Yan Zhongwei

Homogenized Daily Mean/Maximum/Minimum Temperature Series for China from 1960-2008

Abstract Inhomogeneities in the daily mean/maximum/ minimum temperature (Tm/Tmax/Tmin) series from 1960–2008 at 549 National Standard Stations (NSSs) in China were analyzed by using the Multiple Analysis of Series for Homogenization (MASH) software package. Typical biases in the dataset were illustrated via the cases of Beijing (BJ), Wutaishan (WT), Ürümqi (UR) and Henan (HN) stations. The homogenized dataset shows a mean warming trend of 0.261/0.193/0.344ºC/decade for the annual series of Tm/Tmax/Tmin, slightly smaller than that of the original dataset by 0.006/0.009/0.007ºC/decade. However, considerable differences between the adjusted and original datasets were found at the local scale. The adjusted Tmin series shows a significant warming trend almost everywhere for all seasons, while there are a number of stations with an insignificant trend in the original dataset. The adjusted Tm data exhibit significant warming trends annually as well as for the autumn and winter seasons in northern China, and cooling trends only for the summer in the middle reaches of the Yangtze River and parts of central China and for the spring in southwestern China, while the original data show cooling trends at several stations for the annual and seasonal scales in the Qinghai, Shanxi, Hebei, and Xinjiang provinces. The adjusted Tmax data exhibit cooling trends for summers at a number of stations in the mid–lower reaches of the Yangtze and Yellow Rivers and for springs and winters at a few stations in south–western China, while the original data show cooling trends at three/four stations for the annual/autumn periods in the Qinghai and Yunnan provinces. In general, the number of stations with a cooling trend was much smaller in the adjusted Tm and Tmax dataset than in the original dataset. The cooling trend for summers is mainly due to cooling in August. The results of homogenization using MASH appear to be robust; in particular, different groups of stations with consideration of elevation led to minor effects in the results.

Influence of inhomogeneity on the estimation of mean and extreme temperature trends in Beijing and Shanghai

Inhomogeneities in the temperature series from Beijing and Shanghai are analyzed, using the detailed histories of both sets of observations. The major corrections for different periods range from −0.33 to 0.6°C for Beijing and −0.33 to 0.3°C for Shanghai, Annual mean and extreme temperature series are deduced from the daily observations and trends in the adjusted and unadjusted series are compared. The adjusted yearly mean temperatures show a warming trend of 0.5°C/ century since the turn of this century and an enhanced one of 2.0°C/ century since the 1960s. In contrast, the unadjusted data show a twice this value trend for Shanghai but little trend for Beijing at the long-term scale and overestimate the recent warming by 50%–130%. Beijing experienced a decrease of frequency of the extremes together with a cooling during the 1940s–1970s and an increase of frequency of extremes together with a warming since then. The trends of frequency of extremes at Shanghai were more or less opposite. It is implied that the regional trends of strong weather variations may be different even when the regional mean temperatures coherently change.

Trends in Seasonal Precipitation over China during 1961-2007

Abstract Trends in six indices of precipitation in China for seasons during 1961-2007 were analyzed based on daily observations at 587 stations. The trends were estimated by using Sen‧s method with Mann-Kendall‧s test for quantifying the significance. The geographical patterns of trends in the seasonal indices of extremes were similar to those of total precipitation. For winter, both total and extreme precipitation increased over nearly all of China, except for a small part of northern China. Increasing trends in extreme precipitation also occurred at many stations in southwestern China for spring and the midlower reaches of the Yangtze River and southern China for summer. For autumn, precipitation decreased in eastern China, with an increasing length of maximum dry spell, implying a drying tendency for the post-rainy season. Wetting trends have prevailed in most of western China for all seasons. The well-known ‘flood in the south and drought in the north’ trend exists in eastern China for summer, while a nearly opposite trend pattern exist for spring.

