Abstract:Interdecadal variation of the leading modes of summertime precipitation anomalies in the monsoon regions of eastern China and its association with the spatio-temporal variations of summertime water vapor transport fluxes over East Asia are analyzed by using the daily data of the ERA-40 reanalysis and precipitation data at 516 observational stations of China for 1958－2000 and the EOF analysis method. The analysis results show that there are two leading modes in the spatio-temporal variations of summertime precipitation anomalies over the monsoon region of eastern China: The first leading mode exhibits not only a characteristic of obvious interannual variation with a quasi-biennial oscillation, but also a feature of interdecadal variability, and its spatial distribution is of a meridional tripole pattern. And the second leading mode exhibits a characteristic of obvious interdecadal variability, and its spatial distribution is of a meridional dipole pattern. This shows that these two leading modes have a significant interdecadal variability. During the period of 1958－1977, the distribution of summertime precipitation anomalies in eastern China exhibited a “＋－＋” meridional tripole pattern from the south to the north, and the distribution of precipitation anomalies for 1978－1992 showed a “－＋－” meridional tripole pattern in the region, which was opposite to that for 1958－1977, but during the period of 1993－1998, since the role of the second leading mode in summertime precipitation anomalies in eastern China was intensified, the distribution of summertime precipitation in this region showed a combination of “＋－＋” meridional tripole pattern and “＋－” meridional pattern, which caused the increase of summertime precipitation in South China. Moreover, the analysis results also show that the interdecadal variation of these two leading modes is closely associated with the spatio-temporal variations of summertime water vapor transport fluxes over East Asia, which is associated not only with the interdecadal variation of the EAP pattern teleconnection-like wave-train distribution of summertime water vapor transport flux anomalies over East Asia and the western North Pacific, but also with the interdecadal variation of the EU pattern teleconnection-like wave-train distribution of summertime waver vapor transport flux anomalies in the westerly zone over middle and high latitudes of Eurasia.

Abstract:The centered difference scheme for advection transport may cause unphysically negative values when used in an ocean model to simulate oceanic passive tracers. Selecting positive definite schemes can suppress the generation of negative values. In order to examine the difference of advection schemes, second-order Centered Difference (CD), Flux-Corrected Transport algorithm (FCT), Multidimensional Positive Definite Advection Transport Algorithm (MPDATA), Piecewise Parabolic Method (PPM) and Second-Order Moments advection scheme (SOM) are chosen to simulate the distribution of oceanic tritium, respectively. Two cases are set to compare the differences between these schemes. The physical fields are exactly same for these runs with different advection schemes for tritium in a basinwide ocean general circulation model of the Pacific Ocean. In Case 1 that contains full physical processes, all runs give similar horizontal distributions along isopycnals from the subarctic to tropical regions. However, many negative values arise in the CD run, while for other runs much weaker negative values are generated in some grids. During model years 1968－1976, negative values in the CD run gradually become comparable in magnitude to main signals in the subsurface layer of the eastern Tropical Pacific. Those negative noises beneath the depth of maximum tritium concentration within the subsurface layer can disturb the lateral ventilation along isopycnals, leading to much larger subsurface values near 120°W. At 125°W below 200-m depth, negative values also exist, making the CD results meaningless, while the runs with other schemes all give acceptable values. Both FCT and SOM runs show a weaker numerical dissipation in the vertical direction than PPM and MPDATA runs. In Case 2, which does not have physical dissipation, the CD run fails to produce a valid solution, while other runs all succeed in getting nonnegative distributions. The SOM run gives the strongest meridional gradient near the western boundary in the North Pacific, while the PPM run produces the weakest one. In the Antarctic Circumpolar Current region, the results from both PPM and SOM are smooth everywhere, while the results from both FCT and MPDATA have severe fluctuations near the land boundary. By comparisons of these two cases, it can be concluded that the CD shows a poor performance in the simulation of oceanic passive tracers. The PPM and the SOM are much better than the FCT and the MPDATA in numerical stability. The results from the SOM can easily maintain a strong gradient because of its weak numerical dissipation. However, it seems that the PPM is a better choice for the investigation of the long-term transport process of passive tracers in the ocean when the computational efficiency is taken into account.

