Abstract:This study investigates variations in surface solar radiation (SSR) and the effects of cloud on SSR during 1961-2009 based on global radiation and cloud fraction data from 100 radiation observation sites over China. The results indicate a general decrease of -4.3%/10a (7. 87 W m^-2 (10 a)^-1) in SSR over land surfaces since measurements began (in about 1961) until 1990, with a turnover corresponding to a 2.8%/10a (2.4 W m^-2 (10a)^-1) increase from 1990 onward. Moreover, this turnover was more significant in southern China. It seems that cloud was not one of the primary factors inducing global dimming during 1961-1990, based on the similar variation characteristics witnessed under all-sky and clear-sky conditions and the weak correlation between total cloud fraction and SSR. However, there were distinct differences between all-sky and clear-sky SSR results after 1990 and it shows a significant negative correlation between all-sky and clear-sky SSR values. Thus, it may be concluded that cloud had a significant effect on SSR variation, such that it may have inhibited global brightening in some way during 1990-2009.

Abstract:In this study, by using an observing system simulation experiment, a shuffled complex evolution algorithm (SCE-UA) is evaluated in terms of the effectiveness and efficiency of calibrating parameters in one-dimensional Richards equation, including soil components and all physical parameters. The result shows that the ability of calibrating parameters with SCE-UA depends on not only the uncertainty ranges of parameters but also their sensitivity degrees. For sensitive parameters, unique optima parameter estimates can be easily obtained. However, for insensitive ones, there exists an "insensitive range", and only suboptimal parameters are obtained in this range. By increasing training times and reducing the parameters' range, the performance of parameter calibration in the insensitive range can beimproved. Moreover, the overdetermination of the model parameters may result in suboptimal parameter estimates, and a good calibration effectiveness can be archived by appropriately setting parameters and adding constrains and criterions.

Abstract:Simultaneous surface measurements of ice nuclei (IN) and aerosols in Nanjing from November 15 to December 2, 2011, are presented to investigate the characteristics of the IN concentration and the relationship between IN and aerosols. The results show that the mean concentration of IN (N_IN) observed during the campaign was 0.352 L^-1 at the activation temperature (T_a) of -20℃, and the supersaturation with respect to water (S_w) equals 1%;the ratio of the IN to aerosol concentrations in the size range of 0.01-10 μm was 4×10^-8. The IN concentration would be high at low temperatures and high humidities. An obvious scavenging effect of fog and precipitation on IN was observed. A comparison of IN in different air masses showed that the concentration of IN and aerosols in an air mass that originated from a polluted area to the east was the highest, but the ratio of IN to aerosols was the highest when the air mass originated to the northwest, which might be because of the high ice-nucleating ability of dust particles. The correlation coefficient between IN and aerosols in different size ranges showed that IN with larger particle surface concentrations was much more highly correlated with aerosols. The ice nuclei empirical equation considering T_a and the aerosol number concentration (N_{0.5～10 μm}) was obtained.

Abstract:The performance of two multisphere coupled climate system models developed by the Beijing Climate Center (BCC_CSM) to simulate the annual modes of tropical precipitation is evaluated and possible reasons for the resulting simulation biases are discussed. The results indicate that both BCC_CSM1.1 and BCC_CSM1.1(m) can reasonably reproduce the major characteristics of the global annual mean precipitation and annual modes of tropical precipitation. The solstice modes simulated by the two models exhibit equatorial antisymmetric structure, which corresponds to observations. BCC_CSM can reasonably reproduce the relationship between the spring-fall asymmetric mode and tropical sea surface temperature (SST). The biases in air temperature, atmospheric circulation, and tropical SST contribute to the deficiency of the solstice modes. The simulation biases in the SST annual cycle contribute to the biases in the spring-fall asymmetric mode of tropical precipitation. The differences in horizontal resolution of atmospheric model and land model affect the simulation of precipitation annual modes in BCC_CSM to some extent. The comparison shows that the spatial variabilities of climatological monthly precipitation from January to December simulated by BCC_CSM1.1 (m) are closer to the observations than those by BCC_CSM1.1 and the SST annual cycles in the tropical ocean are generally more reasonable in BCC_CSM1.1 (m) owing to its finer horizontal resolution. However, obvious biases remain in BCC_CSM1.1 (m). Therefore, further work is required to improve the performance of BCC_CSM.

