall data in China for the winters of 2000-2011, we first define an index to measure the strength of the South China quasi-stationary front (SCSF), and strong SCSF cases are then selected. The structure and characteristic circulation of strong SCSFs in relation to winter precipitation over South China are analyzed by composite analysis. The results show that strong SCSFs are nearly west-east oriented, are more common in January and February, and their frequency of occurrence shows a clear upward trend over the past 12 winters. The frontal zone of a strong SCSF is indicated by a dense band of potential pseudo-equivalent temperatures in the lower troposphere, slanting northward with altitude, with obvious differences across the band in temperature rather than in moisture. In addition, the frontal zone is often associated with a great temperature inversion, positive relative vorticity, and moisture flux convergence of north and south winds. Updrafts mainly appear above the slanted frontal zone, and disturbance zonal circulations with sub-frontal scale updrafts and downdrafts exist at about 850 hPa below the upper-level westerlies. Strong SCSFs can be classified into three types according to their wind convergence patterns in the frontal zone at 850 hPa. Among these three styles, the northerly convergence type is characterized by stronger cold air from the north and less precipitation in South China; the southerly convergence type features strong south winds, a deeper India-Burma trough, and more precipitation in South China; and the northerly and southerly convergence type falls in between these two. Winter rainfall in South China is closely related to a winter SCSF. In the presence of strong SCSFs, stronger moisture flux convergence and an enhanced ascending motion appear in the lower frontal zone over South China, accompanied by a deeper India-Burma trough in the mid troposphere to the west of the SCSF, leading to increased rainfall in South China.

Abstract:Storms of the Bay of Bengal have an important impact on southwest water vapor transportation, with double-peak periods (May and Oct-Nov) occurring in storm active stage. According to 2001-2010 JTWC (Joint Typhoon Warning Centre) storm data and NCEP (National Centers for Environmental Prediction)/NCAR (National Center for Atmospheric Research) 1°×1° reanalysis data on southwest water vapor transportation in double-peak periods, the storm water vapor is transported most strongly northward, followed by eastward transportation. Transportation in other directions is weaker. In the center and southwestern areas of the storm, each layer of water flux and all layers combined are higher than the climate average value for the southwest. In the double-peak periods, storm meridional moisture transportation is double than that of zonal transport. The actual southwest water vapor transportation for storms in May is twice as large as that in Oct-Nov. The effect of water vapor transport prior to the peak period (May) for storms in the Bay of Bengal is greater than that after the peak period (Oct-Nov). the Bay of Bengal storms are therefore one of the main systems for southwest water vapor transportation in May.

Abstract:The temporal and spatial distributions of the mean annual surface air temperature and annual precipitation over Central Asia during 1948-2011 have been studied using trend analysis and moving average methods based on the Climatic Research Unit (CRU) dataset and the output of the historical experiments from the Beijing Climate Center Climate System Model version 1.1 (BCC_CSM1.1) and the Beijing Climate Center Climate System Model version 1.1 with a Moderate Resolution (BCC_CSM1.1(m)) for the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR5). Heat flux and radiation flux were imported to further assess the capability of the two BCC_CSM versions in simulating the climate over Central Asia. The results show that these two versions effectively simulated the significant upward trend and north-south increasing characteristic of sensible heat flux and radiation flux over Central Asia. The performance of BCC_CSM1.1(m) in simulating the spatial distribution of air temperature, heat flux, and long/short radiation flux improved significantly compared with the results of BCC_CSM1.1. However, the performance of BCC_CSM1.1 in simulating the spatial distribution of the standard deviation of air temperature was better than BCC_CSM1.1(m). The improvement in model resolution more clearly demonstrated the topographic effects and improved the model simulation performance for heat flux and radiation flux. The high-resolution model displayed advantages in simulating the air temperature over Central Asia.

Abstract:Using high-resolution simulation data of typhoon Bilis (0604), the rainfall was separated into convective and stratiform precipitation. By comparing the cloud microphysical characteristics of the two precipitation types, their contributions to torrential rainfall amplification was assessed and determined as follows: (1) Before precipitation amplification, most precipitation are stratiform, with rainfall in only a few small scattered areas convective. During precipitation amplification, the convective proportion of precipitation increases significantly, with the mean precipitation intensity three times of stratiform precipitation. (2) During precipitation amplification, clouds develop more vigorously and the cloud hydrometeor content increases much more than previously. That is, both convective and stratiform precipitations have characteristic levels of growth of cloud hydrometeors, with a more obvious increase in convective precipitation. Meanwhile, both before and during precipitation amplification, hydrometeors content in convective precipitation is greater than that of stratiform precipitation, with the difference between the two rain types enhanced with increasing of surface precipitation intensity. (3) Before and during precipitation amplification, two main sources of rainfall in the convective precipitation region can eventually be traced back to cloud water. Through the interaction and conversion between cloud water and large liquid particles (rain drops), between cloud water and large solid particles (snow) and between large solid particles (snow and graupel), raindrops grow, ultimately generating surface rainfall. The processes associated with raindrop formation in the stratiform precipitation region are notably weaker. However, these processes in stratiform precipitation during precipitation amplification are stronger than those prior, indicating that stratiform precipitation also contributes to precipitation amplification.

