Abstract:This article composed seven El Niño events for the period 1971－2003, and analyzed the Southern－Northern Hemispheres symmetry of general circulation anomalies during the El Niño events. The analysis shows that the tropical (15°S－15°N) general circulation have mainly symmetric anomalies, but less antisymmetric anomalies during El Niño events. And antisymmetry of height and zonal wind anomalies is week; oppositely that of meridional wind anomalies are much stronger. In addition, there are more varieties in height and zonal wind anomalies than in meridional wind anomalies. Comparing the anomalies at 200 hPa and 700 hPa, it can be seen that the anomalies at these two levels are obviously out-of-phase. Furthermore, the Southern-Northern Hemispheres asymmetric effect of El Niño events on general circulation was studied. The Hough function was utilized to expand the observational ERA-40 daily data to equatorial waves for analyzing the evolutive characteristics of equatorial waves during El Niño events. The result shows that the reason of strong tropic symmetric anomalies of general circulation is the presence of strong symmetric Rossby waves anomalies. On the other hand, the mixed Rossby-gravity wave presents the antisymmetric anomalies, especially the meridional wind anomalies. In addition, during the El Niño events the positive sea surface temperature anomalies over the tropical eastern Pacific generated the Gill-type circulation, which consists of symmetric Rossby wave and Kelvin wave. Finally, the Hough function has advantages in analyzing the symmetry and asymmetry of the tropical atmospheric waves.

Abstract:The old radiation scheme in IAP9L-AGCM is replaced by a new one from the CCM3/NCAR codes. And the effects of such modification are evaluated in detail. It shows that there are many improvements in the simulated radiation fields from the new version of IAP9L-AGCM, especially some net radiation fields at the surface. As a result, there is a prevalent enhancement of the atmospheric temperature in the new version. Other fields, such as sea level pressure, surface air temperature, geopotential height, wind field, precipitation, specific humidity and so on, show some corresponding changes, though the magnitudes are not large. The evaluation results put forward the requirement of a further improvement of the IAP9L-AGCM.

Abstract:Based on the daily outgoing long-wave radiation (OLR) dataset from NCEP/NCAR during 1979－2004, the features of the zonal and meridional propagations of the tropical convection during the onset period of the South China Sea (SCS) summer monsoon (SCSSM) are analyzed in this paper. Results indicate that the directions of both the zonal propagation and the meridional counterpart are quite different. For example, for the zonal propagation, the deep convection can propagate either from the Bay of Bengal (BOB) to SCS, or from the western Pacific to SCS, or developed locally. But during the onset period of the Indian summer monsoon, the zonal propagation is relatively simplex with a westward propagation. For the meridional direction, the convection over SCS can be influenced by both the counterpart from the tropical regions of the Southern Hemisphere and from the middle latitudes of the Northern Hemisphere. But in the Indian monsoon regions, the convection comes only from the Southern Hemisphere. These observation results reveal that the convective activities during the onset period of SCSSM are more complex than those of the Indian summer monsoon. Results also show that the different retreat ways of the western Pacific subtropical high in the lower troposphere can lead to the multi-direction propagation of the deep convection over SCS. This will misrepresent the judgment of the onset dates of SCSSM.

Abstract:To study the radiative effect of dust aerosols on synoptic－scale system and the dust transport, an interactive weather-dust model including shortwave radiative effect of dust aerosols is developed. The change of temperature caused by radiative effect of dust aerosols is on-line fed back to the weather model. A dust storm caused by the cold front occurring in the south of Xinjiang during the period of 11 to 16 March 2006 is analyzed with the model. Two parallel simulations are done, one including the radiative effects of dust aerosols (RAD) and the other without it (CTL). A comparison of the two tests showed that: 1) Incorporation of radiative effect of dust can well improve simulations of diurnal changes of surface temperature, vertical changes of air temperature and variation of the sea level pressure during the high floating dust period. 2) During the dust storm period, the radiative effect of dust aerosols can cause large increases in dominant wind speeds at lower model levels but will reduce dust emissions and deposition. 3) A combination of the radiative effect of dust and the basin terrain influences leads to decreasing more deposition of dust aerosols than emission, therefore, it helps maintain the floating dust weather.

