Abstract:The ECMWF daily reanalysis data (1961－2000）are used to study the differences of intraseasonal oscillation (ISO) between the South China Sea (SCS) and South Asia (SA) monsoon regions through the band-pass filtering for 30－60 days. The comparative analysis of the ISO in these two regions is made mainly from the structure and horizontal propagation including zonal propagation and meridional propagation. It is found that the vertical structure of ISO in the SCS tilts westward more obviously than that in SA and that the summer monsoon trough over the SCS is more powerful than that over SA. The meridional and zonal propagations of the ISO in the two monsoon regions in 1996 are compared as an example. It is shown that all of them propagate northward but the ISO over the SCS begins to propagate northward earlier and arrives at the lower latitude than that over SA; the northward propagating is relatively slow over the SCS; the zonal propagation of the ISO is obviously different over these two regions, before July the ISO propagates westward over the SCS while eastward over SA.

Abstract:Southwest Western Australia (SWWA) locates in the southwest corner of Western Australia. Its capital, Perth, is not only the political, economic, cultural, educational, and tourism center of Western Australia, but also the wine region and wheat belt of Western Australia. The observed rainy season precipitation over SWWA has significantly declined since the 1950s. The reduction in austral winter rainfall resulted in an even sharper fall in the stream flow in southwestern Australia and heavily impacted the water resources in the state. Based on the observation data and reanalysis datasets in recent 6 decades, the variability and circulation features influencing the SWWA austral winter rainfall (SWR) are investigated. The impacts of the known modes, which may have impacts on the Australian rainfall, El Niño-Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), ENSO Modoki (EM), and Southern Hemisphere Annular Mode (SAM) on the SWR are analyzed. It is found that they all cannot explain the long term rainfall variation of the SWR in neither early (May to July, MJJ) nor late (August to October, ASO) austral winter. Particularly, the reported significant inverse relationship between the SAM and SWR is caused by an extreme year. Based on the investigation on rainfall distribution over SWWA, seasonal march of the circulation features, dynamical and thermal structures of the atmospheric circulation, and thermal contrast between land and sea over wider southwest Australia, the authors find that the circulation over SWWA shows strong seasonality, and exhibits monsoonal characteristics, that is, alternate wet and dry seasons, seasonal reversal of prevailing winds, and an evident land-sea thermal contrast. Taking account of these monsoonal characteristics, the authors propose the concept of monsoon-like southwest Australian circulation (SWAC), and discuss the relationship between the SWAC and the SWR. The result indicates that the variability of SWAC contributes to the variation of SWR, whose weakening is a possible reason for the SWR decreasing.

Abstract:In part I of this study (Xu et al., 2011), The ensemble-based 3DVar (three-dimensional variational technique) method with SVD (singular value decomposition) technique (SVD-En3DVar) is used for assimilation of the simulated radar velocity data and the results demonstrate that using the initial perturbation samples produced with 3DVar method in SVD-En3DVar can shorten the time interval of assimilation cycle and improve the short-term forecast of precipitation. In the current study the feasibility of using SVD-En3DVar for assimilating radar velocity observations is tested with the real observational data. Two torrential rain cases (June 2008 in South China and July 2003 in the Changjiang－Huaihe region) are chosen for the test and the 18-hour forecast of rainfall is compared with that by WRF-3DVar (3DVar based on the weather research forecasting model) assimilation. For the first case (2008) the observational data from 13 radars are assimilated and the forecast of rainfall within 18 hours is improved after assimilation with SVD-En3DVar, but the improvement is not evident with WRF-3DVar assimilation. For the second case (2003), only single-radar observations are used and the forecast of rainfall is improved in the first 6 hours after assimilation with SVD-En3DVar, however the forecasts are not improved by using either SVD-En3DVar or WRF-3DVar in the subsequent 12 hours.

