Abstract:Based on the Tropical Rainfall Measuring Mission (TRMM) data in the latest twelve years, the characteristics of spatial and temporal distributions of convective rainfall and stratiform rainfall in the Asian monsoon region are analyzed. The results show that: on the multi-year mean timescale, the spatial distributions of convective rainfall and stratiform rainfall in the Asian monsoon region show the characteristics of meridional variation, such as, the stratiform rainfall dominating over the subtropical monsoon regions to the north of 25°N, whose percentiles are above 50% and increase with latitudes, however, the convective rainfall dominating over the regions to the south of 25°N, whose percentiles are about 55% and show homogeneous spatial distribution characterstics. The spatial distributions of convective and stratiform rainfall pixels percentiles show that the stratiform cloud is the main rain cloud all over the Asian monsoon region (the area mean of stratiform rainfall pixels percentiles is above 60%). On the seasonal timescale, the convective rainfall and stratiform rainfall show distinguished seasonal variation (their standard deviations are above 10%) over the northern part of the East Asian monsoon region (to the north of 25°N) which is called the subtropical monsoon region. The stratiform rainfall dominates over this region, whose percentiles are about 85% in winter, but the percentiles of the stratiform (convective) rainfall decrease (increase) gradually and reach the minimum 55% (maximum 45%) with the coming of summer and the northward shift of monsoon rainband. With the advance of time, the percentiles of the stratiform (convective) rainfall increase (decrease) gradually to the winter status. However, the convective rainfall and stratiform rainfall show little seasonal variation (their standard deviations are below 3%) over the South Asian monsoon region and the southern part of the East Asian monsoon region (to the south of 25°N) which is called the tropical monsoon region. The convective rainfall dominates over this region, whose percentiles are from 50% to 65% all the year round. The analyses of dynamic factors and thermodynamic factors show that the spatial and temporal distributions of the convective rainfall and the stratiform rainfall in the Asian monsoon region are controlled by the vertical wind shear dynamic factor of the monsoon circulation. However, the thermodynamic factors, such as temperature, moisture, equivalent potential temperature, and convective available potential energy, influence the intensity, extension, and duration of the convective rainfall and the stratiform rainfall. These dynamic factors and thermodynamic factors seem to have their critical values.

Abstract:To understand the mechanisms responsible for the severe drought in Yunnan Province during the boreal winter (DJF) of 2009/2010, by using NCEP/NCAR reanalysis data and winter precipitation index in Yunnan Province which is calculated from station precipitation data in Yunnan Province, this study investigates the relationship between the North Atlantic Oscillation (NAO) and the precipitation in Yunnan Province during boreal winters. In the 49 winters of 1961/1961－2009/2010 period, the correlation coefficient between the time series of winter mean precipitation indices in Yunnan Province and the NAO indices is 0.373 (exceeds the 95% confidence level), which indicates that the winter precipitation in Yunnan Province is linked to the variability of the NAO. Regression results between the winter precipitation indices in Yunnan Province and the northern hemispheric anomalous geopotential height at 300 hPa show that the circulation systems associated with the winter precipitation in Yunnan Province have a south and a north major components: the southern branch trough and the ridge of Lake Baikal. And the results show that the NAO can impact these two components through the quasi-stationary waves propagating along the Asia－Africa subtropical jet and the wave reflections, respectively. Therefore, the variations of the NAO and the winter precipitation in Yunnan Province are linked. The results also show that the linkage between the NAO and the winter precipitation in Yunnan Province is modulated by ENSO phenomenon. The relationship between the NAO and the winter precipitation in Yunnan Province is much close in the warm ENSO winters, while, they are barely linked in the cold ENSO winters.

