Abstract:TIGGE offers an opportunity to develop methods of applying global ensemble predictions systems (EPSs) from different models to improve the predictions of local hydrological risks. Comparative assessments of data from different EPSs improve the application of the grand ensemble forecasts from multiple sources. Based on the observed rainfall records from 10 stations in the Yishu River basin during August 2007, ensemble forecasts of 6-h precipitation from 5 EPSs, i.e., BABJ (Beijing), ECMF (ECWMF), EGRR (UKMO), RJTD (Japan), and KWBC (NCEP), are compared and assessed using several quantitative methods. Comparing the ensemble mean precipitation rates of all the EPS, EGRR scored highest with a correlation coefficient (R) of 0.48 and Nash-Sutcliffe efficiency (NE) of 0.21 in the lead time of four days; BABJ scored the lowest. For the control predictions of all the EPSs, RJTD scored highest with R = 0.19 and NE = 0.13 in a lead time of four days, followed by BABJ and EGRR. Compared with the control predictions, the ensemble mean of each EPS showed better performance. The multimodal ensemble mean showed a further improvement of the prediction skill. In the lead time of four days the multimodal ensemble mean had an R = 0.49 and NE=0.24. For different threshold values, the analysis of the threat score (TS) and Brier score (BS) showed similar comparative assessment. When the prediction lead time was increased, all the EPS showed a stable decline of prediction skills, with EGRR showing the longest and most stable decline period (9 days).

Abstract:On the basis of the fog droplet spectrum and visibility observations in Zhanjiang during a sea fog event in the spring of 2010, the microphysical characteristics and the correlations among microphysical properties, such as the fog droplet number concentration, the liquid water content, and the fog droplet mean diameter, were analyzed, and the dominant physical processes that govern the sea fog event were discussed. The result showed that the average fog droplet size distribution was in good agreement with the Junge distribution. In the development and mature stages of the sea fog, the broadening process of the fog droplet spectrum and the positive correlation between and showed that activation and condensational growth were the main processes. Turbulent mixing tends to make the droplets homogeneous, with a central diameter of approximately 3.5 μm. Simultaneously, turbulence causes an exchange between the inside and outside air, concentrations from outside air nucleation resulting in higher number concentration, and the condensational growth results in larger droplets.

Abstract:An iterative ensemble square root filter (iEnSRF) is designed on the basis of the latest asynchronous algorithm. In this iterative scheme, the forecast backgrounds at the analysis time and an earlier time are synchronously updated; then, the ensemble forecast is launched from the analysis field at the earlier time; finally, these two steps are repeated to produce an iterative analysis of the background at the analysis time. The performance of this iterative scheme is examined using simulated radar data assimilation with an idealized storm case. The iEnSRF results are compared to those yielded by a traditional EnSRF. In addition, the performance of iteration involving only the background at the analysis time is discussed. The results obtained using data from a single simulated radar show that iEnSRF can effectively retrieve the positive feedback between the vertical motion and the latent heat release in the presence of a poor initial condition that provides no storm information. This improvement significantly optimizes the balance between different variables in the initial analysis and increases the convergence speed of assimilation. Conversely, the traditional EnSRF is unable to retrieve this positive feedback relationship in the initial analysis with the same poor initial condition, resulting in slower convergence and a larger analysis error. Iterative analysis cannot outperform the traditional EnSRF if the iteration considers only the background at the analysis time, indicating that considering two backgrounds at different times is necessary for the iterative algorithm to produce improvement. Results using data from two simulated radars show that the iEnSRF still outperforms the traditional EnSRF, especially in the upper troposphere. A comparison of the results using single radar data and dual radar data indicates that the traditional EnSRF cannot effectively use more data to improve the initial analysis of non-observed variables such as temperature, whereas the iEnSRF can effectively use more data to further improve the initial analysis.