Abstract:Sea breeze fronts are closely related to the severe convective weather that occurs in coastal regions. Changes in the underlying surface of the coastal region, caused by urbanization, influence the characteristics of the sea breeze front. We applied the WRF (Weather Research and Forecasting) model, coupled with a new generation urban physics scheme, the UCP-BEM (Urban Canopy Parameterization-Building Energy model), to study the effects of urban underlying surfaces on sea breeze front characteristics in the Bohai Bay region. The results showed that the low-level sea breeze was weakened noticeably because of the roughness of the urban underlying surface, leading to decreases in low-level convergence, upward motion, and the inland distance of the sea breeze front. The combination of larger upward sensible heat flux, smaller upward moisture flux, and weaker sea breeze, caused by the thermal and dynamic properties of the urban underlying surface, inhibited the cooling and moistening of the low-level atmosphere from the relatively cool and moist sea breeze. The frictional force effect of the urban underlying surface on sea breeze circulation resulted in the lifting of the sea breeze, expanding the vertical atmospheric cooling and moistening range. The available convective potential energy behind the sea breeze front decreased correspondingly, but its vertical distribution expanded, leading to the uplifting of the convective inhibition center. Moreover, the static instability zone thickened while the dynamic instability zone above it thinned, but the thickness of the whole instability zone remained constant.

Abstract:A severe pre-TC (Tropical Cyclone) squall line that preceded Typhoon Matsa (2005) was investigated using the non-hydrostatic "Weather Research and Forecast" model. The results showed that this pre-TC squall line formed in the moist transition zone between the parent TC and a subtropical high. It was found that Matsa may have contributed to convective development in the squall line by producing conditional instability and enhancing low-level moisture convergence, leading to the organization of the early discrete convection into a linear rainband. Although the pre-TC squall line caused fewer surface pressure perturbations at its mature stage than is typically observed at mid-latitudes, it exhibited stronger moisture convergence in the boundary layer and a larger area of inflow in the mid-troposphere. Sensitivity analyses showed that stronger TCs give pre-TC squall lines that are longer lived and faster moving, with more intense rainfall. The results showed that a stronger TC tends to produce more low-level vertical wind shear in the transition zone, which favors the organization of early discrete convection into a squall line. However, low level vertical wind shear in the vicinity of the pre-TC squall line decreases when the mature stage is entered. This, in addition to the likelihood of a reduced low-level moisture supply, appears to be one of the factors that is unfavorable for the pre-TC squall line being maintained.

Abstract:The microphysical characteristics of multi-layer stratiform clouds in autumn are examined using layered vertical observation data of Particle Measuring Systems (PMS) obtained during an airborne observation experiment over the Sanjiangyuan National Nature Reserve. Results show that cirrostratus (Cs) and upper-level altostratus (As) are ice-phase clouds, while lower-level As and stratocumulus (Sc) are supercooled mixed-phase clouds. The particle concentration and the content of supercooled water of the lower level As are the largest in the cloud system. The Sc particle size and spectral width are the largest and shows clear regional characteristics. The median diameters of the Sc (lower As and convective bubble) cloud particles are 3.5-18.5 μm (3.5-21.5 μm) for the liquid phase and greater than 21.5 μm (24.5 μm) for the ice phase. The differences between the higher and lower supercooled water in the cloud particle spectra are distinguished. The high supercooled water area of Sc exhibits an obvious rime growth phenomenon. The effective radius of droplets becomes increasing large in Sc, at the bottom of the lower-level As, and on the top of the convective bubble. The content of supercooled water of the high subcooled water area has a mean ratio and standard deviation of 69.9±19.4%. In addition, the fraction of liquid water in Sc is related to the supercooled liquid water content in the high content of water, but there is no obvious variation in the lower-level As, which shows a mean ratio and standard deviation of 89.2±8.1%. The cloud particle spectrum in the mixed-phase cloud observed by the Forward Scattering Spectrometer Probe (FSSP) and the Two-Dimensional Cloud Probe (2DC) are respectively in single-peak Γ and negative exponential distribution.

Abstract:Autumn precipitation in North China (APNC) is studied for 1951-2011 by using 160-station observed monthly precipitation in China, National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis data, National Oceanic and Atmospheric Administration (NOAA) extended reconstructed sea surface temperature (SST), and the ECHAM5 general circulation model. In addition, the APNC and anomalous circulations recorded in September 2011 are compared with the statistical and numerical results. It is determined that the APNC shows dramatic interannual and interdecadal variability. The APNC was in a positive phase during the 1960s to early 1980s and in a negative phase in the 1950s, mid-late 1980s, and 1990s. Moreover, the strengthened APNC is closely related to westward stretching of the western Pacific subtropical high (WPSH). The cooling SST in the central tropical Pacific depresses the atmospheric convection over the region and exerts Rossby waves to enhance the convection over the western tropical Pacific. The local Hadley circulation is connected to convection over same region. The strengthened local Hadley circulation leads to anomalous anticyclones in the northwestern Pacific and is favorable for the westward movement of the WPSH via East Asia-Pacific (EAP) teleconnection, which increases the APNC. In 2011, negative SST anomalies in the central tropical Pacific and anomalous circulation contributed to extreme above-normal precipitation, which is consistent with the statistical and model experiment results.

