π-Conjugated Organic Semiconductors for Field-Effect Transistors

Organic semiconductors employed as active layers in field-effect transistors （FETs） are of great current interest because such FETs can potentially be fabricated at low cost, over large areas, and on flexible substrates. Such facile fabrication approaches offer a significant advantage over silicon technology in numerous applications. Much progress about organic FETs has been made in various organic-based electronic circuits, such as displays, sensor, inverts and logic elements. Of all the OFET materials reported so far,     

Synthesis and Field-effect Transistor Behavior of New Oligo-selenophene Derivatives

1 Results In recent years,interests in organic semiconductor have increased due to the applications in optoelectronic devices such as organic light-emitting diodes (OLEDs)[1],field-effect transistors (FETs)[2],and photovoltaic devices[3]. These organic electronics have several advantages over conventional inorganic electronics including facile processability,chemical tunability,compatibility with plastic substrates,and low cost to fabricate. Selenophene-based molecules show good π-conjugating electron overlap between adjacent molecules,because they have high electron density and stronger electron-donation property than thiophene moieties. Therefore selenophene-based molecules have recently received significant attention in the research field of organic FETs.     

Synthesis and Field-effect Transistor Behavior of New Oligo-selenophene Derivatives

1 Results In recent years,interests in organic semiconductor have increased due to the applications in optoelectronic devices such as organic light-emitting diodes (OLEDs)[1],field-effect transistors (FETs)[2],and photovoltaic devices[3]. These organic electronics have several advantages over conventional inorganic electronics including facile processability,chemical tunability,compatibility with plastic substrates,and low cost to fabricate. Selenophene-based molecules show good π-conjugating electron o...     

Changes in tropospheric ozone levels over the Three Representative Regions of China observed from space by the Tropospheric Emission Spectrometer (TES), 2005–2010

Tropospheric ozone observations over China from 2005 to 2010 at three pressure levels (484,681 and 825 hPa) from the Tropospheric Emission Spectrometer on board the NASA Aura satellite have been analyzed.Fourier Transform analysis revealed the trends and seasonality of regionally-averaged,monthly-mean ozone concentrations over western,northern and southern China.Significant increases in ozone levels are found over all three regions at 464 hPa and the rate of increase is fastest over northern China,reaching 0.89±0.059 nL/(La).At 681 hPa,ozone shows increases over northern and western China,at a rate of 0.57± 0.065 nL/(La) and 0.41±0.041 nL/(La) respectively,but is constant over southern China.At 825 hPa,ozone increases at a rate of 0.36±0.074 nL/(La) over northern China,while decreasing over southern China at a rate of 0.21±0.061 nL/(La).Over the three regions,ozone levels are generally higher in summer and lower in winter.Over southern China at all three pressure levels and northern China at the 825 hPa level,ozone shows double peaks occurring in spring and autumn as a result of the combined effects of atmospheric chemistry and global transport.This work provides a useful observational dataset and tools for future analysis of changes in tropospheric ozone over China.     

Impact of land surface degradation in northern China and southern Mongolia on regional climate

Clear evidence provided by the singular value decomposition (SVD) analysis to the normalized difference vegetation index (NDVI) and precipitation data identifies that there exists a sensitive region of vegetation-climate interaction located in the transitional zone over northern China and its surrounding areas, where the vegetation cover change has the most significant influence on summer precipitation over China.““ Comparison of reanalysis data with station data provides a good method to assess the impacts of land use change on surface temperature, and the most obvious contribution of land use change may be to lead to notable warming over northern China in the interdecadal time scale. Based on the new statistical results, a high-resolution regional integrated environmental model system (RIEMS) is employed to investigate the effects of land surface degradation over the transitional zone and its surrounding areas (northern China and southern Mongolia) on the regional climate. Land degradation results in the decreases in precipitation over northern and southern China, and the increase in between, and increased and decreased temperature over vegetation change areas and the adjacent area to the south, respectively. Not only would it change the surface climate, but also bring the significant influence on the atmospheric circulation. Both the surface climate and circulation changes generally agree to the observed interdecadal anomalies over the last five decades. These integrated statistical and simulated results imply that land surface degradation over the transitional zone in northern China and its surrounding areas could be one of the main causes responsible for the climate anomalies over China, especially the drought over northern China.     