Abstract:The Ice Nuclei (IN) were observed in Nanjing from May to August 2011 using a 5 L mixing cloud chamber and a static diffusion cloud chamber, and the characteristics of different ice nuclei concentrations were examined. The results show that during the observation period, the mean concentration of total ice nuclei measured by the 5 L mixing cloud chamber was 20.11 L^-1, the concentration of IN under high water vapor conditions (5% calculated supersaturation with respect to water) and low water vapor conditions (5% calculated supersaturation with respect to ice) measured by the static diffusion cloud chamber was 0.928 L^-1 and 0.291 L^-1, respectively. The IN under favorite conditions had a diurnal variation. The daytime concentration of IN was larger than that at night and peaked in the afternoon, which indicates that atmospheric IN is significantly influenced by turbulence intensity, human activities, and industrial pollution. The scavenging effect of precipitation on IN was obvious, and the concentration of IN increased during the typhoon system. The IN levels in the Nanjing area increased with decreasing temperatures and increasing humidity. The back trajectory model showed that the concentration of ice nuclei in the air mass from the northeast marine region was highest, and that the number of ice nuclei in different air masses decreased with decreasing activation temperature. A case analysis focused on the relationship between IN and PM₁ (the particulate matter with diameter less than or equal to 1 mm) produced by biomass burning suggested that the product of the combustion may contribute to IN.

Abstract:A squall line that occurred in eastern China on August 17, 2009, was successfully simulated with the Advanced Regional Prediction System (ARPS) by assimilating high-resolution Doppler Radar data and conventional observations. Using the simulation output, we analyzed both the dynamical and thermodynamical characteristics of the squall line and diagnosed the environmental conditions of a back-building convective line that was a part of the squall line. Our major conclusions are as follows: (1) during the formative stage of the squall line, the southward-moving cold air from the low-level mesoscale convective vortex (MCV) in a clustered convective system, which was located to the north of the squall line, encountered southeastern warm moist air. This triggered strong convections and initiated the squall line. As the squall line propagated south, the role of the cyclonic circulation associated with the MCV in the maintenance of the squall line convection was reduced. Instead, cold surface outflow associated with the cold pool, which is an inherent part of the squall line, played a critical role by converging with the ambient air flow from the south. (2) The evolution of the squall line was enhanced by four convective lines that were formed by back-building convective cells. Several dynamical and thermal processes resulted in environmental conditions that favored the formation of the back-building convective cells. These conditions included: a small quantity of convective inhibition, a large amount of convective available potential energy, moderate vertical wind shear, strong low-level convergence, and large helicity.

Abstract:Based on data of droplet spectra, visibility, temperature field, wind field, and meteorological elements for twelve cases from a comprehensive fog experiment carried out at the radar station on Donghai Island of Guangdong from February to March 2011, we classified the weather synoptic system for fog occurrence and analyzed the meteorological elements, boudary layer structures, and microphysical properties. Furthermore, a typical case was studied. It is summarized that when Zhanjiang is located at forepart of depression, cold front and rearward of high pressure, the frequency of fog occurrence was high. During the fog events, the wind direction was mainly easterly near the surface, and southerly and easterly in the upper air. The range of fog droplets number concentration (N), liquid water content (LWC), and average radius was 170-372 cm^-3, 0.018-0.170 g m^-3, and 1.71-3.28 μm, respectively. The initial stage of the development of Case 11 was dominated by droplet activation and condensational growth. N increased with a slight variation of radius when the water vapor was not sufficient for the growth of fog condensation nuclei. In the mature stage, according to the auto conversion threshold functions, the collision-coalescence efficiency and the number density of large droplets increased, and the drop-size distribution expanded. Combined with the meteorological elements in the boundary layer, we demonstrated that the wind jets were significant at night, inhibiting the growth of the fog droplets, and the LWC was low. The reasons for the sharp increase in average radius might be the strong inversion of the virtual temperature and the low wind speed. Statistical analysis showed that the droplets spectrum had a bimodal distribution and could be described by the Gamma distribution.The relationships between LWC and the extinction coefficient and the effective radius were positive with a correlation coefficient 0.95 and 0.97, respectively.

