Abstract:This paper assesses the performance of a new climate system model, namely CAS-ESM-C (Chinese Academy of Sciences-Earth System Model-Climate system component), which employs the recently improved version of IAP AGCM, namely IAP AGCM4, as its atmospheric component. This paper first describes the development and framework of the model briefly, and then evaluates the performances of the model in simulating the climate mean states of the atmosphere, land surface, ocean, and sea ice. Some aspects of the seasonal cycle and interannual variability are also analyzed. The results indicate that the CAS-ESM-C succeeds in controlling the long-term climate drift and has acceptable performances in realistically reproducing the climate mean states of the atmosphere, ocean, land surface and sea ice. The CAS-ESM-C also successfully reproduces the seasonal cycle of SST over the tropical Pacific and the seasonal cycle of the sea ice cover in the Arctic. The seasonal migration of monsoon rain band is well reproduced in the model, indicating the acceptable performance of the East Asian monsoon simulation. Except for the slight underestimation of the ENSO period and overestimation of the average amplitude, other characteristics of interannual variability over the tropical Pacific are well reproduced in the CAS-ESM-C. It is particularly important that, benefiting from the realistic simulation of the seasonal cycle of SST over the tropical Pacific, a “phase－locked” phenomenon appears in the simulated ENSO, which is hardly reproduced in other coupled models. The main deficiency of the CAS-ESM-C is the tropic bias, which is common in other coupled models. Some analyses are made to reveal the possible reason behind these simulation biases especially the tropical bias. The results suggest that the biases in the atmosphere which are amplified by the ocean－atmosphere feedback are the key reasons of the tropic bias in the coupled system. According to the analyses of the biases, future improvements of the CAS-ESM-C should focus on the treatment of physical processes of cloud and precipitation in the AGCM. From this point, updating or improving the low-level cloud scheme and the convective parameterization of the atmosphere model may be the first step for the future development of the CAS-ESM-C.

Abstract:Timing and climate change with a 2℃ global warming, with reference to the pre-industrial period, have been paid more attention to worldwide. In particular, many countries, including the member states of the European Union, and international organizations have set the greenhouse-gas emission targets for limiting global warming to 2℃. In this study, two sets of simulations performed with 16 atmospheric－ocean general circulation models, i.e., the model outputs from the 20th Century Climate in Coupled Models (20C3M) and from the Special Report on Emission Scenarios (SRES) emission scenarios B1, A1B, and A2, are used to analyze the timing, atmospheric concentration of greenhouse gases, and climate change over China with a 2℃ global warming. Based on the multi-model ensemble mean, a 2℃ global warming is expected to occur in 2064, 2046, and 2049, with equivalent atmospheric CO2 concentrations of 625 ppm, 645 ppm, and 669 ppm, under the SRES B1, A1B, and A2, respectively. At that time, warming is stronger overall in China. Where the spatial pattern is concerned, warming is stronger towards the north and on the Qinghai-Tibetan Plateau. At the national scale, annual mean surface air temperature is elevated by 2.7－2.9°C, and the winter temperature is elevated by 3.1－3.2℃, which is the strongest among the seasons. Annual mean precipitation is reduced by 0－5% in most parts of South China, but increased by 0－20% in the rest of the country. Annual (seasonal) mean precipitation is increased on average by 3.4%－4.4% (0.5%－6.6%) over China.

Abstract:Based on the dual-Doppler radar data of the mobile weather radar (CINRAD/CCJ) and Changle weather radar (CINRAD/SA), the characteristics of three-dimensional wind structure in the puissant and weakening phases of the strong banded echo of super typhoon Sepat (0709) on 18 August 2007 are examined by means of dual-Doppler radar retrieval in the Earth coordinate. The results reveal that: 1) During the puissant phase, there were strong east by north wind centers (strong wind core ) along the right or the back of the right side of strong echoes in their course, even weak wind center existed in the front of the strongest echo. The structure maintained from the lower layer to the mid-layer, thus the strong echoes corresponded to the horizontal convergence and positive vorticity, resulting in significantly upward movement which helped the development and maintenance of convection. And the clouds migrated quickly. Relative to the moving clouds, the anterior and posterior air in the mid－lower layer flowed to the strong echoes, and then converged and ascended with maximum vertical velocity between strong echo center and the wind core on its northern side. In the meanwhile, divergence appeared in the higher layer over the strong echo area, and the air mainly flowed out backward. 2) During the weakening period, the weak wind center was located at the strong echo center or its north side with negative vorticity. The clouds moved slowly and the easterlies went through the clouds. The convergence over the echo area weakened and the obvious updraft appeared in the mid-layer strong echo near the typhoon center. Thus the banded echo has been difficult to maintain. 3) Strong wind core could transport the momentum and water vapor of weakening spiral cloud bands on the posterior side to the developing area of the banded echo; therefore the strong wind core structure was likely to be the main cause for the rapid development of banded echo.