Abstract:Signals of global tropospheric and stratospheric temperature anomalies associated with ENSO events in boreal winter during the period of 1980－1999 are investigated by using the Microwave Sounding Unit (MSU) data. Results derived from five reanalysis datasets (ERA40, JRA25, NCEP1, NCEP2, and MERRA) are compared with the MSU data. The symmetric and asymmetric components of temperature anomalies associated with warm and cold episodes of ENSO are revealed. The MSU data are taken as a benchmark to evaluate the veracity of ENSO signals described by different reanalysis datasets. The results show that, in the middle troposphere, the symmetrical components of temperature anomalies associated with ENSO feature a dumbbell-shaped pattern in the eastern Pacific, while the wave structures are evident, stretching from the Pacific to North America, with 3℃ maximum located in the eastern Pacific and northern North America; on the contrary, negative values turn up in the North Pacific, South Asia, and the South Pacific, with the minimum of -3.2℃ centered in southwestern China. The asymmetrical components are only evident in most of the high latitudes of the Northern Hemisphere. In the lower stratosphere, the symmetrical components of ENSO-related temperature anomalies are negative in the eastern Pacific, Greenland, and the North Atlantic, with the minimum -7℃ located in the North Atlantic. The positive values occupies the region north of 30°N during 70°W－70°E, with the maximum 6℃ centered in northeastern Russia. The asymmetrical components are obvious in the high latitudes of the Northern Hemisphere, and the maximum is 5.7℃. The reverse signals between the middle troposphere and the lower stratosphere are found in the equatorial eastern Pacific, the South Pacific, the North Pacific, southern China, Greenland, and some areas of the North Atlantic, while northern North America sees the same signals. Further studies indicate that temperatures averaged over 850－300hPa and 100－30hPa can well represent the ENSO-related signals in the middle troposphere and the lower stratosphere respectively.

Abstract:Performance of a regional climate model CREM, which was developed at the State Key Laboratory of Numerical Modeling for Atmospherics Sciences and Geophysical Fluid Dynamics/ Institute of Atmospheric Physics (LASG/IAP), in simulating the intraseasonal oscillation (ISO) of summer rainfall over eastern China is assessed by comparing model results against the observations and reanalysis data sets. The model reasonably reproduces the observed intensity, period, and propagation features of summer rainfall climatological ISO (hereafter CISO) in eastern China, but the location of the maximum rainfall center associated with CISO shifts northwestward. The observed high-frequency and local oscillation characteristics of rainfall anomalies associated with the transient ISO (hereafter TISO) are also well simulated, although the amplitude of TISO rainfall center is stronger than the observation, and the location of maximum rainfall center shifts northward relative to the observations. The simulated TISO period along the Yangtze River valley is shorter than the observations, and the associated rainfall centers also shift eastward. The deficiencies in ISO simulation are closely related to the model's biases in both summer mean state and vertical structures of convergence and specific humidity fields associated with ISO, suggesting that the improvements in summer mean state and convection process are crucial for a realistic simulation of ISO. The rainfall data derived from the reanalysis fail in reproducing both CISO and TISO, however, added values are evident in the rainfall downscaled by the CREM model.

Abstract:WRF (Advanced Weather Research and Forecasting modeling system) was used to simulate the local heavy rainstorm process caused by super typhoon Wipha over the eastern coast of Zhejiang Province from 17 to 19 September 2007. The analysis of the simulation result combined with Doppler radar data, TBB (Temperature of Black Body) data, and NCEP (National Centers for Environmental Prediction) reanalysis data shows that the rainstorm was due to the intrusion of north cold－dry current from the upper-level trough into the typhoon circulation. The warm－moist peripheral southeastern jet from typhoon Wipha brought rich moisture and heat, and the north cold－dry current provided kinetic condition and instability energy for precipitation. The distribution of rainfall and circulation of typhoon Wipha showed clear asymmetric structure during the landing period. The rainfall and convection activity mainly occurred in the west quadrant of typhoon. The east part of typhoon circulation was warm and moist while the west part was cold and dry. The energy front contributed by the convergence of cold and warm air in the coast area of Zhejiang Province was the most active convective region, also the main precipitation zone. There were several small scale convective cloud cells lining from north to south along the energy front in the low troposphere, together forming a mesoscale spiral rainband. As the typhoon moved northwestward, the north cold－dry current further intruded into the warm－moist circulations. There were new-born convective cells at the down edge of the rainband. The strong vertical shear of wind along the energy front offered advantageous environment to the development and maintenance of the convective cells. The cold－dry current invaded into low level and imbedded into the bottom of the warm－moist circulations along the energy front, which, together with the beneficial topography, lifted the warm－moist air up forcefully and triggered convection weather.