Abstract:Typhoon Haitang landing Fujian Province from 0800 LST 19 July to 0800 LST 20 July in 2005, gives rise to a significant rain process during the 24 hours, and precipitation intensity on the north side of typhoon is stronger than that on the south side, which shows obvious asymmetric distribution. The rainfall process is simulated using the WRF model and based on simulated outputs, the relative humidity and vertical ascending motion fields are analyzed, meanwhile, the forcing roles of orgoraphic lifting and surface friction are calculated and the diagnosis of the revised wet Q vector is conducted, so the cause of asymmetric distribution formation of rainfall associated with typhoon Haitang is analyzed quantitatively. The results are as follows: (1) The rain asymmetric distribution feature, the fallout region of primary precipitation, and the intensity and the location of extreme rain exceeding 300 mm are simulated successfully, so the simulation effects are considerably inspiring. (2) Vertical ascending motion condition may be the predominant factor leading to asymmetric distribution of precipitation associated with the typhoon. (3) The convergence intensity of the revised wet Q vector divergence on the north side of the typhoon is stronger than that on the south, and from the further calculation and analysis it is found that the rain field forced by the revised wet Q vector divergence also shows marked asymmetric distribution, with intensity on the north stronger as compared with that on the south side of the typhoon. (4) The precipitation intensity forced by orographic factors is about 1.6－2.5 times of the counterpart caused by the revised wet Q vector divergence, and the rain intensity forced by surface friction is approximately 2－3 times of the counterpart caused by orographic lifting.

Abstract:The NCEP/NCAR reanalysis data is firstly used to analyse the persistent diabatic heating in the summer monsoon regions including South China, the Changjiang River-Huaihe River valley and North China. Seeing that condensation heating is the main part of diabatic heating in the monsoon region, then related numerical experiments in the above three regions are made so as to discuss the possible impacting approaches of condensation heating on general circulation. All numerical experiments reveal that, the condensation heating can firstly induce cyclonic anomaly nearby the heating region in the mid-low troposphere, then the anomaly propagates northeastward, and the wave train forms in mid-high latitudes finally, which indicates the northeastward dissipation of latent heat energy. Moreover, the difference of instance experiments shows that the initial response of atmosphere to the condensation heating depends a lot on the general circulation in the initial stage of heating, and the anomaly distribution similar to the vertical circulation structure may be one of the possible response modes.

Abstract:The East Asian Subtropical Westerly Jet (EAWJ) is a narrow and strong westerly wind belt with large horizontal and vertical wind shears over the East Asian subtropical region, which is generally located at the north edge of the Western Pacific subtropical high. It plays a key role in linking weather systems over the mid-latitudes and low latitudes. Its northward jump is the signal of general circulation seasonal transition and the criteria for dividing the natural season of East Asia. Therefore, it is a meaningful work to further study the influencing features for its variations. Based on that climate model GAMIL successfully simulates the pattern and seasonal evolution of East Asian general circulation, the impact of anomalous surface heating in the Kuroshio Current region to the south of Japan and the Tibetan Plateau on the EAWJ is investigated by designing sensitive experiments with the GAMIL. The results indicate that strong heating in the Kuroshio Current region to the south of Japan in winter can cause the eastward shift of the East Asian trough, intensifying of the cold high pressure in the continent, Aleutian low pressure and western Pacific subtropical high, as a result, leads to strong winter monsoon circulation. Corresponding to variations of the geopotential height field, cyclonic difference circulation over the northeastern Pacific Ocean and anticyclonic difference circulation over the mid-high latitudes adjacent to the ocean are observed. The anomaly of geopotential height over the low latitudes south of the EAWJ is positive and the opposite situation occurs to the north of the EAWJ. Therefore, the meridional difference of air temperature is accordingly enlarged and the EAWJ is intensified.

Abstract:Based on the continuous observation data at the 90 meteorological sites in Xinjiang, a selfcalibrating Palmer drought severity index (PDSI) is firstly used to investigate the annual and seasonal variation of drought and wet conditions in the region during 1961－2003. During the concerned period, Xinjiang is characterized by normal climate condition, and both annual and seasonal mean climates are prone to become wetter as a whole. Qualitative analysis reveals that the recorded surface warming tends to induce drought, whereas increased precipitation is favorable for wetness. Under the SRES A2 emission scenario for atmospheric greenhouse gasses and aerosols, projected surface warming tends to induce drought, whereas increased precipitation tends to induce wetness in the 2090s. Taking into account the above two factors together, the PDSI indicates that drought and wet conditions in Xinjiang alter to some extent in the 2090s.

Abstract:In order to investigate the mechanism of impact of sea surface temperature (SST) cooling on typhoon intensity and the ocean response to typhoon, a mesoscale coupled air－sea model is developed based on the non-hydrostatic mesoscale model MM5 (the fifth-generation Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model) and the regional ocean model POM (Princeton Ocean Model). In this study, two experiments are performed, one used the coupled model and the other MM5, in which the SST field at the model initial time is fixed so that the typhoon-induced cooling is ignored. Results reveal that the level of intensity prediction is improved including typhoon-induced SST cooling, the simulated minimum surface pressure and maximum surface wind in the coupled model are better consistent with observation than that in the uncoupled model, and the simulated central pressure from the latter is 20 hPa deeper than that from the coupled model result.  The SST simulated by POM in the coupled experiment is also in good agreement with that from the TRMM/TMI-derived data. The maximum SST cooling is 5.8°C, which is about 95 km away from the right side of the typhoon track. Corresponding to the SST cooling, the depth of mixed layer deepens in the meantime, indicating the effect of entrainment on SST cooling.  Analysis shows that SST cooling reduces the sensible and latent heat fluxes from ocean to the vortex, especially in the inner-core region. In this study, the averaged total heat fluxes in the inner-core region of the typhoon decrease by 32.1%, which causes reduction of the moist static energy and the radial gradient of moist static energy, resulting in weakening of the typhoon.