Abstract:Anomalous summer air temperature (SAT) series at 160 stations of China during 1960－2010 are adjusted, then an Empirical Orthogonal Function (EOF) analysis is made on original and adjusted anomalous SAT series, respectively. The main results show that: 1) The large value areas of the first three typical fields of the adjusted anomalous SAT series distribute uniformly in three northern regions (namely Northeast China, North China, Northwest China), the Tibetan Plateau, and the Yangtze River, which is consistent with that of the mean square deviation field of SAT, while the large value areas of the first three eigenvectors of the original data lie mainly to the east of 95°E and are apparently different from those of the mean square deviation field. Therefore, the spatial features of EOF analysis on the adjusted data are more reasonable. 2) The EOF first time coefficients of the original data are significantly related to those of the adjusted data, but the variance contributions of the linear and interdecadal components of the first two time coefficients increase significantly. 3) The features of the first two typical fields and their time coefficients for the adjusted data can display the main strong warming and cooling areas in summer over China in the last 51 years, and the interdecadal components are more consistent with the evident interdecadal areas than those of the original data. The above results show that the network homogeneity adjustment improves obviously the results of EOF analysis on anomalous SAT series in China.

Abstract:The East Asian upper-tropospheric jet stream (EAJS) jumps northward abruptly in late July with its center shifting from 40°N to north of 45°N in climatology. Year-by-year statistics showed that there are two categories of typical northward jumps: The first is characterized by enhanced westerly to the north of the EAJSs axis and the second is by the weakened westerly in the EAJS's axis. In this study, the physical mechanisms for the two categories of northward jumps of the EAJS are investigated by using the NCEP/NCAR reanalysis data from 1958 to 2002 and the dynamical process is diagnosed based on the wave activity flux. The trough over the high-latitude Far East and the northeastward extension of the South Asia high in the upper troposphere, together leads to meridional gradient of geopotential height enhancing and then the westerly enhancing over the Okhotsk Sea between them, and results in the first category of northward jump of the EAJS. Actually, the high-latitude trough is contributed by the eastward propagating Rossby wave, which is originated from West Europe and the North Atlantic, along the north edge of Eurasian continent, and the extension of the South Asia high is mainly related to the “Pacific-Japan (PJ)” teleconnection pattern induced by the positive rainfall anomaly over the tropical western North Pacific. The second category of northward jump, however, is mostly attributed to the Rossby wave along the Asian subtropical jet stream in the upper troposphere. The eastward propagating Rossby wave is blocked over East Asia and forms an in-situ ridge, reducing the meridional gradient of geopotential height and the westerly over East Asia and leading to northward jump of the EAJS.

Abstract:The characteristics of sub-microsecond electric field change waveforms during the initial stage of intracloud (IC) flashes and preliminary breakdown process of cloud-to-ground (CG) flashes have been studied based on the observation data. The data include fast electric field change and slow electric field change of IC flashes and CG flashes measured in Yangbajing of the Tibetan Plateau in 2008, and in Da Hinggan Ling in 2009. It is found that the electric field change waveforms during the initial stage of IC flashes and preliminary breakdown process of CG flashes appear as a series of pulses. The majority of pulses are narrow with pulse width less than or equal to 10 μs. The majority of pulses are weak with the normalized amplitude value less than 0.5. Compared with the lower altitude area, the number of pulses during the preliminary breakdown process of negative CG flashes measured in the Tibetan Plateau is larger and the proportion of narrow pulses is smaller. The proportion of large bipolar pulses with large peak-to-peak amplitude that is equal to or larger than the peak amplitude of electric field change of the first return stroke is small. Based on the statistics, the mechanism of discharge processes that produce bipolar pulses has been discussed theoretically. It is found that if the propagation speed of current pulse and the damp length of current pulse are large, the peak-to-peak amplitude of bipolar pulse will be large.

Abstract:The impact of stratiform heating on the Madden-Julian Oscillation (referred to as MJO) is investigated. The nonlinear CISK (Conditional Instability of Second Kind)-Kelvin wave equations with both cumulus heating and stratiform heating are reduced to ordinary differential equations by using a spectral truncated method. With the help of the fourth-order Runge-Kutta method, the numerical solutions are obtained. It is found that the MJO could also be excited by the only stratiform heating. Meanwhile, the combined role of both cumulus and stratiform heating can enhance the MJO. However, the chaotic solutions under the cumulus heating can become regular solutions by considering the stratiform heating. In this case, the regular solution looks like a propagating envelope wave packet, whose periodicity is more evident with the period being easily within the range of 30－60 days.