Abstract:The vertical transport of water vapor by a severe convective process occurring in Nagqu area over the Tibetan Plateau has been investigated using the Weather Research Forecast (WRF) model with different cloud microphysical schemes. The simulated characteristics of the storm reveal good agreement with observations, such as the onset and location of convection and precipitation. The results show that when the convective cloud arises, the upward flux of water vapor over the convective region increases at first then declines with altitude, and is not sensitive to cloud microphysical schemes. Similar trend is also found for the total water vapor integrated over a period of 24 hours. A further analysis shows that this trend is correlated to the vertical updraft in the cloud, that is, both the short-time humidifying effect in the upper troposphere and duration are sensitive to cloud microphysical parameterization schemes. The largest difference of the maximum upper tropospheric water vapor mixing ratio can reach 20.3% among different schemes, leading to humidifying lasting from 1.5 to 7 hours. Taking a 24-hour average can reduce the sensitivity of upper troposphere humidity, but the maximum still reaches 14.3%. The results of this study indicate that when the WRF model is used for studies of the effects of deep convection on the upper tropospheric water vapor, the uncertainty induced by using different microphysical schemes cannot be neglected within 24-hour time scale.

Abstract:In reviewing both the observed and predicted summer rainfall in China in 2010, the authors have proposed a judgment that the lack of external forcing factors such as pre-winter sea temperature and snow when using the objective and quantitative prediction method of summer precipitation based on changeable configuration of optimal multi-factors may lead to forecast failure in some regions. Through the diagnostic analysis of possible climate causes of abnormal summer precipitation combined with the results of hindcast, the authors have verified this judgment. In addition, abnormality occurs in varying degrees within each member of Asian summer monsoon system in the context of pre-winter sea temperature and snow anomalies, and the western Pacific subtropical high is the most significant, which mainly induces the abnormal summer precipitation. At last, the authors put forth the possible ways to improve this prediction method.

Abstract:Using NCEP2 reanalysis data from 1979 to 2004, the daily 500-hPa geopotential height in the Northern Hemisphere (20°N－90°N) has been expanded into double Fourier series and the signals in the frequency domain have been analyzed for different spatial scales. For the daily series in the entire year, when both the zonal wave number k and the meridional wave number l equal zero, the signals series are characterized by a significant low-frequency variation (10－30 days). When the meridional wave number l equals zero, the signals in the frequency domains vary with the zonal wave number k significantly. The coefficient series consist of a low-frequency variation at larger spatial scales and a high-frequency variation (4－8 days) at smaller spatial scales. When the zonal wave number k equals zero, the signals in the frequency domains vary little with the meridional wave number l. The coefficient series are characterized by a low-frequency variation. When both k and l are not equal to zero, the spatial scales of low-frequency variations mainly concentrate on l=1, corresponding to the dipole structure of low-frequency flows in the mid-high latitudes. The results for daily series during the winter or summer half year are similar to those during the entire year. The diagnosis results can be explained, to a certain extent, by the dispersion relation for the Rossby wave.