Abstract:The extratropical transition (ET) process of a tropical cyclone will lead to a variation in the rainfall rate and distribution, which has long been a challenge to weather forecasters. Based on Chinese Meteorological Administration T106 data, satellite observed temperature of black body (TBB) data issued by Japanese Meteorological Administration and the Pennsylvania State University / National Center for Atmospheric Research (PSU/NCAR) numerical model MM5 output, the change of rainfall related to typhoon Winnie (9711) during its ET process is analyzed. Results indicate that the rainfall distribution displays a remarkable asymmetric feature. The heavy rainfall centers occur mainly in the northern part of the typhoon at first, and then move to the northeast and southeastern part of the typhoon circulation, revolving around the typhoon center clockwise during the ET process.Study shows that the change of rainfall distribution is related to the strong vertical shear of horizontal winds in the surrounding environment. It is found that the vertical wind shear is enhanced under the interaction between the typhoon remnant and the upper level westerly trough during the ET process, which is favorable for the rainfall concentration on the left side of downstream of the shear. The locations of heavy rain change clockwise along with the direction of the vertical wind shear. Meanwhile, the vertical structure of the typhoon vortex slants with height, inclining considerably to the north at its transformation stage, then tending to become perpendicular at its re-intensification stage, and inclining to the southeast afterward. We found that the heavy rainfall occurs in the slanting direction of the typhoon vortex. On the other hand, the rainfall locations were also related to the thermal advection activities in the typhoon circulation, which have a good relationship with warm air advection at the lower layer. Moreover, the vertical component of the convective vorticity vector can reflect the impact of the wind vertical shear and mesoscale frontal zone in the ET typhoon circulation synthetically. Its high value area at 800 hPa can indicate the heavy rainfall position within the typhoon circulation.

Abstract:A regional climate model (RegCM3) nested in one-way mode within a NASA/NCAR finite volume element AGCM (FvGCM) is used in this study. Two sets of multi-decadal simulations are conducted at 20-km grid spacing for present day (1961-1990) and future climates (2071-2100, A2 scenario). The runoff outputs are used to drive a large-scale routing model (resolution, 0.25°×0.25°), to project the effect of future climate change on hydrological extreme events over the Yellow River and Yangtze River basins. The results show that the annual variability of the streamflow over the Yellow River will increase, and the monthly variability will decrease. The diurnal variability will decrease upstream and increase downstream of Toudaoguai station. A greater possibility of the largest streamflow percentile will exist in the upper reaches of Lanzhou station in sub-humid areas and southeast of the Yellow River basin in humid areas, whereas the possibility is less in the arid and semi-arid areas of the northwestern basin. The flood disaster risk will increase in future over the middle reaches of the Yellow River except in the summer season. The annual variability of the Yangtze River may increase, and the diurnal and monthly variability may decrease in the upper and middle reaches and rise slightly in the lower reaches in future. The annual, monthly, and diurnal variability of the streamflows all increase in future over the Hanjiang River basin. The flood disaster risk may decrease over the Yangtze River, especially in summer, and the probability of flooding in the Hanjiang River basin will increase in each month.

Abstract:The mesoscale operational numerical model known as Global and Regional Assimilation and Prediction System GRAPES-MESO v3.0, from the National Meteorology Center of the China Meteorological Administration (CMA), was used to study the application of a one-dimensional variational (1DVAR) surface-precipitation assimilation scheme in the GRAPES-three-dimensional variational (3DVAR) data assimilation system (Expt ASSI) by using the experimental forecasts of the period June 1-30, 2010. The results are then compared with those of experiments without rainfall assimilation (Expt CNTL) to evaluate the application effects of assimilating 1 h intensive nationwide rainfall data into the GRAPES-3DVAR. The results are summarized in the following points: 1) 1DVAR precipitation assimilation can provide a meaningful modification for the moisture profiles by providing rainfall analysis results that are close to those determined through observation in the limits of moisture background errors and rainfall observational errors. The initial fields were obviously improved in Exp ASSI, in which the temperature, pressure, moisture, and wind values were modified to be closer to the observed values. 2) For the continuous precipitation process south of the lower reaches of the Yangtze River and in South China during June 17-21, 2010, the daily rainfall and hourly precipitation forecast of Exp ASSI was generally stronger than that of Exp CNTL and were closer to the observed values. 3) The threat score (TS) and equitable threat score (ETS) of 0-24 h rainfall forecasts at 0800 BT from Exp ASSI were better than those from Exp CNTL for rainfall levels of 1 mm, 10 mm, 25 mm, 50 mm, and 100 mm. Moreover, its forecasting bias is much closer to 1.0. The TS and ETS of 0-24 h precipitation were increased, and the forecast bias was decreased after assimilation of the 1 h accumulated precipitation in the GRAPES-3DVAR. 4) The distribution, evolution, and intensity variation of the rain region in Exp ASSI were better than those of Exp CNTL. 5) The rainfall ETS score for one month and the verification of typical heavy rain cases indicate that the assimilation of surface precipitation data in the GRAPES-3DVAR by using the 1DVAR precipitation scheme can improve the precipitation forecasts of GRAPES-MESO v3.0.

