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Chao-Lin Wang, Shao-Bo Zhong, Guan-Nan Yao, and Quan-Yi Huang

Abstract

Drought disasters cause great economic losses in China every year, especially in its southwest, and they have had a major influence on economic development, lives, and property. In this study, precipitation and drought hazards were examined for a region covering Yunnan, Guizhou, and Guangxi Provinces to assess the spatial and temporal distribution of different drought hazard grades in this region. Annual precipitation data from 90 meteorological stations in or around the study area were collected and organized for the period of 1964–2013. A spatiotemporal covariance model was calculated and fitted. The Bayesian maximum entropy (BME) method, which considers physical knowledge bases to reduce errors, was used to provide an optimal estimation of annual precipitation. Regional annual precipitation distributions were determined. To analyze the spatiotemporal patterns of the drought hazard, the annual standardized precipitation index was used to measure drought severity. A method that involves space–time scan statistics was used to detect the most likely spatiotemporal clusters of the drought hazards. Test-significance p values for all of the calculated clusters were less than 0.001, indicating a high significance level. The results showed that Yunnan Province was a drought-prone area, especially in its northwest and center, followed by Guizhou Province. In addition, Yunnan and Guizhou Provinces were cluster areas of severe and extreme drought. The most likely cluster year was 1966; it was clustered five times during the study period. In this study, the evolutionary process of drought hazards, including spatiotemporal distribution and spatiotemporal clustering characteristics, was considered. The results may be used to provide support for prevention and mitigation of drought in the study area such as optimizing the distribution of drought-resisting resources, drought monitoring, and evaluating potential drought impacts.

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Qingjing Hu, Peiran Yu, Yujiao Zhu, Kai Li, Huiwang Gao, and Xiaohong Yao

Abstract

In this paper, the concentration, the size distribution, and the formation of dimethylaminium (DMA+) and trimethylaminium (TMA+) ions in atmospheric particles were studied during a cruise campaign over the Yellow Sea and the Bohai Sea of China in May 2012. The concentrations of DMA+ and TMA+ in particles smaller than 11 µm were 4.4 ± 3.7 and 7.2 ± 7.1 nmol m−3, respectively. The two ions had a good correlation (R 2 = 0.86), and both had a moderately good correlation with chlorophyll a fluorescence (R 2 = 0.66–0.67). The observed concentrations were from one to three orders of magnitude larger than the concentrations reported in other marine atmospheres. They were also much larger than the values observed at a coastal site neighboring the Yellow Sea in May–June 2013. The high concentrations of DMA+ and TMA+ observed in the marine atmosphere were probably associated with local biogenic activity instead of the long-range transport of these species from adjacent continents. The calculated mole ratios of (DMA+ + TMA+) to in different-sized particles over the seas indicated that (DMA+ + TMA+) most likely played an important role in neutralizing acidic species in particles less than 0.43 µm but not in particles of other sizes. Size distributions of DMA+ and TMA+ in the marine and coastal atmospheres were analyzed in terms of the respective contribution of gas–particle partitioning, cloud/fog processing of TMA+ and DMA+, bioaerosols, and sea-salt aerosols to the observed concentrations of the two ions over the seas.

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Dan-Qing Huang, Jian Zhu, Yao-Cun Zhang, Jun Wang, and Xue-Yuan Kuang

Abstract

Spring persistent rainfall (SPR) over southern China has great impact on its society and economics. A remarkable feature of the SPR is high frequency. However, SPR frequency obviously decreases over the period of 1997–2011. In this study, the possible causes have been investigated from the perspective of the individual and concurrent effects of the East Asian subtropical jet (EASJ) and East Asian polar front jet (EAPJ). A close relationship is detected between SPR frequency and EASJ intensity (but not EAPJ intensity). Associated with strong EASJ, abundant water vapor is transported to southern China by the southwesterly flow, which may trigger the SPR. Additionally, frequencies of both strong EASJ and weak EAPJ events are positively correlated with SPR frequency. Further investigation of the concurrent effect indicates a significant positive correlation between the frequencies of SPR and the strong EASJ–weak EAPJ configuration. Associated with this configuration, southwesterly flow strengthens in the lower troposphere, while northerly wind weakens in the upper troposphere. This provides a dynamic and moist condition, as enhanced ascending motion and intensified convergence of abundant water vapor over southern China, which favors the SPR. All analyses suggest that the EASJ may play a dominant role in the SPR occurrence and that the EAPJ may play a modulation role. Finally, a possible mechanism maintaining the strong EASJ–weak EAPJ configuration is proposed. Significant cooling over the northeastern Tibetan Plateau may induce a cyclone anomaly in the upper troposphere, which could result in an accelerating EASJ and a decelerating EAPJ.