Abstract:The role of the western Pacific and the Indian Ocean in the South China Sea (SCS) summer monsoon onset is investigated by analyzing various atmospheric and oceanic datasets. The Results show that the impact factor of the SCS summer monsoon onset exhibits interdecadal variation. From 1951 to 1970, the impact factor exists in the Indian Ocean, from 1970 to 1998 it occurs in the western Pacific Ocean. The interdecadal variation of impact factor results from the significant abrupt change of the Arctic Oscillation (AO) and the intensity of the Western Pacific Subtropical High (WPSH). For the period of 1951 through 1970, an anomalous westerly wind over the northern Indian Ocean induced by a negative AO index and the weak WPSH that are helpful to the SCS summer monsoon onset make the zonal wind over the northern Indian Ocean the main factor controlling the SCS summer monsoon onset. Meanwhile, during the period, the southern Indian Ocean Subtropical Dipole (IOSD) is closely related to the SCS summer onset. The positive IOSD (with positive SST anomaly in the southwestern Indian Ocean, a negative SST anomaly in the other regions) results in anomalous westerly wind over the northern Indian Ocean and early onset of the SCS summer monsoon, and vice versa. For the period of 1971 to 1998, an anomalous easterly wind induced by a positive AO index and stronger WPSH offer an unhelpful condition for the SCS summer monsoon onset when the heat content in the western Pacific warm pool become the main factor controlling the SCS summer monsoon onset. The positive (negative) anomalous heat content in the western Pacific warm pool leads to earlier (later) onset of the SCS summer monsoon.

Abstract:Based on the observed precipitation data from 160 China stations, the NCEP/NCAR reanalysis dataset and the sea surface temperature (SST) dataset from the Met Office Hadley Centre, this study investigates two major modes of wintertime precipitation over China through empirical orthogonal function (EOF) method and associated atmospheric circulation and SST through linear regression method. It reveals that the first two EOF modes account for 49.6% and 17.3% of the total variance, respectively, and can be well separated from the remaining modes. EOF1 reflects the strength of wintertime precipitation over southeastern China. The associated principal component (PC1) has significant interannual variability with 2－4 year period. Its interdecadal variations suggest that the wintertime precipitation over southeastern China was below (above) normal before (after) the mid-1980s, and has a slight decreasing trend in recent years. EOF2 delineates an out-of-phase relationship between South China and the middle and lower reaches of the Yangtze River as well as the northern part of Xinjiang. The associated time series (PC2) also has a clear 2－4 year period and an interdecadal component, which experiences a clear trend towards its negative polarity from 1980 to 2005. Further analysis suggests that EOF1 is closely related to the El Niño-Southern Oscillation (ENSO) and the East Asian winter monsoon (EAWM). When warm (cold) ENSO is in its mature phase, the EAWM is weak (strong), the anomalous water vapor transport from the Bay of Bengal and the South China Sea converges (diverges) over southeastern China and favors more (less) precipitation in this region. The SST anomalies over the tropical eastern Pacific have significant relationship with the variation of EOF1, and may act as a potential predictor for this mode. The variation of EOF2 is closely associated with a barotropic wave train across the Eurasian continent originating from the North Atlantic, which can induce anomalous northerlies (southerlies) along the coasts of southeastern China. It then causes the divergence (convergence) of water vapor flux in the lower reaches of the Yangtze River and opposite condition in South China, and therefore favors a positive (negative) phase of EOF2. The SST anomalies around the Norwegian Sea over the North Atlantic have the closest relationship with the variations of EOF2, and may act as a potential predictor with a leading time of 7 months.

