Abstract:This paper explores the modulation of 30－60 day low frequency oscillation (MJO) on tropical cyclogenesis over the western North Pacific (WNP) in terms of dynamics and energy conversion, using the global reanalysis data and tropical cyclones (TCs) archives obtained from Joint Typhoon Warming Center, USA. During the westerly phase in the western part of the WNP, waves in the flow convergence region are characterized by the increase in wavenumber and the decrease in wavelength, which causes the evolution from large-scale waves to synoptic scale ones. The convergence and shear of zonal wind in the westerly phase would accelerate kinetic energy transfer from low frequency to high frequency, favorable for TCs formation in that region. Reversely, tropical cyclogenesis would be inhibited during the easterly phase. In addition, the TCs occurrence probability would increase (decrease) with the strength of westerly flow during the westerly phase (the strength of easterly flow during the easterly phase). However, the modulation of MJO on TCs would weaken in the eastern part of the WNP. The interannual variations of MJO activity suggest that, during the warming state years of the WNP warm pool, MJO is active and the westerly phase is prevalent, which is conducive to tropical cyclogenesis. The circumstances are reversed during the cooling state years of the warm pool.

Abstract:Based on the conception of“Annular Belts of Action (ABAs)”, characteristics of the seasonal variation and main submonthly timescales of Northern and Southern hemispheres annular modes (SAM and NAM) are studied in this paper using NCEP/NCAR reanalysis daily data. The results show that NAM is strong in winter and weak in summer while SAM's intensity has semi-annual cycle. Furthermore, power spectrum analysis is applied year by year to detect the main submonthly timescals of anular modes, the result shows that NAM has quasi-week period and quasi-two-week period variability, which are inclined to accompany each other, the quasi-three-week period is the following time scale; and SAM exhibits similarity of the main submonthly timescales to NAM, that is, the quasi-two-week period and the quasi-week period are the main periods and the quasi-three-week period is the second period. Both the spatial characteristics of NAM and SAM with different submonthly timescales and their temporal evolvement are worth further researching.

Abstract:This paper estimates the uncertainty in the outputs of a dust-transport model due to physical parameterizations (dry deposition) and input data (source intensity and wind fields). Sensitivity experiments are generated from a reference simulation in which one parameter is changed at a time. Comparisons of the simulations and observations allow us to access the impact of each process and the robustness of the model. The case study is a 10-day simulation of dust concentrations over East Asia during 15-24 March 2002. Results show that there is a higher uncertainty due to the input data (source intensity and wind fields) than the dry deposition velocity. Moreover, for this case, there is higher uncertainty due to the source intensity and the dry deposition velocity in the middle area (95°E-110°E) than in other areas, and there is higher uncertainty due to surface wind fields than higher-level ones.

Abstract:By using a regional model named AREM (Advanced Regional Eta-coordinate Model), which was developed by the State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamic, Institute of Atmospheric Physics, Chinese Academy of Sciences, several local heavy rainfall events happening over the western Sichuan basin during August 2003 are simulated and analyzed. The results indicate that the initial local water vapor condition is crucial to the happening and evolution of heavy rainfall events over the western Sichuan basin. Both the intensity of heavy rainfall and the time of maximum precipitation are highly sensitive to the initial condition of local water vapor. An increase of initial local water vapor would not only increase the 24-hour accumulated precipitation amount, but also lead to the main rainfall events happening earlier, resulting in a day-time rather than nocturnal rainfall events. Correspondingly, a decrease of initial water vapor would delay the maximum precipitation time and decrease the total precipitation amount.

