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R. W. Higgins, Y. Yao, and X. L. Wang

Abstract

Key features of the U.S. summer precipitation regime are examined within the context of the evolving North American monsoon system. The focus is on the antecedent and subsequent atmospheric conditions over the conterminous United States relative to the onset of monsoon precipitation over the southwestern United States, which typically begins in early July. The onset of the monsoon in this region is determined using a precipitation index, based on daily observed precipitation for a 31-yr (1963–94) period. Lagged composites of the observed precipitation and various fields from the NCEP–NCAR reanalysis for the period 1979–94 provide a comprehensive picture of atmospheric conditions during the evolution of the U.S. warm season precipitation regime.

The summer precipitation regime is characterized by an out-of-phase relationship between precipitation over the Southwest and the Great Plains–northern tier and an in-phase relationship between precipitation over the Southwest and the East Coast. Changes in the upper-tropospheric wind and divergence fields (mean vertical motion) are broadly consistent with the evolution of this precipitation pattern. Enhanced upper-tropospheric divergence in the vicinity and south of the upper-tropospheric monsoon high coincides with enhanced upper-tropospheric easterlies and Mexican monsoon rainfall after onset. Over the Great Plains and along the northern tier, the middle- and upper-tropospheric flow is more convergent and rainfall diminishes after onset to the north and east of the monsoon high. The frequency of occurrence of the Great Plains low-level jet (LLJ) and southerly moisture transport change little during the evolution. However, LLJ-related precipitation is controlled by changes in the large-scale flow related to the North American monsoon system. There is increased upper-tropospheric divergence and precipitation after onset in the vicinity of an “induced” trough over the eastern United States. The pattern of evaporation minus precipitation from the NCEP–NCAR reanalysis shows broad consistency with the divergence of the vertically integrated flux of water vapor during the monsoon, although the resolution in the NCEP–NCAR reanalysis is inadequate to yield quantitatively accurate regional estimates of these fields. In agreement with earlier studies, the NCEP–NCAR reanalysis indicates that most of the moisture below 850 hPa over the desert Southwest comes from the northern Gulf of California, while most of the moisture at and above 850 hPa arrives from over the Gulf of Mexico.

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Dong-Peng Guo, Ren-Tai Yao, and Dan Fan

Abstract

This paper introduces a wind tunnel experiment to study the effect of the cooling tower of a nuclear power plant on the flow and the characteristics of visible plume regions. The relevant characteristics of the flow field near the cooling tower, such as the plume rise and the visible plume region, are compared with the results of previous experimental data from Électricité de France (EDF) and the Briggs formulas. The results show that the wind tunnel experiment can simulate the top backflow of the cooling tower and the rear cavity regions among others. In the near-wake region, including the recirculation cavity, mean velocity decreases and turbulence intensity increases significantly. The maximum turbulence intensity observed is 0.5. In addition, the disturbed flow extent of the cooling tower top reaches 1.5 times the cooling tower height. Analysis of the visible plume region shows that the wind tunnel experiment can simulate the variation of a visible plume region. The results are consistent with the wind tunnel experiment of EDF. Moreover, the plume rise analysis shows that the wind tunnel experiment data are in agreement with the Briggs formulas for 50–200 m. As a whole, the proposed wind tunnel experiment can simulate the flow field variation of the visible plume region and the plume rise around the buildings with reasonable accuracy.

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Dehai Luo, Yao Ge, Wenqi Zhang, and Aiguo Dai

Abstract

In this paper, reanalysis data are first analyzed to reveal that the individual negative (positive)-phase Pacific–North American pattern (PNA) or PNA (PNA+) has a lifetime of 10–20 days, is characterized by strong (weak) westerly jet stream meanders, and exhibits clear wave train structures, whereas the PNA with rapid retrogression tends to have longer lifetime and larger amplitude than the PNA+ with slow retrogression. In contrast, the wave train structure of the North Atlantic Oscillation (NAO) is less distinct, and the positive (negative)-phase NAO shows eastward (westward) movement around a higher latitude than the PNA. Moreover, it is found that the PNA wave train occurs under a larger background meridional potential vorticity gradient (PVy) over the North Pacific than that over the North Atlantic for the NAO. A unified nonlinear multiscale interaction (UNMI) model is then developed to explain why the PNA as a nonlinear wave packet has such characteristics and its large difference from the NAO. The model results reveal that the larger background PVy for the PNA (due to its location at lower latitudes) leads to its larger energy dispersion and weaker nonlinearity than the NAO, thus explaining why the PNA (NAO) is largely a linear (nonlinear) process with a strong (weak) wave train structure, though it is regarded as a nonlinear initial-value problem. The smaller PVy for the PNA than for the PNA+ leads to lower energy dispersion and stronger nonlinearity for PNA, which allows it to maintain larger amplitude and have a longer lifetime than the PNA+. Thus, the difference in the background PVy is responsible for the asymmetry between the two phases of PNA and the difference between the PNA and NAO.