Abstract:The variations of temperature in winter in eastern Asia during 17 stratospheric weak polar vortex events were investigated based on the standardized temperature departure and the Northern Annular Mode (NAM) index calculated by using NCEP data set. Results show that the circulation anomalies in the stratosphere occurred about 15 days before the temperature variation in the troposphere. In the latter period of the weak stratospheric polar vortex, the surface temperature departure reached its peak. At that time, the regions south of latitude 40°N were warmer than the climatic average and the regions north of 40°N were colder. The variation of potential vorticity and the Asian trough at 500 hPa are also taken into account when discussing the physical process and mechanism of the anomalous surface temperature. In addition, cold air with high potential vorticity remains near 60°N in the early and middle period of the weak vortex and begins to extend southwards in the late period. In the middle and upper troposphere, cold air from the high latitudes can migrate to 45°N. At the same time, the Asian trough at 500 hPa becomes stronger and the low pressure region moves eastward; the ridge near Lake Baikal becomes weaker, the activity of cold air behind the trough influences Northeast China, northern China and more northerly regions and causes the surface temperature to fall. Moreover, warmer air with low potential vorticity moves from the south into the regions south of 40°N causing the surface temperature there to rise.

Abstract:Using the objective definition and auto-identification and tracking technique, the database of extratropical cyclones (ECs) in the winter half-year in northern China from 1951 to 2010 is built on the basis of the NCEP/NCAR reanalysis data. Furthermore, the climatology and interannual variability of ECs are studied. The results show that cyclogenesis is mainly focused in the central-west region of Mongolia and the eastern foot of Yablonoi Mountains. ECs are active mostly over southeast of Mongolia, southeast of Inner Mongolia, and the Songnen Plain; however, cyclolysis and rapid decay of ECs occur over the southeast of inner Mongolia and central-north of Northeast China. The frequency of ECs is the highest in early spring and middle autumn and the lowest in winter. With seasonal evolution, the active region of ECs moves eastward and southward. The eastward path is the most common track of ECs, after which comes the southeastward track, and then the northeastward one. Horizontal winds at 700 hPa can be considered as the steering flow of ECs. The rapid growth of ECs, whose standard deepening rate is greater than 0.5 Bergeron for 24 h, is mainly active over the east of Inner Mongolia, central-north of Northeast China, and the central Japan Sea. However, explosive ECs are spread mainly over the Japan Sea and marginally over the continent of China. The extremely strong EC events occurred mostly in the early spring and mid autumn before the 1980s. However, frequency of ECs was relatively low, the frequency of extremely strong ECs has increased since the 1980s. The decrease of ECs is related to the weakening of the baroclinicity in the lower troposphere of Northeast Asia.

