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2018 Vol. 23, No. 2

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Summer Rainfall-SST Relationships in the Western North Pacific Simulated by the FGOALS Model with Ocean Assimilation
Liwei ZOU, Donghuan LI, Tianjun ZHOU, Bo WU
2018, 23(2): 139-149. doi: 10.3878/j.issn.1006-9585.2017.17006
Abstract(1273) PDF (6275KB)(1400)
This study evaluates the performance of FGOALS (Flexible Global Ocean-Atmosphere-Land surface-Sea ice coupled model) with ocean assimilation in the simulation of summer rainfall-SST relationship during 1979-2005 in the western North Pacific (WNP), and compares the results with corresponding simulations forced by observed sea surface temperature and FGOALS historical simulation. Results show that the FGOALS with ocean assimilation well captures the interannual variability of summer SST over the WNP except that over east of the Philippines. For the interannual variability of precipitation, it barely demonstrates any skill over Asian summer monsoon region, which is comparable to the AMIP (Atmospheric Model Intercomparison Project) simulation. However, for the summer rainfall-SST relationship, the observed negative correlations over South China Sea and east of Philippines are partly reproduced in the FGOALS with ocean assimilation, in particular when the precipitation leads SST by one month and is concurrent with SST. The simulated skill is better than the AMIP simulation, but is inferior to the historical simulation. Based on observations, anomalous convection and circulation in the summer over the WNP are primarily driven by SST anomalies over the area near the dateline and the eastern Indian Ocean-Maritime Continent. The induced anomalous convections affect solar radiation reaching the sea surface, which contributes significantly to local SST anomalies and leads to negative SST-rainfall correlation and SST tendency-rainfall correlation. In the AMIP simulation, the anomalous circulation over the WNP driven by the remote forcing is underestimated. Since the AMIP simulation is forced by observed SST, the anomalous convection and circulation are forced by underlying SST over some places of the WNP, leading to positive rainfall-SST correlation. Although the anomalous circulations over the WNP driven by the remote forcing are also underestimated in both FGOALS with ocean assimilation and historical simulation, weaker than observed negative SST-rainfall correlations are produced since local air-sea coupling is included. In addition, the historical simulation tends to overestimate the forcing from SST anomalies over the WNP south of 20°N, which leads to better simulated SST-rainfall correlation than the FGOALS with ocean assimilation over South China Sea and south of Japan islands.
Possible Impacts of Two Types of El Niño Events on the Western North Pacific Monsoon Trough and Tropical Cyclogenesis
Hongjie ZHANG, Liang WU, Ronghui HUANG
2018, 23(2): 150-160. doi: 10.3878/j.issn.1006-9585.2017.17055
Abstract(1320) PDF (4702KB)(1495)
The present study analyzes the monsoon trough and tropical cyclone (TC) genesis over the western North Pacific to preliminarily investigate the anomalous monsoon trough and its possible effect on TCs during different types of El Niño events for the period of 1948-2015. It is shown that, compared with that in the eastern Pacific warming (EPW) years, the monsoon trough is weaker and its position leans toward the west and north during the central Pacific warming (CPW) years. In these years, the warmer (cooler) sea surface temperature (SST) over the central (western and eastern) Pacific induces anomalous westerly winds from the central to western Pacific in the tropical region and stronger than normal ascending motions and convective activities over the central Pacific, which can induce a strengthened and eastward extending monsoon trough. Meanwhile, the western Pacific subtropical high is weaker than normal and shifted northward, which leads to the northward displacement of monsoon trough. During EPW years, however, warmer (cooler) SSTs occur over the eastern (western) equatorial Pacific; anomalous westerly winds significantly extend eastward in the tropics; the ascending branch of the anomalous Walker circulation shifts eastward to the eastern Pacific; the monsoon activity becomes stronger; the subtropical high intensifies and leans toward the south. All the above changes are favorable for a stronger monsoon trough that extends more eastward compared to that in CPW years. Further study reveals that large-scale environmental factors that are related to TCs genesis will change with variations in the intensity and position of the monsoon trough. During CPW years, the cyclonic vorticity in the lower troposphere, the divergence in the upper-level, the higher relative humidity in the middle troposphere and the lower vertical wind shear all move toward the north with the monsoon trough. In EPW years, the above factors follow the monsoon trough to move southward and westward. These changes prompt the location of TC genesis over the western North Pacific to lean northward and eastward compared to that in EPW years.