Abstract:Climatology of the observed daily precipitation extreme indices (SDII, simple daily intensity index; CDD, the maximum number of consecutive dry days; R10, number of days with precipitation greater than 10 mm; R5d, maximum 5-day precipitation total; R95t, fraction of total precipitation due to events exceeding the 95th percentile of the climatological distribution for wet day amounts) at 550 stations in China during 1961－2000 is used to evaluate the simulation ability of 7 IPCC AR4 Coupled Climate Models, the projected change of the precipitation extreme indices over China under IPCC SRES A2、A1B and B1 is also studied. The results show that the state-of-the-art IPCC AR4 models can simulate the spatial distributions and the linear trends of precipitation extremes well. The multi-model ensemble (MME) shows the best skill, but both the MME and single model fail to simulate the interannual variability and have large biases, such as there are excessive extreme precipitation over the eastern side of the Tibetan Plateau while in the monsoon regions the modeled intensity of precipitation extremes is lower than the observation. In the 21st century the precipitation will become more “extreme”， there would be an overall increasing trend in extreme precipitation events over most of China under a warming environment, and the change scope is scaled to the emissions scenarios.

Abstract:The atmospheric water vapor transports associated with typical anomalous spring rainfall patterns have been investigated using NCEP/NCAR, ERA40 monthly mean reanalysis data and precipitation data of 160 stations in China during 1951－1999. Results show that origins of water vapor supply related to anomalous rainfall patterns are different from the climate mean situation. In anomalous pattern 1, with a heavier rainbelt along the South China coast, the main moisture comes from the Philippine Sea and the adjacent South China Sea. The background large-scale circulation changes include the intensification of the western Pacific subtropical high (WPSH) and the southwest shift of the East Asian jet stream (EAJS). In anomalous pattern 2, with a main rainbelt along the middle and lower reaches of the Yangtze River, the origins of water vapor supply contain the western tropical Pacific and the tropical Indian Ocean. Both the WPSH and the EAJS move to the north of their normal position. In anomalous pattern 3, a rainy region is located in the Huaihe River valley. The moisture originates from the northwestern Pacific. The 500-hPa anticyclone anomaly moves to northeastern China and the EAJS is weaker than its normal condition. Although water vapor transport along the southern edge of the Tibetan Plateau is one of the main branches in the climate mean pattern, none of the typical water vapor transport related to typical anomalous rainfall reflects this water vapor flow.

Abstract:Utilizing soil temperature data collected at a bare ground soil over the Loess Plateau during the period from 22 to 26 July 2005，the soil thermal diffusivity is calculated by using two algorithms (i.e., thermal conduction algorithm combined with mathematical fitting method and thermal conduction-convection algorithm) in order to better understand the physical property and procedures of the soil over the Loess Plateau quantitatively and to form a basis of improvements of soil temperature diagnosis in land surface models. Soil thermal diffusivity is calculated by the two methods mentioned above, and the sum of vertical gradient of soil thermal diffusivity and water flux density is calculated by the thermal conduction-convection algorithm to range from 0.80×10-6 to 2.43×10-6 m/s for the soil layer of 0.05－0.10 m. Taking the soil layer at the depth of 0.05 m as the upper boundary, the temperature for the soil layer at the depth of 0.10 m is modeled by means of the two methods mentioned above. It is found that for the thermal conduction algorithm, owing to the fact that it takes into account thermal conduction alone and considers neither the soil water movement nor the soil vertical heterogeneity, both the diurnal amplitude and the delay of phase are overestimated. For the thermal conduction-convection algorithm, the diurnal amplitude and phase can be well estimated. The bias, standard error and normalized standard error of the modeled value against the observation are respectively 0.19 K, 0.18 K and 0.08% during the daytime (0800－2000 Beijing Time).