Abstract:Characteristics and regular variabilities at different rotation rates are discussed for the horizontal and vertical distributions of the global monsoon by the Dynamical Normalized Seasonality (DNS). Effects of the seasonal changes of meridional and longitudinal wind directions on monsoon areas are also analyzed. Furthermore, a new index is defined to discuss relative seasonal change of wind at different rotation rates compared to the Earth rotation rate. It is showed that typical monsoon areas change with different rotation rates. Horizontally, the DNS is remarkable in the zonal direction at fast rotation rates, and the monsoon areas become smaller and smaller with rotation period increasing. However, at slow rotation rates the DNS is remarkable locally in the meridional direction, and the monsoon areas become larger and larger as the rotation period becomes longer. Vertically, the remarkable DNS distributes irregularly near the surface at fast rotation rates, which is mainly in the equatorial and tropical areas at slow rotation rates with a larger area in the Northern Hemisphere than in the Southern Hemisphere. At levels above 100 hPa and below 500 hPa, the remarkable DNS areas are mainly between 30°S and 30°N, whose intensity and north-to-south width vary regularly with different rotation periods. Above 100 hPa, the remarkable DNS areas, whose maximum intensity changes regularly with different rotation periods, are primarily in the equatorial and subtropical areas with hemispherical symmetrical pattern at fast rotation rates, while they decrease markedly and even disappear at slow rotation rates. Besides, seasonal difference of the zonal wind is the main reason for the remarkable DNS at fast rotation rates. But the seasonal difference of meridional winds causes remarkable DNS at slow rotation rates especially blow 100 hPa. Otherwise, the relative seasonal change of wind at fast rotation rates compared to the Earth rotation rate is remarkable primarily above the equatorial and tropical areas, and the remarkable change areas distribute widely at slow rotation rates and become smaller and smaller above 100 hPa with the rotation period increasing.

Abstract:An improved frozen soil parameterization scheme is introduced and its effects on climate in East Asia are investigated by using the latest version of general circulation model (GCM) CAM3.1 developed at NCAR. Simulation results show that the improved soil freezing process leads to enhanced heating of the earths surface by the atmosphere over most parts of Eurasia in winter, while it leads to enhanced heating of the atmosphere by the earth's surface in summer, especially the abnormal heating is significantly strengthened over the Tibetan Plateau. The climate in East Asia is very sensitive to the frozen soil parameterization with notable change of the surface air temperature in both winter and summer. With the improved soil freezing process, the simulated Siberian high is enhanced and the East Asia trough moves westward in winter; the East Asian summer monsoon is intensified, and the western Pacific Ocean subtropical high extends northwestward, resulting in increasing rainfall in northern China and the middle and lower reaches of the Yangtze River and decreasing rainfall in South China. These results may have meaningful implications for improving the performance of GCM to simulate the climate in East Asia.

Abstract:Using observational and reanalysis data, the variaton of the Tibetan Plateau Summer Monsoon (TPSM) and its relationship with the India Summer Monsoon (ISM) and the East Asian Summer Monsoon (EASM) are investigated by the correlation analysis and the composite analysis. Results show that the TPSM in 2008 is earlier and stronger than normal. The most significant period of the intraseasonal oscillation in the TPSM is biweekly, similar to those of all ISM rainfall and the EASM index. When the TPSM is strong (weak), the precipitation in all India and North China tends to be less (more) than normal. During the active phase of the TPSM, the position of the Indian－Burma monsoon trough is to the east and north of its normal place, so that the moisture transportation in front of the trough is of benefit to the rainfall in the southeast of the Tibetan Plateau, but the ISM breaks for the influence of the high ridge over the Indian continent. Meanwhile, since the Western Pacific Subtropical High (WPSH) is to the east and south of its usual location, the EASM precipitation concentrates in East China. On the contrary, when the TPSM is weaker, with the WPSH expanding westward and shifting northward, the lower southwesterly jet shifts northward, thus both the southwesterly moisture transportation from lower latitudes and the southeasterly moisture transportation from the south edge of the WPSH contribute to the more precipitation in North China. It is also shown that the TPSM oscillation related to diabatic heating modulates the behavior of the South Asia high to a considerable degree.

