J. Meteor. Res.
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2018 Vol. 32, No. 3
Published: 2018-06-28

Near-Term Projections of Global and Regional Land Mean Temperature Changes Considering Both the Secular Trend and Multidecadal Variability
Yajie QI, Zhongwei YAN, Cheng QIAN, Ying SUN
2018, 32(3): 337-350 [Abstract]( 100 ) HTML PDF (4255 KB)  ( 140 Supplemental Material
Abstract:Near-term climate projections are needed by policymakers; however, these projections are difficult because internally generated climate variations need to be considered. In this study, temperature change scenarios in the near-term period 2017-35 are projected at global and regional scales based on a refined multi-model ensemble approach that considers both the secular trend (ST) and multidecadal variability (MDV) in the Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations. The ST and MDV components are adaptively extracted from each model simulation by using the ensemble empirical mode decomposition (EEMD) filter, reconstructed via the Bayesian model averaging (BMA) method for the historical period 1901-2005, and validated for 2006-16. In the simulations of the "medium" representative concentration pathways scenario during 2017-35, the MDV-modulated temperature change projected via the refined approach displays an increase of 0.44℃ (90% uncertainty range from 0.30 to 0.58℃) for global land, 0.48℃ (90% uncertainty range from 0.29 to 0.67℃) for the Northern Hemispheric land (NL), and 0.29℃ (90% uncertainty range from 0.23 to 0.35℃) for the Southern Hemispheric land (SL). These increases are smaller than those projected by the conventional arithmetic mean approach. The MDV enhances the ST in 13 of 21 regions across the world. The largest MDV-modulated warming effect (46%) exists in central America. In contrast, the MDV counteracts the ST in NL, SL, and eight other regions, with the largest cooling effect (220%) in Alaska.
Projected Changes in Extreme High Temperature and Heat Stress in China
Xingcai LIU, Qiuhong TANG, Xuejun ZHANG, Siao SUN
2018, 32(3): 351-366 [Abstract]( 88 ) HTML PDF (10211 KB)  ( 81 Supplemental Material
Abstract:High temperature accompanied with high humidity may result in unbearable and oppressive weather. In this study, future changes of extreme high temperature and heat stress in mainland China are examined based on daily maxi-mum temperature (Tx) and daily maximum wet-bulb globe temperature (Tw). Tw has integrated the effects of both temperature and humidity. Future climate projections are derived from the bias-corrected climate data of five general circulation models under the Representative Concentration Pathways (RCPs) 2.6 and 8.5 scenarios. Changes of hot days and heat waves in July and August in the future (particularly for 2020-50 and 2070-99), relative to the baseline period (1981-2010), are estimated and analyzed. The results show that the future Tx and Tw of entire China will increase by 1.5-5℃ on average around 2085 under different RCPs. Future increases in Tx and Tw exhibit high spatial heterogeneity, ranging from 1.2 to 6℃ across different regions and RCPs. By around 2085, the mean duration of heat waves will increase by 5 days per annum under RCP8.5. According to Tx, heat waves will mostly occur in Northwest and Southeast China, whereas based on Tw estimates, heat waves will mostly occur over Southeast China and the mean heat wave duration will be much longer than those from Tx. The total extreme hot days (Tx or Tw > 35℃) will increase by 10-30 days. Southeast China will experience the severest heat stress in the near future as extreme high temperature and heat waves will occur more often in this region, which is particularly true when heat waves are assessed based on Tw. In comparison to those purely temperature-based indices, the index Tw provides a new perspective for heat stress assessment in China.
Influences of the 11-yr Sunspot Cycle and Polar Vortex Oscillation on Observed Winter Temperature Variations in China
Chunhui LU, Botao ZHOU
2018, 32(3): 367-379 [Abstract]( 71 ) HTML PDF (7395 KB)  ( 148 Supplemental Material
Abstract:Using the NCEP-2 reanalysis data in 1979-2015, we analyze variations in the coupled stratosphere-troposphere system and attribute them to the polar vortex oscillation (PVO) and the 11-yr sunspot cycle (SC). Subsequently, influences of PVO and SC on the near-ground temperature and extreme temperatures are diagnosed based on observations at 2419 surface stations in China over the same period. Empirical Orthogonal Function (EOF) analysis of geopotential height (GH) anomalies indicates that the first and second EOF modes together can explain nearly 50% of the total variance and they have different driving sources, active periods, and regions. The first EOF mode mainly represents variation characteristics of the polar vortex, and its active periods appear in late winter. It is found that a weakened polar vortex (larger amplitude in the positive time series of the first mode) corresponds to lower daily mean, minimum, and maximum temperatures and more frequent cold nights and days. This cooling effect mainly occur in northeastern China. The second EOF mode is closely related to the SC, and its major active periods are late autumn and early winter. The results reveal that strong solar activity (larger amplitude in the positive time series of the second mode) leads to cooling effects in northern China through accelerating seasonal transformation of the stratospheric circulation and enhancing intensity of the subtropical westerly jet in the upper troposphere and lower stratosphere. The near-ground temperature is lower than usual, especially for daily mean and minimum temperatures. The number of warm nights and days is significantly reduced, and cold nights and days become more frequent. Therefore, the first and second EOF mode time series of GH anomalies can be used as indices of PVO and solar activity, respectively; and can provide indications of winter cooling processes in China.