Abstract:The Antarctic Oscillation Index (AAOI) and the westerly index of Antarctic oscillation (Uaaoi) are defined by using the NCEP data of sea level pressure, temperature and 850-hPa westerly wind from January 1951 to December 2002. Long-term trend, interdecadal jump and cycles of AAOI and Uaaoi as well as the effect on the Antarctic circumpolar wave are discussed. Main conclusions show that the relationship between AAOI and Uaaoi is significant with the correlation coefficient of 0.218, which exceeds 99.9% confidence level. While AAOI and Uaaoi are high, the westerly wind increases, vice versa. Long-term tendency exists in AAOI and Uaaoi with trend rate of 0.01915/10 a and 0.009249/10 a respectively. A distinctive jump point happened in 1972, before and after this year the average AAOI was -3.9691 hPa and 2.9107 hPa respectively, the average AAOI is increased by 6.88 hPa, the average westerly wind was -1.09 m/s and 0.93 m/s respectively. The sea level pressure and temperature along 50°S-60°S in 50 years show clear patterns of dominating eastward propagation with 4.4 years rounding a circle before the jump year and 3.6 years after the jump year in term of the time series by filter of 3-5 years. In the first principal component of EOF, wave number 3 and wave number 2 are dominating before and after the jump year, respectively.

Abstract:The roles of wind stress and surface heat flux in generating the Pacific interannual and interdecadal variability are examined by using two sets of simulations performed with LASG/IAP climate ocean model named LICOM. It is found that the interannual variability of SST in the tropical Pacific is mainly controlled by wind stress, however, the simulation of El Niño is improved significantly when surface heat flux is considered. The interannual variability of SST in the North Pacific is generated by the effect of surface heat flux, though it can be reproduced partly in a simulation without interannual variability of surface heat flux. Simulation without interdecadal variability of surface heat flux fails to reproduce the Pacific Decadal Oscillation (PDO) pattern, which confirms that the interdecadal variability of SST is governed by surface heat flux. To reveal the contributions of surface flux, horizontal advection, vertical advection and diffusion (including horizontal and vertical diffusion) to the variability of winter averaged upper ocean temperature in different areas, the upper ocean heat budget is examined by using the output of LICOM_HW run. It shows that SST is controlled by different mechanisms in different regions. The anomaly of temperature trend is determined by both net surface heat flux and horizontal advection in the central North Pacific. But in the coast off California, only the anomaly of net surface heat flux is found to be dominant. In the Kuroshio and Kuroshio Extension (KKE) region, the contributions of net surface heat flux and the oceanic nonlinearity effect are significant, but the contributions of horizontal advection and diffusion should not be neglected.

Abstract:The satellite, radar and NCEP data are used to analyze an unusual snowstorm over Liaoning Province during 3-5 March 2007. The analysis shows that the combination of south and north troughs at 500 hPa, the northbound vortex at 850 hPa and the cyclone at the surface are very favorable to the occurrence of snowstorm. The intense low level jets and ascending motion at 850 hPa are in favor of the moisture transportation and the intensification of snow. The low level jets and the convergence of wind may be considered as triggering mechanism for the snowstorm.

Abstract:This work assesses the performance of the new generation of LASG/IAP coupled climate system model named FGOALS_s in simulating the relationship between the Asian-Australian monsoon (AAM) and ENSO (El Niño-Southern Oscillation). Compared with the observations, FGOALS_s can simulate main characteristics of climatology of AAM. The amplitude of ENSO simulated by FGOALS_s is about 70% of that in observations. The FGOALS_s model reasonably reproduces the irregularity of ENSO period. Main features of ENSO are reasonably reproduced in FGOALS_s model, i.e., when SST in the Niño3 region is higher than normal, the sea surface pressure is higher than normal in the warm pool, but lower in the eastern Pacific. Phase locking of ENSO reproduced by FGOALS_s is in boreal spring or summer. This is the main deficiency of the model. The amplitude of interannual variability of AAM is larger than that in the observation. FGOALS_s fails in simulating the negative correlation between AAM indexes and contemporary Niño3 index, which may be caused by wrong phase locking of ENSO. When El Niño events reach peaks in boreal spring or summer, anomalous subsidence of Walker circulation moves eastward. An anomalous anticyclone stimulated by the anomalous subsidence is located to the east of that in the observation. As a result, the negative correlation between Indian monsoon rainfall and ENSO is not significant. During boreal winter, SST anomalies over the tropical eastern Pacific are weaker than those in observations, so AAM response is also weak. On the other hand, the divergent center, which is located over the Australian monsoon region in the observation, shifts westward in the model, because SST anomalies in the equatorial central and eastern Pacific extend westward, resulting in an insignificant negative correlation between the Australian monsoon rainfall and ENSO in the model. This analysis has provided a guide for the future improvement of the coupled model.