Abstract:The features of diurnal variation of sea surface temperature (SST) are simulated in the LASG/IAP Climate System Ocean Model (LICOM) by resolving the diurnal variation of solar radiation and the influence of weak mixing is investigated. Forced by the ideal diurnal variation of solar radiation, some features of diurnal variation of SST can be captured in LICOM with 10-m vertical resolution in the upper layers. The diurnal variation of solar radiation also results in diurnal variation of currents in the upper layers. The horizontal distribution of diurnal amplitude of simulated SST is consistent with that of observed SST and modulated by the diurnal amplitude of solar radiation. In the eastern Pacific cold tongue, the diurnal amplitude of simulated SST is about 0.3℃－0.4℃ and 0.1℃－0.2℃, smaller than that of observations. The peak of daily SST variation appears during 1500－1600 LST and lags 2－3 hours than the peak time of daily solar radiation, which is close to observation. After reducing mixing, the diurnal amplitude of SST variation increases in the equatorial eastern Pacific cold tongue, closer to observation. This indicates that the features of diurnal variation in the equatorial eastern Pacific cold tongue are mainly controlled by the diurnal variation of solar radiation and vertical mixing. After reducing mixing, under the modulation of solar radiation, the mean states including the mixed layer depth, temperatures and currents are obviously changed. In the northern part of the eastern Pacific cold tongue, the reduced mixing causes heat accumulation in the upper layers and then leads to an increase in SST about 0.3℃. While in the southern part, the enhanced meridional advection leads to a decrease in SST about 0.2℃.

Abstract:This study investigated the snow albedo and hydrological effects on the precipitation in South China in 2010, based on the singular value decomposition (SVD) analysis of the Eurasian snow depth dataset and the 160-station rainfall dataset in China. Three ensemble simulations were conducted by using community atmosphere model 3.1 (CAM3.1). The first ensemble simulation includes both the snow albedo and the snow hydrological effect. The second ensemble simulation only considers the snow albedo effect but ignores the hydrological effect. The third ensemble simulation only considers the snow hydrological effect but ignores the albedo effect. The results indicate that both these effects could have impacts on the South China rainfall. However, there are great differences in amplitudes and ranges of abnormities induced by those three simulations. The magnitude of abnormities caused by the snow hydrological effect is larger than that by the albedo effect. When those two effects work together, the result agrees well with observations and the magnitude is the largest. However, the magnitude is not equal to the sum of that caused by the snow albedo and hydrological effects respectively.

Abstract:Using the NOAA monthly reanalysis geopotential height on 500 hPa, the sea surface temperature (SST)， the Pacific decadal oscillation (PDO) index and the triple mode of North Atlantic SST anomaly (ATM）index from 1950 to 2008, the temporal and spatial evolution of teleconnection in the troposphere during the Northern Hemisphere winter at the end of the 1970s is studied with correlation analysis method, Mann-Kendall analysis method, and coherence wavelet analysis method. The results indicate that the spatial distributions and temporal evolutions of Pacific－North American (PNA), Eurasia－Pacific (EUP), West Pacific (WP), and West Atlantic (WA) teleconnection patterns change significantly at the end of the 1970s. The spatial distribution changes are found in the intensity and range of abnormal centers of these teleconnection patterns. On temporal evolution, the upward tendency of PNA and WP indexes and the downward tendency of EUP and WA indexes are also found. After the end of the 1970s, the key SST areas impacting on the 5 teleconnection patterns during the Northern Hemisphere winter have changed, in which those areas impacting on the EUP, and those areas in the Pacific impacting on WA have significantly decreased; those areas in the middle and southern Atlantic impacting on EA have obviously changed. On the time scale above 16 years, the variation of PDO is the main reason which leads to the variation of PNA, EUP, WP, and WA teleconnection patterns; the variation of ATM is the main reason which leads to the variation of WA and EA teleconnection patterns during the Northern Hemisphere winter.