Influence of Springtime Atlantic SST on ENSO: Role of the Madden—Julian Oscillation
Xin YAN, Juzhang REN, Jianhua JU, Song YANG
2018, 32(3): 380-393 [Abstract]( 62 ) HTML PDF (7183 KB)  ( 88 Supplemental Material
Abstract:Increased evidence has shown the important role of Atlantic sea surface temperature (SST) in modulating the El Niño-Southern Oscillation (ENSO). Persistent anomalies of summer Madden-Julian Oscillation (MJO) act to link the Atlantic SST anomalies (SSTAs) to ENSO. The Atlantic SSTAs are strongly correlated with the persistent anomalies of summer MJO, and possibly affect MJO in two major ways. One is that an anomalous cyclonic (anticyclonic) circulation appears over the tropical Atlantic Ocean associated with positive (negative) SSTA in spring, and it intensifies (weakens) the Walker circulation. Equatorial updraft anomaly then appears over the Indian Ocean and the eastern Pacific Ocean, intensifying MJO activity over these regions. The other involves a high pressure (low pressure) anomaly associated with the North Atlantic SSTA tripole pattern that is transmitted to the mid- and low-latitudes by a circumglobal teleconnection pattern, leading to strong (weak) convective activity of MJO over the Indian Ocean. The above results offer new viewpoints about the process from springtime Atlantic SSTA signals to summertime atmospheric oscillation, and then to the MJO of tropical atmosphere affecting wintertime Pacific ENSO events, which connects different oceans.
Impact of 10-60-Day Low-Frequency Steering Flows on Straight Northward-Moving Typhoon Tracks over the Western North Pacific
Qiao LIU, Tim LI, Weican ZHOU
2018, 32(3): 394-409 [Abstract]( 104 ) HTML PDF (8387 KB)  ( 94 Supplemental Material
Abstract:This study investigates the impact of low-frequency (intraseasonal and interannual) steering flows on straight northward-moving (defined as a meridional displacement two times greater than the zonal displacement) typhoons over the western North Pacific using observational data. The year-to-year change in the northward-moving tracks is affected by the interannual change in the location and intensity of the subtropical high. A strengthened northward steering flow east of 120°E and a weakened easterly steering flow south of the subtropical high favor more frequent straight northward tracks. Examining each of the individual northward-moving typhoons shows that they interact with three types of intraseasonal (10-60-day) background flows during their northward journey. The first type is the monsoon gyre pattern, in which the northward-moving typhoon is embedded in a closed cyclonic monsoon gyre circulation. The second type is the wave train pattern, where a cyclonic (anticyclonic) vorticity circulation is located to the west (east) of the northward-moving typhoon center. The third type is the mid-latitude trough pattern, in which the northward-moving typhoon center is located in the maximum vorticity region of the trough.
Using a Hidden Markov Model to Analyze the Flood-Season Rainfall Pattern and Its Temporal Variation over East China
Lianyi GUO, Zhihong JIANG, Weilin CHEN
2018, 32(3): 410-420 [Abstract]( 64 ) HTML PDF (3065 KB)  ( 81 Supplemental Material
Abstract:The homogeneous hidden Markov model (HMM), a statistical pattern recognition method, is introduced in this paper. Based on the HMM, a 53-yr record of daily precipitation during the flood season (April-September) at 389 stations in East China during 1961-2013 is classified into six patterns:the South China (SC) pattern, the southern Yangtze River (SY) pattern, the Yangtze-Huai River (YH) pattern, the North China (NC) pattern, the overall wetter (OW) pattern, and the overall drier (OD) pattern. Features of the transition probability matrix of the first four patterns reveal that 1) the NC pattern is the most persistent, followed by the YH, and the SY is the least one; and 2) there exists a SY-SC-SY-YH-NC propagation process for the rain belt over East China during the flood season. The intraseasonal variability in the occurrence frequency of each pattern determines its start and end time. Furthermore, analysis of interdecadal variability in the occurrence frequency of each pattern in recent six decades has identified three obvious interdecadal variations for the SC, YH, and NC patterns in the mid-late 1970s, the early 1990s, and the late 1990s. After 2000, the patterns concentrated in the southern region play a dominant role, and thus there maintains a "flooding in the south and drought in the north" rainfall distribution in eastern China. In summary, the HMM provides a unique approach for us to obtain both spatial distribution and temporal variation features of flood-season rainfall.