Interdecadal Variations and Mechanisms of the Seesaw Pattern for Winter Surface Temperature between Northern and Southern Eurasia
Yiling LI, Chenghai WANG, Feimin ZHANG
2018, 23(2): 161-175. doi: 10.3878/j.issn.1006-9585.2017.17059
The characteristics and mechanism of the spatial and temporal seesaw pattern of winter surface temperature between northern and southern Eurasia was analyzed. Results show that the seesaw pattern of winter surface temperature between northern and southern Eurasia is evident with the boundary located at about 55°N. Besides, the interannual and interdecadal variations of winter surface temperature over Eurasia are also significant during 1961-2015. On the interdecadal scale, planetary waves in the Northern Hemisphere such as the "Three Troughs/Ridges in winter" are closely associated with the seesaw pattern. In addition, this seesaw pattern on the interdecadal scale also has an obvious transitional feature on seasonal scale. Results that exclude the interdecadal and global warming signals indicate that winter surface temperature in Eurasia is significantly correlated with the Arctic sea ice extent in autumn. The decrease in Arctic sea ice extent is one of the main external forcing factors that cause decreases in winter surface temperature in Eurasia, especially in central Asia. Further analysis shows that on interannual time scale, the seesaw pattern has a significant correlation with the synchronous sea surface temperature that displays a "three-pole" structure in the North Atlantic Ocean.
The Impact of Land Use/Land Cover Changes under Different Greenhouse Gas Concentrations on Climate in Europe
Zuheng HU, Zhongfeng XU, Zhuguo MA
2018, 23(2): 176-184. doi: 10.3878/j.issn.1006-9585.2017.17010
Abstract(1178) PDF (2225KB)(1540)
Human activities have modified the Earth's climate through emissions of greenhouse gases and land-use and land-cover changes (LULCC). To investigate the impacts of LULCC under different greenhouse gas concentrations (GHG) on regional climate, several numerical experiments are performed using the fully coupled community earth system model (CESM). Results are as follows. Under the concentrations of greenhouse gases around 1850, the LULCC lead to increases in precipitation in central and eastern Europe, while under concentrations of greenhouse gases around 2000, the LULCC lead to decreases in precipitation in central and eastern Europe. Under the increased GHG concentration, LULCC lead to changes in the atmospheric circulation from convergence to divergence and decrease the air temperature and moisture content. These changes play a dominant role in determining the net impact of LULCC on regional precipitation.
Application of Time-Scale Decomposition Statistical Method in Climatic Prediction of Summer Extreme High-Temperature Events in South China
Sisi CHEN, Jingyong ZHANG, Gang HUANG
2018, 23(2): 185-198. doi: 10.3878/j.issn.1006-9585.2017.16220
Abstract(1294) PDF (1652KB)(1839)
A time-scale decomposition (TSD) method to statistically downscale the predictand and predictors is used for seasonal forecast of summer extreme high temperature events (hot days) in South China. The hot days present a significant variability that is associated with distinct possible predictors. Both the hot days and the possible predictors are decomposed into inter-decadal and inter-annual components by fast flourier transformation filtering. Three downscaling regression models are then separately set up for the total hot days and the inter-decadal and inter-annual components of hot days. The downscaling regression model of the total hot days is named as direct regression model, while the downscaled inter-decadal and inter-annual regression models are combined together and named as TSD statistical regression model to obtain the total hot days. The fitting results of the direct regression model and TSD statistical regression model are tested by 10-fold cross-validation. The results show that compared to the direct regression model, the TSD statistical regression model decreases the root-mean-square error (RMSE) from 2.6 d to 2.3 d and increases the correlation coefficient with observations from 0.69 to 0.73 for the inter-decadal component; the TSD statistical regression model also decreases the RMSE from 3.2 d to 2.9 d and increases the correlation coefficient from 0.4 to 0.48 for the inter-annual component; for total hot days, the TSD statistical regression model decreases the RMSE from 4.1 d to 3.7 d and increases the correlation coefficient from 0.48 to 0.68. The hindcast results of hot days during 1979-2010 show that the correlation coefficient between observations and outputs of the direct regression model is 0.57, while the value is improved to 0.72 by the TSD statistical regression model. The forecast results of hot days during the independent validation period (2011-2013) show that the relative RMSE is 26.4% by the direct regression model, and it is 12.3% by the TSD statistical regression model. Compared with observations, both of the direct regression model and the TSD statistical regression model can predict the hot days to some extent in South China, and the TSD statistical regression model performs better for forecasts during 1979-2013.