Abstract:Using the CloudSat tropical cyclone crossing dataset from 2006 to 2010 and dividing the data into different evolutionary stages according to wind speed, the cloud, precipitation, and thermal structure characteristics of typhoons at different stages in the eastern Pacific are comprehensively analyzed. The results show that radar reflectivity has opposite distributions above and below 5 km in height, and the echo intensity and top height decrease along the radial direction. Large differences appear between the distribution of different types of clouds along the radial and vertical directions, and the vertical scale and occurrence probability of deep convective clouds are always large. The effective radius, distribution width parameter, and ice water content decrease but the particle number concentration increases as the height increases. Each ice cloud parameter and the rainfall rate decrease gradually along the radial direction. The rainfall rate at each stage in summer is generally larger than that in autumn, and along the latitudinal direction in each season, it varies within different stages. The rainfall rate in the inner core area follows a nearly exponential distribution and is sensitive to warmer sea surface temperatures, and its scatter plot with radar reflectivity converges in three regions. There is a hot core structure at 5-10 km in height in the inner core area; below it a wet core area exists, whereas an area with a larger anomaly in the relative humidity corresponds to the cirrus canopy at the top of the typhoon. Atmospheric stratification is convectively stable above 4.5 km in height at each stage; below that, it varies between stages, and the pseudo-equivalent potential temperature decreases along the radial direction.

Abstract:Observational and National Centers for Environmental Prediction (NCEP) analysis data are used to diagnose the characteristics of synoptic circulations, sources of water vapor, and mesoscale convective systems (MCS) related to the formation of heavy rainfall on July 21, 2012. Results indicate that the locations and intensities of upper- to lower-level synoptic circulations and mid- to low-latitude systems are favorable for the heavy rainfall formation over North China that occurred under typical weather patterns with high pressure to the east and low pressure to the west. Synoptic systems such as vortices, shear lines, troughs, cold fronts, and low-level jets affect the formation and intensity of heavy rainfall. Tropical cyclones in the Bay of Bengal and the western Pacific Ocean provide favorable conditions for the transportation of water vapor from the ocean to the mainland. The sources of water vapor to North China are the Bay of Bengal, the Bohai Sea, and the Yellow Sea. Water vapor in the lower troposphere is mainly transported from the latter two sources, and water vapor in the middle troposphere is mainly transported from the former. The entire heavy rainfall process involves two stages. During the first stage, the triggering of MCS is associated with cold air invasion, terrain, and easterly wind in the prefrontal area. During the second stage, the organization and intensification of MCS are forced by the cold front. Under favorable synoptic circulations, the frequent development of stable and persistent MCS is important for the formation of the heavy rainfall event examined in this study.

Abstract:The relationships of the March precipitation anomalies over South China with the associated circulation and the global sea surface temperature (SST) field are analyzed by using observations of March rainfall over South China, NOAA SST data,Niño3.4 index, and NCEP reanalysis data for the period 1951-2005. The possible causes of the March precipitation anomalies over South China were revealed from the point of view of the atmospheric circulation and SST field. The results show that when March rainfall over South China is above (below) normal, a cyclonic circulation (an anticyclonic) anomaly exists in the middle-low-level of the troposphere over the North Pacific Ocean, and an anticyclonic (a cyclonic) circulation anomaly appears over the western Pacific Ocean and the South China Sea. These circulation anomalies are favorable (unfavorable) to the southeast warm-moist air flow and the abnormal cold air in the east of northern China which form a water vapor convergence over South China, leading to a significant increase (decrease) of precipitation. Further analysis showed that the El Niño-Southern Oscillation (ENSO) and North Indian Ocean and South China Sea SST are important external factors that influence the March precipitation anomalies over South China. The influence of ENSO on the March precipitation anomalies over South China is through the western Pacific subtropical high and low-level wind field anomalies, and the influence of the North Indian Ocean and South China Sea SST on the March precipitation anomalies over South China is affected by the vertical circulation and low-level wind field anomalies, as well as the abnormal western Pacific subtropical high.