Abstract:The anomalies of moisture flux and atmospheric circulations linked to the subsequent snowstorms occurring in Altay area of Xinjiang during winter 2009 are discussed on the basis of station-observed and NCAR/NCEP reanalyzed data sets, and the possible causes associated with ENSO and Arctic Oscillation (AO) are addressed in the framework of historical analog analysis. Results suggest the enhanced anomalous cold trough in the west of Lake Baikal is a critical circulation system, and its maintenance resulted in the west cold surge moving southward, meeting the southwest warm and wet air coming from the Caspian Sea in North Xinjiang, and caused the successive snowstorms. Correlation analysis suggests that the enhanced anomalous cold trough over the west of Lake Baikal is closely related to the weakened AO index. In negative AO index phase, the 500-hPa geopotential height around Lake Baikal decreases, a meridional circulation anomalies pattern prevails in the Northern Hemisphere, which maintained the Ural blocking high and gave rise to the southward moving clod surge along the west of Lake Baikal, and results in the successive cold waves and snowstorms in the north of Xinjiang. Analysis suggests that during the El Niño year, the enhanced western North Pacific subtropical high results in the southeast moisture flux, but the northward component could not arrive in North Xinjiang, therefore the El Niño event possibly has little benefit to the successive snowstorms in this area.

Abstract:The influence of Arctic Sea Ice Concentration (SIC) anomalies on the atmospheric general circulation during spring and summer is investigated with version 3.0 of the NCAR Community Atmosphere Model (CAM3.0). Regress the second EOF mode of ICE to observed ICE, and then added to climatic ICE as the external force of test. The results show that on the intraseasonal time scale, the atmospheric circulation anomalies evolved from spring to summer, and the spring Arctic SIC anomalies were thermally and dynamically consistent with atmospheric circulation, surface temperature, and rainfall anomalies in spring and summer, especially contributing to less spring rainfall in eastern China and more summer rainfall in northeastern and central China. While on short time scales, the initial adjustment of the atmospheric circulation is characterized by relationship of out of phase in geopotential height anomalies in the lower and upper troposphere locally, and after two weeks, the out of phase turned progressively to more barotropic with the responses propagating to remote areas, at last, the quasi-equilibrium stage of adjustment is reached in the sixth week. The remote responses are regarded as a stationary Rossby wave generated thermally and dynamically through an anomalous turbulent heat fluxes and atmospheric circulation internal varieties. At first, the surface heat fluxes were changed by anomalies in the Arctic SIC, and then a large scale stationary Rossby wave was triggered through the interaction with atmospheric circulation. The lower tropospheric response is baroclinic and thus favors upward emanation of wave activity flux in the negative height anomaly area; while, in upper levels, the energy is dispersed to East Asia through teleconnection, the internal varieties keeping the energy, and then affects the climate in this area.

Abstract:By using cirrus measurements with a Micro Pulse Lidar (MPL-4B) at the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) from April to November in 2007, the statistical analysis is made for both cirrus height and thickness. The optical depth of cirrus is derived through transmittance method, and the relationship of the cirrus optical depth depending on the cirrus thickness and cloud base height is got. The results show that the mean value of the cirrus height at the SACOL is 10.16±1.32 km above sea level. The cirrus thickness varies between 0.3 and 2.8 km, and the mean value is 1.10±0.49 km. The cirrus optical depth varies between 0.003 and 1.057, and the mean value is 0.17±0.16. Most cirrus observed at the SACOL is thin cirrus whose optical depth is less than 0.3. From June to August, the cirrus height is higher and the optical depth and thickness are less than those in other months. The results show that the cirrus optical depth and thickness follow a positive linear relation.

Abstract:The performance of GAMIL 1.0, a grid atmospheric model developed by the State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, in simulating the western North Pacific summer monsoon is investigated. Influence on the simulation exerted by the inclusion of wind gustiness parameterization is discussed. Results show that major features of both the mean state and interannual variability of WNPSM are reasonably reproduced in the original version, but the strength is underestimated. After the inclusion of wind gustiness parameterization, improvements are obvious and mainly displayed as follows: the regional mean precipitation increases from 5.71 mm/d to 8.35 mm/d, which is more close to the observation; the correlation coefficient between the simulated and observed monsoon indices increases from 0.66 to 0.82; the amplitude of interannual variability becomes larger; more important, the simulated “ENSO－monsoon” relationship is almost the same as the observation. Analyses on the latent heat flux, heating rate, and divergent circulation indicate that the improvement in the mean state should be attributed to the enhancement of surface latent heating over the western North Pacific region. The improvement of interannual variability is related to the better mean state. The inclusion of wind gustiness parameterization leads to stronger surface wind speed, which enhances the surface latent heating. Then the mean precipitation is increased, indicating that the climate mean precipitation is improved. Following the improvement of climatological mean precipitation, the model's response to El Niño-type SST forcing is significantly improved, which is evident for both the heating field and corresponding upper-tropospheric convergence center in the western North Pacific.