Responses of South and East Asian summer monsoons to different land-sea temperature increases under a warming scenario

We used outputs from climate models that participated in the fourth assessment (AR4) of the Intergovernmental Panel on Climate Change (IPCC) to evaluate the responses of the South Asian summer monsoon (SASM) and the East Asian summer monsoon (EASM) circulations to different warming over land and ocean under a medium warming scenario, SRES A1B. Our results suggest that, even though near-surface warming over the Tibetan Plateau (TP) is greater than that over the tropical Indian Ocean (TIO) and the northwestern Pacific (NWP), the upper-tropospheric land-sea thermal contrasts between the TP and the TIO (TP-TIO) and between the TP and the NWP (TP-NWP) will decrease. At interdecadal and longer time scales, the change in the SASM circulation is consistent with the TP-TIO upper-troposphere thermal contrast. Conversely, the change in the EASM circulation is consistent with the TP-NWP lower-troposphere thermal contrast. However, at the interannual time scale, both changes in the EASM and SASM are significantly correlated with the upper-troposphere thermal contrast. Further analyses suggest that increases in moisture and changes in cloud cover induced by global warming may cause amplified upper-tropospheric warming over the TP and the oceans resulting in inconsistent changes in the vertical temperature distribution over these regions. Because the warming over the TIO and NWP is greater than that over the TP, the TP-TIO meridional and TP-NWP zonal thermal contrasts will both decrease. However, at the lower layer, the difference in thermal capacity between land and sea result in a larger thermal effect in the near-surface region of the TP than those over the surrounding oceans. We showed that a range of factors that affect thermal conditions will likely cause changes in the Asian monsoon across a range of time scales under a warming scenario. These changes reflect differences in the influence of the greenhouse effect and natural variability.     

The leading correlation of the winter Aleutian Low with surface air temperature during the subsequent summer over the Arctic and its possible mechanism

The variations of surface air temperature(SAT)over the Arctic are closely related to global climate change.Based on reanalysis datasets and a newly defined Aleutian Low intensity index,we found a good correlation between intensity of winter Aleutian Low and the SAT over the Arctic during the subsequent summer.Explanations were given using correlation analysis,composite analysis,and singular value decomposition methods.When intensity of winter Aleutian Low was weaker,sea surface temperature appeared higher in the North Pacific in the subsequent spring and summer,resulting in mean meridional circulation anomalies and 500 hPa geopotential height anomalies in spring and summer.Anomalous upward motion in mid-latitudes and downward motion in high latitudes(Ferrel cell weakening)transported the warmer air to the north from lower layer to the upper layer followed by increases in the SAT over the Arctic.Anomalous downward motion over about 75°N also caused consequent adiabatic warming and contributed to inhibit the heat transportation from surface to upper layer.Negative 500 hPa geopotential height anomalies existed in mid-latitudes and positive anomalies existed in high latitudes.The pattern(low-in-south and high-in-north)benefited from increasing the inflow volume flux of the Bering Strait,which also made the SAT over the Arctic increase.The results of this study reveal the process that the summer SAT over the Arctic was modulated by interannual variability of intensity of winter Aleutian Low.     

Interdecadal change of summer precipitation over Eastern China around the late-1990s and associated circulation anomalies, internal dynamical causes

Observational study indicated that the summer precipitation over Eastern China experienced a notable interdecadal change around the late-1990s. Accompanying this interdecadal change, the dominant mode of anomalous precipitation switched from a meridional triple pattern to a dipole pattern, showing a "south-flood-north-drought" structure (with the exception of the Yangtze River Valley). This interdecadal change of summer precipitation over Eastern China was associated with circulation anomalies in the middle/upper troposphere over East Asia, such as changes in winds and corresponding divergence, vertical motion and moisture transportation (divergence), which all exhibit remarkable meridional dipole structures. Furthermore, on the internal dynamic and thermodynamic aspects, the present study investigated the influence of the midtroposphere zonal and meridional flow changes over East Asia on the interdecadal change around the late-1990s. Results suggested that, during 1999-2010, the East Asia subtropical westerly jet weakened and shifted poleward, forming a meridional dipole feature in anomalous zonal flow. This anomalous zonal flow, on one hand, induced changes in three teleconnection patterns over the Eurasian continent, namely the "Silk Road" pattern along the subtropical upper troposphere westerly jet, the East Asia/Pacific (EAP) pattern along the East Asian coast, and the Eurasia (EU) pattern along the polar jet; on the other hand, it brought about cold advection over Northern China, and warm advection over Southern China in the mid-troposphere. Through these two ways, the changes in the zonal flow induced descent over Northern China and ascent over Southern China, which resulted in the anomalous "south-flood-north-drought" feature of the summer precipitation over Eastern China during 1999-2010.     