Simulation of Precipitation in Monsoon Regions of China by CMIP3 Models

Abstract The output of 25 models used in the Coupled Model Intercomparison Project phase 3 (CMIP3) were evaluated, with a focus on summer precipitation in eastern China for the last 40 years of the 20th century. Most models failed to reproduce rainfall associated with the East Asian summer monsoon (EASM), and hence the seasonal cycle in eastern China, but provided reasonable results in Southwest (SW) and Northeast China (NE). The simulations produced reasonable results for the Yangtze–Huai (YH) Basin area, although the Meiyu phenomenon was underestimated in general. One typical regional phenomenon, a seasonal northward shift in the rain belt from early to late summer, was completely missed by most models. The long-term climate trends in rainfall over eastern China were largely underestimated, and the observed geographical pattern of rainfall changes was not reproduced by most models. Precipitation extremes were evaluated via parameters of fitted GEV (Generalized Extreme Values) distributions. The annual extremes were grossly underestimated in the monsoon-dominated YH and SW regions, but reasonable values were calculated for the North China (NC) and NE regions. These results suggest a general failure to capture the dynamics of the EASM in current coupled climate models. Nonetheless, models with higher resolution tend to reproduce larger decadal trends and annual extremes of precipitation in the regions studied.

Multidecadal variability of dry/wet patterns in eastern China and their relationship with the Pacific Decadal Oscillation in the last 413 years

We analyzed dryness/wetness indices at 55 sites across eastern China (east of 105°E) for the period AD 1600-2012 using the Ensemble Empirical Mode Decomposition (EEMD) method to investigate decadal-to-centennial timescale variations of dry/wet patterns in this region and their relationship with the Pacific Decadal Oscillation (PDO). Empirical Orthogonal Function (EOF) analysis shows that the most important regional dry/wet pattern is a monopole (consistently dry or wet throughout eastern China), with a center of variability in northern China. The southern-flood-northern-drought (SFND) pattern represents a secondary type of variability for the historical period; however, this tended to intensify and has become the dominant pattern over the 20th century. Multidecadal SFND events, such as that prevailing from the late 1970s to the early 21st century, have rarely occurred during the last 413 years. Multi-decadal dry/wet variations in northern China exhibit positive correlation with an ensemble mean PDO index of various reconstructed indices throughout this historical period.

Updated Homogenized Chinese Temperature Series with Physical Consistency

Abstract Most methods of homogenization of climate data are applied to time series of a single variable, such as daily maximum temperature (Tmax) or daily minimum temperature (Tmin). Consequently, the physical relationship among different variables, e.g., Tmax>Tmin, may be distorted after homogenization of climate series of individual variables. The authors develop a solution to improve consistency among diurnal temperature records, while using the Multiple Analysis of Series for Homogenization (MASH) method to homogenize the observation series of daily mean temperature (Tm), Tmin, and Tmax at 545 stations in China for the period 1960–2011, called CHTM2.0. In the previous version of this homogenized dataset based on MASH (CHTM1.0) for the period 1960–2008, there are a few records (0.039% of the total) that are physically inconsistent. For developing CHTM2.0, the authors apply additional adjustments for each day with inconsistent temperature records, in order to hold Tmax>Tm>Tmin. Although the additional adjustments are barely influential for estimating long-term climate trends in China as a whole (because very few records are additionally adjusted), the newly introduced solution improves the physical consistency throughout the dataset. It is also helpful for developing more reasonable homogenized climate datasets with regard to physical consistency among multiple variables. Based on CHTM2.0, the annual Tmax/Tm/Tmin series averaged over China for the period 1960–2011 show significant warming trends of about 0.19/0.25/0.34°C per decade, respectively. Large warming trends of up to 0.425/0.596/0.704°C per decade occur in northeastern and northwestern China.