Abstract:In this study, we use observational datasets to evaluate the performance of 34 models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) in simulating dry- and wet-season precipitation over southwestern China during 1986-2005. Of the 34 CMIP5 models, 30 and 25 models overestimate dry- and wet-season precipitation, respectively. The ability of 34 models to simulate dry- and wet-season precipitation is found to differ significantly. Moreover, approximately half of the models show that spatial correlation coefficients with the observations are significant at the 99% confidence level and the ratios of the simulated standard deviations to the observed values are less than 2. On the basis of the two skill scores, we select the nine best models for simulating dry- and wet-season precipitation. The 9-model ensemble mean performs better than the ensemble mean of all 34 models and most of the individual models. Therefore, we further use nine-model ensemble mean to project dry- and wet-season precipitation changes under Representative Concentration Pathways RCP4.5 and RCP8.5 scenarios over southwestern China. Compared with the climatology of 1986-2005, the dry-season mean precipitation is projected to increase in the west Sichuan plateau and decrease in the Sichuan Basin, the Panxi region, Chongqing, Guizhou, and the most of Yunnan province during 2016-2035. During the same period, the wet-season mean precipitation is projected to increase in west Sichuan plateau and most of Guizhou and Guangxi and decrease over Sichuan basin, the Panxi region, and Yunnan Province. Both dry- and wet-season precipitation is projected to increase throughout nearly the entire southwestern China region under the two scenarios in the middle and late 21st century. Under the RCP8.5 scenario, the changes in magnitude of dry- and wet-season precipitation are stronger than those under the RCP4.5 scenario.

Abstract:The Beijing Lightning NETwork (BLNET) consisting of ten stations has recently been developed, with each station composed of a fast antenna, a slow antenna and a lightning VHF (Very High Frequency) radiation detection system, forming a lightning network with capability formulti-band integrated lightning observation, including VLF, LF, HF, and VHF frequency bands.In this paper, a detailed configuration of the infrastructure of BLNET is described, and atheoretical analysis of lightning location error using the Monte Carlo methodis presented. The simulation resultsshowed that the 2-D location error was less than 200 m within the network and 3 kmat the range of 100 km outside the network. Finally, using a combined method incorporating Chan and Levenberg-Marquardt algorithms, we located all the cloud-to-ground lightning and intra-cloud lightning that accompaniedthe thunderstorm which passed over the network on July 7, 2013. A comparison of the location results with the corresponding radar data showed good agreement, validating a high performance of the BLNET and the proposed location algorithm.

Abstract:We used simulated data of high spatio-temporal resolution to analyzing the convective instability, conditional symmetric instability, and triggering mechanisms of a torrential rainfall event occurring in Beijing on July 21, 2012. The results indicate that convective instability played the leading role at the initial time of the precipitation. This instability weakened with the occurrence of the heavy rainfall, and conditional symmetric instability was enhanced by the increase in moist baroclinicity and the low-level jet, which maintained and strengthened the subsequent precipitation. Moreover, during the process of precipitation, strong vertical wind shear caused the baroclinic component of the moist potential vorticity anomaly, thus leading to the generation of conditional symmetric instability. Further, during the initial rainfall of the convective instability stage, the terrain lifting force combined with the ascending air on the shear line to boost and stir up the convective instability. In addition, dry air invaded Beijing at the midlevel, which contributed to the convective precipitation. The conditional symmetric instable precipitation resulted in a long-term clash between the cold and warm air over Beijing, which gave rise to the persistent precipitation. The warm air was lifted up by the cold air, which triggered the conditional symmetric instability. Moreover, at 0900 UTC, the wind shifted to an east wind and was abruptly enhanced. The wind was then lifted by the terrain coupled with the ascending air on the shear line, causing intensive upward motion and agitated instability, which is the primary reason for precipitation enhancement during 0900-1300 UTC.