Abstract:Using the National Snow and Ice Data Center (NSIDC) snow cover and snow water equivalent data, monthly four-layer soil temperature from ERA40 reanalysis dataset, the mechanisms of the linkage between the Winter Northern Eurasian Fresh Snow Extent (winter TFSE) and subsequent summer climate anomalies in China are investigated. Correlation analysis shows that, the consistent seasonal evolvements of land anomalies act as a bridge for the lag correlation. A large winter TFSE is followed by a slower northeastward retreating progress of snow and frozen soil over northern Eurasia. Consequently, in summer, snow and frozen soil over East Asian middle and high latitudes melt more intensively, the soil temperature correspondingly gets lower than normal, which influences local temperature by cooling and by influencing local meridional wind, and through enhancing East Asian jet, is related to the subtropical high in the western Pacific, even climate anomalies in China, and vice versa. As well as local variations of land factors over East Asian middle and high latitudes are concerned, the signal of winter TFSE is significantly strengthened during snow-and-frozen-soil melting process.

Abstract:A severe occlusion type of squall line is investigated by using observational data analysis and WRF model simulation, which occurred in Henan Province in China on June 3, 2009 and produced surface damaging high wind. The observational data analysis result shows that: The right branch of this system has some similar features to typical squall lines. Some features such as thunderstorm high, boundary outflow, cold pool, and wake depression are presented in the mature stage. By contrast, for the right branch there are not significant features such as stratiform cloud, thunderstorm high, and cold pool in the left branch of the squall line. The surface wind within the region of left branch is weaker than that in the right branch. High wind was mainly caused by the right branch of the system. High-resolution model outputs reveal that the formation of occlusion morphology of this system is caused by cold wind shear and warm wind shear near the ground. These shears lead to the existence of right branch with stratiform cloud and left branch without stratiform cloud, respectively. Furthermore, the formation mechanism of the surface high wind and the organization mode of the right branch of this system in the mature stage have been revealed: The downward motion of the convective region in the right branch is intense in developing and mature stage. Rear inflow in the middle troposphere may intensify such downward motion. Rear inflow is an important factor for surface high wind. There are three major air flows within the cross section of the right branch, including front-to-rear inflow in front of the squall line, rear-to-front inflow in the middle layer, and inflow in the lower layer which changes to outflow in the high layer.

Abstract:In the framework of impacts of sea-land-atmosphere factors on the following summer rainfall in the middle and lower reaches of the Yangtze River (MLYR), the authors reconstructed 40 factors on the basis of sea surface temperature (SST), snow depth in the Tibetan Plateau, and NCEP/NCAR reanalysis datasets, and evaluated their possible impacts on the summer rainfall in the MLYR. Firstly, the authors discussed the stability of relationships between these factors and the following summer rainfall in the MLYR before and after 1979. Secondly, the authors selected several important factors by correlation analysis, and discussed the relationship between these factors and East Asian summer monsoon (EASM). In addition, the potential forecast capability of these factors for the summer rainfall in the MLYR was discussed by historical hindcasts. The results suggest that the important winter predictors for the summer rainfall in the MLYR mainly come from the external forcing, such as the SST in Niño3 and Kuroshio Current regions, and the snow depth in Tibetan Plateau. In contrast, the potential spring predictors are almost the general circulation indices, such as Arctic Oscillation (AO), polar vortex, and the western North Pacific subtropical high (WNPSH). Running correlations between these potential predictors and the summer rainfall in the MLYR vary on interdecadal time scale, however, the SST in Kuroshio Current region, the snow depth in the Tibetan Plateau, and the surface air temperature near Lake Baikal show the significant antecedents for summer rainfall in the MLYR by affecting the EASM circulations, and jointly suggest the better forecast capability for the rainfall anomaly in the MLYR after 1979.

Abstract:The control experiment and two assimilation experiments of WRF regional mesoscale model prediction are verified for typhoon Haitang (0505). Based on the WRF-3DVAR system, the FY-2C infrared and water vapor cloud motion winds are assimilated into WRF model. The assimilation data of one assimilation test are the cloud motion winds before quality control and those of the other are cloud motion winds after quality control. The effect of the cloud motion winds on the prediction of rain and wind field is analyzed through comparing the three tests. Results show that the assimilation of the quality-controled cloud motion winds into the initial fields has positive impact on prediction of the area and intensity of rain, and also indicate that different grades of Ts score are improved by far than the other tests. The assimilation makes improvement in the forecast of wind. The same approach as that for Haitang is taken to simulate other two typhoon cases and the Ts scores of the forecast rainfall are given. And the results for the two typhoon cases show similar conclusions. Therefore, assimilating cloud motion winds with reasonable control and selection can well improve the descriptive possibility of wind field in WRF model, supply the mesoscale information in the initialization field to improve the simulation result of precipitation and wind field and have positive impact on the forecasting ability of WRF model finally.