Large-Scale Circulation Characteristics Corresponding to the Cold and Rainy Summer of 2014 in Zhejiang Province and the Relationship with SST Anomalies
Zhuolei QIAN, Xiaofen LOU, Jiehua MA, Lili JIANG, Ling LUO
2018, 23(2): 199-209. doi: 10.3878/j.issn.1006-9585.2017.17012
Abstract(1207) PDF (3730KB)(1290)
Using NECP/NCAR reanalysis data, NCC (National Climate Center) and NOAA (National Oceanic and Atmospheric Administration) relevant data, large-scale circulation characteristics and sea surface temperature corresponding to the cold and rainy summer of 2014 in Zhejiang Province are analyzed. The results are as follows. The anomalous cyclonic circulation in the middle and low latitudes from eastern China to southern Japan was conducive to cold and rainy summer in Zhejiang, while the abnormally strong and southward expanding western Pacific subtropical high was also an important factor for the extremely cold and rainy August; EAP and EU were two main teleconnection patterns that affected the abnormal cold and rainy summer in Zhejiang Province. The negative EAP (East Asia-Pacific teleconnection pattern) phase and positive EU (Eurasian teleconnection pattern) phase were in favor of cold air accumulation and southward moving. The cold and warm air intersected over Zhejiang Province, leading to cold rainy weather. The rare cold rainy August was the synergistic result of negative EAP phase and positive EU phase. Further analysis reveals that the warm ENSO phase triggered a strong abnormal descending airflow and an anticyclone over the western Pacific in August, which forced the western Pacific subtropical high to move southward and become stronger, while EAP wave trains occurred over East Asia in response. The maintenance of the warm IOBW (Indian Ocean Basin Wide) phase further weakened convective activities over the Maritime continent in August. Intraseasonal SST changes in the central North Atlantic might be associated with the shifts of the EU phase.
Analysis on the Characteristics and Formation Mechanisms of Air Pollution Episodes in Tianjin during Spring Festival of 2015
Wan CAO, Miao TANG, Peng XUE, Zirui LIU, Lili WANG, Kui CHEN, Guiqian TANG, Dongsheng JI
2018, 23(2): 210-220. doi: 10.3878/j.issn.1006-9585.2017.16109
Abstract(1334) PDF (6744KB)(1461)
To study the impact of firework discharge on air quality in Tianjin, fine particulate matter (PM2.5, aerodynamic diameter is less than or equal to 2.5 microns particulate matter) and its major chemical compositions as well as meteorological variables were measured in Tianjin from 18 Feb to 7 Mar using various instruments like the Tapered Element Oscillating Microbalance PM2.5 instruments with Filter Dynamic Measurement System, an online Rapid Collector of Fine Particles and Ion Chromatography system, an online OC/EC analyzer, a ceilometer and a microwave radiometer and so on. In this study, three air pollution episodes are selected for analysis. The results show that the implementation of policies of prohibition and restriction on firework discharge resulted in a decline in firework discharge volume and the hourly maximum value of PM2.5 concentration on the Chinese New Year's Eve of 2015 was lower than that in 2014; the set-up of areas of prohibitions and restrictions on firework discharge led to obvious spatial differences in PM2.5 concentration in Tianjin with the maximum difference of 394 μg/m3. The attenuated backscatter densities were similar everywhere from the surface to 80-m height due to the impact of fireworks discharge, suggesting identical vertical distribution of PM2.5 below 80 m. This result is consistent with previous measurements. Air pollution episode 1 (EP 1) was caused by dense discharge of fireworks, in which the main chemical species in PM2.5 are K+, SO42-, and Cl-. Air pollutants like SO2 and CO evidently increased during this episode. The formation of air pollution episode 2 (EP 2) was attributed to unfavorable meteorological conditions including temperature inversion, wetness inversion, convergence and high relative humidity, which promoted the secondary transformation of SO42- and NO3- on the surfaces of particles from firework discharge via heterogenous reactions during EP 2. The third air pollution episode (EP 3) was initiated by the widespread discharge of fireworks on the Lantern Festival and characterized by complex pollution. Collaborative growth of the secondary inorganic components and the O3 concentration were found during this episode. In addition, regional transport also played an important role in EP 3.
Decadal Variability of Atlantic Meridional Overturning Circulation (AMOC) Simulated with FGOALS-s2 Model
Xiaolan LI, Yongqiang YU
2018, 23(2): 221-234. doi: 10.3878/j.issn.1006-9585.2017.17047
Decadal variability of Atlantic Meridional Overturning Circulation (AMOC) has been studied in the Flexible Global Ocean-Atmosphere-Land System model, version s2 (FGOALS-s2) preindustrial simulation. Traditionally, the streamfunction on depth coordinate is used to represent AMOC. According to correlations between the AMOC index on density coordinate at 49.5°N and AMOC index at all latitudes north of 34°S, the authors find that AMOC variations propagate southward from the deep-water formation region. Furthermore, the AMOC variability on density coordinate is larger at high latitudes North Atlantic than at low latitudes North Atlantic and South Atlantic. Moreover, the AMOC displays a low-frequency variability with a period of about 70 years. The low-frequency oscillation is related to the interactions between temperature and salinity variations that are associated with the AMOC variations and surface winds. The physics of this decadal variability is as follows. Surface winds in the Greenland-Iceland-Norwegian (GIN) seas are anomalously strong, leading to abnormally large evaporation and thus the positive sea surface salinity (SSS) anomalies. The SSS anomalies strengthen the convection in the GIN seas and subsequently intensify the AMOC. This leads to a strengthened northward heat transport and decreases the meridional temperature gradient and surface winds, which completes the phase reversal.