Abstract:The East Asian intraseasonal oscillation (ISO, 30－60 day) in the summer of 1998 is simulated by using a variable-resolution atmospheric general circulation model LMDZ4 developed by the French National Center for Scientific Research (CNRS) Laboratoire de Météorologie Dynamique (LMD).  The model can simulate the evident periodic signal of intraseasonal oscillation in East Asia in the summer of 1998, and the basic propagating features including the meridional northward propagation and zonal westward propagation. The simulation of the ISO kinetic energy strength is strong in low latitude region but weak in middle latitude region. In addition, the model can reproduce the dominant scale of the precipitation intraseasonal oscillation although with some deviations. The observation results show that there are three precipitation centers at about 15°N, 27°N, and 37°N in the course of northward propagating, corresponding to the three rain belts. But the simulations are weak to the south of 30°N and strong in the north area, which causes the distributions of ISO high value centers to the north of 30°N. The reason of this deviation may be related to the simulation of the precipitation strength and rain belt location.

Abstract:Asia is currently the fastest growing economy region and is also the most concentrated population area in the world, and consequently, regional and potential global climate and environment issues caused by Asian rapid economic development, become an important topic of concern in the scientific community. Originally, it was assumed that the Tibetan Plateau is a key region for the tropospheric air into the stratosphere in summer. However, recent satellite observations and numerical simulations suggested that the Asian summer monsoon is an important pathway for lower tropospheric water vapor and pollutions into the global stratosphere. These chemical mass will have important effect on the regional and global climate and environment through chemical and radiative processes in the stratosphere.

Abstract:Satellite remote sensing is currently the most important way of global-scale precipitation observations. The identification of precipitating clouds based on the satellite-borne measurements is still one of the most challengable problems. In order to get a universal precipitating-cloud identification method available for common optical satellite measurements, the relationship between cloud parameters and precipitating-cloud pixels is analyzed by using matched TRMM Visible and Infrared Scanner (VIRS) and Precipitation Radar (PR) long time scale measurements in the selected regions. According to the derived characteristic cloud parameters of precipitating clouds that is contrast to non-precipitating clouds, a daytime precipitating clouds detection scheme, called Identification of Precipitating Clouds from Optical Thickness and Effective Radius (IPCτRe), is proposed relying on both cloud optical thickness and effective radius. As the cloud parameters are retrieved from the visible and infrared signals that cannot penetrate the precipitating clouds, the IPCτRe scheme can be used operationally over both ocean and land areas. Comparison to PR standard rain products is conducted to verify the IPCτRe results, in which three dimidiate-forecast factors are utilized and two other precipitating-cloud identification schemes are also evaluated, with one proposed by Inoue and Aonashi (2000) and the other proposed by Nauss and Kokhanovsky (2006). The study proves that IPCτRe scheme gives better spatial depiction of precipitating clouds. Especially, in oceanic areas, precipitating and nonprecipitating clouds are well separated by current method, with the probability of detection near 0.84 and probability of false detection remaining just 0.06, indicating a satisfying accuracy for satellite monitoring and forecasting of precipitation operations.