Abstract:By using synthesis, EOF analysis, numerical simulation and the other methods, this paper investigate the intraseasonal climate characteristics of two typical persistent flow patterns, pattern E (there are a longwave ridge, trough, ridge over the Ural Mountains, Lake Baikal, the Sea of Okhotsk, respectively) and pattern C (the positions of trough and ridge is opposite to pattern E’s), over Eurasian middle and high latitudes in summer. The main conclusions are as follows: (1) The deviation of the actual frequency from the theoretic normal distribution probability of the circulation pattern index shows a bimodal feature, i.e., the actual frequency of patterns E and C is more than the theoretical value.(2) The indices averaged over 1959-2000 presents obviously an intraseasonal staggered variation in resemblance to the northward advance of the subtropical high over the western Pacific and summertime rain belt in China. The result of numerical simulation shows this feature too. (3) The obvious contrast between pattern E and pattern C exists not only over Eurasian middle and high latitudes but also over the other areas of the Northern Hemisphere. The occurrence and change of these persistent anomalous circulations are closely correlated with the positive and negative phases of the East Asia－Pacific (EAP) pattern, as well as the two modes of the South Asia high and the western Pacific subtropical high. It is a new focus for understanding the interaction of the systems over different latitudes of Eurasia and its cause of formation.

Abstract:Based on the American NASA monthly precipitation grid data, the monthly mean surface temperature anomalies grid data from the CO2 Information Analyses Centre, USA, the NCEP/NCAR reanalysis global grid data and the stations observational soil temperature data from the Cold and Arid Regions Environmental and Engineering Research Institute of the Chinese Academy of Sciences, the authors study the Asian-African summer monsoon meridional circulation weakening in the mid-1960s on interdecadal time scale and the related global climatic anomalies phenomena. The results show that the tropospheric temperature reduced obviously and the global oceanic temperature abnormal changes in the mid-1960s are mainly characterized by SST increasing in the Indian Ocean and SST decreasing in the northern Pacific Ocean and the northern Atlantic Ocean. Simultaneously, the soil temperatures at 1.6-m and 3.2-m depths in Chinese continent and the surface temperature in the Tibetan Plateau decreased respectively and remarkably in the mid-1960s, which reduces the thermal contrast between Asian continent and the Indian Ocean. Accordingly, the easterly jet weakened, finally the Asian-African summer monsoon weakened too and the monsoon meridional circulation weakened obviously.

Abstract:There are two major sea surface temperature anomaly (SSTA) patterns over the tropical Indian Ocean (TIO): basin-wide SSTA variation which peaks in the boreal spring, and the dipole mode that is phase-locked in the boreal autumn. This paper explores possible impacts of these two SSTA patterns on the South China Sea (SCS) summer monsoon onset. Using composite analyses, it is revealed that basin-wide warming (cooling) in the TIO apparently induces an anomalous reversed (intensified) Walker circulation over the tropical Indo-Pacific region, leading to anomalous descending (ascending) motion, and hence suppressed (increased) convection over the western Pacific. The intensified (weakened) western Pacific anticyclone in April and May prevents (favors) the extension of the Indian Ocean westerly flow into the SCS region, thereby causing a late (an early) SCS summer monsoon onset. The results suggest that the basin-wide SSTA mode in the Indian Ocean should play an important role in prolonging the El Niño-Southern Oscillation (ENSO) effects on the subsequent Asian summer monsoon, mainly through modifying the strength of the western Pacific anticyclone. Further, impacts of the Indian Ocean dipole (IOD) case on the SCS summer monsoon onset in 1994 are carried out. Coupled to the exceptionally strong positive IOD over the tropical Indian Ocean in early May, the Asian summer monsoon circulation around the Indian Ocean is significantly weakened, especially the Somali jet along the east coast of Africa and the low-level westerlies across the equatorial Indian Ocean. Anomalous anticyclones dominate the 850-hPa wind field in the tropical Indian Ocean, accompanied by easterly anomalies along the equatorial Indian Ocean. These features, induced apparently by the positive IOD event, are unfavorable for the establishment and advancement of the Asian monsoon westerlies, and lead to a delay of the SCS summer monsoon onset in 1994.