Abstract:Differences in monthly evolution of the western Pacific subtropical high (WPSH) in summer for different modes of intraseasonal variation of the summer monsoon rain belt (IVSMRB) are examined by using the composition method. For such analysis, the rainfall data from 160 stations in China, 850 hPa wind derived from the National Centers for Environmental Prediction and the National Center for Atmospheric Research (NCEP/NCAR) reanalysis dataset, and the National Oceanic and Atmospheric Administration (NOAA) extended reconstructed sea surface temperature (SST), are used. In addition, the characteristics of the pre-winter East Asia monsoon, Pacific sea surface temperature anomalies (SSTA), and rain styles in summer relative to these modes are investigated by using composition and correlation methods.Results show that the twice northward jumps of WPSH from spring to summer differ significantly among the modes, and that for each mode is relatively independent. The first jump of WPSH has an important influence on rain belt distribution in June, while the second jump has decisive influence on that in July and August.
The influence of SSTA in the Pacific on IVSMRB in China is multi-modal. In different phases of the Monsoon/El Niño-Southern Oscillation (ENSO) cycle, the IVSMRB presents dissimilar features. The first mode appears mainly in the decay phase of El Niño, and the second generally appears in the developing phase of La Niña. The third occurs in the developing phase of El Niño;the fourth, in the decay phase of La Niña;the fifth, at weak ENSO signals;and the sixth, in the persisting phase of La Niña. Moreover, the SSTA in different key regions has different impact on the IVSMRB. The SSTA in Kuroshio has a strong correlation with the rain belt variation in June, while the ENSO cycle in eastern tropical Pacific has an important influence on the rain belt variation in July and August. Analysis of the relationship between the IVSMRB modes and summer rain styles reveals some linkage between the two;however, these results do not imply that they can be substituted for each other.

Abstract:The influence of the background flow and the boundary layer top parameterization error on the low-frequency oscillation of the tropical atmosphere is discussed using a simple three-dimensional numerical model including a wave-CISK mechanism. The results show that the velocity of horizontal background flow (U) has an important impact on the frequency of low-frequency oscillation. When the horizontal wind U =2 m/s, the oscillation period reduces from 50-60 days to 30 days, while when U =-1 m/s the oscillation period increases to 70 days. Because the original oscillation moves eastward, a westerly flow can increase the oscillation while an easterly flow can restrain the oscillation and thus increase the oscillation period. Moreover, the low-frequency oscillation appears to be sensitive to the parameterization error in the model. When the selected value of the boundary layer is larger (1070 m) than the standard value (1030 m), the feedback of heat forcing is large. Thus, the oscillation moves faster than the standard speed and stops moving and increases toward infinity when the wave reaches 80°-90°E. On the contrary, when the selected value of boundary layer is smaller (1000 m) than the standard value, the feedback of heat forcing is not sufficiently large to maintain the oscillation. Thus, the wave will disappear within 110°-120°E, and the period is less than standard.

Abstract:Characteristics of wind profiles were studied using the Global Positioning System (GPS) sonde data from typhoons Wipha in 2007, Morakot in 2009, Fanapi in 2010, and Muifa in 2011. The gradient wind speed and corresponding level were first obtained. The wind profiles were then fitted by the power law and the logarithmic law. The exponential index, the ratio between the wind speed at 10 m and the gradient wind speed, and the height of the constant flux layer defined by the logarithmic law were further calculated. The possible difference between the gradient wind level and the height of the constant flux layer was investigated. The results show an obvious difference between the height of the constant flux layer and the gradient wind level for the typhoons landing along East China. Moreover, the parameters of the mean wind profiles from the typhoons were calculated, and the reason for the deviation of the wind profile below the gradient wind level from the logarithmic law was discussed.

Abstract:A continual rainstorm process influenced by the terrain in Sichuan Province that occurred during July 16-18, 2010, is analyzed. Results show that the process took place under a good high-low system configuration. In addition, a numerical simulation performed with National Centers for Environmental Prediction (NCEP) data used for the initial field accurately simulated this rainfall process. The numerical simulation can not only indicate where the rainfall region is but also the center of storm. Generalized moist potential temperature, the vertical integral of the vertical component of generalized convective vorticity vector and quality vertical helicity are chosen to perform diagnostic analysis for the rainfall region, which lies in northeast Sichuan and is influenced by Daba Mountain. The results show that the inverse Ω shape areas of the moist potential temperature isolines in the vertical sections coincide well with the heavy rainfall region, and the steepness of the isolines and the abnormal height of the moist potential temperature can qualitatively indicate the rainfall intensity. The vertical integral of the moist potential temperature from 800 hPa to 500 hPa can more accurately trace the heavy rainfall region in the northeast by using the improved vertical integral rather than the traditional vertical integral. Because the vertical helicity quality can effectively describe the vertical structure of the typical dynamical field of the heavy rainfall system over northeast Sichuan, it strongly correlates with the heavy rainfall region in the same location.