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Chao Liu, Bin Yao, Vijay Natraj, Fuzhong Weng, Tianhao Le, Run-Lie Shia, and Yuk L. Yung

Abstract

With the increasing use of satellite and ground-based high-spectral-resolution (HSR) measurements for weather and climate applications, accurate and efficient radiative transfer (RT) models have become essential for accurate atmospheric retrievals, for instrument calibration, and to provide benchmark RT solutions. This study develops a spectral data compression (SDCOMP) RT model to simulate HSR radiances in both solar and infrared spectral regions. The SDCOMP approach “compresses” the spectral data in the optical property and radiance domains, utilizing principal component analysis (PCA) twice to alleviate the computational burden. First, an optical-property-based PCA is performed for a given atmospheric scenario (atmospheric, trace gas, and aerosol profiles) to simulate relatively low-spectral-resolution radiances at a small number of representative wavelengths. Second, by using precalculated principal components from an accurate radiance dataset computed for a large number of atmospheric scenarios, a radiance-based PCA is carried out to extend the low-spectral-resolution results to desired HSR results at all wavelengths. This procedure ensures both that individual monochromatic RT calculations are efficiently performed and that the number of such computations is optimized. SDCOMP is approximately three orders of magnitude faster than numerically exact RT calculations. The resulting monochromatic radiance has relative errors less than 0.2% in the solar region and brightness temperature differences less than 0.1 K for over 95% of the cases in the infrared region. The efficiency and accuracy of SDCOMP not only make it useful for analysis of HSR measurements, but also hint at the potential for utilizing this model to perform RT simulations in mesoscale numerical weather and general circulation models.

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Junchen Yao, Tianjun Zhou, Zhun Guo, Xiaolong Chen, Liwei Zou, and Yong Sun

Abstract

Simulating the East Asian summer monsoon (EASM) rain belt has been proven challenging for climate models. In this study, the impacts of high resolution to the simulation of spatial distributions and rainfall intensity of the EASM rain belt are revealed based on Atmospheric Model Intercomparison Project (AMIP) simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5) models. A set of sensitivity experiments is further performed to eliminate the potential influences of differences among CMIP5 models. The results show that the high-resolution models improve the intensity and the spatial pattern of the EASM rainfall compared to the low-resolution models, further valid in the sensitivity experiments. The diagnosis of moist static energy (MSE) balance and moisture budgets is further performed to understand the mechanisms underlying the enhancements. Both analyses indicate that the improved EASM rainfall benefits from the intensified meridional convergence along the EASM rain belt simulated by the high-resolution models. In addition, such convergence is mainly contributed by intensified stationary meridional eddy northerly flows over the central northern areas of China and southerly flows over the south of Japan due to increased model resolution, which is robust in the sensitivity experiments. Further analysis indicates that the stationary meridional eddy flow changes in high-resolution simulations are related to the barotropic Rossby wave downstream of the Tibetan Plateau resulting from increased resolution.

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Lei Wang, Zhi-Jun Yao, Li-Guang Jiang, Rui Wang, Shan-Shan Wu, and Zhao-Fei Liu

Abstract

The spatiotemporal changes in 21 indices of extreme temperature and precipitation for the Mongolian Plateau from 1951 to 2012 were investigated on the basis of daily temperature and precipitation data from 70 meteorological stations. Changes in catastrophic events, such as droughts, floods, and snowstorms, were also investigated for the same period. The correlations between catastrophic events and the extreme indices were examined. The results show that the Mongolian Plateau experienced an asymmetric warming trend. Both the cold extremes and warm extremes showed greater warming at night than in the daytime. The spatial changes in significant trends showed a good homogeneity and consistency in Inner Mongolia. Changes in the precipitation extremes were not as obvious as those in the temperature extremes. The spatial distributions in changes of precipitation extremes were complex. A decreasing trend was shown for total precipitation from west to east as based on the spatial distribution of decadal trends. Drought was the most serious extreme disaster, and prolonged drought for longer than 3 yr occurred about every 7–11 yr. An increasing trend in the disaster area was apparent for flood events from 1951 to 2012. A decreasing trend was observed for the maximum depth of snowfall from 1951 to 2012, with a decreased average maximum depth of 10 mm from the 1990s.