Abstract:Using the CloudSat/CALIPSO data products during the period of September 2006－August 2009, seasonal variations of cloud properties （including occurrence frequency, vertical location, physical thickness, distance between cloud layers, and vertical distribution of radar reflectivity) are contrasted among four regions: East Asian Monsoon Region（EAMR), Indian Monsoon Region（IMR), the Western North Pacific Monsoon Region（WNPMR), and the Tibetan Plateau Region（TPR). The geographical distribution of low-level clouds over the Asian monsoon regions and its correlation with atmospheric stability of the lower troposphere（LTS) are also analyzed. The major findings are as follows:（1) During the period, the total cloud amounts are 69% （EAMR), 72%（IMR), 83%（WNPMR), and 69%（TPR), respectively, to which single-layer clouds contribute 56%（IMR and WNPMR) to 77%（TPR). The multi-layer clouds are mostly double-layered or triple-layered（fractions≥95%). In the IMR, the total cloud amount in summer （greater than 90%) is significantly larger than in winter（about 50%); the total cloud amounts are larger during spring and summer（>90%) than autumn and winter（about 50%) in the EAMR and TPR; the seasonal variation is negligible over the WNPMR.(2) Clouds located above 10 km containing small ice crystals prevail in the tropical monsoon regions （IMR and WNPMR) all the year round. Marine boundary layer clouds are common during each season in the WNPMR, in contrast to the IMR where low-level clouds occur mainly during summer. The EAMR clouds are located mostly below 10 km from autumn to spring. Although more clouds are present in the upper troposphere during summer than other seasons over the EAMR, their occurrences and vertical locations are lower than those in the IMR and WNPMR（cloud amount of 60%－70% from 12 to 16 km heights). The TPR clouds are mostly located in the height range of 4－11 km, with cloud particles at the same height covering a wide range of size.(3) The geographical distributions of low-level clouds in the Asian monsoon region are similar between spring and autumn, and significantly distinct between summer and winter. Low-level clouds are the most abundant during winter（45%－70%), located mainly in the western North Pacific, southern Chinese mainland and the oceans to its east, and the regions around Japan. The low-level cloud amount is correlated with LTS quite well except for winter.(4) The cloud layers are geometrically thin in the four regions, with 30%－36% being thinner than 1 km. Moreover, the vertical distance between two consecutive layers in multilayered clouds with values less than 1 km accounts for about 10%. Both suggest a need to improve the vertical resolution of current general circulation models.

Abstract:To examine the sensitivity of cumulus parameterization scheme in climate simulating over the Tibetan Plateau, three cumulus parameterization schemes in RegCM3 (The ICTP Regional Climate Model, V3.0), Anthes-Kuo (AK), Grell-Fritsch&Chappell (GFC), and MIT-Emanuel (MIT) are compared and analyzed in temperature and precipitation simulation over the Tibetan Plateau. The results show that AK, GFC and MIT schemes well reproduce the distribution of temperature and precipitation over the Tibetan Plateau, and the spatial distribution and annual change of the simulated temperature and precipitation with AK scheme are more closed to observations than other schemes. The simulated temperature shows that there is a systematic cold bias over the Tibetan Plateau about the three schemes. There is a cold bias about 4℃ over Tibet and a warm bias about 2℃ over Tarim and Junggar Basin, the cold bias over the south is large than the north over Tibet, and the variability of temperature is slightly smaller than the observed values. Precipitation simulation results indicate that the simulated precipitation is 0－6 mm/d more than observed one, the simulated one is high in the north and low in the south, the variability of precipitation is also slightly smaller than the observed values. In addition, RegCM3 distinctly improved the MICRO3.2 simulation results, the distribution details of temperature and precipitation are better than the MICRO3.2 simulations for complex terrains, such as Tianshan, Tarim Basin, Junggar Basin, Kunlun Mountain, Gangdise Mountain, and southeastern Tibetan Plateau, which implies that the RegCM3 performance is successful in downscaling.

Abstract:A new nudging assimilation adjustment technique is proposed, which may be used to assimilate the lightning observations of TRMM satellite into the initial fields of the mesoscale numerical model. The technique combines the physical initialization technology with the nudging technology. According to the observed lightning intensity and spatial location, the assimilation technique adjusts the content and spatial distribution of water vapor and five cloud hydrometeors in the initial fields of the numerical model, and thereby improves the initial condition of model and increases the accuracy of 0－6-hour short-term rainfall forecast of the mesoscale numerical model. The nudging assimilation adjustment technique is further used to assimilate the lightning observations of TRMM satellite into the initial conditions of three short-term precipitation events simulated by the ARPS model. The results show that, the assimilation of observational lightning data may increase water vapor and cloud hydrometeors in the initial condition and reasonably rearrange the spatial distribution of water substances. The short-term forecast of precipitation and cloud hydrometeors by the mesoscale numerical model is effectively enhanced.