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Jie Cao, Ping Yao, Lin Wang, and Kui Liu

Abstract

Based on reanalysis and observational datasets, this study proposes a reasonable mechanism for summer rainfall variations over the low-latitude highlands (LLH) of China, in which a subtropical Indian Ocean dipole (SIOD)-like pattern is the key external thermal forcing. In summers with a positive SIOD-like pattern, sea surface temperature (SST) anomalies may lead to lower-tropospheric divergence over the tropical Indian Ocean and convergence over the subtropical southwestern Indian Ocean and Arabian Sea. The convergence over the Arabian Sea can induce easterly anomalies of the divergent wind component off the eastern coast of the Bay of Bengal (BOB), while the divergence over the tropical Indian Ocean can change the interhemispheric vertical circulation and produce a descending motion over the same area and cyclonic anomalies in the rotational wind component over the Indian peninsula. The combined effect of the divergent and rotational wind anomalies and enhanced interhemispheric vertical circulation facilitates easterly anomalies and weakens climatological water vapor flux to the northern BOB. Therefore, anomalous water vapor divergence and less precipitation are observed over the LLH. In summers with a negative SIOD-like pattern, the situation is approximately the same but with opposite polarity and a weaker role of the divergent wind component. Further analyses indicate that the summertime SIOD-like pattern can be traced to preceding seasons, especially in positive SIOD-like years. The SST–wind–evaporation feedback mechanism could account for maintenance of the SIOD-like pattern. These results provide efficient prediction potential for summer rainfall variations over the LLH.

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Donghai Wu, Shilong Piao, Yongwen Liu, Philippe Ciais, and Yitong Yao

Abstract

Earth system models (ESMs) from phase 5 of the Coupled Model Intercomparison Project (CMIP5) were diagnosed as having large discrepancies in their land carbon turnover times, which partly explains the differences in the future projections of terrestrial carbon storage from the models. Carvalhais et al. focused on evaluation of model-based ecosystem carbon turnover times τ eco in relation with climate factors. In this study, τ eco from models was analyzed separately for biomass and soil carbon pools, and its spatial dependency upon temperature and precipitation was evaluated using observational datasets. The results showed that 8 of 14 models slightly underestimated global biomass carbon turnover times τ veg (modeled median of 8 yr vs observed 11 yr), and 11 models grossly underestimated the soil carbon turnover time τ soil (modeled median of 16 yr vs observed 26 yr). The underestimation of global carbon turnover times in ESMs was mainly due to values for τ veg and τ soil being too low in the high northern latitudes and arid and semiarid regions. In addition, the models did not capture the observed spatial climate sensitivity of carbon turnover time in these regions. Modeled τ veg and τ soil values were generally weakly correlated with climate variables, implying that differences between carbon cycle models primarily originated from structural differences rather than from differences in atmospheric climate models (i.e., related to temperature and precipitation). This study indicates that most models do not reproduce the underlying processes driving regional τ veg and τ soil, highlighting the need for improving the model parameterization and adding key processes such as biotic disturbances and permafrost–carbon climate responses.

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Zhanyu Yao, Wanbiao Li, Yuanjing Zhu, Bolin Zhao, and Yong Chen

Abstract

The Tibetan Plateau is a unique location for studying the global climate and China's severe weather. The precipitation on the Tibetan Plateau can be studied conveniently with the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI). It is shown that the TMI brightness temperature at 85 GHz in the vertical polarization (TB85V) is negatively correlated to the surface rain rate, but a very low value of TB85V does not correspond to very intense surface rain rates on the Tibetan Plateau, a result that is different from what is observed in other areas of the world. For surface precipitation retrieval on the Tibetan Plateau from TMI, the effect from snow cover on precipitation retrieval is removed before analysis of precipitation. Using the dynamic cluster K-mean method, five categories of surface types and rain areas are identified on the Tibetan Plateau: dry soil, wet soil, water area, stratiform rain area, and convective rain area. The precipitation areas are screened by classification before the precipitation retrieval. Two datasets of rain-free areas and precipitation areas are formed after surface classification. Based on the dataset of rain-free areas, the value of TB85V can be simulated well by TB10V, TB19V, and TB21V when it is not raining. By means of the dataset of precipitation areas, it is revealed that the scattering index over land (SIL) is positively correlated and the polarization-corrected brightness temperature at 85 GHz (PCT85) is negatively correlated with the surface rain rate. With SIL, PCT85, and their combinations as retrieval algorithms, three precipitation retrieval formulas are proposed in which the SIL algorithm is most suitable for small rain retrieval, the PCT85 algorithm is most suitable for moderate rain retrieval, and the combined SIL and PCT85 algorithm is most suitable for relatively large rain retrieval on the Tibetan Plateau. By means of two thresholds, 265 and 245 K, for TB85V, the combination of the three formulas is applied to precipitation retrieval on the Tibetan Plateau during the Tibetan Plateau Experiment Intensive Observing Period of 1998, resulting in acceptable and encouraging surface rain-rate retrievals. Intercomparison among the TMI algorithms and the 17 Special Sensor Microwave Imager algorithms from the second Precipitation Intercomparison Project demonstrates that the comprehensive application of the TMI algorithms has good precision and error index and is suitable for precipitation retrieval on the Tibetan Plateau.