Abstract:A daily gridded precipitation dataset (APHRO) for Asia was created by the Asian Precipitation-Highly Resolved Observational Data Integration Towards the Evaluation of Water Resources (APHRODITE) project in Japan. The resolution of this dataset is 0.25°×0.25°, and the length is from 1951 to 2007. This study aims to assess the quality of the APHRO rainfall in contiguous China from the perspective of climatological mean, rainfall classes, and long-term trend. Daily rainfall records observed by 559 rain gauges are used for the comparison. The results are as follows: (1) For the mean states, the APHRO shows similar distribution of precipitation amount to station data, and can accurately characterize the seasonal migration of rain-belt. However, compared with station data, this dataset underestimates precipitation intensity, but overestimates precipitation frequency.(2) For the distribution of different classes of precipitation, annual mean precipitation amount for heavy rainfall derived from the APHRO data is lower, while the amounts for light and moderate rainfall are higher than the station data.(3) The trends of precipitation amount in China during 1956－2005 derived from two dataset are identical, the trends of precipitation frequency are also highly consistent, and both show an “increase in the west but decrease in the east” pattern. A large difference between the two datasets is found in the spatial pattern of precipitation intensity trends. The precipitation intensity derived from station data shows an increasing trend throughout the Chinese mainland in the past 50 years; the APHRO data exhibit a similar trend in the southeastern coastal region and northwestern China, but a different pattern in northern China, northeastern China, and Jianghuai region. In addition, analysis on the seasonality of interdecadal variability indicates that the characteristics of “southern China food and northern China drought” and “Jiangnan late spring drought” derived from the APHRO data are different from those derived from station data. The APHRO data tend to underestimate the trends.

Abstract:Using a nonlinear quasi-geostrophic barotropic model, the paper investigates numerically how the spatial pattern of synoptic scale waves and the westerly wind strength affect the phase transition of the North Atlantic Oscillation (NAO), and the role played by the large-scale topography. It is found that the weak basic westerly wind is favorable for the westward shift of the downstream large-scale system, favoring the transition of the NAO from positive (negative) phase into negative (positive) phase. It is noted that when the synoptic scale waves are stronger and concentrated in a wider region, the NAO event can transform more easily from the negative phase into positive phase; when the synoptic scale waves are stronger and shift northward, the NAO can transform more easily from the positive into negative phase. In the presence of a large scale topography, the number of the cases in which the NAO transforms from positive (negative) phase to negative (positive) phase is reduced.

Abstract:A time-scale decomposition (TSD) approach was introduced to statistically downscale the predictand which contains distinct variablity linked with distinct large-scale predictors. It decomposed both the predictand and the predictors into distinct components through filtering and calibrated distinct predictive equations, respectively. Due to the interannual and inter-decadal variability in July-August North China rainfall, it was used as a case to be downscaled by TSD approach. Sea level pressure, 500-hPa geopotential height, 850-hPa meridional wind, and sea surface temperature were considered as predictor parameters; several well-known large-scale climate indices were also taken as potential predictors. An approach of cross-validation-based stepwise regression was used to formulate the regression equations. The downscaling model for the interannual rainfall variability was linked to the sea surface temperature over the mid-eastern tropical Pacific in June and the 850-hPa meridional wind over East China in July－August, while that for the inter-decadal rainfall variability was related to the sea level pressure over the southwestern Indian Ocean in June under the effect of sea surface temperature over the Indian Ocean－Pacific warm pool. The downscaled interannual and inter－decadal rainfall components were added together to obtain the downscaled total rainfall. The results in the independent validation period (1991－2008) showed that the TSD approach performed well to downscale July－August North China rainfall with the correlation coefficient of 0.82 and relative root-mean-square error of 14.8%. With the hindcasted predictors by general circulation models (GCMs), the downscaling model was used to hindcast July－August North China rainfall over 1991－2001. Compared to GCM-hindcasted rainfall, the downscaling model showed better performance, which improved the original bias in terms of insufficient interannual variation in GCM hindcast. The correlation coefficient between the observed and downscaled rainfall reached 0.45, much higher than 0.12 in GCM hindcast.