Abstract:Satellite observations of sea surface temperature (SST) have recently revealed an existence of unstable oscillations in the equatorial eastern Pacific and Atlantic. These oscillations appear as 20－40-day periodical fluctuations with westward phase speed of 0.4－0.6 m/s and wave length of 1000－2000 km during the boreal summer and fall. They are generally called tropical instability waves (TIWs). They are believed to play an active role in the oceanic heat fluxes in the tropical ocean, locally inducing strong air－sea coupling. In this study, TIWs simulated by a high-resolution atmosphere－ocean coupled general circulation model are investigated. The horizontal resolution of the model is 120 km in the atmosphere and 30 km by 20 km in the ocean. Model simulations show good agreement with observed main features associated with TIWs. In the numerical simulations, it is estimated that the strong cooling effect induced by the equatorial upwelling is partly (about 30%－40%) offset by the equatorward heat flux due to TIWs eddies in the eastern tropical Pacific. Furthermore, the TIWs cast a positive feedback on the atmosphere through changing the vertical mixing just above the TIWs. Though the eddy oscillations induced by TIWs south of equator are much weaker than those north of equator, a similar atmospheric response to TIWs is simulated in the atmospheric mixed layer.

Abstract:This paper evaluates the simulations of East Asian Summer Monsoon (EASM) for the new version of climate system model, namely FGOALS_s, developed at the State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics (LASG/IAP), which employs the recently improved version of LASG/IAP spectral AGCM SAMIL as its atmospheric component. To understand the impacts of air－sea coupling, the results of coupled system are then compared to AMIP-type SST-forced simulations with SAMIL. FGOALS_s is shown to capture the main climate mean states and seasonal features of EASM, though the bias is obvious. The main deficiency of the FGOALS_s model is that the simulated temperature is systematically colder than the observation in the middle and upper troposphere, which notably weaken the atmosphere circulations and winds. In addition, the insufficiency of meridional air temperature gradient directly influences the simulation of subtropic westerly jet. The comparison between FGOALS and SAMIL demonstrates that there are similar biases in the stand-alone AGCM, though the simulation of SAMIL is more realistic. This indicates that the biases included by the atmosphere model have great influence on the coupling system performance. Besides, the spatial pattern of EASM precipitation is more reasonable in the coupled model, which illuminates that air－sea interaction can improve the simulation of precipitation in EASM while amplify the biases in the temperature field. Therefore air-sea coupling also plays a vital role in the performance of the climate system model. The results suggest that the biases in the atmosphere are the key reasons for the insufficiency of coupled system. From this point, updating cloud and radiation processes to improve the simulation of temperature in the atmosphere model, may be the first step for the future development of FGOALS_s.

Abstract:The goal of this paper is to analyze the differences of atmospheric radiative distributions between the scattered-cloud sky and the clear sky by simulations of a 3D radiative transfer model. The two parameters, radiance and radiative ratio of 450-nm to 650-nm radiance, are used to express the differences. Analysis results show that the radiative value of atmosphere will suffer the effect of surrounding cloud and the degree will depend on the parameters like cloud cover, cloud optical thickness as well as aerosol optical depth (AOD). It is found that radiance in most regions varies between ±2％ in the cloudy sky when the cloud cover is small, in comparison with the same fields in the clear sky. It indicates that the threshold value obtained from the 1D radiative transfer model, used to recognize cloud from sky in the all-sky image, is applicable for the inhomogeneous cloudy sky. Moreover, the “non-cloud” radiance in the cloudy sky can be used to make other retrieving researches like the AOD retrieval.

Abstract:The mechanism for the transient response of the Atlantic Meridional Overturning Circulation (AMOC) to the enhanced and continuous freshwater input into the Arctic Ocean and the Nordic Seas for 150 years is investigated using a fully coupled climate model (Bergen Climate Model, BCM for short). The responses of the Sea Surface Temperature (SST), Salinity (SSS), Potential Density (SPD), the North Atlantic Deep Water (NADW) formation, Diapycnal Mixing (DM) and the wind stress are analyzed. The transient response of AMOC follows a quick dropping down during the first 50 years, with a gradual recovery for the later 100 years in the freshwater perturbation experiment (FW1). The authors find that the initial weakening of AMOC in the FW1 is mainly caused by decreasing of SSS and SPD which leads to a stable vertical stratification and then to the weakening of the NADW formation; however, AMOC recovers though the enhanced freshwater input is continuous and constant during the following 100-year integration by means of a series of feedbacks, which can be summarized as follows: 1) With the reduction in the NADW formation, the vertical density stratification in the mid-to-deep ocean at the mid-low latitudes of the North Atlantic is getting weaker, as a result, the strength of DM, parameterized by stratification-dependent method, increases and then leads to the increased upwelling; 2) the strengthened westerly and weakened easterly over the Southern Ocean and the strengthened easterly over the North Atlantic Ocean contribute together to the recovery of the northward Ekman transport; 3) the increased northward salt transport and the reduced precipitation over the North Atlantic subpolar region cause the recoveries of the SSS and SPD at northern high latitudes, and then the recovery of NADW formation, mainly in the Irminger Sea.