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Yao Ha, Zhong Zhong, Yijia Hu, and Xiuqun Yang

Abstract

This study investigates the influences of ENSO on tropical cyclone (TC) kinetic energy and its meridional transport in the western North Pacific (WNP) using the TC wind field obtained after a method for removing TC vortices from reanalysis data is applied. Results show that ENSO strongly modulates TC kinetic energy and its meridional transport in the WNP, but their effects and regions differ. The TC kinetic energy is positively correlated with the Niño-3.4 index in the entire WNP, and its poleward transport is positively (negatively) correlated with the Niño-3.4 index in the eastern WNP (the western WNP and the South China Sea); these correlations are statistically significant. The maximum TC kinetic energy is located around 25°N, 135°E (25°N, 125°E) in the warm (cold) year, showing an east–west pattern during different ENSO phases. The meridional transport of TC kinetic energy exhibits a dipole pattern over the WNP, with the poleward (equatorward) transport in the eastern (western) WNP. Both poleward and equatorward transports strengthen (weaken) and shift eastward (westward) in El Niño (La Niña) years. Therefore, El Niño has strong influences on TC kinetic energy and its meridional transport.

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Shangfeng Chen, Renguang Wu, Wen Chen, and Shuailei Yao

Abstract

The present study reveals a marked enhancement in the relationship between Eurasian winter and spring atmospheric interannual variability since the early 1990s. Specifically, the dominant mode of winter Eurasian 500-hPa geopotential height anomalies, with same-sign anomalies over southern Europe and East Asia and opposite-sign anomalies over north-central Eurasia, is largely maintained to the following spring after the early 1990s, but not before the early 1990s. The maintenance of the dominant atmospheric circulation anomaly pattern after the early 1990s is associated with a triple sea surface temperature (SST) anomaly pattern in the North Atlantic that is sustained from winter to the subsequent spring. This triple SST anomaly pattern triggers an atmospheric wave train over the North Atlantic through Eurasia during winter through spring. Atmospheric model experiments verify the role of the triple SST anomaly in maintaining the Eurasian atmospheric circulation anomalies. By contrast, before the early 1990s, marked SST anomalies related to the winter dominant mode only occur in the tropical North Atlantic during winter and they disappear during the following spring. The triple SST anomaly pattern after the early 1990s forms in response to a meridional atmospheric dipole over the North Atlantic induced by a La Niña–like cooling over tropical Pacific, and its maintenance into the following spring may be via a positive air–sea interaction process over the North Atlantic. Results of this analysis suggest a potential source for the seasonal prediction of the Eurasian spring climate.

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Haijun Yang, Xingchen Shen, Jie Yao, and Qin Wen

Abstract

As the most extensive highland in the world, the Tibetan Plateau (TP) plays an important role in shaping the global climate. Quantifying the effect of the TP on global climate is the first step for a full understanding of the TP’s standing on planet Earth. Through coupled model sensitivity experiments, we draw a panorama of the TP’s global impact in this paper. Our model results show that the absence of the TP would result in a 4°C colder and 10% drier climate in the Northern Hemisphere (NH). The TP has a striking remote effect on the North Atlantic. Removing the TP would enhance the westerlies in the mid- to high latitudes of the NH and weaken the easterlies over the tropical Pacific. More moisture would be relocated from the tropical Pacific to the North Atlantic, shutting down the Atlantic thermohaline circulation, which would eventually result in more than 15°C colder and 20% drier climate over the North Atlantic. Our model results suggest that the presence of the TP may have contributed greatly to the hospitable modern climate in the NH, by promoting the establishment of the thermohaline circulation in the Atlantic, and therefore enhancing the northward ocean heat transport and atmosphere moisture transport across the equator.

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Lung-Yao Chang, Kevin K. W. Cheung, and Cheng-Shang Lee

Abstract

A total of 40 out of 531 tropical cyclones that formed in the western North Pacific during 1986–2005 have accompanied trade wind surges located 5°–15° latitude to the north of the pretropical cyclone disturbance centers. Composite and empirical orthogonal function analyses indicate that the trade wind surges are related to a midlatitude eastward-moving high pressure system often found during the East Asian winter monsoon. Therefore, these trade wind surge tropical cyclones tend to occur in late season (with one-third of them in December), and at lower latitudes (7° latitude lower than the climatological average formation position).

The evolution of mesoscale features during formation of trade wind surge tropical cyclones is examined. Various satellite datasets show similar mesoscale patterns during their formations. A few convective lines form by convergence between the trade wind surges and the strengthening cyclonic circulation associated with incipient vortex within the 24 h before formation. Some mesoscale convective systems are embedded in the convective line with lifetimes of about 5 h, and these are illustrated through case studies. Formations usually occur when the trade winds start to decrease in magnitude and a short period after the major episodes of convection in the convective lines and mesoscale convective systems. The relationships between the temporal variability of synoptic-scale trade wind surges, the mesoscale features, and associated tropical cyclone formations are discussed.

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