Numerical Simulation of a Freezing-Rain Process in Beijing Capital International Airport and the Mechanism for Its Formation
Guijun LIU, Fei WANG, Han SHEN, Xiangyang FENG, Wei ZHANG
2018, 23(2): 235-240. doi: 10.3878/j.issn.1006-9585.2018.17051
Abstract(1124) PDF (2377KB)(1329)
In order to investigate the possible mechanism of freezing rain process, the WRF model (Weather Research and Forecasting) was used to simulate a freezing-rain process occurred on 31 Jan 2013 in the area of the Beijing Capital International Airport. The NECP reanalysis data, the Black Body Temperature (TBB) of FY-2E, the daily average cloud height data of FY-3A are used to verify the model simulation. Major conclusions are as follows:1) The cloud top temperature of the simulation is consistent with TBB of FY-2E. During the freezing rain process, the cloud top temperature varied between 0--6℃, and the simulated vertical distribution of the hydrometeors agree well with observations. According to the cloud top height derived from FY-3A observations, it is found that the cloud top in the freezing-rain process basically remained at around 3 km, and the clouds contained less microphysical solid contents. 2) Observed cloud top temperature and height and the simulation results show that this is a typical warm freezing rain process. This result indicates that the mechanism for the formation of freezing rain in northern China may not be single. Ice-crystal processes and warm-rain processes may simultaneously exist during the formation of freezing rain.
Scenario Analysis of Drought/Flood Change in the Middle and Late 21st Century in South China
Meishuang WU, Weibiao LI, Yinlong XU, Xiuzhen LI, Min ZHANG
2018, 23(2): 241-251. doi: 10.3878/j.issn.1006-9585.2017.17018
Abstract(1189) PDF (1009KB)(1497)
This paper exploits the regional climate mode PRECIS (Providing Regional Climates for Impacts Studies) to analyze the changes of the drought and flood indices in South China in the middle and later period of the 21st century under the SRES A1B (Special Report on Emissions Scenarios A1B) scenario compared with the climatic reference period (1961-1990). Plenty of drought and flood indices are utilized first to verify the capacity of PRECIS for drought and flood indices simulation. Possible changes of the future drought and flood conditions in South China are preliminarily discussed. Results indicate that PRECIS can well simulate yearly and monthly changing features of the selected drought and flood indices. Under the SRES A1B scenario, the frequency and intensity of extreme precipitation event in South China in the middle and later period of the 21st century will increase dramatically, and the period of precipitation extreme will increase too, thereby the possibility of flood disaster occurrence will grow to a large extent. Meanwhile, possible changes of meteorological drought in spring, summer and autumn in South China is not obvious, but in winter the possibility will rise, especially in the winters of the last thirty years of the 21st century, the possibility will be higher.
A Comparative Study on Three Types of Remote Sensing Solar Radiation Products
Jing LI, Dan WANG
2018, 23(2): 252-258. doi: 10.3878/j.issn.1006-9585.2017.17038
Abstract(1274) PDF (4407KB)(1489)
The verification of remote sensing radiation products mainly focuses on comparative analyses of single products and measured data in the Qinghai-Tibet Plateau region. The lack of comparison between the products make it hard to determine the applicability of these products. Therefore, a systematic comparative study of the applicability of these products in China is conducted in the present study. GEWEX-SRB (Global Energy and Water Cycle Experiment-Surface Radiation Budget), CERES-EBAF (Clouds and Earth's Radiant Energy Systems-Energy Balanced and Filled), and GLASS-DSR (Global LAnd Surface Satellite-Downward Shortwave Radiation) radiation products are chosen to verify their accuracy. The correlation coefficient, the mean bias error, and root mean square error are taken as evaluation indexes, and the accuracy of the temporal and spatial distribution of these products in China is discussed. GEWEX-SRB and CERES-EBAF over the period from 2000 to 2007 are compared and analyzed. Results indicate both of them have the same overestimation and underestimation tendencies. The most severe underestimation and overestimation both occur at Mount Emei Station and Taiyuan Station, and the accuracy of CERES-EBAF is higher than that of GEWEX-SRB. GLASS-DSR and CERES-EBAF over the period from 2008 to 2010 are also compared and analyzed. It is found that CERES-EBAF has a higher accuracy. Overall, these three radiation products all have a high accuracy. The underestimated sites are mostly located at high altitudes. The CERES-EBAF product covers a long time period with a relatively high accuracy, especially in eastern China and most of the coastal region in China.