Abstract:In order to improve climate monitoring during key periods and meet the needs of meteorological services, a criterion for determination of the onset and end of the rainy season in Southwest China is investigated based on daily rainfall data from 92 stations in Southwest China during 1961-2011 and daily atmospheric circulation reanalysis data from NCEP/NCAR for 1981-2010. The results indicate that, for the mean climate, the rainy season in Southwest China begins in the third pentad in May (pentad 27) and ends in the fourth pentad in October (pentad 57). Variations in atmospheric circulation confirm the above result. Moreover, a criterion for determination of the annual onset and end of the rainy season in Southwest China is proposed based on the results of comparative analysis of different definitions and the seasonally changing characteristics of atmospheric circulation. Simultaneously, possible mechanisms influencing the onset and end of the rainy season are also discussed preliminarily.

Abstract:The spatiotemporal evolution of Somali cross-equatorial flow (SMJ) in summer at five levels (1000 hPa, 925 hPa, 850 hPa, 700 hPa, 600 hPa) are studied on the basis of National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data and ERA40 reanalysis monthly mean data. The results show that vertical structures of SMJ in the middle and lower troposphere differ, with interannual and interdecadal change characteristics. Two main vertical structure modes are clarified. The first is a consistent mode in the entire region ("A" mode), and the second is the vertically opposite mode ("B" mode). Thus, four vertical anomalous types of SMJ are observed to include consistently strong pattern ("A+" pattern), consistently week pattern ("A-" pattern), higher weak and lower strong pattern ("B+" pattern), and higher strong and lower weak pattern ("B-" pattern). It is also indicated that the vertical structures of SMJ are closely related to East Asia summer monsoon activity. Specifically, the "A+" pattern (“A-" pattern) correlates strongly to the strong (weak) East Asia summer monsoon with more (less) rainfall in eastern Inner Mongolia and North China. The "B+" pattern ("B-" pattern) correlates strongly to the weak (strong) East Asia summer monsoon with less (more) rainfall in the southern area of the Yangtze River and Japan.

Abstract:The fourth-generation atmospheric general circulation model, developed by the Institute of Atmospheric Physics, Chinese Academy of Sciences, was used to simulate decadal variations in the East Asian summer monsoon (EASM) and in summer rainfall in eastern China during the late 1970s. It was shown that the model simulated well the decadal weakening of the EASM, a northerly anomaly along the East Asian coast, and changes in the western Pacific subtropical high. The model also reproduced the anomalous summer rainfall pattern in eastern China (i.e., an increase in the Yangtze River basin and a decrease in North China and South China), except that the modeled pattern was located somewhat south of the observed pattern. Further analysis, using singular value decomposition, indicated that the decadal weakening in the EASM was mainly driven by warming in the tropical ocean, which is related to the Pacific decadal oscillation (PDO) phase transition in the late 1970s. The model also simulated the cooling trend in the Yangtze River basin, which led to a decrease in the thermal contrast between the continent and the ocean, and a weakened EASM.

Abstract:We developed a novel nonlinear artificial intelligence ensemble prediction (NAIEP) model based on multiple neural networks with identical expected output created by using the genetic algorithm (GA) of evolutionary computation. We extracted the main signal feature from the meteorological fields with random noise and eliminated the random disturbance by principal component analysis (PCA). We set up the NAIEP model based on data of typhoons that occurred in the South China Sea from June to September in the period 1980-2010. The predictors were selected by the stepwise regression method and PCA both in the predictors of climatology persistence and Numerical Weather Prediction (NWP) products to predict the typhoon tracks for each month. Under the condition of identical model samples and independent prediction sample cases, we compared the genetic neural network ensemble prediction (GNNEP) model by selecting the predictors with both the method of Stepwise regression with PCA and the climatology and persistance(CLIPER) prediction model. The result showed that the former method was more accurate than the latter, and the average absolute error of the typhoon track from June to September decreased by 7.4%, 4.8%, 12.4%, and 17.0%, respectively. Under the condition of identical primary predictors and sample cases, we compared the prediction accuracies of the new model, the model of Stepwise regression, and the model of GNNEP (using only the method of PCA for the input predictors), and theoretically proved that the new model is more accurate than the other two. In the method which uses forecast information in all the alternative predictors and in the GNNEP model in which the resultant prediction from the ensemble integrates the predictions of the multiple ensemble members, the network structure is determined through the optimizing computation of GA;therefore, the generalization capacity of the ensemble prediction model is improved, leading to better availability and improved weather prediction