Organic Field-effect Transistors Based on Tetrathiafulvalene Derivatives

1 Restults Tetrathiafulvalene (TTF) and its derivatives have been extensively investigated in the field of organic conductors and superconductors since 1973. Recently, their application in organic field-effect transistors (OFETs) has attracted considerable attention. So far, on the one hand, the fabrication techniques of the TTF-based FETs have been primarily limited to high vacuum evaporation, which is a relatively expensive process. On the other hand, low FET performances, such as the low on/off ratios, large off-current at zero gate voltage and the lack of saturation currents in the output curves, need to be improved.     

Comparison of sensible and latent heat fluxes during the transition season over the western Tibetan Plateau from reanalysis datasets

The sensible and latent heat fluxes during the transition season over the western Tibetan Plateau from the NCEP-1 (NCEP/NCAR Reanalysis 1), AR-II (NCEP-DOE Reanalysis 2) and ERA40 reanalysis datasets were compared and analyzed. The results show that the phase change in soil moisture has a significant effect on the sensible and latent heat fluxes over the western Tibetan Plateau (TP) due to the freezing–thawing processes during the transition from the dry to the wet period. The uncertainties in the sensible and latent heat fluxes over the western TP are quite high in the reanalysis data, and depend largely on the success of the soil moisture simulations in the models. Improving the hydrological process simulations in the land-surface models in seasonally frozen ground and in the active frozen soil layer may be an effective way of enhancing the reliability of the surface heat fluxes from the reanalysis data over the Tibetan Plateau.     

Relationship between the atmospheric heat source over Tibetan Plateau and the heat source and general circulation over East Asia

On the basis of NCEP/NCAR version I daily reanalysis data from 1971 to 2000 and by the methods of inverse calculation, correlation analysis and comparative analysis, the influences of atmospheric heat source （AHS） over the Tibetan Plateau on the large-scale AHS and the general circulation in summer are studied in this paper. The results show that AHS over the plateau in summer may trigger a heat source wavetrain propagating northeastward along the coast from the East Asian continent and West Pacific to Bering Strait-Arctic or even North America. In addition, if AHS over the eastern plateau is intense, South Asian High moves to southeast and West Pacific subtropical high moves to southwest; on the contrary, if AHS over the eastern plateau is weak, South Asian High moves to northwest and West Pacific subtropical high moves to northeast. Therefore, South Asian High and West Pacific subtropical high move in the horizontally-opposite directions in terms of interannual variation, for which AHS over the eastern plateau seems to be thermodynamically responsible.     

The terraced thermal contrast among the Tibetan Plateau, the East Asian plain, and the western North Pacific and its impacts on the seasonal transition of East Asian climate

The heating sources over the Tibetan Plateau （TP）, the East Asian plain, and the western North Pacific （WNP） form a terraced thermal contrast in the west-east direction. Over East Asia and the WNP, this zonal thermal contrast contributes as high as 45 % to the seasonal variance based on the EOF analysis and exerts a significant impact on the seasonal transition of the East Asian climate through the enhancement of the year-round southerly to the southeast of the TP in late March and early April. This effect is investigated in this study using a high-resolution regional atmospheric model by doubling the surface sen- sible heat flux, respectively, over the TP, the East Asian plain, and the WNP in three sensitivity experiments. Comparisons among the experiments reveal that doubling the surface sensible heat flux over the WNP has little upstream response over East Asia. The increased zonal thermal contrast between the TP and the East Asian plain due to doubled heat flux over the TP would induce anomalous northerly over the region with year-round southerly to the southeast of the TP and weaken its seasonal enhancement. Doubling the surface sensible heat flux over the East Asian plain decreases the zonal thermal contrast and leads to southerly anomaly over the region with year-round southerly to the southeast of the TP and South China, which is favorable for the enhancement of the year-round southerly and its eastward extension.     