Abstract:The initial condition sensitivity leads to a limit for the prediction of mesoscale weather systems. The rapid growth of initial errors and associated mesoscale predictability depend on the features of flow. The generation of Meiyu heavy rainfall is the result of interaction of many different-scale weather systems. Therefore, there are different mechanisms for the formation and the sensitivity to initial condition of Meiyu heavy rainfall.  The mechanisms of error growth for five different types of Meiyu rainfall are investigated. It is the larger scale cold air lifting, low level vortex, boundary layer cold pool, and gravity wave that play important roles in the error growth of Meiyu rainfall systems. They promote the scale growth of initial errors from small to larger scales in different ways. The perturbation simulations with random or systematic initial errors demonstrate that the initial errors can quickly reach its optimal scales in the systematic perturbation experiment.  The cold air lifting controls the scale of error in the moving trough type of Meiyu rainfall, as a result, the scale of error is rather large and the predictability is rather high. The low-level vortex advection inthe Meiyu rainfall of the moving-out Southwest China vortex type and the north-trough with south-vortex type can offset the deviation within the vortex, and accelerate the transport of error energy from the small scale to large scale, and then the forecast skill of warm-season heavy rainfall is improved. The self-organizing rainfall system has a smaller saturation scale of error and lower predictability than those of the first two types of rainfall, because the immanent coupling mechanism of gravity wave and the cold pool can accelerate the dispersion of error energy, which results in the co-growth of errors at many scales. It is difficult for small scale error to reach its saturation in the non-organizing local rainfall systems, which is usually influenced by local terrain and thermal-dynamic conditions, and therefore it has the lowest predictability. The structure of error in Meiyu heavy rainfall is mainly dependent on the sub-weather systems, which play the most important role in the formation of Meiyu rainfall. The error in the moving trough type slants vertically along the front. Cold air intruding in the moving-trough and north-trough with south-vortex types leads to a prominent increasing of temperature errorat the mid-levels. The genesis of cold pool in the moving-out Southwest China vortex, north-trough with south-vortex, self-organizing, and non-organizing local system types results in a high momentum difference at the lower levels. During the later stage of all types of rainfall, there will be a large error zone at the upper levels, due to the transportation from the lower levels to upper levels.

Abstract:Time series of the intensity of North Atlantic transient eddy activity and a daily index of the North Atlantic Oscillation (NAO) are calculated by using the NCEP/NCAR reanalysis data. The results suggest that peaks in the intensity of eddy activity are followed by strengthening of the NAO mode, accompanied by a collapse in eddy energy. To determine whether the interaction between eddies and low-frequency flow results in the transformation of synoptic-scale energy to low-frequency-scale energy, the eddy vorticity flux is used to investigate the transfer of vorticity and energy. The results show that transient eddies can induce low-frequency anomalies and that vorticity transfer can enhance the anomaly, resulting in an increase in the NAO index. However, eddies, low-frequency flow, and the vorticity flux show continuous variations, meaning that they cannot attain equilibrium all the time. Accordingly, the authors analyze the total contribution of stochastic eddies to the process that induces an NAO-like response to external forcing. The steady solution suggests that eddy forcing can induce a response more similar to the NAO mode.

Abstract:A warm cloud microphysical parameterization was incorporated into a regional model to study the sensitivity of cloud-radiative properties and precipitation to aerosols. Assuming a tri-modal lognormal aerosol size distribution, the aerosol numbers are explicitly calculated from prognostic aerosol masses, considering advection, diffusion, and cloud and rain drop activation/deactivation processes. Sensitivity experiments of a cold front passing through northern China during 25－27 June 2005 with different initial conditions for clean, continental, and urban types of aerosols were then conducted to study the Giant Cloud Condensation Nuclei (GCCN)’s effects. It is found that the presence of giant nuclei enhances the condensation, collection, and cloud-rain auto-conversion process, leading to the decrease of cloud drop numbers and cloud water by 40% and 20% respectively, to the increases of cloud drop radius by 30%, and increases accumulated precipitation both in polluted and clean environment.

Abstract:By using routine observation, FY2C TBB and NCEP reanalysis data, a diagnostic analysis and simulation study have been performed to study the mesoscale systems producing the heavy rainfall event in North China during 22－24 July 2005. The results show that two mesoscale convective systems which developed within the inverted typhoon trough directly resulted in the heavy rainfall event. The mesoscale vortexes were firstly formed in the middle troposphere when the mesoscale convective system matured, and then developed to the lower troposphere. Water vapor convergent centers are mainly concentrated near the surface boundary layer. Easterly inflow pushed water vapor to the rainfall area and upward branches of the secondary circulation transported the water vapor to the middle and upper troposphere. Although the water vapor convergence in the surface boundary layer is very important, the rainfall amount depends on the vertically integrated moisture flux convergence. Dry and cold air apparently intruded into the rainfall area from the high troposphere in the development of heavy rainfall and it was located between 600 and 300 hPa. The dry intrusion in upper levels overruns that in low levels. The dry intrusion enhanced moist instability and promoted the development of convection and the formation of mesoscale vortexes, and then the enhancement of precipitation. In the development of the heavy rainfall, the boundary layer jet was induced due to the local heating of terrain in North China in the night. Precipitation and dry intrusion further enhanced the boundary layer jet and strong wind in upper levels emerged accordingly. Jets and heavy rainfalls have positive feedback through the secondary circulation. The coupling among the boundary layer jet, upper and lower tropospheric jet streaks played an important role in this heavy rainfall event.