Abstract:Based on the monthly mean rainfall data at 160 China stations, the National Centers for Environmental Prediction /National Centers for Atmospheric Research (NCEP/NCAR) reanalysis dataset, and HadISST1 dataset for 1951－2009, the temporal evolution and spatial distribution of precipitation over astern China (15°N－40°N, east of 100°E) during the Meiyu season (June and July) are investigated with the empirical orthogonal function (EOF). It shows that there are two dominant modes, the first EOF mode depicting a precipitation oscillation between the Yangtze River valley and South China, and the second EOF mode describing a consistent precipitation anomaly over the south of the Yangtze River. In this study two time scales are separated as: interannual component whose period is shorter than eight years and interdecadal component whose period is longer than eight years. After analyzing the precipitation at these two time scales during Meiyu season, it is found that for both of the annual scale and the decadal scale, the first two leading modes are a precipitation oscillation between the Yangtze River valley and South China, and a consistent precipitation anomaly over the south of the Yangtze River. The result of the comparison between these two time scales shows that the pattern for the precipitation oscillation between the Yangtze River valley and South China has clear interannual and interdecadal variations, and the pattern for the consistent precipitation anomaly over the south of the Yangtze River is mainly characterized by interdecadal variation. The atmospheric circulation and the sea surface temperature (SST) patterns associated with these two modes are investigated through the regression/correlation method. It shows that although the leading modes of interannual component are similar to those at the decadal scale, the associated circulation and SST anomalies are quite different. The atmospheric circulation anomalies associated with the pattern which shows the precipitation oscillation between the Yangtze River valley and South China for the interannual component include the double blockings at mid－high latitudes and the meridional wave train over Eurasia, and the East Asian subtropical jet shows significant positive anomaly. The atmospheric circulation anomalies associated with this pattern of the interdecadal component include the consistent positive geopotential height anomaly over Eurasia, and the subtropical high over East Asia strengthens and shifts northward. The SST anomalies associated with this pattern of the interannual component is mainly represented by the positive SST anomaly over the ocean around Japan and the Indian Ocean during the preceding winter and spring, and over the South China Sea during the simultaneous Meiyu period. The SST anomalies associated with this pattern of the interdecadal component is mainly represented by the negative horseshoe SST anomaly over the North Pacific during the simultaneous Meiyu period. The atmospheric circulation anomalies associated with the pattern which shows the consistent precipitation anomaly over the south of the Yangtze River for the interannual component are characterized by the Okhotsk blocking high and the meridional wave train in East Asia. And the atmospheric circulation anomalies associated with this pattern of the interdecadal component include the positive geopotential height anomaly over Eurasia and the East Asian subtropical jet weakens and shifts southward. The SST anomalies associated with this pattern of the interannual component mainly are represented by the negative SST anomaly over the North Pacific and the positive SST anomaly over the east coast of Australia during the preceding winter and spring, and the positive SST anomaly over the South China Sea during the simultaneous Meiyu season. The positive SST anomalies over the coast seas of East Asia during the preceding winter and spring, and those over the South China Sea and the tropical western Pacific during the simultaneous Meiyu period are highly correlated with this pattern of the interdecadal component.

Abstract:Thunderstorm is defined as the severe convective weather with thunder and lightning. On the one hand, it brings much useful precipitation in spring and summer in many regions, especially in semi-arid areas. On the other hand, however, its accompanying hails, heavy rain, tornados, and strong wind bursts cause much loss in life and property over the world wide. So, thunderstorm is one of severe weather phenomena to be continuously investigated in meteorological and atmospheric sciences. The observational data of thunderstorm are always required in studying storm initiation and development mechanism, developing mesoscale models, and predicting and warning thunderstorm hazards. This paper aims to review the advance in thunderstorm observation investigations with focus on the description of some field experiments over the past 6 decades.

Abstract:Based on the monthly mean NCEP/NCAR reanalysis dataset and the surface air temperature and precipitation data from 160 China stations, the interannual variations of winter Eurasian teleconnection pattern (EU) and its possible influence on the climate in China are investigated. Wavelet analysis reveals that the significant periods of Eurasian teleconnection pattern index (EU index) are 2－4 years. The result suggests that the interannual variation of the EU is dominant, whereas the interdecadal component is weak. In a winter with positive EU phase, the East Asian westerly jet stream at 200 hPa tends to be enhanced and the East Asian trough at 500 hPa becomes stronger. In the meantime, there are the surface northerly anomalies in East Asia which lead to a cooling condition over there. The opposite situations tend to occur in a negative EU winter. Hence, during the boreal winter the cooling and less precipitation are likely to occur in most of eastern China associated with a positive phase of EU.