Effect of Mesoscale Land Use Change on Characteristics of Convective Boundary Layer: Semi-Idealized Large Eddy Simulations over Northwest China
Bangjun CAO, Shuwen ZHANG, Deqin LI, Yanlin LI, Linfan ZHOU, Jiemin WANG
2018, 32(3): 421-432 [Abstract]( 147 ) HTML PDF (3451 KB)  ( 89 Supplemental Material
Abstract:Although large-scale topography and land use have been properly considered in weather and climate models, the effect of mesoscale and microscale heterogeneous land use on convective boundary layer (CBL) has not been fully understood yet. In this study, the influence of semi-idealized strip-like patches of oases and deserts, which resemble irrigated land use in Northwest China, on the CBL characteristics, is investigated based on the Weather Research and Forecasting (WRF)-large eddy simulation (LES) driven by observed land surface data. The influences of soil water content in oases on aloft CBL flow structure, stability, turbulent kinetic energy (TKE), and vertical fluxes are carefully examined through a group of sensitivity experiments. The results show that secondary circulation (SC)/turbulent organized structures (TOS) is the strongest/weakest when soil water content in oases is close to saturation (e.g., when the oases are irrigated). With the decrease of soil water content in oases (i.e., after irrigation), SC (TOS) becomes weak (strong) in the lower and middle CBL, the flux induced by SC and TOS becomes small (large), which has a dramatic impact on point measurement of eddy covariance (EC) fluxes. The flux induced by SC and TOS has little influence on EC sensible heat flux, but great influence on EC latent heat flux. Under this circumstance, the area averaged heat flux cannot be represented by point measurement of flux by the EC method, especially just after irrigation in oases. Comparison of imbalance ratio (i.e., contribution of SC and TOS to the total flux) reveals that increased soil moisture in oases leads to a larger imbalance ratio as well as enhanced surface heterogeneity. Moreover, we found that the soil layer configuration at different depths has a negligible impact on the CBL flux properties.
Validation of Column-Averaged Dry-Air Mole Fraction of CO2 Retrieved from OCO-2 Using Ground-Based FTS Measurements
Yanmeng BI, Qian WANG, Zhongdong YANG, Jie CHEN, Wenguang BAI
2018, 32(3): 433-443 [Abstract]( 37 ) HTML PDF (2750 KB)  ( 38 Supplemental Material
Abstract:In order to correctly use the column-averaged atmospheric CO2 dry-air mole fraction (XCO2) data in the CO2 flux studies, XCO2 measurements retrieved from the Orbiting Carbon Observatory-2 (OCO-2) in 2015 were compared with those obtained from the global ground-based high-resolution Fourier Transform Spectrometer (FTS) participating in the Total Carbon Column Observing Network (TCCON). The XCO2 retrieved from three observing modes adopted by OCO-2, i.e., nadir, target, and glint, were separately validated by the FTS measurements at up to eight TCCON stations located in different areas. These comparisons show that OCO-2 glint mode yields the best qualitative estimations of CO2 concentration among the three operational approaches. The overall results regarding the glint mode show no obvious systematic biases. These facts may indicate that the glint concept is appropriate for not only oceans but also land regions. Negative systematic biases in nadir and target modes have been found at most TCCON sites. The standard deviations of XCO2 retrieved from target and nadir modes within the observation period are simi-lar, and larger than those from glint mode. We also used the FTS site in Beijing, China, to assess the OCO-2 XCO2 in 2016. This site is located in a typical urban area, which has been absent in previous studies. Overall, OCO-2 XCO2 agrees well with that from FTS at this site. Such a study will benefit the validation of the newly launched TanSat products in China.