Abstract:Severe floods occurred in the Yangtze River valley in 1998 and 2010. The atmospheric circulation patterns responsible for the disastrous events share a common feature: frequent occurrence and persistent presence of the blocking high over the Ural Mountains. To understand the formation and longstanding presence of the Ural blocking high, a comparative analysis was conducted by utilizing the daily data from the NCEP/NCAR reanalysis. The results suggest that (1) a significant positive persistent anomaly event occurred over the Ural in the summer of 2010, which was the second longest (37 days) since 1948 with the peak of the anomaly located slightly west of the Ural, whereas the 1998 event was shorter (22 days). (2) Significant abnormal planetary wave activities were seen in both cases. The planetary wave components with wavenumbers 2 and 4 played a dominant role in the 2010 case, but those with wavenumbers 1 and 2 were more important in the 1998 case. (3) Significant abnormal convections occurred over the tropics, which may have modulated the planetary wave activity and were responsible for the difference between the cases. (4) An intensified convection pattern emerged in the east tropical Indian Ocean region west of Sumatra in both cases. It might have significantly contributed to the geopotential height anomaly chain originating from the tropical Indian-Pacific Ocean and propagating toward the middle and high latitudinal Pacific; thus, the anomalous planetary wave activities excited a wave-train similar to the well-known East-Asian Pacific pattern.

Abstract:As the main long-lived greenhouse gases, CH₄ and N₂O are included in the Kyoto Protocol, and countries are required to limit the rapid increase in their emissions since the Industrial Revolution. In this work, a radiative transfer model with a high resolution of 998 bands is used to calculate the instantaneous radiative efficiencies, stratospheric- adjusted radiative efficiencies, and lifetime-adjusted radiative efficiencies of CH₄ and N₂O for clear and cloudy skies, as well as their global warming potentials (GWPs) and global temperature potentials (GTPs). Simple fitting formulas for calculating the adjusted radiative forcing due to CH₄ and N₂O are given on the basis of the model results in this work. It is shown that the radiative efficiencies of CH₄ and N₂O for cloudy skies are 4.142×10^-4 W m^-2 ppb^-1 (1ppb=10^-9) and 3.125×10^-3 W m^-2 ppb^-1 after stratospheric adjustment, and 3.732×10^-4 W m^-2 ppb^-1 and 2.987×10^-3 W m^-2 ppb^-1, respectively, after lifetime adjustment, which are highly consistent with those of the IPCC (2007). Moreover, the 100-year GWPs of CH₄ and N₂O are 16 and 266, respectively, and their corresponding 100-year GTPs are 18 and 268 for sustained emissions, and 0.24 and 233 for pulse emissions. These results indicate that CH₄ and N₂O will still play a critical role in future global warming, second only to CO₂.

Abstract:The National Centers for Environmental Prediction (NCEP) reanalysis data and the National Oceanic and Atmospheric Administration (NOAA) Extended Reconstructed Sea Surface Temperatures (SSTs) are used in this study to investigate characteristics of the East Asian winter monsoon (EAWM) at various latitude zones. On the basis of two indices describing the strengths of the low-latitudinal EAWM (EAWM-L) and mid latitudinal EAWM (EAWM-M), their variation characteristics, associated atmospheric circulation, and related SST patterns are examined. The results indicate that although both the EAWM-M and EWAM-L indices can reflect the intensity of the EAWM, the atmospheric circulation associated with the two indices is clearly distinct. In the lower troposphere, the EAWM-L is closely related to anomalous circulation around the South China Sea and the Philippines, while the EAWM-M presents a closer relationship with the circulation adjacent to Lake Baikal, known as the Baikal blocking high. In the middle troposphere, the close relationship of EAWM-M and the Baikal blocking high remains persistent, while the EAWM-L has a relatively closer linkage with a major Asian trough. In the upper troposphere, variation in the intensity of a subtropical westerly jet may modulate secondary circulation and is eventually linked with the EAWM-L index, while the variation of EAWM-M is mainly related to a shift of the north boundary of the westerly jet. In addition, the EAWM-L is significantly correlated with SSTs in the central and eastern equatorial Pacific and that in the tropical Indian Ocean, while the EAWM-M has a closer relationship with the latter than with the former. The close relationships of the EAWM-L with the central and eastern equatorial Pacific and with the tropical Indian Ocean are clearly revealed on interannual and interdecadal scales; however, the significant linkage of the EAWM-M with the tropical Indian Ocean exists mainly on the interdecadal scale.