Abstract:The HALOE (Halogen Occultation Experiment) dataset is used to study the relation of stratospheric aerosols with trace gases (O3, H2O, HCl, NOx, CH4, and HF) and temperature in a period of relative volcanic quiescence. The lag-correlation analysis shows trace gases and temperature are significantly related to aerosol surface area density. Characteristics of the relevance vary with different components. Temperature is negatively correlated with aerosol for 70－20 hPa over low-and mid-latitudes. Standardized multiple linear regression demonstrates that greenhouse gases are the primary direct contributor for temperature change, and the direct contribution of aerosols is much less. The impact of aerosols on the stratosphere is simulated with the two-dimensional model SOCRATES (Simulation of Chemistry, Radiation, and Transport of Environmentally important Species). The response of trace gases and temperature coincides with the analysis of the HALOE. The rangeability of temperature is much less than that of aerosols. The result suggests that the stratospheric heterogeneous processes on aerosol surfaces are important for the relation between aerosols and trace gases in the aerosol-enriched layers. The radiation effect of aerosols is cooling in the period of relative volcanic quiescence, which is different from the warming effect after volcanic eruptions. The phenomenon depends on the aerosol contents. The indirect radiation effect via heterogeneous processes is stronger than the direct effect of aerosols themselves, but the total radiation effect makes little impact on temperature.

Abstract:Initial errors and model errors are the limiting factors to the improvement of the numerical weather prediction（NWP). Traditional NWP and variational data assimilation（VDA) always ignore the influence of model errors, but it is important to investigate the dynamics of model errors for deep research. Based on the nonlinear dynamic model, the model error equation and the mean quadratic error expression for short time which are independent of the particular model are obtained on the assumption that there exist model parameters error and physical processes lacking error in the prediction model, and the non-Markovian character of the probability density of the model error is proved with Liouville equation. Taking the Hopf bifurcating system and Lorenz low-order atmospheric systems as examples, the results indicate that the mean quadratic error is bound to behave like t2 in a short time; the mean quadratic error tends to a stable value after a long time; the parameters error is equivalent to the physical processes lacking error under certain condition. It will afford valuable information for the research of forecasting error correction and weak constraint VDA from the theoretic aspect.

Abstract:The authors compared the simulations of seasonal evolution of rainfall over eastern China and the associated large-scale circulation during summer by a coupled general circulation model (CGCM, MIROC3.2_hires) and the nested regional climate model (RegCM3). Results show that both the MIROC3.2_hires and RegCM3 can reproduce the basic features of summer mean atmospheric circulation, and the spatial distribution of precipitation in China characterized by a decrease from the southeast to northwest of China. Compared with observations, the MIROC3.2_hires simulates a more extensive area of heavy rainfall over South China, while the RegCM3 presents more reasonable spatial distribution of rainfall over this region.  The improvement of simulation by RegCM3 is more evident in the seasonal evolution than that by the driving CGCM. It is found that the MIROC3.2_hires fails to reproduce the seasonal evolution of rain band by presenting the precipitation maximum stagnating over South China during the whole summer period. Meanwhile, the RegCM3 can basically capture the seasonal evolution of rainfall over eastern China during summer, characterized by three stepwise stages, i.e. pre-summer rainy season in South China, Meiyu period in the Yangtze River valley, and rainy season in North China. It suggests that the better simulation of seasonal evolution of wind in both the lower and upper troposphere may be important for the improvements of RegCM3 in simulating the seasonal rainfall evolution.