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Anthony D. Del Genio, Yonghua Chen, Daehyun Kim, and Mao-Sung Yao
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Junchen Yao, Frédéric Vitart, Magdalena Alonso Balmaseda, Tongwen Wu, and Xiangwen Liu

Abstract

This study investigates the impact of coupled initialization on the extended-range prediction of the Madden-Julian Oscillation (MJO). A set of reforecasts using combinations of the oceanic and atmospheric initial conditions produced with coupled and uncoupled data assimilation (DA) are conducted to evaluate the impact of coupling in the different domains, from the perspective of MJO forecasts. The coupled initial conditions are provided by CERA-SAT pilot coupled reanalysis for the satellite era recently produced by ECMWF. We focus on the prediction skill of the MJO using the Real-time Outgoing Long-wave Radiation (OLR) MJO index in a series of re-forecasts. The impact of atmospheric initial conditions produced by coupled DA shows slight benefit for the MJO prediction. However, compared with the operational ocean reanalysis, the ocean initial conditions created by CERA-SAT degrade the MJO prediction skill during the first 2-3 weeks of the re-forecast by 1.5% to 5.8%. A moist static energy budget analysis revealed that the underestimation of 0.2 K sea surface temperature, 1.4 W m-2 top of atmosphere downward longwave radiation, and 3.8 W m-2 latent heat flux over the Maritime Continent lead to small but statistically significant degradation of the MJO forecast skill. The results demonstrate that the MJO is sensitive to ocean initial conditions, and illustrate the value of the extended range MJO prediction for evaluating the quality of coupled data assimilation, and suggest that future efforts on coupled data assimilation pay special attention to the balance of air-sea interaction processes over the warm pool area, in terms of modeling, observational needs and system.

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Anthony D. Del Genio, Yonghua Chen, Daehyun Kim, and Mao-Sung Yao

Abstract

The relationship between convective penetration depth and tropospheric humidity is central to recent theories of the Madden–Julian oscillation (MJO). It has been suggested that general circulation models (GCMs) poorly simulate the MJO because they fail to gradually moisten the troposphere by shallow convection and simulate a slow transition to deep convection. CloudSat and Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) data are analyzed to document the variability of convection depth and its relation to water vapor during the MJO transition from shallow to deep convection and to constrain GCM cumulus parameterizations. Composites of cloud occurrence for 10 MJO events show the following anticipated MJO cloud structure: shallow and congestus clouds in advance of the peak, deep clouds near the peak, and upper-level anvils after the peak. Cirrus clouds are also frequent in advance of the peak. The Advanced Microwave Scanning Radiometer for Earth Observing System (EOS) (AMSR-E) column water vapor (CWV) increases by ~5 mm during the shallow–deep transition phase, consistent with the idea of moisture preconditioning. Echo-top height of clouds rooted in the boundary layer increases sharply with CWV, with large variability in depth when CWV is between ~46 and 68 mm. International Satellite Cloud Climatology Project cloud classifications reproduce these climatological relationships but correctly identify congestus-dominated scenes only about half the time. A version of the Goddard Institute for Space Studies Model E2 (GISS-E2) GCM with strengthened entrainment and rain evaporation that produces MJO-like variability also reproduces the shallow–deep convection transition, including the large variability of cloud-top height at intermediate CWV values. The variability is due to small grid-scale relative humidity and lapse rate anomalies for similar values of CWV.

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Dan-Qing Huang, Jian Zhu, Yao-Cun Zhang, and An-Ning Huang

Abstract

To investigate the concurrent impacts of the East Asian polar front jet (EAPJ) and subtropical jet (EASJ) on the summer rainfall over eastern China, positive (strengthened EAPJ with weakened EASJ) and negative (weakened EAPJ with strengthened EASJ) configurations are identified. In the positive configuration, rainfall decreases in the northern part of eastern China and increases in the southern part, vice versa in the negative configuration. The possible mechanisms maintaining the two jet configurations are further proposed from the perspectives of sea surface temperature (SST) and synoptic-scale transient eddy activities (STEA). In the positive configuration, meridional distributed cold–warm SST anomalies over the eastern North Pacific may induce regional circulation and meridional temperature gradient anomalies, which can strengthen the EAPJ and weaken the EASJ. The central Pacific La Niña–like SST anomalies are related with the Arctic vortexlike anomalies in the stratosphere, which may strengthen the EAPJ. Furthermore, the divergence of Eliassen–Palm vectors and the conversion from eddy kinetic energy to mean kinetic energy over the active region of the EAPJ may strengthen the EAPJ, vice versa for the weakened EASJ. In the negative configuration, associated with the warm SST anomalies over the western North Pacific, the enhanced convective activities may lead to a strengthened EASJ via meridional teleconnection. The teleconnection may be intensified by the strengthened easterly vertical shear. Additionally, eastern Pacific La Niña–like SST anomalies may intensify the Walker circulation, which may strengthen the EASJ via the Hadley circulation. The STEA-related anomalies are almost opposite those in the positive configuration, especially for the weakened EAPJ.

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