Abstract:Based on the analyses of relationship between summer precipitation and upper tropospheric temperature, North Atlantic Oscillation (NAO) in the preceding spring, a statistic predictive model for summer precipitation over China is proposed and the forecast results for years 2004－2009 are tested. The results show that the large scale precipitation over China mainly changes with NAO in May, and the meridional dipole pattern of precipitation is associated with NAO in May and the anomalous cooling during 500 hPa－200 hPa in the preceding spring over North China and Northeast China. The NAO influences the summer precipitation over China on both interannual and interdecadal time scales, while the upper tropospheric temperature influences the precipitation mainly on interdecadal time scale. The anomalous condition of spring NAO and upper tropospheric temperature determines the pattern and magnitude of summer precipitation over China via large scale circulation and water vapor transport. A predictive model for summer precipitation over China is proposed based on both this statistic and physical relationship, with the first three principle components (PCs) of summer precipitation as the predictands, and with the first four PCs of upper tropospheric temperature over Asia (10°N－50°N, 60°E－130°E) and the monthly NAO index in the preceding spring as the predictors. The periods of these series with different time scales are also considered to remove the noise and extract useful information. The averaged anomaly correlation coefficient for years 2004－2009 is 0.335, indicting a good skill in forecasting the summer precipitation over China.

Abstract:Aerosol particle size distribution function is a key environmental parameter. Retrieval of the aerosol particle size distribution function is an improperly posed inverse problem. To deal with this problem and obtain the stable approximation of the particle size distribution function, proper inversion technique is necessary. In this paper, the author studies the constrained optimization methods for retrieval of aerosol particle size distribution functions. The ill-conditioning properties are discussed and a regularizing model is established. To solve the minimization problem, the author develops a BFGS method. Numerical experiments indicate that the method introduced in this paper can be used for solving the inversion problem in retrieval of aerosol particle size distribution functions.

Abstract:The tropical low frequency oscillation and its possible influence on the South China Sea (SCS) summer monsoon are studied by using the daily NCEP/NCAR-2 (National Centers for Environmental Prediction and National Center for Atmospheric Research) reanalysis data and the outgoing long-wave radiation (OLR) data from NOAA (National Oceanic and Atmospheric Administration) for the time period of 1979－2008, and the CPC Merged Analysis of Precipitation (CMAP) data for the time period of 1979－2007. Results reveal that there are significant low frequency oscillations with 30－60 day period over the SCS region. When the 30－60 day oscillation is in an active state, the low frequency components of southwesterly prevail over the SCS and the cyclonic circulation appears in northern SCS. Hence, in the active state the low frequency oscillation tends to strengthen the South China Sea summer monsoon (SCSSM) trough and the SCS summer monsoon. In the inactive state of the low frequency oscillation, the situation tends to be opposite. Further researches indicate that the spatial pattern for the variability of the low frequency oscillation is analogous to that for the interannual variations of the SCS summer monsoon. Particularly the variance of low frequency oscillation activity in the SCS region can account for nearly half of that for the interannual variations of the SCS summer monsoon. The comparison between strong and weak SCS summer monsoon years presents that the occurrence probability of active state is higher than that of inactive state for the low frequency oscillation in strong monsoon years. However, in weak monsoon years the occurrence probability of inactive state is higher than that of active state. Therefore, the low frequency oscillation with 30－60 day period likely has an important influence on the SCS summer monsoon. When the active state is dominant for the low frequency oscillation, the SCS summer monsoon tends to be strong. On the contrary, the SCS summer monsoon tends to be weak when the inactive state is dominant for the low frequency oscillation.