Abstract:A method of retrieving aerosol optical depth (AOD) is developed using daily horizontal broadband direct solar radiation, including an “equivalent” instantaneous solar zenith angle model. Furthermore, a cloud-contaminated reduction method, which is based on aerosol scaling height, is proposed to calculate monthly mean AOD. Numerical simulation and error analysis show that: 1) The mean value of AOD retrieving errors due to instability of the “equivalent” instantaneous solar zenith angle model is 3.66%; 2) AOD diurnal variation has little influence on the mean value of AOD for a relatively long time; 3) the AOD retrieving error is less than or equal to 4％ when the error of diffuse solar radiation is less than or equal to 20％. A comparison between AODs retrieved from this method and the AERONET shows that the AOD retrieving method and the cloud-contaminated reduction method are both effective. The correlation coefficient and mean bias error between AODs from this method and the AERONET at 0.75 μm in clear days are 0.95 and 0.02, respectively. With the cloud-contaminated reduction method, the seasonal and yearly mean AODs show a good consistent with those from AERONET.

Abstract:The temporal evolution and spatial distribution of the frequency of extreme maximum temperature (EMT) days as well as mean maximum temperature (MMT) during the EMT days in East China have been analyzed by using daily maximum surface air temperature from 1960 to 2005, the western Pacific subtropical high intensity and area index, the equatorial Pacific sea surface temperature and the number of landfall typhoons. The results show that the EMT days and the MMT during the EMT days have large temporal variation and regional difference. In the past 45 years, the EMT days show more-less-more interdecadal variability and the MMT during the EMT days has a higher-lower-higher interdecadal variability. There are more EMT days in the south-central and southwest parts of East China, and less ones in the east coastland area and the north part. The MMT during the EMT days is relatively higher in the most parts of Zhejiang, Anhui and Jiangxi provinces in the middle and west parts of East China. In addition, the EMT days and the MMT during the EMT days show different types of jump change and different jump change time in different areas. In East China, particularly, in its southern part, the EMT days are positively correlated with the western Pacific subtropical high intensity and area index, the Niño 4 SST in the last half part of previous year, and the number of landfall typhoons in China. Urbanization also increases the frequency of EMT days.

Abstract:Using multi-scale analysis, several indicators of cold air activities during the East Asian summer monsoon (EASM) are compared, and the results show that potential vorticity (PV) is the optimal indicator in tracing cold air activities during EASM. The further study shows that it is best that the low-frequency oscillations of PV (20－80 day and 40－80 day oscillations) represent cold air activities. Then, the source and role of cold air during EASM are studied by use of the indicator, and the process and mechanism of interactions between EASM system and extratropical and mid-upper level systems during the Meiyu period are analyzed in detail. The conclusions are as follows: the two high PV pools are the main sources. One exists in the extratropical low stratosphere and tropopause, and the other exists in the low troposphere over the east of the Asian continent. To the intrusion paths, different rainy seasons show different features: during the pre-flood season in South China, the cold air intrudes twice respectively from the northeast at mid-lower levels and the high PV pool of the tropopause; during the Meiyu period, the cold air origins mainly from the high PV pool in the extratropical low stratosphere and tropopause during the onset of Meiyu, and from the high PV pool in the low troposphere over the Sea of Okhotsk during the mid-late period of Meiyu; during the rainy season in North China the cold air origins chiefly from the upper troposphere. Inclined isentropic surfaces are the areas where vertical vorticity grows quickly most easily, and the paths along which the upper cold air intrudes. Each rainy season over eastern China in summer is likely the reflection of the intraseasonal oscillations of the interactions between the EASM system and extratropics, especially the low-frequency part playing a more important role. Finally, the sources and characteristics are sketched.