Characteristics and Changes of Cold Surge Events over China during 1960-2007

Abstract This paper demonstrates regional characteristics, a long-term decreasing trend, and decadal variations in the frequency of cold surge events based on daily mean temperature and daily minimum temperature data in mainland China from 1960 to 2008. During these 48 years, four high frequency centers of cold surge events were located in Xinjiang, central North China, northeast China, and southeast China. A main frequency peak of cold surge events occurs in autumn for the four regions and another peak is detected in spring over northeast China and southeast China. The regional pattern of cold surge frequencies is in accordance with the perturbation kinetic energy distribution in October—December, January, and February—April. The long-term decreasing trend (–0.2 times/decade) of cold surge frequencies in northeast China and decadal variations in China are related to the variations of the temperature difference between southern and northern China in the winter monsoon season; these variations are due to the significant rising of winter temperatures in high latitudes.

A Spatial Cluster Analysis of Heavy Rains in China

Abstract Clustered heavy rains (CHRs) defined using hierarchical cluster analysis based on daily observations of precipitation in China during 1960–2008 are investigated in this paper. The geographical pattern of CHRs in China shows three high-frequency centers—South China, the Yangtze River basin, and part of North China around the Bohai Sea. CHRs occur most frequently in South China with a mean annual frequency of 6.8 (a total of 334 times during 1960–2008). June has the highest monthly frequency (2.2 times/month with a total of 108 times during 1960–2008), partly in association with the Meiyu phenomenon in the Yangtze River basin. Within the past 50 years, the frequency of CHRs in China has increased significantly from 13.5 to 17.3 times per year, which is approximately 28%. In the 1990s, the frequency of CHRs often reached 19.1 times per year. The geographical extent of CHR has expanded slightly by 0.5 stations, and its average daily rainfall intensity has increased by 3.7 mm d-1. The contribution of CHRs to total rainfall amount and the frequency of daily precipitation have increased by 63.1% and 22.7%, respectively, partly due to a significant decrease in light rains. In drying regions of North and Northeast China, the amounts of minimal CHRs have had no significant trend in recent years, probably due to warming in these arid regions enhancing atmospheric convectivity at individual stations.

Changes in Local Growing Season Indices in Shanghai Due to Urbanization during 1873–2013

Abstract The contribution of urbanization to the changes in growing season indices (e.g., the start date of the growing season, Ds) in Shanghai, eastern China, for the period 1873–2013 is assessed. The urban-related Ds advancement (referred to as the urbanization contribution, UC) is reconstructed based on the changing urban-land fraction within a grid box around Shanghai for 1873–2013. After removing the UC from the raw Ds time series, the secular trend (ST)representing the long-term climatic warming and multidecadal variability (MDV) representing an oscillatory component with a period of several decades are obtained by using the Ensemble Empirical Mode Decomposition (EEMD) method. Results show that the UC is comparable with the contribution of the ST for the period 1873–2013, when Ds exhibits an advancing rate of about 0.44 days per decade. MDV plays an important role in regulating changes in Ds on the decadal time-scale, e.g., with a contribution of about 56.1% for 1981–2013.

Changes in Seasonality in China under Enhanced Atmospheric CO2 Concentration

Abstract Seasonality changes in China under elevated atmospheric CO2 concentrations were simulated using nine global climate models, assuming a 1% per year increase in atmospheric CO2. Simulations of 20th century experiments of season changes in China from the periods 1961–80 to 1981–2000 were also assessed using the same models. The results show that the ensemble mean simulation of the nine models performs better than that of an individual model simulation. Compared the mean climatology of the last 20 years in the CO2-quadrupling experiments with that in the CO2-doubling ones, the ensemble mean results show that the hottest/coldest continuous-90-day (local summer/winter) mean temperature increased by 3.4/4.5°C, 2.7/2.9°C, and 2.9/4.1°C in North-east (NE), Southwest (SW), and Southeast (SE) China, respectively, indicating a weakening seasonal amplitude (SA), but by 4.4/4.0°C in Northwest (NW) China, indicating an enlarging SA. The local summer lengthened by 37/30/66/54 days in NW, NE, SW, and SE China, respectively. In some models, the winter disappeared during the CO2-quadrupling period, judging by the threshold based on the CO2-doubling period. The average of the other model simulations show that the local winter shortened by 42/36/61/44 days respectively, in the previously mentioned regions.