Abstract:Annual CH4 and N2O emission fluxes were measured from double rice cropping systems under six fertilization treatments by using the method of static chamber-gas chromatograph techniques. The different treatments including non-fertilizer(CK), conventional chemical fertilizers (NPK), fresh rice straw with chemical fertilizers (RS+NPK1), mushroom residue with chemical fertilizers (MR+NPK1), fresh cow dung with chemical fertilizers (CD+NPK2), and biogas digestion with chemical fertilizers (BD+NPK2) were dedicated to this experiment in order to study the characteristics of CH4 and N2O emissions and the relationship between gas emission fluxes and related environmental factors. The results showed that, seasonal variation of CH4 emission fluxes under different treatments displayed the same trend, which can be described as “lower in early rice growing period, higher in late rice growing stage, and negligible in the fallow season”. The majority of N2O emissions (more than 78%) occurred in rice growing periods, and small amounts of N2O emission occurred in the fallow season. Although the annual accumulated CH4 emission amount for NPK was increased by 35% compared with CK, the difference between them was not significant at 0.05 level. Among different treatments of organic and mineral fertilizers, the annual accumulated CH4 emission amount for RS+NPK1 was 2.44 times more than that for MR+NPK1 (P<0.05, P is the significance level), while the annual N2O emission amount for the latter was 59% higher than that for the former (P>0.05). The annual accumulated CH4 emission amount for CD+NPK2 was 2.45 times more than that for BD+NPK2 (P<0.05), but the annual N2O emission amount for the latter increased by 102% compared with CD+NPK2 (P>005). The fertilizer-induced direct N2O emission factor (E) was reduced by 45%－80% under the treatments of combined application of chemical and organic fertilizers in paddy fields compared to NPK. In this analysis, water status and temperature were main driving factors which affected the seasonal variation of CH4 emission fluxes. However, N2O emission fluxes did not have direct relation with water status in double rice paddies and a temporal trade-off relationship between the two gas emission fluxes was not observed in this study. CH4 was the dominant greenhouse gas which contributed more than 75% to the integrated global warming potential of CH4 and N2O emissions in local double rice fields. By using composted mushroom residue and biogas digestion to substitute for fresh rice straw and cow dung for returning fields, CH4 emission and integrated global warming potential of CH4 and N2O emissions reduced by approximately 60% and 50%, respectively, and the grain yield will not be affected.

Abstract:The distribution of negative correlation center in the North Pacific Ocean from the low level to high level and characteristics of other anomalous centers which have strong correlations with the North Pacific Ocean are analyzed using the NCEP/NCAR global height field data and surface pressure data. Research results show that the distributions of the centers which have remarkable correlations with the North Pacific Ocean are always the same in each height field, and the three correlation centers are: negative centers in the central equatorial Pacific and the Bering Strait, positive center in the southeast of the North America continent. The correlations between grid points in the North Pacific Ocean and grid points in these three centers show that there is an obvious negative correlation center in the North Pacific Ocean at each height which reflects a character of barotropic structure, and the negative center moves eastward with the height increasing in the meridional direction while the ambulation in the zonal one is irregular; in these anomalous centers, the most remarkable center at 700 hPa、 500 hPa, and 200 hPa which influences the the North Pacific Ocean is the negative center in the central equatorial Pacific while that in the surface pressure field is the negative center in the Bering Strait. The correlation center first influencing the North Pacific Ocean is the positive one in the southeast of the North America continent for surface pressure and 700 hPa, and is the negative one in the central equatorial Pacific at 500 hPa and 200 hPa.

Abstract:Using the monthly gridded precipitation data in China, the NOAA extended reconstruction sea surface temperature (SST) data, and the NCEP/NCAR reanalysis data, the authors investigate the impact of the ENSO on the interannual variability of late spring－early summer rainfall in Yunnan Province and the relevant physical mechanism. The analysis of composite and correlation study reveal a strong negative correlation between the rainfall anomaly in Yunnan Province and the Niño index. An El Niño (La Niña) event is concurrent with less (more) precipitation during late spring－early summer (April－May) in Yunnan Province. Further analysis of atmospheric circulation demonstrates that an El Niño (La Niña) event is coincident with an anomalous easterly (westerly) over the Arabian Sea and the Bay of Bengal in the lower troposphere, a stronger anomalous anticyclone (cyclone) over the Bay of Bengal, as well as a stronger (weaker) subtropical anticyclone in the northwestern Pacific Ocean. The correlation between height at 200 hPa and rainfall anomaly during late spring-early summer (AprilñMay) in Yunnan Province reveals a relevant Indian Ocean－Asia teleconnection (IA) pattern. The water vapor transport indicates that during an El Niño (La Niña) event there is a stronger water vapor divergence (convergence) over the region. All above evidences demonstrate that, in comparison with the physical processes in controlling the interannual variation of the summer rainfall in southeastern China and the Yangtze River－Huaihe River valley, the conversion of Indian Ocean－Asia teleconnection pattern from winter to spring is dominant in determination of the rainfall anomaly in Yunnan Province during late spring－early summer, and particularly the increased water vapor transport associated with the intensification of the anomalous cyclone over the Bay of Bengal plays an important role.