An Algorithm for Automated Identification of Gust Fronts from Doppler Radar Data
Yue YUAN, Ping WANG, Di WANG, Huizhen JIA
2018, 32(3): 444-455 [Abstract]( 51 ) HTML PDF (5521 KB)  ( 74 Supplemental Material
Abstract:Gust fronts are weak narrow-band echoes of increased reflectivity at the background levels in the low-elevation fields of Doppler radar. An automated approach to gust front detection that relies on the image features of radar observations is presented in this paper. The algorithm is not sensitive to the variations in reflectivity values and gust front widths. The approach includes the following steps. First, a novel local binary with dual-template (LBDT) algorithm is designed as the fundamental algorithm to identify the potential areas of narrow-band echoes. Second, based on the disadvantages of the LBDT algorithm, several modifications are made, including splitting the intersecting lines, connecting the fragments, and filtering the edges and radial interference noise. Third, an optical flow method is used to determine whether a weak narrow-band echo is a gust front according to the prior knowledge that a gust front usually propagates in front of the associated generating storm. The results of experiments show that the proposed method can automatically identify gust fronts with a high probability of detection and a low false alarm rate. The automatic identification of gust fronts is potentially useful for accurate short-term weather forecasting, particularly in the forecasting of storm winds.
Simulation of Non-Homogeneous CO2 and Its Impact on Regional Temperature in East Asia
Xiaodong XIE, Xiaoxian HUANG, Tijian WANG, Mengmeng LI, Shu LI, Pulong CHEN
2018, 32(3): 456-468 [Abstract]( 62 ) HTML PDF (4480 KB)  ( 102 Supplemental Material
Abstract:Carbon dioxide (CO2) is an important greenhouse gas that influences regional climate through disturbing the earth's energy balance. The CO2 concentrations are usually prescribed homogenously in most climate models and the spatiotemporal variations of CO2 are neglected. To address this issue, a regional climate model (RegCM4) is modified to investigate the non-homogeneous distribution of CO2 and its effects on regional longwave radiation flux and temperature in East Asia. One-year simulation is performed with prescribed surface CO2 fluxes that include fossil fuel emission, biomass burning, air-sea exchange, and terrestrial biosphere flux. Two numerical experiments (one using constant prescribed CO2 concentrations in the radiation scheme and the other using the simulated CO2 concentrations that are spatially non-homogeneous) are conducted to assess the impact of non-homogeneous CO2 on the re-gional longwave radiation flux and temperature. Comparison of CO2 concentrations from the model with the observations from the GLOBALVIEW-CO2 network suggests that the model can well capture the spatiotemporal patterns of CO2 concentrations. Generally, high CO2 mixing ratios appear in the heavily industrialized eastern China in cold seasons, which probably relates to intensive human activities. The accommodation of non-homogeneous CO2 concentrations in the radiative transfer scheme leads to an annual mean change of -0.12 W m-2 in total sky surface upward longwave flux in East Asia. The experiment with non-homogeneous CO2 tends to yield a warmer lower troposphere. Surface temperature exhibits a maximum difference in summertime, ranging from -4.18 K to 3.88 K, when compared to its homogeneous counterpart. Our results indicate that the spatial and temporal distributions of CO2 have a considerable impact on regional longwave radiation flux and temperature, and should be taken into account in future climate modeling.
Evaluation of the Forecast Accuracy of Near-Surface Temperature and Wind in Northwest China Based on the WRF Model
Haixia DUAN, Yaohui LI, Tiejun ZHANG, Zhaoxia PU, Cailing ZHAO, Yuanpu LIU
2018, 32(3): 469-490 [Abstract]( 50 ) HTML PDF (9275 KB)  ( 62 Supplemental Material
Abstract:This study investigated the performance of the mesoscale Weather Research and Forecasting (WRF) model in predicting near-surface atmospheric temperature and wind for a complex underlying surface in Northwest China in June and December 2015. The spatial distribution of the monthly average bias errors in the forecasts of 2-m temperature and 10-m wind speed is analyzed first. It is found that the forecast errors for 2-m temperature and 10-m wind speed in June are strongly correlated with the terrain distribution. However, this type of correlation is not apparent in December, perhaps due to the inaccurate specification of the surface albedo and freezing-thawing process of frozen soil in winter in Northwest China in the WRF model. In addition, the WRF model is able to reproduce the diurnal variation in 2-m temperature and 10-m wind speed, although with weakened magnitude. Elevations and land-use types have strong influences on the forecast of near-surface variables with seasonal variations. The overall results imply that accurate specification of the complex underlying surface and seasonal changes in land cover is necessary for improving near-surface forecasts over Northwest China.