Abstract:Rainfall, especially heavy rainfall (torrential rain), has great impact on economy, society and people's routine life. While owing to the complicated physical processes related to rainfall, the studies and predictions of rainfall are rather difficult. Previous relevant studies mainly focused on the impacts of water vapor and its convergence. Gao et al.(2005a) proposed a so-called surface rainfall equation by combining the tendency equations of water vapor and hydrometeors (cloud water, rain water, cloud ice, snow and graupel), which can be used to study quantitatively the water vapor variation and cloud evolution related to surface rainfall together.  In this article, 21-day tropical cloud-resolving simulation data are used to calculate the local change and convergence rates of water vapor, surface evaporation rate and cloud variation rates in the surface rainfall equation, and their impacts on surface rainfall are discussed. The results show that local change rates of water vapor and cloud hydrometeors, water vapor convergence rate and surface evaporation rate all have great influence on surface rainfall. The co-existence of water vapor convergence and local vapor loss leads to heavy rainfall, and the co-existences of water vapor convergence and local vapor gain or water vapor divergence and local vapor loss lead to medium rainfall, and the co-existence of water vapor divergence and local vapor gain leads to weak rainfall. The partition of convective and stratiform rainfalls shows that the convective rain rate is normally bigger than the stratiform rain rate. Water vapor convergence is the main water vapor source for convective rainfall, while local vapor loss is the main vapor source for stratiform rainfall. Model domain mean local vapor loss mainly occurs in the raining stratiform region, while the strongest local vapor gain occurs in the convective and clear sky regions. The strongest local hydrometeor loss occurs in the stratiform region, while the strongest local hydrometeor gain occurs in the convective region.

Abstract:Precipitable water vapor (PWV) is one of important green-house gases in the atmosphere. It affects the energy balance of the land-atmosphere system. The evolution of cloud and precipitation has a close relationship with the PWV. In the climate change, the PWV has a positive effect on temperature. Ground-based microwave radiometer can make all day unattended measurement of the PWV. Three different PWV measurements are compared. The PWV mean bias between the radiometer and the radiosonde is 0.281 cm. The PWV mean bias between the radiometer and the GPS is 0.322 cm. The PWV mean bias between the GPS and the radiosonde is 0.728 cm. In Beijing, there exist different diurnal variations in different seasons. The mean diurnal variation is about 0.162 cm in autumn, 0.130 cm in winter, 0.229 cm in spring, and 0.276 cm in summer. Moreover, the maximum PWV appears mostly at 0000 LST or 2300 LST in the four seasons. The minimum PWV is the same as the maximum PWV, except that the minimum PWV appears at 1000 LST in winter. The variation of PWV per hour decreases in the order of summer, autumn, spring and winter. Moreover, in every season, both percentages of the positive and negative PWV variations per hour are nearly 50%. The hourly correlations between the PWV and temperature are analyzed for the four seasons. The hourly correlation coefficients decreases in the order of autumn, spring, winter and summer. In autumn, spring and winter, all the hourly correlation coefficients are positive. However, in summer, the hourly coefficients are negative from 0900 LST to 2200 LST.

Abstract:The evolution of summer rainfall quasi-biennial oscillation (TBO) is studied over eastern China using monthly mean rainfall data of China 160 stations from China Meteorological Administration. The maximum entropy spectral analysis and the relative maximum entropy spectral analysis denote that the summer rainfall in eastern China has a marked TBO signal, the areas with high values distribute basically like a belt, and the variance maximum value centers are located in the Yangtze-Huaihe valley and the south littoral. Based on Rotated EOF analysis (REOF) of summer rainfall TBO, eastern China is divided into 7 sections including Northeast China, the Hetao basin, the Huaihe River basin, the Yangtze River basin, western South China, centre South China and eastern South China. The further study shows: (1) the eastern China summer rainfall amplitude possesses TBO features. (2) The summer rainfall TAO has different evolutions and interdecadal features for different sections. TBO is distinct in the Huaihe River basin, the Yangtze River basin and central South China; the TBO signal is weak in western South China and Northeast China; TBO was marked in the Hetao basin before the 1990s, but after the 1990s TBO weakened gradually; TBO was remarkable in eastern South China before the 1970s and it weakened after the 1970s. (3) The Huaihe River basin is the transition zone from south to north over eastern China, and is also the sensitive area to summer rainfall TBO in eastern China.