Abstract:Based on the radiation evidence from the monthly ISCCP (International Satellite Cloud Climatology Project) data, the performances of three state-of-the-art Atmospheric General Circulation Models (AGCMs), namely BCC AGCM, IAP SAMIL, and IAP GAMIL, in simulating the cloud radiative forcings (CRFs) are evaluated in terms of climatology and the response to ENSO. It shows that all of the three models can reasonably simulate the spatial patterns of annual mean long-wave and short-wave cloud radiative forcings and net cloud radiative forcing. The spatial pattern correlation coefficients between the model results and ISCCP data are all statistically significant at the 1% level. However, the differences between models results and ISCCP data in the magnitude of cloud radiative forcings are significant over most of the regions, especially over the subtropical oceans. In addition, there exist significant discrepancies in cloud radiative forcings over the off-equatorial central northern Pacific as well as over the southeastern and southwestern Pacific in the three models. Further analysis shows that the discrepancies in cloud radiative forcings are caused by unrealistic cloud fraction and the vertical structure of cloud which are determined by the convection bias and choice of cloud parameterizations. The spatial patterns of the responses of net cloud radiative forcing to ENSO are well reproduced by the three models, except that the magnitude of the negative response over the tropical western Pacific is overestimated (underestimated) by BCC AGCM and IAP SAMIL (IAP GAMIL), and the location of negative response in IAP GAMIL shifts westward to the tropical central Pacific.

Abstract:The ensemble-based 3DVar (three-dimensional variational technique) method with SVD (singular value decomposition) technique (SVD-En3DVar) is used to assimilate simulated Doppler radial velocity observations. A localization scheme is introduced to the method to reduce spurious error covariance among distant points. The impact of different methods of producing initial perturbation and integration time lengths for forecast samples on the assimilation is emphatically investigated. A new scheme producing initial perturbation samples is proposed. This scheme takes the pseudo-random perturbation fields of temperature and specific humidity as the observation innovation and a 3DVar system is utilized to yield the initial perturbation fields of all variables from the observation innovation. Experiments using the simulated observations by WRF(weather research forecasting) model demonstrate that in the initial perturbation fields produced by using the new scheme the compatibility between different variables is better and the perturbation will not decay quickly in the forecast, so the spin-up time is cut down and the time interval of assimilation cycle can be shortened. The forecasts of temperature, humidity, horizontal wind, and rainfall within 12 hours are improved after assimilating the radar velocity data.

Abstract:The subtle forecast of high-impact weather events by the mesoscale numerical simulation is sensitive to the surface momentum and heat fluxes, so the development of reliable surface turbulent flux parameterization schemes has been a very important research subject. Using the Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) data, the authors compared the Blackadar scheme of the Weather Research Forecast (WRF) model against the air－sea turbulent flux parameterization scheme developed recently (i.e. LGLC scheme). The results show that the turbulent fluxes are better estimated with the new scheme than with the Blackadar scheme, especially for the wind stress and the latent heat flux, because the new scheme accurately describes the relationship between the Obukhov length and the Richarson number and takes account of the difference between dynamic roughness and thermal roughness. Meanwhile the LGLC scheme is a non-iterative scheme, and can save more CPU time in comparison with the iterative scheme in the WRF model. This offline test results form a basis for further online tests.

Abstract:The annual modes of tropical precipitation are simulated with a Spectrum Atmospheric General Circulation Model (SAMIL) developed by LASG/IAP. Sensitivity of the model's response to convection schemes is discussed. Two convection schemes, i.e. the revised Zhang-McFarlane (ZM) and Tiedtke (TDK) convection schemes, are employed in two sets of AMIP-type SAMIL simulations, respectively. The major characteristics of the annual mean precipitation can be reasonably reproduced in both simulations. There are some uniform simulation biases when using the ZM and TDK schemes. Both of them show bad performance in the simulation of spring-fall asymmetric mode. The simulated Indian monsoon and northern Australian monsoon are weaker, while the western North Pacific, African and American monsoons are stronger than the observations. The simulated bias of monsoon mode is derived from the simulation of the western North Pacific (NWP) precipitation in the boreal summer. In the boreal summer, the simulated temperature with both the schemes is cooler than that from the NCEP data in the upper troposphere and warmer in the lower troposphere, which results in convective instability in the NWP and heavier precipitation there. The mean temperature from the surface to the tropopause is cooler than the observations in the simulations. It leads to the weaker temperature gradient and weaker westerly jet over the NWP. The weaker westerly jet provides favorable dynamic condition for the convection anomaly over the NWP. Nonetheless, there are also some differencs between the TDK and ZM schemes. The simulated monsoon mode in the western North Pacific is stronger with the ZM scheme than with the TDK scheme. The simulated specific humidity is larger with the ZM scheme than with the TDK scheme. It is one of the reasons that the simulated NWP summer monsoon precipitation is heavier with the ZM scheme. The performance of the SAMIL in simulating the tropospheric temperature and humidity over the NWP deserves further improvement.