Land-atmosphere coupling amplifies hot extremes over China

Climate extremes, such as extreme hot temperatures and heat waves, can have dramatic societal, economic, and ecological consequences. China has experienced remarkable interannual and decadal changes in hot extremes during the last several decades. However, the underlying mechanisms responsible for changes in the hot extremes over China have not been clearly identified. In this study, we investigate the role of land-atmosphere coupling for hot days and heat waves during summer over China using two long-term Weather Research and Forecasting model simulations with and without interactive soil moisture. Results indicate that land-atmosphere coupling mainly amplifies hot extremes over China. In particular, significant amplifying effects appear over most of eastern and southwestern China. Over these areas, land-atmosphere coupling generally accounts for 30%-70% of the numbers of hot days and heat waves. This study highlights the critical importance of land-atmosphere interactions for the occurrence of hot extremes over China.     

Precursory influence of the Antarctic Oscillation on the onset of Asian summer monsoon

The Asian summer monsoon(ASM) begins firstly over the Indo-China Peninsula in early May and over the South China Sea(SCS) in mid-May.The different monsoon onset dates can exert distinct effects on the summer rainfall in Asia.Statistical results indicate that the Antarctic Oscillation(AAO) in the boreal winter has a significant precursory influence on the ASM onset dates.In stronger AAO years,both the Mascarene high and the Australia high in March are stronger owing to the "see-saw" structure of atmospheric circulation over the subtropics and higher latitudes in the Southern Hemisphere,and the tropical intertropical convergence zone(ITCZ) is deeper.Thus,the pressure gradient between the subtropical and tropical regions increases in spring.As a result,the Somalia cross-equatorial flow(SCEF) occurs earlier,strengthens,and enhances the westerlies over the tropical Indian Ocean.The enhanced westerlies impel an eastward withdrawal of the western Pacific subtropical high and intensify the convergence and rising motion at the lower troposphere,accelerating the burst of ASM.Differently,weaker AAO weakens the pressure gradient between the tropical and subtropical regions and delays the establishment of SCEF,resulting in a delayed onset of ASM.This study extends the leading time of seasonal forecast of ASM onset from the previous spring to winter and provides useful information about precursory signals in climate prediction operation.     

Rainfall spectrum change in North China and its possible mechanism

The seasonal distribution of the rainfall in North China has changed greatly since 1977, with more rainfall in spring and less in July, August, and September (JAS). Wavelet analysis showed that the JAS rainfall underwent an abrupt spectrum change in the mid-1960s. Its interannual variability has declined while the interdecadal component has become the dominant mode, associated with the dry climate. Correlation analysis found that the JAS rainfall is negatively correlated with the departures of the 500 hPa geopotential height significantly over the northwest and southwest of China, and positively correlated with the suer monsoon over eastern China. Therefore, the interdecadal ascending of the air pressure in northwestern China and the monsoon decaying over East Asia in the past 20 years may be the two major causes of North China drought.     

The spring soil moisture and the summer rainfall in eastern China

The relation between the soil moisture in spring and the rainfall in summer in eastern China is investi- gated. Results show that the summer rainfall in eastern China is closely related to the spring soil moisture in the area from North China to the lower reaches of Yangtze River (NCYR). When spring soil moisture anomalies over NCYR are positive, the summer precipitation exhibits positive anomalies in Northeast China and the lower reaches of Yangtze River, and negative anomalies in southern China and North China. The higher soil moisture over NCYR cools land surface and reduces the land-sea tem- perature gradient, which weakens East Asian summer monsoon. The western Pacific Subtropical High (WPSH) is located to the south and shifts westward, resulting in more rainfall in the lower reaches of Yangtze River and less in southern China and North China.     

Extracting change information of land use and soil erosion based on RS & GIS technologystration

Rapid landuse change has taken place in many arid regions of China such as Yulin prefecture over the last decade due to rehabilitation measures. Landuse change and soil erosion dynamics were investigated by the combined use of remote sensing and geographic information systems (GIS). The objectives were to determine landuse transition rates and soil erosion change in Yulin prefecture over 15 years from 1986 to 2000. Significant changes in landuse and soil erosion occurred in the area over the study period. The results show the significant decrease in barren land mainly due to conversion to grassland. Agricultural land increased associated with conversions from grassland and barren land. The area of water erosion and wind erosion declined. The study demonstrates that the integration of satellite remote sensing and GIS is an effective approach for analyzing the direction, rate, and spatial pattern of landuse and soil erosion change.     