The 200-hPa Wind Perturbation in the Subtropical Westerly over East Asia Related to Medium-Range Forecast of Summer Rainfall in China
Ronghua JIN, Yan LI, Qingrou LONG, Sijia LIU
2018, 32(3): 491-502 [Abstract]( 31 ) HTML PDF (3991 KB)  ( 30 Supplemental Material
Abstract:The 200-hPa wind perturbation (WP) in the subtropical westerly over East Asia (SWEA) has seldom been examined in previous studies, especially in connection with forecast of the summer rainfall in China. Based on the daily NCEP/NCAR reanalysis data and precipitation observations in China from 1 June to 31 August of 1960-2015, this study first systematically analyzes the spatiotemporal distribution features of the 200-hPa WP in the SWEA on different scales, especially during the Meiyu season in the Yangtze-Huaihe region and during the rainy period in North China, by using spectral decomposition and period analysis. It is found that in the 56-yr mean fields, the 200-hPa WP in the SWEA is collocated with the East Asian subtropical jet (EASJ), with the centers of the two systems coincidentally overlapped. The WP filed in the subtropical westerly mainly comprises planetary- and synoptic-scale waves. The quasi-stationary planetary-scale wave seems to determine the shape and intensity of the WP in the SWEA, while the synoptic-scale wave is closely related to the local central intensity of the WP. The daily evolution of the 56-yr mean fields shows that, following the northward (southward) movement of SWEA from 1 June to 31 August, the planetary-scale WP in the SWEA becomes gradually weakened (intensified) whereas the synoptic-scale WP is slightly intensified (weakened). The results also reveal that during the Meiyu season in the Yangtze-Huaihe region, the WP in the SWEA moves northward slowly around 37°-39°N, demonstrating a quasi-biweekly oscillation in its geographic location, which is largely attributed to the strong (weak) planetary-scale (synoptic-scale) component. On the contrast, in the summer rainy season over North China, the WP in the SWEA further strives northward beyond 40°N, showing both quasi-biweekly and weekly oscillations in its position; meanwhile, the planetary-scale wave in the SWEA becomes weakened whereas the synoptic-scale wave is enhanced. These salient variational features of the WP in the SWEA and its scale-dependent components may be useful for the medium-range forecast of the rain belt migration in eastern China.
A Heavy Rainfall Event in Autumn over Beijing—Atmospheric Circulation Background and Hindcast Simulation Using WRF
Xiangrui LI, Ke FAN, Entao YU
2018, 32(3): 503-515 [Abstract]( 65 ) HTML PDF (7703 KB)  ( 119 Supplemental Material
Abstract:Heavy rainfall events often occur in Beijing during summer but rarely in autumn. However, during 3-5 September 2015, an exceptionally heavy rainfall event occurred in Beijing. Based on the reanalysis data and the Weather Research and Forecasting (WRF) model simulations, the main contributing factors and the predictability of this heavy rainfall event were examined through comprehensive analyses of vorticity advection and water vapor transport/budget. The results indicate that a "high-in-the-east-low-in-the-west" pattern of 500-hPa geopotential height over the Beijing area played an important role. The 850-hPa low-level jet (LLJ) provided a mechanism for rising motion and transported abundant water vapor into the Beijing area. Two-way nested hindcast experiments using WRF well reproduced the atmospheric circulation and LLJ. Quantitative analysis indicates that the WRF model with the rapid update cycle (RUC) land surface scheme and the single-moment 6-class (WSM6) microphysics scheme exhibited the best skill, and the model performance improved with a higher resolution. Further analysis indicates that the bias in the precipitation forecast was caused by the bias in water vapor transport.
Erratum to: Regional Meteorological Patterns for Heavy Pollution Events in Beijing
Ting YOU, Renguang WU, Gang HUANG, Guangzhou FAN
2018, 32(3): 516-516 [Abstract]( 32 ) HTML PDF (1229 KB)  ( 9
Abstract:This erratum is to report a figure error in You et al. (2017) entitled "Regional Meteorological Patterns for Heavy Pollution Events in Beijing." In You et al. (2017), Fig. 3 and its caption should be replaced by the figure (Fig. 3) here in the left column and the caption above.
We sincerely apologize for the mistake and hope this erratum does not reflect poorly on the paper's overall quality. It is important to note that the conclusions about regional meteorological patterns for heavy pollution events in Beijing still hold.
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