Abstract:To explore effects on summer climate from land use changes in various metropolitan areas and the possible mechanisms, an atmospheric general circulation model (LMDZ) developed by the French dynamic meteorology laboratory was used. East Asia climate change induced by land use changes in the Pearl River Delta, Yangtze River Delta and Beijing-Tianjin-Hebei metropolitan area were simulated by the LMDZ. Results showed that changes of the underlying surface type in the Pearl River Delta, Yangtze River Delta and Beijing-Tianjin-Hebei metropolitan area significantly reduced the surface latent heat flux. In order to balance the surface energy budget, the ground temperature increased and the sensible heat flux and surface effective long-wave radiation were enhanced. The surface energy budget was re-equilibrated with an elevation of the surface temperature. The main changes were situated in areas of modified underlying surface type. The temperature response has a significant local characteristic. Among the different areas, there was a good relationship between the regional surface temperature change and changes in surface heat flux. Changes in the total heat flux of the Yangtze River and Pearl River Deltas were much greater than those in the Beijing-Tianjin-Hebei metropolitan zone. Local warming of the Yangtze River and Pearl River Deltas were also more than doubled in the Beijing-Tianjin-Hebei metropolitan zone. Although a local temperature increase is favorable to a thermal low pressure and induces significant upward motion in the lower layers of the atmosphere, the decrease in evaporation significantly reduces the local water vapor decreasing precipitation. In fact, the change in moisture conditions was the primary factor contributing to the precipitation reduction. Since the geopotential height field in high layers exhibited negative anomalies in the north and positive anomalies in the south, the Western Pacific Subtropical High extended westward and strengthened. As such precipitation reduction was not limited to local areas, but extended to large zones in the eastern part of the domain. This was particularly true in the three-area combined experiment.

Abstract:Based on previous studies (Mu and Zhou, 2012), the linkage mechanism between winter northern Eurasian total fresh snow extent (T_FSE) and subsequent summer climate anomalies in China is investigated in depth using empirical orthogonal function (EOF) and correlation analysis, with the primary purpose of finding the key region of fresh snow extent over northern Eurasia. The results show that the correlation between mid-latitude fresh snow extent in Europe (T_FSE-1) and subsequent summer climate anomalies in China is unremarkable. However, T_FSE-1 variability supports the physical mechanism for T_FSE summer climate correlation. Thus, T_FSE-1 provides a crucial influence on the climatic effects of T_FSE. The role of mid-latitude fresh snow extent in Asia (T_FSE-2) is unknown. However, winter fresh snow extent in Eurasian middle-latitude regions (T_FSE-1-2), which significantly affects the summer climate in China, dominates climatic effects of T_FSE because the correlation between T_FSE-1-2 and the summer climate in China has a clearer physical mechanism than the correlation between T_FSE and the summer climate, which suggests that T_FSE-1-2 could replace T_FSE in climate forecasts.

Abstract:Using datasets from the NCEP/NCAR Reanalysis, we study conventional observational datasets of snow depth, surface air temperature, and snowfall, and the spatio-temporal characteristics of wintertime snow depth over China. We found that snow depth is out of phase between Sinkiang, Northeast China and the area south of the Yellow River with respect to the leading mode of empirical orthogonal functions. Namely, the positive (negative) anomalies in snow depth over northern Sinkiang and Northeast China are associated with negative (positive) anomalies over the area south of the Yellow River valley. Spatial distributions of snow cover in the leading mode are inversely correlated with Arctic Oscillation (AO) in the past thirty years. During the negative AO phase, a cyclonic circulation with its center located at Lake Baikal is observed on the 500-hPa isobaric surface north of 40°N. Meanwhile, an anticyclonic circulation with its center located in Northwest China is also observed south of 40°N. As a result, North China and South China are controlled by cyclonic and anticyclonic circulation, respectively. In North China, anomalous cyclonic circulation associated with AO can result in excessive snowfall, lower temperatures, and ultimately increased snow depth. Meanwhile, in South China, anticyclonic circulation associated with AO may result in less snowfall, higher temperatures, and decreased snow depth. Our studies show that AO may influence wintertime snow depth over China significantly by affecting snowfall and surface air temperature.

Abstract:Features of Huaxi Autumn Rain (HAR) quasi-four-year period and their relationship to Sea Surface Temperature (SSTs) over the equatorial Pacific Ocean are examined using data from meteorological stations in Huaxi, NCEP/NCAR reanalysis data, NOAA SST data and the Multi-Taper Method-Singular Value Decomposition (MTM-SVD) method. Results show that central equatorial Pacific SSTs cause a notable quasi-four-year period in the HAR, with yearly characteristics that are stronger, a bit strong, weaker, and a bit weak, during each year within the period, and SST joint features that are lower, a bit low, higher, and a bit high during each year. The signal is present from early summer until late fall. Circulation analysis indicates that the HAR is strong (weak) when the summer central equatorial Pacific SST anomaly is low (high), the 500-hPa height field anomaly is positive (negative), the west Pacific subtropical winds are westerly (easterly), and moisture transport from the south China Sea and Bay of Bengal is more (less). The HAR also responds to the El Niño-Southern Oscillation (ENSO), and is mainly evident during strong ENSO events.