Temperature and precipitation in Mongolia based on dendroclimatic investigations

Recent tree-ring studies in Mongolia provide evidence of unusual warming that is in agreement with large-scal reconstructed and recorded temperaturec for the Northern Hemisphere and the Arctic.One Mongolian proxy record for temperature extends back over 1000 years and several others are over 350 years in length.Precipitation reconstructions based on thee rings reflect recent increases but also indicate that the increases are within the long-term range of varlations.Spectral analyses of recorded preciplta-tion data and the reconstructions support the hypotheses of quasi-solar periodicity in precipitation variation,previously suggested by others.     

Climatological distribution and diurnal variation of mesoscale convective systems over China and its vicinity during summer

The climatological distribution of mesoscale convective systems （MCSs） over China and its vicinity during summer is statistically analyzed, based on the 10-year （1996-2006, 2004 excluded） June-August infrared TBB （Temperature of black body） dataset. Comparing the results obtained in this paper with the distribution of thunderstorms from surface meteorological stations over China and the distribution of lightning from low-orbit satellites over China and its vicinity in the previous studies, we find that the statistic characteristics of TBB less than -52℃ can better represent the spatiotemporal distribution of MCSs over China and its vicinity during summer. The spreading pattern of the MCSs over this region shows three transmeridional bands of active MCSs, with obvious fluctuation of active MCSs in the band near 30^oN. It can be explained by the atmospheric circulation that the three bands of active MCSs are associated with each other by the summer monsoon over East Asia. We focus on the diurnal variations of MCSs over different underlying surfaces, and the result shows that there are two types of MCSs over China and its vicinity during summer. One type of MCSs has only one active period all day long （single-peak MCSs）, and the other has multiple active periods （multi-peak MCSs）. Single-peak MCSs occur more often over plateaus or mountains, and multi-peak MCSs are more common over plains or basins. Depending on lifetimes and active periods, single-peak MCSs can be classified as Tibetan Plateau MCSs, general mountain MCSs, Ryukyu MCSs, and so on. The diurnal variation of multi-peak MCSs is very similar to that of MCCs （mesoscale convective complexes）, and it reveals that multi-peak MCSs has longer life cycle and larger horizontal scale, becomes weaker after sunset, and develops again after midnight. Tibetan Plateau MCSs and general mountain MCSs both usually develop in the afternoon, but Tibetan Plateau MCSs have longer life cycle and more active MαCSs. Ryukyu MCSs generally develop after     

Characteristics of decadal-centennial-scale changes in East Asian summer monsoon circulation and precipitation during the Medieval Warm Period and Little Ice Age and in the present day

Using meteorological observations, proxies of precipitation and temperature, and climate simulation outputs, we synthetically analyzed the regularities of decadal-centennial-scale changes in the summer thermal contrast between land and ocean and summer precipitation over the East Asian monsoon region during the past millennium; compared the basic characteristics of the East Asian summer monsoon (EASM) circulation and precipitation in the present day, the Little Ice Age (LIA) and the Medieval Warm Period (MWP); and explored their links with solar irradiance and global climate change. The results indicate that over the last 150 years, the EASM circulation and precipitation, indicated by the temperature contrast between the East Asian mainland and adjacent oceans, had a significant decadal perturbation and have been weaker during the period of rapid global warming over the past 50 years. On the centennial time scale, the EASM in the MWP was strongest over the past 1000 years. Over the past 1000 years, the EASM was weakest in 1450-1570. When the EASM circulation was weaker, the monsoon rain belt over eastern China was generally located more southward, with there being less precipitation in North China and more precipitation in the Yangtze River valley; therefore, there was an anomalous pattern of southern flood/northern drought. From the 1900s to 1920s, precipitation had a pat- tern opposite to that of the southern flood/northern drought, with there being less precipitation in the Yangtze River valley and more precipitation in North China. Compared with the case for the MWP, there was a longer-time-scale southern flood/northern drought phenomenon in 1400-1600. Moreover, the EASM circulation and precipitation did not synchronously vary with the trend of global temperature. During the last 150 years, although the annual mean surface temperature around the world and in China has increased, the EASM circulation and precipitation did not have strengthening or weakening trends. Over the past 1000 years, the weakest EASM occurred ahead of the lowest Northern Hemispheric temperature and corresponded to the weakest solar irradiance.